The Brakes That Stop a 1,000 MPH Bloodhound SSC

cartechboy writes: "The problem: How do you stop the 1,000 mph Bloodhound SSC? The solution: Apparently you use steel rotors from AP Racing, which managed to absorb 4.6 kilowatts of energy on a test stand without failing although the Bloodhound team hasn't spun them up to the full 10,000 rpm just yet. During testing, a set of carbon rotors from a jet fighter shattered under the stress during a half-speed, 5,000-rpm test, thus the team switched to steel rotors. It's like stopping a bus from 160 mph on a wet road. That's how the engineers behind the Bloodhound SSC—the British land-speed record car designed to break the 1,000-mph barrier—described the task of stopping their creation once it's finished breaking the sound barrier. We'll have to wait to see if the steel rotors can handle the full 10,000 rpm run, but until then, it looks like steel is stronger than carbon when it comes to some instances."

Comments

Killowatts are power, not energy

[YesIAmAScript]

And 4.6kW isn't that much power anyway. About 60HP.

I've seen resistor boxes used for testing EVSEs that take 6.6kW and of course don't fail.

Re:Killowatts are power, not energy

[Anonymous Coward]

Power is energy over time.

Re:Killowatts are power, not energy

[Anonymous Coward]

Power is energy per time. But 6HP is correct.

Re:Killowatts are power, not energy

[hackertourist]

Brakes on ordinary cars are typically several times more powerful than the car's engine, so we're talking about several hundred kW of available braking power for an ordinary saloon. On one hand, Bloodhound is a 6-ton machine going 250 km/h when the brakes are applied which would suggest the figure needs to be higher than that. On the other hand, it'll have far less grip than rubber tires on tarmac can generate so it's not the maximum power dissipation that counts.

Re:Killowatts are power, not energy

[Connie_Lingus]

watch TFV...

it clearly states that air brakes will slow the machine from 1000mph to 160mph and the brakes are simply used for fine-tuning the stop location.

Re:Killowatts are power, not energy

[hackertourist]

As others have said, Bloodhound already uses airbrakes for higher speeds. The disk brakes are used when the airbrakes become ineffective at lower speeds.
NASCAR is 200 mph, not 300 (and 1/4 the weight). And NASCAR brakes don't have to survive rotating at 1600 km/h. At that speed, the centrifugal force is more than most materials can handle. Bloodhound's wheels are some of the biggest engineering challenges in the project, they have to withstand something like 50,000 G. The brakes are a bit easier because they're smaller, but still a major problem.

Re:Killowatts are power, not energy

[dskoll]

Hold on for a metre while I think about this... Yes, lazy reporters who get their units wrong should be sprayed with five square metres of water and then shocked with a 20-coulomb current. Maybe then they'll spend the small extra mass needed to do proper research...

Re:Killowatts are power, not energy

[saider]

Saying power is energy is like saying speed is distance.

Re:Killowatts are power, not energy

[mlyle]

getting zapped with 20 coulombs kinda makes sense, though.

Re:Killowatts are power, not energy

[dskoll]

Umm... no.

Re:Killowatts are power, not energy

[AK Marc]

Why not a non-contact inductive system? Use a regenerative braking system. But the techs working on this are automotive techs, so they think of the standard answers. It looks like braking performance isn't the hard part, but having the materials survive the top speed. So change the braking system to not use rotors at all.

Re:Killowatts are power, not energy

[AK Marc]

Anchors, or a really really compliant tailhook system.

Re:Killowatts are power, not energy

[AK Marc]

I didn't say they should have done it, but the question was why didn't they? That's not arm-chairing, but trying to learn. Why do you hate learning?

Re:Killowatts are power, not energy

[AK Marc]

Inductive brakes have calipers. Why do you assume they are necessary for all inductive braking systems?

Re:Killowatts are power, not energy

[parabyte]

The unit kilowatt is fine, but the number is ridiculously low. Cars breaks are typically designed to have a breaking power of four times the engine power, so we are talking about 500-1000 kW of breaking power in typical cars.

Modern high speed trains have a breaking power in the order of 10 - 20 MW using their engines for regenerative dynamic braking. The german ICE3 has about 16,4 MW dynamic breaking power, which is only slightly higher than the 16 MW propulsion power. It also has additional eddy current brakes, but they there breaking power is just 1600 kW, about 10% of the the dynamic breaking power. There are even additional disc brakes, but they are only used in emergency situatuions.

The largest burner on my gas stove has 5 kW power, but I don't think I can make a pan to glow in seconds, if at all.

Watching the video I would assume that they are talking about 4.6 MW oder 4600 kW of breaking power this disk can handle.

Re:Killowatts are power, not energy

[AK Marc]

Which article discusses it? None? Thought so.

Re:Killowatts are power, not energy

[pslytely psycho]

Well they seem to be very open to new technologies, and are combining quite a few to achieve their goal, I wonder if they maybe did look at it and dismissed it due to complexity/failure analysis/weight constraints?
Disk brakes are relatively simple, proven technology.

How much braking power do you think they could get within the same reliability and weight? Even though this is possibly (opinion, not fact checked) one of the heaviest vehicles to try with 3 types of motive force (piston/jet/rocket), weight will still have to play an important role in design. After all, they tried carbon.
Of course even if heavier they may have to select something along the lines you suggested regardless of weight if it turns out the steel can't support that RPM either...or maybe bigger parachutes (unless the rules require an active wheel based braking system..don't know and didn't google)?

Hmm, maybe the troll was right, perhaps you should talk to them.

Just interested in your opinion.

Re:Killowatts are power, not energy

[AK Marc]

You just described regular brakes, not regenerative ones. You are so busy proving me wrong that you forgot you should be right first before you correct someone.

Re:Killowatts are power, not energy

[cheater512]

Then why on earth are they shattering aircraft brakes? Just whack on any old brakes from your local auto shop.
Braking from 160mph is a lot for a regular car, but isn't out of the ordinary.

Re:Killowatts are power, not energy

[AK Marc]

I would have considered something in the axle, though no idea what they used for it. To better spread the stresses, longer than minimum axles are generally used, and putting small magnets embedded into them with copper wire around would have minimal effect on rotating mass and be able to provide non-contact stopping power. The other option I though of was putting the same thing in the wheel, but even the smallest weight in the wheel could have large knock-on effects.

Or what about going with friction braking. Have a roller come down on the top of the wheel, and generate resistance. Use the wheel itself as the braking surface. There are 100 ways I can think of for stopping a car without having brake disks. Drum brakes started out by having the calipers work from the outside in, before it was reversed to make the "drum" appearance style widely called drum brakes. Putting them in a drum increased performance, but increased cost and complexity. Going back to the basics and re-inventing automotive brakes could give them something better than adapting current brakes to a special situation. The precursers to drum brakes were lighter and cheaper than their replacement, but were bad for wear and wet weather performance, but something tells me they won't be taking their runs in the rain (but may need to consider the large amount of cast-off of the ground surface that could pollute the braking surface in them.

This article is devoid of scientific and engineering details. It's "ooh look, this car is so fast that it breaks fighter-jet brake rotors before even trying to use them." Yeah, cool. So if it's so hard, why didn't you try other ways? What are the pads, as regular pads will no work at the temperatures given. Also, I noted the rotors were vented, but not slotted or drilled. Is this because survivability is more important than effectiveness?

Re:Killowatts are power, not energy

[pslytely psycho]

Informative +1
Thank you.

"Also, I noted the rotors were vented, but not slotted or drilled. Is this because survivability is more important than effectiveness?"
As a former brake mechanic...I think you hit squarely on that.
The projects homepage appears to have quite a bit of information on the tech. I have not had time to read it yet. Perhaps you will find it interesting. (I think it should of been a linked page in the article personally) The linked pages were all 'gee whiz, looky here.'

http://www.bloodhoundssc.com/project/facts-and-figures/vehicle-technical-specification

Re:Killowatts are power, not energy

[pslytely psycho]

I am an idiot. I stated 3 types of motive force, when there is only two...Jet/Rocket. The piston engine is for the APU.

Re:Killowatts are power, not energy

[jeremyp]

Regenerative braking systems work by having a generator driven by the wheels that drives an electrical load - typically a battery charger. Charging the battery generates a current through the generator making it act like a motor but in the opposite direction to the way the wheels are making it spin.

Clearly storing charge in a battery is useless in a car whose only motive power is a rocket engine, so we can do away with that. We can just put a wire across the terminals of the generator. The generator itself can incorporate the axle as one of its parts, so it seems like it might work.

Of course, the wire will get very hot, so some form of cooling arrangement will be needed and since the wire is a continuous loop that goes through the generator, the cooling arrangement needs to keep the generator cool too.

Also, regenerative braking effect drops off at low speeds, so you'll need some ordinary disc brakes to bring the car to a complete halt.

This is all looking very complicated and heavy compared to the simple solution of metal discs and callipers.

Re:Killowatts are power, not energy

[AK Marc]

The projects homepage appears to have quite a bit of information on the tech. I have not had time to read it yet.

A lot of statistics and specifications, but not too much on why, or other more interesting questions.

I've seen more than one set of brakes break at a cross-drilled hole. Though it's possible to cast them with the holes, and lose the weak points induced by drilling. And slotting was selected because it gives most of the benefits of drilling, with no reduction in strength because the metal is carved, but not pierced. And the pads. Even racing car pads will start outgassing at those temperatures, and then without holes, your performance willl drop a lot. Though they are aiming for 0.3g of deceleration, about 1/10th the max for a race car, so aside from them falling apart at top speed, standard race-car (F1) brake systems should work fine. I also found it interesting that they put the caliper on the frame, not the suspension. But again, no reasoning for that unusual choice was given.

Re:Killowatts are power, not energy

[AK Marc]

So, nobody can ever be right if they weren't asked to work on the project? Your logic doesn't work.

Re:Killowatts are power, not energy

[AK Marc]

How do you have a wheel without an axle? The wheel must mount to something.

Re:Killowatts are power, not energy

[AK Marc]

This is all looking very complicated and heavy compared to the simple solution of metal discs and callipers.

Embedding magnets in the hub and a coil near. The weight is much less than discs and callipers. Yes, when you do it poorly, it will be a poor solution.

Re:Killowatts are power, not energy

[Strider-]

Regenerative braking systems work by having a generator driven by the wheels that drives an electrical load - typically a battery charger. Charging the battery generates a current through the generator making it act like a motor but in the opposite direction to the way the wheels are making it spin.

Actually, your typical freight train is running regenerative braking. If you look down on a locomotive going by, you can see huge fans in the top of the cab that are used to blow air over load coils. Figure a maximum sized train weighs in at 19,000 tonnes (130 car coal train), that means that when it's operating at 60km/hr it has roughly 2.6 gigajoules of kinetic energy. To stop it, that energy has to be dumped. Some of it is done by the train's air brakes, but most of it is done through regenerative braking in each of the locomotives.

This is all irrelevant when it comes to the SSC though, since it's using air resistance to drop its speed to 260kph, then slowing down with traditional brakes. The hard part is going to be making the brakes survive spinning at 10,000rpm, not dissipating the energy from slowing down.

Re:Killowatts are power, not energy

[michelcolman]

The problem was that, even though they don't use the brakes at high speeds, those break disks are still on the wheels and spinning at whatever speed the wheels are spinning at, for the entire duration of the run. And apparently just that centrifugal force was enough to shatter carbon brakes. Vibrations at 1000 mph over desert ground certainly didn't help either.

Re:Killowatts are power, not energy

[L4t3r4lu5]

Regular cars don't go 160MPH, and don't weigh 6 tonnes.

Re:Killowatts are power, not energy

[pslytely psycho]

Yes, I was disappointed when I got back and had time to read it. I was really hoping for some in-depth discussions of the technologies and reasoning behind their design choices.
A quick glance at the site before I posted gave me false hopes.

Re:Killowatts are power, not energy

[tibit]

They do, although they are not really called cars anymore. The problem isn't 160MPH nor 6 tonnes. The problem is that the brakes have to survive "storage" (not braking) at 10kRPM, since the car will be going ~1000mph at some point. Everyone is focusing at the low speed or relatively forgettable weight. Those are not the problems, even my "little" Volvo XC90, when loaded, weighs about 2.5 tonnes, and I'm sure its brakes would survive slowing it down from 160MPH with a 3 ton brakeless trailer attached to it. Meh. But I don't really know if the front brake discs would survive being spun up to 10kRPM.

Re:Killowatts are power, not energy

[tibit]

Of course if you actually looked at what you propose you'd realize that any inductive system still needs to use bulky rotors. It's the rotor's survivability that is the problem. The fact that it's a friction brake is rather inconsequential here. It's not the braking that is the problem. It's mere survivability of a disc brake at rotational speeds of an enterprise hard drive.

Re:Killowatts are power, not energy

[tibit]

"There are 100 ways I can think of for stopping a car without having brake disks."

You miss the forest for the trees. The article is just mumbo-jumbo. The wheel itself is a much larger diameter metal structure, subject to much larger stresses. Since the wheel isn't a problem, seemingly, then the friction disc brake isn't either. Remember that the disc brake doesn't have to operate at 1000mph, and doesn't have to endure the high centrifugal forces while being hot. Operation at 160mph is peanuts. Why did they goof and go with carbon discs I don't know, but the brakes aren't an issue at all. If the wheel survives, a similarly constructed brake disc will, too. The entire reason for having a separate braking disc and not using the wheel itself is the wear. The wheel has much larger diameter than the brake disc, so any braking wear on its circumference would require wheel rebalancing. That's an expensive, time consuming operation, since the wheel has to be balanced better than a hard drive spindle is balanced.

Re:Killowatts are power, not energy

[tibit]

The hard part is going to be making the brakes survive spinning at 10,000rpm, not dissipating the energy from slowing down.

Finally someone who gets it :) But that's not even the hard part. The hard part is making a much larger diameter wheel that will survive this. The brake disc is, comparably speaking, peanuts.

Re:Killowatts are power, not energy

[tibit]

Do you really need to write like an american teenager?

brakes, not breaks
braking, not breaking
their, not there

Sheesh :)

Re:Killowatts are power, not energy

[cellocgw]

Saying power is energy is like saying speed is distance.

But we all know Time is Money.

And speed is measured in parsecs.

Not to mention mass in eV .

Re:Killowatts are power, not energy

[AK Marc]

I've looked at it. I've actually done it (not for a car, but other purposes). The constraints you claim are self-imposed, not imposed by it being inductive.

Re:Killowatts are power, not energy

[AK Marc]

The wheel has much larger diameter than the brake disc, so any braking wear on its circumference would require wheel rebalancing.

The wheel has a solid rubber (looking) band around for traction. It's funny (to me at least) that they made rubber tread that can go 1000mph (well, actually 2000 mph relative to the ground at the top of the wheel) but their brakes fell apart. Proves to me that they thought about some things in great detail, and ignored others. But once you've slowed the wheel down to 150 mph, what's wrong with a brake that is a rollerblade wheel extended above the ground wheel, that contacts it and uses friction on the brake wheel's axle to slow the car. The complexity would be higher, but the massive sprung weight would be avoided.

But they proved they didn't consider anything "interesting" when they grabbed a carbon disc, expecting "carbon is better" and it failed. They didn't pre-think about the problem, but tried to grab the closest automotive solution. Smart people aren't very smart when they don't think about the problem.

Re:Killowatts are power, not energy

[AK Marc]

There's still an axle within the hub, even if it's smaller than the width of the wheel. The upright attaches to the wheel's axle, proving the wheel has an axle.

Re:Killowatts are power, not energy

[tibit]

So, you made a 1 megawatt inductive brake that was smaller than this disc brake was? You must be filthy rich then :)

Re:Killowatts are power, not energy

[tibit]

What I mean is: anyone able to pull off such a feat is on way to getting rich, rich, rich.

Re:Killowatts are power, not energy

[AK Marc]

It was scale, and this one, in a test case (harder than expected usage) was kW, not MW. Why are you demanding more for a hobby project than needed for a 1000 mph car, just to prove it's possible?

Face it, these guys grabbed the brakes off a 1000MPH+ vehicle, without thought, then they broke, so they tried something else. They didn't "research" anything. They iteratively tried things based off intuition until something worked. That's not science. That's not even very interesting.

Re:Killowatts are power, not energy

[patniemeyer]

FYI, Tesla automobile can generate about 60kW while in max regeneratively braking
and 4.5kW is only about 5hp.

Pat

Stronger?

[hubie]

If I had to guess, it isn't that steel is stronger in this case, but better at heat conduction/dissipation.

Re:Stronger?

[Drethon]

If heat is a problem, it seems like regenerative breaking could be a better option.

Re:Stronger?

[The Grim Reefer]

If heat is a problem, it seems like regenerative breaking could be a better option.

It's a jet and rocket powered car. How are you going to regenerate those with brakes? How much weight will they add? And finally, have regenerative brakes been built that would even be practical at 10K RPM?

Re:Stronger?

[Anonymous Coward]

When the brake pedal (or control or whatever) is pushed, redirect the jet / rocket exhaust out the front and accelerate in the opposite direction. It is just force vectors. You might need some ablative shield on the front where the exhaust exits. Sure, there will be some amount of loss as you do this (with a U shaped pipe or whatever) and it will need to handle the heat, but should prove more robust than exploding brakes.

Re:Stronger?

[Drethon]

Was thinking it could power internet egos, though I forgot they need far more energy.

Re:Stronger?

[xaxa]

Possibly he's mistaken regenerative brakes for resistive/rheostatic brakes. On diesel-powered railway locomotives (which are almost always electric motors powered by a diesel generator) there's a bank of resistors. The motors are run as generators, the electricity put through the resistors and lost as heat: http://en.wikipedia.org/wiki/D...

However, this vehicle doesn't have electric motors, so it's not applicable.

Re:Stronger?

[holmstar]

Why not just dump the electricity into a resistive coil? No need to store it.

Re:Stronger?

[Bengie]

Electric Heating coils, like what trains use.

Re:Stronger?

[Drethon]

Well the more responses the more people might read what is being responded to. So I guess those responding think it is important for some reason.

Re:Stronger?

[The Grim Reefer]

When the brake pedal (or control or whatever) is pushed, redirect the jet / rocket exhaust out the front and accelerate in the opposite direction. It is just force vectors. You might need some ablative shield on the front where the exhaust exits. Sure, there will be some amount of loss as you do this (with a U shaped pipe or whatever) and it will need to handle the heat, but should prove more robust than exploding brakes.

Uh huh. While that sounds great in theory, you need to keep in mind this is a 1000 mph land vehicle. What happens if at full speed the redirected thrust doesn't function quite right? At best the pilot/driver will need to change his/her underwear. Worst case he/she has to be hosed out of what is left of the car as it will become one fast moving uncontrollable centrifuge of death.

Steel rotors may not be elegant, but they are also fairly simple and we've understood the tech for a long time. I think I'd prefer a drag chute and rotors over some vectored thrust contraption. It's not like they are going to be doing continuous laps in this thing. Plus they won't need to entirely redesign the vehicle to accommodate a bunch of duct work and heat shielding.

Re:Stronger?

[Kielistic]

I assure you that dumping waste energy as heat as in classical braking systems will be far more effective and robust than any "regenerative" system. If you are having heat dissipation problems you will likewise have overload problems anywhere else you try to direct that energy- along with heat dissipation problems. Although most likely your regenerative system would just be destroyed by the forces involved.

Re:Stronger?

[Bartles]

Does it just sit in there until someone lets it out? You're dumping enough energy to get a car up to 1000mph hour into a coil. That coil better be very large, and have a huge heatsink or you're going to have a fire.

Re:Stronger?

[gl4ss]

have it melt some stuff in a crucible? or what?
or launch the energy as emf?

Re:Stronger?

[NatasRevol]

A freaking lightning bolt coming out of the tail as it slows would be spectacular.

Re:Stronger?

[BronsCon]

Of course, once he's learned to spell, you'll have no further quarrel with him, so step two should really be option two (and with a slight modification), as he really only need complete one step or the other.

Re:Stronger?

[AK Marc]

regenerate into a resistor. A regenerative system doesn't need to use contact or rotors.

Re:Stronger?

[AK Marc]

Putting magnets into the wheels and slowing the wheels through inductive forces would solve the rotor issue (though introduce its own). I think that was the core of the suggestion.

Re:Stronger?

[holmstar]

The "car" is traveling at high speed. It should be relatively simple to divert some of that fast moving air through the resistive coil to cool it. Probably a LOT easier than trying to dissipate the same amount of heat from the brake disks.

Re:Stronger?

[Zynder]

A KABOOM! There's supposed be an Earth-shattering KABOOM!

Re:Stronger?

[Zynder]

Excuse me AC, but would your name happen to be Wile E Coyote?

Re:Stronger?

[holmstar]

Also, we're not talking about 1000 to 0, there are parachutes to do most of the deceleration. The "regenerative" style braking would be for something like 200-0.

Re:Stronger?

[Bartles]

Acttually I went to the article, the summary is pretty misleading. The carbon brakes have to rotate with the wheels. At 1000mph they are turning 10,000 rpm and failed under the stresses, When it's time to stop the car airbrakes are deployed which slow the car to 160mph when conventional disk brakes are employed. The carbon brakes would certainly be more effective from 160mph to 0mph, but can't withstand 10,000rpm.

Re:Stronger?

[pjbgravely]

"Steel rotors" is probably another typo. I have heard of stainless steel rotors and the commonly used cast iron rotors but a solid steel rotor that is used on a motorcycle sounds like it would heat up too fast in this application. My guess that it is a standard cast iron vented rotor.

Re:Stronger?

[AK Marc]

You are incorrectly applying the technical term to a generic usage. Your ignorance doesn't demonstrate my error.

Re:Stronger?

[The Grim Reefer]

"Steel rotors" is probably another typo. I have heard of stainless steel rotors and the commonly used cast iron rotors but a solid steel rotor that is used on a motorcycle sounds like it would heat up too fast in this application. My guess that it is a standard cast iron vented rotor.

It's hard to say. They didn't state they were "solid" steel. They could be a 2 piece design that are separate from the hubs. . Those come in both solid and ventilated.

Re:Stronger?

[Demonantis]

Trains use regenerative breaking. The power is transferred to a bank of high amperage resistors. It works better since the heat is spread out to an area larger than the surface of the break pads.

Re:Stronger?

[jeremyp]

Because you still need ordinary brakes to bring the car to a complete stop.

Re:Stronger?

[tibit]

So, let me think. A simple chunk of metal that heats itself by friction, vs. a generator and a bunch of resistor coils. Yes, I thought so. Proposing regen braking for this project is insane. BTW, who the heck told you that heating is a problem? Disc brakes work just fine. On my wife's car, on dry pavement, I can hit 1MW of braking power for a second or two, no biggie. Braking a 6 ton vehicle from 160mph is no problem for disc brakes. Let me repeat: braking is not a problem at all. It's the survivability of a brake disc that wasn't necessarily designed for operation in enterprise hard drive spindle type of a job. A major problem with off-the-shelf brake discs in this application is fatigue cracking. They have various holes and radii that are not designed for the hard drive spindle operation called for here. The whole article is IMHO a big fat decoy, most likely an inadvertent one - due to ignorance, not malice.

Re:Stronger?

[wagnerrp]

Well... 160mph anyway... All you need to do is open a vent, and there will be plenty of convection to keep that coil cool.

Re:Stronger?

[tibit]

Let's get some perspective.

A well-loaded SUV doing pedal-to-the-metal ABS emergency braking from 100mph on dry pavement with summer tires can easily hit 1MW total braking power.
A passenger car doing some only mildly distracted late braking in city traffic easily pulls off 50kW braking power.
An old grandma's scooter can easily exceed 5kW of braking power in city traffic.

The 4.6kW figure is a typo, and anyone who takes it on face value is silly. I don't know where the heck it came from. A 6 ton vehicle doing 15 second braking from 160mph to stop needs to dissipate 1MW.

All figures are average power per vehicle, not per brake. Uneven braking will produce higher peak power.

Re:Stronger?

[tibit]

So, to anyone dumb enough to propose regen braking: take a good look at the size and weight of a 1MW generator, even a small duty cycle one, even water-cooled.

Re:Stronger?

[tibit]

It wouldn't solve anything, because you have no clue about mechanics of such composite assemblies. Just forget it. There is no rotor issue. Remember that the article doesn't mention any issues at all. They managed to shatter a carbon disk that wasn't designed for the job. Big deal. There are no other problems at all. It's just sensationalism and innuendo. Get over it.

Re:Stronger?

[AK Marc]

Yeah, the "smart people" guessed without planning and screwed up. They "solved" it by trying other things until one worked. They didn't demonstrate any forethought or planning. That proves to me that they didn't consider any unconventional brakes that would have worked much better for their application.

Watts is not energy

[Anonymous Coward]

4.6 kilowatts of energy

Kilowatts is a unit of power, not energy. Sheesh.

Re:Watts is not energy

[tibit]

There's no 4.6 anything. A 15 second braking of a 6 ton vehicle from 160mph to a stop dissipates 1MW (1,000 kilo Watts) on average. The vehicle energy at 160mph is 15MJ (15,000,000 Joules). The energy at 1000mph is 0.6GJ (600,000,000 Joules). The 4.6 is someone's figment of imagination.

Aerodynamics

[Anonymous Coward]

I think aerodynamic drag, even without a parachute, will help out a lot with the stopping.

Re:Aerodynamics

[Ralph Wiggam]

I don't think any materials for the parachute or the shroud lines could handle the jolt of when it first opens at those speeds.

Re: Aerodynamics

[Anonymous Coward]

Sure it could, they're called ballistic parachutes. Trick is you don't just open the whole thing all at once, you might start with a small drogue or even a steamer. They also have devices designed to gradually open a chute. I agree though, I wonder why they don't think aerodynamic drag is enough. Hell, throw some speedbrakes on there like an airplane, they go these speeds all the time and don't have the luxury of landing in the salt flats.

Re:Aerodynamics

[gandhi_2]

So open it gradually.

Re:Aerodynamics

[jandrese]

That's what I was thinking. Some smallish spoilers that can be extended out of the sides of the car would add a tremendous amount of braking power at 1000mph.

Re: Aerodynamics

[KingOfBLASH]

Actually if your airplane is going 1,000 mph into your descent, you, sir, have a problem.

I don't know the top speed you can land, but I would bet it's not much more than 200mph....

Re: Aerodynamics

[Firethorn]

The SR-71 Blackbird has one of the highest take off and landing speeds going. Around 200 KEAS, or 230mph.

Passenger jets are 120-150 mph.

Unless you're flying high performance military aircraft 'under 200mph' is a good bet.

"it looks like steel is stronger than carbon"

[Anonymous Coward]

Um, duh? Materials science 101, chubs. But I guess when you have people talking about space elevators in all seriousness, you can't expect too much realism...

Steel is stronger than carbon in many instances

[holophrastic]

I think people forget that "stronger" is meaningless. In the case of steel vs carbon, carbon is going to be stronger for a given weight, but that just makes the word "stronger" even more meaningless.

Steel usually wins out against most materials when it comes to survival. Steel bends, and bends back. Just about everything else loses by being brittle. Aluminum is the best example, being about three times lighter, but incredibly brittle. Carbon is also very brittle, just at the microscopic level. It'll fray, and slowly degrade until it comes a part -- like most fabrics.

Steel deforms, and then melts back together and deforms again. In order for friction to destroy steel, it needs to actually wear it away one particle at a time. Being so much heavier/denser, there are that many more particles to wear away. That's the win.

Why are people surprised when mass wins in a mass-bound effort? The challenge here is to get a heavy car to go really fast, and to then slow it down. That's always been a mass vs mass game. More mass always wins.

My question remains: if the carbon solution were as heavy as the steel solution, would it survive? But we all know that you can't cram that much carbon fibre into the same style of braking system.

Re:Steel is stronger than carbon in many instances

[holophrastic]

Umm, whatever your industry's definition of "strength", you'll find that it morphs across the three disciplines involved here. You'll also note that your own industry's definition of "strength" doesn't distinguish between various directions. So if I were to say that steel is stronger laterally, vs carbon's strength longitudinally, I'd still be within your industry.

Your industry also flexes in terms of the definition of the term "failure under load". Failure in some instances means breaking under the stress applied, but in others in means breaking as a result of the stress applied. In this case, we're talking about a spinning disk. If the material can withstand the load without the spinning, but then breaks due to the spinning, the term "strength" either does or does not cover the actual environment being discussed.

All of that aside, you'll note that this is not a materials sciences web-site, nor is it a theorhetical sciences journal. It is a site specifically for autistic yaps who make billions of dollars by transforming complicated depths into familiar fundamentals.

So why are you here?

Re:Steel is stronger than carbon in many instances

[holmstar]

In order for friction to destroy steel, it needs to actually wear it away one particle at a time. Being so much heavier/denser, there are that many more particles to wear away.

Or, you know, heat it up so much that it starts to melt. That's a real possibility for this application. A previous poster suggested rheostatic brakes (basically regenerative braking, where the electricity is dumped into a big resistor instead of being stored for later use). It would add weight and complexity, but if regular brake disks can't dissipate the energy fast enough, then something like that might be necessary.

Re:Steel is stronger than carbon in many instances

[holophrastic]

oops, my mistake. I didn't mean to type fibre. Thanks!

Re:Steel is stronger than carbon in many instances

[LordLimecat]

That's always been a mass vs mass game. More mass always wins.

So, lead brakes then.

Re:Steel is stronger than carbon in many instances

[Cramer]

In order for friction to destroy steel, it needs to actually wear it away one particle at a time.

Not entirely correct. While that may be the most common aging / failure method on a road car. On a race car, heat effects are what kills rotors -- of all construction. When you heat a steal (cast iron) rotor near (or past) the glass transition point (the point where it "melts", or transitions from solid to liquid) it will wear quickly and unevenly, begin to warp, and start developing cracks. Look at any used rotors from a race car; almost all of them have small, spider cracks in the braking surface from the repeated heat cycles. (heat causes metal to expand, but the heat isn't applied evenly over the entire disc, and it's a circle so the inside will expand more than the outside.)

But yes, in this equation, mass wins. Carbon fibre is great for many repeated, brief, super high heat cycles -- which is why F1 uses them. In this case, it's one HUGE prolonged dump of energy. That sort of thing will shatter a carbon rotor.

Re:Steel is stronger than carbon in many instances

[holophrastic]

I think you'd have a hard time keeping the lead there. Lead's pretty soft and melts easily, if I remember correctly. So it'd probably fall off faster than it would do anything else.

Re:Steel is stronger than carbon in many instances

[holophrastic]

No, you're absolutely right. I meant that in more of the current technical application for brakes in racecars and such. Personally, and professionally, I love aluminum. And because it's a lot more sturdy, it holds a structural shape much better than steel does.

Though it does have some properties that just totally killed it in one of my projects. It shields radiation so well that I couldn't get a wi-fi signal through it at all. Steel didn't have that issue.

But by far, the funniest part was when I grabbed a steel nibbler and tried to use it on aluminum sheet. I've never seen a professional tool break so easily in all my life. I was bewildered.

Re:Steel is stronger than carbon in many instances

[AK Marc]

The issue isn't how it works under load. The issue is that the wheels turn so fast that the wheels at top speed is a materials problem. Slowing is easy. Surviving top speed is harder.

Re:Steel is stronger than carbon in many instances

[Iniamyen]

Watch the video. The issue with the carbon rotors wasn't the amount of energy they can absorb - they'll always beat steel (iron) in that regard.

The issue was that at the higher (non-braking) speeds that they have to handle, because they'll still be on the wheels even when they aren't being used, the centripetal forces across the disc are enough to shatter the carbon.

Re:Steel is stronger than carbon in many instances

[Cramer]

Well, yeah, that's other drawback... a ceramic rotor won't take much abuse before shattering. (that said, my brake pads are ceramic.) And they aren't balanced for 10k RPM. The steel ones can cope with it better, but I wouldn't trust them at that speed for long -- enough for a run, sure -- and never more than once. (after a full braking, I would never trust them again.)

Re:Steel is stronger than carbon in many instances

[Anonymous Coward]

Did you even read the article you linked? "Strength of materials" is a name of an area or subject of study and sub-field of engineering. Beyond a very generic definition of what "strength" means, there is no precise or quantitative definition given for "strength." There are many precise definitions for strength with various qualified names, which gets back the to the GP's point: unqualified, "strength" is virtually meaningless in when discussing actual applications.

Re:Steel is stronger than carbon in many instances

[slinches]

AAAAH!!! So much wrongness!!! (okay, now that I've gotten that out of my system :) )

To preface: I'm an aerospace engineer and I have significant experience with the differences in capabilities of metal alloys.

I think a lot of the confusion comes from the overlap of engineering terminology and the general popular usage of the same words. In engineering strength (or more precisely, ultimate tensile strength) is defined as the load per unit sectional area at which a material will fracture, regardless of how far it has stretched (which is called ductility. The general usage of "strength" is rather ill-defined, but I think it most closely correlates to the material property called toughness, which is a combination of tensile strength and ductility and is calculated as the area under the stress-strain curve.

In the case of supersonic car brake disks aluminum isn't even option. Its melting point is only ~900F and even the brakes in a normal car can get above that occasionally. That leaves steel and carbon composite of the options discussed, both of which are actually entire categories of materials with vast ranges of properties within each. Although, a few of the main differences are that; carbon composites are almost all brittle (very low ductility, which means they snap rather than bend or stretch) while steels tend to have at least some ductility, thermal conductivity is generally higher in metals than carbon composites, and carbon composites tend to expand less as they heat up than steels. Strength probably isn't the limiting factor in this case have high enough strength to keep from tearing themselves apart at 10k rpm. What was probably the most critical factor in the failure of the carbon brakes was how rapidly they heat up and that they got to much higher peak temperatures because they can't conduct the heat away. This is called thermal shock which some steels may be better able to withstand because the increased thermal conductivity and higher ductility compensates for the higher thermal expansion.

One other thing to consider is the failure mode. A steel in this sort of scenario will soften as it overheats, reducing braking force gradually until it can no longer support the loads. Carbon composite, on the other hand, will shatter causing a sudden complete loss of brake force giving the driver far less time to notice and react to the problem.

Re:Steel is stronger than carbon in many instances

[slinches]

Correction: Aluminum melts at ~1200F rather than the 900F value I stated above, but the point still stands since the strength at that temp is so low that it would no longer function as a brake disk.

Re:Steel is stronger than carbon in many instances

[Neil Boekend]

You'd risk tearing the breaks apart during the top of the 1000 mph run, purely due to centrifugal force.

Re:Steel is stronger than carbon in many instances

[tibit]

You're correct, but even then I find the video rather weird. They're supposed to be braking from 160mph down to 0mph. They don't freaking need to apply the brakes at 5000RPM or anything like that, even though they seem to do just that in the video. To me, that's silly, or the superimposed numbers are someone's fantasy. The testing regime for their brakes is spinup to 10kRPM, the dyno braking down to 1600RPM to simulate air drag, then actual braking down to 0RPM at ~1MW braking power, if my assumption of 15s braking is correct, and the assumption that 1000mph = 10kRPM. The highest I've ever recorded my SUV braking at was 1.2MW IIRC, during some emergency braking tests. Of course the brakes probably wouldn't last if I could keep at it for 15 seconds, but for 3 second they were just fine (that was 100mph to 0mph).

Re:Steel is stronger than carbon in many instances

[slinches]

I didn't watch the video (blocked by the network filter).

True, if everything functions as planned, standard passenger car brakes are probably all that's really necessary here, possibly with a design or material change to handle the higher RPM. Although, I do see one reason to test the brakes at high speeds. If they can survive that test, that means they may last long enough to stop the car in case any of the aero-braking system components fail.

The solution

[cdrudge]

The problem: How do you stop the 1,000 mph Bloodhound SSC?

Friction brake, electromechanical brake, eddy current brake, drogue parachute, inclined plane, arrester bed, rubber bands, brick/stone wall, etc. You'd think engineers would have been able to think of these things...

If they use a really long bungee cord not only could they use it to brake the vehicle at the end of one run, but use it for initial acceleration on the return run too!

Re:The solution

[NotDrWho]

They tested the brick wall stopping method. It did not end well.

Re:The solution

[bswarm]

You forgot to include a boat anchor.

Re:The solution

[cdrudge]

But it stopped. And depending on the thickness of the wall and size of the subsequent debris field, it probably stopped it the quickest compared to other methods. Subsequent runs became much more difficult though.

Re:The solution

[MiniMike]

I heard that they were actually mortarfied with the results...

Re:The solution

[Charliemopps]

Right, that's the first thing I thought of. This is an incredibly stupid way to stop a high speed vehicle. They're going to have to replace those things every run.

Many moons ago I worked for a bicycle company building bikes for the Olympics down-hill racing team that year. (yes, I've had every weird job you can think f if you follow my posts at all.) Those breaks and wheels had to be replaced after 2 runs, and cost $800 per set.

I suspect whats happening here is some sort of endorsement. Their putting their driver at risk in exchange for investment. Hopefully they have a backup chute in case these silly brakes fail.

Re:The solution

[EvilSS]

But it stopped. And depending on the thickness of the wall and size of the subsequent debris field, it probably stopped it the quickest compared to other methods. Subsequent runs became much more difficult though.

Yes, the problem was cost. Using the brick wall meant that all parts of the car, including the driver, were single use only. They at least need the car to be reusable. Driver optionally so.

Re:The solution

[cdrudge]

Those breaks and wheels had to be replaced after 2 runs

Perhaps if you used brakes instead of breaks they'd last longer. Just a thought...

Re:The solution

[NotDrWho]

Nonsense. Two screws, a bolt, and an eyeball were successfully recovered for reuse.

Re:The solution

[freeze128]

It seems to be propelled by a rocket engine. Why not just apply thrust in the opposite direction?

Re:4.6 kilowatts of energy

[Anonymous Coward]

It was a typo. It was suppose to say "4.6 jiggawatts"

Journalism students attempting technical reporting

[mpoulton]

4.6kW, eh? That's 6.2 horsepower. I'm gonna go out on a limb and say that number is wrong by several orders of magnitude. 4.6MW is more likely.

Why no parachute?

[kruach aum]

Why don't they just invest in strong ropes, good bolts and a parachute, like literally every other rocket car?

Re:Kilowatts per ...?

[Russ1642]

Killowatts per parsec. Something, something, Kessel Run.

It's a monster

[Ralph Wiggam]

My favorite thing about the Bloodhound SSC is that it uses a 4.2L V12 engine producing 750bhp...to run its fuel pump.

Re:It's a monster

[njnnja]

Slight correction - it's a 2.4L engine...to run the fuel pump.

Re:It's a monster

[KingOfBLASH]

Hey! You should label posts like that NSFW. I just creamed my pants at an inopportune time.

Re:It's a monster

[Russ1642]

Also it's a V8 not V12.

Re:It's a monster

[EvilSS]

My favorite thing about the Bloodhound SSC is that it uses a 4.2L V12 engine producing 750bhp...to run its fuel pump.

Even with the subsequent corrections, that actually is very cool.

Re:It's a monster

[Threni]

So you're saying it's pretty much just a souped up Ford Cortina...

Re:It's a monster

[Ralph Wiggam]

Doh. I got the details from a 2009 article. They apparently changed it.

Re:It's a monster

[bill.e.gloat]

In that case, I invite you to read up on the Apollo space program with the Saturn rocket engines. An excellent book is https://www.goodreads.com/book.... This gets behind the engineering diffuclties and solutions. It depresses me greatly that I'll never get to see one launch first hand.

Re:It's a monster

[Ralph Wiggam]

Yes. Even getting the Saturn V from the assembly building to the launch pad was a massive engineering challenge.

Let's not compare a government project with an unlimited budget to a group of hobbyists with corporate sponsorship.

Who needs brakes?

[macraig]

Why not just skip the brakes, save the money, and eject the driver/pilot and let the sucker crash and burn? Could be an awesomely popular YouTube video.

Re:Who needs brakes?

[l0ungeb0y]

Because then they wouldn't have a shiny vehicle to send on world tour as a money making exhibit

Re:Who needs brakes?

[Drethon]

As many hits as that YouTube video would get, I don't think it will pay for the replacement.

Re:Who needs brakes?

[jklovanc]

Because the land speed record requires you to run in both directions in the same vehicle within an hour.

The record is standardized as the speed over a course of fixed length, averaged over two runs (commonly called "passes"). Two runs are required in opposite directions within one hour, and a new record mark must exceed the previous one by at least one percent to be validated.

Re:Who needs brakes?

[jovius]

I suggest they build wings to that machine. A machine of that size would be easily lifted from the ground at even lower speed than 1000 mp/h. There's less friction higher in the air anyway and they could reach speeds well exceeding 1000 mp/h. The team seems to be stuck with the car paradigm which is already well over 100 years old. I believe that humans will be able to fly with the aid of modern technology. All it needs is a change in thinking, an evolution of mind.

Re:Who needs brakes?

[LordLimecat]

I might be wrong, but my middle school level of physics leads me to believe that would not solve the problem of the pilot's forward momentum.

Re:Who needs brakes?

[bra1n]

Land speed record attempts require 2 passes, in opposite directions. So the vehicle has to be reusable.

Re:Who needs brakes?

[macraig]

So you saw what I did there.

not a car

[deadweight]

IMHO these are not cars and the records are fairly meaningless. It is a low flying aircraft being precisely controlled to keep the landing gear down on the runway. Don't believe me - watch what happens if the design is wrong. it will definitely be flying and not in a good way.

Re:not a car

[Animats]

Right. There are wheel-driven land speed records, currently 470.444 MPH with a turboshaft engine, 462 MPH with a piston engine, 307.666 with an electric motor, and 139.843 MPH with a steam turbine.

The 139.843 MPH steam speed record was set in 2009, by a British team. This is embarassingly low for a custom-built steam turbine powered land speed record car that looks like an aircraft. They brought the car out to the salt flats at Edwards for this.

Re:not a car

[Russ1642]

Lots of wheel powered cars can take-off if something upsets them.

Re:not a car

[operagost]

The thing with the steam engine is that it produces insane amounts of torque, when HP is what we need for top speed. Jay Leno described it as the "hand of God pushing you along".

Re:not a car

[trawg]

That actually makes me wonder if that would be a more efficient way of making it stop. Instead of trying to brake, deploy some surfaces that give lift and just point straight up, pop some parachutes and deploy some landing bags.

I'm sure trying to make it aerodynamic enough to do that would just be massively complicating the whole design to the point that it is worthless, but it'd be a fun way to stop.

Re:not a car

[CaptainLard]

The 139.843 MPH steam speed record was set in 2009, by a British team. This is embarassingly low for a custom-built steam turbine powered land speed record car that looks like an aircraft. They brought the car out to the salt flats at Edwards for this.

Embarrassing to who? The team? Steam engine builders local 402 circa 1897? Humanity? The fact that the Land Speed World Record is what it is for a steam engine means that it might be harder than it looks. Now if society had spent hundreds of $billions over the past century optimizing the steam engine like they have the ICE, you might have a point. From the site you listed:

"No one is going to suggest that this vehicle represents a major technical breakthrough, a relatively small improvement has been won at a cost of enormous complexity but it is unquestionably a triumph of determination, persistence and absolute refusal to give up in the face of adversity. Does it exemplify the "spirit of adventure"? Unquestionably!"

Good on them. I don't know about you but I don't have any world records to my name. I also never thought I'd get so fired up (no pun intended) defending a steam engine...

Re:not a car

[Type44Q]

It is a low flying aircraft being precisely controlled to keep the landing gear down on the runway

Ekranoplan^H^H^H^Hcar? :p

Re:not a car

[deadweight]

The Stanley Steamer is not impressed with a steam car going 137 MPH. >>

Re:not a car

[deadweight]

The Stanley twins fascination for speed insured that the earliest models included racers and roadsters while later production centered on touring cars and their unique Mountain Wagon that was both a bus and a truck. A Stanley car set a land speed record of 127 MPH in 1906 and the following year one was clocked at nearly 150 MPH before it crashed near Daytona Beach.

Red Lectroids drool!

[Thud457]

pish, the obvious design is to dump the energy into the oscillation overthruster. That way you don't have sudden deceleration when you collide with the mountain.

Re:Red Lectroids drool!

[tedgyz]

Nice!

Friction brakes, that's unusual

[GameboyRMH]

Very high-end landspeed cars usually use eddy current brakes and only have friction brakes for coming to a complete stop.

More "mundane" (like up to 700kph) landspeed cars use conventional friction brakes - after parachutes have done most of the work of course.

FLAPS!

[TheRealSteveDallas]

Why aren't they using analog flaps to gradually increase drag? At 1000mph you will decelerate pretty quickly by just not applying thrust. Other aircraft can slow down pretty well using them and they don't have the friction of maintaining contact with the ground to help. Bonus for using the same flaps to increase downforce as they open so the mechanical brakes will work better or just deploy a dragster chute once you get below the operating envelope.

Re:FLAPS!

[GameboyRMH]

You don't want downforce on a landspeed car, adding downforce is almost like dragging the brakes as far as they're concerned. Also air brakes make the vehicle they're attached to squirm around a little - not a problem on a fighter jet or a supercar, but a big problem on a vehicle travelling at speeds you don't want to be on the ground for and that can't turn worth a damn at any speed.

I'm sure it already uses a parachute. Usually these kinds of cars use eddy current brakes to slow to the point that the chutes can be opened, then after the parachutes have done most of their work they use conventional friction brakes to come to a complete stop.

Re:FLAPS!

[caseih]

Umm yeah that's what the OP was suggesting. The flaps cause drag and increase downforce. So of course you might want that... while slowing.

Re:FLAPS!

[GameboyRMH]

The problem is that the kind of suspension that can handle significant downforce wouldn't do well on a landspeeder, and if you start generating a meaningful amount of drag it means you've made an air brake...

you know not what you speak of

[SuperBanana]

Steel bends, and bends back. Aluminum is the best example, being about three times lighter, but incredibly brittle. Carbon is also very brittle, just at the microscopic level. It'll fray, and slowly degrade until it comes a part -- like most fabrics.

I'm sorry, but you know not what you speak. Aluminum is used on millions of planes for, what, almost a century? There are very malleable forms of steel (like the springs in your car) and very brittle forms of steel (like some kitchen knives.) Go and look at the carbon fiber wings on thousands upon thousands of aircraft.

Go look at the carbon fiber rear seat/chain stays and front forks on millions of bicycles.

People commonly attribute specific qualities to broad material categories like "steel" or "aluminum" like you just did, which is completely ignorant of the fact that all these materials can be engineered for different properties.

Carbon fiber is the most engineer-able material available, just about. Choosing a fighter jet part was pretty stupid, given it was engineered for weight, very occasional use, and lots of airflow, etc. They could almost certainly have a proper ceramic rotor designed for them, but it's probably too expensive or they got sponsorship with AP (given the article etc. this seems likely.)

Re:you know not what you speak of

[Bengie]

Airplanes need to use very special flexible joints because Aluminum is so brittle. Aluminum becomes very weak very fast when it flexes to the point of being deformed. Steel, not so much, unless you put a ton of carbon in it.

He was correct when it comes to the general properties of steel, but not all cases.

Re:you know not what you speak of

[bluefoxlucid]

You can use brittle materials on airplanes. Some airplanes are carbon fiber framed.

Re:you know not what you speak of

[holophrastic]

I find it interesting to note that all of your examples are of structural, non-frictional components, which doesn't really apply here. I'd have argued that while most metals are readily engineered for differing properties, 90% of those efforts fail miserably in frictional, high-heat, high-wear applications, where the base material undergoes chemically-significant physical forces -- like friction.

In any event, my comments were not intended to describe all steel and all carbon. Instead they were meant to describe the steel and carbon being used in the report. I don't really care about any others in this thread.

On the other hand, if you'd like to get into my personal experiences with aluminum on hang gliders, or my professional expreiences with aluminums and steels in commercial environments, or my pseudo-professional experiences with aluminums, steels, and irons in kitchen environments, my knowledge is narrow but deep in the first, broad and deep in the second, broad and shallow in the third.

Re:you know not what you speak of

[Firethorn]

Have you seen the video of the stress test for the carbon fiber wings? They might be very brittle - they shatter amazingly when they finally break, but they're also very flexible, taking an amazing(for me at least) amount of force and distance of flex before finally snapping.

In this case though they're looking at resistance to heat/friction, so the types of damage resistance necessary is different.

Re:you know not what you speak of

[bluefoxlucid]

Aluminum will flex, too, if it's long or thin. A material's profile has a lot of impact on how it behaves.

Aluminum has severe cycling issues. When steel flexes, nothing happens; when aluminum flexes, it weakens. You can flex steel until deformation and weaken it, or you can flex it within deformation limits infinitely; aluminum flexed at all will weaken, and eventually crack.

Claiming that aluminum is not brittle because it's used on airplanes is silly. The aluminum used on airplanes isn't different; it's a grade of aluminum suitable for planes, with some of aluminum's weaknesses more pronounced and some less. It's not some kind of light steel with lower strength but otherwise all the behavior of steel; for that you use titanium steel alloys.

Re:you know not what you speak of

[AK Marc]

This article is about non-friction materials as well. There's no question about whether either rotor can stop the vehicle. That's trivial. The question is about whether it can survive the stresses of spinning at top speed.

Sometimes the failure mode doesn't matter. I had a friend that hit a pothole on a bicycle. His bike shattered (mutliple failure points, top tube and down tube failures at multiple points). Another friend did the same on a steel bike. It didn't fare any better, but was in one piece. One mangled and useless piece. The material didn't matter, both failed spectacularly.

Re:you know not what you speak of

[tibit]

There are specific terms for it. Aluminum generally has finite fatigue life no matter what the cyclic stress amplitude is. That means that in presence of cyclic stresses, it will always eventually fail. If you really overdesign things, it may take a very, very large number of cycles - so large that they won't occur in a human lifetime, but still, if you keep cycling the stress, you'll get failure. Many kinds of steel, though, have infinite fatigue life at sufficiently small cyclic stress amplitudes. If you design things properly out of steel, they'll literally "last forever" - or at least they won't fail due to fatigue.

Re:you know not what you speak of

[tibit]

"when aluminum flexes, it weakens." Nope. The finite fatigue life of aluminum has nothing to do with weakening, unless you simply use the wrong term to really mean fatigue. Fatigue has little to do with strength of the material itslef. A typical fatigue failure mode is fatigue cracking, and it most definitely doesn't make the material weaker. The part gets weaker, but that's because it changes shape. Cracking produces new surfaces and thus changes the shape of the part. A part with a crack in it is not the same part as one without a crack, even if the material is no weaker.

Re:you know not what you speak of

[tibit]

I hope that you do realize that the braking job described here is something done routinely by disc brakes in large trucks, and occasionally done by disc brakes in run-of-the-mill SUVs. The stuff that happens at the braking speeds is inconsequential. You got taken by a very inaccurate and misleading article. What they worry about is what the disc does when it's not braking at all and is merely spun fast without any braking action. An emergency braking on my SUV dissipates as much power as a 15 second braking would on their vehicle. You can't look at those things without running some numbers. The video is also rather misleading since it overlays thermal imagery on visible image. Nothing is really glowing in visible light. The brake testing that they do is also slightly over-the-top: the brakes will not be used at 5kRPM at all.

Re:you know not what you speak of

[bluefoxlucid]

Point.

Re:you know not what you speak of

[tibit]

This shows the bulk stress needed to fail the part. It will fail by a fatigue crack. The effect is that of a "weakening" but only if you view it in bulk. The effect is as if the material was weaker, but it's really a material that's not set up the same anymore. It has new internal surfaces that didn't exist before.

Re:you know not what you speak of

[bluefoxlucid]

The number of times the stress must be applied is shown on the X axis. Essentially, steel can handle anything below some 30ksi for about infinite cycles--you can keep flexing and relaxing the steel *forever* and it won't break. Aluminum, not so much: even low amounts of stress repeatedly applied will cause it to break eventually.

The failure mode of steel is to deform a little. Repeat fatigue stress on steel will eventually start to bend it. Aluminum eventually cracks. As stated, steel has a rather high stress tolerance: it can cycle significant loads without experiencing any fatigue. Aluminum can't, and will steadily near its failure mode.

This doesn't make steel a better material for airplanes or bike frames. Aluminum bike frames will break eventually, but are lighter than steel; an aluminum frame can last 30 years under heavy non-professional use. The duty cycle a road warrior will put on a bicycle is a hell of a lot different than the duty cycle a highly-tuned professional athlete will put on a bicycle. Likewise, you can design an aluminum frame to handle the stresses provided by the duty cycle of a commercial airliner, such that the plane doesn't break in half over 30 years of flights.

Re:you know not what you speak of

[tibit]

The reason that the S-N graph is slightly deceptive is this: I can give you a pre-cracked steel part where the material is perfectly sound except that there is a crack, and I can size the crack so that the part will fail at an arbitrarily low load, in just one half-cycle of loading (you only load up, it fails before you cycle load back to zero). It looks as if I gave you "weaker" steel, but the steel is fine, it's the geometry of the part that's wrong. It may appear to the naked eye that the geometry is fine, because I can make the crack in such a way that you won't see it.

When aluminum fails due to fatigue, the tensile yield stress appears to be lowered, and thus we talk of fatigue "weakening", but only because you ignore the presence of cracks. Say you have a 1 inch square aluminum rod under a 20,000lb normal load, so you think the longitudinal tensile stress in the rod is 20ksi. But in fact it's not, it's very high, at the level of a yield stress, since there are cracks in the material, and the crack surface is a stress-free boundary. So the load has to find elsewhere to go, figuratively speaking. So it looks as if you had a weak rod. But then you can apply a compressive load just under the nominal yield (not any weakened yield), and guess what, if the rod doesn't buckle, nothing else will happen. So the material is not weaker. The part is. That's a subtle difference.

So, there's a very easy way to tell if a material is weaker, or just the part is precracked and thus weaker on average: just apply compressive load instead of a tensile one. In metals, the compressive and tensile strength should be similar. When it isn't, you have a precracked part. When it is, and both are low, you have true material weakening at the microscopic level.

Re:you know not what you speak of

[longbot]

This is why I trust my old car with a cast iron engine block more than new ones with aluminum.

Serial parachutes

[JimSadler]

Why not a small, strong parachute to start the slowing followed by progressively larger parachutes? If we are considering only brake shoes or pads then I would think ceramic with some embedded metal pieces might be the way to go. If pre-warmed the ceramic should hold together nicely.

parachutes or water-cooled rotors

[redelm]

Traditionally, parachutes (strictly, drogue chutes) are used from these speeds. Other drag increasers (flaps) would also work. So would shutting off the motive force!

If you insist upon friction brakes, then you know you'll have a problem with heat removal. For that, water is best. Either pumped supply or static fill, just let the steam blow out of hub-wards pressure valves at 15-100 psig on hollow rotors..

Time unit

[g8oz]

"managed to absorb 4.6 kilowatts of energy"......per what? The number is meaningless without a unit of time.

Re:Time unit

[wonkey_monkey]

The same as is the case with this gem?

It's like stopping a bus from 160 mph on a wet road

which is probably relatively easy if you're given 5 minutes to do it.

Re:Time unit

[Twinbee]

Nah, it's a rate. So 4.6 kilowatts increase for every ten seconds (9.2 kW after 20 seconds, 13.8 kW after 30 seconds). Well either that or the article's dumb - sigh....

Re:Time unit

[Twinbee]

It's even more meaningless with a unit of time.

SSC?

[Geoffrey.landis]

http://www.acronymfinder.com/S...

Couldn't they mention what they are talking about in the first sentence or two of the summary?

steel is stronger than carbon...

[DeTech]

A better way to make the comparison would be to say, "steel is a better material choice in this application".

you solved it -

[rewindustry]

at the end of the run, or if in trouble, simply take off, and recover by parachute.

They are using chutes

[Firethorn]

It's right in the article - the brakes are for below 160mph, before that it'll be air brakes and parachutes.

Turn it into a plane!

[dskoll]

They could make the car into a plane. Want to stop? Just flip the wings to the flying position and take off. You lose lots of kinetic energy as you ascend; when the speed is reasonable, you glide back down to earth.

Though... I guess the engineering challenges in making a plane that suddenly takes off at 1600km/h are quite substantial.

Brakes

[Firethorn]

Right, that's the first thing I thought of. This is an incredibly stupid way to stop a high speed vehicle. They're going to have to replace those things every run.

Not that big of an issue for a 'car' that's essentially a rocket engine designed to break the land speed record.

Hopefully they have a backup chute in case these silly brakes fail.

Actually, that's being deployed before the brakes. The article mentions air brakes and parachutes. Presumably the stop sequence will be air brakes first, then parachute, then wheel brakes that they're searching for a suitable solution for now, which per the article only start when it's slowed to 160mph.

Re:SSC?

[NoImNotNineVolt]

And couldn't you actually post what it stands for instead of linking to some other site?!

Re:4.6 kilowatts of energy

[Zynder]

Great Scott!

Re:SSC?

[Geoffrey.landis]

And couldn't you actually post what it stands for instead of linking to some other site?!

I would, but since the article didn't ever say what it stands for, I don't actually know what it stands for.

"Superconducting Supercollider" is my best guess. Why they call it that is a mystery to me. Maybe because they're both expensive high-tech things that are go round in circles in a desert.

Re:SSC?

[NoImNotNineVolt]

I was thinking more along the lines of Super Sonic Car (along the same lines as the Concorde SST being a Super Sonic Transport).

But I guess we'll never know.

SSP Ram Jet!

[CronoCloud]

I may be feeling a bit of childhood nostalgia, but this car reminds me of the SSP Ram Jet toy.

Re:SSC?

[swell]

irritating summary

And what is the braking problem? Whatever an SSC is, it will stop by itself eventually. Or, perhaps someone wants it to stop within a particular distance; but of course speed and mass would have to be considered in addition to materials technology and heat dissipation.

Neither the summary nor the comments seem to offer a holistic picture of the problem (if there is one) or a solution. If you expect readers to follow three links to piece this together, count me out.

Mostly missed the point

[Firethorn]

Claiming that aluminum is not brittle because it's used on airplanes is silly.

It would be, but this statement is attacking a strawman, since none of us claimed it. For that matter, I suggest rereading my post for any mention of Aluminum. You won't find any. I addressed Carbon Fiber. SuperBannana covered Aluminum, but your post agrees with him - summarized as various materials can be engineered to perform various roles, empasizing or minimizing various characteristics within limits.

As for Aluminum flexing too, I agree, I see it frequently. The difference is one of magnitude. Aluminum wings will fail at far lower stress levels and certainly less bend than CF wings. You also have the problem that Aluminum is known to fatigue far quicker even with less flex than steel, much less CF.

Steel is mostly good because it's cheap and has one of the more forgiving failure modes when overstressed.

The aluminum used on airplanes isn't different; it's a grade of aluminum suitable for planes, with some of aluminum's weaknesses more pronounced and some less.

Contradicting yourself here. Airplane aluminum IS different than what you'll find in something like a Soda Can. The alloy will be different, as well as treatment and forming techniques.

Re:Mostly missed the point

[bluefoxlucid]

It would be, but this statement is attacking a strawman, since none of us claimed it.

Here was the original quote:

Aluminum is the best example, being about three times lighter, but incredibly brittle.

Nothing else was said about aluminum. The poster went on to talk about carbon fiber.

Here was the response:

I'm sorry, but you know not what you speak. Aluminum is used on millions of planes for, what, almost a century?

"Aluminum is brittle" becomes "You don't know what you're talking about; they use aluminum in airplanes!" Either the response is wholly stupid or it's claiming aluminum is not brittle. Aluminum's particular weakness doesn't exclude it from aerospace applications: the load put on the fuselage may not be high enough to fail an aluminum frame. I don't care if the load's high, as long as it's not high enough to break the plane.

Steel is mostly good because it's cheap and has one of the more forgiving failure modes when overstressed.

Yeah, bent steel is easy to fix: bend it back and weld to strengthen. Not optimal, but the repairs are nearly good as new. But it also cycles well: if you flex steel less than what's needed to deform it, it just springs back. When you do this with aluminum, you stress the metal; it gets weaker each time, and eventually cracks.

Contradicting yourself here. Airplane aluminum IS different than what you'll find in something like a Soda Can. The alloy will be different, as well as treatment and forming techniques.

No, I covered that. Aluminum is brittle: it flexes until it bends and quickly breaks, and it handles cycling by weakening until it breaks. Different grades of aluminum will snap or will deform before breaking; but they'll all break with less effort than steel in substantially similar application (i.e. a steel fuselage versus aluminum fuselage, a steel bike frame versus aluminum). The materials also behave differently at different profiles, for example a soda can versus an H beam.

The grade of aluminum used on planes is different from other grades of aluminum; however, it compares to steel in the same way as the aluminum used for storm doors versus steel for storm doors: it's more brittle than suitable steel. It likely handles more cycling than the aluminum for your storm door--i.e. you can fly that plane more than five times without the fuselage cracking--but, eventually, that aluminum will age and wear down, and it will fail by simply snapping at a stress point. Steel won't.

The grade difference just means this aluminum will get you 20 or 30 or 50 years of operation, while that aluminum would get you 5 flights. Steel would undoubtedly get you INFINITE operation, because it can load cycle forever. You'd have to use a corrosion-resistant steel, or protect it (oil, galvanize, etc.), which is imperfect and will corrode if exposed to salt or acids or just dripping water (likely not pH neutral); aluminum is corrosion resistant, and will not fail in that way.

That's the point: It's still aluminum. It's still brittle. This grade of aluminum can handle this workload for this operating cycle without succumbing to its weaknesses; this other grade of aluminum can't. It has all the same weaknesses, some more or less pronounced. Steel is entirely different, and can carry carbide or titanium or chromium or molybdenum, and so steel and its alloys have their own properties which can be more or less pronounced. Engineered metal allows us to select for not just a set of properties, but a degree of expression of those properties: we can select aluminum or steel, and we can adjust their properties within the strengths and weaknesses of aluminum and steel.

Re:Mostly missed the point

[Firethorn]

No, I covered that. Aluminum is brittle: it flexes until it bends and quickly breaks, and it handles cycling by weakening until it breaks.

I think we're considering different scales of difference here. Depending on how you alloy and shape it you can play around with it's characteristics quite a bit, which I view as different but you don't. You even mention a lot of it yourself. For some reason you don't consider them different while I do.

As for the lectures on Aluminum, I'm not sure why you're writing them. I know the general properties of Aluminum. By the way, a mostly steel airplane would suck - while it could theoretically last forever it'd be too heavy to be useful.

Steel is entirely different, and can carry carbide or titanium or chromium or molybdenum, and so steel and its alloys have their own properties which can be more or less pronounced.

And you can mix Aluminum with copper, magnesium, manganese, silicon, zinc, and scandium to do some of the same.

I'm still not sure why you responded to my post about carbon fiber with a rant about Aluminum.

Re:Journalism students attempting technical report

[Macman408]

4.6kW, eh? That's 6.2 horsepower. I'm gonna go out on a limb and say that number is wrong by several orders of magnitude. 4.6MW is more likely.

And, as others have noted, kW is a unit of power anyway, and so is fairly meaningless for a braking system, which is taking huge amounts of kinetic energy and trying to convert them to something else (eg heat) without that something else causing some sort of spectacular show.

But maybe it's just the journalist's error - 4.6 *kWh* would be a reasonable number; eg the equivalent of slowing down a 1000 kg vehicle from 400 mph to 0. Or, in their example, the 160 mph bus must weigh about 6500 kg. Not coincidentally, Wikipedia lists the curb weight of the Bloodhound SSC as 6,422 kg.

(Of course, whether the road is wet or dry has nothing to do with the amount of energy dissipated in stopping the bus. They might as well have said "It's like stopping a bus from 160 mph on a Tuesday.")

I am a geek attorney, but not your geek attorney unless you've already retained me. This is not legal advice.

Sir, you will be hearing from my attorney shortly on the basis that you have provided me with illegal advice. I will be seeking PUN-ative damages.

heat

[kqc7011]

Braking is the generation and dissipation of heat. If the rotors are getting to hot, why not dump metered gas (nitrogen?) to maintain the proper temperature? This is not for repeated use, generally in a two way speed run system the brakes are used twice. Once in each direction. The dispensing system does not have to be that large or heavy. This system would activate when the parameters that the engineers set are met otherwise the brakes work without chilling.

What cute little car

[brambus]

From TFA:

While most of the retardation will be done by air brakes and parachutes, a set of car-like disc brakes still have to haul it down from 160 mph to a standstill on the slippery earth of South Africa's Kaksken Pan. At that speed, the car's steel wheels will still be spinning at 10,000 rpm.

If at 160 mph the wheels are spinning at 10,000 rpm, then it means that the wheel are about 14 cm in diameter, which, looking at the Bloodhound SSC side on (http://www.car-addicts.com/wp-content/gallery/bloodhound_ssc/Bloodhound_SSC_01.jpg) means that the car's body is about 30cm tall. Truly a marvel of miniaturization, including the driver! In related news, motoring journalists still suck at delivering factual information to the public.

Re:Kilowatts per ...?

[wagnerrp]

Huh? Watts are already "per time". A 4.6kW bulb is what you find on outdoor spotlights.

Re:Journalism students attempting technical report

[Macman408]

4.6 kW is a bit low for a system

I nominate this for "understatement of the year".
A 4.6 kW braking system would be good for a *bicycle*, which could then stop in about half a second at full braking. As I noted in the GP, the total energy of their vehicle at 160 mph is 4.6 kWh, so it would take an HOUR to stop it at a rate of 4.6 kW. Even if you had 8 discs, it would still take you 7.5 minutes to stop. You'd go well over 10 miles in that time.