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Elon Musk Explains Why SpaceX Prefers Clusters of Small Engines (arstechnica.com)
An anonymous reader quotes a report from Ars Technica: The company's development of the Falcon 9 rocket, with nine engines, had given Musk confidence that SpaceX could scale up to 27 engines in flight, and he believed this was a better overall solution for the thrust needed to escape Earth's gravity. To explain why, the former computer scientist used a computer metaphor. "It's sort of like the way modern computer systems are set up," Musk said. "With Google or Amazon they have large numbers of small computers, such that if one of the computers goes down it doesn't really affect your use of Google or Amazon. That's different from the old model of the mainframe approach, when you have one big mainframe and if it goes down, the whole system goes down."
For computers, Musk said, using large numbers of small computers ends up being a more efficient, smarter, and faster approach than using a few larger, more powerful computers. So it was with rocket engines. "It's better to use a large number of small engines," Musk said. With the Falcon Heavy rocket, he added, up to half a dozen engines could fail and the rocket would still make it to orbit. The flight of the Falcon Heavy likely bodes well for SpaceX's next rocket, the much larger Big Falcon Rocket (or BFR), now being designed at the company's Hawthorne, California-based headquarters. This booster will use 31 engines, four more than the Falcon Heavy. But it will also use larger, more powerful engines. The proposed Raptor engine has 380,000 pounds of thrust at sea level, compared to 190,000 pounds of thrust for the Merlin 1-D engine.
For computers, Musk said, using large numbers of small computers ends up being a more efficient, smarter, and faster approach than using a few larger, more powerful computers. So it was with rocket engines. "It's better to use a large number of small engines," Musk said. With the Falcon Heavy rocket, he added, up to half a dozen engines could fail and the rocket would still make it to orbit. The flight of the Falcon Heavy likely bodes well for SpaceX's next rocket, the much larger Big Falcon Rocket (or BFR), now being designed at the company's Hawthorne, California-based headquarters. This booster will use 31 engines, four more than the Falcon Heavy. But it will also use larger, more powerful engines. The proposed Raptor engine has 380,000 pounds of thrust at sea level, compared to 190,000 pounds of thrust for the Merlin 1-D engine.
Redundancy is always good
Seems like a good idea to me but I'm no rocket scientist.
I thought the F in BFR stood for something else than Falcon...?
Yes but Falcon doesn't get censored in interviews
Pain is merely failure leaving the body
up to half a dozen engines could fail and the rocket would still make it to orbit
Not if the fail catastrophically. If one blows up you've had it. This is probably more likely than a computer failing and burning down your data centre, so a factor worth considering,
Obviously SpaceX has calculated this but Id like to see a graph of the probability of flight failure of a rocket with 5 big engines and a rocket of 31 small engines. The more engines the higher the chance one will not work but also the higher the redundancy. The fewer engines the less chance one will not work but also the greater the chance one going out dooms the flight.
I came to the datacenter drunk with a fake ID, don't you want to be just like me?
Do you have to shut down computers opposite to the one that fails to maintain "computing balance"? Can a computer that fails blow up and take a few adjacent computers with him? Can a failing computer cause a cascading effect by sending bogus signals through the network that makes other computers fail? Does a failing computer fundamentally alter your mission profile to the point that you have to change the computations for ALL other computers?
Maybe we should stay with car analogies. They aren't any better, but at least we're used to them being garbage.
We used to have a Bill of Rights. Now, with the rights gone, all we have left is the bill.
I read the headline and said to myself, "Because there's not a single point of failure..." *pats self on back* (gotta do that sometimes...)
"That's different from the old model of the mainframe approach, when you have one big mainframe and if it goes down, the whole system goes down." Except that mainframe doesn't and AWS does.
Us Gen Xers just say it: big fucking rocket...
Wow Grandpa, that's so badass. You're using that .. word .. like it's just a word. I'm terribly impressed.
when kids are in ear shot.
Friendly reminder, millenials aren't kids anymore.
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It's basically for the same reasons as tesla batteries are made from thousands of the small lithium cells that are already mature.
Fictional?
Fake?
Faux?
Fishy?
Farcical?
Fanciful?
Fantasy?
Far-fetched?
Folly?
Fruitless?
Frustrating?
Fireball?
Firebomb?
Firework?
Flack?
Fragmenting?
Flameout?
Failing?
Falling?
Free-falling?
Flipping?
Flopping?
Floppy?
Flawed?
Fiasco?
Finicky?
Faulty?
-Fatalities?
Funerary?
Fanart?
Fanservice?
Fuckup?
(Just kidding of course. I think SpaceX is great and they'll get there eventually, although I seriously doubt their BFR timeline, and expect plenty of fireworks en route to their desired reliability level)
It's time for Operation Crazy Plan.
Friendly reminder, millenials aren't kids anymore.
Yes they are. Get off my lawn!
It would be interesting to know why this engine redundancy wasn't leveraged to save the center core of the Falcon Heavy when it attempted to land on the drone ship. They claim two of three engines failed to fire. If so, why wasn't the system programmed to automatically try to fire two alternate engines in that failure mode? Unless the failures where of a more catastrophic nature of course...
If nothing else, this shows Elon knows nothing about mainframe computers.
Still a kid until they stop acting like a kid.
Millenials will always be kids
The real reason they are taking this approach is larger engines are ruinously expensive and remain fundamentally outside the reach of private companies and the domain of nation states. The materials science research required to replace those 31 engines with say 3 or 5 engines would run up a bill pushing $100 billion. The Chinese and the Russians will remain in the game because at the end of the day that's exactly what they will do while SpaceX sits back and watches. With smaller engines you pay a substantial weigh penalty for the buttressing and gimballing that goes with each engine. All else equal a design with few engine is more efficient. And the redundancy argument is frankly nonsensical because the probability of failure goes up with the number of parts and smaller engines = more parts = a greater chance of your QC missing defects = a greater chance of BOOM. This is why the US always went with fewer but larger engines and the cash strapped Soviets went with smaller but more numerous engines. It was wholly a question of available funds.
yes. You do.
That is why you have sensors on the engines to make sure that WE shut them down before a RUD.
As such, the chances of being forced to shut down 1 is higher, but, with the massive number of sensors, it is very small chance of not making it to orbit.
I prefer the "u" in honour as it seems to be missing these days.
This preference for engine clusters is like using a stuff-ton of laptop cells instead of a much smaller number of automotive cells in the Tesla battery pack?
Even the Russians used multiple smaller engines in their space program. And if I'm not mistaken the Saturn V used five of them.
I always heard it was the same basic acronym as BFG the gun from Doom.
Calvin:Do you believe in the devil? Hobbes:I'm not sure man needs the help.
No! There are two extreme cases. One, where the probability of total system failure is the sum of the probabilities of failure of components (bad), the other where it is the product (good) 1% chance of failure + 1% chance of failure = 2% chance of total failure
You can't sum chance of failure like that. That's not how the math of it works. (think about it - if you take that to it's logical conclusion with 200 failure modes each at 1% chance of failure you can end up with a >100% chance of failure which isn't possible) First you have to determine whether the failure modes are genuinely independent or not. But even if you have two completely independent failure modes with a 1% chance of Failure A and a 1% chance of Failure B, the total chance of Failure is NOT F(A)+F(B) = 2% because there is a probability of both failure occurring simultaneously so the real probability will be less than 2%.
1% chance of failure * 1% chance of failure = %0.01 chance of total failure
It doesn't work like that either unless those failure modes are such that both have to occur for a failure to occur.
That's different from the old model of the mainframe approach, when you have one big mainframe and if it goes down, the whole system goes down
In the case of the mainframe the redundancy is build in. You don't have to use 100's of mainframes because they almost never go down. I've been working on mainframes for the last 20 years, and I can count on one hand the number of times the mainframe was down in a production environment in that period.
Try it! Library of Babel
I don't think this article is particularly newsworthy for anyone who's familiar with the subject or even stopped for a few seconds to think about it.
But here are a few points why multiple smaller engines are better in this case.
* Mass production makes things cheaper and sometimes better.
If you look at the cost of an engine, the raw materials are a pretty small percentage of the total cost, It's more about the manpower and tolerances required.
If you have a guy performing a certain task maybe once every two months, he'll be slower and less proficient at doing it than say every few days.
And overall, economies of scale make more engines cheaper to manufacture.
* Redundancy, as mentioned already in the thread.
SpaceX has already lost one of the engines on one of the earlier flights and continued to complete the mission.
They have walls between them that prevent the explosion of one from damaging the next.
And it's only going to improve for Falcon Heavy and BFR, they'd be able to lost multiple engines and compensate for the imbalance.
* Telemetry collection.
You get to build up a history of past performance a lot faster with ten engines than you do with one or two.
After 54 flights, SpaceX has gathered operational telemetry on 486 first stage engine firings and 54 second stage ones (Not including all the test firings).
* Throttling, maneuverability, unique thrust characteristics.
In the early stages of landing R&D SpaceX had a rocket called the Grasshopper, which was modified Falcon 9, that was able to fly up, hover, and then land.
Most larger engines would not be able to throttle low enough to maintain a hover.
This one is just a guess, but I imagine it's much easier and faster to gimbal a smaller engine, and you don't have to put the smaller, cheaper gimbaling hardware on all the engines.
It seems that the exhaust from the multiple engines behaves somewhat similarly to an Aerospike engine, which gives the configuration some extra efficiency.
That's all I could think of, but I'm sure there are more reasons.
1) more engines is more parts, which is generally a recipe for more failures
Only if you hold everything else in the system constant which is clearly not the case in most real world systems. To use a car example, modern cars have a LOT more parts in them than cars from 40 years ago but they also are demonstrably more reliable. Same with jet engines. Modern ones are more complex and with (usually) more parts but they also are more reliable. The relationship between number of parts and reliability is not a simple linear one. Many of those added parts actually contribute to the reliability of the overall system.
The most extreme example of this sort of thing ever attempted was OTRAG.
John Carmack had some interesting things to say about that at his now-defunct Armadillo Aerospace website, some of which have been preserved at Wikipedia here.
Ceci n'est pas une signature.
If nothing else, this shows Elon knows nothing about mainframe computers.
I think that comment says more about you than it does about Elon. Do you seriously think Elon doesn't get that it's an imperfect analogy used to make a rhetorical point?
No, ULA uses Russian RD-180 and RS-68 on their rockets. SpaceX's Merlin is a newly designed engine that is similar to some that NASA have used in the past.
I'm a good cook. I'm a fantastic eater. - Steven Brust
I love the fact that they include "pricing" on the SpaceX website, like your just buying a refrigerator...
Or a flamethrower.
I'm a good cook. I'm a fantastic eater. - Steven Brust
> Friendly reminder, millenials aren't kids anymore.
I know your mom came over to my house and yelled at me for calling you a kid.
She was likely just uselessly seeking attention. After all, she's gen X.
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The classic IBM mainframe had super reliability, which included redundancy.
-cranky old guy
All this debate from Slashdot rocket scientists over whether Musk is properly designing his rockets, whether he "understands" the finer points of his (imperfect!) metaphor, whether he really understands computers at all, and yet...
he launches rockets (and lands them) again and again and again and again.
How about a moderation of -1 pedantic.
Bob Truax argued that the most cost-effective way to build a big rocket was using one huge engine, which is what was planned for the Sea Dragon booster. He said cost is driven not by size but by parts count. More engines equals more parts that have to be produced, inventoried, tested, assembled, etc., and that leads to higher cost.
However. . . The way SpaceX are returning their boosters to earth wouldn't work with one huge engine. There would be no feasible way to throttle it down enough for the return and landing burns! Each Falcon 9 core typically lights up just one engine for the landing.
yet still no "bag o' glass"...
the preceding comment is my own and in no way reflects the opinion of the Joint Chiefs of Staff
I love the fact that they include "pricing" on the SpaceX website, like your just buying a refrigerator...
I clicked the shop button, but I couldn't add one to my shopping basket.
Finally! A year of moderation! Ready for 2019?
He announced the next Bigger Falcon Heavy lunch vehicle will be made entirely of the rocket engines sold to high school science projects. 3.2 million of these rocket motors glued together will form the launch vehicle.
He said, "We know Elon got his start by building an electric car by duct taping 8000 laptop batteries together. Same thing here, back to the basics. man!"
sed -e 's/Chuck Norris/Rajnikant/g' joke > fact
It took the US and the USSR years to solve instability problems with engines larger than the V2 rockets.
Also, why would anyone trust the opinion of a software developer with regard to hardware development?
The flight of the Falcon Heavy likely bodes well for SpaceX's next rocket, the much larger Big Falcon Rocket (or BFR), now being designed at the company's Hawthorne, California-based headquarters. This booster will use 31 engines, four more than the Falcon Heavy. But it will also use larger, more powerful engines. The proposed Raptor engine has 380,000 pounds of thrust at sea level, compared to 190,000 pounds of thrust for the Merlin 1-D engine.
Does anyone know what this means relative to the lift capacity of this new rocket they're working on? The Falcon Heavy was already a huge leap over the competition and this doubles the thrust with a few more engines (understanding that some of that thrust is going to come at the cost of carrying additional fuel too).
Weight of Rocketdyne F-1 engine on Saturn V (moon rocket): 18,000 pounds, thrust:1,500,000 pounds
Weight of Merlin 1-D engine on Falcon 9: 1,000 pounds, thrust: 190,000 pounds
The specific impulse of a Merlin engine is 282 seconds, the specific impulse on an F-1 engine is 263 seconds.
TL;DR:
18 Merlin engines weigh the same as one F-1 moon rocket engine but only 8 of them are needed to provide the same thrust.
Gen *Y* is the Millennial generation. Gen Z is the "little shits" turning 18 now.
Boomer 1946-1964
Gen X 1965-1981
Gen Y/Millennial 1982-1998
Gen Z/Post-Millennial 1999-2016(?)
Give or take a couple of years on either end of the ranges depending on what specific cultural element you are judging the generations by. (The defining characteristics of a post-gen-z generation aren't yet known, so we don't know when to draw a line on gen z.)
And the B is actually for Belgium
Indeed, Anonymous Coward.
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From what I've seen, Gen Z are WAY less into authoritarian political correctness than Gen Y millennials. On average of course. It's a hopeful sign for civilization.
Haha, I designed lithium smart battery packs in use in the field right now and I dare you to make less sense. Tesla packs not only have series and parallel strings, it can dynamically rewire itself around problem cells. It is insulated and has a heating and cooling system akin to a radiator. Take a look at a teardown before posting stuff you have no idea about.
:{ is
Only this time with reliable technology and safety systems.
"Falcon doesn't get censored in interviews"
Which is why SpaceX are the best in the falcon business.... :)
"Turbines are routinely designed to contain rotor failure (ever see the jet engine tests?)"
Yup.
Turbines are most emphatically NOT designed to contain rotor failure. This would make them so heavy they can't fly, so they design to make the rotors are reliable as possible and to try and protect critical components (uncontained compressor/turbine rotor failure won't usually down an aircraft but it makes for an exciting day as far as the pilot's concerned and the cause is usually oil starvation)
What they ARE designed to contain is _fan_ blade failure - these are more than capable of bringing down an aircraft if not contained and the fan takes the brunt of anything being ingested (birds, humans, baggage carts....)
In both rocketry and aviation, statistics is used to determine what may fail and what the consequences of failure might be. So far SpaceX has been getting it right and that has a lot to do with not being told how to make every part of the rocket that NASA or the USAF is ordering.
These engines are lighting off at the *front* of the rocket, in the sense that they're open end is in the direction of travel (down), correct? Naively I would think that the air blowing into them would tend to blow them out, like blowing air at a candle you're trying to light. I suppose when it's supersonic, there's a shock wave that prevents that; but the final burn is surely at subsonic speed. But maybe the engines are designed so that this doesn't happen, even subsonic?
"modern cars have a LOT more parts in them than cars from 40 years ago but they also are demonstrably more reliable": Interesting point. I'm sure the reason for this is known by somebody, but it isn't known by me. I always assume it has to do with more QC in the factory, and that this QC effort was started by the Japanese in the 70s or so, and only taken up by Detroit (and the European mfgs) because they were loosing market share so badly. Can you (or someone) enlighten me? Why are cars more reliable now?
PURRRGE.... :P
[($)]
Boomers aren't the judge of cool.
Typical gen-xer, obsessed with cool for some bizarre reason. If cool is all you've got, you've got nothing.
Circle the wagons and fire inward. Entropy increases without bounds.