Slashdot Mirror
Carmack's Throatless Rocket Engine
Baldrson writes "John Carmack is working a potentially disruptive technology: A throatless rocket engine. Its made from plain aluminum pipes with few machined fittings. Carmack says: "The great thing about these engines is that it only takes me two nights to machine the parts, so we can test two engines a week if necessary." It scales too: "If this line of tube engine development works out, we can make a 5,000 lbf engine with very little more effort than the test engine." This is what makes disruptive technology development work: Cheap, fast turnaround on on redesign producing technologies that scale. If this works, the NASCAR guys may really start entering space competitions like the X-Cup."
If you would have read through armadillo's website you would see that he has been putting a serious effort in. As an aerospace engineer who has been keeping tabs on John for several years I can assure you he's got his design well thought out.
Throatless rockets aren't new... they've been around for awhile. They aren't as efficient as a throated rocket but they offer some operational advantages (namely in throttling, which is nice for a powered reentry).
-everphilski-
Before.....
After.
If you read the link, the X-Prize people are talking about starting the X-Cup, a regular space competition.
Well then, that's no problem, as lbf means "pounds-force", not pounds*feet (which would be a measure of torque).
Here's a hint: a serving of beer weighs about a pound. One lbf is how much force you must use to hold it up (assuming you're drinking it somewhere on Earth).
It's also equivalent to about 4.44 newtons, but that unit is too small to provide a satisfactory serving.
This "throatless" engine seems more useful for testing injectors than actually extracting impluse (propulsion). The narrow throat of engine followed by a expanding nozzle allows for the chamber pressure to be high (good) while the exhaust pressure is lower (also good). This site explains much this and in fact says, "If the pressure ratio (and thus expansion ratio) [like Carmak's design] is 1, then F = 0. The only thrust produced by such a nozzle is the pressure thrust, or Ftotal = (Pe-Pa)Ae. Such a nozzle, of course, would have no divergent portion, since A*/Ae=1, and would be a badly designed rocket nozzle!"
;)
Simon
Ok, first, you don't get shockwaves in nozzles- not unless you've got a rough nozzle surface, which is a bad idea, because the hot gas comes to a screaming halt ("stagnates") and the local temperature goes way up, and then the nozzle melts. And yeah, Carmack knows that a nozzle and throat needs to be smooth, this isn't the first bipropellent engine he's built, and he's widely known not to be stupid. :-).
Oh yeah and actually, even these 'throatless' engines has a throat, but it's kinda hard to spot :-), the gas makes up its own mind where to put the throat, in realtime- the throat is defined to be where the gas goes sonic, and this always happens when the combustion pressure is more than 2.7 times the ambient.
You mainly get shockwaves in air inlets in jet engines, not in the nozzle. You also get shockwaves in the exhaust plume of rocket engines where the exhaust kinda bounces of the external atmosphere, but that's harmless (actually kinda pretty google on "mach diamonds"), and they form wayyy downstream of the exit. Oh yeah, and a rocket launching, once it passes about mach 0.85 gives transonic shockwave around its nosecone, and then later supersonic shockwaves there, those can cause damage, but they rarely do.
So, these non existent shockwaves can't damage any equipment, or waste any energy. Oh yeah, and did I mention there aren't any shockwaves? :-)
-WolfWithoutAClause
"Gravity is only a theory, not a fact!"He reports only an ISP only in the low 200s, this is not efficent enought to get to orbit.
TFA is unavailable due to slashdotting, but low 200's will get you ~5km/s with a 90% mass ratio. It's plenty for sub-orbital work, and useful for the first stage if you're not trying for Single Stage To Orbit.
The shuttle SRBs have an ISP of 273 seconds.
I think that what Carmack is trying to do is actually to explore a lot of options in terms of engine design, trying to find out if he can come up with one that is actually symple and efficient.
Of course, there's absolutely no assurance that he'll actually find one, but that's the the risk of any kind of research.
The whole point is to actually move away from the existing methods, so he can't possibly use them.
Yes, However the Shuttle's 470ish ISP SSMEs do most of the work in getting it to orbit. If he could get the rating around 250 I would say he has a chance at maybe a first stage.
It's actually a misnomer; provided the chamber pressure is more than 2.7x the atmospheric pressure (which it always will be if you stuff enough propellant in through the injectors) then a throat spontaneously forms near where the nozzle widens out. The throat is defined to be the place in the combustion chamber where the gas goes faster than sound. Normally that would happen at the narrowest point of the nozzle, but in this case it may even move around in the combustion chamber, but it can't leave because the nozzle widening out stops it.
-WolfWithoutAClause
"Gravity is only a theory, not a fact!"From Mirror:
http://www.mirrordot.org/stories/8f5373b24e35f5c45 3edf914cc953eff/index.html
Armadillo Aerospace News Archive
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Servo regulator, Throatless engines, Hold down test
Aug 4, 2005 notes
Despite not having time to do an update for a while, we have been steadily working...
Servo regulator
When we last worked with it, the setup showed what seemed to be a valve lash problem - flow would begin when the high pressure ball valve reached 15% open, but it wouldn't shut off until it was closed all the way back to 5%. Since we had fabricated our own actuator to valve adapter, we thought we might have allowed too much lash into the coupling. We built a new mount using helical beam couplers with zero lash, but that turned out not to help. The coupling seems tighter, with the valve following every little jitter of the actuator, but the flow behavior seems to be an aspect of the seals in the ball valve, not the linkage between the actuator and the valve.
This cracking problem is only really an issue at very low flow rates, so we were able to do some flow tests at roughly the performance levels that our single-man space shot vehicle will use. With a single large nitrogen bottle feeding the servo regulator, we did the following test:
2700 psi initial bottle pressure
60 gallons of water at 230 psi and 215 gpm flow rate
1800 psi final bottle pressure
2" plumbing, 1" valve
The small fittings at the bottle valve became the limiting factor as the pressure dropped below about 2200 psi, with the servo valve eventually going wide open and still not quite being able to keep up. Our flight vehicle pressurant tanks will manifold directly out of bottle necks with a -10 fitting, so they won't become flow limited at all. When our new 36" hemispheres arrive, we will be welding up the full tankage and pressurization system for the big vehicle and doing water flow tests in preparation for testing a 5,000 lbf class engine.
Speaking of spheres, here are a couple pictures of the tear area on the burst one:
http://media.armadilloaerospace.com/2005_08_03/tor nSphere.jpg
http://media.armadilloaerospace.com/2005_08_03/tor nSphere2.jpg
Throatless engine
I was recently looking at the table in Sutton regarding losses due to small chamber to throat contraction ratios, and they weren't as significant as I had remembered them. A chamber with no contraction ratio at all will lose 20% of its thrust due to pressure losses from accelerating gasses in the straight section, but the Isp loss is only 1.5%. The text mentions "throatless rockets" being used in some missile applications to minimize chamber length and dry mass at the expense of Isp. The text doesn't say if these were liquids or solids, but I assume they were solids.
However, this does open up the question of building liquid engines like that. If L* remained constant, you would have an extremely long engine that would probably be impossible to cool, but I could imagine the accelerating, high speed flow could reduce required combustion stay times significantly. A 1.5% Isp loss is utterly meaningless for our purposes, so a configuration that traded that for fabrication benefits could be quite useful.
We fired a few crude throatless lox / ethanol chambers, and the results were surprisingly encouraging. With a very crude injector (a spray nozzle for the lox and four straight horizontal jets for the ethanol), we measured a 190 Isp from a 12" long straight pipe combustion chamber. It melted in a couple seconds, but this was still very impressive. With a 3:1 expansion cone added, performance should increase about 15% to around 220 Isp. That would be right at theoretical va
"If all the world's a stage, I want to operate the trap door." - Paul Beatty
It's more what he's doing than what he has discovered (which is nothing.)
For amateur rocket work, you spend about $1000 to burn $1 worth of propellant. Think about the logistics: site costs, setup costs, safety planning, data acquisition, etc.
Streamlining the process is where you make big wins: accept a 2% ISP loss, and test 10x more frequently. This is how you gain knowledge fast and avoid expensive dead-ends. A lot of this work is just learning skills -- build, launch, avoid dying, repeat.
More tech (GPS, computers, digital video) makes the process much easier: John is now doing 1970s era work after starting at a 1950's level a few years ago. There's a good chance that he will be able to reach earth-orbit level within a decade.
It impacts manufacturing costs, but in an interesting way. If you are NASA or General Dynamics, it would be a little bit cheaper to make, but no big deal. The interesting bit is that you should be able to make a decent nozzle with 1/10th the manufacturing/machining capability. It reduces the costs of entry, probably down to the level of a NASCAR crew's machine shop.
So, not truly revolutionary, but "disruptive" tech in the sense that it puts the ability to make decent nozzles in the hands of many many more people.
-- your Web browser is Ronald Reagan
According to the X Cup Schedule, Armadillo will be conducting a demo flight out in New Mexico. (Check out Oct 9th activities).
I wonder if he'll be showing off the BFG as well... =p
A concise explanation of shock diamonds(mach diamonds) here.
It's strange how you don't realize that the term "Disruptive technology" is a real term and refers to an idea or product shaking the mainstream and becoming the dominant. It even has it's own wikipedia entry.
- what is the definition of simultanagnosia?! I've been meaning to look it up!
It is a pound-force, as distinguished from the pound-mass, which weighs 1 pound-force when sitting in 1 normal g acceleartion at Earth's surface.
It is true, this is a hard to learn and obsolete system of units. But since some of the most advanced machinery in the history of the world was developed in it, and we have a whole industrial base that can crank out devices and gadgets practically on demand that revolve around it, we are loathe to give it up just so we can be like the French.
By the way, Pound-feet is abbreviated lb-ft and indicates a torque as opposed to ft-lbs, which would be an energy. Probably. You need to actually Understand what you are doing over here.
ps. 1 pound (lbf) is equal to 4.448 Newtons. Which most metric people would call 2.205 kilos anyway, because they don't know a force from a mass.
Behold, this dreamer cometh. Come now, and let us slay him... and we shall see what will become of his dreams.
The problem was hydrogen peroxide. His first engines were built around the stuff. The way hydrogen peroxide works is you catalyze it - that is run it through a mesh of material that reacts with it to liberate steam and hot oxygen, which you then combust with a fuel. Hydrogen Peroxide is a nasty beast. It's hard to find vendors to sell to you (at rocket grade concentrations, 90-98%), and combustion is tricky. After a lot of experimenting (and he himself will tell you - a lot of valuable data gained; he was able to test at rates higher than using other fuel combos) they gave up on it.
Now they are using liquid oxygen as an oxidizer. They aren't stalled. They are exploring their options. If you look at NASA they have really only done things one way, the convergent-divergent regeneratively cooled nozzle, using O2 and H2, occasionally kero. He's sticking his neck out trying something new, it just takes awhile with limited funds. He's not stalled now.
-everphilski-
NASA did things based on previous research by Goddard, Von Braun, and others. H2/LOX fueled engines didn't materialize until the Apollo program. Kero/LOX was the status quo for big liquid fueled boosters before that. And it is still used by the Russians. Is your argument that NASA has only done things one way, and hasn't explored other options? What about the Soviet space program, did they steal the design from NASA, or did the also come up with it through research? If alien cultures ever made cars, do you think they would have round wheels? Some times the optimum solution is the same, no matter where it is invented. I don't think that chemical rockets are the end all be all of rocketry, but they are a mature technology, and everyone who has achived orbit has used convergent-divergent nozzles. The revolutionary step is mass production, to bring the cost down. Making parts from aerospace alloys is difficult. Tubular shapes are easy because tubing is mass produced. If you want to bring the cost down, one needs to find cheaper ways of making rockets. Liquid fueled rockets are complex and require turbo machinery. Maybe he should look to solid fueled rockets. There isn't as much complexity in them.
Russia, i would assume. Despite monetary problems, they are testing some *revolutionary* engines, of the kind that will avoid the combustion or whatnot.
A more serious effort for SSTO was DC-X. The full size version probably wouldn't quite have made it to orbit, but it would have been close enough to know if it was possible to do it with a reasonable payload. Unfortunately, after two successful Air Force flights, NASA took over and the craft was destroyed because a technician didn't hook up a hydraulic line to one of the landing "legs". Then NASA cancelled it in favor of X-33, a project with no hope of success. X-33 allowed NASA to say, with a straight face, "SSTO doesn't work", when what they proved was X-33 doesn't work.
Staging helps. Two stages will get you to low earth orbit. Beyond low orbit usually requires three. This reduces the fuel fraction, but by less than one would hope. The Shuttle's fuel fraction is around 89%.
Yes, and staging also complicates the design, making it more expensive. You get those one-shot parts you throw away, which means doing lots of extra work (ie spending $$) to make sure they work the first and only time. I suppose you could have some kind of flyback reuseable stage, but that's complicated enough that it won't save you any money.
So space flight is all about weight reduction. Which is why everything is so fragile and unreliable. If you could build a launch system with a fuel fraction of 50%, which is roughly where most aircraft live, it would be a straightforward job.
Everything is fragile and unreliable because the design philosophy is wrong. It's a question of designing for perfomance when we should be designing for operational efficiency. In the end the mass fraction doesn't matter - what matters is reliability and $/lb. to orbit. There will always be a market for heavy lift launchers, but for manned flight you'd rather have frequency and reliability.
The benefit to VTVL SSTO is you can launch it more frequently, since all you have to do for the next flight is inspect it and fill up the tanks. The reentry is powered, so you don't have thermal problems, and since you don't need a runway you can land it on the same spot you launched it.
Look at it this way - the amount of fuel it takes to get to orbit will get you from the US to Australia in a 747. The reason it's cheaper to go to Australia is they don't throw away the plane when you get there (expendables) or take it apart and rebuild it (the shuttle) before the next flight.
This also has implications for safety. Would you rather fly a 747 for its maiden flight or its 100th? If you fly the same craft more than once you're much less likely to be bitten by manufacturing defects.
We've been using staged rockets for fifty years now, and the price is still a huge multiple of fuel costs. Time to try something different.
Psst, here is another secret: Most of the rocket engines made weren't designed by NASA either. The were built by Pratt and Whitney, Rocketdyne, and others. But NASA managed the project and doled out the money because rocket engine design and manufacturing is expensive and not very profitable unless you're working for the government, which has all this taxpayer money to spend and wants some rockets. So it isn't a government bureacracy that has a monopoly on rockets. There is nothing stopping a private company building rockets, but they are expensive to build and not very profitable.
There was never suppose to be a fleet of shuttles. They were supposed to have such a fast turnaround that the capital cost of each shuttle would be amortized over zillions of launches. It was originally sold to Congress as having a turnaround of a week. It was never sold as being cheap to mass produce a fleet of them. You wouldn't need a fleet if they had a one week turnaround.
Infuriate left and right
The DC-X saw a lot more than two successful flights. At one point in its SDIO (not just Air Force) operated test series, it did two flights with a 24 hour turnaround.
I had the privilege of attending the first public flight, which was the second real flight. Seeing a rocket climb out and then just stop in mid-air is quite something. Then it flew sideways a hundred yards or so and descended tail first to a perfect landing.
Later flights went higher and faster, and one demonstrated its survivability when an at-launch explosion of vented hydrogen blew off part of the aeroshell, and the thing was dropping bits and pieces as it climbed out. The remote pilot (Pete Conrad) just clicked the emergency autoland button and the thing hovered until it had burned off enough fuel to land (the landing gear wasn't designed to support fully fueled weight, it sat on a "milk stool" for launch).
Then NASA took it over and, as you mention, fucked up their first flight. The unconnected leg folded up on landing and the thing fell over, broke apart and burned.
Given the huge workforce that NASA keeps employed to fly the Shuttle (or rather, to act like they're keeping it flying while keeping the actual number of launches to a minimum -- reduces the career risk for NASA managers), it's not surprising that they don't like anything that might threaten that turf. Not that, as you point out, the ridiculous design of X-33 ever remotely threatened it, and gave NASA engineers (and their LockMart, etc, buddies) something else to spend money on.
-- Alastair
It's a force equivalent to that of Earth's gravity (at the surface) exerted on a roughly half-kilogram mass (0.454 kg, to be precise). About four and a half newtons.
(lbf = pounds force, not pounds feet)
-- Alastair
Is ANYONE doing rocketry at a 2000's level today?
Nobody is even doing it at a 1990's level. Or even 1980's.
Shuttle is pretty much 1970's technology, although the SSME (Space Shuttle Main Engine) is about the only part that isn't 1960's technology, at least as far as the launch phase goes. Aerospike SSTO designs were being explored (on paper) in the 1960s, with some limited engine testing in the early 1970s. Some early nuclear engine testing (NERVA, too low thrust for launch) was done in the 1960s, with higher thrust designs (eg DUMBO) being studied.
The only really new technology (ie, that wasn't at least studied on paper in the 1960s or earlier) is laser-launch stuff, which has put masses of a kilogram or two a few hundred or thousand feet in the air. To be useful it would require mind-bogglingly large laser systems and their power supplies.
But there's plenty of life in "old" technologies if coupled with modern materials and different design trade-offs. (Heck, gunpowder has been around for six or seven hundred years, and the basic technology -- with materials and design improvements -- is still the way to make portable weaponry.)
-- Alastair
In mechanical engineering, a throat is a channel through which fluids move that has a narrowing followed by an expansion (so that the cross sectional area of the channel decreases and then increases). When a compressible fluid flows through a channel that narrows, its velocity increases. Normally, when the channel expands again, the fluid will slow back down. However, with a properly designed throat and a low enough backpressure, the fluid will accelerate to speed of sound at the throat and then, instead of slowing down, continue to accelerate beyond Mach 1 as the channel expands. I don't know anything about "throatless" rockets, but I suspect that they are able to accelerate the rocket exhaust above Mach 1 by forcing the exhaust gases to contract and then expand even though there is no channel forcing them to.
XCOR engines have throats. Some of the engines look like a straight tube from the outside, but that's just the profile of the cooling jacket. Inside that jacket, between the chamber and atmosphere, there is a throat.