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Q & A - Aerospike engines
by
kraisee
on 19 Feb, 2006 04:25
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I'd like to ask anyone here with real-world rocket experience about annular aerospike engines (as opposed to the linear aerospike used on the X-33) and the possibile double-use of them as re-entry shields for re-usable stages.
My knowledge stretches to the fact that aerospike engines tend to be slightly more efficient across the broad range of altitudes and atmospheric conditions than their regular engine bell brethren. I also understand that the aerospike 'bell' experiences temperatures in the 5,000F ball-park and can support thrust forces which would seem to indicate that it could serve as a re-entry shield.
I know that a hypothetical cylindrical stage could have an annular aerospike engine attached under it, precisely the same diameter as the stage itself. If that stage were to be launched, and were then to be placed into a trajectory where it would follow a re-entry profile, the significant weight of the engine mechanicals would cause such an empty stage to orientate its CofG engine-first during re-entry. If that were to occur, the aerospike engine bell could be designed to provide a very good structure to use as a TPS to protect the stage and allow it to re-enter safely - and thus to be made re-usable.
I'm still quite sketchy on the detailed specifics behind the idea, and I'd like to know more because the cocept has had me quite intrigued for quite a while now. I can sure see some places where such a design could be really advantageous in the future.
I'm still trying to work out how the concept prevents the plasma created during re-entry from penetrating the injectors around the edge of the aerospike and destroying the vehicle from the inside-out. I also don't know how much heating the stage would have to withstand around the sides during re-entry.
And is there a practical limit to the maximum diameter of an annular aerospike? I understand that multiple engines could be integrated into a single large aerospike bell, each supplying a quadrant of the larger spike, but I don't know what would occur in such a situation if you had one quadrant shut down early.
So what I'm asking is does anyone here have any real insight into this interesting concept?
Thanks in advance for any assistance in helping me understand this rather complex idea better.
Ross.
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#1
by
Jim
on 19 Feb, 2006 04:48
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Some/most spike designs had fuel cooling the spike vs TPS
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#2
by
simonbp
on 19 Feb, 2006 14:04
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#3
by
Rabidpanda
on 16 Mar, 2010 23:26
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I'm a little fuzzy on the design aspects of an aerospike and I thought this would be the perfect thread to ask a question. My question is, where is the combustion chamber located on an aerospike engine?
The illustrations I've seen have the combustion chamber at the top of the spike at the widest part, does this mean that there are many small combustion chambers positioned around the top?
Thanks in advance.
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#4
by
Proponent
on 16 Mar, 2010 23:46
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My question is, where is the combustion chamber located on an aerospike engine?
Many of the original aerospike designs called for a single annular combustion chamber just above the spike. Recent designs tend to feature multiple chambers, also at the top of the spike.
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#5
by
Ronsmytheiii
on 17 Mar, 2010 12:23
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Some/most spike designs had fuel cooling the spike vs TPS
Would it be possible to still cool the engine bell with fuel with out combusting it? For instance, using excess H2 to cool the bell without using O2 to prevent combustion, it would not need to flow as long as liftoff.
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#6
by
Rabidpanda
on 19 Mar, 2010 01:20
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All right I have another question.
I have often heard of aerospike engines as being efficient at all altitudes as oppose to normal nozzles only being efficient at a certain altitude, whether that's sea level or a vacuum. However when I look at the specs for existing engines it always shows their sea level Isp as being lower than their vacuum Isp, even if it's a first stage engine. Is any engine, no matter what kind of nozzle it has, going to have a lower sea level Isp than vacuum Isp? Or does that only apply to traditional bell nozzles and an aerospike would be different?
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#7
by
Namechange User
on 19 Mar, 2010 01:25
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Engines are engines. Same cycles, etc. The only think that makes it an aerospike is the nozzle. Nozzle plays into efficiency but I bet whatever you are looking at assumes that it is the standard bell nozzle that the engine was baselined to have.
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#8
by
Downix
on 19 Mar, 2010 03:13
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All right I have another question.
I have often heard of aerospike engines as being efficient at all altitudes as oppose to normal nozzles only being efficient at a certain altitude, whether that's sea level or a vacuum. However when I look at the specs for existing engines it always shows their sea level Isp as being lower than their vacuum Isp, even if it's a first stage engine. Is any engine, no matter what kind of nozzle it has, going to have a lower sea level Isp than vacuum Isp? Or does that only apply to traditional bell nozzles and an aerospike would be different?
Aerospike engines are equally efficient at all altitudes, which is to say, are not as efficient as an optimized bell. Their isp/thrust is pretty consistant throughout the whole stage. A closed bell, however, is optimized for a set altitude, and looses performance when outside of that envelope. That is why aerospikes are studied.
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#9
by
Rabidpanda
on 19 Mar, 2010 03:40
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All right I have another question.
I have often heard of aerospike engines as being efficient at all altitudes as oppose to normal nozzles only being efficient at a certain altitude, whether that's sea level or a vacuum. However when I look at the specs for existing engines it always shows their sea level Isp as being lower than their vacuum Isp, even if it's a first stage engine. Is any engine, no matter what kind of nozzle it has, going to have a lower sea level Isp than vacuum Isp? Or does that only apply to traditional bell nozzles and an aerospike would be different?
Aerospike engines are equally efficient at all altitudes, which is to say, are not as efficient as an optimized bell. Their isp/thrust is pretty consistant throughout the whole stage. A closed bell, however, is optimized for a set altitude, and looses performance when outside of that envelope. That is why aerospikes are studied.
I understand all of that but even engines that are designed for first stages seem to have lower sea level Isp than vacuum Isp. I'm asking if this is the same for aerospikes.
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#10
by
Rabidpanda
on 19 Mar, 2010 03:41
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Basically, is there some rule that all engines, no matter what nozzle they have will have a lower sea level Isp than vacuum Isp?
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#11
by
hop
on 19 Mar, 2010 05:01
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Basically, is there some rule that all engines, no matter what nozzle they have will have a lower sea level Isp than vacuum Isp?
I believe this is correct. If you aren't going down to zero pressure, you can't extract all the available energy from the exhaust. I'm not a rocket scientist tho
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#12
by
Max_Peck
on 19 Mar, 2010 06:05
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A typical bell nozzle is designed to operate best at one specific altitude. Either side of that altitude it will perform sub-optimally.
An aerospike nozzle is also optimized for a given altitude as well, but will not be quite as good as the bell nozzle at that given altitude. Where the aerospike gets its advantage, is that the curve which indicates loss in efficiency either side of its optimal point, is far less steep with the aerospike than with the bell nozzle.
So the aerospike should be substantially more efficient throughout the ranges either side of optimal (the majority of the flight), but for a short period of the flight, a bell nozzle operating at its optimal altitude would briefly out-perform the aerospike.
At least, that is the theory. Nobody as so far ever launched an aerospike on a space launch vehicle and sent it to any serious altitude yet, so there is no solid data to prove the theory either way. Only one or two sounding rockets have flown with aerospike nozzles, but the data from those is still currently inconclusive.
Here is a good writeup:
http://www.aerospaceweb.org/design/aerospike/compensation.shtml
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#13
by
KelvinZero
on 19 Mar, 2010 10:15
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Basically, is there some rule that all engines, no matter what nozzle they have will have a lower sea level Isp than vacuum Isp?
This probably isnt directly relevant to what you are actually talking about, and I don't know enough to judge how accurate the article is, but I thought it was interesting:
http://en.wikipedia.org/wiki/Air-augmented_rocketIt uses air simply as additional working mass, not as oxidizer.
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#14
by
strangequark
on 19 Mar, 2010 13:40
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Basically, is there some rule that all engines, no matter what nozzle they have will have a lower sea level Isp than vacuum Isp?
Yes, even if you design a nozzle for sea level operation, its Isp will increase with increasing altitude. It just won't be as good as a nozzle designed for that altitude.
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#15
by
Rabidpanda
on 19 Mar, 2010 14:39
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Basically, is there some rule that all engines, no matter what nozzle they have will have a lower sea level Isp than vacuum Isp?
Yes, even if you design a nozzle for sea level operation, its Isp will increase with increasing altitude. It just won't be as good as a nozzle designed for that altitude.
Thank you, that's what I wanted to know.
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#16
by
baldusi
on 06 Jan, 2011 15:03
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Sorry to bump up this thread. But I seem to remember that NASA even flight tested an aerospike in an SR-71. I would presume that with all the studies that they did they concluded aerospikes were not worth it. Does anyone know what were the causes? the T/W eat on any improved efficiencies? The gimballing was too heavy? It was not worth the extra cost? They simply cut it because the marginal improvement was no worth to keep the research? It was again one of those political decisions that made no technical sense?
As a side question. If it were worth it, wouldn't work on solids? I mean, instead of a single 140" tube to use four 60" at higher pressure with a common aerospike nozzle? It would need hold on to make sure all start at the same time. But you could use carbon casing and each tube should be simplier. I'm sure it's a stupid idea.
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#17
by
Jim
on 06 Jan, 2011 15:08
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Sorry to bump up this thread. But I seem to remember that NASA even flight tested an aerospike in an SR-71.
It was never tested hot, only cold flows
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#18
by
Proponent
on 11 Jan, 2011 02:43
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s a side question. If it [an aerospike nozzle] were worth it, wouldn't work on solids?
This is not a direct answer to the question, but I recall a few years ago a launch (by Cal Poly students, maybe?) of a solid with a spike nozzle. In other words, a full-length spike extended out the aft end of the rocket (i.e., the spike was not terminated, as would be the case in an aerospike, where most of the spike consists of gas).
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#19
by
Propforce
on 12 Jan, 2011 14:08
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s a side question. If it [an aerospike nozzle] were worth it, wouldn't work on solids?
This is not a direct answer to the question, but I recall a few years ago a launch (by Cal Poly students, maybe?) of a solid with a spike nozzle. In other words, a full-length spike extended out the aft end of the rocket (i.e., the spike was not terminated, as would be the case in an aerospike, where most of the spike consists of gas).
I think you are referring to the work by John Garvey and the Cal State Long Beach students. It was a LOX/Ethanol engine with full aerospike, so it is not a solid propellant system.
Their work was published in AIAA Space 2004 conference
http://www-rohan.sdsu.edu/~sharring/nlv.pdfIn theory, if aerospike works for liquid, it should work for solids as well.
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#20
by
Proponent
on 13 Jan, 2011 00:40
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Some discussion of solid (aero-) spike nozzles
here, including (see link in first post) a photo of a launch at Dryden.
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#21
by
john smith 19
on 22 Jul, 2011 12:54
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Some discussion of solid (aero-) spike nozzles here, including (see link in first post) a photo of a launch at Dryden.
There were actually 2 flight tests of an aerospike design.
One was the LOX/Ethanol the other was the solid fuel design at Dryden.
AFAIK these constitute the only *flight* tests of the concept *ever* and had limited funding and instrumentation.
LASRE was the linear design on the SR71 as part of the X33 programme which never got to hot fire. the core hardware dates from 1973.
BTW NASA and the USAF tested actual aerospike hardware up to 250Klbs in the 1960s including both LO2/LH2 and the insanely dangerous LF2/LH2. One space tug design wrapped the aerospike segments around a conventional bell , both with throttling of 9:1, giving a 81:1 throttling range.
In 1974 the USAF's AFRL had Rocketdyne build a *flight* weight LH2/LO2 gimbaled unit with dual cooling in the 25Klb (RL10) class. Project director was Dr David Hwang (that's the Hwang of Hwang & Huzel).
It was later badly damaged with several segments destroyed in tests but the early tests confirmed performance at flight weight.
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#22
by
strangequark
on 24 Jul, 2011 21:09
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AFAIK these constitute the only *flight* tests of the concept *ever* and had limited funding and instrumentation.
I know they've been used on small, in-space monoprop engines. That has been for length limited applications, because the aerospike can be extremely short for it's expansion ratio.
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#23
by
john smith 19
on 05 Aug, 2011 14:30
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I know they've been used on small, in-space monoprop engines. That has been for length limited applications, because the aerospike can be extremely short for it's expansion ratio.
Is there a reference for this work? They've certainly been *considered* for use in space specifically due to their short length, but I've never heard of them being deployed.
Some of the NASA space tug ideas looked at multiple "scarfed" conventional nozzles (like the RCS pod in the nose of the Shuttle) around an expansion surface while the plus with a conventional nozzle in the centre of it was another space tug concept.
Most organizations would view that as *very* high risk option.
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#24
by
demorcef
on 05 Aug, 2011 14:54
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Would it be possible to design a rocket engine with a variable geometry nozzle that could be adjusted on the fly to suit the performance needs of the altitude at that moment?
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#25
by
strangequark
on 05 Aug, 2011 18:28
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Would it be possible to design a rocket engine with a variable geometry nozzle that could be adjusted on the fly to suit the performance needs of the altitude at that moment?
Possible yes. Weight disadvantage, complex effects on nozzle flow, and hit to reliability outweigh advantages. Jet engine nozzles are convergent only. Much harder to do this on a convergent-divergent nozzle and maintain good nozzle flow.
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#26
by
strangequark
on 05 Aug, 2011 18:29
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Is there a reference for this work? They've certainly been *considered* for use in space specifically due to their short length, but I've never heard of them being deployed.
No.
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#27
by
D_Dom
on 05 Aug, 2011 21:31
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California State University Long Beach has a flight test program that flew aerospike experiments several years ago. On this website, scroll down almost to the bottom of the page to see pictures.
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#28
by
john smith 19
on 06 Aug, 2011 09:36
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Is there a reference for this work? They've certainly been *considered* for use in space specifically due to their short length, but I've never heard of them being deployed.
No.
Can you elaborate at all, or is it up for publication? Even what country or body did it. An actual flight test on orbit changes the game quite a bit.
I can't really think of a reason for secrecy. People have talked about them (and their expected flight properties) for decades. Their *expected* benefits are quite well understood, as are their problems.
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#29
by
strangequark
on 06 Aug, 2011 10:10
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Can you elaborate at all, or is it up for publication? Even what country or body did it. An actual flight test on orbit changes the game quite a bit.
I can't really think of a reason for secrecy. People have talked about them (and their expected flight properties) for decades. Their *expected* benefits are quite well understood, as are their problems.
The fact that it's been used on flight was in some materials that have been publicly distributed (quick Google search doesn't turn it up, though). As for where, that's not for me to delve into. The secrecy is not about the engines, as they were very standard.
It doesn't change the game at all. The fluid mechanics of aerospikes are well understood. Having a small RCS thruster use an aerospike nozzle for packaging reasons doesn't provide any substantial data.
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#30
by
john smith 19
on 07 Aug, 2011 13:43
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The fact that it's been used on flight was in some materials that have been publicly distributed (quick Google search doesn't turn it up, though). As for where, that's not for me to delve into. The secrecy is not about the engines, as they were very standard.
It doesn't change the game at all. The fluid mechanics of aerospikes are well understood. Having a small RCS thruster use an aerospike nozzle for packaging reasons doesn't provide any substantial data.
Technically it does not. But the fact they were used with *no* problems on orbit is the story. It breaks the classic "won't use them on my vehicle unless proven on orbit"/"can't launch them because they are not proven" mobius strip that seems to affect programme managers.
Hence my surprise. Packaging would have had to be *very* tight to consider something so (apparently) radical.
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#31
by
Jim
on 07 Aug, 2011 13:54
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I would like to know where they were used. I haven't seen such documentation.
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#32
by
mmeijeri
on 06 Dec, 2014 14:13
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Bumping. Is there any reason Glenn's Spacecraft Propulsion Research Facility couldn't be used for testing aerospike nozzles over the entire range of conditions they would encounter during launch? I'm assuming yes, or they would have done it, but it would be good to know the precise reason.
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#33
by
Proponent
on 06 Dec, 2014 16:39
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California State University Long Beach has a flight test program that flew aerospike experiments several years ago. On this website, scroll down almost to the bottom of the page to see pictures.
That's what I would call a spike rather than an aerospike, the difference being whether the tail of the spike is solid, as in Cal State's experiments, or gaseous, as in just about all of the other engines discussed in this thread.
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#34
by
mmeijeri
on 10 Dec, 2014 11:18
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If the "virtual bell" of an aerospike nozzle is much wider than the diameter of the rocket, does that mean that you'll get the same drag you'd get with a physical nozzle of the same size? And how does this affect the possibility of parallel staging? Do the magical self-tuning powers of aerospikes extend to allowing multiple overlapping aerospikes without ill effects?
