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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.
