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Post Info TOPIC: On the efficiency of aerospikes nozzles for rocket engines and hypersonic vehicles


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On the efficiency of aerospikes nozzles for rocket engines and hypersonic vehicles
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Ok still mega bored from the coronavirus lockdown and need the mental stimulationnnnnnnn.

As I closed the Aurora thread for the inevitable road it went down we left off from a very interesting conversation about aerospike engine nozzles and their efficiency on rocket engines and theoretical hypersonic vehicles. I'm reopening this discussion since its such a rich topic with many approaches... Its highly theoretical subject but still worth the indulgence imo. :)

Rafale D kicked it off with the question of why the linear aerospike is not used on most rocket engines and went further to explain why he thought it could work, saying that it would increase efficiency because it "adjusts to variable air pressure with differential thrust, it is lighter and consumes less fuel = an efficient answer to conventional space shuttle engines and could be an aswer to efficient hypersonic vehicles."

I replied:In short?

1. they are a massive engineering hassle for what theyre worth (see no 5)

2. they are very expensive to build and integrate, again for what they're worth (see no 5)

3. they are NOT lighter, for the most part. They can be significantly heavier with rocket engines despite the reduced profile from a conventional bell nozzle, but turbojets may find more benefit regardless of the weight penalty. Unless it is fully integrated into the system though, like on the Lockheed X-33 then the weight is reduced a lot... weight is a variable from the specific design, really... they are not just all universally lighter.

4. they ARE fuel efficient yes, but with this comes the issue of high temperatures and another engineering hassle in figuring out an effective cooling solution. Besides what if you used it as a reentry shield? it could work in theory but good luck dealing with the frictional heat on top of everything :)

5. they don't really work. Ok, they do partially but not enough to be worth it. Yes they are a one-up from conventional engines because they adapt to ambient pressures at varying altitudes sufficiently but at the cost of some pretty negative factors. And they reduce base drag a little bit, woooo. Bottom line theit total efficiency is nothing too spectacular.

In the end better fuel efficiency and partial base drag reduction are overshadowed by too many negatives to make them worth regular usage, especially facing the factor of cost... if its not the least bit cost effective its essentially a dead end. Most of the time anyway... :/

Maybe not as short of an answer that i planned but it does the job xD

( yes this was a copy-pase from the Aurora thread instead of retyping it all over again. :P  )

Rafale D replied again asking why you would want to use an aerospike as a heat shield and asked about the NASA LASRE (Linear Aerospike SR-71 Experiment) .

I replied (another copy-paste sue me :P )

Because you can.....

Think about it, it CAN withstand the extreme temperatures so you can design a craft that will utilize it as a reentry shield if you need to... especially if it were a plug nozzle (see Phil Bono ROMBUS) they are sometimes classified differently from an aerospike but they are essentially the same thing just truncated. Remember they are heavy relative to a craft's center of gravity so you can use this mass to an advantage... doesnt work so great with SSTO design like the X-33 though and was a major concern that prompted full integration into the structure. Cooling can be achieved by actively circulating liquid hydrogen through the engine cooling system at the base while the engines are still active, before it enters the combustion chambers. You don't really need cryogenic fuel for this kind of cooling method, hell water circulation will work. Obviously the linear aerospike of the X-33 would not work with ballistic reentry... hypothetical SSTOs of this type are usually designed for noseward reentry per Air Force/DOD requirements for cross-range distance anyway. It's not so much that an aerospike can't work at all, as a shield, but more of whta kind of aerospike and the craft it was designed for. The cooling method I described is fine and dandy in theory, but it's still an engineering hassle to integrate efficiently and quite costly.

It doesnt matter, all of this is purely theoretical since the LASRE is the only form of aerospike to have ever been flight tested. the SR-71 aerospike was never hot-fired during flight tests because of a liquid hydrogen leak in the LASRE pod... they only cold-flow tested it plus some hot tests on the ground. Not much data is available because the programme is essentially dead, so the results are inconclusive. Bottom line is cost and politics... the biggest issues to overcome in any development in aerospace over all else. :P

this answer confused Rafale D because i retyped this waiting for AB servers to come back online and i was dead tired watching the sun come up xD

Retroistic replied with an explanation of water circulation :

Water circulation is dual purpose for cooling and injecting into the airflow to increase air density, which increases thrust because the compressor operates at higher than normal levels. Really old trick but it works if done right (you can be faced with affected booster ISP.) DARPA RASCAL Phase 2 ground testing was able to achieve around Mach 5 through LOX and water injection. The specific form of water matters too: ordinary tap water was used in cooling the LASRE unit and caused burn-through due to CaCO3 buildup in the coolant vessels, and decreased combustion pressure. De-ionized water is safer and more efficient.

So then i returned with clarification on my last post and replied to Retro:

Yes retro taht technically works but only with a breathing engine system like SABRE and RASCAL's F100 turbofan or something like these. Solid/liquid chemical propulsion is a bit different... you could mix in antifreeze and circulate through a closed cycle system but you suffer a weight penalty since the system's complexity adds excess mass to an already heavy aerospike nozzle, and besides water would most likely be wasted in the burn process so it's only useful as a closed cycle coolant (higher thermal absorption rate). TBH you'd be better off with a regenerative liquid oxygen cycle or a mix of both... a hydrolox system.

Both aerospike designs could work as a shield but a plug nozzle would be more efficient for its shape in combination with an aerodynamic body that allows such. like i said, anything like the X-33 would obviously not work since the lifting body is designed for noseward entry. ROMBUS is a better candidate for ballistic reentry.

But like i said also... all theoretical. No real life basis to go on since the LASRE was the only aerospike that was flight tested.

the thread left off on a cliffhanger when Hillberg started talking about a diode cooling system for flight instruments and pilots.

Ok so lets keep the discussion going but without the arguing like last time! probably wont happen again now that the A-word is out of the equation, but better safe than sorry. :)



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Leela25 wrote:

you could mix in antifreeze and circulate through a closed cycle system but you suffer a weight penalty since the system's complexity adds excess mass to an already heavy aerospike nozzle, and besides water would most likely be wasted in the burn process so it's only useful as a closed cycle coolant (higher thermal absorption rate). 


 Wait I just now caught this... how could water be wasted through the burn process (internal combustion itself or the outward thrust?) if it were closed-cycle?



-- Edited by Pepper on Friday 15th of May 2020 04:46:36 PM

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*sigh* i really need to proofread more often before posting.... X_x

I was saying that in a rocket engine the cooling/injection combo would result in wasted water because it doesn't burn and the principle of its thrust works differently than on a breathing engine type where injection basically puts the compressor on overdrive to increase thrust. On a closed cycle cooling system its regenerative and circulates to the major components like an infinite loop, so no wasted fluids. Hydrolox on the other hand might be a better alternative. I hope that makes more sense. :/



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It's all good. Yeah that clears it up a lot better, thanks. 



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Leela25 wrote:

*sigh* i really need to proofread more often before posting.... X_x


 It would help a litle bit. We all appreciate your knowledge and input on these subjects (do you happen to be an employed aviation professional by chance? you don't have to answer that smile) But if I can lend a little bit of honest criticism, you do sound like you're in a rush when you post or that you're half-asleep (which you admitted to with some earlier posts). You obviously have a passion for aerospace, that enthusiasm should be expressed in your writing. I know it might be out of place for me to say to a staff member, but please understand that I'm only trying to help. Hope you're taking things well under the coronapacalypse (TM).

Since you clipped my response to Hillberg I'll post another version of it here:

The principle in diode cooling involves hardware that would literally melt when used for cooling the engine because of the extreme temperatures. There is a reason that radiative-cooling has been used for disposable, single-use rocket engines, using expansion and transfer of generated heat to advantage instead of a heavier and more expensive internal cooling system. But for the application described in that test, as a cooling system for individual flight components, it has much more promise if it can be integrated efficiently.



-- Edited by retroistic on Sunday 31st of May 2020 01:58:32 PM

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I would be willing to bet that she is an engineer or at least a very knowledgeable technician. I often find her posts at this forum among the most educational for me.smile



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Im flattered but i would never claim to be at such a high calibre. I work at a bloody convenience store ffs xD

ive just always have been interested in aerospace but i just lack the drive to pursue it professionally. I don't think i have the intelligence for it and im certainly not qualified at present. you may think im smart but i only know what i read from the crapton of books ive collected over the years. i wouldnt know the first thing on ho to apply what i know aoutside of my limited skillset. X_x

There are far more intelligent people than me at this forum... I consider myself a hobby historian more than anything near an aerospace expert, i just know some useful things. We used to have some very intelligent, experienced aero pros in the past but they dont come here anymore and i think its a huge loss. i appreciate the ego boost though xD

BTW thanks for the advice, I will try to improve my writing clarity a bit. Didnt know it was so lacking in cohesiveness. :)



-- Edited by Leela25 on Sunday 31st of May 2020 02:55:41 PM

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Wow, I was not expecting that. You are a lot smarter than you think, Leela. I think you have what it takes to apply what you know and develop those skills. You have potential to go far. I hope you consider it, I really do.

smile



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ty for the support guys it means a lot :)



-- Edited by Leela25 on Monday 8th of June 2020 01:49:26 PM

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In attempt to take us back to the original topic, I got a question (no not about the cooling methods)

What other types of aerospikes are there and which aerospike is more efficient over the other? Would the linear aerospike for its versatility? Since it uses multiple jet nozzles that can be adjusted individually for speed and corrections... the plug nozzle appears to use a single outlet that doesn't allow much for efficient control, even the conical plug type.



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The "conical plug type" is called a toroidal aerospike. There are only three types, toroidal, linear, and the plug.... judging which is more efficient over the other greatly depends on the design it was built for. For example the X-33 benefits from the Rocketdyne RS-2200 concept for its dynamic requirements as a SSTO RLV. the toroidal and plug types fit best for smaller and narrower vehicles and could work with hybrid engine types like a combined cycle rocket/turbofan. The plug IMO would be the least efficient but gains an advantage for a vehicle designed for ballistic reentry in which you can theoretically use the aerospike as a shield.

There are varying factors to consider... design of the engine, design of the vehicle, dynamic requirements, range requirements, etc. 

I wish i could give a more detailed answer for that one but with such limited real world testing most data is purely theoretical and open to speculation/debate. :P



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Fair enough. I think another factor to consider is the weight of the nozzle, like you said before they can be heavy. The plug and toroidal nozzles could be built at half the scale of a bell nozzle which would cut down the mass and increase efficiency.



-- Edited by Pepper on Wednesday 10th of June 2020 01:54:26 PM

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A question I have been speculating about is if aerospikes can find more efficient maneuverability if they were gimballed instead of variable thrust vectoring and RCS thrust. The added weight penalty should balance out.



-- Edited by Fornax on Thursday 11th of June 2020 04:52:42 PM

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But without the variable thrust you also sacrifice efficient altitude compensation, right? I can see it working for both if it can be simplified, but IMHO a gimballed nozzle would likely add more complexity than it's worth.



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Not if you gimballed the entire engine instead of just the nozzle. Best of both worlds, full control over pitch & roll plus differential throttling for adapting to changing atmospheric pressure.



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Pepper wrote:

Fair enough. I think another factor to consider is the weight of the nozzle, like you said before they can be heavy. The plug and toroidal nozzles could be built at half the scale of a bell nozzle which would cut down the mass and increase efficiency.



-- Edited by Pepper on Wednesday 10th of June 2020 01:54:26 PM


 what are we even talking about anymore, chemical propulsion or breathing engines with aerospikes??confuse

That's just one example, again it greatly depends on the design of the engine and vehicle that the aerospike is designed for.



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Rafale D wrote:

But without the variable thrust you also sacrifice efficient altitude compensation, right? I can see it working for both if it can be simplified, but IMHO a gimballed nozzle would likely add more complexity than it's worth.


 THIS

Linear aerospikes for example are designed for both altitude compensation and efficient manuverability without the complexity of a gimbal, so less mass but retaining similar control of all dimensions. Since the linear aerospike has multiple chambers with their nozzles aimed at the wedge (instead of one conventional chamber) you can downthrottle them to achieve this control... decrease thrust on one side of the aerospike = pitch, opposite corners = roll, and both = your altitude compensation. Some toroidal types can have multiple chambers too and achieve the same control, its design is not limited to conventional layouts like the single-chambered plug type. With this control achieved why need a gimbal? Is redundant beyond any sense of practicality. :P

Now lets look at the toroidal and the plug... unless the toroidal type has multiple chambers like the linear type, it is basically the plug type with a conventional engine layout in which case it might benefit from a gimbal since its not that much different from a standard bell nozzle with respect to control of pith/roll. BUT a single multi-chambered toroidal type will obviously not be able to achieve roll, so i guess a gimbal could work here too unless the vehicle design calls for two sources of thrust instead of one, then you could just downthrottle a number of chambers from the separate units. The annular toroidal and plug types might find more efficiency in separate control surfaces or RCS thrust to affect pitch/roll instead of manipulationg the engine though, because whether you gimballed the engine or the nozzle only (doesnt matter) they both add complexity and a weight penalty that is wholly unnecessary.

Another useful tidbit about the toroidal/plug types, aerospikes originated from a truncated layout that was based on the principle of base bleed. Most modern aerospikes consist of a physical point to direct the exhaust around, especially the annular toroidal type with their long cones/spikes to increase efficiency, but theoretically you can achieve the same level of efficiency by creating a "virtual" spike... this is done by venting a small amount of gas from an exit at the centre of the plug. "aero... spike" :D Saves weight but uses more fuel.



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Leela25 wrote:
Now lets look at the toroidal and the plug... unless the toroidal type has multiple chambers like the linear type, it is basically the plug type with a conventional engine layout

 You know that combustion chambers don't have to have just one throat right? You can have an aerospike nozzle with a single chamber but more that one throat and exhaust oriented down the spike.



-- Edited by retroistic on Saturday 13th of June 2020 02:57:54 PM

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Yes you can have an engine with one chamber and several throats but you lose the advantage of differential throttling, i.e. it needs a gimbal. Unless I am wrong and there is another option that I am overlooking, in that case I encourage some enlightenment. By the way, thanks Leela25 for your input. You are right, it does depend on the design of the aerospike and the rocket.



-- Edited by Fornax on Monday 15th of June 2020 01:02:18 AM

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retroistic wrote:
Leela25 wrote:
Now lets look at the toroidal and the plug... unless the toroidal type has multiple chambers like the linear type, it is basically the plug type with a conventional engine layout

 You know that combustion chambers don't have to have just one throat right? You can have an aerospike nozzle with a single chamber but more that one throat and exhaust oriented down the spike.



-- Edited by retroistic on Saturday 13th of June 2020 02:57:54 PM


 Points for the newbie for covering this one (hai Fornax :D) his response pretty much sums up the issue, that the single chambered multi exhaust toroid gains little advantage over a classic toroid/plug because you cant downthrottle individual outlets like you can with multi chambers. BTW like i said a gimbal could probably work, or separate control surfaces/RCS thrust, but if its an airbreather there are a number of theoretical workarounds if one is inclined to be a bit imaginative. ;)



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Fornax wrote:

Not if you gimballed the entire engine instead of just the nozzle. Best of both worlds, full control over pitch & roll plus differential throttling for adapting to changing atmospheric pressure.


 I think you misunderstood what I was saying. bY altitude compensation I was not referring to it adjusting to the ambient pressure, but basic pitch & yaw control. Not sure how gimballing the engine vs. the nozzle only will make much of a difference. AFAIK they can all adjust to ambient pressure at varying altitudes regardless of the control method. Aerospikes just happen to be more efficient at it than bells.



-- Edited by Rafale D on Monday 15th of June 2020 03:33:48 PM

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What if it had valves?



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