Reusable Single Stage to Orbit Concept

[93143]

Do you mean because of cube-square effects?

Pretty much.  They don't hit everything equally, but they are a factor.

What is it about the engines that precludes scaling them down easily? I thought that engines typically scaled up badly, twice as large being more than twice as difficult.

I don't know exactly, but Hempsell has mentioned this more than once, and it's in their FAQ.  Whatever it is, it's specific to SABRE.

...How much did SpaceX spend on the Merlin 1?  They seem to think they can do an engine with about 15 times the thrust for $1B.

I was thinking of manned suborbital hops.

Not worth it.  SABRE is an SSTO engine.  Suborbitals don't need that kind of technology, so it would be underutilized and overpriced for that market.

I can believe there may be systems for which this is true. Even so cost/kg isn't the only variable. ROI demands by investors may be a tougher constraint to live with. And you don't have to achieve a reduction in price by an order of magnitude all at once, merely enough to build up enough market share. I think the suborbital RLV people are being very sensible in how they go about their work which they ultimately want to lead to orbit.

Actually, you do pretty much have to achieve an order of magnitude all at once.  Otherwise SpaceX will eat your lunch...  What do you suppose a Falcon 9 Heavy will cost by the time Skylon flies?

[marginally-relevant rant about TSTO RLV vs. SSTO RLV deleted]

You may argue that Reaction Engines should be concentrating on simpler technology and trying to build up market share before tackling the big one.  I say that would just distract them.  The technological commonality wouldn't be high enough to justify it in an Ares I/Ares V sense, and it's not like these guys need the engineering experience.  Besides, there's no way operating a suborbital vehicle in this soon-to-be-highly-competitive market would net them a significant fraction of the 12 billion dollars they need to bring Skylon to market in any reasonable timeframe...

[RanulfC]

Seward – Philip Bono was a good engineer and I am sure the basics of his ballistic plug nozzles designs were credible. However they are pure rocket (I am not sure that works), they had not been trimmed (see comment above), and aerospikes are unlikely to work in the simple manner that was assumed in the 1960s.  When MacDonnell Douglas (heirs to his work) got a second go with Delta Clipper, they saw Bono as the heritage, but decided nose first was best and preferred bell engines (I was on team in Huntington Beach the initial stages of this project).

Couple of points on plug-nozzle engines;

Aerospikes work, they work well and they have been extenisivly tested using various fuel combinations and nozzle types. There was some question of the "round" (annular) design having thrust fall offs between Mach3 and Mach5 but that hasn't appeared to be an actual issue outside a wind-tunnel. The "flat" (linear) aerospike engine worked as advertised there was an issue however in that instead of planning on thrust-vector control it was decided that actual actuators would be used to move the entire engine as per a normal engine gimbeling system. (Why? Not a clue on where that came from)

This greatly increased the already heavy engine structure, which was critical on the X-33 design as it made the tail structure too heavy to balance properly on glide back.

Mr. Hempsell; On the Delta Clipper design, the "nose-first" entry wasn't a "design" decision that was a requirement to meet the Air Force specified cross-range ability so there was no reason to use a plug-nozzle even though it would have been easier all around to use for both engine and heat-shield. The Air Force had done a lot of the original work on plug-nozzle engines as both propulsion and heat-shielding and a purely ballistic "Delta-Clipper" would have met the requirements except for the stated cross-range ability.

Since neither DARPA nor the Air Force had actually intended to pursue the Delta Clipper to a flight testing stage there wasn't even a need for the stated cross-range requirement but it's interesting that General Dynamics had already submited a study of an SSTO ballistic vehicle using a plug-nozzle engine/heat-shield combination requested by DARPA and the Air Force in conjunction with NASA. Alternative engine lay-outs and types were studied but the combination engine/heat-shield was by far the suprior design in terms of payload to orbit.

Randy

[mmeijeri]

Question for Hempsell (or anyone else who might know): could Skylon be profitably combined (or even at all) with mass injection precooling?

[RanulfC]

Question for Hempsell (or anyone else who might know): could Skylon be profitably combined (or even at all) with mass injection precooling?

The concept is already "deep-cooling" the incoming air so I don't see precooling as working for the engine.

Randy

[mmeijeri]

Couldn't it allow higher mach numbers in air breathing mode, or maybe less sophisticated heat exchangers?

[RanulfC]

mmeijeri wrote:
Couldn't it allow higher mach numbers in air breathing mode, or maybe less sophisticated heat exchangers?

MIPCC (Mass Injection Pre-Compressor Cooling) is mainly used to extend the Mach envelope of a standard turbo-fan/jet engine by allowing the turbo-machinery to operate closer to normal temperatures even when at high Mach and altitude.

The injected water cools and increase the air density while also increasing the mass flow, while the injected LOX stabilizes the combustion in both the combustion chamber and the afterburner. The SABRE engine already densifies the air by deep-cooling the incoming air and it doesn't use the same temperature limited turbomachinery that regular jet engines do so it never reaches it temperature limit. In order for water injection to work at all the MIPCC duct has to be long enough to allow the water to vaporize, but with the deep-cooling of the SABRE engine the water would re-condense out of the air stream and turn to ice which would be VERY BAD on the mechanicals of the engine system.
(Note that one of the mentioned "show-stoppers" was issues with icing of the heat exchangers )

SABRE (and SKYLON) wouldn't get any benifit from using MIPCC so the extra equipment and such would be dead-weight.
Here is a High Speed Propulsion Cycles report:
http://ftp.rta.nato.int/public//PubFullText/RTO/EN/RTO-EN-AVT-150///EN-AVT-150-02.pdf

This one shows the majority of the mentioned types of propulsion and their good points and bad with a focus on MIPCC. The majority of the cycles shown DO NOT WORK WELL TOGETHER with only a few exceptions because of the mechanics of the cycles themselves. (About the only ones are MIPCC and the Turbine Engine With Rocket Augmentation cycles. This since the MIPCC does a good job fooling the engine into thinking it's operating at Mach-2 and 50Kft when it's actualy Mach-4 at 80Kft, and TEwRA basiclly turns the After-Burner into a crude rocket engine from there.)

Now... Were this thread opened up a bit more to allow 1.5 or TSTO ideas I'd be pushing my latest line of research, the Spacejet concept.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810020560_1981020560.pdf
(There's a low speed subsonic study someplace too but it gives the idea)

Add MIPCC to the turbojet packages and TEwRA and you can release the Spacejet at higher Mach and altitude for a more effcient spaceplane.
But that ain't this thread )

Here is some other high speed air-breathing and such reports to take a look at:

Innovative Airbreathing Propulsion Concepts for Access to Space Report, 2001
http://gltrs.grc.nasa.gov/reports/2001/TM-2001-210564.pdf

Populsion Systems For Hypersonic Flight:
http://ftp.rta.nato.int/public//PubFullText/RTO/EN/RTO-EN-AVT-116///EN-AVT-116-09.pdf

Here is a LAPCAT (Mach-5 cruiser version of SKYLON, sort of) report:
http://ftp.rta.nato.int/public//PubFullText/RTO/EN/RTO-EN-AVT-150///EN-AVT-150-12.pdf

Paper on Advanced Rocket Motors:
http://ftp.rta.nato.int/public//PubFullText/RTO/EN/RTO-EN-AVT-150///EN-AVT-150-06.pdf

And a download site for a series of breifings and reports given at NATO on high speed flight and propulsion:
http://www.rta.nato.int/pubs/rdp.asp?RDP=RTO-EN-AVT-150

Randy

[lkm]

So why not Lox injection? Couldn't you inject Lox from Mach 5 to 8 with greater efficiency than the SABRE rocket mode? It doesn't introduce new tankage, it won't freeze in the heat exchanger, what's wrong with it?

[mmeijeri]

Thanks for the links. I had read a number of them already and I don't really understand your explanation. I thought that Skylon was limited at M5.5 by a compressor temperature limit and I was wondering if MIPCC couldn't help with that. Water would obviously not work (unless you precool less deeply, which might require less advanced heat exchangers), but LN2 or LOX could. I have no good feel how much this would help as I'm not sure what the limiting factor for Mach number is: skin heating, drag losses, air-breathing/rocket Isp or T/W.

[RanulfC]

lkm wrote:
So why not Lox injection? Couldn't you inject Lox from Mach 5 to 8 with greater efficiency than the SABRE rocket mode? It doesn't introduce new tankage, it won't freeze in the heat exchanger, what's wrong with it?

Actually would it be more 'efficient' than the rocket mode? By Mach-5 your aeroheating on the hull itself is becoming an issue so going straight to rockets and climbing out of the atmosphere would probably be better.

(And wouldn't it add tankage? The LOX you use would no longer be available for use in the rocket and you're haveing to add to densify and augment the incoming over-heated air? I'd think it would be less effective. YMMV )

mmeijeri wrote:
Thanks for the links. I had read a number of them already and I don't really understand your explanation. I thought that Skylon was limited at M5.5 by a compressor temperature limit and I was wondering if MIPCC couldn't help with that.

That may be my bad then because I was under the impression Mach-5+ was getting to hot for the airframe for extended aeroheating.

Water would obviously not work (unless you precool less deeply, which might require less advanced heat exchangers), but LN2 or LOX could. I have no good feel how much this would help as I'm not sure what the limiting factor for Mach number is: skin heating, drag losses, air-breathing/rocket Isp or T/W.

I'll look around but I did find a paper on LN2 addition to UAV engines for added thrust. The issue with less pre-cooling (and simpler heat exchangers) runs into (as I understand it, again YMMV ) is that less deeper cooling effects your overall engine efficincy, which effects your overall payload to orbit. The added mass would certainly help the thrust, but it reduces your overall ISP by about the same amount that it helps the thrust. One study I've seen shows a linear progression, added thrust from mass injection equals the overall loss in ISP over the studied range.

You could probably get away with it easier if you DON'T go SSTO but that need for single-stage tends to make ISP over the whole envelope pretty damn critical.

Randy

[lkm]

For SABRE the energy recovered from the precooling drives the compressor, less precooling means less pressure in the rocket engine. The engine is limited by the temperature limit of 1350 K of the compressor and helium cycle. If Lox was bled in at the shock cone to keep to that limit it should require less Lox than the pure rocket mode requires up to around mach 8, I think. As to the Aeroshell I don't know, doesn't  it already face greater temperatures from re-entry?

[Hempsell]

RanulfC  “Aerospikes work?”
Well yes. They work well on static test stands but once on flying vehicle complications arise like base drag and the interaction of the two supersonic flows they get more complicated. That is not to say they cannot be made to work it is just a lot more complex than simple theory suggests. There are similar complexities with Expansion/Defection nozzles and we may yet still use these, so it is not that we are anti-advanced nozzles .

RanulfC  “On the Delta Clipper design, the "nose-first" entry wasn't a "design" decision that was a requirement to meet the Air Force specified cross-range ability”
I agree it was not driven by the UASF requirements my point was that the team most motivated to follow Bono’s design decided it was more difficult to do than the nose first approach. (Again I would point out I was in Huntingdon Beach working with McDAC working on this programme and saw these decision being made). I am sure “Aerospike as a heat shield” might be made to work but, once the realities have been engineered in, it might not be as attractive as Bono initially thought.

LOX injection: This was actually used on HOTOL as the frost control system (there is a UK patent on this). The problem with using it to much beyond Mach 5 is not temperature but Momentum drag;  that is the force needed to stop the air so it can be fed to engine is getting comparable to the force the engine provides and acceleration slows down to the point it is not worthwhile.  If you want to go much beyond the Mach 5 or 6 region you need to go to Scramjets where the air is not stopped.

Also RanulfC is correct that heating on the way up is also an issue it is why the medium temperature TPS extends over the whole airframe rather than just the re-entry areas as on the Shuttle. This is another reason why Mach 5 is looking like a good transition point.  In the end it is difficult to firmly establish whether it is the journey up or down that is driving the SKYLON temperature control.

Another point RanulfC is correct to point out is that once you start to use LOX the effective Isp starts drop dramatically.

[mmeijeri]

To what speed do you need to slow down the air before you can feed it to the engine? I recall reading somewhere that there were concepts for high subsonic combustion, but I don't remember if it was in the Skylon documentation or somewhere else.

[RanulfC]

RanulfC  “Aerospikes work?”
Well yes. They work well on static test stands but once on flying vehicle complications arise like base drag and the interaction of the two supersonic flows they get more complicated. That is not to say they cannot be made to work it is just a lot more complex than simple theory suggests. There are similar complexities with Expansion/Defection nozzles and we may yet still use these, so it is not that we are anti-advanced nozzles .

Actually FLIGHT testing has proven the "complications" aren't as large an issues as was suspected. The test-stand testing didn't address some of those concerns that's WHY they were flight tested. Flight tests have been done with both liquid bi-propellant, mono-propellant,and solid aerospike engines and they do work.

I don't think you're "anti-advanced-nozzles" and don't mean to give that impression. I just am tossing the information out there because it seems that many people aren't aware of how far the research has gone because the majority of the work hasn't been done under large, big-budget programs run by the major space agencies. The current research programs are private industry and collage/university level endevours for the most part. Current research is focusing on multi-nozzle "plug" and "dynamic" aerospike engines scalable for larger launch vehicles.

RanulfC  “On the Delta Clipper design, the "nose-first" entry wasn't a "design" decision that was a requirement to meet the Air Force specified cross-range ability”
I agree it was not driven by the UASF requirements my point was that the team most motivated to follow Bono’s design decided it was more difficult to do than the nose first approach. (Again I would point out I was in Huntingdon Beach working with McDAC working on this programme and saw these decision being made). I am sure “Aerospike as a heat shield” might be made to work but, once the realities have been engineered in, it might not be as attractive as Bono initially thought.

Uhm, the "Nose-First" reentry WAS an Air Force requirement because a base first ballistic entery did not have the necessary "cross-range" as required by the Air Force. Sorry if that was confusing the issue. I understand that you were there when the decsions were made but the REASON the base-first entry wasn't considerd was that the Air Force had set a required "cross-range" ability for the vehicle and there was no way to make that requirement with a base-first ballistic entry. So a lifting "Nose-First" entry was required to meet the specified cross-range distance.

While there may have been 'motivation' to use the previous work related to "plug-nozzle aerospike as engine and heat-shield" from the 60s (which, BTW was mostly funded by the Air Force) the given requirements would not allow consideration of that method.
As part of the problems with Delta Clipper was the added TPS requirements that the "nose-first" entry required which added much more mass to the system than the simpler "ballistic entry" with the plug-nozzle and base activly cooled did. The neccessary manuevers (end-over-end flip required to go from "nose-forward" glide to "base-first" for landing) also added complexity and increased the danger, and difficulty of recovery versus the more straight forward base-first all the way landing that the Bono style engine/heat-shield system would have used.

It's not that MacDac didn't want to use the system or that they didn't feel it was technically sound, the given requirements for the flight parameters diss-allowed the idea of ballistic "base-first" entry from the design.

I hope this clears up what I was meaning.

Randy

[Hempsell]

mmeijeri “To what speed do you need to slow down the air before you can feed it to the engine?”

To all intensive purposes it is brought to a stop in the Nacelle before it is drawn through the heat exchanger.

RanulfC The Delta Clipper  "Nose-First" reentry

As I recall it (and it was 20 years ago and my notes are no longer with me - property of what is now Astrium) the underlying motivation of the team for the nose first was they were not convinced about things like using a low level burn to protect the combustion chambers and on other work I have found these low level protective burns consume more propellant than I for one first thought. Although the Delta Clipper did baseline an aerospike plug several team members, including Max Hunter if I remember correctly, did not really like the engines and were arguing for conventional bells.

I have to admit I joined the team after the nose first decision had been made and they may have been more influenced by the Air forces requirements than I was aware, but I am certain they saw it as a better technical approach to “engine first” regardless of meeting the Air Force’s Cross Range requirements which were so modest I suspect an “engine first” configuration could have been found to meet them.

My job on the team was to explore a HOTOL like alternative, which of course had massive cross range but that feature gave us no brownie point whatsoever!

[Warren Platts]

Mr. Hempsell, I'd just like to say that we really appreciate you taking the time to keep us informed on this discussion forum. Thank you sir!

[Cinder]

I'll second that.  A real privilege, thank you.

[RanulfC]

RanulfC The Delta Clipper  "Nose-First" reentry

As I recall it (and it was 20 years ago and my notes are no longer with me - property of what is now Astrium) the underlying motivation of the team for the nose first was they were not convinced about things like using a low level burn to protect the combustion chambers and on other work I have found these low level protective burns consume more propellant than I for one first thought. Although the Delta Clipper did baseline an aerospike plug several team members, including Max Hunter if I remember correctly, did not really like the engines and were arguing for conventional bells.

Well I actaully 'see' an issue with something in the above information that doesn't bode well for the design team ACTUALLY checking the work on a plug-nozzle/Aerospike engine!

Plug-nozzle/Aerospike engines don't DO a "burn" to protect the combustion chambers of the rocket nozzles. You ONLY use that with more conventional bell nozzles and it is very much less for protecting the combustion chambers as it is for generating a pressure bubble to defelect heat from the nozzles themselves.

In "heat-shield" mode Liquid Hydrogen (in an LH2/LOX vehicle, you can also use Lox, fuel, even water) is circulated behind the "plug" base where the majority of heating is taking place and then flows around the engine system and finally is dumped into the combustion chambers and into the vehicle slipstream. As noted if you're not using LH2 other cryogenic and some non-cryogenic propellants can be used instead. It was shown that in some cases (mostly the low-density LH2 designs) that it might actually be better to carry a volume of water instead of more LH2.

I'm wondering if the design baseline wasn't actually a "plug-cluster-nozzle" system instead of an aerospike design?

I have to admit I joined the team after the nose first decision had been made and they may have been more influenced by the Air forces requirements than I was aware, but I am certain they saw it as a better technical approach to “engine first” regardless of meeting the Air Force’s Cross Range requirements which were so modest I suspect an “engine first” configuration could have been found to meet them.

I don't think an 'engine-first' ballistic design would have made the cros-range because by thier nature ballistic's don't have good cross-range. And especially in the case of the Air Force even "modest" cross-range can be rather a 'broad' defninition depending on the bias of the Air Force people involved. They have a rather 'bad' habit of waiting till a team settles on a  good design and then asking that the cross-range be increased... A lot!

I have a suspicion that there was an overall in-group opposition to the possible use of a plug-nozzle aerospike no matter what the original "baseline" design called for. As I recall Max Hunter for one, though being a SSTO-VTOL believer was (IIRC) unconvinced that ANY aerospike engine was a technically sound idea. I seem to recall him being quoted at one point saying a working "aerospike-rocket-engine" of any type would never fly because the theory and design concept were inheirently flawed.

Which ends up injecting what I generally call (after a Sci-Fi short story of the same name, synopsis below*) "Paper Virus" into the mix.
When an "expert" or respected manager/engineer/whatever passes judment on a system, concept, or idea there is a tendency for those around (even if they have another opinion) to accept that judment at face value with no questions asked.

This is similar to an issue with Space Suits that I have long been fighting, with some success. I have studied all the available data on, and confured with the designer and original researcher on a concept space suit called: The Space Activity Suit which is also called a "Mechanical Counter-Pressure Space Suit. This was designed in the early 70s as an alternative to the heavy and difficult to move Apollo and previous space suits and rather than trying to surround the body with small space ship filled with air it used the fact that the human skin, as long as it is reinforced to equalize lung and internal pressure with pressure from the outside is immune to the effects of vacuum.

Very basiclly it is a leotard garment made up of layers of stretchable cloth that squeezes the body and reinforces the skin to match the helmet and internal organs pressure. The suit is extremly light weight, easier to move and do activities in by orders of magnitude than any "pressure" suit ever designed. Thermal control is regulated by already existing body function such as sweating so this does not have to be included in the life support for the suit. If you get cold you put on an inulated set of coveralls. Simple, easy and pretty cheap. Once a way is found to allow the mechanical pressure to 'relax' when out of a vacuum, (current work is looking at air-tubes or memorty metal strips used in a manner similar to the 1950s "capstan" emergency pressure suit) it can be work comfortably as normal "clothing" until needed.

I had a chance to call into the Space Show at one point and ask some questions of the man who is considerd to be the "foremost" expert on Space Suit design and asked him about commercial and NASA work on the Mechanical Counter-Pressure suit. He quickly informed me that the entire concept was UNWORKABLE and had never passed any research test. When I mentioned by research on the subject and that it HAD passed NASA testing he snapped that the "astronauts" had been to "macho" during testing to report any problems and that the information I had was totally false. He then repeated that the concept was a dead and an would never work and went on to another question.

I should note that I have emailed him, tried asking on forums or message boards I happen to find him on to name WHICH astronauts participated in the test program as I could find none listed in the data as having even been asked about participation in the testing let alone having worn the suits at all. The only response I've ever recieved basiclly told me all the research I've mentioned is a lie, that the concept was never successfully tested and that he had no time to argue with someone who would obviously not listen to the 'facts' as given by an expert.
I can understand that he's busy, since he is now designing space suits for the Indian Human Space Flight program

The main point is this: Despite his being the worlds "foremost" expert on Space Suit design and having written all the text books used for teaching everyone ELSE how to design a space suit, the "concept" he does not believe will ever work HAS been tested extensivly (even in vaccum) and has passed every test with much higher marks than any "balloon" pressure suit can possibly achieve. Even more important is the fact that NASA and Russia are using more and more "low-pressure" balloon suits with bits and pieces of mechanical counter-pressure encorperated into it. The last winner of the NASA sponsored "glove" competition walked away easily with a design that surpassed every checkpoing and was a Mechanical Counter-Pressure glove!

Yet there are people being trained professionally to design the next generation space suit being told (and taught!) specifically that MCP does not work!

Similarly if "experts" are against using aerospike rocket engines they tend to get listened to, and believed even if they have no evidence to back them up. It's not a conspiricy, not a plot, nothing more sinister than common human emotionalism

My job on the team was to explore a HOTOL like alternative, which of course had massive cross range but that feature gave us no brownie point whatsoever!

Of course not... It's not like we actaully WANT anyone to win now is it? )

Randy

*Story was: People on space station that were flying probes around he sun to gather data. One "visiting" proffessor is trying to prove/disprove a theory that might allow for FTL flight. Most of the probes with data for this experiment come back damaged and the majority of what little data comes through seems to indicate that the theory is wrong.

Proffessor is going to declare the theory wrong and use the data gathered to refute the theory which he's "sure" will put him in the text-books for his efforts.

Turns out the "proffessor" is no such person and is an alien sent to discredit the suggested theory because it leads to the discovery of an FTL drive. By 'proving' the theory wrong and getting that conclusion entered into educatioin, reference materials, and databases the "wrong" conclusion becomes in fact a "paper-virus" whos main purpose is to stunt and marginalize any work done which might prove more succesful and therefor lead to a working FTL theory and drive!

[Hempsell]

Thanks for the comments on my participation I am flattered and very glad it helps out.  This also helps us out by reflecting back how our project is perceived externally.

Randy

You are right about just hydrogen bleeding rather than burning I should have been more precise.  I have never looked at this in detail' my experience with real burning of a Hydrazine NTO engine to protect it during reentry and was a surprise at the amount it consumed, but I accept the hydrogen case may be different.

The nozzle on Delta Clipper was segmented (you can see the fences on the plug skirt) and it was expected that on landing only some of the segments would be used. To be honest I cannot remember if the nozzle was fed with a full ring chamber (also segmented of course) or a cluster of conventional chambers.

You are right about the inherent suspicion of plug nozzles by some of the team especially (as I have ready pointed out) Max Hunter.  I was not one of those sceptics, however research work I started when at the University of Bristol has sobered me up a bit.  As I have said before I am sure they can be made to work, but they are complex and small changes to parameters can massively change the nozzle behaviour.  Anyone can design a working bell nozzle with a pencil, piece of paper and a calculator. Advanced nozzles must have advanced CFD verified by extensive experimental work on the precise configurations you expect to use in the real external flow regimes.

I agree with you on the potential of Mechanical Counter-Pressure Space Suits.

[Jim Davis]

Mr. Hempsell,

If memory serves one of the problems of air liquefaction and/or precooling schemes has been the icing up of heat exchangers because of moisture in the air.

How does Skylon/SABRE deal with this issue?

[Hempsell]

Mr. Hempsell,

If memory serves one of the problems of air liquefaction and/or precooling schemes has been the icing up of heat exchangers because of moisture in the air.

How does Skylon/SABRE deal with this issue?

You are right it is a big problem.

On HOTOL we injected liquid oxygen to take the air below the critical temperature so the ice crystals acted like dust that blew through rather than sticky particles that attached to the tubes.

Both Japan and USA have successfully tested ethanol (and methanol) with glycol sprayed on the tubes.

How it is done in SABRE is a commercial secret – Sorry to be evasive.