The Reaction Engines Skylon Master Thread (1)

[john smith 19]

That's not even getting into the changes you'd need to make to the airframe to fly that fast...

Sorry about the Isp. It was the only figure he gave.

IIRC a big part of the SABRE4 redesign was to improve air breathing performance. I'd guess the transonic range with maximum drag remains an area they are keen to improve in any way possible.

I sometime wonder if people appreciate that it's *pointless* improving performance over a couple of Mach numbers if you loose it everywhere *else*. Keeping that balance positive is a *lot* tougher than it might seem.

With reference to scramjets I think Hempsell was fairly lenient. Turbojets have flown since the late 1930s, ramjets before that. Scramjets have only achieved *positive* thrust in the last decade in an actual flight test.

That's far too risky to pin a key part of a $12Bn dollar development programme on and any diligent investor would know that.

[93143]

Sorry about the Isp. It was the only figure he gave.

Well, to be fair, SABRE is in rocket mode over basically the entire useful Mach number range of a scramjet...  The coincidence of the switchover point is probably what causes people to think adding a scramjet might be a good idea...

Looking at the Isp curve for a LACE engine makes me wonder how anyone ever managed to convince himself that HOTOL was a good idea...  on the other hand, clearly someone did - in fact the design itself seems to have been only marginally unsuccessful, which bodes well for Skylon...

[john smith 19]

Looking at the Isp curve for a LACE engine makes me wonder how anyone ever managed to convince himself that HOTOL was a good idea...  on the other hand, clearly someone did - in fact the design itself seems to have been only marginally unsuccessful, which bodes well for Skylon...

The short answer is because it is *not* LACE.

That would be down to Alan Bond and some thermodynamic modelling he did on his Sinclair Spectrum (Spaceflight 1989 for more details).

The key was the recognition that LACE is hard because of the *huge* amount of energy you need to extract to trigger the phase change but if you just pushed the air *close* to its condensing point you'd get most of the volume & temperature reduction without needing a huge heat sink IE LH2 tank. Trouble is there is no common term for this neo-LACE? Pseudo-LACE? REL tend to use deeply pre-cooled instead.

HOTOL was stuffed by its engine location at the back. SOP for rockets and the Shuttle (the only actual flying vehicle). Then came the recognition of how big the forces and their moments on the vehicle would be during the trajectory, and (worse still) how much they would *shift* during the 0-M23-0 flight.

Once the scale of the problems were recognized re-shaping the vehicle to put more mass at the centre of pressure or mass, put the payload in the middle and split both LO2 and LH2 tanks, with the engines at the sides to deliver substantial reduction in the forces and sizes of the aerosurfaces needed.

At least one of the more modern NASA concept vehicles (WB400?) uses roughly this layout for pretty much the same reasons.

REL have always been adamant that SABRE is a bad air breather but a very good rocket. Their criteria for its air breathing function seems to be "Good enough to get the job done over the Mach range we need." It's the balance they believe a flight vehicle must have to meet the mission.
It may not sound very glamorous but getting *something* flying is the first goal.

Some civil servants compared Skylon to Concorde (and it seems progress only started to take off when they retired) but it's interesting to note that the 17th Concorde (call it Block 2) would have been a very different machine. Longer range (bit more fuel but much better fuel economy) more powerful engines and *no* afterburner to get through M1 ("supercruise" in the late 1970's! Even the SR71 could not do that), lighter due to more composites, more refined stress analysis needing smaller structural margins etc.

Sklyon would not be the state of the art in HTOL SSTO.

It would be the *start* of the art.

[mmeijeri]

Trouble is there is no common term for this neo-LACE? Pseudo-LACE? REL tend to use deeply pre-cooled instead.

Looks like a deeply precooled air turborocket. I'm not convinced it will give them SSTO though and if you go with TSTO you could go with a less ambitious "undeeply" cooled hydrocarbon air turborocket, perhaps using LOX/methanol or methanol/peroxide.

[93143]

@john smith 19:  I know most of that.  I've been interested in Skylon for a long time.

But I could have sworn the RB-545 was a LACE engine...  oh well...

@mmeijeri:  The ESA doesn't see a problem, provided the engine works.  Have you picked up on something they missed?

[flymetothemoon]

It's there!

Very funky looking new website and skylon / sabre animation.

Quite a headline statement on the front page too:

"THE GREATEST ADVANCE IN PROPULSION SINCE THE JET ENGINE"

http://www.reactionengines.co.uk/

No news update, so we're waiting for Farnborough!

[Crispy]

Here's that new animation

http://vimeo.com/45371849#

Farnborough starts today, so there should be news as soon as an interested journalist can get it to us!

[mmeijeri]

@mmeijeri:  The ESA doesn't see a problem, provided the engine works.  Have you picked up on something they missed?

SSTO is considered very ambitious and expensive and there was talk of maybe needing a kick stage. RE's logic (at least publicly) is that they can only expect significant cost savings with SSTO. I don't think that's true and I'm not confident they can make an SSTO, either technically or financially.

[jrc14]

@mmeijeri:  The ESA doesn't see a problem, provided the engine works.  Have you picked up on something they missed?

SSTO is considered very ambitious and expensive and there was talk of maybe needing a kick stage. RE's logic (at least publicly) is that they can only expect significant cost savings with SSTO. I don't think that's true and I'm not confident they can make an SSTO, either technically or financially.

So that would be a "no", then? 

[mmeijeri]

So that would be a "no", then? 

A possibly. I don't find an appeal to authority convincing.

[simonbp]

SSTO is considered very ambitious and expensive and there was talk of maybe needing a kick stage.

I think you might be misrepresenting the report. As least as I understood it, the concern was that the current commercial market was for GEO, and so an upper stage is needed, and that's an extra layer of complexity/risk. IIRC, responding to this was part of the reason for the bump up to 15 tonne payload.

But I can't recall them (ESA) actually questioning the ability of the final vehicle to get to orbit. IIRC, the two prototypes will initially be suborbital, but that's a matter of testing.

[93143]

SSTO is considered very ambitious and expensive

That's just a boilerplate statement.  It doesn't imply any specific knowledge of Skylon at all.

It will be expensive.  It's an airplane the size of an A-380, and it will probably cost a similar amount to develop, SSTO or not.

and there was talk of maybe needing a kick stage.

For GTO payloads, yes, but the "kick stage" is supposed to come back to LEO to meet up with the waiting Skylon so it can be returned to Earth and reused.

It could also be used to increase Skylon's LEO payload with suborbital deployment of satellite+US in cases where subdivision of the payload is not feasible, but that would increase the cost more than the capacity, so it wouldn't be the preferred mode of operation.

As far as I recall, no one who knows anything about the project has ever suggested that suborbital deployment with a kick stage might be required in normal operation.

RE's logic (at least publicly) is that they can only expect significant cost savings with SSTO. I don't think that's true

Well, that's a separate argument.  But I will say that the operational characteristics of Skylon look to be in a different league when compared with TSTO.  No stage management (ground processing or landing-site management, not to mention the actual staging event), intercontinental self-ferry, very large cross-range, Shuttle-like LEO operational and low-G-loading downmass capabilities, recovery and reuse of GTO transfer stages, intact abort from engine start to orbit including engine-out scenarios, airplane-like ground operations, LOV of one in 10,000 at worst, likely much better...  it's enough like an airliner that they're getting it certified as one (more or less)...

And, of course, there's only one large atmosphere-transiting flight vehicle to develop, rather than two, which in the usual rSSTO case doesn't signify because of the difficulty of closing the design, but in this case the SABRE engine enables a vehicle design with fairly robust margins and relatively low technical risk.

I'm not confident they can make an SSTO, either technically or financially.

Again, no rationale, just disparagement.  Have you read this?

http://www.bis.gov.uk/assets/ukspaceagency/docs/skylon-assessment-report-pub.pdf

The review ended with a consensus that no technical or economic impediments to the development of SKYLON or SABRE had been found.

If you're "not confident", you have to say why not or it's just hot air.  Of course the project could be unsuccessful for some reason not currently obvious; it won't be certain to be a success until it actually is one.  But there is currently no basis for pessimism so far as I can tell.

Every now and then someone suggests that REL should scale back their ambitions with respect to Skylon, either making it a TSTO or going for the suborbital market.  What doesn't seem to be understood in these cases is that REL are not making an SSTO RLV.  They're making an engine that enables an SSTO RLV with reasonable margins and technical feasibility measures, and is overkill for anything else.  (They've been designing the LV in detail to make sure it works, but their main contribution is the engine.)  Scaling back either the engine or the LV removes the whole point of the exercise.

Go ahead and suggest that someone should try building a TSTO RLV using an ATR first stage.  But REL is not that someone.

A possibly. I don't find an appeal to authority convincing.

Appeal to authority is not always a logical fallacy.  Or are you going to quibble over the definition of "convincing"?

Do you actually know anything about this project that might give your opinion comparable weight to that of the ESA?

[jrc14]

REL press-release at http://www.reactionengines.co.uk/press_release.html .
Looks like the second series of pre-cooler tests has produced a satisfactory result.

[Seer]

It's there!

Very funky looking new website and skylon / sabre animation.

Quite a headline statement on the front page too:

"THE GREATEST ADVANCE IN PROPULSION SINCE THE JET ENGINE"

http://www.reactionengines.co.uk/

No news update, so we're waiting for Farnborough!

I like the sabre animation too, very enlightening.

[mmeijeri]

That's just a boilerplate statement.  It doesn't imply any specific knowledge of Skylon at all.

I've read the RE documentation, and it's not enough (obviously) to come to a definitive conclusion. Many people have dreamt of fully reusable SSTO launch vehicles, and no one so far has succeeded in building one. Maybe RE has the magic ingredient that others don't, but it's not the safe way to bet.

It will be expensive.  It's an airplane the size of an A-380, and it will probably cost a similar amount to develop, SSTO or not.

It will cost a lot more, because it will include development of a new engine, and not just any old engine, an engine of a type that has never been flown before.

It would be the first completely reusable RLV, it would be SSTO and it would be large (EELV class).

For GTO payloads, yes, but the "kick stage" is supposed to come back to LEO to meet up with the waiting Skylon so it can be returned to Earth and reused.

I meant in discussion in this thread.

It could also be used to increase Skylon's LEO payload with suborbital deployment of satellite+US in cases where subdivision of the payload is not feasible, but that would increase the cost more than the capacity, so it wouldn't be the preferred mode of operation.

That's RE's reasoning, and they might be right. It's part of the reason they give for not being interested in a TSTO vehicle.

As far as I recall, no one who knows anything about the project has ever suggested that suborbital deployment with a kick stage might be required in normal operation.

SSTO concepts to date have had very little or even negative margin, so it's only logical to count with that possibility.

Well, that's a separate argument.  But I will say that the operational characteristics of Skylon look to be in a different league when compared with TSTO.  No stage management (ground processing or landing-site management, not to mention the actual staging event), intercontinental self-ferry, very large cross-range, Shuttle-like LEO operational and low-G-loading downmass capabilities, recovery and reuse of GTO transfer stages, intact abort from engine start to orbit including engine-out scenarios, airplane-like ground operations, LOV of one in 10,000 at worst, likely much better...  it's enough like an airliner that they're getting it certified as one (more or less)...

Certainly, if they can pull it off. It will be a great day for spaceflight if they do.

Again, no rationale, just disparagement.  Have you read this?

I gave the rationale above. Unless they have something that others that have tried before didn't.

http://www.bis.gov.uk/assets/ukspaceagency/docs/skylon-assessment-report-pub.pdf

I've only skimmed reactions to it, but I believe the gist is that if RE can achieve the intended effective specific impulse with SABRE, then the rest of the vehicle doesn't look excessively optimistic. But that's not saying much of course. There can be little doubt that the SABRE concept is workable in principle, but that doesn't mean we already have reliable data on Isp and T/W.

If you're "not confident", you have to say why not or it's just hot air. 

Hot air is actually part of the reason to be skeptical, given that this is a hypersonic airbreather...

Of course the project could be unsuccessful for some reason not currently obvious; it won't be certain to be a success until it actually is one.  But there is currently no basis for pessimism so far as I can tell.

I find that an incredible statement based on the track record of prior work.

Every now and then someone suggests that REL should scale back their ambitions with respect to Skylon, either making it a TSTO or going for the suborbital market.  What doesn't seem to be understood in these cases is that REL are not making an SSTO RLV.  They're making an engine that enables an SSTO RLV with reasonable margins and technical feasibility measures, and is overkill for anything else.  (They've been designing the LV in detail to make sure it works, but their main contribution is the engine.)  Scaling back either the engine or the LV removes the whole point of the exercise.

Well, it's their call, but I disagree it removes the point of the exercise. A reusable first stage with an undeeply precooled hydrocarbon air turborocket could be much cheaper to develop. On the flip side it could also be more expensive to operate per kg launched of course. But it would be less ambitious and could be very useful. Look how successful XCOR has been with their incremental approach.

Go ahead and suggest that someone should try building a TSTO RLV using an ATR first stage.  But REL is not that someone.

Well, again, it's their call and I'm not saying they should. I do think it would be less risky.

Appeal to authority is not always a logical fallacy.  Or are you going to quibble over the definition of "convincing"?

One study does not overturn decades of failure. It's an interesting opinion of course, from a notable organisation.

Do you actually know anything about this project that might give your opinion comparable weight to that of the ESA?

Well, obviously my opinion carries nowhere near as much weight as that of ESA, but that doesn't mean I have to give up my opinion. Especially as the track record of similar projects has been unanimously bad. And yes, I have actually read the publicly available Skylon documentation, since I find the concept very interesting. But the adulation here has VASIMR written all over it.

[strangequark]

That's just a boilerplate statement.  It doesn't imply any specific knowledge of Skylon at all.

I've read the RE documentation, and it's not enough (obviously) to come to a definitive conclusion. Many people have dreamt of fully reusable SSTO launch vehicles, and no one so far has succeeded in building one. Maybe RE has the magic ingredient that others don't, but it's not the safe way to bet.

The engine is that ingredient.

As far as I recall, no one who knows anything about the project has ever suggested that suborbital deployment with a kick stage might be required in normal operation.

SSTO concepts to date have had very little or even negative margin, so it's only logical to count with that possibility.

The SSTOs that have seen any level of development have relied on pure rocket concepts. A workable combined cycle engine is a game changer, and that's the point.

Again, no rationale, just disparagement.  Have you read this?

I gave the rationale above. Unless they have something that others that have tried before didn't.

The engine.

http://www.bis.gov.uk/assets/ukspaceagency/docs/skylon-assessment-report-pub.pdf

I've only skimmed reactions to it, but I believe the gist is that if RE can achieve the intended effective specific impulse with SABRE, then the rest of the vehicle doesn't look excessively optimistic. But that's not saying much of course. There can be little doubt that the SABRE concept is workable in principle, but that doesn't mean we already have reliable data on Isp and T/W.

T/W may be a little speculative, but provided that the precooler works (and it has so far), the Isp is pretty well given. It's all about pushing back the T3 limit. There's nothing in the conceptual design of SABRE that is questionable, and all the technologies required, except the precooler, are existing and very well understood.

If you're "not confident", you have to say why not or it's just hot air. 

Hot air is actually part of the reason to be skeptical, given that this is a hypersonic airbreather...

It's not a scramjet, which is why it's interesting.

Of course the project could be unsuccessful for some reason not currently obvious; it won't be certain to be a success until it actually is one.  But there is currently no basis for pessimism so far as I can tell.

I find that an incredible statement based on the track record of prior work.

Previous work has required much more speculative technologies, and razor thin margins.

Every now and then someone suggests that REL should scale back their ambitions with respect to Skylon, either making it a TSTO or going for the suborbital market.  What doesn't seem to be understood in these cases is that REL are not making an SSTO RLV.  They're making an engine that enables an SSTO RLV with reasonable margins and technical feasibility measures, and is overkill for anything else.  (They've been designing the LV in detail to make sure it works, but their main contribution is the engine.)  Scaling back either the engine or the LV removes the whole point of the exercise.

Well, it's their call, but I disagree it removes the point of the exercise. A reusable first stage with an undeeply precooled hydrocarbon air turborocket could be much cheaper to develop. On the flip side it could also be more expensive to operate per kg launched of course. But it would be less ambitious and could be very useful. Look how successful XCOR has been with their incremental approach.

There's no point to a turborocket without precooling for this application. You have dramatically less mass flow because of the density problem, and then you hit the T3 limit in the low supersonic regime; same reason a jet engine first stage is silly.

Sure, there are things that could go wrong, but the whole reason that this is exciting is that it's not just another rocket SSTO concept like X-33, or a monumental technical challenge like a scramjet. If the precooler works within parameters, then the engine can  be made to work, it is that simple.

[mmeijeri]

The engine is that ingredient.

Yes, obviously. Or rather, it could be that ingredient.

The SSTOs that have seen any level of development have relied on pure rocket concepts. A workable combined cycle engine is a game changer, and that's the point.

That's not true, NASA has spent a lot of time and money on various airbreathing concepts, and not just scramjets. And even if it were true, it bodes ill for Skylon, because the all-rocket concepts at least got to the point of having any level of development.

T/W may be a little speculative, but provided that the precooler works (and it has so far), the Isp is pretty well given. It's all about pushing back the T3 limit.

Effective Isp also depends on how well the precooler works over the whole intended air-breathing Mach range. And as I understand it predicting off-design performance of airbreathing engines is not yet an exact science.

There's nothing in the conceptual design of SABRE that is questionable, and all the technologies required, except the precooler, are existing and very well understood.

That's what I said and the precooler is a big unknown, as RE themselves say.

It's not a scramjet, which is why it's interesting.

I know, and that's the main reason I'm interested in it.

Previous work has required much more speculative technologies, and razor thin margins.

LACE, MIPCC and ATREX have also received attention, to no avail as yet. The ATREX ambitions have been scaled down to a deeply precooled turbojet (presumably to save on the design of an ATR), MIPCC used an existing turbojet too. Both seem to have gone nowhere.

There's no point to a turborocket without precooling for this application. You have dramatically less mass flow because of the density problem, and then you hit the T3 limit in the low supersonic regime; same reason a jet engine first stage is silly.

I didn't say without precooling, I said with "undeep" precooling.

Sure, there are things that could go wrong, but the whole reason that this is exciting is that it's not just another rocket SSTO concept like X-33, or a monumental technical challenge like a scramjet. If the precooler works within parameters, then the engine can  be made to work, it is that simple.

That precooler is a big unknown (which is why it's a good thing they're testing it), but there is no doubt you could have a precooled ATR that could function at least up to Mach 4-5, which would be good enough for reusable first stage. I'm not saying the SSTO is impossible, I'm saying it's not a done deal and that I think people are far too optimistic about it. That said, I'd love to see them succeed.

[simonbp]

REL press-release at http://www.reactionengines.co.uk/press_release.html .
Looks like the second series of pre-cooler tests has produced a satisfactory result.

And the next series will get down to -150 C (123 K). The boiling point of O2 is 90 K (and N2 is 77 K), so that's what they mean by "almost-liquid".

Next step after that is a full-scale SABRE!

EDIT: Phase diagram of O2; it would only take a few 10s of atms of pressure to liquefy at -150 C, so it's probably closer than the 1 atm boiling point would imply.

[93143]

@simonbp:  A little over 12 atmospheres, if Peng-Robinson is any guide.  But the precooler is upstream of the compressor, and the inlet isn't going to hit 12 atmospheres by itself...


@mmeijeri:  What's different this time is that the combined-cycle engine has both high Isp (as opposed to LACE) and high T/W (as opposed to everything else).

The precooler is the secret sauce, the key to the whole thing.  They've figured out how to manufacture it, they've solved the icing problem, and they're two-thirds of the way through testing it with no problems in sight.  It's not as big an unknown as it used to be.

Your pessimistic assessment rests entirely on historical examples.  It is thus manifestly invalid when dealing with an approach that specifically claims to solve the problems encountered in those examples.

No one here thinks Skylon is a done deal.  It appears that you have no new information to add to the discussion, which means your pronouncements basically amount to a variant of "contempt prior to investigation".

there is no doubt you could have a precooled ATR that could function at least up to Mach 4-5

That's virtually the same specification as SABRE; something seems out of proportion here...  Can you clarify?

[mmeijeri]

Your pessimistic assessment rests entirely on historical examples.  It is thus manifestly invalid when dealing with an approach that specifically claims to solve the problems encountered in those examples.

No, until it flies all they've shown is a potentially successful new approach. It might work, it might not. I didn't dispute the former, I merely pointed out the latter.

Earlier approaches too have all had their own secret sauce, their own reasons to believe they would make it. Airbreathing has been tried before, unsuccessfully to date, but the concept remains attractive, though not self-evidently the right approach. The supercharged ejector ramjet people had reason to believe they had a unique approach to airbreathing that could work. It could indeed still work, just like SABRE could, but just like SABRE so far it hasn't. ATREX isn't self-evidently a bad idea, neither is KLIN nor any of a large number of related approaches. Each have their own unique characteristics you could point to and say "See, that's why it will be successful where others haven't been". It might even be true, but you won't know for sure until someone has demonstrated it.

Others have attacked mass ratios, believing composites would hold the solution. They may yet turn out to be right, but so far they haven't been. Using dense propellants instead of high Isp cryogens for better mass fractions was a new approach a little over a decade ago, and it might still work, it certainly has its own unique new aspect to it.

No one here thinks Skylon is a done deal.  It appears that you have no new information to add to the discussion, which means your pronouncements basically amount to a variant of "contempt prior to investigation".

I feel no contempt of Skylon at all, I said I found the concept very interesting. It is among the more interesting airbreathing concepts, and I'm intrigued by airbreathers, though not by scramjets, which this one of course isn't. All I said was that I wasn't convinced it would give them SSTO, not that it could never work. So if you describe that feeling as contempt, I think you are being overly enthusiastic and overly defensive about the concept.

That's virtually the same specification as SABRE; something seems out of proportion here...  Can you clarify?

It doesn't have to go up to 5.5 (and stagnation temperature is a quadratic function of Mach number) and it doesn't have to power an SSTO. Performance of the engine, including T/W and Isp, is a strong function of precooler efficiency over the whole airbreathing Mach range. Effective Isp and T/W strongly affect total delta-v to orbit, thermal protection requirements and the necessary mass ratio, and SSTO is a very ambitious target. Even if SABRE works over the entire Mach range, it still doesn't guarantee SSTO. LACE could work over the entire Mach range too, but it is too fuel-rich to give you the necessary effective Isp to give you SSTO. It still could be very useful on a TSTO. SABRE could be expected to do better than LACE, but is not self-evident that it will be sufficiently successful. It's clearly not a ridiculous approach, but still a very ambitious one in a field where many others have made related, different but also reasonable attempts. Only time will tell if it will be more successful than the other approaches, and if its special characteristics will win out, where the equally reasonable special characteristics of other approaches so far haven't. Let's hope someone or multiple someones will be successful or we'll never have large scale manned spaceflight. If RE turn out be be some of those someones, they deserve lasting fame and maybe a Nobel Prize.