The Reaction Engines Skylon Master Thread (1)

[Robotbeat]

TSTO vehicles have an operational disadvantage. HOWEVER, for the same dry weight, same engine Isp, same lift-off mass, they get a significantly higher proportion of payload to orbit. You might get 1% to orbit with a typical SSTO (using conventional rocket technology, just so we can have a fair comparison with historical figures). For a TSTO using roughly the same technology, you may well get 4% to orbit for the same lift-off mass. Four times as much payload, for probably less than twice as much operational cost, so traditionally a TSTO vehicle should be twice as cost-effective. For RLVs, with the fact you need to carry recovery mechanisms with you, this effect is probably exaggerated even more. What happens if you automate the stage integration, so stacking the stages becomes a pieces of cake? Then, TSTO doesn't look so bad, does it?

There are ways to allow intact-abort for even a TSTO launch vehicle.

Skylon has an enormous obstacle of an estimated development cost that is WAY up there, and honestly an operations cost that isn't free, either. If they can beat Blue Origin and SpaceX in the long run, I will be very surprised. Delighted, but surprised.

And 30 Skylons is a LOT of RLVs to sell before you become profitable. I thoroughly expect that even if Skylon is somehow successful, they most likely will not have made a real profit when all is said and done. We'll all be better for it, of course, but just like how people say the Airline industry has never made a profit, neither will RE likely be able to recuperate all of their development costs.

[mmeijeri]

Still, Skylon could be at least an excellent reusable first stage. Aircraft-like economics (on a sunk cost basis) for a first stage could be very helpful, even with an expendable second stage.

[MP99]

TSTO vehicles have an operational disadvantage. HOWEVER, for the same dry weight, same engine Isp, same lift-off mass, they get a significantly higher proportion of payload to orbit. You might get 1% to orbit with a typical SSTO (using conventional rocket technology, just so we can have a fair comparison with historical figures). For a TSTO using roughly the same technology, you may well get 4% to orbit for the same lift-off mass. Four times as much payload, for probably less than twice as much operational cost, so traditionally a TSTO vehicle should be twice as cost-effective.

Surely a better way to state this would be "...for the same payload to orbit, same engine Isp, they need 4x the dry/lift-off mass". From your last sentence you're saying that would be twice the price.

cheers, Martin

PS Bonus points for stating your assumptions in your statement. Critical question is whether SABRE's higher Isp counts as "the same technology" or a true technological step.

Edit: re-phrased.

[adrianwyard]

Lots of potentially very bad things can happen. Heck, someone should write a book about it!

http://www.amazon.com/Perigee-ebook/dp/B006PNL48I/ref=pd_rhf_gw_p_t_3

 

A fun read.

A central part of the story relates to a discussion in several threads: point-to-point hypersonic transport is very nearly as hard to do as orbital. So which of these two is likely to happen first, i.e. which market is more easily tapped? 1] small mass fraction Reusable SSTO (Skylon), or 2] Super-high-end high-speed executive travel (i.e. LAPCAT, or Clipper in the book).

It's clear that the Scimitar engine is much farther down the road than SABRE, due to its greater complexity. And I'm pretty sure I recall Mark Hempsell saying Skylon would not work well for point-to-point for various reasons, but I do wonder if the super-fast civil transport market might be easier to open. Easier at least than massively expanding the LEO launch market to the point where many carriers wish to buy many tens of Skylons...

[93143]

Incoming tl;dr...

I'll try to rephrase:  You've criticized Skylon for reasons that don't apply to it,

Rephrasing is good, but I still don't think it's true my arguments don't apply to Skylon and I'd be happy to discuss it further.

Up until the point I said that, your arguments were mostly of this sort:

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

(Until you point me to where in this 50-page thread someone mentioned a kick stage, I will assume it was suggested in ignorance by someone unconnected with the project.)

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.

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

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.

Especially as the track record of similar projects has been unanimously bad.

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.

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.

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.

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.

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.

Emphasis added.

None of these arguments have anything at all to do with Skylon.  They sound almost as if you are refusing to consider the possibility that the SABRE engine actually is a major breakthrough; that REL's idea really is more likely to succeed than other ideas.

The only arguments you had made that had any bearing on Skylon at all were:

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

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.

Performance of the engine, including T/W and Isp, is a strong function of precooler efficiency over the whole airbreathing Mach range.

No concrete objections; certainly nothing justifying the implication that your skepticism is based on a firmer understanding of the project than is possessed by the rest of us on this thread.

This precooler has decades of engineering behind it, starting with the RB545 and progressing through four iterations of the SABRE design.  And it doesn't have to work over the whole Mach number range because it's behind the inlet.  It's just a subsonic counterflow heat exchanger; it's not that hard to model...  the hard part was building it.

ESA notes that the REL heat exchanger dimensions have already been optimised for SABRE 3 as part of the overall engine design process and in conjunction with trajectory modelling.

The engine is optimized for the trajectory it's flying, not for a cruise condition.  The variable-geometry inlet, helium flow control and bypass ramjet mean that at no point should the engine be significantly "off-design".

We don't know beyond doubt that it will work as advertised.  But you seem to be treating it as if it's not much better understood than the X-30's scramjet.

  And the only radical technological challenge is (was?) the precooler.

Which is a very serious challenge.

Perhaps you missed the fact that what they've been testing is a full-size flight-weight precooler.  Not an early prototype - the real thing.  If the tests go well (and it looks like they've been going well), they're done.

SABRE really is a game-changer if it works.  And right now it looks like it will probably work, at least technically.

I don't think this confidence is justified by the known facts. Perhaps you have access to information I haven't seen?

Yep.  The ESA report, which you admit to not having read.

But I do think the statement was justified aside from that.

The precooler is the critical piece, and it looks like it's working.  The rest of the engine is much more conventional engineering.  We know how to build rockets.  We know how to build turbojets.  We know how to build ramjets.  We know how to build translating-centerbody inlets.  Sure, it's not a cakewalk, but it does look feasible; there are no giant leaps.  And a team of the best engineers in the business has been working it for 20 years.

Most people here know it might not work.  Most people here are at least passingly familiar with the history of attempts at rSSTO, and as you don't seem to have any special command of the thread topic

For the nth time I know damn well what I'm talking about, thank you very much. I find your repeated suggestion to the contrary presumptuous and offensive.

I never said you didn't know what you were talking about.  I said you didn't know more than the rest of us on here.  In fact, if you haven't read the ESA report and listened to all the interviews, etc., you probably know less.

And instead of admitting this, you dismissed a reference to the ESA report as "appeal to authority", as if this were a bad thing.

Given your evident lack of unique expertise, your early pronouncements amount to an insult to the intelligence of not only everyone on this thread, but REL as well (I refer to your off-the-cuff attempt to come up with a better idea than the one they've spent twenty years patiently advancing and are on the cusp of bringing to fruition).

I don't understand how the development cost of Skylon could be comparable to that of A-380, even if it excludes the cost of SABRE development, but I'd interested to hear why others find it plausible.

john smith 19 has addressed that reasonably well, I believe.  REL's cost modelling appears to be well regarded.  They don't pull numbers out of thin air.

Besides, the A-380 was a pan-European project with integration issues; it was probably significantly more expensive than it needed to be...

If you're going to claim something like this is implausible, you need a reason other than just gut feel.

But let me give some specific scenarios: what if rocket mode Isp turns out to be slightly disappointing (as it was with RS-68) and if that has structural implications (as it did with Delta)? What if the precooler only works efficiently up to Mach 4.5 instead of 5.5? What if it uses more LH2 than expected? What if lower than expected T/W leads to a slower ascent, more drag losses and a need for beefed up TPS? In my opinion all of these could reasonably happen.

According to my calculations, to reduce the payload to zero you'd need to lose roughly 50 seconds of Isp off the rocket mode, or have the airbreathing mode efficiency reduced by ~20% (leading to a transition speed closer to Mach 4.5), or have the engine mass more than double, or have the structural mass increase by about a third, or some combination thereof.

The airbreathing efficiency could probably take a significantly greater hit if the vehicle were redesigned to hold a bit more hydrogen and a bit less LOX; I explicitly held everything else constant, so it didn't have any more fuel to burn than the baseline design.

That's based on C1, which apparently didn't quite hit its payload target when analyzed in greater detail, but the D1 reportedly has a better payload mass fraction after structural and performance margins than the C1 was supposed to, due to technological advances including improvements in the airbreathing performance of the engine.

This is not your average SSTO concept.

[QuantumG]

None of these arguments have anything at all to do with Skylon.  They sound almost as if you are refusing to consider the possibility that the SABRE engine actually is a major breakthrough; that REL's idea really is more likely to succeed than other ideas.

Sigh.. it's not a breakthrough yet. When it flies and the specifications are proven it may be a breakthrough. Until then it's a science project.

Perhaps you missed the fact that what they've been testing is a full-size flight-weight precooler.  Not an early prototype - the real thing.  If the tests go well (and it looks like they've been going well), they're done.

Not even close.

Whether you like it or not, you have to wait and see before you can declare victory. Until the engine has been tested in flight, it is just a lump of metals.

[93143]

Sigh.. it's not a breakthrough yet. When it flies and the specifications are proven it may be a breakthrough. Until then it's a science project.

Semantics.  You know perfectly well what I meant.

The precooler by itself, though, looks like it is a breakthrough.  Whether it's useful for its intended purpose or not remains to be seen, though I think it's likely.

Perhaps you missed the fact that what they've been testing is a full-size flight-weight precooler.  Not an early prototype - the real thing.  If the tests go well (and it looks like they've been going well), they're done.

Not even close.

Whether you like it or not, you have to wait and see before you can declare victory. Until the engine has been tested in flight, it is just a lump of metals.

The subject was the precooler, not the whole engine.  Pay attention.

And yes, the design and development are done, assuming a successful completion of the ground test series and no unanticipated issues in flight.  The precooler exists.  That's a major step, even if they have to tweak it later.  Even if the rest of the design never works, they have done this.

...

I will take this opportunity to point out that one of the advantages of SABRE is the fact that it can be ground-tested.  Would you consider a full-scale extensively ground-tested engine "just a lump of metals" until it had actually been installed on a Nacelle Test Vehicle and put through its paces in the air?

...

I am getting really tired of the "false until proven true" attitude.  That's not how critical thinking works.

[QuantumG]

The precooler by itself, though, looks like it is a breakthrough.  Whether it's useful for its intended purpose or not remains to be seen, though I think it's likely.

No.. it hasn't even been tested at the intended temperature yet.

The subject was the precooler, not the whole engine.  Pay attention.

Your attitude problem is unacceptable, reign it in please.

I am getting really tired of the "false until proven true" attitude.  That's not how critical thinking works.

Yeah it is. Anyone can make claims, a skeptical mind demands evidence before accepting them as fact. Otherwise you might as well just go read sci-fi.

[93143]

No.. it hasn't even been tested at the intended temperature yet.

1) Context.  Continuing from my earlier statements, which made it clear I meant in the event of successful completion of the testing phase, which looks moderately likely given that they're two-thirds through it with no problems in sight.

2) Even if it falls apart under deep cryo conditions, they have made an extremely high-performance heat exchanger, leagues better than any other.  Expensive?  Sure.  Not super useful without the rest of the picture?  Perhaps.  But impressive nonetheless.

I am getting really tired of the "false until proven true" attitude.  That's not how critical thinking works.

Yeah it is. Anyone can make claims, a skeptical mind demands evidence before accepting them as fact. Otherwise you might as well just go read sci-fi.

No, it is not.  I said "false until proven true".  Your attitude seems to be that until everything is completely demonstrated and working, they've got nothing.

Real critical thinking involves assessing plausibility based on available evidence.  It's not just leaving the probability meter at zero until the finished product hits the market.

...

I should probably take a break from this thread until I've calmed down a bit and/or gotten some freaking research done...

[krytek]

Probably what TRL is for. Multiple levels of "proven", "not proven" and "unknown". SABRE isn't high on that scale.

[mmeijeri]

Emphasis added.

I don't understand what you're trying to say here.

None of these arguments have anything at all to do with Skylon.

They most certainly do. They may not be unique to Skylon, but they certainly apply to it.

They sound almost as if you are refusing to consider the possibility that the SABRE engine actually is a major breakthrough;

I'm not at all refusing to consider that possibility, I'm unwilling to take it for granted. Extraordinary claims require extraordinary evidence.

that REL's idea really is more likely to succeed than other ideas.

Than some other ideas. I think it's more likely to succeed than SSTO scramjets for instance. I still don't know how likely it is they will achieve SSTO. If you drop SSTO, I don't think it is necessarily more likely to succeed than other mixed cycles incorporating a compressor.

No concrete objections;

How can you call uncertain performance in the face of severe delta-v targets not a concrete objection? You may not agree with that objection, which is fair enough, but it is certainly concrete.

certainly nothing justifying the implication that your skepticism is based on a firmer understanding of the project than is possessed by the rest of us on this thread.

What implication? I never said or implied anything of the sort. At least three people here implied or openly said they knew more about it than I did, not me. And I had to correct one of them on SABRE's turbine inlet temperature...

This precooler has decades of engineering behind it, starting with the RB545 and progressing through four iterations of the SABRE design.

And only now are we starting to see ground tests of anything resembling flight hardware. I don't think we've seen anything close to simulating the airflow behind the inlet at Mach 5.5 and I don't think we've yet seen the HX4 in action. I'm not saying we should expect it to fail, I'm saying we shouldn't be confident it will succeed. And in any event it will still take a lot of time and work.

And it doesn't have to work over the whole Mach number range because it's behind the inlet.

Of course it does, it doesn't have to touch supersonic flow, but it has to cool the very hot subsonic flow that results from supersonic flow entering the inlet at high Mach numbers. And not only that, it has to do so without requiring excessive amounts of LH2.

  It's just a subsonic counterflow heat exchanger; it's not that hard to model...  the hard part was building it.

Now let's see if it works.

The engine is optimized for the trajectory it's flying, not for a cruise condition.  The variable-geometry inlet, helium flow control and bypass ramjet mean that at no point should the engine be significantly "off-design".

Huh? It has to operate as an airbreather from 0 to Mach 5.5 and as a rocket from then on. It can't very well be optimised for all of these conditions.

We don't know beyond doubt that it will work as advertised.  But you seem to be treating it as if it's not much better understood than the X-30's scramjet.

Not at all, I think it is better understood than scramjets. All I'm saying is that it will take a lot of time and work to make something fly and that its performance is still uncertain.

Perhaps you missed the fact that what they've been testing is a full-size flight-weight precooler.

No I haven't.

  Not an early prototype - the real thing.  If the tests go well (and it looks like they've been going well), they're done.

Early tests, nothing approaching flight conditions at Mach 5.5. I don't think they can do that entirely on the ground, which is why I believe they are doing some work on a subscale test vehicle.

Yep.  The ESA report, which you admit to not having read.

OK fair enough, I'm skeptical it will convince me, but I'll have to read it first to be sure.

The precooler is the critical piece, and it looks like it's working.

It looks as if it could still work.

The rest of the engine is much more conventional engineering.  We know how to build rockets.  We know how to build turbojets.  We know how to build ramjets.  We know how to build translating-centerbody inlets.  Sure, it's not a cakewalk, but it does look feasible; there are no giant leaps.  And a team of the best engineers in the business has been working it for 20 years.

Well, the fact that it has taken them 20 years so far doesn't suggest it's all that easy. But sure, it looks feasible, and I have no doubt that given time and money such an engine could be built. Whether it will be built is another matter, many other feasible designs have never been built. But my main point is that until it flies we cannot assume it will have the desired performance it will need to power an SSTO. I find it odd that people who were skeptical of Falcon 9 are so confident about Skylon.

I never said you didn't know what you were talking about.  I said you didn't know more than the rest of us on here.

Fine, I never claimed I did.

  In fact, if you haven't read the ESA report and listened to all the interviews, etc., you probably know less.

Perhaps, so enlighten me.

And instead of admitting this, you dismissed a reference to the ESA report as "appeal to authority", as if this were a bad thing.

Acting as if settles the matter would indeed be a bad thing. And it's not as if ESA are the world's experts on RLVs or mixed cycle engines.

Given your evident lack of unique expertise, your early pronouncements amount to an insult to the intelligence of not only everyone on this thread,

Where the heck did I insult your intelligence? So far you have been the one throwing around insults.

but REL as well (I refer to your off-the-cuff attempt to come up with a better idea than the one they've spent twenty years patiently advancing and are on the cusp of bringing to fruition).

I did no such thing.

john smith 19 has addressed that reasonably well, I believe.  REL's cost modelling appears to be well regarded.  They don't pull numbers out of thin air.

Why does it "appear" well-regarded?

Besides, the A-380 was a pan-European project with integration issues; it was probably significantly more expensive than it needed to be...

Certainly, as were Ariane and ATV. Do we have any reason to believe Skylon would be different?

If you're going to claim something like this is implausible, you need a reason other than just gut feel.

It strikes me as implausible that a spaceplane with capabilities far exceeding that of A-380 could be built for a similar amount of money. The mere fact that RE's costing models "appear well-regarded" in the eyes of unspecified observers doesn't convince me. Studying those models might. Is there a publicly available link?

According to my calculations, to reduce the payload to zero you'd need to lose roughly 50 seconds of Isp off the rocket mode, or have the airbreathing mode efficiency reduced by ~20% (leading to a transition speed closer to Mach 4.5), or have the engine mass more than double, or have the structural mass increase by about a third, or some combination thereof.

I'm happy to believe rocket mode performance won't be off by 50s, though I'd have to see your calculations to be confident that's the margin they have. I'm also happy to believe engine mass will likely not double, unless airbreathing T/W is disappointing at high Mach numbers. Structural mass and airbreathing efficiency look like more serious risks. And I see nothing about T/W.

This is not your average SSTO concept.

I never claimed it was.

[mmeijeri]

But impressive nonetheless.

Certainly, and I don't think anyone here has suggested otherwise. It looks very useful, even if SABRE and Skylon in its current form were not to work out.

I am getting really tired of the "false until proven true" attitude.  That's not how critical thinking works.

Not false until proven true, unproven until proven true.

I should probably take a break from this thread until I've calmed down a bit and/or gotten some freaking research done...

Probably a good suggestion.

[jded]

mmeijeri: I think the problem people have with your posts is that you are predicting general project failure modes that don't apply to this particular project.

There is no scenario under which Skylon gets built with a lawnmower engine instead of SABRE.

There is no scenario under which a Skylon has unsatisfying precooler performance therefore (something).

Precooler is built first. If it can't be made to work as needed, it's over. If it can (and they've almost confirmed that it works already), they start working on SABRE. If that doesn't work, SKYLON doesn't get built.

"Normal" SSTO performance depended on having razor thin weight margins on everything. There were multiple ways of almost having it right - but not good enough in the end. This is different. Either SABRE works as needed, or it does not. If it does, the rest is relatively straightforward. If it does not, there is no rest.

The project might fail, but there won't be a TSTO SKYLON.

BTW, cost vs. A380 - perspective of having every precooler pipe built in different country may be a reason why ESA doesn't want to take over the project while it seems to be moving ahead on its own.

[mmeijeri]

mmeijeri: I think the problem people have with your posts is that you are predicting general project failure modes that don't apply to this particular project.

I'm not predicting any failure modes will materialise. I'm pointing out various ways Skylon could fail.

There is no scenario under which Skylon gets built with a lawnmower engine instead of SABRE.

Huh? I wasn't suggesting anything like that.

There is no scenario under which a Skylon has unsatisfying precooler performance therefore (something).

Precooler is built first. If it can't be made to work as needed, it's over.

Sure, but it cannot be completely tested on the ground. Which is why subscale test vehicles are important.

If it can (and they've almost confirmed that it works already),

I think a lot of testing remains to be done on the precooler, including some test flights where the precooler is integrated with other engine systems on a subscale test vehicle.

"Normal" SSTO performance depended on having razor thin weight margins on everything. There were multiple ways of almost having it right - but not good enough in the end.

Or spectacular effective Isp, or some combination of the two. Skylon is not the first or only combined cycle concept.

This is different. Either SABRE works as needed, or it does not. If it does, the rest is relatively straightforward. If it does not, there is no rest.

Yes, Skylon, like other airbreathers, is predicated on an engine with spectacular effective specific impulse, as I've said several times now.

The project might fail, but there won't be a TSTO SKYLON.

What's your point?

[jded]

What's your point?

 The objection you started with ("i'm not convinced it will give them SSTO") and some further post sounded like you were suggesting that REL might try to build a Skylon and end up with a TSTO or otherwise sub-performing plane. This is not the case.

Ok, so what was your original point? That Skylon project might still fail? Well...

[mmeijeri]

The objection you started with ("i'm not convinced it will give them SSTO")

It wasn't an objection, just an observation. And one people even seemed to agree with factually.

and some further post sounded like you were suggesting that REL might try to build a Skylon and end up with a TSTO or otherwise sub-performing plane. This is not the case.

Maybe you thought I was suggesting Skylon was too risky to try because it might fail only at the end, after you'd spent all the money? That's not what I was saying.

I do think an incremental approach would be better, and that even if a SSTO Skylon turned out to be too ambitious, something like SABRE could still be very useful. That's a positive comment about SABRE, yet it seems to have provoked the ire of the locals.
 

Ok, so what was your original point? That Skylon project might still fail? Well...

Yes, that it might fail and still be useful...

[Tnarg]

This thead has seems of gone a little crasy last time I look up we where hoping for some new info at the Farnborough air show?  any news on that?  I might of missed it with all the it might not work/it might work posts.

[strangequark]

Sure, but it cannot be completely tested on the ground. Which is why subscale test vehicles are important.

The engine isn't a super/hypersonic combustion device, which means you can test everything downstream of the inlet on the ground relatively easily. You don't need a hypersonic wind tunnel, if that's what you're implying. The inlet will take the flow down below transonic (Mach 0.5 is pretty typical for most airbreathers). All you'd have to do is provide heated air at subsonic velocity, which is a much easier proposition. Yes, maybe you're not "completely" testing it then, but building a subscale just to test the inlet is a waste of money.

[Jim Davis]

The engine isn't a super/hypersonic combustion device, which means you can test everything downstream of the inlet on the ground relatively easily. You don't need a hypersonic wind tunnel, if that's what you're implying. The inlet will take the flow down below transonic (Mach 0.5 is pretty typical for most airbreathers). All you'd have to do is provide heated air at subsonic velocity, which is a much easier proposition. Yes, maybe you're not "completely" testing it then, but building a subscale just to test the inlet is a waste of money.

You're correct as far as you go. What you're leaving out is that the temperature of the air that the precooler will have to deal with is strongly dependent on the efficiency of the inlet. Inlet design has to tested in a wind tunnel and in flight to insure the design specification is met. Even in this age of computational fluid dynamics there is a great deal of uncertainty in inlet design at high Mach numbers, both in pressure recovery and installed weight.

[simonbp]

Probably what TRL is for. Multiple levels of "proven", "not proven" and "unknown". SABRE isn't high on that scale.

It is about at "Component and/or breadboard validation in relevant environment", which is TRL 5 (on the NASA scale).

When they get a full SABRE up and running, they will be at TRL 6 ("System/subsystem model or prototype demonstration in a relevant environment (ground or space)").