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.
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
Quote from: 93143 on 07/11/2012 05:57 amI'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.
I'll try to rephrase: You've criticized Skylon for reasons that don't apply to it,
SSTO is considered very ambitious and expensive and there was talk of maybe needing a kick stage.
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.
QuoteOf 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.
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.
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.
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.
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.
Quote And the only radical technological challenge is (was?) the precooler.Which is a very serious challenge.
And the only radical technological challenge is (was?) the precooler.
QuoteSABRE 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?
SABRE really is a game-changer if it works. And right now it looks like it will probably work, at least technically.
QuoteMost 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 topicFor 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.
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
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.
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.
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.
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.
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.
QuotePerhaps 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 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.
The subject was the precooler, not the whole engine. Pay attention.
I am getting really tired of the "false until proven true" attitude. That's not how critical thinking works.
No.. it hasn't even been tested at the intended temperature yet.
QuoteI 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.
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.
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.
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.
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.
Yep. The ESA report, which you admit to not having read.
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.
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).
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.
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.
This is not your average SSTO concept.
But impressive nonetheless.
I should probably take a break from this thread until I've calmed down a bit and/or gotten some freaking research done...
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),
"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.
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...
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.
Probably what TRL is for. Multiple levels of "proven", "not proven" and "unknown". SABRE isn't high on that scale.