The Reaction Engines Skylon/SABRE Master Thread (6)

[e of pi]

Exactly. What makes SABRE unique is the air breathing phase. Once you are above the atmosphere (or cease collecting O2), then it is no different than a normal rocket engine. And that is where staging - if one wanted to do it - would make sense.

There's also the thing that makes Skylon's airframe unique: astoundingly low dry mass. To use the D1 specifications, which I know are no longer the latest but are the most complete I have to hand:

SABRE makes a T/W of about 14, with a thrust of 2000 kN. That's a dry mass of about 14,500 kg per engine. Two is 29 metric tons.
Landing gear is about 2-7% of the GTOW, typically in the 4% ballpark. On the low side, that's 6,500 kg, but on the high side as much as 22,700 kg. A reasonable guess might be 12,000 metric tons.
Thus, the mass of the remainder of the airframe--engines and tanks--is the 53 metric ton total structure minus these. That's about 12 metric tons using the 4% GLOW landing gear assumption.

That'd make Skylon--including its wings, engine nacelles, and cargo bay all wrapped up in an orbital-capable TPS--about as mass efficient as the Space Shuttle SLWT. To be fair, the SLWT isn't common bulkhead and was designed to deal with the TAOS Shuttle, so it has an intertank and heavy thrust beam. OTOH, Skylon has an equally large (actually larger) cargo bay and a wing spar in about the same spot.

It seems like the real magic of Skylon isn't its engine--it's the structure.

[ChrisWilson68]

It just seem that we armchair engineers who really have no experience on real rockets seem to think we known more about developing rockets and spacecrafts than guys like Alan Bond who have done it for >35 years

No, we think the thousands of engineers who have successfully built and flown all the dozens of multi-stage launch vehicles that have actually flown to space know more than Alan Bond, who hasn't actually designed anything that has successfully flown to space, in spite of trying for > 35 years.

[ChrisWilson68]

Thank God for China, rescuing this thread from whining about SpaceX:

Looks like they're developing their own version of Skylon.

And good for them, as Europe is too timid to fully fund such a huge sum for the full-up space plane and Reaction Engines refuses to come up with a more practical, gradual plan to develop a reusable orbital vehicle.

What's more practical in having three sets of heavy engines instead of one set of engines?

The correct answer to this question is "it depends".

Anyone who thinks the answer is always that one set of engines is better than three is a fool.  It absolutely depends on the details.  In real life there are trade offs.  Having only one set of engines has obvious advantages.  There are other advantages to having multiple sets of dissimilar engines, including the fact that each can be optimized for a different purpose and the costs can be lower by not trying to have one designed do three different things.

[Paul451]

Do you have a successful comparison?

So we're clear you DO know who Marquardt was, right?

Bankrupt.

After being quite lucrative over decades and it actually still exists IIRC so my point stands.

The US company that made ramjets went quite spectacularly bankrupt and was sold for about $1m. The unrelated German company that makes electronics still exists and is quite successful.

so my point stands.

I'm pretty sure at this stage neither of us is anywhere near the point. The point has wandered off to get a drink and is now standing by the front door, glaring and tapping its watch.

[Paul451]

Perhaps because every time someone suggested an alternative (especially TSTO), they were jumped over by the true-believers who insisted that Skylon was the one-true-design, perfect in every way, and anything else was madness and ignorance.

If you want runway to orbit you need a SABRE cycle. If you do not then whatever it is is not a SABRE cycle. REL are probably the lead developers on those as well with the Scimitar. Call it whatever you want, but it's not a SABRE.

John, Scimitar lacks a rocket component. At what altitude do you believe a TSTO stages at? (I wonder if you aren't instead picturing a carrier aircraft for an air-launch system like Pegasus or Stratolaunch or VG.)

It's just a series of logical questions.  It's a question of what a prospective customer wants and do they understand what they are getting. Saying you want TSTSO and SABRE is simply illogical.

[Lars-J]

It seems like the real magic of Skylon isn't its engine--it's the structure.

Perhaps, but that is also the part of Skylon that has had the least amount of work, and the most amount of hand-waving. And there is much skepticism about its mass efficiency as well.

[RanulfC]

It's just a series of logical questions.  It's a question of what a prospective customer wants and do they understand what they are getting. Saying you want TSTSO and SABRE is simply illogical.

Eh, no. Because it could be VERY likely that the prospective customer WANTS a TSTO powered by SABRE because he fully understands what he WANTs to get and those pushing SABRE as usable only on an SSTO airframe are unaware of, or just don't care what the CUSTOMER wants or needs. Saying it's illogical[/] is an attempt to 'intimidate' the argument into arriving at the preferred conclusion rather than actually using, you know, 'logic' and more importantly customer/market inputs which may in fact be the opposite of your own!

(And ya, Paul I've seen this stuff quite often but having been on the SpaceX boards I can say it mostly hasn't been as "jumped-on" as it gets there )

JS19 and others are simply making some assumptions that not even REL has made based on some assumptions that REL has made which pertain to operations and economics which themselves are based on some limited modeling for a specific outcome. And outcome which actually hasn't reflected the actual market or business planning needs of known users for several decades. REL knows this as they have been playing with words since the start to cover this point which is one of the reasons people have general doubts about how serious REL is and what, exactly, they are attempting to sell.

I'm quite sure there is plenty about Skylon that has deliberately been left low resolution so the vehicle consortium can impose their own mark on the design but I also expect it would take a lot of effort to come up with a design that looked radically different that can do the job as well as Skylon is expected to.

Actually it's ALL pretty 'low-resolution' since REL is and continues to be, (and they state this pretty regularly) NOT an airframe designer/maker. Therefor while what REL has done so far can be used as a beginning, anyone who takes on the actual design is going to make significant changes as it progress. Assuming they don't go with a different design all together once ALL the trades are done. It might eventually look somewhat like RELs "Skylon", (I'd think more like the cFASST in that case) but going to an actual flying vehicle will take about as much work with or without the REL work.

Once you understand the massive CoG and CoL shifts during the M0-M23-M0 flight path and you want an easily controlled vehicle the Skylon configuration is tough to beat.

Eh, again, that's not really accurate as the CoG and CoL shifts are well known and can be effectively countered with a number of configurations. I know you're aware of all the aerospace work on airframe design done over the decades and controllability of the various designs was only an issue with one specific rather recent design concept that started with some fundamental flawed assumptions. (And no we aren't talking NASP as aerodynamically it was fine, the main problems were unrealistic propulsion assumptions and requiring flying inside the atmosphere at Mach-25 in order to 'justify' afore mentioned propulsion system)

Any configuration which places its propulsion system near the CoG/CoL interface tends to negate the severity of the shift during high speed flight in a aerodynamic, lifting trajectory. Most combined cycle propulsion system vehicles are designed in this manner for that reason.

"Skylon's" general design is, (and REL has stated this before) very basic for a number of reasons. They wanted to be conservative, they didn't want to put to much effort into refining an airframe as they are not an airframe company but an engine company and needed an 'airframe' for reference purposes, but most importantly because it IS a reference design rather than the required design for the use of the SABRE engine. Not to put too fine a point on it but the way "Skylon" is designed is so that what "engines" it has in the place it has them does not matter very much to the calculations of the overall flight and trajectory modeling.

"Skylon" is designed the way it is simply to allow REL to ignore most vehicle effects on the engine operation and vice versa to allow a very simple and quick calculation set to be used with minimum changes during design iteration. That in no way means it is in fact the best, or most optimum design and in fact it's obviously not with what we know of high speed design.

I know this was mentioned but can't find it but that's the main reason the engines are on the wingtips because by placing them there you can in fact simply change certain assumptions and values and arrive at nearly the same outcome at the checkpoints along the flight trajectory without having to make significant changes to the overall "design" which has far to many 'down-stream' effects in any other position. "Skylon" is not an optimized design by any means and as e of pi points out the dry mass is optimistic to say the least, especially for a vehicle that is supposed to be able to be rapidly turned and serviced. Significant mass growth from design to operation is a given unless someone is willing to spend a LOT of money on materials and technology and that's not (as we're all well aware) very often conducive to economic construction or operation.

I seem to recall that the "Skylon" design can handle up to 15% lower engine performance and still be able to meet the given design goals. (I'll be corrected if I'm wrong, probably several hundred times I'm sure ) I thought I'd mentioned this but if so it probably got lost, but while that's a good margin it's actually not the one that people who actually design vehicles tend to be worried about. (A worry but not THE worry in other words)

http://www.sciencedirect.com/science/article/pii/S0094576509004998

Ballpark inert mass growth during development can be anywhere from 10% (highly unlikely) to 50% (same) but is averages around 20% to 30% which also cuts into your margins. 5% engine short-fall and 25% inert mass growth combined means "Skylon" has no payload, not to orbit anyway and there are standardized formulas for figuring a rough set of growth parameters for a vehicle design. I'm betting REL used them too. It doesn't really matter because aerospace designers and manufacturers use them too, for very preliminary designs and they rarely carry over to the actual vehicle. And the bigger the vehicle the more likely for mass growth is. (Yes in some ways a bigger vehicle can absorb more and it averages out but as always ANY SSTO design is more sensitive to the combined factors. Less than a pure-rocket VTVL SSTO but it is not inconsequential. And yes a two or more stage design is also susceptible to mass growth but it actually starts with higher margins to begin with)

When someone suggests something stupid(like using SABRE on TSTO craft) he reveals his ignorance about Skylon/SABRE.

Really? Considering that to actually PERFORM the comparable mission that REL 'designed' "Skylon" to perform, (matching EELV payload to DESTINATION, not ORBIT) REL uses a second stage that "Skylon" has to recover, (there's an operational glitch there a TSTO doesn't have) and return you really think that? Let me point out that almost ALL the advantages cited for SSTO ignore the obvious fact that simply getting a payload to 'orbit' is in fact not very useful and the proper amount of payload has to be delivered to a usable orbit or its worthless? The main point of SSTO has always been based on the idea that a single-stage would be more economic to operate and in fact the amount of payload delivered, and where it is delivered is secondary. That turns out to be quite opposite of the requirements of the people who would be using them though. SSTO's, or pretty much any LEO orbital deliver systems, require to carry or be paired with on-orbit infrastructure, (usually in the form of some sort of Space-Tug or carried propulsion stage, Skylon's US, Shuttle Centaur, Shuttle Agene and PAM being examples) to meet market/customer requirements while, (obviously) multi-stage vehicles have this capability inherent. This is why straight up comparisons rarely work.

And John Smith 19 explained well in his post why SABRE on TSTO craft is stupid.

Your premise would seem to be fundamentally flawed. Nothing in JS19's post shows that SABRE is "stupid" to use in a TSTO design he simply STATES that it is by inferring it is ONLY capable of being used in that role with no supporting evidence. (He's also got an issue with inferring that it is the ONLY system capable of doing so which is also unsupported) Nothing inherent in the SABRE prevents its use in a TSTO vehicle and REL has stated that it's not an optimum solution but have never given the impression that it's "stupid" or impossible. The main issue is that there is an assumption that a TSTO would not use the SABER in a similar manner to an SSTO for portions of the flight.

Check me here:
Take off from a runway
Accelerate to @Mach-5 using deep-cooled air breathing rocket power
Switch to straight Hydrolox rocket power and continue accelerating
Continue accelerating to orbital speed
Deploy payload with upper stage
Reentry atmosphere
Land on runway

Compare:
Take off from a runway
Accelerate to @Mach-5 using deep-cooled air breathing rocket power
Switch to straight Hydrolox rocket power and continue accelerating
Deploy upper stage with payload
Upper stage continues to accelerate to orbital speed (Note US can be fully reusable and capable of returning to Earth by itself)
Reentry atmosphere
Land on runway

Which parts can SABRE not do?

It just seem that we armchair engineers who really have no experience on real rockets seem to think we known more about developing rockets and spacecrafts than guys like Alan Bond who have done it for >35 years and also learned lot from earlier failures, and when our stupid ideas are shot down by other people, we cannot take it well.

Just so you are aware, Alan Bond has NEVER said that using SABRE on a TSTO is "stupid" or can't be done. That would be the "armchair engineers" around here. He said it wasn't the optimum use but not that it wouldn't or couldn't work. Nothing that he's said, written or done indicates that SABRE ONLY works on SSTO vehicles and he's experienced enough to know better than to make such a claim where as some "armchair engineers" obviously don't so quoting his experience and wisdom to support something he himself never said or inferred IS rather 'stupid' don't you think?

Further and probably more importantly Alan Bond (and this is NOT a dig or dissing him) is NOT actually a aerodynamics or spacecraft engineer, he's a mechanical and propulsion engineer. Lets be clear at least. No one at REL is in fact an experienced hypersonic aircraft designer or engineer (and there ARE a lot of those around) and the Skylon is "designed" to fulfill a set of criteria based on certain assumptions using the simplest possible vehicle design and standard DSMC modeling without that experience and expertise. Given the fact that Skylon as designed is only a very low resolution basic vehicle design for calculations of what is possible with the assumed performance of the SABRE engine the work done is sufficient to show that if the all the assumptions hold up the design is probably viable in basic function.

That does not translate to being viable or even desirable until a lot more 'variables' have been defined completely by people who have the applicable skill sets. Before that however the engine has to be proven and actual performance parameter data gathered which is what REL is really all about.

Rejection is a part of being on these forums and trying to interact with professionals and knowledgeable amateurs is part of the fun of being here. The people posting back and forth for the most part have been on here and "won" and "lost" quite often and fully understand that most of what we discuss here won't effect those working on the real projects one bit so I'd say we all assume our "stupid" ideas will be stressed tested on a regular basis. Those that can't handle it well simply don't stay.

When people propose something smart/reasonable alternative to skylon, like different aerodynamic configuration for SSTO craft using SABRE, they are treated well

Have you actually read and understood what that paper is about? First of all it's essentially attempting to use a greatly simplified aero-heating program in place of the standard, more complex one in regards to a "simple" aerodynamic shape as compared to a more "complex" one. (Bottom line is if you're going for very simple and quick calculations as long as you keep in mind the variables and limitations and there are a lot of them, the new program works to a degree) Secondly note that the configuration changes were to make "concessions to ease manufacturing and structural efficiency" but that it is essentially a re-skinned Skylon with more efficient aerodynamic design.

The paper doesn't show much new as REL was/is well aware that the Skylon is NOT an optimized design but a general one. It has issues which will require someone with more time, money and engineers to address sufficiently. And once having done so the result may (probably in fact) will look very little like the Skylon as currently designed.

Despite the look the cFASST-1 design changes very little of the basic Skylon design and already (this is shown both with the simpler HyFlow and the more complex and encompassing DSMC models) large changes are observed. It is obvious that more fundamental changes using well known high-speed/hypersonic methods will yield equally large changes and efficiencies.

For example:
Note the engines remain in the same place on cFASST. That is NOT because that is the only place that SABRE will work, nor is it because that position is the perfect position for an air breathing engine because it's very much not. Again it's to keep the calculations and formula simple so there is no need to calculate airframe/engine interaction. Experts are well aware that there are large increases in efficiency when engines and airframe are more integrated than when they are not, but the interaction also gets very complex very quickly. This is not in fact a bad thing as the more integrated the engines/airframe the greater efficiency overall both have. (For example both inlet and exhaust design gets simpler and overall propulsion efficiency increases when you can use the airframe as part of the system. It can cost some complexity in airframe thermal management but it can yield double digit efficiency % in overall performance)

None of this precludes the design from being used in a TSTO launch system. The paper notes that "Optimization to include low-speed, low altitude flight will almost inevitably result in a configuration that is at least partially aircraft like and thus a vehicle which geometrically is more complex than space vehicles that have been designed in the past." That's very true but if you follow that logically then the conclusion also follows that something designed to be optimized for one regime will, by it's nature, NOT be optimized for different regimes. So compromises will be required at various points to allow a single airframe to cover all regimes. Whereas two vehicles both optimized for different regimes can, when used in combination be more efficient than a single vehicle.

Staging gets more efficient the higher and faster it's done. A SABRE powered lower stage, air breathing to @Mach-5 before switching to pure rocket can then accelerate outside the effective atmosphere to speeds approaching Mach-10 before releasing a space/very-high hypersonic optimized stage that carries the payload to LEO and beyond. Nothing in the SABRE design prevents this from being possible and it may in fact happen when the actual people who will be building the airframe get done with the trades and design studies.

Randy

[RanulfC]

It seems like the real magic of Skylon isn't its engine--it's the structure.

Perhaps, but that is also the part of Skylon that has had the least amount of work, and the most amount of hand-waving. And there is much skepticism about its mass efficiency as well.

Yes but the POINT here is that it doesn't actually matter because it's not the important bit. That would be SABRE and what seems to confuse people the most. REL is and always has been about SABRE with Skylon there to hang it all together as a concept with some fairly viable numbers to work with, but it has always been SABRE and not Skylon that was the point of the exercise.

If SABRE works as advertised then Skylon is in the ballpark as a viable concept that will require the proper skillset to refine. (Or reject in favor of a more viable design which is always possible as well) But it all boils down to SABRE and Skylon is just a concept to set that into a proper context.

If SABRE test out anywhere near what REL hopes then they won't care if it's hung on a totally different design because they are selling SABREs and that's the end goal they have been working towards.

Randy

[ChrisWilson68]

It seems like the real magic of Skylon isn't its engine--it's the structure.

Perhaps, but that is also the part of Skylon that has had the least amount of work, and the most amount of hand-waving. And there is much skepticism about its mass efficiency as well.

Yes but the POINT here is that it doesn't actually matter because it's not the important bit.

It is important, because if the mass fraction of Skylon isn't realistic, Skylon can't reach orbit and is total non-viable.  In that case, SABRE would only have a use in the first stage of a multi-stage vehicle.

Skylon proponents would like everyone to believe that the only thing that was ever in doubt about Skylon was the heat exchanger, and testing that in a room on the ground means there's no more development risk in Skylon, it's just a matter of adding money and it will undoubtedly work as advertised.  Many people don't believe that.

[Rocket Science]

I'll be interested if anything pans out with the RFP for the XS-1...
http://www.space.com/32115-skylon-space-plane-engines-air-force-vehicle.html

[t43562]

This isn't really news but it's an interesting video about the testing of the ED nozzles:

[john smith 19]

Landing gear is about 2-7% of the GTOW, typically in the 4% ballpark. On the low side, that's 6,500 kg, but on the high side as much as 22,700 kg. A reasonable guess might be 12,000 metric tons.
Thus, the mass of the remainder of the airframe--engines and tanks--is the 53 metric ton total structure minus these. That's about 12 metric tons using the 4% GLOW landing gear assumption.

Why are using that figure for brake mass. Where does it come from?

[lkm]

Exactly. What makes SABRE unique is the air breathing phase. Once you are above the atmosphere (or cease collecting O2), then it is no different than a normal rocket engine. And that is where staging - if one wanted to do it - would make sense.

There's also the thing that makes Skylon's airframe unique: astoundingly low dry mass. To use the D1 specifications, which I know are no longer the latest but are the most complete I have to hand:

SABRE makes a T/W of about 14, with a thrust of 2000 kN. That's a dry mass of about 14,500 kg per engine. Two is 29 metric tons.
Landing gear is about 2-7% of the GTOW, typically in the 4% ballpark. On the low side, that's 6,500 kg, but on the high side as much as 22,700 kg. A reasonable guess might be 12,000 metric tons.
Thus, the mass of the remainder of the airframe--engines and tanks--is the 53 metric ton total structure minus these. That's about 12 metric tons using the 4% GLOW landing gear assumption.

That'd make Skylon--including its wings, engine nacelles, and cargo bay all wrapped up in an orbital-capable TPS--about as mass efficient as the Space Shuttle SLWT. To be fair, the SLWT isn't common bulkhead and was designed to deal with the TAOS Shuttle, so it has an intertank and heavy thrust beam. OTOH, Skylon has an equally large (actually larger) cargo bay and a wing spar in about the same spot.

It seems like the real magic of Skylon isn't its engine--it's the structure.

I think it needs to be pointed out that your numbers are wildly wrong.
Over the years there have been a large number of published papers detailing Skylon construction so there's no need to guesstimate a whole lot of numbers when you can just read them.
According to documentation the SABRE 3 engines mass 10870 kg for both which at peek can produce a gross thrust of about 4000kN and a net T/W of about 14 (not at the same point in time), and the much discussed innovative undercarriage is 1.5% of GTOW thank to the use of the dumpable water coolant.
A full breakdown of the the Skylon C1 mass budget is available in the documentation  down to a component level in places and  can be reasonably scaled up for the D1.
Skylon C1
Item       Mass (kg)   
         
Main engines 2      10870   
Nacelle, Inlet, Bypass      3922   
Wing      5115   
Fuselage: aeroshell, insulation, structure, payload bay      8130   
Main Tankage, cryogenic insulation      2816   
Undercarriage       4170   
Aerodynamic control, hydraulics      2660   
Auxiliary systems, pressurants, coolants etc      5016   
   Basic Mass      42699
OMS/RCS propellants      2357   
Ascent Fuel      66807   
Ascent Oxidiser      150235   
Propellant margins and residuals      1282   
   Total fluids      220681
         
Mass Margin         0
Payload         11620
   Gross Takeoff mass      275000

[john smith 19]

I'll be interested if anything pans out with the RFP for the XS-1...
http://www.space.com/32115-skylon-space-plane-engines-air-force-vehicle.html

there is no mention of the XS-1 in this article.  There is a claim that SABRE is a LACE, which is disappointing given how long SABRE has been around   

[john smith 19]

Here Arguing that while it's "non-optimum" it's not only a consideration but, (and I've said this consistently) that the Skylon design itself while it is technically an SSTO vehicle is specifically a TSTD (Two-Stage-To-Destination) design as it's based on the idea of equaling EELV satellite delivery capability. That requires delivery to GTO and Skylon can't do it alone so needs a stage/transfer-vehicle/what-have-you.

True, but GTO is only part of the market and it's very challenging. Using the Skylon Upper Stage splits the design problem into the very tough part (Earth to LEO) and the relatively well understood process of going from LEO to GEO.

SABRE is an engine designed to accelerate a vehicle from runway to orbit. It is also designed to be testable on the ground. Either it will meet those performance specs or it does not, before you have to install it in a vehicle and fly it.

Either you want an engine that can go from runway to orbit or you do not.

Rather simplistic actually. It keeps being said that if you want an engine for a TSTO you want the Scimitar and not the SABRE because of the mistaken belief that "if you want to go to orbit need SABRE, if you're not going all the way you don't" which is, pardon me BS.

Fair point  I was assuming they were drawing the line right on M5 while in the atmosphere. And I know what happens when you assume. If you want to go higher and above then SABRE would be the option. I was also thinking that this is already covered in the DARPA XS-1 programme.  I was also recalling  that REL are aware of the results of the cost modelling in the MUSTARD programme, specifically that if 2 stages are not identical you more than double the budget, because not only do you need their development and engineering budgets, you need one to cover the combined interactions of the stages.

I can see a TSTO architecture where the staging Mach number is gradually raised, lowering the gross weight of the 2nd stage or increasing its payload up to the structural and thermal limits of the 1st stage carrier. But a reusable 2nd stage will still need the full orbital rated TPS anyway while an expendable 2nd stage will never give the cost per flight level of a fully reusable system.

SABRE isn't a single niche engine no matter what people think though that's what REL WANTS the simple and very obvious truth is it's not that limited of an engine cycle. SABRE is capable of air-breathing up to around Mach-5 pretty easily followed by rocket powered flight, technically up to orbital speed but that should be noted that includes every Mach number short of that speed as well. Note the main difference between SABRE and Scimitar is the latter is designed for extended CRUISE at hypersonic speed while the former is designed as an acceleration engine.

A big difference but I'd say the biggest is that's it's solely an air breather, with no provision to run on LO2.

"Runway-to-Orbit" is NOT about the engine, as I've often pointed out the SERJ and several other combined cycle engine system are perfectly capable of performing the same mission and have been since the mid-50s.

True. But SABRE's pretty good T/W (for an airbreather) and excellent Isp relax the limits on structural mass quite a lot, and even a TSTO with a rocket only 2nd stage will still be a demanding structure, if only from the heating angle.

The major problem is they almost all got caught in the two main traps of thinking at the time, (and currently if we are being honest) liquid-air-cycle and SCramjets as "requirements" for operation. "Runway-to-Mach-10-and-500,000ft" would be something that SABRE could and would do as well as orbital flight so trying to say a SABRE is ONLY good for runway-to-orbit and nothing else is simply a bias on the part of the person saying it.

That's fair also. It just seems like overkill once you've built a fully orbital engine to not use it as such.Looking deeper into it I just can't see the architecture.

Not going the whole way in a single stage suggests but you are using SABRE suggests you're OK with the engine but don't believe the structure.

The only large reusable high Mach structures I know are the Shuttle, the X37b, X15 and XB70. Except the X37b all have done powered flight inside the atmosphere but only the Shuttle did the whole potential SABRE speed range and only the XB70 could lift its own weight.

 If you're that nervous only a rocket would be acceptably safe for the 2nd stage.

It's this mix of optimism and caution that I'm having trouble with.

So an LH2 powered engine is OK. A truss structure with fibre reinforced glass skin is not OK.
Horizontal separation at high Mach number (that's an assumption but I've hear nothing about a VTO SABRE concept) is OK.

There's optimum and then there's good-enough either of which SABRE is perfectly capable of handling and still being SABRE.

True. I'd never really thought of Skylon's design being selected because it was easy to anlayse. It seemed to address quite a few problems.  I don't know about "optimum" but I'd certainly say "Good enough" and given the design goals I think it would be difficult to come up with something that looked much different but still gave undisturbed airflow to the engines and a well balanced design.

That statement actually hurts my brain because it points up a lot of misunderstandings about what REL has done, what they are proposing, and what their goals are. In reverse order; TSTOs have proven advantages over SSTO designs when directly compared and vice-versa so the 'call' to go one way or the other is a business, engineering, and operations decision. Not something you base on optimum use of a specific engine design. RELs "efforts" have amounted to what would be a very preliminary study of a possible airframe that can maybe do "this" with an assumed performance of "this" from the propulsion system.

AFAIK the perceived advantages are an easier design problem as your design is split in two and you can have more structure and a higher payload fraction.

But Skylon was designed to deliver an ELV payload fraction, not the 1% of the Shuttle. For an equal payload I find it very hard to believe a TSTO will be simpler to design, build or test.  I doubt it would be cheaper to operate either.  The big question is the perception of how risky is the Skylon structural design

I know you hate the launch-vehicle-like-an-aircraft analogy but no one has ever built a large 2 stage cargo aircraft, despite the benefits of an enormous 1st stage to get a heavily loaded (and fueled) 2nd stage off the runway and airborne. 

REL has gone a bit further due to being biased towards an SSTO design but because they are not in fact airframe designers/makers they have missed some obvious areas. Any actual airframe designer/engineer/maker is going to take the "Skylon" design under consideration and then do actual trade studies to define a REAL design capable of doing what the overall business/market/user requires.

I've never doubted that would be the case.

The REL "Skylon" has a leg up in the preliminaries because of the work REL has done but they didn't do the work to actually DESIGN an airframe, they did the work to define something that by using SABRE engines that meet expectations could do "this" operationally. Someone who's going to make an actual Skylon vehicle will do a lot more detailed and in-depth work which may or may not validate RELs assumptions and combine that with marketing and operational inputs which may not in fact recommend an SSTO design at this time or a different airframe configuration.

If the design assumptions on Skylon are as conservative as you think that seems unlikely barring (again) the perception that HTOL SSTO  is risky and would essentially put the airframer at square 1 in design.

Again I'd be highly surprised if the actual "Skylon" design gets built as it has some issues that need more work, but the work REL has already done address some general known issues relating to hypersonic, high altitude, and trans-orbital flight, just not to the level that Skylon would be considered an "actual" design in it's present form.

An interesting point. NASA pointed out that SABRE plume heating of the rear fuselage might be an issue, but did you have any others in mind?

More in the next post because addressing the assumptions in the rest is going to be a bit long... And "I" said that so you've been warned

Noted.

[john smith 19]

Skylon proponents would like everyone to believe that the only thing that was ever in doubt about Skylon was the heat exchanger, and testing that in a room on the ground means there's no more development risk in Skylon, it's just a matter of adding money and it will undoubtedly work as advertised.  Many people don't believe that.

"Proponents" know the difference between the Skylon vehicle and the SABRE engine.

However someone floating a strawman argument would not care. 

Please stop with the strawman arguments.

[ChrisWilson68]

Skylon proponents would like everyone to believe that the only thing that was ever in doubt about Skylon was the heat exchanger, and testing that in a room on the ground means there's no more development risk in Skylon, it's just a matter of adding money and it will undoubtedly work as advertised.  Many people don't believe that.

"Proponents" know the difference between the Skylon vehicle and the SABRE engine.

However someone floating a strawman argument would not care. 

Please stop with the strawman arguments.

REL identified designing and making the pre-cooler as the unknown element in the SABRE design so that's what they went for.

liked by john smith 19:

And for Skylon, they have built the preocooler which is the most important part of the craft.

the SABRE engine is a dual air-breathing AND rocket engine. The air-breathing cycle has a rather nifty and advanced heat-exchanger to stop things melting. If it performs to expectations, SABRE should do SSTO.

I don't think REL have every really doubted their ability to build SABRE provided a)The pre cooler worked as expected and b) The could get the funding.

The pre cooler has now been extensively tested and worked as expected. Progress milestones then depend on their getting the necessary funding when it's needed.

The *real* unknown (and highly worrying) item would be the pre-cooler and it's frost control system.

Which might explain why they went to tackle it first.
I'd suggest a lot about Skylon is beyond the current state of *practice* in *LV* design. A lot of it is well *inside* the state of the art in other areas *if* you broaden your viewpoint and realisze that LV design is *very* conservative.

Methods exist in the *aircraft* industry to manage risk in development and schedule. Will they (if *fully* funded) deliver a Skylon to the  *day* set in their schedule? I'd suggest they have as good a chance of doing that as Spacex did when they started.

[Rocket Science]

I don't get the post above??

[ChrisWilson68]

I don't get the post above?? ???

John Smith 19 accused me of making a strawman argument.  In response, I listed several posts where he and others made the argument I was disputing, showing my argument wasn't a strawman argument.

[Rocket Science]

I don't get the post above??

John Smith 19 accused me of making a strawman argument.  In response, I listed several posts where he and others made the argument I was disputing, showing my argument wasn't a strawman argument.

Oh, ok Chris, thanks... You guys carry on!