It would also allow the design to have loiter and significant cross-range launch capability which is another thing the military loves for an "on-demand" launch system.
Often in this thread there is assumption: if they would go two stage everything would be easier and better. This is reasoning from analogy to VTO rocket Launch Vehicles. The case is: if they go TTO then nor Skylon nor Sabre has any sense. All concept is build around using air, not fighting with it, what makes it unique and first phase super efficient - and since engines are the same then what you can throw away? Airframe? How much fuel is used in first phase? How much mass could be shaved? This is two phases to orbit in one stage to orbit.
Quote from: SICA Design on 03/16/2016 10:49 amYou have mentioned "single engine possibility" more than once now. It should be pointed out that it was the rear-heavy design failure of HOTOL - which became a "means for lifting hydraulics into orbit" (huge canards to compensate CoM issues) - which principally drove the twin-engined Skylon design.True.For a demonstration vehicle what is essentially a winged nacelle would be possible.To preserve the CoG protecting features you'd need to put it on top of the payload bay, like a sort of V1 cruise missile. But that would leave the issue of how you open the bay to get the payload out. The latter is a slightly closer to Skylon so might shorten the test programme of the real Skylon, while the former is likely to be cheaper.Both would probably make the test programme more expensive overall.
You have mentioned "single engine possibility" more than once now. It should be pointed out that it was the rear-heavy design failure of HOTOL - which became a "means for lifting hydraulics into orbit" (huge canards to compensate CoM issues) - which principally drove the twin-engined Skylon design.
Shuttle has a problem because of margins. Mass to orbit was not just the payload at 50klb (round numbers) but the 250klb orbiter. So if the orbiter came in too heavy at 5%, that means the payload loses 12.5klb mass. that is why they had to increase the performance of the system. For SSTO, it is worse.
The engineering challenge of developing an SSTO is so astoundingly far beyond the engineering challenge of TSTO. Not just twice as hard, but orders of magnitude.
REL is at least doing one form of iterative development: subscale component testing. It also seems like they are happy to have other players use their technology on lower risk iterations, ironically unlike many of their defenders here.
Engineering margins.
Quote from: TrevorMonty on 03/16/2016 03:24 pmA strapon booster for existing/future LV is one possibility. No it's not. This was a running theme of WIlliam Escher. It's a bad notion given the poor T/W of airbreathers (although SABRE's is excellent by turbofan standards).
A strapon booster for existing/future LV is one possibility.
Quote from: RanulfC on 03/14/2016 06:37 pmIt would also allow the design to have loiter and significant cross-range launch capability which is another thing the military loves for an "on-demand" launch system.The cross range capability is a result of the Skylon design, not the SABRE engine.
But a pre cooled engine gives you the T/W to allow such a design to take off from a runway.
It took me years to find a report that would finally state what a SCRamjet T/W was. ONce I saw "2:1" It thought, "how did they ever get funding to pursue this?" Yet $Bn have been spent.
QuoteEngineering margins.Last I heard, the airframe was being designed to a safety factor of 1.5, the same as a commercial airliner.
Quote from: Jim on 03/16/2016 12:29 pmShuttle has a problem because of margins. Mass to orbit was not just the payload at 50klb (round numbers) but the 250klb orbiter. So if the orbiter came in too heavy at 5%, that means the payload loses 12.5klb mass. that is why they had to increase the performance of the system. For SSTO, it is worse.Not this SSTO.
Engineering or design margin is in addition to the safety factor. If they design has something wrong or under calculated, they might not have 1.5 safety factor
Yes, this SSTO and all others. They are more sensitive to mass increases than other LVs, especially first stages.
Quote from: TrevorMonty on 03/16/2016 03:24 pmA strapon booster for existing/future LV is one possibility. Does anybody know what type performance increase 2 1xsabre boosters would give Vulcan assuming they separate at Mach5.SABRE has a projected thrust-to-weight ratio of around 14. That's double the F119 engine, for example, which itself it higher than most commercial engines.But rocket engines are up around 80-100. (The record is either Merlin 1D (180:1) or one of the Russian engines.)[edit2: to clarify that. It means that if your SABRE-booster and fuel masses 14 times the mass of the engine, the booster will only be able to hover, it won't add any lift. If it masses more than 14 times the engine, the boosters will actually be hanging off the core stage, merely adding to the weight.]SABRE's Isp is massively better than any rocket during its air-breathing mode, but for a vertical launch that precisely when you are most willing to sacrifice Isp for thrust. (The amount of fuel you burn in your first three minutes is much less important than the amount consumed in the last three minutes, because the fuel burnt in the last three minutes has to be carried during the rest of the flight.)Skylon can cope with low thrust/weight because it gets lift from aerodynamic surfaces to compensate for gravity losses. And that long horizontal climb is where you need high-Isp.[edit1: added the parenthesised comments]
A strapon booster for existing/future LV is one possibility. Does anybody know what type performance increase 2 1xsabre boosters would give Vulcan assuming they separate at Mach5.
To preserve the CoG protecting features you'd need to put it on top of the payload bay, like a sort of V1 cruise missile. But that would leave the issue of how you open the bay to get the payload out.
but dropping off the excess mass of the booster stage and continuing the mission profile with a more efficient second stage that is fully reusable is as valid an assumption as not.
Quote from: john smith 19 on 03/16/2016 07:21 pmThe cross range capability is a result of the Skylon design, not the SABRE engine.No that's actually because the SABRE is an air-breathing engine which allows them to MOVE the launch point around to where they need it.
The cross range capability is a result of the Skylon design, not the SABRE engine.
Quote from: john smith 19 on 03/16/2016 07:11 pmTo preserve the CoG protecting features you'd need to put it on top of the payload bay, like a sort of V1 cruise missile. But that would leave the issue of how you open the bay to get the payload out.Belly doors, obviously. (Loading would be via an elevator-pit.)
Quote from: Paul451 on 03/16/2016 04:07 pmThe engineering challenge of developing an SSTO is so astoundingly far beyond the engineering challenge of TSTO. Not just twice as hard, but orders of magnitude.The whole point of SABRE is that it makes this not true any more. Insisting otherwise without any actual math as backup (as people like to pop in here and do now and then) is cargo cultism - associating the difficulty with the label rather than with the technological requirements.
The "case" is a lot more complex than you make it out to be. First of all there is a difference between "orbit" and "destination" that is often ignored in these comparisons. Far more often than not the former is not the latter. The adage that once you're in orbit you are half way to anywhere is often cited as a truism but really we don't currently USE LEO as much as that was assumed when that was said. GTO/GEO and escape trajectories, serviced by multi-stage, direct ascent vehicles has become the default method instead of LEO rendezvous, fueling, and orbital assembly.
video from SABRE – The Next Leap Forward in Powered Flight15th March 2016, Aerospace and Transport Technologies Research Priority Area, Faculty of Engineering, Nottingham University.(I've not seen it yet, but passing on the link)
Belly doors, obviously. (Loading would be via an elevator-pit.)
If you're using SABRE, then you've got that backwards. You'd have a reusable first stage (the Skylon-derivative), with an expendable all-rocket, vacuum-optimised upper-stage.
JS19's point was that any air-launch system has the same cross-range, rapid deployment ability; not just a SABRE-based launcher.
Quote from: Radical_Ignorant on 03/16/2016 09:18 am...And unfortunately Sabre is not very scalable. Nor down (engine reasons) nor up (infrastructure reasons) except single engine possibility.You have mentioned "single engine possibility" more than once now. It should be pointed out that it was the rear-heavy design failure of HOTOL - which became a "means for lifting hydraulics into orbit" (huge canards to compensate CoM issues) - which principally drove the twin-engined Skylon design.
...And unfortunately Sabre is not very scalable. Nor down (engine reasons) nor up (infrastructure reasons) except single engine possibility.
Quote from: Radical_Ignorant on 03/16/2016 07:53 amproof of concept and tech demonstrator [...] but I suspect it wouldn't save much development.This is, IMO, one of the most destructive myths in aerospace. The belief that developing directly to the end goal is going to be cheaper than passing through three or four operational stepping-stones.Reality is the opposite. Unless what you are developing is completely mundane and your designers, engineers, technicians and ops people are all experienced in that technology, it will always cost less to pass through multiple iterative stages before even attempting the final design. (Indeed, before attempting to design the final design.)(It's the same "one is less than two" mentality.)REL is at least doing one form of iterative development: subscale component testing. It also seems like they are happy to have other players use their technology on lower risk iterations, ironically unlike many of their defenders here.Quote from: Radical_Ignorant on 03/16/2016 09:18 amIn short if you don't push envelope you don't have benefits. You much prefer Space X way of pushing it in small steps. But then we have Space X and they won't stop doing it, so duplicating their way is doing what other does and makes no sense if you are not established player on the market.That's silly. It's their method you'd be copying, iterative development, not the product.Quote from: Radical_Ignorant on 03/16/2016 09:18 amCan you go to investors and tell them: hey I need few hundred millions and I'll do things like Space X is doing except I'm much late into the gameVersus going to investors and saying: hey I need a ten billion (probably) and I'll (try to) develop something that no-one (including me) has ever done before, and without doing any risk-reducing steps? Oh, and because I'm developing directly to the end-goal, you won't see any return for twenty years. Oh, and it involves sustained high-mach flight, which has been traditionally horribly expensive. Oh, and it involves a hybrid engine, which has traditionally been horribly expensive. Oh, and we're not airframe designers, so we'll need to outsource that anyway, but don't worry, this computer model says my design is perfect.[edit: typos]
proof of concept and tech demonstrator [...] but I suspect it wouldn't save much development.
In short if you don't push envelope you don't have benefits. You much prefer Space X way of pushing it in small steps. But then we have Space X and they won't stop doing it, so duplicating their way is doing what other does and makes no sense if you are not established player on the market.
Can you go to investors and tell them: hey I need few hundred millions and I'll do things like Space X is doing except I'm much late into the game
Please consider that in Europe there is no something like NASA or DARPA which will pay for tech demonstrators.
Secondly, there is what Zurbin is arguing against: let's do multiple middle steps. Let's push the final goal so far away that till this time will come, everything will change.
How many projects with approach let's-first-develop-demonstrator were never continued because of, "nah, we are not interested in this anymore" from politicians.