Author Topic: The Reaction Engines Skylon Master Thread (1)  (Read 781643 times)

Offline ArbitraryConstant

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Re: The Reaction Engines Skylon Master Thread
« Reply #1620 on: 12/04/2012 10:43 pm »
Musk should not have made a comment since he really didn't know anything about what REL is doing. He has a tendency to express an opinion even when he doesn't have the facts. "Bad assumptions" always equals "invalid opinion." In that sense, he can't be right because he didn't reason properly. He should simply keep his mouth shut when he doesn't know the facts.
Regardless of vertical versus horizontal takeoff, it still involves carrying the intakes/precoolers/etc, wings, and all the other atmospheric trappings all the way to orbit. That still constitutes the vast majority of the delta-v. Adding that mass still incurs gravity losses, even without the vertical ascent from sea level. Getting everything working that has to work within the necessary margins before money runs out is still extremely challenging. It's not dishonest or wrong to point this out. More than one scheme that could have theoretically worked didn't because money ran out, in many cases because superfluous stuff was put on the critical path (such as composite tanks on the X-33), so it's important that SpaceX has a launch vehicle that's gone to orbit and can continue to do so even if reusability is a bust or partial success.

REL has an extremely interesting concept and probably the best approach to a hypersonic airbreathing engine that I've seen, but I don't think it's lying or misinformed to suggest that there's drawbacks to using it for space launches and that it's extremely challenging.

I'm glad to see REL exploring terrestrial passenger vehicles. They should be ready to monetize the technology as quickly as possible by any means available. Even without a SSTO vehicle, it's still an important advancement if they can make it work.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread
« Reply #1621 on: 12/04/2012 10:56 pm »
It also should be noted that SKYLON will be licensed as an airliner, not a rocket like Falcon. I don't think there's a valid reason for it to carry explosives, while most assume the Falcon will.
Interesting point. People see the big engines and the huge tanks on launchers but miss the myriad of little rockets and pyrotechnics. Spacex has a gas operated stage separation system. Rockets don't need to carry a self destruct system (but it looks good in movies) but they do need to carry a flight termination system, which can be substantially less violent. In essence a command receiver wired to the main propellant valves. Valves close engine dies (dammed expensive for what they and I don't think Spacex can make them in house as they have to be flight certified).


Actually, the math is generally simplified.  In CFD, for instance, a complete and correct simulation is totally impossible and will remain so until we develop the requisite quantum computing capabilities.  This means we have to use approximate modelling techniques to describe turbulence and combustion, not to mention liquid-gas interaction, radiative heat transfer, plasma dynamics...  As for accurate equations of state, high-temperature air isn't too tough, but liquids are brutal...

I'm not quite so well versed in solid mechanics simulations, but just off the top of my head, FEA models have a tendency to use the linear strain approximation, and composites could get interesting...  Wear on rotating joints?  I don't even know...
Given the ratio of laminar to turbulent heat transfer knowing where that happens will be quite important. IIRC REL had the German DLR do modelling with their codes. Hempsell said temperatures were lower than expected but they have a backup plan if there are problems in flight test.

I'm surprised that liquids are tough to model at anything below thousands of bar, except for Hydrogen.

I'm wondering how well combustion chamber modelling works given at least one propellant (and possibly both) in a modern chamber is supercritical.
Quote
Experimentum solum certificat in talibus.
I don't think REL would have it any other way.
Brief article on non-space applications of SABRE technology here:

Interesting.
Given that a) getting Skylon into space will almost certainly be more expensive and take longer than REL are currently anticipating,
and b) licensing this technology to existing aeroengine manufacturers would presumably be lucrative, I wonder if we'll actually see RE-designed precoolers flying on airplanes before spaceplanes.
a) REL have spent a lot of time on cost and schedule modelling. They describe the schedule as (I paraphrase) "Fast for a launcher, but slow for an airliner" (3 Rocketeers film). So unless you know how much float is built into any development stage that's not a given. The core team is well aware of the idea of "unknown unknowns."
b) Actually the article talks about blade cooling and recuperation. Note the REL HX design is a different pattern of pipes & pipe sizes. It might make such a blade design more difficult to mfg. Recuperation uses exhaust gas to heat an incoming fluid but what fluid? If it's fuel than increases the blade cooling problem (the fuel is boiling before it gets to the combustor). A closed loop (He ?) could assist driving the inlet fan but you don't have the LH2 to chill it.

New design blades might be a drop in replacement but the question becomes how much do all the other parts change by? I think other options demand a new engine.
MCT ITS BFR SS. The worlds first Methane fueled FFSC engined CFRP SS structure A380 sized aerospaceplane tail sitter capable of Earth & Mars atmospheric flight.First flight to Mars by end of 2022 2027?. T&C apply. Trust nothing. Run your own #s "Extraordinary claims require extraordinary proof" R. Simberg."Competitve" means cheaper ¬cheap SCramjet proposed 1956. First +ve thrust 2004. US R&D spend to date > $10Bn. #deployed designs. Zero.

Offline flymetothemoon

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Re: The Reaction Engines Skylon Master Thread
« Reply #1622 on: 12/04/2012 11:19 pm »
I'm glad to see REL exploring terrestrial passenger vehicles. They should be ready to monetize the technology as quickly as possible by any means available. Even without a SSTO vehicle, it's still an important advancement if they can make it work.

Thinking about it, it would almost be a surprise if the Americans weren't already talking to them about tieing them up for a test Mach 5 hydrogen fuelled military test plane!... oh yeah... apart from the ITAR problem...
« Last Edit: 12/04/2012 11:20 pm by flymetothemoon »

Offline flymetothemoon

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Re: The Reaction Engines Skylon Master Thread
« Reply #1623 on: 12/04/2012 11:29 pm »
Regardless of vertical versus horizontal takeoff, it still involves carrying the intakes/precoolers/etc, wings, and all the other atmospheric trappings all the way to orbit. That still constitutes the vast majority of the delta-v. Adding that mass still incurs gravity losses, even without the vertical ascent from sea level. Getting everything working that has to work within the necessary margins before money runs out is still extremely challenging. It's not dishonest or wrong to point this out. More than one scheme that could have theoretically worked didn't because money ran out, in many cases because superfluous stuff was put on the critical path (such as composite tanks on the X-33), so it's important that SpaceX has a launch vehicle that's gone to orbit and can continue to do so even if reusability is a bust or partial success.

Ok, but according to REL:

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

"This approach enables SABRE-powered vehicles to save carrying over 250 tons of on-board oxidant on their way to orbit."

Even with my limited knowledge, it's clear that 250 tons is quite a lot of savings and clearly enough to compensate for all the additional paraphenalia (assuming it works - can we assume this disclaimer is a given?!). Looks pretty clear Elon Musk doesn't know about that.

Also, the wings provide lift... The whole air-breathing package...

"...can be used until the engine has reached over 5 times the speed of sound and an altitude of 25 kilometres which is 20% of the speed and 20% of the altitude needed to reach orbit."

20% is a decent chunk of speed and altitude too...

Offline QuantumG

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Re: The Reaction Engines Skylon Master Thread
« Reply #1624 on: 12/04/2012 11:43 pm »
No-one cares about how much oxidizer you don't have to load onto it.
Human spaceflight is basically just LARPing now.

Offline flymetothemoon

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Re: The Reaction Engines Skylon Master Thread
« Reply #1625 on: 12/04/2012 11:57 pm »
No-one cares about how much oxidizer you don't have to load onto it.
You would care if you had to carry it   ;)

Online Elmar Moelzer

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Re: The Reaction Engines Skylon Master Thread
« Reply #1626 on: 12/04/2012 11:59 pm »
Quote
Also, the wings provide lift...
and a lot of drag. Rockets leave the atmosphere are quickly as possible to avoid the drag. An air breather needs air. That air also means drag. It is a blessing and a curse. Yes they need less oxidizer because they can use air. In return they loose some of the advantage due to added drag and heaver engines. You gain some, you loose some. The discussion whether air breathing makes sense or not has been ongoing for decades now and a lot of very smart people have expressed very strong opinions on this either way. Therefore, I would not call what Musk said uninformed (if he referred to the Skylon and not Stratolaunch which is more likely given their recent "divorce"). It is merely an opinion that he shares with many in the industry. That does not mean that he is right. Personally I am happy to see as many and as diverse approaches to the problem as possible. That increases the chance that at least one of them will make it.

Offline tnphysics

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Re: The Reaction Engines Skylon Master Thread
« Reply #1627 on: 12/05/2012 12:26 am »
I have not found any reference to these kinds of problems/solutions on the REL website. This could be a show-stopper for the reusability of the SABRE engine. Mark Hempsell, if you're reading this, has REL a plan for dealing with this problem?

check this page.
http://www.reactionengines.co.uk/sabre_howworks.html
click on the 3rd picture.

It's complicated because you're looking at the hardware for 2 different engine modes superimposed on each other. The bit you're worried about is in the bottom left hand corner labelled "pre-burner" like the bit in the SSME whose output drives the main pump turbines.

SABRE does not drive any turbine directly with H2 rich superheated steam. All are driven either by Helium or what they are pumping (at different physical conditions to the pumped fluid).

This has 2 effects. a) Parts exposed to the H2 rich super heated steam don't experience the multiplying effect of high mechanical stress. Everything happens through heat exchangers (which is why it has so many of them). Keep in mind these are REL's low pressure loss/high heat transfer heat exchangers so the difference between the pre-burner pressure and the main thrust chamber can be less that in the SSME.
b)There are no "Criticality 1" seals like the ones on the SSME between the preburner driving the HP LOX pump and the LOX flow. Poor seal design here required He purge gas to be 4x it's design flow, partly contributing to the 270lb GHe tank on each engine.

Eliminating such seals, de-coupling fluid heating from turbine drive and using fluid bearings should eliminate most of the major reasons the SSME was taken apart after every flight (although that got better as time went on NASA could have installed many more improvements).
Designing in IVHM sensors to both the engine and the structure (with engine data collection as part of the primary test goals from day 1 on the test stand to drive the health models)

"SSME The first 10 years" teaches a lot of lessons if read carefully.

Is this why the SABRE does NOT have a heat exchanger to reheat the hydrogen after the turbine but before the preburner? The purpose would be to improve airbreathing mode Isp and especially to shrink the LH2 tanks.

Offline 93143

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Re: The Reaction Engines Skylon Master Thread
« Reply #1628 on: 12/05/2012 01:31 am »
I'm surprised that liquids are tough to model at anything below thousands of bar, except for Hydrogen.

They're tough to model accurately.

If you know roughly what the conditions are going to be like, you can use a stiffened-gas equation of state or some such, and if you really don't care about compressibility you can just assign it a density (though this restricts the solution to incompressible methods unless you're doing multiphase and can guarantee the presence of at least a little gas in each cell).

But if you want a general model that can go from atmospheric or below all the way to the critical point and beyond, you need a general equation of state, and the accurate ones tend to be full of transcendental functions and high-order polynomials and are quite time-consuming to solve in each cell at every timestepping stage, not to mention horribly finicky with multiple roots near the critical point...  I use the Peng-Robinson equation of state, which is a cubic EoS and thus not terribly accurate (just using the original form of the equation, liquid oxygen at 90 K comes out 12% denser than it should), but it handles liquids, gases and supercritical fluids more or less seamlessly, it doubles as a decent high-temperature gas EoS, and it's relatively quick and easy to solve.  Even so, it seems to account for roughly two-thirds of my solver's total runtime.

That's not even getting into multiphase modelling...

You've also got to be careful about the stiffness of the fluid, if you're solving compressible.  Modern low-Mach-number methods can handle it, though.

Quote
Also, the wings provide lift...
and a lot of drag.

The wings take gravity losses and convert them to drag losses at an exchange rate dictated by the lift/drag ratio.  L/D tends to be much higher than 1, so this is generally extremely favourable.

The tradeoff comes in with having to stay in the atmosphere so much longer, which is why a slight increase in Isp from airbreathing doesn't help.  The improvement needs to be big.
« Last Edit: 12/05/2012 02:30 am by 93143 »

Online Elmar Moelzer

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Re: The Reaction Engines Skylon Master Thread
« Reply #1629 on: 12/05/2012 02:14 am »
Quote
he tradeoff comes in with having to stay in the atmosphere so much longer, which is why a slight increase in Isp from airbreathing doesn't help.

Yes

Offline 93143

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Re: The Reaction Engines Skylon Master Thread
« Reply #1630 on: 12/05/2012 02:35 am »
...well, that and the extra dry mass...

....according to my calculations, Skylon C1's orbital ascent takes almost 12 km/s of delta-V (airbreathing is net after intake losses, if I did it right).  Nearly half of that is before the switchover, yet the mass ratio for the airbreathing phase is only 1.185.
« Last Edit: 12/05/2012 02:59 am by 93143 »

Offline Jim

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Re: The Reaction Engines Skylon Master Thread
« Reply #1631 on: 12/05/2012 03:16 am »
It also should be noted that SKYLON will be licensed as an airliner, not a rocket like Falcon. I don't think there's a valid reason for it to carry explosives, while most assume the Falcon will. Given a choice, passengers will prefer a vehicle that isn't carrying a bomb onboard.

Wrong on both counts.
a.  Since Skylon is an unmanned vehicle and until it is proven, it will probably carry explosives.
b.  The same is true for Falcon, but once the reusable Falcon is proven, there should be no reason for explosives.

Offline joek

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Re: The Reaction Engines Skylon Master Thread
« Reply #1632 on: 12/05/2012 03:53 am »
It also should be noted that SKYLON will be licensed as an airliner, not a rocket like Falcon.

Launches on orbital and sub-orbital vehicles are "permitted" or "licensed"; vehicles that provide such launches are not "licensed" or "certified" (as commercial airliners).  The broader question of how those issues will be dealt with is a work-in-progress and of keen concern to potential commercial providers.  To state that Skylon will or could be "licensed as an airliner" presumes a regime that does not exist, and AFAICT is without basis.

Offline 93143

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Re: The Reaction Engines Skylon Master Thread
« Reply #1633 on: 12/05/2012 04:15 am »
REL has explicitly stated that they're working towards certification airliner-style - as close to airliner certification as they can get given the differences.  There is, as you note, no regulatory regime for spaceplanes; they're working on that.
« Last Edit: 12/05/2012 04:25 am by 93143 »

Offline grondilu

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Re: The Reaction Engines Skylon Master Thread
« Reply #1634 on: 12/05/2012 04:27 am »
Quote
Also, the wings provide lift...
and a lot of drag. Rockets leave the atmosphere are quickly as possible to avoid the drag. An air breather needs air. That air also means drag. It is a blessing and a curse. Yes they need less oxidizer because they can use air. In return they loose some of the advantage due to added drag and heaver engines. You gain some, you loose some.

Except that you forget to mention than this drag is also very useful when it comes to reentry and landing.

Wings would not make much sense for a vehicle that is not supposed to come back to Earth.  But Skylon is.   Therefore wings are not a luxury at all.    So if you assume that during ascent the benefits of air-breathing just balance the drawbacks of drag, then in the end you win because you must add the benefits during the descent (with no drawbacks there).

To me the alternative envisioned by Musk, i.e. using rockets to break pretty much all the way down to ground, seems much less realistic.  This strategy considers Earth more or less like an atmosphere-less planet, which is not very accurate a model.
« Last Edit: 12/05/2012 04:50 am by grondilu »

Offline joek

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Re: The Reaction Engines Skylon Master Thread
« Reply #1635 on: 12/05/2012 04:46 am »
REL has explicitly stated that they're working towards certification airliner-style - as close to airliner certification as they can get given the differences.  There is, as you note, no regulatory regime for spaceplanes; they're working on that.

So also are XCor, VG, etc.  That they're working towards it or want it doesn't mean much as yet.  In particular, a reading of COMSTAC records suggests there is a significant gulf... The FAA proposed rules way-back-when and the reaction from commercial was less than joyful.  There doesn't appear to have been much movement since; although the FAA is on the hook to provide some guidance, I don't see much progress.  In short, given the present and foreseeable situation, REL is not going to get any sort of "certification" for their vehicle, airliner-like or otherwse.  While I wish it were otherwise, that appears to be the state of affairs.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread
« Reply #1636 on: 12/05/2012 06:37 am »
The FAA proposed rules way-back-when and the reaction from commercial was less than joyful.  There doesn't appear to have been much movement since; although the FAA is on the hook to provide some guidance, I don't see much progress.  In short, given the present and foreseeable situation, REL is not going to get any sort of "certification" for their vehicle, airliner-like or otherwse.  While I wish it were otherwise, that appears to be the state of affairs.
FAA rules apply in the US, not the UK, where REL is based. While such organisations aim to harmonize globally, like the telecomms regulators, I think differences are allowed.

IIRC REL is one of 2 companies in the UK looking at some form of winged launch vehicle and I guess between them they can lobby the CAA. Note that REL tend to state worst case assumptions. A 5.5Km runway is for a fully loaded Skylon aborted takeoff. Quite a bit of the test programme would be without significant payload, possibly on LH2 alone during airbreathing tests only. That would require a much reduced runway spec.

The joker (IMHO) is the unmanned requirement. But CAA & FAA do certify systems that have total control of passenger aircraft in some phases of flight. Autoland systems have a failure spec of 1x10^-9 failures per hour. IOW 1 failure in 1 billion hours of operation across all vehicles carrying this equipment.

Note that Shuttle software demonstrated this is possible and embedded processors and their programming are much more pervasive in life threatening situations (and the techniques to mitigate failure) than they ever were during Shuttle's design.
MCT ITS BFR SS. The worlds first Methane fueled FFSC engined CFRP SS structure A380 sized aerospaceplane tail sitter capable of Earth & Mars atmospheric flight.First flight to Mars by end of 2022 2027?. T&C apply. Trust nothing. Run your own #s "Extraordinary claims require extraordinary proof" R. Simberg."Competitve" means cheaper ¬cheap SCramjet proposed 1956. First +ve thrust 2004. US R&D spend to date > $10Bn. #deployed designs. Zero.

Offline Kharkov

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Re: The Reaction Engines Skylon Master Thread
« Reply #1637 on: 12/05/2012 07:46 am »
IIRC REL is one of 2 companies in the UK looking at some form of winged launch vehicle and I guess between them they can lobby the CAA. Note that REL tend to state worst case assumptions. A 5.5Km runway is for a fully loaded Skylon aborted takeoff. Quite a bit of the test programme would be without significant payload, possibly on LH2 alone during airbreathing tests only. That would require a much reduced runway spec.

Two companies looking at a winged launch vehicle? Assuming you're not talking about Virgin Galactic, who's the 2nd?

As for runway lengths, perhaps Mark Hempsell, should he return, or someone else would care to speculate on the takeoff run of a (prototype?) Skylon with... oh, let's say a 50% liquid hydrogen load & nothing in the cargo bay.

For that matter, does anyone know how the Expansion/Deflection nozzle program is going?
Even Entropy Isn't What It Used To Be

Offline flymetothemoon

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Re: The Reaction Engines Skylon Master Thread
« Reply #1638 on: 12/05/2012 01:38 pm »
To me the alternative envisioned by Musk, i.e. using rockets to break pretty much all the way down to ground, seems much less realistic.  This strategy considers Earth more or less like an atmosphere-less planet, which is not very accurate a model.

On the other hand, if Elon Musk makes it work, it fits his cost-saving, commonality model. A generic, flexible, modular, extensible design with re-usable components that fits multiple scenarios. For example, the man rated capsule to be able to escape, land and take-off using the same (or very similar) system.

You can't scale Skylon, but Alan Bond hopes this Skylon design is but version 1 of a whole new fleet of space-planes. Here's to hoping the Jumbo-Skylon and Jumbo-A2 style craft are flying before I croak it.

« Last Edit: 12/05/2012 01:47 pm by flymetothemoon »

Offline flymetothemoon

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Re: The Reaction Engines Skylon Master Thread
« Reply #1639 on: 12/05/2012 01:44 pm »
Two companies looking at a winged launch vehicle? Assuming you're not talking about Virgin Galactic, who's the 2nd?

Possibly Bristol Spaceplanes?

http://www.bristolspaceplanes.com/


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