The Reaction Engines Skylon Master Thread (1)

[Elmar Moelzer]

The differences between the two ideas wont be that large in the end.

Akin's first law of spacecraft design.  Go find it.

Point taken. It is only an opinion, but it is based on the numbers provided by the two competitors. Given, they might be wrong and not all numbers needed are available. This is why it is only an opinion.

[93143]

What two competitors?  SpaceX is using a two-stage kerolox design.  Not comparable in the way you seem to be trying to compare it.

The goal with Skylon is SSTO.  REL sees major advantages in avoiding the multi-stage paradigm, at least for LEO launch, and with these engines they can.  All-rocket might work too, but the performance will not be close.

I replaced my previous post; the new one may be clearer and slightly more polite...

[BobCarver]

@RanulfC: "Not exactly following where he's going here, cooling the intake air isn't going to "help" the combustion process all that much in fact it will make it less efficent overall. Which would pretty much REQUIRE the ability to re-add the combustion heat prior to the air coming into the combustion chamber. So ... More equioment for less performance???"

According to the Carnot theory (see http://en.wikipedia.org/wiki/Carnot_efficiency#Efficiency ), efficiency is increased by upping the difference between the cold and hot sides of a heat engine. Therefore, reducing the inlet temperature results in increasing that difference.

[93143]

The cold side of this particular heat engine is the exhaust, not the intake.

[Elmar Moelzer]

What two competitors?  SpaceX is using a two-stage kerolox design.  Not comparable in the way you're apparently trying to compare it.

They are still competing in the launch market.
Yes, I know that they are very different approaches to the same problem (lowering the cost of putting payloads into LEO). If you read back to what I said, then you would understand how I meant that and that you are arguing semantics more than actual opinion.

[RanulfC]

The differences between the two ideas wont be that large in the end.

You're handwaving.  My math (based on REL's math) seems to say otherwise.  I'm seeing a payload that's twice what an all-rocket SSTO can get as a fraction of dry mass, or four times as high as a fraction of GTOW, with correspondingly less sensitivity to structural mass growth and engine underperformance.

One thing I've noted in the "discussion" here is that there is a strict tendency to try and "seperate" SSTOs into "all-rocket" and "SKYLON" when in fact there are and were many proposals for various OTHER "not-all-rocket" SSTOs.

This includes VTVL air-"augmented" SSTOs such as the "Spaceliner" which used a Supercharged-Ejector-Ramjet engine with or without Scramjets and/or LACE (Liquid Air Cycle Engines which are what the SABRE actually is) for propulsion.
Skylon's SABRE engines are actually "air-breathing" rockets but they are far from the only ones that have been developed or tested.

One thing to keep CLEARLY in mind during the discussin is that while MATH doesn't "lie" it only works to the limits of the assumptions made during the formula set up

Randy

[RanulfC]

@RanulfC: "Not exactly following where he's going here, cooling the intake air isn't going to "help" the combustion process all that much in fact it will make it less efficent overall. Which would pretty much REQUIRE the ability to re-add the combustion heat prior to the air coming into the combustion chamber. So ... More equioment for less performance???"

According to the Carnot theory (see http://en.wikipedia.org/wiki/Carnot_efficiency#Efficiency ), efficiency is increased by upping the difference between the cold and hot sides of a heat engine. Therefore, reducing the inlet temperature results in increasing that difference.

The cold side of this particular heat engine is the exhaust, not the intake.

What HE said

Especially in this case as the reduced input temperature of the air into the combustion chamber has to be made up with added heating to reach the desired temperature. Which leads right back to the later suggestion of adding a heat-exchanger to "pre-heat" the incoming air by taking it from the exhaust

Deep-Precooling of incoming air works for high mach speeds and hypersonic speeds but it's not so great for subsonic speeds. It is supposed to increase air density for greater mass ejection, but actually adding mass (such as water or inert gas injection) seems to help more and of course it actually reduces you ISP because of the addition of fluids into the stream. (It CAN however give some pretty kick-arse thrust augmentation though! )

Randy

[Elmar Moelzer]

You're handwaving.  My math (based on REL's math) seems to say otherwise.  I'm seeing a payload that's twice what an all-rocket SSTO can get as a fraction of dry mass, or four times as high as a fraction of GTOW, with correspondingly less sensitivity to structural mass growth and engine underperformance.

Also, it's "lose", not "loose".  This is an extremely common error, but it is an error.

Where was I talking SSTO? I was comparing Skylon to the reusable Falcon 9 which is TSTO!
I hope your second language is as good as my English.

[BobCarver]

The cold side of this particular heat engine is the exhaust, not the intake.

Wrong.

[93143]

What two competitors?  SpaceX is using a two-stage kerolox design.  Not comparable in the way you're apparently trying to compare it.

They are still competing in the launch market.
Yes, I know that they are very different approaches to the same problem (lowering the cost of putting payloads into LEO). If you read back to what I said, then you would understand how I meant that and that you are arguing semantics more than actual opinion.

Where was I talking SSTO? I was comparing Skylon to the reusable Falcon 9 which is TSTO!

No, you may have meant that originally, but your attempt at a technical argument sounded like it was disregarding staging and fuels entirely and just comparing airbreathing+extra losses to all-rocket.  To anyone who knows multivariable calculus (and probably many who don't), this implies that everything else is held constant, so as to create a fully-constrained argument.  Given that, airbreathing with SABRE comes out very much on top.

The argument was originally about SpaceX vs. REL, but the way it was being argued was apples-to-oranges.  An all-rocket SSTO makes for a sensible comparison consistent with the argument that was being made.

I hope your second language is as good as my English.

It's not just you.  Sometimes it seems like half the people on the Internet get that wrong.  I've seen veteran Apollo engineers use "loose" instead of "lose" too.  Very aggravating...  but off topic.  Just take it as an FYI.

One thing I've noted in the "discussion" here is that there is a strict tendency to try and "seperate" SSTOs into "all-rocket" and "SKYLON" when in fact there are and were many proposals for various OTHER "not-all-rocket" SSTOs.

We're not comparing Skylon with alternatives.  We're comparing airbreathing with all-rocket, in the context of a Skylon thread.  Naturally Skylon is going to end up the champion of airbreathing...

The cold side of this particular heat engine is the exhaust, not the intake.

Wrong.

Who cares what the intake temperature was?  It's an open cycle.  You can only use the energy that comes out before the working fluid leaves the system.

[Elmar Moelzer]

No, you may have meant that originally, but your attempt at a technical argument disregarded staging and fuels entirely and just compared airbreathing+extra losses to all-rocket.  To anyone who knows multivariable calculus (and probably many who don't), this implies that everything else is held constant, so as to create a sensible argument.  Given that, airbreathing with SABRE comes out very much on top.

I dont know about you, but I was talking about Musk who was quoted as seeing little advantage in the Skylon over his approach.
I argued that they are two very different approaches and that the Skylon will loose some of its advantages over Musks concept for the reasons I mentioned. Becaue of that, Musks opinion is not "completely uninformed" as some argued.
Capiche?

[93143]

Dammit, slow down.  Read my post again.  I edit a lot after posting...

Musk's big advantage is staging.  It's such a big advantage that airbreathing is actually not very useful for a TSTO.  And from his statements, that seems to be what he was talking about; he gave no indication of being aware that the question was about an SSTO.

They aren't even approaches to the same thing.  SpaceX may get a fully-reusable launcher.  But REL is aiming for a "space shuttle" with any-time intact abort, orbital maneuvering, satellite retrieval, automated rendezvous and docking, and low-gee-loading heavy downmass capabilities, not to mention self-ferry capability, all in the same vehicle.

[simonbp]

Yes, that's exactly it. If you restrict yourself to two-stages, and have a not-ridiculous flight rate, air-breathing doesn't make that much sense. If you want to do one stage, air-breathing is an enabling technology.

Skylon has a much higher initial cost, but potentially much lower operating costs, than something like a reusable Falcon. SpaceX's approach is to build the simplest system and iterate with it, while REL is putting a lot of effort into the front end to try to outsmart the problem. Since neither has been demonstrated flying to orbit and back, it's hard to really argue which will do better.

[Elmar Moelzer]

Dammit, slow down.  Read my post again.  I edit a lot after posting...

Musk's big advantage is staging.  It's such a big advantage that airbreathing is actually not very useful for a TSTO.  And from his statements, that seems to be what he was talking about; he gave no indication of being aware that the question was about an SSTO.

And they aren't even approaches to the same thing.  SpaceX may get a fully-reusable launcher.  But REL is aiming for a "space shuttle" with orbital maneuvering, satellite retrieval, automated rendezvous and docking, and low-gee-loading heavy downmass capabilities, all in the same vehicle.

I still dont get your problem with what I said.
I was talking about Elon Musks comments not being uninformed for the very reason that you are mentioning. I am not sure about the capabilities that you are attributing to Skylon here. It will for the most part be unmanned. So I am not sure about the retrieval part.
SpaceX dragon has some downmass capabilities too.
Yes, it is apples and oranges, which is what my point was (both systems have advantages and disadvantages and both have somewhat different goals).
Grrr, so can we please stop arguing about what I meant ( I do know that quite well, thanks) and about the actual issue.
Thanks

[Elmar Moelzer]

Yes, that's exactly it. If you restrict yourself to two-stages, and have a not-ridiculous flight rate, air-breathing doesn't make that much sense. If you want to do one stage, air-breathing is an enabling technology.

Skylon has a much higher initial cost, but potentially much lower operating costs, than something like a reusable Falcon. SpaceX's approach is to build the simplest system and iterate with it, while REL is putting a lot of effort into the front end to try to outsmart the problem. Since neither has been demonstrated flying to orbit and back, it's hard to really argue which will do better.

Thanks, that was the point, I was trying to make.

[93143]

People are not saying he was uninformed because he got the disadvantages of airbreathing wrong.  He didn't.  They're saying he was uninformed because he gave an answer that seemed to assume a generic TSTO, when the question was about a specific SSTO.

I am not sure about the capabilities that you are attributing to Skylon here. It will for the most part be unmanned. So I am not sure about the retrieval part.

It has to be able to retrieve its own upper stage after a geostationary mission.  Presumably it wouldn't be that hard to have it do the same with other satellites, especially if it were equipped with an RMS (no indication that it will be, but it certainly could be.  With Falcon the option simply doesn't exist; you'd need to design a separate spacecraft to do the retrieval and reentry).

[Elmar Moelzer]

People are not saying he was uninformed because he got the disadvantages of airbreathing wrong.  He didn't.  They're saying he was uninformed because he gave an answer that seemed to assume a generic TSTO, when the question was about a specific SSTO

I think there were cases of both. I am pretty sure that Musk referred to advantages of an air breathing system compared to his TSTO approach, not to some non existing off topic VTOL SSTO.
I actually think that he was thinking about Stratolaunch which he had just parted ways with at the time (and thus assumed that people were likely asking him about that).

[93143]

some non existing off topic VTOL SSTO.

If you answer a question about Skylon by saying that airbreathing offers no real advantage, without mentioning staging, an all-rocket VTOL SSTO is the obvious implication.  (And the discussion here was all in that vein; not mentioning any difference in staging strategy, which is a dominant difference and invalidates basically all of the discussion.  Keep in mind that this is a Skylon thread).

Except that he was plainly talking about a TSTO, what with his "larger first stage" and "air-breathing hybrid stages" comments.  Hence the inference that he didn't know what Skylon was when he answered the question (the guy in the video, at least, didn't happen to mention the SSTO aspect when he asked his question; he just asked about SABRE).  He did say that he might be working from incorrect assumptions...

[john smith 19]

I am and will continue to be quite impressed with the work that RE is doing and where they are going with it, but the various "claims" of this being "ground-breaking" and "paradigm-shifting" technology just are not being supported and I think they might want to take a step back and review the idea of "selling" it as such.

The correct comparison is with LACE engines. Designs for this (Linde IIRC were active in this area) never worked without frosting up or meeting their weight goals. They used spinning heat exchangers in an attemp to "fling" the cooling water off the tubes. It did not work. The heat exchanger architecture and mfg techniques are 6x better in terms of heat capacity per unit mass than any other designs.

Frankly? All the stuff RE has done still has not reached the TRL of the Supercharged-Ejector-Ram-Jet (SERJ) engine which was ready for flight testing in the mid-60s. And the "problem" area remains the same for the SABRE as the SERJ; Flight testing is going to be very expensive and getting funding and support is not going to be at all easy. SABRE just like the SERJ is going to require a flight test vehicle capable of all-aspect runway-to-hypersonic and back again flight regime and my "read" is that the proposed "Nacelle-Test-Vehicle" is going to be very much less than that.

SERJ was not designed to achieve SSTO. It was proposed to use as the 1st stage of a 2 stage LV. Uncooled airflows are much larger in volume. And "variable geometry" does not come much more so than retracting the turbine into a protective casing (SABRE variable geometry is mostly about changing valve settings). On the subject of "historical" air breathers the last (airbreathing mostly to orbit) US programme was the X30. As "facing the heat barrier" records it did not end well.
The Nacelle Test Vehicle is a rocket powered vehicle to refine the design of the Nacelle. It is not designed to go above the transition speed. 
The rest of the flight path and vehicle development will be done by what REL refer to as the "boilerplate" or Y Skylon vehicle. That will be expensive but as  you may have noticed each phase  builds on the last and yes the cost multiplies as the vehicles get bigger.

[Elmar Moelzer]

If you answer a question about Skylon by saying that airbreathing offers no real advantage, without mentioning staging, an all-rocket VTOL SSTO is the obvious implication.

I answered a question about Musks comment and I never said that it offers no real advantage. I said that it looses some of its advantages because of all the reasons me and others named. All that means that is not the miracle cure it all super solution to the problem. It is just one possible solution and a very expensive one to develop.
Grrr and can we please end this discussion about what I allegedly meant to say. It is pointless and OT.