Reaction Engines Sabre/Scimitar
These are pre-cooled turbojet engines; they use a pre-cooler that uses liquid helium to cool down the airflow going into it, the heat is transferred to liquid hydrogen which acts as a heat sink -- that vaporizes and is burned off as fuel.
Incoming-air is cooled just above the point at which it would liquefy (-150 C) as I understand. There was a guy who worked at Reaction Motors/Reaction Engines (I forgot which it was called) and he described the airflow going through the Sabre engine going from a few thousand degrees down to -150 C, so I assume the pre-cooler adjusts to keep cooling the air down to -150 C regardless of mach number. Since LH2 is used as a heat-sink for the refridgeration system which runs on liquid helium, wouldn't that LH2 get really hot and as a result drive up the turbine temperatures? |
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I recently attended a Royal Aeronuatical Society Lecture on Reaction engines. Interesting stuff, the main aim of the cooling (helium) is so that the engines can go hypersonic as hot air entering them would be uncompressable. The options then are to fuel the engines with hydrogen and make them a hybrid.
The technology that is going into producing small but very efficient coolers I thought was amazing. The gentleman that did the lecture was Alan Bond from Reaction Engines Ltd in Oxfordshire. This is their website: Reaction Engines Ltd : Space Propulsion Systems |
Why would the air be uncompressible?
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I'd imagine uncompressible in the sense that by compressing already hot air you make it so hot as to melt the structure. Hence the need to cool it prior to compression. Think intercooler on piston engine supercharging.
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Ah, SSTO yet again. I always cringe at these Holy Grail projects, ssto, supersonic airliners, over-elaborate vaccum cleaners etc.
The simple arithmetic of rocket science says everything you throw away is advantageous. Carry no surplus baggage. Works well in planetary life, works even better in space. You need to minimise the amount you bring back to earth and you maximise payload by dumping as much as you can on the way up. Perhaps breathe a little air at low altitude, but dump the air-breathing bits before they become dead mass. Manufacturing cost is minimised because the bits you throw away can be produced in greater volumes and become vanishingly cheap. It's what the Russians do, it's what the Chinese do. We would be fools not to copy the world's leading nations. |
Originally Posted by mike-wsm
(Post 6290201)
The simple arithmetic of rocket science says everything you throw away is advantageous.
And we've already built SSTOs: both Atlas and S-II were capable of reaching orbit without dropping anything even if they were never used that way. What we've never built is a reusable SSTO, and if you're throwing the whole thing away anyway there's not much benefit to an expendable SSTO. BTW, I went to a talk on Skylon (the follow-on to Hotol) about ten years ago, and it seemed to be a pretty clever design. The problem as I see it is that there's no confirmed market big enough to justify anyone investing the amount of money required to actually build something like it and make it work. |
The Scimitar isn't intended as an SSTO engine; it's a pre-cooled variable cycle engine (turbojet/turbofan) to power a hypersonic airliner as part of the E.U.'s LAPCAT program.
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MG23: SSTO isn't about maximising payload it's about minimising cost, ... MG23: ... and the way to do that is to have a reusable spacecraft with fast turnaround. MG23: ... The problem as I see it is that there's no confirmed market big enough to justify anyone investing the amount of money required to actually build something like it and make it work. (In case I haven't been obvious, I'm with the lot that's just plain fed up with this pipe-dream swallowing up NASA's budgets, and then having to listen to yet another "But wait - we've got this New Technology. One more demonstrator and we will be at Technology Readiness Level 9.8. Here, we hired some artists to draw pictures so you can believe us.") |
Originally Posted by balsa model
(Post 6291937)
Blanket statement with no shred of supporting evidence in our entire history of orbital launches. And quite a bit of clues that it isn't so.
And demonstrating new technology is precisely what NASA should be doing. |
Originally Posted by Jane-DoH
(Post 6291922)
The Scimitar isn't intended as an SSTO engine; it's a pre-cooled variable cycle engine (turbojet/turbofan) to power a hypersonic airliner as part of the E.U.'s LAPCAT program.
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MG23: You won't get cost to orbit below a hundred dollars a kilo by throwing away your spacecraft ever time you launch. Our propulsion technology hasn't seen any revolutionary invention since the 60's. That's our limit. At least with dumb-boosters, we have some way to go in cost reduction via economies of scale, i.e. mass production. No such luck with SSTO. Each one (of the very few) would be build nearly by hand, and will be overbuild to take the repeated cycles and for extra fear of losing "the precious". MG23: Would you seriously suggest that throwing away an airliner every time it flies across the Atlantic would be an economical business model? MG23: And demonstrating new technology is precisely what NASA should be doing. Enter fiscal constraints and the issues are: 1. Is this the new technology we should be demonstrating? We can't afford to demonstrate them all. Expensive demonstrations of white elephants take away from the rest. 2. Is NASA still an efficient organisation to do it / define the goals then assign and oversee the contractors? I think that we are heading towards JB. Sorry Jane-DoH for this drift. More on the subject of your Reaction Engines, they sure seem to be pushing the edges of where airbreathing engines can go. IIUC, hellium is used as a heat circulation fluid: it moves the heat from the intake air to the cold "sink" of LH2. It won't boil until you get the whole content of the LH2 tank above its boiling point. It is presumably large, although I imagine that this must be a concern in the final phase of the cruise. I like their attempt at forecasting of costs, on their web page. There is an estimated R&D cost and eventual passenger ticket prices. Faster, longer legged, and cheaper than Concorde? |
http://i.telegraph.co.uk/multimedia/...l_1717952c.jpg balsa_model "Here, we hired some artists to draw pictures so you can believe us." |
MG23
And SABRE is the Skylon engine. balsa model Absolutely! If it cost less to produce a new expandable one than to recover and overhaul a reusable one that carries the same payload, then Yes! they sure seem to be pushing the edges of where airbreathing engines can go. IIUC, hellium is used as a heat circulation fluid: it moves the heat from the intake air to the cold "sink" of LH2. It won't boil until you get the whole content of the LH2 tank above its boiling point. I like their attempt at forecasting of costs, on their web page. There is an estimated R&D cost and eventual passenger ticket prices. |
Where does all the ice go? I mean at -150degC isn't all the water vapour going to solidify? What provision is there for dealing with this issue? (I'm old enough to remember the Britannia and that was only a U-bend) |
mike-wsm
I'm not sure exactly what measures they have put in place to deal with icing, but they have talked about dealing with that issue. BTW: The pre-burner is only used with the rocket-cycle right? |
Looks like it, Robyn. Not sure about those heat exchangers. I can remember the abject failure of the Rover gas turbine, ok on the track but when they tried to add heat exchangers for efficient road use they just couldn't make the throughput. And they were big complicated spinning things. We had one at college, running but without the exchanger. |
mike-wsm
Looks like it, Robyn. Not sure about those heat exchangers. |
Jane-DoH
And this goes to show why obsessing on economics is not always the best course of action. By the way, engineering vs. science: engineering has to contend with a plethora of diverse parameters to optimize. It's a dream when a customer says "cost is no object". Usually, it is an issue. If we followed our lives strictly by economics, old people, sick people, and mental ill people would probably be euthanized -- after all it costs a lot of money to keep these people alive and take care of them A plane/launcher being destroyed after their useful lifetime is over is sad. But let's not get overboard by comparing them with euthanasia of living people. You're right about ...transfer the heat into LH2 heading towards the engine... About pre-burners in jet engines: unless I mixed up the terms, such devices were used at least in the early designs. It had something to due with difficulty of injecting liquids into the chamber and getting a good mixing. With a pre-burner, they were injecting gas and the mixing was easy(ier). At least that's what I read. When I find where, I'll post it, but I'm sure we have real experts here who could clarify the issue. |
balsa model
roponents of SSTO argue for it because, they claim, that it must be a more economical way to get to space and some of us argue the opposite. Let's try to keep this in the spirit of Tech Log. A plane/launcher being destroyed after their useful lifetime is over is sad. But let's not get overboard by comparing them with euthanasia of living people. You're right about Honestly, it seems more practical to just tap the heat off the liquid helium, transfer it to the LH2 (as well as combustion chamber temperatures) to just vaporize it, then drive an expansion turbine off that. It looks like it would simplify things and would best exploit the extremely low temperatures of the Liquid Helium and LH2 in the cycle. About pre-burners in jet engines: unless I mixed up the terms, such devices were used at least in the early designs. It had something to due with difficulty of injecting liquids into the chamber and getting a good mixing. With a pre-burner, they were injecting gas and the mixing was easy(ier). At least that's what I read. When I find where, I'll post it, but I'm sure we have real experts here who could clarify the issue. |
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