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Reaction Engines Sabre/Scimitar

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Reaction Engines Sabre/Scimitar

Old 6th Mar 2011, 05:26
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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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Old 6th Mar 2011, 15:47
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Old 6th Mar 2011, 16:50
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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
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Old 7th Mar 2011, 04:31
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Why would the air be uncompressible?
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Old 7th Mar 2011, 05:27
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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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Old 7th Mar 2011, 10:12
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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.
 
Old 8th Mar 2011, 03:18
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Originally Posted by mike-wsm View Post
The simple arithmetic of rocket science says everything you throw away is advantageous.
To maximise payload. SSTO isn't about maximising payload it's about minimising cost, and the way to do that is to have a reusable spacecraft with fast turnaround.

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.
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Old 8th Mar 2011, 04:19
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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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Old 8th Mar 2011, 04:39
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MG23: SSTO isn't about maximising payload it's about minimising cost, ...
How about being more precise about our goals: minimising cost per pound to orbit. Don't forget NRE.

MG23: ... and the way to do that is to have a reusable spacecraft with fast turnaround.
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.

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.
Hm.. I guess the world is just too small for all you SSTO thinkers.

(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.")
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Old 8th Mar 2011, 05:30
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Originally Posted by balsa model View Post
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.
You won't get cost to orbit below a hundred dollars a kilo by throwing away your spacecraft ever time you launch. Would you seriously suggest that throwing away an airliner every time it flies across the Atlantic would be an economical business model?

And demonstrating new technology is precisely what NASA should be doing.
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Old 8th Mar 2011, 05:32
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Originally Posted by Jane-DoH View Post
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.
And SABRE is the Skylon engine.
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Old 8th Mar 2011, 18:44
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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.
I don't have to get the cost where you want it, to win this argument. I just have to point out that all reusable options have shown to be more expensive than using the throw-away variants. By a lot.
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?
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!

MG23: And demonstrating new technology is precisely what NASA should be doing.
In the ideallistic world of 1960's, yes.
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?
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Old 8th Mar 2011, 22:00
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"Here, we hired some artists to draw pictures so you can believe us."
 
Old 9th Mar 2011, 02:09
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And SABRE is the Skylon engine.
Yes, I am aware of this. I was pointing out that this kind of technology wasn't relegated only to SSTO's, but hypersonic air-breathing aircraft as well.


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!
And this goes to show why obsessing on economics is not always the best course of action. 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

they sure seem to be pushing the edges of where airbreathing engines can go.
They are very advanced concepts. If I recall correctly, McDonnell/McDonnell-Douglas did some research with some kind of engine-precooling back in the 1960's. It had to do with cooling the airflow to keep the turbine temperatures within limits, and maximizing ram-compression by cooling and densifying the air. I don't know the exact details but a professor who worked for McDonnell/McDonnell-Douglas named Paul Czysz did make mention of such a proposal and remarked that it was similar to Skylon (though I don't know exactly how similar -- figurative or literal)

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.
Okay, I thought the heat would be transferred into the fuel heading towards the engine's combustion chamber (On this note, could you use a pre-cooler using liquid-helium, transfer the heat into LH2 heading towards the engine, vaporize the LH2, and use the expansion of the H2 to drive an expansion turbine?)

I like their attempt at forecasting of costs, on their web page. There is an estimated R&D cost and eventual passenger ticket prices.
How much would the R&D cost run, and how would the passenger ticket process compare to the Concorde?
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Old 9th Mar 2011, 17:21
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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)

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Old 10th Mar 2011, 02:29
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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?
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Old 10th Mar 2011, 11:34
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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.
 
Old 11th Mar 2011, 02:40
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Looks like it, Robyn.
The Scimitar doesn't have a rocket-cycle. I don't really understand what the function of a pre-burner would be without a rocket cycle.

Not sure about those heat exchangers.
Yeah me neither. Especially with the HX3 mentioned on the Scimitar. I assume it's function is different because on the SABRE, HX3 was part of the rocket-cycle, which the Scimitar lacks.
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Old 11th Mar 2011, 04:30
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And this goes to show why obsessing on economics is not always the best course of action.
Proponents 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.
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
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
...transfer the heat into LH2 heading towards the engine...
It makes more sense too (now that I'm looking at the diagram and not shooting from the hip).

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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Old 11th Mar 2011, 08:47
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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.
Well, it isn't. Still, I wouldn't want to be junking too many spacecraft. There are only so many resources available on earth.

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.
I wasn't comparing the loss of an expendable plane to euthanasia; I was simply saying that if life was lived purely by economics... well you get the idea. I personally think anything taken to excess is generally bad.

You're right about
Which keeps bringing me back to one main issue: Wouldn't that be a lot of heat to be transferring into the LH2 that's going to be heated additionally by burning it in the engine's combustion chamber? You'd be absorbing a couple of hundred degrees from the outside air temperature when you're just running the engines on the tarmac -- when you get up to Mach 5, you'd have another 600 degrees extra to drain away from the airflow and transfer into the LH2. Would that have an adverse effect on driving up turbine temperatures?

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).
So they used a small pre-burner to vaporize the fuel so it would burn easier?

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.
Yeah
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