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?

Reaction Engines SABRE - Wikipedia, the free encyclopedia
Reaction Engines Ltd : LAPCAT - SCIMITAR Engine

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?

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.

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.

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.

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.

How about being more precise about our goals: minimising cost per pound to orbit. Don't forget NRE.

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.

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.")

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.

And SABRE is the Skylon engine.

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".

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!

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?

MG23

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

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 :eek:

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)

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?)

How much would the R&D cost run, and how would the passenger ticket process compare to the Concorde?

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

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.

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.

Jane-DoH
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.

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 It makes more sense too (now that I'm looking at the diagram and not shooting from the hip:O).

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

Well, it isn't. Still, I wouldn't want to be junking too many spacecraft. There are only so many resources available on earth.

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.

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.

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

Yeah

The faqs (frequently asked questions) on Sabre/Scimitar include:

This appears to be a fudge. It is icing that is the biggest reason why Sabre/Scimitar will not in my considered opinion work and they simply dodge the issue by claiming to have a "secret" solution.

I don't believe they will get working heat exchangers and I don't believe they have any way of dealing with the icing problem.

Edit - The above opinion was based on the originally cited -150 deg C precooling and does not apply for 665 deg C precooling.

mike-wsm

Heat Exchanger, Scimitar Engine (Not-Classified)

Page 2 discusses the temperature changes encountered. From what it seems, the pre-cooler takes the temperature from 1,250 K down to 665 K (Truthfully those temperatures seem a touch high for Mach 5 flight as skin temperatures usually just come out to 600 C or 873.15 K), not down to -150 C/123.15 K regardless of mach number (I think the SABRE did operate this way though). This would probably eliminate the turbine-inlet temperature issue I was mentioning earlier (I don't know what the solution for the SABRE was, but it would explain why at around Mach 6, they switched over to rocket propulsion -- that fuel would be awfully hot)

So they didn't make one yet.

Paul Czysz Hypersonic Interview

mike-wsm

I've been looking for this for a long time! Thanks

Out of curiosity, do you remember anything about the Japanese ATREX engine, which had a pre-cooler and worked on a hydrogen-expander cycle? As I understand it, the design was cancelled in favor of a pre-cooled turbojet similar to the Scimitar, but it's pre-cooler seemed a lot smaller, lighter and simpler than the version used by Reaction Engines.

mike-wsm

Yesterday, I stumbled upon another link which better illustrates both the SABRE and Scimitar Engines as well as the aircraft designs involved :ok:

Voila!

mike-wsm

Fascinating design, though puzzling if you ask me. Firstly the inlet that it has seems to have a tiny throat with what must be enormous duct pressures, I don't know why you'd have so much air go through the porous part of the duct into the pre-cooler (though I understand why you'd have the air from the inlet going through). The pre-cooler looks smaller than the SABRE and Scimitar's though (though this could be an optical illusion).

Why do you have the air going through the pre-cooler from the sides and in a spiral-trajectory rather than just straight through? I don't really get it.


BTW: In the link about SABRE/Skylon and Scimitar/LAPCAT A2 -- there's a mathematica formula listed: T = 220 (1 + 0.2M^2N)

I don't really know what this formula is or what units the 220 is in (I assume Kelvin though I'm not sure); I don't really understand what the function of the N is. It looks almost like an exponent, but it's a subscript instead of a superscript. I don't know that math function.

Jane-DoH

I'll try to find some answers. May take a little time, I need to download and print the references so I can study them more fully, there is a limit to what my tiny human brain can do on-screen.

You do raise some interesting questions!

Jane-Doh
First, I'd like to express my appreciation for making us old guys (at least this one:}) catch up on the more recent developments on the cutting edge of technology.

The formula you posted is not quite correct, but that is caused by the problems with cutting and pasting mathematical formulas with limited editing tools. The original formula read: T= 220(1+ 0.2*Machnumber squared)

This is the stagnation temperature formula expressed in terms of Mach number (Mn). In this case, the 220 is the assumed initial (Freestream) temperature Kelvin. Check out the Wiki on stagnation temp here:
Stagnation temperature - Wikipedia, the free encyclopedia
Using the formula for stagnation temperature T0, they have assumed a starting free stream temperature, and simplified into terms of Mach number. (Good enough for back of the envelope work until you start to get disassociation of the working fluid)

Machinbird

Glad to be of help.

Okay the equation would go like this
1.) T0 = 220 (1 + 0.2(M)^2)
2.) T0 = 220 (1 + 0.2(5)^2)
3.) T0 = 220 (1 + 0.2(25))
4.) T0 = 220 (1 + 5)
5.) T0 = 220 (1 + 5)
6.) T0 = 220 (6)
7.) T0 = 1320 K

Which I'm guessing is the outside air temperature they're going to be flying Mach 5 at?

Jane-DoH

This is close enough to the figures you quoted above 1250K to 665K and removes any niggles from me about icing.

mike-wsm

Yeah, it's hot enough...

BTW: Dunno about the SABRE or Scimitar, but the ATREX, as I understand it to combat frost were using some kind of methanol injection system

Jane-DoH

Thin-walled high operating temperature heat exchanger and now methanol injection. This is one incredibly complex engine. What is the reliability over how many re-usage cycles?


I've had a chance to examine the references you gave and the spiral construction of the pre-cooler appears to be an attempt to make it behave like a counterflow heat exchanger. A conventional counterflow heat exchanger is continuous in that one fluid is gradually heated along its length as the other fluid travelling in the opposite direction is gradually cooled, At all points along the heat exchanger the temperature difference between fluids is the same, yielding the best performance. In the Sabre pre-cooler, the flow temperatures are stepped but tend towards the counterflow ideal. See spiralflow/crossflow in the wiki link above.

The formula does look ambiguous, but if you look carefully is Mn^2.

Check out the admiring video on jetblast/hamsterwheel, sung by heart-throb Christofer Drew / Never Shout Never.

mike-wsm

Well, the methanol injection was ATREX, not SABRE or Scimitar. Regardless, yes, it is a very complicated engine.

So, the air goes one way, and the coolant goes the opposite way?

Yes, in the diagram above the air is traveling radially inward and the coolant is traveling outward.

I don't know exactly how this relates to this but I remember reading something Paul Czysz mentioned about a pre-cooling system which entailed cooling the airflow into the engine above Mach 1.8 to avoid an excessive rise in temperature and optimize the benefits of ram compression or something and simultaneously increasing the engine RPM while doing this and likened it to Skylon. Maybe I got the exact explanation wrong, but do you know what I'm talking about?

Yes, something like that. It's all down to physics, if you want to go that fast and do it by burning fuel with air you have to do what is necessary.

Happily nobody has found a way of using air as both fuel and oxidant, by burning the nitrogen and oxygen, that would leave a real nasty NOx trail. Oops, why did I say that? Now they'll start doing it . . .

mike-wsm

How exactly does that work, increase density, then increase the RPM due to the density change, or to allow it to pull in more air, or something else?

Why does the LAPCAT A2 fly at such a low altitude? While 85,000 feet is high up, for Mach 5 it seems relatively low...

The only known mass market for spaceflight requires launch costs below $100 a kilo. At that point tourism starts to become viable enough to sustain a huge increase in payload to orbit, and your flight rate can be high enough to make an SSTO more profitable than an expendable.

Saying 'I can prove SSTOs don't make sense so long as I define the question so that SSTOs don't make sense' seems rather silly. In the medium term they're the only things that make sense if your goal is to build a sizable human population in space (longer than that and something like a space elevator may be viable and cheaper).

The big problem with launcher development right now is that there's no proven large market between communication satellites and tourists. If you're launching a billion-dollar comsat which will produce billions in revenue over its lifetime, then the difference between paying $200 million or $20 million to launch it is small, whereas if you're in the launch business, the difference between charging $200 million or $20 million a launch is enormous if it won't result in any significant increase in the number of launches.

So there is little economic incentive for developing new technologies to dramatically reduce launch costs until you can get them down to levels where ordinary mortals can afford them. Skylon, if I remember correctly, was estimated to cost around $500 per kilo back in the 90s, which puts it squarely into the 'who do you think your customer is?' range. Put a $10,000,000,000 price tag on developing a launcher with no known profitable use, and no-one in their right mind is going to fund it... particularly when you consider that the development cost is almost certainly optimistic.

Which is presumably why RE are now pushing for work on a hypersonic airliner instead of an SSTO. The problem is that there's not much more of a proven market for hypersonic airliners than for spaceflight at $500 a kilo.

The only operational 'reusable' launcher is the shuttle, which is barely reusable and has to carry a human crew with all the extra cost that entails yet still costs less per flight than a comparable expendable launcher. The problem with the shuttle is the huge fixed costs of the facilities and standing army required to support it; the variable cost of flying an extra shuttle launch is around $200,000,000 while the average cost over the entire program is closer to ten times that amount because the fixed costs have to be spread over a small number of launches.

That is why both reusability and high flight rate are important if you want to dramatically cut costs. So, for that matter, is not running your own 'spaceport' that adds a couple of billion dollars a year to your fixed costs when you could just buy launch slots elsewhere when you need them.

No, that's as far as we've had any incentive to go. When the only real markets for your services are happy to pay $200,000,000+ a launch with a 98% success rate, there's simply no incentive to do any better.

And even the shuttle, which is at best refurbishable and throws away around $100,000,000 in hardware each flight, costs less to overhaul than the cost of building an equivalent expendable launcher.

Should I even need to add that SpaceX are aiming to recover and reuse both stages of their Falcon launchers because they believe it will reduce costs compared building new ones each time?