Talk:Reaction Engines SABRE

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Ok, this piece seems to me to be difficult to write, it's a fairly complex engine, and the information probably isn't complete. But we do the best we can.

I've made some corrections to the piece which I believe are correct. They include:

a) the helium loop is gaseous, not liquid (see section 4.1 JBIS_v57_22-32.pdf)

b) the air isn't separated into nitrogen and oxygen anymore with Sabre, unlike LACE

c) the helium loop isn't pumped, on the contrary it does the pumping ("Brayton cycle"). Check out the diagram 4 and the discussion in section 4.1 in the above document

There's still some stuff I don't know for sure:

I'm still not clear whether SABRE actually liquifies the air- I've a feeling that it only cools and compresses it; but I haven't seen anything written in black and white either way. Liquifying it would probably be a bad idea, since it uses more hydrogen and doesn't actually improve the efficiency. I noticed that the experimental precooler only cools the air down to -80C; AFAIK it's not written in black and white anywhere whether that's the same temperature that they would employ on SABRE, but if so, then that's far too warm to liquify the air.

If anyone knows of a reference we can point to, I'd appreciate it.

Incidentally, it might be a good idea for someone to ask Bond and co. whether we could include their engine diagram and an external view of the engine here- a diagram or two would be very desirable.

-WolfKeeper

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The only reference I can find on the liquifaction/cooling the air issue is section 4.1, where they talk about an air compressor after the precooler. Given that liquid air is incompressible; this implies that the output from the precooler is gaseous.

-WolfKeeper

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Actually, there's another, rather clearer description of why SABRE does *not* liquify the air in JBIS_v56_108-117.pdf, page 115.

Given this, I'm going to change the article to remove reference to liquid air with SABRE.

-WolfKeeper.

[edit] Looking very good now

Thanks for the heads up WolfKeeper. I have read the articles in question and I think the article really reflects the actual system much better now.

I do find it interesting that dragging around "dead" engines for much of the flight profile still results in massively better performance overall. But I suppose that should not be expected, after all, we have reheat/afterburners.

Maury 22:21, 13 July 2005 (UTC)

just one comment -80°c may liquify air at high pressure, at Mach 5,and 60,000ft 20,000m the stagnation pressure should be about 31.7 bar stagnation temperature 1335 k (isentropic compresion), liquifyed air (any liquid) is easier to pump up to a high pressure (as they are incompresable), and high presures are important for cycle efichiencies. the problem with liquifying air however is that not all the gasses will condence at the same temperature, and much erosion can be expected if the liquid droplets are traveling at any speed in the heat exchanger. incidently the loss of entropy for such heat exchangers are probably horrific.

They're not going to liquify the air.- (User) WolfKeeper (Talk) 22:02, 10 June 2008 (UTC)

[edit] no sign of ramjet.

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

the cycle show a conventional turbojet mode and a rocket mode. there is no direct air to chamber bypass which is what a ramjet is. rather air either enter the turbo or bypass the entire engine in the 3d model. another unusual point is that 'ramjet' was not mentioned once on the official site. therefore is the details on this article correct? no source appears to be given. Akinkhoo (talk) 02:07, 22 February 2008 (UTC)

The diagram as it says, is a simplified cycle. If you read the papers they do describe it- the problem is that the amount of hydrogen you need to boil off to cool the air down is too much to burn in the main engine, and they use a ramjet for that.- (User) WolfKeeper (Talk) 03:43, 22 February 2008 (UTC)

[edit] Description [inaccuracies]

1) I think that the next statement in the present version of this article is wrong: 'SABRE uses two "pure" rocket engines surrounded by a ring of smaller engines similar to ramjets'.

If you look here: http://www.reactionengines.co.uk/sabre.html (look at the diagram of the engine)

and here: http://www.reactionengines.co.uk/downloads/JBIS_v57_22-32.pdf (read last paragraph in page 29)

it is clear that their design (Reaction Engines Limited's) is not the merge of two different kind of engines, rockets and jets, there is only one, the rocket, but capable of burning either compressed_air+LH2, or LOX+LH2.

In fact they claim that THAT is one of its main advantages, due to the reduction in weight from not having two whole systems, one for the airbreathing mode and another for the rocket mode. The only two main components that dont "double" (don't work in rocket mode) are the precooler and the air turbocompressor.

2) Another statement that I think is wrong, under 'The Engines', it's read: "After being launched and brought to speed by a short burst of the rockets, the jets are started".

In the last link I provided they say that this engine (unlike ramjets) is capable of giving static thrust, that means that it works and give push even at cero speed, so there is no need to use the rocket for takeoff (and of course there are no "jets").

3) And finally, under 'Performance' is this: "The losses from carrying around a number of engines that will be turned off for some portion of the flight would appear to be heavy...".

Again, there are not multiple whole engines in the Sabre design, just one kind with a few added components that only work in the airbreathing mode.

Since I'm not a registered user in this version of the WP I'm not going to make the changes right away, but let you check them up and decide.

Regards. CharlieM. —Preceding unsigned comment added by 62.175.122.58 (talk) 01:13, 2 June 2008 (UTC)

If you look at:

Around the central engine there's all these conical shapes with holes in them. I'm pretty sure that they're flameholders. If you trace the airflow path it comes in around the outside of the heat exchanger after passing through the shock cone.

I'm pretty sure that they're the ramjets- they're used to burn off the excess fuel produced by the hot air boiling the hydrogen off in the heat exchanger. This also matches what is described in Reaction engines literature. These ramjets aren't on the simplified schematic though.- (User) WolfKeeper (Talk) 02:47, 2 June 2008 (UTC)

You might be right, although it's difficult to be sure since the available info is so scarce. What I make of it (see "4.3 SABRE Installation" in pages 31 and 32) is that at low machs the Sabre engine swallows too much air, and that the excess is let to scape through "an internal bypass system" (sic), but first it is mixed with hidrogen and burned. They call it a "bypass burner system" (sic) not a ramjet, and explain that its function is to heat the air so its exhaust velocity is enough to not have a drag effect (so I understand it's not to produce thrust, I suppose the main engine is more efficient at that). Maybe it also doubles as the system to get rid of the excess hidrogen, but that is unclear.

I think the 'bypass burner system' certainly constitutes a ring of ramjets by most people's standards. It gets its compressed air direct from the inlet shock, and it doesn't even go through the heat exchanger, and each one has its own de laval nozzle.- (User) WolfKeeper (Talk) 21:56, 10 June 2008 (UTC)

About the rocket assisted takeoff, according first paragraph on page 29 they contemplated it on the first versions of the Skylon, but later on drop it.

CharlieM.