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The BBC News article Rolls-Royce and Boeing invest in UK space engine about Reaction Engines Limited (REL) and its Sabre engine states

REL is developing what it calls the Sabre engine. This power plant is designed to push a vehicle from a standing start all the way to orbit in a single step.

It would work like a conventional jet engine up to about Mach 5.5 (5.5 times the speed of sound) before then transitioning to a rocket mode for the rest of the ascent.

Key technologies include a compact pre-cooler heat-exchanger that can take an incoming airstream of over 1,000C and cool it to -150C in less than 1/100th of a second. This would permit Sabre to use oxygen direct from the atmosphere for combustion instead of carrying it in a tank with the weight penalty that implies. (emphasis added)

A temperature change of 1000C to -150C expressed in absolute terms is about 1273 K to 123 K, or more than a factor of 10 in absolute temperature, and in less than 10 milliseconds.

How does the pre-cooler work? How can it cool a continuous flow of air so quickly at such high volume, and still remain physically small and sufficiently light weight to be just one component of this amazing engine?

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above: "Sabre would work like a jet engine in the lower atmosphere and like a rocket motor in the high atmosphere." From BBC. Credit: REL

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    $\begingroup$ "How does the pre-cooler work?" – As far as I know, it doesn't. There has been a viability demonstration, which basically means that a bunch of scientists have waved their hands furiously enough to convince a bunch of politicians that given unlimited amounts of money, manpower, and time, there is a chance that it might actually work at some unspecified time in the future. Or something like that. $\endgroup$
    – Jörg W Mittag
    Commented Apr 15, 2018 at 9:51
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    $\begingroup$ @JörgWMittag That's incorrect. They've shown the precooler works, by running one on a test stand. That attracted investment not from politicians, but from American defence companies. $\endgroup$
    – Hobbes
    Commented Apr 15, 2018 at 10:54
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    $\begingroup$ They've apparently demonstrated it to DARPA, USAF, and Boeing's satisfaction. Just because they're not going to divulge hard-earned proprietary technology to the press doesn't make it "handwavium." $\endgroup$
    – PHChilly
    Commented Apr 16, 2018 at 15:43

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It uses lots of ridiculously fine tubing - about 50km in a unit with walls thinner than a human hair.

Each unit consists of about 20-30 modules, each consisting of thousands of closely arranged parallel tubes arranged in a single revolution spiral. Helium, chilled by the liquid hydrogen fuel (at below 20K) enters the spiral at the inside edge of the spiral and leaves at the outer edge.

The modules are interleaved, and the air passes through 20 sets of progressively cooler tubing from the outside inwards.

So basically it's a massive surface area that the air is passing over very closely. The short time it takes is due to the speed the air is passing through the system.

IIRC, more recent quotes from the company have it dropping 1000 C in 1/25th of a second.

For a slightly more technical description, the IAC paper "HEAT EXCHANGER DEVELOPMENT AT REACTION ENGINES LTD" is worth a read.

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    $\begingroup$ Do you have some references for further reading? $\endgroup$
    – Jack
    Commented Jul 25, 2018 at 22:20
  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$
    – user20636
    Commented Jul 26, 2018 at 10:57
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There is some information online. Summarising that, it seems to be a matter of incremental improvements. Compared to other heat exchangers, they use more smaller thinner-walled coolant pipes, have a clever geometry, etc. Wikipedia has some complementary information, including the reason for using a closed helium loop for the cooler, rather than directly using the LH2 fuel (avoiding hydrogen embrittlement). A key technology is the icing prevention system which involves injecting methanol into the air path. This article describes this a bit. They economise on methanol by capturing (now somewhat damp) methanol and reusing in places where the deicing requirements are less demanding.

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  • $\begingroup$ "incremental improvements" over what? This is a nicely sourced answer but without an understanding of how pre-coolers of previous air-breathing to vacuum transitioning engines work, this answer doesn't help me understand how this one works. Is it possible to add an explanation of the underlying principle? Thanks! $\endgroup$
    – uhoh
    Commented Apr 15, 2018 at 11:47
  • $\begingroup$ Is it just that the tubes are so small in diameter that molecules collide so frequently with them? Is laminar flow prevented in these tubes, or does cooling still take place convectively transverse to the flow, without direct contact? $\endgroup$
    – uhoh
    Commented Apr 15, 2018 at 11:56
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    $\begingroup$ Incremental over the state of the art in heat exchangers, I guess. Obviously this is a first in this application, but heat exchangers of one form or another are a very well developed technology. en.wikipedia.org/wiki/… $\endgroup$
    – Steve Linton
    Commented Apr 16, 2018 at 8:49

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