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Comment by Mithaldu | Hacker News Reader

Can someone explain for a layman what this cooling technology is useful and necessary for?

Edit: Thanks for the excellent explanations. Upvotes all around. :)

The guys at Reaction Engines are trying to build a jet engine that will operate at very high speeds. In order to make this work the incoming supersonic air has to be slowed to subsonic speeds and compressed. This has the unfortunate effect of also making the air very hot and much less dense than it might otherwise be. This limits the effectiveness of the engine. This cooling technology can be used to cool the hot air before injecting fuel for combustion. This can increase the maximum speed at which the jet engine can operate.

The reason Reaction Engines is working on this type of jet engine is because they want to use it for an airbreathing single-stage-to-orbit re-usable spaceplane. (Technically, the Reaction Engines design is a hybrid that switches to a pure rocket mode at higher speeds.)

Some further reading:

Precooled jet engine: https://en.wikipedia.org/wiki/Precooled_jet_engine

SABRE (rocket engine): https://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29

Single-stage-to-orbit: Airbreathing SSTO: https://en.wikipedia.org/wiki/Single-stage-to-orbit#Airbreat...

Skylon (spacecraft): https://en.wikipedia.org/wiki/Skylon_%28spacecraft%29

Perhaps not all of them are 'guys'. Can we stick to the gender-neutral 'boffins' please?

'guys' is gender neutral in the US. 'boffins' is not used at all in the US, and sounds vaguely offensive, even if its meaning is not.

Well that is bizarre. When did guys become gender neutral in the US? How did that happen?

At some point in the last twenty years or so. I can perceive the gender of the term. However, my wife is much younger than I, and she and her female friends exclusively use the term "guys" to refer to themselves and cannot comprehend that I can perceive gender in the term. I mean, intellectually, they know it use to connotate gender, but it is silly to them in the same way that it's silly to us that strawberries are berries that grow in straw. In the US, and apparently also depending on your age, "guys" is gender neutral. And as a hobbyist linguist, "we do not prescribe language, we describe it"...

As a hobbyist linguist, do you have any thoughts about how it might have happened?

Honestly I think it's just a matter of a younger crowd being exposed to the term only in its gender-neutral form, and seizing on that use.

Started exactly in June 1985. In the film Goonies, Sloth called out 'hey you guys' to a gender mixed group. Never been the same.

https://www.youtube.com/watch?v=OmTNWVh2gSw

Well that explains it, thanks :(

My comment was mostly in jest, but to address one point seriously, 'boffins' is fine, if nowadays rare, in British English, and since Reaction Engines is British company, its use would be completely fine here.

'guys' is only gender neutral in specific circumstances. See http://jvns.ca/blog/2013/12/27/guys-guys-guys/

Many members of my female-only netball club use 'guys' to refer to ourselves...

"Folks" is better. I use it as a drop-in replacement for "guys". Not that I've met anyone who cares.

I'll give it a try.

Rockets require dense oxygen to burn. Near the edge of the atmosphere the air is not dense. Cooling the air increases the density. Cooling the air a lot will produce the dense oxygen required by a rocket. Air also contains water vapor. Cooling the air will increase the density of this water vapor. Forming liquid water then ice. This water and ice will block air flow. Which will prevent a cooler from producing the oxygen required. To prevent ice this cooler uses antifreeze. Methanol is sprayed (?) into the air being cooled. The methanol is then collected from the air and recycled. The collected liquid will contain water absorbed from the air. This prevents the ice and removes the water. Producing the dry, cold, and dense air for the rocket. The innovation is using the methanol efficiently. The methanol introduced to the air absorbs water. This decreases it's antifreeze ability. This also increases the liquids volume. Where the oxygen is collected for the rocket the air is dense, low volume, and dry. This is where a low volume of pure methanol is required. From that point forward the air is less dense, warmer, and less dry. A larger volume of less effective antifreeze is required. By recycling the liquid and pumping the liquid forward they match the requirements with the liquid.

That's the best I can do YMMV :-) Cool stuff!

For a spaceflight revolution?

'The design comprises a single combined cycle rocket engine with two modes of operation.[4] The air breathing mode combines a turbo-compressor with a lightweight air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram-compressed air leading to an unusually high pressure ratio within the engine. The compressed air is subsequently fed into the rocket combustion chamber where it is ignited with stored liquid hydrogen. The high pressure ratio allows the engine to continue to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed which gives a very lightweight engine—essential for reaching orbit. In addition, unlike the LACE concept, SABRE’s precooler does not liquefy the air letting it run more efficiently.

...

The combination of high fuel efficiency and low mass engines permits a single-stage-to-orbit approach, with air breathing to mach 5.14+ at 28.5 km altitude, and with the vehicle reaching orbit with more payload mass per take-off mass than just about any non-nuclear launch vehicle ever proposed'

https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

It's been a while since I read about the sabre engine so someone please correct me.

One of the most promising developments in aviation and (one day) rocketry are ram jets and scram jets which breathe in atmosphere for fuel oxydizer instead of keeping it on board along with the fuel. Scram jets operate above the speed of sound so the air rushing into any intake will essentially be superheated which is a problem since the fastest you go the more fuel you need to burn and thus you need more and more oxidizer the faster you go (meanwhile, your intake air is explosively expanding). A passive supercooler aplows you take in more air, compress it, and feed the oxygen to your engine.

Considering that the space shuttle booster rockets used a 6:1 mass ratio of oxydizer to fuel, air breathing engines powered by these supercoolers can change the economics of rockets.

> Considering that the space shuttle booster rockets used a 6:1 mass ratio of oxydizer to fuel, air breathing engines powered by these supercoolers can change the economics of rockets.

It doesn't change it too much because it's not feasible to go above Mach 10 or so in the atmosphere. (In fact, another comment on this page says air-breathing typically cuts out before Mach 6.) Low Earth orbit is something like Mach 25, so most of the accelerating is done in a vacuum where there's no air anyways. My vague understanding is that the performance enhancement is fairly modest on an absolute scale, but it happens to be important because it is just enough to make single-stage-to-orbit feasible.

The benefits may be small or large depending on how you look at them. Suppose you need 95% of your launch mass to be propellant for a pure-rocket SSTO design, and suppose you only need 85% of your launch mass to be propellant for a hybrid SSTO like Skylon. This seems like a small savings, but if you look at the "dry-weight" instead -- the mass that you put all the way into orbit -- the air-breathing design puts three times as much mass into orbit. This simplifies the design greatly, allowing you to add things like wings, a more robust fuselage, thermal protection tiles, and covers the extra mass-cost of the heavier engines that you need.

So you may end up with two designs with about the same launch mass and the same payload to orbit, but one of them is re-usable and one isn't.

I'm personally more than a little skeptical that the economics will ever work out, but it's an interesting idea, and it's not completely crazy.

Here are some details: https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

In summary, cooler air is more dense than hot air, leading to a high (desirable) compressor pressure ratio. It also allows to make the engine more lightweight due to reduced heat tolerance requirements, making it possible to reach orbit.

"I'm not a rocket scientist", but to me this sounds like it would be a leap forward in the "rocket equation" department. Anyone care to comment?

the rocket equation is merciless as usual, that's why this approach is so helpful - it allows to accelerate without shedding mass, or more precisely, a fraction of mass that would be required for the same delta-v using a conventional rocket engine. in the end you have to accelerate less fuel before switching over to rocket mode. mach 5 is about 1.8 km/s, which is 20% of orbital velocity. falcon 9 reusable first stage separates at mach 6 (at a much higher altitude, though - i hear 80km and also heard mach 10 numbers). if you can do what the falcon 9 can do without the first stage... that's pretty huge.

The liquid oxygen is mixed with liquid hydrogen and burned directly by the rocket as fuel. It's highly reactive and combustible so it makes a pretty good one.

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