Cool Your Jets


Reaction Engines in the UK seems to be excited about something:

The Biggest Breakthrough in Propulsion Since The Jet Engine

Can you tell they might think this is a big deal? I for one hope they’re not exaggerating because this does have the potential to be a generational leap in engine technology.

If their pre-coolers can be made to work outside the lab, then the concept of airbreathing rockets (or rocket-based combined cycle, RBCC) isn’t so farfetched anymore. And cooling a fast-moving mass of air by over 1,000°C in a hundredth of a second ain’t no small potatoes.

Take a look at this cutaway view of their SABRE engine: the heat exchangers are those baffled rings between the inlet spike in front and the compressor in the middle. It’s meant to function like the intake of a normal jet engine – but at hypersonic speeds, temperature becomes more limiting than just about anything else (presumably the inlet spikes are managing the shock waves that are just itching to bounce around inside that engine while it’s moving through the air at Ludicrous Speed).

So anyways, air tends to get kind of hot when it’s being pushed and squeezed at high velocities. And when air gets really hot, jet intakes tend to not work very well. That was a big reason the SR-71 was limited to around Mach 3.5 (or so they say). For a combined-cycle engine, precoolers are pretty much ball game.

SABRE engine. Credit: Reaction Engines UK

A functioning SABRE engine would enable the kind of suborbital spaceplanes that I wrote about in Perigee. In fact, Reaction’s ideas were used extensively in my mental world-building while the story took shape. I’m a big believer in the potential for suborbital point-to-point airline service – if you’d be willing to spend a quarter-mil for a 30 minute joyride on Virgin Galactic, wouldn’t you spend that much to actually go somewhere?

Yes, I’m kind of excited about this. Could it eventually lead to a single-stage-to-orbit spaceplane like they propose with Skylon? Maybe. The energy needed to make orbit is exponentially greater than that needed for a 6,000 mile hop at Mach 10.

Skylon spaceplane. Credit: Reaction Engines UK

So my answer would be, “beats me, ask a real engineer.” I just play one at work. But hanging those heavy engines out on the ends of the wing strikes me as not being a real good idea. Any twisting moments (which will happen in atmospheric flight) would just be amplified. Which means beefier wing structure, which adds weight, which increases minimum runway, which also requires more power from the engines, etc…this is the kind of circular reasoning that is otherwise known as a “trade study.” Every decision about one aspect of a system’s design affects all sorts of other stuff in the system. This is especially true in aeronautics.

But perhaps the biggest hurdle to overcome (in my view) is the apparent operating assumption that a passenger-carrying version of Skylon wouldn’t have a pilot aboard. They’d just pop in a passenger cabin, program the airplane, and send it on its way. There has been a tremendous amount of progress in the UAV world, but I have a hard time seeing how people would pay big money for an inherently risky ride with nobody up in the front office to deal with stuff when it all goes sideways. I have an even harder time seeing how FAA or EASA would ever certify such a bird to carry passengers (and that’s coming eventually, we can be certain). Unexpected bad stuff will happen, you can bet on it: flying is hours of boredom interrupted by moments of sheer terror. Thus shall it ever be.

11/30 UPDATE: io9 has more, but it sounds like they’re confusing Skylon with another Reaction proposal called LAPCAT.

12/01 UPDATE: Wired is on the case as well.

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