Redemptive Engineering

I find this picture pretty striking. This is what NASA calls the "Common Extensible Cryogenic Engine." It's really a variant of the fairly venerable RL10 rocket engine, built to test various methods of modifying the RL10 to achieve wider throttle ratios.

Maybe the most striking thing about this picture is the lack of overall rumpus. The thing is running at full throttle and yet it seems as placid and harmless as the burner on a gas stove. No smoke, no roiling clouds of flame, no explosions. This is what a well-controlled rocket engine should look like.

Another striking thing is the blue color of the exhaust plume. I believe the blue color is produced by what are called "Swan bands", bands of light at specific colors produced by highly excited hydrogen atoms. Unlike the orange glow of a campfire, which is produced mainly by black body radiation coming from glowing bits of soot, the color here is atomic in nature and not any kind of black body radiation. It's the same blue color as a blowtorch flame, the characteristic blue of highly efficient hydrocarbon combustion.

But let's look a bit deeper. At high magnification, one can see that there are actually icicles hanging from the skirt of the nozzle. The nozzle is so well cooled that the superheated steam in the exhaust plume condenses into water and then freezes into ice, even though the temperature of the exhaust plume is on the order of several thousand degrees. That's some serious regenerative cooling.

The heat flux staggers the imagination. The temperature in the exhaust plume is high enough to melt the engine, but the cooling system can draw heat out of the metal nozzle so fast it not only doesn't melt, but it actually runs below room temperature.

Another striking feature of this engine is its "deep throttling". Throttling a rocket engine is exactly like throttling a car engine - making it produce more or less power as circumstances dictate. But unlike car engines, rocket engines are notoriously difficult to throttle. On the one hand, if your throttling system goes haywire on the high side, the engine can over-pressurize and blow up (though RL10s and other expander-cycle engines don't really have this problem, as the square-cube law means that even with the propellant valves thrown wide open, the engine can't really run away and blow up). On the other hand, reducing the engine's power creates all sorts of complications, like flow separation in the nozzle (bad), and periodic combustion instabilities like "chugging" and "screeching". Screeching isn't really a combustion instability; it's a regenerative acoustic effect, but it can happen by accident (it's always by accident) as you throttle an engine.

The X-15 rocket plane was designed to have a pretty wide throttle range, from about 30% to 100%. But the mission logs of the X-15 program reveal that the XLR99 engine just didn't like to run at low throttle. It wouldn't start reliably at low throttle, and would occasionally just cough and die when throttled back (and the X-15 fell so fast after drop that you didn't get many restart attempts before it was time to start dumping propellant and get ready to land the thing).

But this engine has been run from 8% to 104% throttle. That's pretty amazing to me. 8% is particularly striking - I can't begin to imagine how they do that without employing exotic variable geometry in the nozzle throat (my understanding of de Laval nozzles is that the gas flow rate through the throat must be sufficient to cause sonic choking. How they can keep the flow sonic at 8% throttle without necking down the nozzle is a mystery to me - either that, or the engine is seriously underexpanded at full throttle - or something...)

My point is that this photograph shows off some very advanced rocket science. Whenever I start to feel that modern life has turned into a cornucopia of dung designed to appeal to the least common denominator, I look at pictures like this and think "Man, we really are pretty smart, aren't we?"

Well, I'm not that smart. But I'm smart enough to recognize staggering engineering achievements when I see them, and be cheered up by them.