Single-stage-to-orbit has been just around the corner for decades. Where is it?
Hypersonic commercial space planes are just within reach using current technology, with a leading propulsion contender being Reaction Engines’ SABRE combined cycle power plant. The critical enabling technology, the pre-cooler, has been successfully tested and there is no technical reason why development can’t move forward rapidly. But even if the engine works, it will be very complex and expensive, with no way to know if aircraft that it powers can be scaled sufficiently to make commercial passenger carrying operations economically viable.
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Episode Transcript:
Remember Stanley Kubrick’s brilliant 2001: A Space Odyssey? In that film, passengers flew a Pan Am spaceplane to a gigantic orbiting space station, an idea not very far-fetched from the perspective of the late 1960s.
While the space station did indeed happen, the hypersonic orbital airliners are dead on arrival. Why?
I pondered this question the other day when I came across a mention of the Reaction Engines company’s SABRE engine concept, a very novel propulsion system that promises to work in both low altitude and low-speed flight regimes, as well as at hypersonic speeds, potentially up to orbital velocity. If it can be made to work, it can be a game changer.
The technology is complex, but the key is the use of a novel intake air pre-cooler that dramatically lowers intake air temperature. Why refrigerate the intake air? Well, heat has been the perpetual problem of all very high-speed aircraft and jet engine design since the very first turbojets in the 1940s. Originally, the problem was the materials in the jet engine hot section turbines. By the 1950s, airframe heating became not just a problem, but an absolute speed limiter for very high-performance airplanes.
To get to suborbital and orbital space planes, not only will the airframe get extremely hot, but so will the engines. Supersonic combustion ramjets, or “scramjets,” are currently the hot ticket to hypersonic flight in military applications like air to surface missiles. But the dirty secret to their performance is that they can operate successfully over a life expectancy of three or four minutes, with the missile impacting before the engine melts itself into liquid.
For true hypersonic flight in the region of Mach 5 or so, in a reusable and commercially viable aircraft, managing the heat rise caused by both frictional heating and the very high compression of the intake air charge is what hypersonic technology is going to depend on.
The SABRE engine does this using a gaseous helium coolant loop, chilling intake air from 1,000 °C down to −150 °C in 0.01 seconds. That helium coolant loop is the secret sauce of this design, capturing the absorbed heat and using it to drive pumps and compressors for both the engine’s liquid hydrogen fuel and for the cooling system itself. Just preventing the ambient water vapor from icing the engine is something of a miracle, but for a commercial application, the technology will have to combine some of the features of a turbojet, a ramjet and a rocket engine.
Conceptually, the most difficult part of the program is the pre-cooler and it has been tested successfully, but the system is going to be very complex and expensive.
Will it produce hypersonic aircraft that you or I can buy a ticket on? Eventually, yes. But market forces aside, it’s worthwhile to remember that a century ago, commercial aviation only existed because of government subsidies. In the U.S., airmail service was paying the overheads for early airlines, until the airplanes became big enough to carry an economically viable number of passengers.
If similar scaling factors apply with hypersonics, it’s going to require huge airplanes and be a lot more difficult than just adding horsepower to piston engines.
I hope it happens before I die. But I wouldn’t bet the farm on it.
