The dream of airline-style orbital flight operations has come a step closer to reality with a successful demonstration of the critical technology behind a radical air-breathing rocket engine concept.
After years of work that has consumed some £250 million, Oxford-based Reaction Engines has declared success in its attempt to devise a pre-cooler that can liquidise oxygen from intake air, before mixing it with tanked liquid hydrogen to generate thrust like a normal rocket engine. A spaceplane powered by such engines would leave a runway under rocket power and liquidise its own oxygen until reaching M5.5 at 26km altitude, when tanked liquid oxygen would take over for the journey to low-Earth orbit.
Reaction Engines is now seeking another £250 million from investors to develop a demonstrator of its SABRE (Synergetic Air-Breathing Rocket Engine) powerplant concept. Founder Alan Bond, whose vision of the air-breathing rocket engine goes back 30 years to his work on British spaceplane concepts including HOTOL - which bears much resemblance to Reaction's current Skylon concept - reckons that with parallel airframe development a spaceplane with a 200-trip lifespan could be flying by 2020 and operational by 2030.
SABRE engine technology could also power an aircraft to multi-Mach atmospheric cruise, potentially realising London-Sydney travel times of 4h. Technical director Richard Varvill, a former Rolls-Royce engineer, adds that the intercooler technology might also transform more normal sub-Mach aero engines by recovering heat from the exhaust to put "free" energy back to the combustion chamber. Such recuperative architectures are used in electricity generation plants and could cut aero engine fuel burn by 5-10%, says Varvill.
The critical technology is the pre-cooler, essentially a radiator made of many hundreds of kilometres of 1mm tubing capable of cooling the 1,000ºC Mach 5 air to a nearly-cryogenic -150ºC. Reaction Engines has also mastered a process to actually manufacture such a structure which in its full-scale form will have a million brazed joints and must be leak-free.
But, according to Varvill, the really "magic" piece of the puzzle is the ability to prevent the cooler from being completely closed by frost. That would occur within seconds were if not for a technique which, he stresses, is being kept absolutely secret. Steady-state tests lasting more than 10min have shown the cooler to work perfectly, he says.
The European Space Agency is backing Reaction's claims, having supported its test programme with a small amount of funding and what Reaction describes as a highly significant amount of expertise. While stressing that there is a very long engineering road to any operational system, ESA's head of propulsion engineering, Mark Ford, is adamant that the SABRE engine is a "potentially disruptive" technology.
Reusable launch systems under development today, he says, are all based on traditional rocket engines, and ESA sees little or no chance of success going down that route. With SABRE, however, "one of the main obstacles to reusability has been removed", he says.
What makes SABRE so potentially valuable is the fact that it needs to carry from the ground only a fraction of the liquid oxygen needed for a trip to space. As Ford puts it, if we can liquidise our oxidiser from the air, we would be "mad" not to.
And, he adds, a SABRE engine could be wing-mounted as in the Skylon concept, which would allow quick and easy swap-out of powerplants to enable an airline-style maintenance regime. One of the reasons NASA's Space Shuttle proved to be so slow and expensive to turn around between trips was the hugely time-consuming task of maintaining its internal main engines.