Explosions get you much more bang from your fuel buck than combustion – if your engine can withstand them. NASA believes the rotating detonation engine could be the future of deep space travel, and it's getting strong results in prototype testing.
Combustion engines are tried and true, and however angry they might look and sound in a top-fuel dragster or space rocket booster, the combustion process of oxidizing fuel in air is relatively slow and predictable. Detonation, on the other hand, is as chaotic and destructive as it sounds. It's how most bombs work; you take an explosive fuel and hit it with a jolt of energy, and the chemical bonds holding each molecule together break apart, releasing wild amounts of energy in a shockwave that expands at supersonic speed.
NASA, along with many other groups, wants to harness these explosions for a couple of key reasons. Firstly, detonation engines have a considerably higher theoretical level of efficiency than combustion engines, perhaps as much as 25%; they should be able to produce more thrust using less fuel and a smaller rocket. In the engineering and economics of space flight, that means cheaper launches, more billable payload, and greater distances.
They're also relevant to hypersonic flight. Combustion engines can only operate at subsonic airspeeds. To go supersonic or hypersonic, the intake air needs to be rapidly decelerated to a subsonic speed for combustion to take place. This generates heat and drag. Detonation occurs at supersonic speeds, so in addition to greater efficiency, you also reduce heat and drag in hypersonic applications since you don't need to slow the air down nearly as much.
Rotating detonation engines (RDEs), as opposed to oblique wave detonation engines or pulse detonation engines, use ring-shaped chambers and precisely-timed fuel injection to generate constant thrust. Each explosion sends out a shockwave that produces thrust, but it also travels around the ring to trigger the next explosion.
A number of groups are now reporting successful test firing of rotating detonation engines, from the University of Central Florida, working with the Air Force Research Laboratory, to Australia's RMIT, working with DefendTex, to Houston company Venus Aerospace, Aerojet Rocketdyne, and others... Jaxa, the Japanese space agency, has even gone so far as to test a small one in space.
NASA is keeping its testing on terra firma for the time being, but it's now announced the successful testing of a small RDE last year, in partnership with Indiana company In Space LLE. The engine was fired "over a dozen times, totaling nearly 10 minutes in duration," so it's clearly handled the major challenge for RDE development with aplomb – that being not to let your engine blow itself to bits.
The engine is built using powder bed fusion 3D printing, incorporating NASA's own GRCop-42 copper alloy, which the agency says is key to its ability to withstand the extreme conditions of sustained detonation without overheating.
At full throttle, says NASA, the RDE produced "over 4,000 pounds of thrust for nearly a minute at an average chamber pressure of 622 pounds per square inch, the highest pressure rating for this design on record." The testing included "successful performance of both deep throttling and internal ignition."
With promising results in the bag, NASA has announced it's scaling up to a fully reusable RDE in the 10,000-pound thrust class, where the team hopes it'll be able to start demonstrating performance benefits over regular rocket engines. You can watch this thing fire in the video below.
Source: NASA
