Aerojet Rocketdyne and the US Air Force Research Laboratory (AFRL) have achieved a new record thrust output from a scramjet engine, providing a boost for hypersonic flight. A year of ground testing saw the engine generate 13,000 lb of thrust with over an hour of sustained combustion under various hypersonic flight conditions.
Hypersonic technology is regarded as one of the great game changers in 21st century warfare. Just as supersonic flight rendered all previous warplanes and anti-aircraft defenses all but obsolete overnight, aircraft and missiles that can fly at over five times the speed of sound would require an equally great advance in radar, data processing, air defenses, and even artificial intelligence to counter.
However, one major challenge is finding a way to make a craft maintain hypersonic velocity. Currently, most test vehicles are actually hypersonic gliders, which are dropped from a mothership at high altitude, then accelerated to above Mach 5 (3,709 mph, 5,969 km/h) either by rocket or by going into a dive to build up speed. That's fine for a test program, but for something practical, gliding has its limitations – not least of which is restricted range and the inability to operate at lower altitudes.
The approach of the AFRL-Aerojet Rocketdyne team to maintain flight is to use a scramjet. A decade ago, they produced the first hydrocarbon-fueled and cooled air-breathing hypersonic flight test with the Air Force's X-51A Waverider. Now they are working on an 18-foot (5.5-m) Generation-3 version, which can propel a vehicle 10 times the size of the X-51 to hypersonic speed. The new engine is claimed to have increased performance at a lower price.
A scramjet is essentially a ramjet designed to operate at much higher velocity. In a conventional jet engine, a spray of fuel is combined with air that's compressed by a series of turbine blades and ignited. A ramjet is, put simply, a hollow tube where the air is compressed by the forward velocity of the craft instead of turbines, which means that the engine has to already be flying before it will work, and it will work most efficiently at supersonic speeds.
However, the air traveling into the ramjet has to be decelerated to subsonic speeds. In a scramjet, the air only has to be slowed to supersonic, which allows it to operate with greater acceleration and lower engine temperatures, though it still needs an advanced cooling system to avoid heat damage. The advantage of this is that a scramjet is basically a rocket engine that doesn't need to carry oxygen along for the ride, only the fuel, which gives it a specific impulse, or efficiency, that is an order of magnitude higher than a rocket.
The recent AFRL-Aerojet Rocketdyne scramjet engine tests were carried out at the Arnold Engineering Development Complex (AEDC) Aerodynamic and Propulsion Test Unit (APTU) at Arnold Air Force Base.in Tennessee. The engine is being developed as part of the Air Force's Medium Scale Critical Components (MSCC) program.
"AEDC and the APTU facility have been proud to partner on this challenging and rewarding test program," says Kirk Butler, director of Operations for the AEDC’s Hypersonic Systems Test Branch and Ground Test Team Section Lead. "The test team has worked through COVID-19 restraints, shared base resources and hardware limitations to make the test successful. The knowledge gained from this testing will be studied for years and will drive future designs."
Source: Aerojet Rocketdyne
Update (Dec. 22, 2020): This article originally stated that ramjet engines need to be traveling at supersonic speeds before they will work. A number of readers have pointed out this is incorrect and that ramjets can operate at subsonic speeds. We apologize for the error and have updated the article accordingly.
