GUY NORRIS / LOS ANGELES
Historic flight aims to prove PDE engines produce thrust
The US Air Force Research Laboratory (AFRL) and Scaled Composites plan to conduct around late September what is described as the first-ever flight of a pulse detonation engine (PDE)-powered aircraft.
The AFRL's primitive four-tube PDE is to be installed in a Scaled Composites-owned Long-Ez over the next month, before being shipped back to Mojave, California. "This demonstration is to show that PDE engines produce thrust and that an aircraft can really fly with a PDE engine," says AFRL propulsion directorate combustion science branch chief Robert Hancock.
In static tests the engine hasgenerated100-200lb-thrust (0.45 0.89kN), with each detonation tube operating at 20Hz for a total of 80Hz.Although Hancock says this is "quite an uncomfortable" frequency, he adds that the flight test is a vital milestone towards proving the basic PDE concept for eventual application in Mach 2-4 missiles, manned and unmanned air vehicles and as a replacement for afterburners.
The PDE, in basic form, is an air-breathing jet engine with no moving parts. Combustion takes place in an open-ended tube in which fuel is mixed with air and detonated. As the detonation wave travels down the tube at supersonic speed, it reduces inlet pressure and draws in fresh air and fuel, generating thrust. Each pulse lasts milliseconds.
"In its current form this is a very inefficient engine, and no-one is going to make PDEs for Long-Ezs. People say it is going to be way too loud, and that there are vibration issues, so this test will hopefully satisfy some people that you can do it," Hancock says. Valve control for the firing sequence is operated through an adapted General Motors Quad 4, 16-valve dual overhead cam shaft car engine, with power provided by a Hirth auxiliary power unit attached in a cradle beneath the fuselage.
Despite rumours that PDEs are among several exotic or advanced propulsion systems already developed by the USA for classified air-breathing aircraft programmes, Hancock says "to our knowledge this will be the first flight of a PDE-powered aircraft".
The programme has been dubbed "Borealis", however, in recognition of the popular mythology surrounding the fabled "Aurora" and the use of the PDE concept. Although later identified as a funding line for the Advanced Tactical Bomber, the Aurora classified budget line item is still widely used to describe purported advanced hypersonic reconnaissance and strike platforms developed in secret.
Source: Flight International
