Take-off and climb to use conventional engine for flights that could last one month

The US Air Force has revealed studies of a hybrid nuclear/conventional-powered high-altitude, long-endurance (HALE) unmanned air vehicle (UAV) that could cruise and loiter for more than a month.

The Air Force Institute of Technology at Wright-Patterson AFB, Ohio, examined the application of a triggered isomer heat exchanger (TIHE) as part of a hybrid, long endurance, dual heat-source combustor in an otherwise conventional engine. The study examined a concept in which the UAV would take off and climb using a conventional engine before switching to the TIHE combustor for cruise and loiter. On the return leg, the UAV's conventional engine would take over for descent and landing.

The USAF says the impetus for the study came from successful experiments at the University of Texas where researchers triggered rapid energy release in various isomers using a low-power X-ray device. Several isomers were bombarded with X-rays and produced power levels up to 60 times the energy input. The study group says the reaction differed from fission reactions in that while some gamma radiation was produced, there was no significant neutron release. With no fission products and negligible neutrons, the study says, this "allows for reduced shielding weight and safer maintenance and removal of used fuel".

These results, combined with studies at the University of Sacramento, California, into the replacement of an engine's combustor with a solid-state heat exchanger, suggest a hybrid system for a mission requirement similar to that of the Northrop Grumman RQ-4AGlobal Hawk may be possible. To address regulatory and environmental issues, the mission called for restricting TIHE operation to a minimum altitude of 20,000ft (6,100m) as well as limiting flights over domestic and allied airspace to conventional power.

After studying adaptions of the TIHE to engines ranging from turboprops and turbofans to turbojets and rockets, the team identified a single Mach 0.5-optimised turbofan with the best switchover condition at M0.4 and 40,000ft. Although not specified, the parameters of the chosen engine are similar to the Global Hawk's Rolls-Royce AE3007.

The HALE-UAV would need a lead radiation shield around 70mm (2.8in) thick and weighing 1,250kg (2,750lb) - about 11% of the UAV's reference take-off weight and around 19% of the fuel weight of the conventional Global Hawk.

The team estimated that a HALE-UAV designed for a seven-day mission could shed more than 2,250kg by using the TIHE-turbofan hybrid.

Source: Flight International