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98 June/July 2018 | Unmanned Systems Technology PS | Nuclear-powered unmanned systems N uclear-propulsion for unmanned vehicles is back on the agenda, according to Vladimir Putin. In March, he claimed Russia had flight-tested a nuclear-propelled and armed cruise missile, and that it is now developing a UUV capable of delivering a nuclear warhead over global ranges (writes Peter Donaldson). The US meanwhile has been quietly researching a little-known, relatively clean nuclear power source that could dramatically improve UAV performance. In the US, nuclear propulsion for aircraft has a history stretching back to the late 1940s, one that includes some horrifically dirty and dangerous engine designs and some that at least paid lip service to safety. From 1948 until the early 1960s, airborne reactor technology took two directions, dubbed direct and indirect, in government projects given to General Electric and Pratt & Whitney. GE ground-tested several J-47 turbojets modified to allow the airflow from the compressor to be directed either through the combustor or a small fission reactor to heat it before passing through the turbine and out of the exhaust. The aim was to allow the aircraft to take off on conventional power and then switch to nuclear power at a safe distance from base. Inevitably though, the exhaust gas was extremely radioactive. In Pratt & Whitney’s indirect system, the jet’s intake airflow would pass through a liquid metal heat exchanger instead of the reactor core. Although progress was made with reactor and heat exchanger designs, no test reactor was ever built. While this engine would have produced much less radioactive exhaust, like its direct counterpart it would have needed heavy shielding to protect ground and air crews. Although neither nuclear engine flew, a modified Convair B-36 bomber fitted with a reactor but not powered by it did fly several times to test radiation shielding. With nuclear test ban treaties in the offing, the effort was cancelled in 1961 and little was heard of the idea until the early 2000s, when the US Air Force revealed that it was looking at a new (and immediately controversial) technology. This was a form of nuclear power that did not rely on fission, fusion or isotopic decay, instead extracting energy from an isomer of hafnium known as 178m2 Hf by bombarding it with X-rays in a triggered isomer heat exchanger (TIHE). In nuclear physics, an isomer of an element is one where some of its protons and neutrons are in an excited state, meaning they have the potential to decay to a lower energy state, releasing energy in the process. The energy release is potentially 100,000 times greater than any chemical reaction but 1000 times less than a nuclear fission reaction, and produces no particulate radiation. Although little detailed information was made public, in 2002 the Air Force Institute of Technology published a Master’s thesis by USAF Captain Christopher E Hamilton, then another in 2004 by Ensign Jonathan C Cox of the US Navy Reserve. Both addressed the use of TIHE devices to supplement or replace the combustor as the heat source in jet engines for UAVs – respectively a Global Hawk with a modified turbofan, and a conventionally armed air-launched supersonic cruise missile with a TIHE ramjet. For the past 15 years, arguments about the fundamental science and costs involved in producing enough 178m2 Hf have created an atmosphere of uncertainty – in the public domain at least – over whether the technology could ever be practical. Nonetheless, behind the scenes, the combination of unmanned systems and nuclear power retains its fascination. Now, here’s a thing “ ” Aircraft history includes some horrifically dirty and dangerous engine designs, and some that at least paid lip service to safety