Background

Because the design of unmanned aerial vehicles (UAVs) can be much smaller than manned aircraft and therefore more easily and more cost-effectively stored and transported, the role of UAVs is expanding. Understandably, it is becoming more common for UAVs to conduct potentially dangerous security and surveillance missions that previously relied on man. These same practical and economic characteristics are leading the way to further UAV missions - here on Earth and to other planets.

However, UAV space travel presents unique challenges. Specifically, in an oxygen-free environment like space, Earth-style combustion power and propulsion systems will not work. Additionally, the UAVs' proximity to the Sun throughout its mission could prohibit its reliance on solar energy as a power source.

Solution

To address this issue, USRA researchers at the Idaho National Laboratory, working in collaboration with NASA, are investigating radioisotope-powered engines. By replacing an Earth-style combustion chamber with a radioisotope driven heat exchanger, jet engines would be able to operate in an oxygen-free environment. As the isotope decays, heat will be produced and thrust will be generated from the expansion and release of high pressure, high temperature exhaust gases. Approximately 95% of the heat generated would be utilized for powering the craft.

Although radioisotope thermoelectric generators have been used since the days of Apollo and as recently as the New Horizons mission to Pluto, current USRA studies are giving particular consideration to total mass size of the craft, flight velocity that is required to fly the UAV, and to length of its mission. When narrowing the options for suitable isotopes for use in UAVs, scientists are looking for fuel forms with a half-life that can support extended flight missions, can operate at temperatures above 2500 degrees Kelvin, possess self-shielding properties, and can provide 20-40 kWt of thermal power.

Results

Radioisotope-heated engines are viable for long duration flight in non-oxygenated atmospheres and are relatively easy to construct. Additionally, the propulsion characteristics of radioisotope-powered engines are ideally suited for loitering over specific targets to map and explore an entire planet over multiple years.

Scientists are specifically reviewing flight conditions surrounding Saturn's moon, Titan. With an atmosphere containing hydrocarbon and complex organic materials, Titan has all the basic elements to support single cell microbial life. Flying relatively small (~200kg) UAVs across Titan is a very cost effective approach to studying how life may be evolving here. Radioisotope-powered engines that could keep a UAV airborne on Titan, allowing for extended scientific study of the moon.