Saturday, January 17, 2015

high energy to solve the challenge of the planets


wired |  Today we know that Americans can reach the “ends” of the Solar System without resort to nuclear propulsion (though a radioisotope system is handy for generating electricity in the dark beyond Jupiter, where solar arrays become impractical). When President Kennedy gave his speech, however, it was widely assumed that “high-energy” propulsion – which for most researchers meant nuclear rockets – would be desirable for round-trip journeys to Mars and Venus and a necessity for voyages beyond those next-door worlds. In his speech, President Kennedy referred specifically to the joint NASA-Atomic Energy Commission (AEC) ROVER nuclear-thermal rocket program. As the term implies, a nuclear-thermal rocket employs a nuclear reactor to heat a propellant (typically liquid hydrogen) and expel it through a nozzle to generate thrust.

ROVER had begun under U.S. Air Force (USAF)/AEC auspices in 1955. USAF/AEC selected the Kiwi reactor design for nuclear-thermal rocket ground testing in 1957 – a major step forward for the U.S. nuclear rocket program – and USAF relinquished its role the program to NASA in 1958. As President Kennedy gave his speech, U.S. aerospace companies competed for the contract to build NERVA, the first flight-capable nuclear-thermal rocket engine.

Nuclear-thermal propulsion was not the only form of nuclear-powered high-energy propulsion. Another was nuclear-electric propulsion, which can take many forms. This post examines only the form known widely as ion drive.

An ion thruster electrically charges a propellant and expels it at nearly the speed of light using an electric or magnetic field. Because charging propellant and generating electric or magnetic fields require a great deal of electricity, only a small amount of propellant can be ionized and expelled. This means in turn that an ion thruster permits only very gradual acceleration despite the speed at which propellant leaves it; one can, however, in theory operate an ion thruster for months or years, enabling it to push a spacecraft to high velocities.

American rocket pioneer Robert Goddard first wrote of electric propulsion in his notebooks in 1906. By 1916 be had begun experiments with “electrified jets.” Interest faded in the 1920s and resumed in the 1940s. The list of ion drive experimenters and theorists reads like a “Who’s Who” of early space research: L. Shepherd and A. V. Cleaver in Britain, L. Spitzer and H. Tsien in the United States, and E. Sanger in West Germany all contributed to the development of ion before 1955.