Nuclear power in space is a controversial subject. Mention Project Prometheus, and NASA is quick to point out that a nuclear rocket is not in the plans. One reason lies in the technology's chequered history.
The advantages of nuclear over chemical propulsion for interplanetary travel have been recognised since the earliest days of rocketry, even before the atom was split. But time and again, environment concerns and budget issues have halted development.
While the best chemical rockets have a specific impulse - a measure of efficiency - of about 450s, solid-core nuclear rockets can reach around 900s. Long-duration space missions need high-specific impulses.
Nuclear fission-powered rockets were first considered seriously in the late 1940s. But work on space nuclear propulsion did not begin in earnest until 1955, when the US Air Force and Atomic Energy Commission began the Rover programme to develop a nuclear reactor for a missile engine.
NASA took over responsibility for the project from the USAF in 1958, and in 1960 an Aerojet/Westinghouse team was selected to develop the Nuclear Engine for Rocket Vehicle Application (NERVA). The aim was to develop a flight-rated 75,000lb-thrust (335kN) nuclear rocket that used a solid-core reactor to heat hydrogen and expel it through a nozzle.
Over the next decade, under the Rover/NERVA programme, a series of engines were tested under codenames such as Kiwi, Phoebus, Pewee and the Nuclear Furnace. In most of the tests, conducted at Jackass Flats in Nevada, the nozzles were pointed upwards, exhausting hot hydrogen into the atmosphere. The maximum thrust achieved was 250,000lb.
As the US public lost interest in manned spaceflight after the Apollo moon landings, NASA's budget was cut and with it funding for NERVA. The programme was cancelled in 1972 after almost $500 million had been spent. The programme had demonstrated all the elements of a nuclear rocket, achieving a specific impulse of 850s.
NERVA was not the only nuclear propulsion concept pursued during that period. In 1957 the US Air Force and Atomic Energy Commission launched Project Pluto, to develop the engine for a Mach 3, 13,000km-range nuclear missile. The simple concept behind Pluto was to heat the air in a ramjet using an unshielded nuclear reactor. The first nuclear ramjet ran in 1961, but the project was cancelled in 1964.
Even more outlandish was Project Orion, an effort to develop a interplanetary spaceship powered by exploding nuclear bombs behind the vehicle, which was accelerated by the plasma hitting a pusher plate. The concept promised theoretical specific impluses of 10,000s, and possibly as high as 1 million seconds. Begun in 1958, with the goal of reaching Mars by 1965 and Saturn by 1970, Orion was cancelled after the atmospheric test ban treaty of 1963 outlawed the concept.
The nuclear rocket concept did not die with NERVA. In 1987, the US Strategic Defense Initiative established a secret nuclear space propulsion programme, codenamed Timberwind. The aim was to develop a rocket motor for space-launched ballistic missile interceptors that used simpler particle-bed reactor technology.
The Timberwind programme was declassified in 1992, after the US Air Force established the Space Nuclear Thermal Propulsion programme with the goal of demonstrating a nuclear upper-stage rocket using liquid hydrogen propellant and capable of 75,000-85,000lb-thrust and a 930-1,000s specific impulse. The programme was killed in 1993.
In 1990, under US president George Bush's abortive Space Exploration Initiative, NASA studied the option of nuclear propulsion for a mission to Mars. Although chemical propulsion was ultimately favoured, it was judged that a nuclear thermal rocket (NTR) based on NERVA solid-core technology would be feasible by 2010-20.
NASA has continued to study the nuclear thermal rocket, focusing on a 10,000-15,000lb-thrust engine with a 915-955s specific impulse. As well as thrust, this "bimodal" NTR would generate up to 25kW of electricity to power the spacecraft. A "LOX-augmented" NTR would burn liquid oxygen in an "afterburner" nozzle to increase thrust by at least 40%.
Despite several mission analyses that have illustrated the speed advantages of the NTR for moon and Mars missions, NASA has elected to pursue nuclear electric propulsion under Project Prometheous. The principal reason is the stigma that still attaches to the term "nuclear rocket".
Source: Flight International
