Project Rover: Main Series of Nuclear-Rocket Engines
Named after the large, flightless bird, Kiwi was the first phase of Project Rover. Kiwi consisted of eight reactors that scientists tested between 1959 and 1964. The first reactor, dubbed Kiwi-A, was fired for the first (and only) time on July 1, 1959, at Jackass Flats in the Nevada Test Site (now the Nevada National Security Site).
The Kiwi-B series increased power by ten-fold while maintaining the same size of the Kiwi-A series. The Kiwi-B reactors experienced a problem similar to Kiwi-A: Internal vibrations caused by dynamic flow instability fractured portions of the fuel elements. Scientists resolved this problem when they developed Kiwi-B4.
During the 1960s, scientists developed the Phoebus series of nuclear reactors to meet the needs of an interplanetary mission, such as a manned mission to Mars. Phoebus-1 was developed to study how best to increase power density. Phoebus A-1 was successfully tested on July 25, 1965, at Jackass Flats.
Scientists increased power density even further with the Phoebus-2 series. However, a limiting factor proved to be the cooling in the aluminum pressure vessel. Despite this limitation, tests run with the Phoebus-2 were considered highly successful. The final Pheobus-2 test in June 1968 ran for more than 12 minutes at 4,000 megawatts—for its time, it was the most powerful nuclear reactor ever built.
Considered as a smaller version of Kiwi, Pewee was fired several times at 500 megawatts to test coatings made of zirconium carbide. Scientists also increased Pewee's power density. Easy to test and compact, Pewee was ideal for unmanned scientific interplanetary missions.