Nuclear Thermal Rocket - Test Firings

Test Firings

KIWI was the first to be fired, starting in July 1959 with KIWI 1. The reactor was not intended for flight, hence the naming of the rocket after a flightless bird. This was unlike later tests because the engine design could not really be used; the core was simply a stack of uncoated uranium oxide plates onto which the hydrogen was dumped. Nevertheless it generated 70 MW and produced an exhaust temperature of 2683 K. Two additional tests of the basic concept, A' and A3, added coatings to the plates to test fuel rod concepts.

The KIWI B series fully developed the fuel system, which consisted of the uranium fuel in the form of tiny uranium dioxide (UO₂) spheres embedded in a low-boron graphite matrix, and then coated with niobium carbide. Nineteen holes ran the length of the bundles, and through these holes the liquid hydrogen flowed for cooling. A final change introduced during the KIWI program changed the fuel to uranium carbide, which was run for the last time in 1964.

On the initial firings immense reactor heat and vibration cracked the fuel bundles. Likewise, while the graphite materials used in the reactor's construction were indeed resistant to high temperatures, they eroded under the heat and pressure of the enormous stream of superheated hydrogen. The fuel bundle problem was largely (but not completely) solved by the end of the program, and related materials work at Argonne National Laboratory looked promising. Fuel and engine coatings never wholly solved this problem before the program ended.

Building on the KIWI series, the Phoebus series were much larger reactors. The first 1A test in June 1965 ran for over 10 minutes at 1090 MW, with an exhaust temperature of 2370 K. The B run in February 1967 improved this to 1500 MW for 30 minutes. The final 2A test in June 1968 ran for over 12 minutes at 4,000 MW, the most powerful nuclear reactor ever built. For contrast, the Itaipu Dam, one of the most powerful hydroelectric plants in the world, produces 14,000 MW, enough to supply 19% of all the electricity used in Brazil, and 90% of that used in Paraguay.

NERVA NRX (Nuclear Rocket Experimental), started testing in September 1964. The final engine in this series was the XE, designed with flight design hardware and fired in a downward position into a low-pressure chamber to simulate a vacuum. SNPO fired NERVA NRX/XE twenty-eight times in March 1968. The series all generated 1100 MW, and many of the tests concluded only when the test-stand ran out of hydrogen propellant. NERVA NRX/XE produced the baseline 75,000 lbf (334 kN) thrust that Marshall required in Mars mission plans.

A smaller version of KIWI, the Pewee was also built. It was fired several times at 500 MW in order to test coatings made of zirconium carbide (instead of niobium carbide) but Pewee also increased the power density of the system. An unrelated water-cooled system known as NF-1 (for Nuclear Furnace) was used for future materials testing. Pewee became the basis for current NTR designs being researched at NASA's Glenn and Marshall Research Centers.

The last NRX firing lost a relatively small 38 pounds of fuel in 2 hours of testing, enough to be judged sufficient for space missions by SNPO. Pewee 2's fuel elements reduced fuel corrosion still further, by a factor of 3 in Nuclear Furnace testing, but Pewee 2 was never tested on the stand. Later designs were deemed by NASA to be usable for space exploration and Los Alamos felt that it had cured the last of the materials problems with the untested Pewee.

The NERVA/Rover project was eventually cancelled in 1972 with the general wind-down of NASA in the post-Apollo era. Without a manned mission to Mars, the need for a nuclear thermal rocket was unclear. To a lesser extent it was becoming clear that there could be intense public outcry against any attempt to use a nuclear engine.