improving the Isp to between 930 and 1000 seconds. Test firings KIWI was the first to be fired, starting in July 1959 with KIWI 1. The reactor was not intended for flight and was named after the flightless bird, Kiwi. The core was simply a stack of uncoated uranium oxide plates onto which the hydrogen was dumped. The thermal output of 70 MW at an exhaust temperature of 2683 K was generated. Two additional tests of the basic concept, A1 and A3, added coatings to the plates to test fuel rod concepts. The KIWI B series was fueled by tiny uranium dioxide (UO2) spheres embedded in a low-boron graphite matrix and coated with niobium carbide. Nineteen holes ran the length of the bundles, through which the liquid hydrogen flowed. On the initial firings, immense heat and vibration cracked the fuel bundles. The graphite materials used in the reactor's construction were resistant to high temperatures but eroded under the stream of superheated hydrogen, a reducing agent. The fuel species was later switched to uranium carbide, with the last engine run in 1964. The fuel bundle erosion and cracking problems were improved but never completely solved, despite promising materials work at the Argonne National Laboratory. NERVA NRX (Nuclear Rocket Experimental), started testing in September 1964. The final engine in this series was the XE, designed with flight representative hardware and fired 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 thrust that Marshall Space Flight Center required in Mars mission plans. The last NRX firing lost of nuclear fuel in 2 hours of testing, which was judged sufficient for space missions by SNPO. 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 and 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 4000 MW, at the time the most powerful nuclear reactor ever built. A smaller version of KIWI, the Pewee was also built. It was fired several times at 500 MW to test coatings made