Nerva

The Nuclear Engine for Rocket Vehicle Application, known as NERVA, was a major development program for nuclear thermal rocket engines. This project lasted for roughly twenty years. It was a joint effort between the Atomic Energy Commission (AEC) and the National Aeronautics and Space Administration (NASA). The program was managed by the Space Nuclear Propulsion Office (SNPO) until it ended in January 1973. The main goal was to build a technology base for nuclear rocket systems. Scientists wanted to design propulsion systems for future space missions.

Nuclear thermal rockets work differently than the chemical rockets we use today. Conventional rockets create thrust through a chemical reaction. This reaction produces heat, which makes a fluid expand and escape through a nozzle. NERVA used a nuclear reactor to provide that heat instead. Because nuclear reactions are much more powerful than chemical ones, a small reactor can replace large amounts of chemicals. This allows the engine to be much more efficient. In fact, these engines aim for at least twice the specific impulse of a chemical engine.

To make the engine work, engineers had to manage many complex steps. First, they had to store a working mass, usually liquid hydrogen, in a large tank. Because hydrogen must stay extremely cold, it is kept in liquid form below 20 Kelvin. A turbopump then moves this liquid hydrogen into the reactor. Inside the reactor, the hydrogen is heated to about 2,700 Kelvin. As the hydrogen heats up, it expands rapidly. This hot gas is then pushed out of the rocket nozzle to create thrust.

Designing the reactor required choosing very specific materials. For the fuel, engineers chose uranium-235. They rejected plutonium because it forms compounds too easily and cannot reach high enough temperatures. For the reactor structure, they chose graphite. Graphite was a great choice because it is inexpensive and actually gets stronger at temperatures up to 2,800 Kelvin. To control the reactor's power, they used control drums. These drums were coated with a neutron moderator on one side and a neutron poison, called boron, on the other.

Safety was a major priority during the development process. The engine emitted intense neutron and photon radiation. To protect people and other parts of a spacecraft, engineers developed a radiation shield. They created a material called BATH. This was a mixture of boron carbide, aluminum, and titanium hydride. The titanium hydride acts as a neutron moderator, while the boron carbide absorbs neutrons. This material was very strong and could withstand high temperatures.

The history of these ideas began long before NERVA. During World War II, scientists at the Los Alamos Laboratory speculated about nuclear rockets. In 1946, Stan Ulam and C. J. Everett wrote a paper about using atomic bombs for propulsion. In 1953, physicist Robert W. Bussard wrote a detailed study on nuclear energy for rocket propulsion. By 1955, a committee led by Mark Mills recommended moving forward with development. This led to Project Rover at the Los Alamos Scientific Laboratory. Project Rover eventually became the foundation for the NERVA program.

NERVA was considered a highly successful program because it met its goals. It proved that nuclear thermal rockets were a reliable tool for space exploration. By the end of 1968, the XE engine was deemed ready for a human mission to Mars. The engines were built with components that were flight-certified. However, the program was cancelled by President Richard Nixon in 1973. Because of this cancellation, the NERVA engines never actually flew in space.