Nuclear Engine for Rocket Vehicle Application (NERVA)

In 2014 SpaceX announced that it planned to put humans on Mars by 2026. However by 1965 NASA had designed a rocket that would have been able to do just that, and rather than taking the estimated 300 days that modern chemical rockets use, this would have been able to do it in just 125. NERVA was a series of test conducted in the 1950-70’s meant to demonstrate that nuclear thermal rocket engines were fully feasible and reliable for space exploration. By the end of 1968 SNPO had certified the latests of the NERVA engines as meeting the requirements for a human mission to Mars.

The NERVA engines although never given the chance to fly were built and tested in environments as close to a complete flight system as physically possible. The latest engine, the NERVA NRX/EST was even equipped with a flight-design turbopump. As well as during its testing it was reoriented to fire down into a compartment that was meant to simulate firing in a vacuum. The engine utilised a fission reactor that would super-heat hydrogen gas, which would then escape through a small nozzle to generate thrust. The engines were so powerful that one test the Phoebus-2A engine was able to dump 4,000MW of electrical load in only 12 minutes, enough to energy power 3 million homes. The more practical NRX/XE delivered 1,100MW of energy with 75,000 lbf of thrust, the baseline that would have been required to effectively get astronauts to Mars.

Throughout testing, even with such powerful engines there were almost no accidents. The most serious injury sustained during testing was by a hydrogen explosion where two employees received foot and eardrum injuries.

The engines worked so incredibly well that they were quickly deemed ready for integration into spaceflight. The applications of such engines were almost limitless, some wanted them to replace the J-2 boosters that were used as the second and fourth stages of the Saturn rockets. Others wanted to use them as space tugs, towing objects from low earth orbit all the way to opper orbit, even to the moon and further.

Unfortunately the engines were never used as in the late 1960’s the primary protector of the program Clinton P. Andersen, a New Mexico senator had become seriously ill. And Lyndon B. Johnson, another advocate of human space exploration had declined to run a second term. So when Congress reduced NASA program funding in 1969 and the Nixon administration reduced it further in 1970, the project was forced to disband by 1972

Resources:
http://gizmodo.com/5992441/how-nasas-nuclear-rockets-will-take-us-way-beyond-mars
http://www.gizmag.com/nasa-nuclear-cryogenic-propulsion/25772/