| In the 1950s, scientists began to experiment with nuclear rocket propulsion systems as an alternative to chemical rockets. If interplanetary travel were to ever become practical, a more efficient means would have to be developed. Starting in 1955, scientists at Lawrence Livermore National Laboratory experimented with designs for a solid core nuclear thermal rocket, and by the time the experiment was finished, they had managed to produce a rocket with a higher specific impulse than today's most advanced chemical rockets.
Unfortunately, these experiments also revealed many limitations to the solid core design, and the technology available at the time made experiments with gas core nuclear rockets unfeasable, so the project was shelved.
Recent advancement in the understanding of the behavior of plasma as well as the advent of computer modeling has rekindled interest in gas core nuclear propulsion systems, which promise more than a tenfold increase in specific impulse over chemical propulsion systems. A trip to Mars could take as little as three months, could carry a much larger payload, would minimize the negative effects of weightlessness and exposure to radiation on the crew, and cost far less.
The concept behind nuclear thermal propulsion is simple. Fission in the reactor core produces energy. That energy heats the propellant. The propellent proceeds to exit the vehicle, and Newton's Second Law of Motion kicks in.
The design, however, is tricky. Inside the core, a vortex is created, allowing the propellant to pass through while isolating the hot uranium plasma. Liquid hydrogen is the most likely candidate for the propellant, since it has a low molecular mass, and does not become radioactive. The intricacies of creating the vortex, injecting the uranium and increasing the efficiency of the system are still being worked out.
| 
