Gas heated by small nuclear explosions provides thrust for the pulsed nuclear rocket launcher. The explosions produce extreme temperature, vibration, and pollute atmosphere with radioactive waste. The rocket cannot be scaled down to less than 1000 tons due to the critical mass of a nuclear explosion.
The pulsed nuclear rocket launcher is prohibited by the Nuclear Test Ban Treaty, because it pollutes the atmosphere with radioactive waste. Moreover, radiation produced by the nuclear explosions is a serious threat to the cargo. To minimize the exposure, the cargo must be placed far away from the explosions. For example, the cargo may be attached to the rocket with a long tether. The thrust-to-weight ratio is low, so the rocket cannot carry much cargo.
Alexander G. Parlos and John D. Metzger, "Feasibility Study of a Contained Pulsed Nuclear Propulsion Engine," Journal of Propulsion and Power, Vol. 10, No. 2, March-April 1994, pp. 269-278.
The nuclear-thermal rocket launcher is powered by a nuclear reactor rather than nuclear explosions. Hydrogen propellant is in direct contact with the reactor, so it picks up some radioactive waste and releases it into the atmosphere. The thrust-to-weight ratio is so low that it is questionable whether the rocket could lift itself off the ground. To improve the ratio, some designs run so hot that the nuclear fuel is liquid or even gaseous. The nuclear-thermal rocket launcher is popular with nuclear engineers despite the high probability of melt-down in low Earth orbit. The minimum mass is 10 tons.
Stanley K. Borowski, Edward A. Gabris, and John Martinell, "Nuclear Thermal Rockets: Next Step to Space," Aerospace America, June 1989, pp. 16-29.
Nuclear Thermal Propulsion History, AIAA, 1992.