Thermal Nuclear Propulsion

Thermal Nuclear Propulsion (TNP) uses a nuclear reactor to heat up hydrogen (or another reaction mass) to provide thrust. The most famous TMP was NASA's NERVA program which was ground tested, but did not fly in space.

Description

To build a rocket, you need a energy source, and you need reaction mass to be blasted out the back of your rocket. With chemical rockets, fuel and oxidizer are burnt which provides the energy, and then the burnt fuel is ejected via nozzles as reaction mass.

Let us say you heat two gasses to the same temperature. One is carbon dioxide (CO₂) and the other is molecular hydrogen (H₂). The temperature is the average momentum of the gas particles. Momentum = mass times velocity. The gram molar mass of CO₂ is 48, where as H₂ is 2. So at a given temperature, the hydrogen must be moving faster at a given temperature.

In other words, small, light molecules are moving faster at a given temperature, then larger molecules. The faster the reaction mass leaves our rocket, the more efficient it is.

Thus, the ideal reaction mass is hydrogen.

Robert A. Heinlein, in his novel "Space Cadet" used a fuel and reaction mass of atomic hydrogen (H₁). (How atomic hydrogen was stored was never discussed.) It combined with other hydrogen, producing H₂ exhaust as reaction mass. All of his ships flying around the solar system really worked with this ideal fuel.

With Thermal Nuclear Propulsion, hydrogen gas (H₂) is flowed thru a active nuclear reactor. It gets very hot, expands, and then is ejected thru a nozzle to provide thrust. The low mass of the hydrogen molecules improves efficiency. The power source (the reactor) stays with the ship while the reaction mass is ejected.

Advantages of Thermal Nuclear Propulsion

The reactor can provide energy for the ship. For example, some of the heat could provide electrical power.
The efficiency is good since the reaction mass has a low atomic mass. (The ISP is about double that got by burning hydrogen and oxygen.)
This higher efficiency can allow missions to carry more payload.
The thrust is quite high, potentially enough to allow the ship to launch to orbit from Earth or from Mars.
The science is straightforward and well understood. It is simply a matter of engineering.

Disadvantages of Thermal Nuclear Propulsion

It is hard to get most of the heat of the reactor to go into the hydrogen. The heat exchange surfaces between the nuclear reactor elements and the flowing gas need to have a maximum amount of area, since gases are inherently bad thermal conductors. This has mostly been solved by the early nuclear engine tests. However, engine reuse may be limited and refurbishing a nuclear thermal engine in space may be difficult.
Any heat not expelled with the reaction mass must be radiated away. Radiators have significant mass, which make the ship heavier. However, this applies much more directly to Nuclear Electric Propulsion, as the high exhaust volume of hydrogen propellant can carry away most of the heat.
Nuclear reactors tend to be large, thus this is best for large ships.
Hydrogen is not very dense, so large tanks are required to hold it.
Hydrogen is not space storable, as it warms it expands and must be bled off, wasting reaction mass. This might be alleviated using refrigeration and cooling system, but this adds complexity.
Nuclear reactors can not be instantly turned on and off. Thus the thrust will taper on and off.
Many people fear nuclear power, sometimes beyond the bounds of reason, which may cause political complications. As the engines need to be tested on the ground, this creates risks that can be perceived as too high.
Nuclear fuel, in particular enriched nuclear fuel, is hard to obtain for civilians. This makes the development of nuclear engines difficult.
If high thrust is not needed, NEP offers much higher exhaust velocities for the same energy input. So the propellant efficiency is higher.

Summary

TNP is a worthy form of rocket, which deserves further development. However, as its advantages grow as the size of the ship increases, it is less practical for smaller space craft. This suggests that early, small ships are unlikely to use it. For short trips in the Cis-Lunar space it may be an excellent long term solution.