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.

Variants

LANTR

The Lunar Oxygen Augmented Thermal Nuclear Propulsion engine was proposed in the 1990s as an improved design compared to TNP(1). The engine uses Lunar, or Martian, oxygen, injecting it in the nozzle of the NTR engine to increase thrust by adding mass to the propellant and adding the chemical energy of the Oxygen-hydrogen reaction. By varying the oxygen-to-fuel mixture ratio (MR), the LANTR concept can provide variable thrust and specific impulse (Isp) capability with a LH2-cooled NTR operating at relatively constant power output. For example, at a MR = 3, the thrust per engine can be increased by a factor of 2.75 while the Isp decreases by only 30 percent. With this thrust augmentation option, smaller NTR's become more acceptable from a mission performance standpoint (e.g., Earth escape gravity losses are reduced and perigee propulsion requirements are eliminated). Hydrogen mass and volume is also reduced resulting in smaller space vehicles. As soon as any form of industrial base is established, Oxygen from the Moon, or Mars, will be much cheaper than hydrogen from Earth.

MITEE

The MITEE (MIniature Reac Tor EnginE) was a design for a very light nuclear Thermal rocket. While the thrust to weight ratio of the NERVA engines was close to one (the Nerva engine can barely lift itself), the MITEE theoretically achieved a thrust to weigh ratio of 10, making it a candidate for missions that could lift from the surface of practically any planet, including the atmosphere of Uranus or Neptune. Or obviously, Mars. The MITEE engine heats hydrogen propellant to 3000 K, achieving a specific impulse of 1000 seconds, with a thrust of 28 000 Newtons and a total engine mass is 200 kg, including reactor, pump, auxiliaries and a 30% contingency. MITEE requires significant amounts of very highly enriched uranium, and therefore is difficult to test and implement, in particular for a civil space program.

Discussion

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.
If propellant is cheap, then there is no economical advantage in 'saving' propellant. Therefore, the cost of developing a full TNP engine and rocket must be compared to the cost of just using more propellant for chemical rockets.

Summary

TNP is a worthy form of rocket, which deserves further development. However, it is difficult for it to compete with chemical propulsion using cheap propellant mass, such as used by fully reusable Methane-Oxygen engines. This suggests that early ships are unlikely to use it. For short trips in the Cis-Lunar space or as a Mars shuttle it may be an excellent long term solution.