Nuclear Thermal Rockets NTRs

The nuclear thermal rocket is another propulsion technology where the energy is provided by a separate source, once again a nuclear reactor. But the mode of operation is different from that in the NEP system. To operate a NEP device, a reactor is used to produce electricity to power the system. However, in the NTR the heat produced by the nuclear reactor is not converted to electricity. Instead it is used directly to heat the rocket propellant, energizing it to produce thrust. NTRs have been developed and tested over many years, but again the technology has not been flown in space.

The NTR shown in Figure 10.11 is referred to as a solid-core configuration, which is the simplest design to construct. In concept, the operation of the rocket is relatively straightforward. Liquid hydrogen propellant is passed through the reactor core, acting to cool the reactor and to heat the hydrogen to temperatures of around 3000°C. This superheated hydrogen is then expanded out of the engine nozzle to produce thrust. This engine can produce a high thrust for relatively long periods of time, up to about an hour, and has a specific impulse of around 1000 seconds. This specific impulse is an improvement over the best chemical propulsion by a factor of two, so that about twice the A V is achievable for a given mass of propellant. To give an example of what this means in terms of numbers, let's suppose we use a 200 kN thrust solid-core NTR with a specific impulse of 1000 seconds to propel a spacecraft with an initial mass of 150 metric tons. If we fire the engine for an hour, the burn would result in a AVof 6.5 km/sec (4.0 miles/ sec), so that we are getting a performance that is really useful for the propulsion of manned exploration missions. The mass breakdown for this example is about 73.5 metric tonnes of fuel, 6.5 metric tonnes of NTR, and 70 metric tonnes of useful payload. Also, by using different designs of NTR, the specific impulse can be further increased up to around 2000 seconds, so in principle the mass of fuel can be reduced further.

Nuclear reactor core

Nuclear reactor core

Engine nozzle

Figure 10.11: A cutaway diagram of a solid-core nuclear thermal rocket. (Backdrop image courtesy of NASA.)

Engine nozzle

Figure 10.11: A cutaway diagram of a solid-core nuclear thermal rocket. (Backdrop image courtesy of NASA.)

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