Image Credit: Project Orion
Compared with the best chemical rockets, nuclear propulsion systems (NPS's) are more reliable and flexible for long-distance missions, and can achieve a desired space mission at a lower cost. The reason for these advantages in a nutshell is that NPS's can get "more miles per gallon" than a chemical rockets.
For any space mission, basic questions must be answered:
1 - What is the destination?
2 - What is the trip time?
3 - Do we want to return?
4 - the mass of the payload we want to send there & bring back?
In chemical rocket engines such as the Space Shuttle Main Engine (SSME), the chemical reaction between the hydrogen and oxygen releases heat which raises the combustion gases (steam and excess hydrogen gas) up to high temperatures (3000-4000 K). These hot gases are then accelerated through a thermodynamic nozzle, which converts thermal energy into kinetic energy, and hence provides thrust. The propellant and the heat source are one in the same.
Because there is a limited energy release in chemical reactions and because a thermodynamic nozzle is being used to accelerate the combustion gases that do not have the minimum possible molecular weight, there is a limit on the exhaust velocity that can be achieved.
The maximum specific impulse Isp that can be achieved with chemical engines is in the range of 400 to 500 s. So, for example, if we have an Isp of 450 s, and a mission delta-V of 10 km/s (typical for launching into low earth orbit (LEO)), then the mass ratio will be 9.63. The problem here is that most of the vehicle mass is propellant, and due to limitations of the strength of materials, it may be impossible to build such a vehicle, just to ascend into orbit.
Early rocket scientists got around this problem by building a rocket in stages, throwing away the structural mass of the lower stages once the propellant was consumed. This effectively allowed higher mass ratios to be achieved, and hence a space mission could be achieved with low-Isp engines. This is what all rockets do today, even the Space Shuttle. In spite of the relatively low Isp, chemical engines do have a relatively high thrust-to-weight ratio (T/W).
A high T/W (50-75) is necessary for a rocket vehicle to overcome the force of gravity on Earth and accelerate into space. The thrust of the rocket engines must compensate for the weight of the rocket engines, the propellant, the structural mass, and the payload. Although it is not always necessary, a high T/W engine will allow orbital and interplanetary space vehicles to accelerate quickly and reach there destinations in shorter time periods.
Nuclear propulsion systems have the ability to overcome the Isp limitations of chemical rockets because the source of energy and the propellant are independent of each other. The energy comes from a critical nuclear reactor in which neutrons split fissile isotopes, such as 92-U-235 (Uranium) or 94-Pu-239 (Plutonium), and release energetic fission products, gamma rays, and enough extra neutrons to keep the reactor operating.
The energy density of nuclear fuel is enormous. The heat energy released from the reactor can then be used to heat up a low-molecular weight propellant (such as hydrogen) and then accelerate it through a thermodynamic nozzle in same way that chemical rockets do. This is how nuclear thermal rockets (NTR's) work.
Solid-core NTR's (See Figure 2) have a solid reactor core with cooling channels through which the propellant is heated up to high temperatures (2500-3000 K). Although solid NTR's don't operate at temperatures as high as some chemical engines (due to material limitations), they can use pure hydrogen propellant which allows higher Isp's to be achieved (up to 1000 s).
In gas-core NTR's, the nuclear fuel is in gaseous form and is inter-mixed with the hydrogen propellant. Gas core nuclear rockets (GCNR) can operate at much higher temperatures (5000 - 20000 K), and thus achieve much higher Isp's (up to 6000 s).
Of course, there is a problem in that some radioactive fission products will end up in the exhaust, but other concepts such as the nuclear light bulb (NLB) can contain the uranium plasma within a fused silica vessel that easily transfers heat to a surrounding blanket of propellant. At such high temperatures, whether an open-cycle GCNR, or a closed-cycle NLB, the propellants will dissociate and become partially ionized.
In this situation, a standard thermodynamic nozzle must be replaced by a magnetic nozzle which uses magnetic fields to insulate the solid wall from the partially-ionized gaseous exhaust.
NTR's give a significant performance improvement over chemical engines, and are desirable for interplanetary missions. It may also be possible that solid core NTR's could be used in a future launch vehicle to supplement or replace chemical engines altogether4. Advances in metallurgy and material science would be required to improve the durability and T/W ratio of NTR's for launch vehicle applications.
An alternative approach to NTR's is to use the heat from nuclear reactor to generate electrical power through a converter, and then use the electrical power to operate various types of electrical thrusters (ion, hall-type, or magneto-plasma-dynamic (MPD)) that operate on a wide variety of propellants (hydrogen, hydrazine, ammonia, argon, xenon, fullerenes) This is how nuclear-electric propulsion (NEP) systems work.
To convert the reactor heat into electricity, thermoelectric or thermionic devices could be used, but these have low efficiencies and low power to weight ratios. The alternative is to use a thermodynamic cycle with either a liquid metal (sodium, potassium), or a gaseous (helium) working fluid. These thermodynamic cycles can achieve higher efficiencies and power to weight ratios.
No matter what type of power converter is used, a heat rejection system is needed, meaning that simple radiators, heat pipes, or liquid-droplet radiators would be required to get rid of the waste heat. Unlike ground-based reactors, space reactors cannot dump the waste heat into a lake or into the air with cooling towers.
The electricity from the space nuclear reactor can be used to operate a variety of thrusters. Ion thrusters use electric fields to accelerate ions to high velocities. In principle, the only limit on the Isp that can be achieved with ion thrusters is the operating voltage and the power supply. Hall thrusters use a combination of magnetic fields to ionize the propellant gas and create a net axial electric field which accelerates ions in the thrust direction. MPD thrusters use either steady-state or pulsed electromagnetic fields to accelerate plasma (a mixture of ions and electrons) in the thrust direction. To get a high thrust density, ion thrusters typically use xenon, while Hall thrusters and MPD thrusters can operate quite well with argon or hydrogen.
Compared with NTR's, NEP systems can achieve much higher Isp's. Their main problem is that they have a low power to weight ratio, a low thrust density, and hence a very low T/W ratio. This is due to the mass of the reactor, the heat rejection system, and the low-pressure operating regime of electrical thrusters.
This makes NEP systems unfeasible for launch vehicle applications and mission scenarios where high accelerations are required; however, they can operate successfully in low-gravity environments such as LEO and interplanetary space.
In contrast to a chemical rocket or an NTR which may operate only for several minutes to less than an hour at a time, an NEP system might operate continuously for days, weeks, perhaps even months, as the space vehicle slowly accelerates to meet its mission delta-V. An NEP system is well suited for unmanned cargo missions between the Earth, Moon and other planets.
For manned missions to the outer planets, there would be a close competition between gas-core NTR's and high-thrust NEP systems.
The performance gain of nuclear propulsion systems over chemical propulsion systems is overwhelming. Nuclear systems can achieve space missions at a significantly lower cost due to the reduction in propellant requirements.
When humanity gains the will to explore and develop space more ambitiously, nuclear propulsion will be an attractive choice.
Source: Nuclear Propulsion from Astro Digital. - Quasar9
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Innovative Nuclear Space Power and Propulsion Institute University of Florida
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