With recent speculation about a Russian nuclear-powered cruise missile, we look at the ways nuclear energy can be used to provide propulsion—from the brute force approach of nuclear pulse propulsion, through nuclear thermal rockets and towards possible future direct fusion propulsion. Nuclear propulsion may have applications both in defense and also for long-range space exploration.
Early Aircraft Engines
Nuclear propulsion actually has a long history that goes back to the start of the cold war period. Both the USA and the USSR developed working nuclear-powered aircraft engines during the 1950’s. These engines had compressor air intakes, similar to conventional turbojet engines. Instead of a combustion chamber, they used a small nuclear reactor with a heat exchanger. This is similar to a nuclear power station but instead of heating water to drive a steam turbine, air is heated as it passes through the jet engine.
Engines were divided into Direct Air Cycle (which pass air directly through the reactor core) and Indirect Air Cycle (which use an external heat exchanger with a coolant fluid such as molten metal or salt).
The third Heat Transfer Reactor Experiment (HTRE-3) engine was a direct air cycle nuclear engine that was successfully ground tested in the 1950’s.
Nuclear aircraft were intended to stay airborne for long periods of time, on top of carrying nuclear bombs. A major issue was with them was the radioactive pollution that their use would cause as well as the risk of major contamination in a crash. The development of intercontinental ballistic missile systems meant they were no longer required and the development was stopped before they were used to power aircraft.
Nuclear Pulse Propulsion
Nuclear pulse propulsion would use nuclear explosions, detonated behind a reaction surface on a spacecraft, to push the craft forward. Both internal and external pulse engines have been considered. External pulse—also known as external pulsed plasma propulsion (EPPP)—is the original and simplest. Small directional nuclear explosives would be ejected from the spacecraft and detonated some distance behind it. These explosives would contain a disc of propellant material designed to produce a jet of plasma on detonation. The plasma cloud would impact on a large pusher plate at the back of the spacecraft, providing huge amounts of thrust at a very highly specific impulse (requiring very little reaction mass). A crewed spacecraft using this type of propulsion would require a very large shock absorber between the pusher plate and the crew module, to even out the pulses of accelerations produced by the nuclear detonations.
A nuclear pulse propulsion unit from Project Orion—a directional nuclear explosive. The nuclear explosion would vaporize the propellant resulting in a jet of plasma which would impact on a pusher plate at the bottom of the spacecraft. Many of these devices would be required to accelerate and then decelerate the spacecraft.
External nuclear pulse propulsion was first studied by Project Orion between 1958 and 1965. This project showed that it was possible, with the technology available at that time, to build a spacecraft with a crew of more than 200 that could complete a round-trip mission to Mars in 4 weeks or around Saturn’s moons in 7 months. Issues with crew shielding and pusher plate ablation appeared to be solved. However, nuclear fallout from detonations within the atmosphere was still an issue and the program was shut down.
Johndale Solem created his Medusa design concept as an improved form of nuclear pulse propulsion. It uses a huge sail deployed in front of the spacecraft to more efficiently catch the plasma from the nuclear detonations. This has the advantage of the shock-absorbing structure becoming a simple cable in tension. Electricity can also be generated on the spacecraft by reeling out this cable as the explosions impact.
In the 1970’s internal nuclear pulse propulsion began to be considered. At the time it was believed that inertial confinement fusion would soon be a viable form of energy. This uses high-energy beams such as lasers to initiate fusion in a fuel pellet. Project Daedalus suggested that such small fusion detonations could produce a steadier stream of plasma from a containment vessel mounted directly to the back of a spacecraft. In the 1980’s NASA took this idea further with Project Longshot which would use a fission reactor to generate electricity to power inertial confinement lasers as well as other on-board systems including a communications laser.
Research into nuclear pulse propulsion is continuing with ideals such as using antimatter to catalyze nuclear reactions and hybrid magneto-inertial fusion under consideration.
Nuclear thermal rockets
Nuclear thermal rockets (NTR) use the heat from a nuclear reaction to heat a propellant. Typically, a fission reactor is used to heat liquid hydrogen which then expands through a conventional rocket nozzle to produce thrust. This is similar to the early nuclear aircraft engines discussed above. The key difference is that while the nuclear-powered jet engines designed for aircraft use air from an intake as a propellant, NTRs must carry their propellant. Although nuclear power provides many times more energy than chemical fuels, the need to carry propellant means that rocket payload is only expected to be increased by a factor of 2 to 3.
Early NTRs were developed in parallel with nuclear aircraft engines during the cold war in which many ground tests were performed. These used a fission reactor with control rods to heat the propellant and were conceptually quite similar to the aircraft engines. The most powerful nuclear rocket tested was the Phoebus-2A Project Rover engine at 4.5 GW, with an exhaust temperature of 3,311 K (5,500°F). This produced 250,000 pounds of thrust with 850 seconds of specific impulse. Its longest burn time during testing was 90 minutes.
NASA stopped actively developing NTR technology in 1972. However, in the last two years, the agency has been given $225M for NTR research with the aim of a test flight in 2024. Elon Musk seems to suggest that private companies should move forward with the mature technologies of chemical rockets while NASA focuses on NTR development.
Within the last 10 years there has also been some effort directed at developing a direct fusion drive. This concept would use magnetic confinement to control a fusion reaction and direct a jet of plasma from it. It shows the potential to achieve very high thrust and specific impulse while also being controllable and producing auxiliary power. The plasma undergoing fusion would be confined in a toroidal magnetic. Charged reaction products would be magnetically directed into a jet and augmented by additional propellant.
The Future of Nuclear Propulsion
It has been suggested that, compared to a chemical rocket, an NTR could reduce transit time from Earth to Mars from 240 days to 100 days. Reduced transit time is important for crew health, reducing exposure to radiation and time in microgravity. However, this transit time is still far more than the 28 days that Project Orion showed could be achieved using nuclear pulse propulsion. It’s looking like we may be entering the age of nuclear-powered space flight. External pulse propulsion remains the only current technology feasible for crewed missions to more distant destinations such as Saturn’s moons and even the stars. A direct fusion drive may provide even higher performance in the future.