Nuclear pulse propulsion (NPP) is a radical propulsion concept that utilizes the highly energetic and efficient energy release from nuclear explosions to produce thrust. NPP can deliver specific impulses between 10,000 to 100,000 seconds with average power densities equal to or greater than chemical rockets, using existing technology.
Origin of the Concept
The idea of using explosive pulses to propel a rocket vehicle dates back to the 1890s, but the key breakthrough was the proposal to use nuclear explosives, which have specific energies orders of magnitude higher than chemical explosives. Physicist Stanislaw Ulam first proposed using fission bombs for propulsion in 1946, and the first full mathematical treatment was published by Cornelius Everett and Ulam in 1955.
The specific energy of uranium fission is around 7.8 x 10^7 MJ/kg, corresponding to a maximum theoretical specific impulse of approximately 1.3 million seconds. In comparison, the specific energy of the best chemical explosives is only on the order of 10 MJ/kg. This enormous difference in energy density is what makes nuclear pulse propulsion so compelling from a performance standpoint.
Types of NPP Concepts
There are two basic types of NPP concepts:
- External NPP: The nuclear detonation occurs at a distance from a pusher plate, which absorbs the momentum of the explosion. Specific impulse is limited to 3,000-10,000 seconds by pusher plate ablation. Adding a magnetic field to shield the pusher plate can enable specific impulses over 100,000 seconds.
- Internal NPP: The explosion occurs inside a pressure vessel and heated propellant is expanded through a nozzle. Performance is limited to under 1,500 seconds specific impulse by radiation heating of the vessel walls.
External NPP
In the external concept, the expanding plasma from the nuclear detonation impinges on a pusher plate at high velocity, typically 100-200 km/s. The interaction time is extremely short, on the order of 0.1 milliseconds, so despite the plasma temperatures of tens of thousands of Kelvin, only a thin surface layer of the plate is ablated.
The fraction of the pulse unit mass that strikes the pusher plate is determined by the design of the explosive charge and the standoff distance, and is typically in the range of 10-50%. Combined with the impingement velocity, this results in the specific impulse limit of 3,000-10,000 seconds.
Experiments in the late 1950s demonstrated the survivability of graphite-coated steel plates subjected to nearby nuclear explosions. It was found that the ablation was limited to a very thin surface layer due to the short interaction time, and active cooling of the plate was unnecessary.
External NPP with Magnetic Shielding
The specific impulse limits of the basic external concept can be overcome by using a magnetic field to protect the pusher plate from direct contact with the high-energy plasma. The magnetic field lines are generated parallel to the surface of a conducting plate. As the explosion plasma expands, it compresses the field lines against the conductor, increasing the magnetic flux density. This increased magnetic pressure decelerates the plasma and reflects it, transferring momentum to the plate via magnetic forces rather than direct particle impact.
With magnetic shielding, the plasma can have much higher temperature and velocity since direct contact with the plate is prevented. Specific impulses over 100,000 seconds become theoretically possible. However, the confinement is not perfect, as any high-energy neutral particles will be unaffected by the magnetic field.
Internal NPP
The internal nuclear pulse concept involves detonating the explosive inside a pressure vessel filled with a working fluid, and exhausting the heated propellant through a nozzle. It was originally thought that this configuration would be more efficient by eliminating the losses associated with an external, non-confined explosion.
In the internal design, propellant, typically liquid hydrogen or water, is fed into the vessel where it also serves as a coolant. The nuclear charge is detonated in the center, creating a shockwave that propagates through the propellant and reflects off the chamber walls multiple times. This increases the internal energy of the propellant until an equilibrium pressure is reached, at which point the propellant is exhausted through the nozzle. The cycle is then repeated.
The performance of internal nuclear pulse engines is severely limited by the need to cool the chamber walls, which are subjected to intense neutron and gamma radiation heating. Studies in the 1960s concluded that specific impulses greater than 1,400 seconds would require very heavy engines. The internal designs also tend to have a much higher inert mass compared to external pulse vehicles.
Project Orion (1958-1965)
The most extensive development effort on NPP was Project Orion at General Atomics. Key highlights include:
- Originally envisioned vehicles up to 80 meters high with 40 meter diameter pusher plates, capable of transporting 150 people
- Built and flew “Putt-Putt” chemical explosive flight models to demonstrate impulsive flight stability
- Extensively tested pusher plate durability and ablation countermeasures
- Estimated development costs of $1.2-24 billion, comparable to Apollo program
- Designed “first generation” vehicle compatible with Saturn V, capable of taking 8 astronauts to Mars and back in 125 days
- Project ended in 1965 due to the Partial Nuclear Test Ban Treaty and lack of political support
Early Orion Designs
The Orion concept originated in 1957 with physicist Ted Taylor at General Atomics. Taylor envisioned a vehicle that was simple, rugged, roomy and affordable – in contrast to the expensive and cramped capsules being pursued by NASA at the time. The original design called for a ground launch of a massive 10,000 ton vehicle, 80 meters tall and with a 40 meter diameter pusher plate.
At launch, 0.1 kiloton pulse units would be ejected and detonated behind the vehicle at a rate of 1 per second. As the vehicle accelerated, the pulse rate would decrease and the yield would increase, up to 20 kiloton charges every 10 seconds. The vehicle would fly straight up through the atmosphere to minimize fallout. Taylor and physicist Freeman Dyson developed plans for missions throughout the solar system, with the motto “Mars by 1965, Saturn by 1970.” The 10,000 ton vehicle could carry 150 people and thousands of tons of cargo.
To prove the viability of external pulsed propulsion, the Orion team built a series of chemical explosive driven models called “Putt-Putts.” A November 1959 flight test, powered by six charges, successfully demonstrated stable impulsive flight. These tests also showed that the pusher plate should be thick in the center and tapered at the edges for optimal strength-to-weight.
Extensive experiments were done on pusher plate survivability using explosively-driven plasma generators. A key discovery was that spraying a graphite-based grease on the plate between shots could substantially reduce ablation. The tests found that the plate would be subjected to extreme temperatures for only about 1 millisecond, ablating only a thin surface layer with negligible heat transfer to the rest of the plate.
Downsizing Orion
By late 1959, it was clear that support from the newly-formed NASA would be required to build Orion. However, NASA was hesitant to pursue such a grandiose project, especially given the concerns over nuclear fallout from a ground launch. In an effort to make the concept more palatable, Taylor and Dyson developed a “first-generation” Orion design that would be launched into orbit by Saturn rockets.
This “NASA Orion” featured a 10 meter diameter pusher plate, set by the Saturn V payload envelope. While this constrained the specific impulse to 1800-2500 seconds, it was still far better than any other nuclear propulsion concept at the time. The 100 ton propulsion module would be launched by two or three Saturn Vs and assembled in orbit.
The most developed mission plan was for a 125 day round trip to Mars, carrying 8 astronauts and 100 tons of supplies. The payload fraction was an impressive 45% of the gross vehicle mass in orbit. Despite the enthusiasm of Wernher von Braun and others at NASA’s Marshall Space Flight Center, the agency ultimately decided not to pursue Orion, likely due to concerns over the optics of using nuclear explosives in space.
The End of Orion
Orion faced major challenges from the start due to its reliance on nuclear explosives. The 1963 Partial Nuclear Test Ban Treaty banned detonations in the atmosphere and space, making Orion illegal under international law. The classified nature of the project also meant it had little support from the broader scientific community.
A final blow came in 1965, when the newly-formed NASA decided not to pursue the project, and the Air Force discontinued its funding. In total, about $11 million had been spent over 7 years developing Orion. According to Freeman Dyson, it was the first time in modern history that a major expansion of human technology had been suppressed for political reasons.
Fusion Propulsion Successors
Following Orion, interest shifted to using fusion microexplosions for propulsion to avoid fission political issues and potentially reach even higher performance. Fusion NPP concepts like Project Daedalus and VISTA pushed specific impulses toward 1,000,000 seconds, but relied on major advances in driver technology and fusion ignition.
Fusion reactions release even more energy per unit mass than fission – for example, the D-He3 reaction has a specific energy of 3.5 x 10^8 MJ/kg, about 4.5 times higher than fission. Fusion also allows much smaller pulse units since there is no minimum critical mass as with fissionable materials. Yields on the order of a single ton of TNT equivalent (4.2 GJ) or less are possible.
The British Interplanetary Society’s Project Daedalus study in the 1970s designed a fusion-powered interstellar probe that would achieve 12% of light speed. The 54,000 ton vehicle would be powered by electron beam-ignited D-He3 fusion microexplosions, with the He3 fuel mined from Jupiter’s atmosphere. Daedalus would accelerate for 4 years, reaching 7,600 km/s and covering 5.9 light years in 50 years.
Other fusion pulse propulsion studies have investigated laser ignition (VISTA) and antimatter-catalyzed fission-fusion. While offering even better performance than Orion, fusion propulsion still faces major feasibility challenges, including the need for high fusion energy gain, which has not yet been demonstrated in the laboratory.
Reconsidering Fission NPP
Despite the allure of fusion propulsion, the practical challenges are immense. It may be worth revisiting fission NPP with modern technologies and political realities:
- Advances in actuation of fissionable charges could enable smaller yield explosions
- Improved materials could reduce pusher plate mass and increase ablation resistance
- Peaceful international cooperation on NPP could help with political acceptance
- Microfission NPP using onboard compression of subcritical fissile targets is a promising avenue to mitigate proliferation concerns
Microfission NPP
Perhaps the most promising modern approach to nuclear pulse propulsion is microfission, where subcritical fissile targets are compressed and ignited by an onboard driver, similar to inertial confinement fusion concepts. The key advantage is that the energy required to compress fissile material to supercriticality is orders of magnitude less than for fusion ignition.
With microfission, there are no preassembled nuclear explosives onboard, addressing a major concern with the original Orion design. The fissile targets would be compressed by a driver (e.g. chemical explosives, electromagnetic coils, lasers) to achieve a supercritical geometry and ignite a small-yield detonation. This approach could potentially achieve specific impulses in the range of 10,000-100,000 seconds while avoiding many of the political issues of Orion.
Microfission NPP would still require advancements in high energy density drivers, high-temperature materials, and spacecraft engineering to be feasible. However, the fundamental physics and technologies are much more mature compared to fusion propulsion. Microfission may represent a logical intermediate step between Orion-style fission NPP and the fusion systems of the distant future.
International Cooperation
The political challenges of developing and testing nuclear pulse propulsion cannot be ignored, given the understandable concerns about nuclear proliferation and the militarization of space. However, the current international climate may be more amenable to peaceful, multilateral development of NPP than during the Cold War era of Orion.
A cooperative international effort to develop NPP for space exploration, with full transparency and a focus on safety and non-proliferation, could help alleviate some of the political obstacles. The Orion team estimated development costs of $1.2-24 billion, comparable to the Apollo program – a level of funding that would likely require collaboration between spacefaring nations. The potential benefits of NPP in enabling rapid interplanetary transit and ambitious exploration missions could make the case for such an undertaking.
Fission NPP could also provide a productive way to dispose of the many tons of weapons-grade fissile material stockpiled around the world, by using it as pulse unit fuel. Detonations far outside of Earth’s atmosphere would have negligible environmental impact. Of course, any serious effort to revive fission NPP would need to grapple with these political and security issues from the start.
Summary
Nuclear pulse propulsion, especially using fission, remains a compelling option for ambitious space missions requiring very high speed. While political and environmental challenges exist, modern technologies and approaches could make NPP more feasible and acceptable than during the Orion era. For rapid Mars missions or exploration of the outer solar system and beyond, NPP deserves renewed consideration.
