Pulse propulsion is an area that captures many peoples’ imaginations, because it is the only near-term option for developing a drive system that is high in both specific impulse and in thrust – indeed, especially with earlier designs, the challenge was making the ship BIG ENOUGH to handle the power of the drive system!
Ever since the early days of the Manhattan Project, many nuclear physicists (who were in the process of becoming nuclear engineers) envisioned harnessing the incredibly powerful and energy-dense explosive capability of the atom to propel a spacecraft. After the war, a number of them – including Stanislaw Ulam, who seems to be the originator of the concept – went to work at General Atomics under Freeman Dyson to develop a spacecraft to take men to the moon and beyond. This was Project Orion, the first of the nuclear pulse propulsion designs. Even today, the sheer scale of this craft boggles the mind, and for good engineering reasons: the bigger the ship, the EASIER it is to make the traditional Orion design work.
The reason for this is that Orion (and many designs after it) uses self-contained, shaped nuclear explosives to propel the ship forward. A propellant material, either heavy (like tungsten) for greater thrust, or lighter and rich in hydrogen (such as polystyrene) for greater specific impulse, is placed in the path of the explosive, to convert the energy of the explosive (which at this point is mostly neutrons and gamma radiation) into a shock wave of plasma, which impacts a pusher plate. This plate has two sets of shock absorbers to slow the impulse of the blast enough that it won’t simply destroy the ship (nuclear explosives, even the smallest ones – which these were – result in large explosions by any human standard), and instead give it a highly efficient and hearty kick forward.
Many tests were carried out, and much design work was put into the pulse units (which is still classified), to make them as economical as possible, to ensure that the design was feasible. However, the Partial Test Ban Treaty of 1963 resulted in the cancellation of the program, and it was shelved. We’ll look at this design more once I’ve got time to do so, but in the meantime, George Dyson’s book “To Mars by A-Bomb,” and the accompanying BBC documentary, are excellent sources of information on this fascinating design, as is the Atomic Rockets page on the drive system and another, even more detailed write-up on the spacecraft options themselves here. The General Atomics reports on the concept (that have been declassified at least) are available here and here. In addition, Aerospace Projects Review has extensively covered the design in three issues, which are available here, here, and here (paywall warning, but if you want the dirty details, this is the place to find them!). Finally, a number of 3d animators have done excellent renderings of this drive. Stephen Philips has done a very good animation using Space Shuttle SRBs for the initial launch here, and DrRhysy has done some wonderful animations of this system on YouTube, which you can find here, and an epic Cold War space battle between Orion-drive battleships around Jupiter that is definitely worth watching! Many people, from Curious Droid to SciShow and beyond, have covered this drive, because it combines epic capabilities with the fearsome (and feared) power of nuclear weapons.
This was far from the end of the story for pulse propulsion, however. One of the major concerns was that the pulse units were self-contained explosives; if this could not be the case, then the concept could fall outside the test ban treaty limitations. The first to explore this concept is the MiniMag Orion, which carried a nuclear reactor on-board to charge a bank of capacitors. These capacitors then discharged into a device called a Z-pinch, which would force the fissile material into a small enough volume to cause the nuclear pulse to occur. One other advantage to this design is that it can use a smaller amount of fissile material, which in turn means that a smaller spacecraft is possible. Other improvements over the years were evident in the design as well, such as the electromagnetic nozzle used in place of the heavy pusher plate. Tests were carried out at Sandia National Laboratory’s Z Machine in the early 90s, and were promising. Unfortunately, as with most nuclear propulsion concepts, there wasn’t enough political will to continue funding the concept, and it has not been pursued on any scale since. This doesn’t mean that it isn’t still one of the more attractive propulsion concepts, though, or a clever way to eliminate many of the problems that the traditional Orion had. The original paper can be found here, and of course Winchell Chung has done an excellent write-up of the concept at Atomic Rockets as well.
This is also where I’m going to break my general “no fusion before I cover fission” rule, because while in general the two processes are different enough, with different enough challenges, that I want to hold off on addressing the different complexities of the other nuclear processes, for pulse propulsion there are a few designs that are similar enough that they fit easily in this section.
The first concept is Medusa, developed by Johndale Solem of Los Alamos National Lab. This concept basically flops the Orion concept around, and uses a much larger explosive. Instead of having a pusher plate, with all of the heavy compression structural members that add mass, Solem uses a sail, with hundreds of cables (known as spinnakers) connected through a shock absorber ring to the payload/crew compartment at the rear of the spacecraft. This results not only in mass savings, but also the capability to use thermonuclear warheads from decommissioned weapons to travel across the solar system – a true swords to plowshares concept. However, the political implications of dozens or hundreds of actual thermonuclear weapons (albeit ones being used for peaceful purposes) make this concept a difficult one for many people to swallow. In addition, while the sail may seem like a simple concept, the hundreds of spinnakers and the complex firing process ends up making this a complex drive system to use. However, it DOES offer attractive capabilities for crewed missions to the outer solar system. The original papers are available here and here, Paul Gisler at Centauri Dreams has written two articles on the concept here and here, and the Atomic Rockets page for the concept is available here. Also, check out Nick Stevens’ excellent animation of the drive sequence on YouTube here.
The second of these is a US Air Force Academy design study for an interstellar probe to Alpha Centauri largely modeled off the Orion concept, but using fusion charges instead of fission ones. This leads to a higher specific impulse, higher thrust, and the capacity for interstellar missions. Known as Longshot, this is a serious contender for interstellar missions – something that fission, and many fusion, designs find simply out of reach. The original paper is available here.
Finally, there is a hybrid fission-fusion pulsed drive currently being researched by NASA, known as PuFF (Pulsed Fission-Fusion). In thermonuclear warheads, a fission primary is used to initiate the intense pressures and temperatures needed for the deuterium and tritium fusion fuel to ignite. This, in turn, releases neutrons and additional pressure, causing the uranium that didn’t undergo fission to split, increasing the temperature and pressure to cause the unburned D and T to fuse, and so on, until all the fuel is spent. This drive uses this fission-fusion cascade to turn liquid lithium into plasma, which is then ejected out the back of the spacecraft through a magnetic nozzle. The NIAC Phase I final report is available here, the presentation slides are available here, and the Atomic Rockets write-up is available here.
Many other fusion drives are pulse drives, using inertial, magnetic, or electrostatic confinement, but the physics behind these drives is different enough that we’ll leave them be for now.
This page is a work in progress, and I hope to cover the individual systems (as well as a few others that have been skipped) in their own pages in the future; this drive class has too much potential, though, to not be covered at all on this page even in its’ early stages.