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| Fig. 1: - Project Orion Rocket Artistic Rendering. [3] (Source: Wikimedia Commons) |
Project Orion was a feasibility study of a nuclear pulse propulsion interstellar spacecraft conducted at General Atomics, lead by Ted Taylor and physicist Freeman Dyson from 1958 to 1963. [1] Nuclear pulse propulsion is where the spacecraft is directly propelled by a series of nuclear (mainly atomic bombs) explosions behind the aircraft. Although bizarre and even implausible upon first hearing about it, it warranted the scientific calculations and engineering that went into the study. After all, even traditional chemical rockets rely on a series of controlled explosions. Nuclear pulse propulsion is like traditional chemical rockets in the way that both are turning densely stored energy, chemical fuel in traditional rockets and atomic bombs in nuclear pulse rockets, into kinetic energy to power the space flight.
Specific impulse is a measure of how efficient a rocket engine uses propellant in order to accelerate the spacecraft. It is defined as the thrust integrated over time per unit weight-on- Earth of the propellant. This is important to mention as gravity is no longer constant when space travelling is considered, so to calculate specific impulse, a fixed gravitational acceleration is used. Specific impulse is measured in seconds. Higher specific impulse on a rocket means that it is more efficient at converting the stored propellant energy into kinetic energy that propels the rocket. To establish a baseline comparison, Saturn V rocket, which was used for all the Apollo missions has a specific impulse of 263 seconds. [2] A Project Orion rocket would have a specific impulse of 2000 seconds conservatively. [3] To put this number into perspective, a payload comparison between Saturn V rockets and the three main rockets proposed by Project Orion is shown in Table 1.
To harvest the extreme power generation of Project Orion, it had to use external detonations of stored energy without trying to contain it inside of the internal structure of the rockets such as a traditional chemical rocket would do. The entire propulsion sequence is detailed as follows: First an atomic bomb is ejected outside of the rocket body, right behind a huge metal plate call the pusher. The atomic bomb is then detonated as it flies right past the pusher. The pusher then absorbed the energy released by the atomic bomb, propels the rocket forward. To smooth out the acceleration from each atomic bomb detonation, a series of multi-stage shock absorbers connect the pusher to the main rocket body. [3] This is illustrated in Fig. 1, a key components drawing from the original study proposal. To achieve this type of rocket performance, the key question is how many atomic bombs each rocket would have to carry. In the feasibility study, a mid-range Orion which is capable of Mars orbit return of 400 tons of payload would use 1080 atomic bombs each weighing in the range of 0.37 to 0.75 tons. [4]
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| Table 1: Comparison between various Project Orion rockets and Saturn V. [1] |
One main concern and focus of the study is the wear and tear on the metal plate absorbing the energy to propel the rocket forward. One solution is to coat the pusher surface with an ablation coating to get rid of the excess heat. This solution leads to a theoretical calculation of an Orion rocket able to achieve 10% of the speed of the light, thus able to reach Alpha Centauri, the nearest star system to the Sun, in the range of 100 to 200 years. [5]
There are three main known issues from this project proposal. First is the ablation of the pusher plate. And the second one is the nuclear pollution it creates during launch into space from the Earth and shielding of the crew from the gamma and neutron radiation in flight and third, is the spallation, which is when the shock broke out of the surface and creates flying tiny metal shrapnel.
The absorption of carbon and hydrogen minimize heating, and the design temperature of the shockwave resulted from the nuclear explosion would reach about 67,000°C and emit ultraviolet light. [3] Given the extreme pressure of the shockwave, most materials are opaque to ultraviolet light, thus preventing the plate from melting.
As expected, the glaring issue of this proposal is the potential nuclear pollution that a rocket launch like this would cause on Earth. First, there is no place on Earth where thousands of nuclear explosions could be tested for a potential design of this rocket to fully validate the concept and design. Second, it was estimated that in order to launch a rocket through nuclear pulse propulsion, it requires approximately one atomic bomb a second, albeit small, in order for the rocket to reach outer space.
The termination of Project Orion was due to two separate reasons, despite the issue of unsolved potential nuclear fallout. The first is the signing of Partial Test Ban Treaty in 1963. The second was due to a lack of need and specific mission. U.S. government, at the time, has no need for such a rocket capable of delivering that much payload to Mars and beyond.
Nuclear pulse propulsion was again later studied by the British Interplanetary Society in 1963 with Project Daedalus and U.S. Navy and NASA in Project Longshot. [6,7]
© Tony Chen. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Nuclear Pulse Space Vehicle Study, Volumn I - Summary," General Atomic, GA-5009-1, September 1964.
[2] "Technical Information Summary AS-501 - Apollo Saturn V Flight Vehicle," U.S. National Aeronautics and Space Administration, R-ASTR-S-67-65, September 1967.
[3] "Nuclear Pulse Space Vehicle Study: Volume IV - Mission Velocity Requirements and System Comparisons," General Atomic, GA-5009-4, February 1966.
[4] G. Dyson, Project Orion: The True Story of the Atomic Spaceship (Henry Holt and Co., 2002).
[5] C. Sagan, Cosmos (Ballantine Books, 2013).
[6] J. Langford, "The Daedalus Project: A Summary of Lessons Learned," American Institute of Aeronautics and Astronautics, AIAA-89-2048, August 1989.
[7] K. A. Beals et al., "Project Longshot: An Unmanned Probe To Alpha Centauri," U.S. National Aeronautics and Space Administration, NASA-CR-184718, 1988.
