Project Orion is a space vehicle propulsion system leveraging exploding atomic bombs behind the vehicle. The apparent absurdity of this idea is one of the reasons why Orion failed; yet, many prominent physicists who worked on the concept were convinced that it could be made practical.
Initiated in 1958, the project was led by Ted Taylor at General Atomics and physicist Freeman Dyson. In principle, Orion could have placed 150 people on the moon or sent expeditions to Mars and Saturn. The concepts developed during the seven-year life of the project are solid and they deserve serious consideration today.
Before examining the Orion system, let’s examine a traditional ship with a chemical propulsion system. A chemically-propelled ship accelerates by expelling part of its mass at high speed. The Tsiolkovsky rocket equation evaluates the propellant required to perform a given orbital maneuver. For a given mass ratio
R = take-off mass/final mass
and an effective exhaust velocity u, the maximal change of velocity of the ship is
v = u ln R .
To achieve a large v, either the mass of fuel must be huge (growing exponentially as v rises) or u must be very high, or some combination of the two. Staging (decreasing mass by throwing out the previous stage) can improve the effective mass ratio, but it increases risk and complexity and still leads to an exponential increase in fuel with the maximal velocity.
A chemical propulsion system has a typical exhaust velocity of about 3–4 km/s. Even with multiple staging, mass ratios are of the order of R = 16 for low earth orbit or R = 1024 for landing on the moon and returning.
Because of this exponential increase in the mass ratio with the required velocity, chemically propelled ships are very expensive and have very limited payloads. They work well for near Earth travels, but become very uneconomic for anything beyond that.
“Nuclear bomb detonations could take over from chemical propulsion as an energy source for long-range space travel” - Freeman Dyson, 1968
In contrast, the core virtue of an Orion ship is that it has only one stage, with a mass ratio well under 10 for long trips around the solar system. Nuclear fission fuel makes this possible as it contains more than a million times as much energy per unit mass as chemical fuel.
The idea is to propel a spaceship forward by detonating nukes ~100 meters behind it. Since atomic bombs are discrete entities, the system has to operate in a pulsed rather than a continuous mode. If you doubt that bombs can be stable enough for any kind of useful propulsion, take a look at the video of the “hot rod” prototype the Orion team built to prove just that:
The Orion design comprises a pusher plate connected via a series of shock absorbers to propellant magazines and a payload/crew section. The shock absorbers store the impulse on the pusher plate and transfer the momentum gradually to the payload and crew module.
The advantage of this system is that no attempt is made to confine the explosions, implying that relatively high-yield (hence high-power) bombs may be used. It is also not temperature-limited. Rather, it relies on the brevity of the explosions to confine the thermal damage to a thin surface layer. The lifetime of the design is then limited by the erosion of the pusher plate by the heat of the nuclear detonation, but coating the pusher plate with a layer of oil has been found to dramatically reduce the ablation. As a result, the pusher plate design can simultaneously produce high thrust with high exhaust velocity, two highly desirable features.
The Orion workers wanted a spaceship that was simple, rugged, capacious, and above all affordable (Image credit: scottlowther@ix.netcom.com)
The performance of the ship is mainly restricted by the capacity of shock absorbers to transfer momentum from an impulsively accelerated pusher plate to the smoothly accelerated ship. Dyson’s calculations show that the properties of available materials limit the velocity transferred by a single explosion to any fragile extended structure to about 30 m/s, independent of the nature and size of the explosion.
If we assume a mean acceleration of 1 g for the ship, with a velocity transfer of 30 meters/sec per explosion, the interval between explosions will be 3 seconds, a reasonably practical interval. Velocities of 10000 km/s can then be achieved in around 10 days. Missions in the 10000 km/s class could reach Mars or Saturn in a matter of weeks and nearby stars in the course of a few centuries. The Orion vehicle is thus capable of long missions around the solar system and beyond.
Of course, detonating nuclear bombs on Earth is not acceptable for obvious reasons. An actual implementation of the Orion concept would probably happen in two stages, first using traditional propulsion before switching to nuclear propulsion when far enough from Earth. In any case, the technology required to build an Orion-type spaceship has existed for over forty years, and progresses in materials and structural design since Dyson’ time can only improve the concept’s viability. And even if not used in practice, the Orion project is a testimony to the creativity of scientists.
Further reading
- Freeman J. Dyson, “Interstellar transport”, Annals of the New York Academy of Sciences 163, 347 (1969)
- George Dyson, “Project Orion: The True Story of the Atomic Spaceship” (2003)
- Erik S. Pedersen, “Nuclear Propulsion in Space” (Englewood Cliffs, NJ: Prentice-Hall Inc., 1964)
