By Kelvin LongThe idea of using atomic bombs to propel a spacecraft was first proposed by Stanislaw Ulam in 1946 and then extended further by Ulam and Cornelius Everett in 1955, both physicists working on the Manhattan project during the Second World War. They talked about using low yield detonations located tens of meters from the external spacecraft at a frequency of one per second and producing an exhaust velocity of around 10 km/s. The initial concept called Helios was to see nuclear detonations occur within a combustion chamber, but with a performance limit due to the surrounding materials.
Project Orion was the name given to the research project in the 1950′s that was initially studied as a research program at the General Atomic Division of General Dynamics in 1958. The British born Physicist Freeman Dyson was one of the people that worked on this exciting research, along with the founder of Project Orion; Theodore Taylor.
The idea here is to detonate many nuclear bombs at the rear of the spacecraft of varying yield and with perhaps a few seconds between each detonation. Each unit would contain a solid layer of tungsten propellant that can be accelerated towards a pusher plate similar to a shaped charge over a cone angle of around 22 degrees. Once around 80% of the expanding explosive products reach the pusher plate they then give it a ‘kick’ and provide the momentum transfer to push the vehicle. This is continued in a succession of such pulses until the required total impulse is achieved.
The energy source would be displaced some distance, 50-100 m, rear of the spacecraft by ejecting it through a central hole in the pusher. It would then detonate and some fraction of the expanding explosion propellant would be intercepted by a large circular Aluminium or Steel pusher plate over a solid angle. There were several issues, but the main one was whether or not the explosively generated hot plasma would melt the pusher plate through ablative mass loss.
The Orion vehicle designs varied in mass and there were several types of space transport vehicles designed throughout its history. This included a nearly 900 tons orbital test version and a 4,000 tons interplanetary version. Advanced 10,000 tons interplanetary versions were also designed with a pusher plate some 56 m in size and capable of visiting most of the planets in the solar system – “to Saturn and back within seven years” went the slogan – and the people involved in Project Orion really believed that it could fly in their lifetimes.
When the project came to an end, Freeman Dyson continued the study for a while to include an interstellar version. This had a 100 m size pusher plate detonating a Hydrogen-based bomb every three seconds and accelerating the spacecraft to one-gravity. The 400,000 tons gross mass vehicle would reach a speed of over three percent of light and reach the nearest stars in a little over a century.
Project Orion was killed off by the existence of several international nuclear test ban treaties and for these political reasons it is not considered a contender for interstellar flight today.
However, it remains a fact that Project Orion did demonstrate in theory that the required performance for deep space interplanetary and interstellar missions was possible using this technology. The only time it may see the light of day is if Earth is threatened by a large asteroid extinction event and in which case Project Orion may be called upon as the only technology that could have been built in decades past.
Kelvin Long is a physicist/aerospace engineer, who is one of the founders of Project Icarus. He is a Fellow of the BIS, Member of the BIS Technical Committee, as well as an active Practitioner of the Tau Zero Foundation.