Project Orion (nuclear propulsion)

Project Orion was a study conducted in the 1950s and 1960s by the United States Air Force, DARPA,^[1] and NASA into the viability of a nuclear pulse spaceship that would be directly propelled by a series of atomic explosions behind the craft.^[2][3] Early versions of the vehicle were proposed to take off from the ground; later versions were presented for use only in space. The design effort took place at General Atomics in San Diego,^[4] and supporters included Wernher von Braun,^[5] who issued a white paper advocating the idea.^[2][6] Non-nuclear tests were conducted with models,^[7] but the project was eventually abandoned for several reasons, including the 1963 Partial Test Ban Treaty,^[8] which banned nuclear explosions in space, amid concerns over nuclear fallout.^[2]

Physicist Stanislaw Ulam proposed the general idea of nuclear pulse propulsion in 1946,^[9] and preliminary calculations were made by Frederick Reines and Ulam in a Los Alamos memorandum dated 1947.^[9][3][10] In August 1955, Ulam co-authored a classified paper proposing the use of nuclear fission bombs, "ejected and detonated at a considerable distance", for propelling a vehicle in outer space.^[4][9] The project was led by Ted Taylor at General Atomics and physicist Freeman Dyson who, at Taylor's request, took a year away from the Institute for Advanced Study in Princeton to work on the project.^[11] In July 1958, DARPA agreed to sponsor Orion at an initial level of $1 million per year, at which point the project received its name and formally began.^[11][7] The agency granted a study of the concept to the General Dynamics Corporation,^[8] but decided to withdraw support in late 1959.^[11] The U.S. Air Force agreed to support Orion if a military use was found for the project, and the NASA Office of Manned Spaceflight also contributed funding.^[4] The concept investigated by the government used a blast shield and shock absorber to protect the crew and convert the detonations into a continuous propulsion force.^[12][13] The most successful model test, in November 1959, reached roughly 100 meters in altitude with six sequenced chemical explosions.^[7] NASA also produced a Mars mission profile for a 125 day round trip with eight astronauts, at a predicted development cost of $1.5 billion.^[5] Orion was canceled in 1964, after the United States signed the Partial Test Ban Treaty the prior year; the treaty greatly reduced political support for the project.^[8][5] NASA had also decided, in 1959, that the civilian space program would be non-nuclear in the near-term.^[11]

The Orion concept offered both high thrust and high specific impulse, or propellant efficiency: 2,000 pulse units (Isp) under the original design and an Isp of perhaps 4,000 to 6,000 seconds according to the Air Force plan, with a later 1968 fusion bomb proposal by Dyson potentially increasing this to more than 75,000 Isp, enabling velocities of 10,000 km/sec.^[5] A moderate-sized nuclear device was estimated, at the time, to produce about 5 or 10 billion horsepower.^[11][14] The extreme power of the nuclear explosions, relative to the vehicle's mass, would be managed by using external detonations, although an earlier version of the pulse concept did propose containing the blasts in an internal pressure structure, with one such design prepared by The Martin Company.^[5][11] As a qualitative power comparison, traditional chemical rockets, such as the Saturn V that took the Apollo program to the Moon, produce high thrust with low specific impulse, whereas electric ion engines produce a small amount of thrust very efficiently. Orion, by contrast, would have offered performance greater than the most advanced conventional or nuclear rocket engines then under consideration. Supporters of Project Orion felt that it had potential for cheap interplanetary travel.^[15]

From Project Longshot to Project Daedalus, Mini-Mag Orion, and other proposals which reach engineering analysis at the level of considering thermal power dissipation, the principle of external nuclear pulse propulsion to maximize survivable power has remained common among serious concepts for interstellar flight without external power beaming and for very high-performance interplanetary flight.^[5] Such later proposals have tended to modify the basic principle by envisioning equipment driving detonation of much smaller fission or fusion pellets, in contrast to Project Orion's larger nuclear pulse units (full nuclear bombs). In 1979 General Dynamics donated a 26 inch (56 cm) tall wooden model of the craft to the Smithsonian, which displays it at the Steven F. Udvar-Hazy Center near Dulles International Airport in Northern Virginia.^[8]