(For instance, the most fundamental reason that current and proposed electric propulsion systems of high I sp tend to be low thrust is due to their limits on available power. For any rocket propulsion, since the kinetic energy of exhaust goes up with velocity squared ( kinetic energy = ½ mv 2), whereas the momentum and thrust go up with velocity linearly ( momentum = mv), obtaining a particular level of thrust (as in a number of g acceleration) requires far more power each time that exhaust velocity and I sp are much increased in a design goal. Specific impulse ( I sp) measures how much thrust can be derived from a given mass of fuel, and is a standard figure of merit for rocketry. Many spacecraft propulsion drives can achieve one of these or the other, but nuclear pulse rockets are the only proposed technology that could potentially meet the extreme power requirements to deliver both at once (see spacecraft propulsion for more speculative systems). The Orion nuclear pulse drive combines a very high exhaust velocity, from 19 to 31 km/s (12 to 19 mi/s) in typical interplanetary designs, with meganewtons of thrust. 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) based on less speculative technology. However, 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. The Partial Test Ban Treaty of 1963 is generally acknowledged to have ended the project. Supporters of Project Orion felt that it had potential for cheap interplanetary travel, but it lost political approval over concerns about fallout from its propulsion. Orion would have offered performance greater than the most advanced conventional or nuclear rocket engines then under consideration. As a qualitative 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. The unprecedented extreme power requirements for doing so would be met by nuclear explosions, of such power relative to the vehicle's mass as to be survived only by using external detonations without attempting to contain them in internal structures. The Orion concept offered high thrust and high specific impulse, or propellant efficiency, at the same time. The actual project, initiated in 1958, 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. ![]() Reines and Ulam in a Los Alamos memorandum dated 1947. Real life proposals of nuclear propulsion were first made by Stanislaw Ulam in 1946, and preliminary calculations were made by F. Heinlein mentions powering spaceships with nuclear bombs in his 1940 short story " Blowups Happen". The idea of rocket propulsion by combustion of an explosive substance was first proposed by Russian explosives expert Nikolai Kibalchich in 1881, and in 1891 similar ideas were developed independently by German engineer Hermann Ganswindt. ![]() The project was eventually abandoned for multiple reasons, including the Partial Test Ban Treaty, which banned nuclear explosions in space, and concerns over nuclear fallout. Six non-nuclear tests were conducted using models. Early versions of this vehicle were proposed to take off from the ground later versions were presented for use only in space. Project Orion was a study conducted between the 1950s and 1960s by the United States Air Force, DARPA, and NASA for the purpose of measuring the efficacy of a starship directly propelled by a series of explosions of atomic bombs behind the craft- nuclear pulse propulsion. Artist's conception of the NASA reference design for the Project Orion starship powered by nuclear propulsion
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