Science Fair Project Encyclopedia
- This article is about Project Orion the spacecraft propulsion project. For the laser broom project, see Laser broom
Project Orion was the first engineering design for a spacecraft powered by nuclear pulse propulsion, led by a team at General Atomics in the 1950s. The design appeared to offer capabilities well in excess of even the most advanced conventional or nuclear rocket engines then under study, making "routine" interplanetary travel a possibility.
In the 1954 Operation Castle nuclear test series at Bikini Atoll, a crucial experiment by Lew Allen proved that nuclear explosives could be used for propulsion. Two graphite-covered steel spheres were suspended near the test article for the Castle Bravo shot. After the explosion, they were found intact some distance away, proving that engineered structures could survive a nuclear fireball.
From 1957 through 1964 this information was used to design a spacecraft propulsion system called "Orion" in which nuclear explosives would be thrown through a pusher-plate mounted on the bottom of a spacecraft and exploded underneath. The shock wave and radiation from the detonation would impact against the underside of the pusher plate, giving it a powerful "kick," and the pusher plate would be mounted on large two-stage shock absorbers which would transmit the acceleration to the rest of the spacecraft in a smoother manner.
Radiation shielding for the crews was thought to be a problem, but on ships that mass more than a thousand tons, the material of the pusher plate is sufficiently thick to shield the crew from the explosives' radiation. Radiation shielding goes up as the exponent of the thickness (see gamma ray for a discussion of shielding).
At low altitudes, during take-off, the fallout was extremely dirty, and there was a grave danger of fluidic shrapnel being reflected from the ground. The solution was to use a flat plate of explosives spread over the pusher plate, to get two or three detonations from the ground before going nuclear. This would lift the ship far enough into the air that a focused nuclear blast would avoid harming the ship.
A preliminary design for the explosives was produced. It used a fusion-boosted fission explosive. The explosive was wrapped in a beryllium oxide "channel filler", which was surrounded by a uranium radiation mirror. The mirror and channel filler were open ended, and in this open end a flat plate of tungsten propellant was placed. The whole thing was wrapped in a can so that it could be handled by machinery scaled-up from a soft-drink vending machine.
At 1 microsecond after ignition, the gamma bomb plasma and neutrons would heat the channel filler, and be somewhat contained by the uranium shell. At 2-3 microseconds, the channel filler would transmit some of the energy to the propellant, which would form a cigar-shaped explosion aimed at the pusher plate.
The plasma would cool to 25,000 °F (14,000 °C), as it traversed the 75 ft (25 m) distance to the pusher plate, and then reheat to 120,000 °F (67,000 °C), as (at about 300 microseconds) it hit the pusher plate and recompressed. This temperature emits ultraviolet, which is poorly transmitted through most plasmas. This helps keep the pusher plate cool. The cigar shape and low density of the plasma reduces the shock to the pusher plate.
The pusher plate's thickness decreases by about a factor of 6 from the center to the edge, so that the net velocity of the inner and outer parts of the plate are the same, even though the momentum transferred by the plasma increases from the center outwards.
Deep in the atmosphere where the surrounding air is dense gamma scattering could potentially harm the crew.
Stability was thought to be a problem due to random placement errors of the bombs, but it was later shown that over time the random errors would tend to cancel out.
A one-meter model using RDX (chemical explosives), called "put-put", flew a controlled flight for 23 seconds, to a height of 185 feet at Point Loma.
The shock absorber was at first merely a ring-shaped airbag. However, if an explosion should fail, the 1000 ton pusher plate would tear away the airbag on the rebound. A two-stage, detuned shock absorber design proved more workable. On the reference design, the mechanical absorber was tuned to 1/2 the bomb frequency, and the air-bag absorber was tuned to 4.5 the bomb expulsion frequency.
Another problem was finding a way to push the explosives past the pusher plate fast enough that they would explode 20 to 30m beyond it, and do so every 1.1 seconds. The final reference design used a gas gun to shoot the devices through a hole in the pusher plate.
The Orion drive combines high effective exhaust velocity (6000 seconds ) with thousands of tonnes of thrust. Many drives can achieve one or other, but no other drive can achieve both simultaneously.
Missions that were designed for an Orion vehicle included single stage to Mars and back, from the surface of the Earth, and a trip to one of the moons of Saturn.
A larger version of the Orion drive was planned for Daedelus, an interstellar mission.
The expense of the fissionables was thought high, until Ted Taylor proved that with the right designs for explosives, the amount of fissionables used on launch was close to constant for every size of Orion, from 2000 tons to 8,000,000 tons. Smaller ships actually use more fissionables, because they cannot use fusion bombs (though the later Project Daedalus design used fusion explosives detonated by electron beam inertial confinement, which could be scaled down much smaller than self-contained bombs). The large size bombs used more explosives to super-compress the fissionables (reducing the fallout). The extra explosives simply served as propulsion mass. The expense of launch for the largest size of Orion was 5 cents per pound (11 cent/kg) to Earth orbit in 1958 dollars. In 2005 dollars, the cost would be 32 cents/lb or 70 cents/kg.
Sizes of Orion vehicles
A 1959 report by General Atomics, "Dimensional Study of Orion Type Spaceships," (Dunne, Dyson and Treshow), GAMD-784 explored the parameters of three different sizes of hypothetical Orion spacecraft:
|Ship Diameter||17-20 m||40 m||400 m|
|Ship Mass||300 T||1-2000 T||8,000,000 T|
|Number of bombs||540||1080||1080|
|Individual Bomb Mass||0.22 T||0.37-0.75 T||3000 T|
The most amazing to consider is the "super" Orion design; At 8 million tons, it could easily be a city. In interviews, the designers contemplated the large ship as a possible interstellar ark. This extreme design was buildable with materials and techniques that could be obtained or anticipated in 1958. The real upper limit is probably larger now.
Most of the three thousand tons of each of the "super" Orion's propulsion units would be inert material such as polyethylene, or boron salts, used to transmit the force of the propulsion unit's detonation to the Orion's pusher plate, and absorb neutrons to minimize fallout. One design proposed by Freeman Dyson for the "Super Orion" called for the pusher plate to be composed of uranium or a largely transuranic element so that upon reaching a nearby star system the plate could be converted to nuclear fuel.
Exposure to repeated nuclear blasts raises the problem of ablation (erosion) of the pusher plate. However, calculations and experiments indicate that a steel pusher plate would ablate less than 1 mm if unprotected. If sprayed with an oil, it need not ablate at all. The absorption spectra of carbon and hydrogen minimize heating. The design temperature of the shockwave, 120,000 °F (67,000 °C), emits ultraviolet. Most materials and elements are opaque to ultraviolet, especially at the 50,000 lbf/in2 (340 MPa) pressures the plate experiences. This prevents the plate from melting or ablating.
One issue that remained unresolved at the conclusion of the project was whether the turbulence created by the combination of the propellant and ablated pusher plate would dramatically increase the total ablation of the pusher plate. According to Freeman Dyson, whilst back in the 1960s they would have had to actually perform a test with a real nuclear explosive to determine this, with modern simulation technology this could be determined fairly accurately without such.
The unsolved problem for a launch from the surface of the Earth is the nuclear fallout. Freeman Dyson, an early worker on the project, estimated that with conventional nuclear weapons, each launch would cause fatal cancers in ten human beings from the fallout.
However, the fallout for the entire launch of a 6000 short ton (5500 metric ton) Orion was only equal to a ten-megaton of TNT (40 petajoule) blast, and he was assuming use of weapon-type nuclear explosives.
With special designs of the nuclear explosive, Ted Taylor estimated that it could be reduced ten-fold, or even to zero if a pure fusion explosive could be constructed. However, bomb designers are reluctant to design such an explosive, because it is thought to be destabilizing, and tempting to terrorists. Project Daedalus solved this problem through the use of electron beam inertial confinement, which is not suitable for use in weaponized explosives.
The vehicle and its test program would violate the International test ban treaty as currently written. This could almost certainly be solved, if the fallout problem were solved.
The launch of such a craft from the ground or from low Earth orbit would generate an electromagnetic pulse that could cause significant damage to computers and satellites, as well as flooding the van Allen belts with high-energy radiation. This problem might be solved by launching from very remote areas. EMP footprints are only a few hundred miles wide. The Earth is well-shielded from the Van Allen belts.
True engineering tests of the vehicle systems were said to be impossible because several thousand nuclear explosions could not be performed in any one place. However, experiments were designed to test pusher plates in nuclear fireballs. Long-term tests of pusher plates could occur in space. Several of these almost flew. The shock-absorber designs could be tested full-scale on Earth using chemical explosives.
Assembling a pulse drive spacecraft in orbit by more conventional means and only activating its main drive at a safer distance would be a less destructive approach. Such a system would be much less efficient than the pure pulse approach, because no chemical rocket could conceivably launch a big enough pusher plate to take full advantage of the thrust of the explosions. Adverse public reaction to any use of nuclear explosives is likely to remain a hindrance even if all practical and legal difficulties are overcome.
The Plumbbob Test
A test similar to the test of a pusher plate apparently happened by accident during a series of nuclear containment tests called "Plumbbob" in 1957. A low-yield nuclear explosive accelerated a massive (900 kg) steel capping plate above escape velocity. See the account by the experimental designer, Dr. Robert Brownlee. Although his calculations showed that the plate would reach six times escape velocity, and the plate was never found, he believes that the plate never left the atmosphere. It probably vaporized from friction. The calculated velocity was sufficiently interesting that the crew trained a high-speed camera on the plate, which unfortunately only appeared in one frame. Brownlee estimated a lower bound of 2 times escape velocity.
Appearance in Fiction
A Project Orion spaceship features prominently in the science fiction novel Footfall by Larry Niven and Jerry Pournelle. In the face of an alien siege/invasion of Earth, the humans must resort to drastic measures to get a fighting ship into orbit to face the alien fleet.
In the novel King David's Spaceship by Jerry Pournelle inhabitants of a planet that is to be re-admitted to the Empire plot to build the spaceship based on an Orion project concept in order to qualify their planet as a higher developed, class one Imperial World with self-government.
Poul Anderson's novel Orion Shall Rise features a post-collapse confederation gathering forbidden nuclear materials for some unknown end -- although the title gives away the true nature of their mysterious project.
Another use of the Orion project was in The Stone Dogs by S. M. Stirling. The spacecraft is created during an arms race between The Domination of the Draka and the Alliance for Democracy, and used by both sides in their explorations of the solar system and as warships.
In the movie Deep Impact the United States launches a deep-space vessel, powered by an Orion drive and called Messiah, to destroy a rogue comet that threatens to destroy Earth.
- "Project Orion: The True Story of the Atomic Spaceship", George Dyson, 2002, ISBN 0805072845
- "Nuclear Pulse Propulsion (Project Orion) Technical Summary Report" RTD-TDR-63-3006 (1963-1964); GA-4805 Vol. 1, Reference Vehicle Design Study, Vol. 2, Interaction Effects, Vol. 3, Pulse Systems, Vol. 4, Experimental Structural Response. (From the National Technical Information Service, U.S.A.)
- "Nuclear Pulse Propulsion (Project Orion) Technical Summary Report" 1 July 1963- 30 June 1964, WL-TDR-64-93; GA-5386 Vol. 1, Summary Report, Vol. 2, Theoretical and Experimental Physics, Vol. 3, Engine Design, Analysis and Development Techniques, Vol. 4, Engineering Experimental Tests. (From the National Technical Information Service, U.S.A.)
- Problems with the Orion project
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