Nuclear pulse propulsion

Nuclear pulse propulsion is a form of spacecraft propulsion that uses nuclear explosions for thrust. It was briefly developed as Project Orion by ARPA. It was invented by Stanislaw Ulam in 1957, and is the invention of which he was most proud.

Calculations show that this form of rocket would combine both high thrust and a high specific impulse, a rarity in rocket design. ISPs from 2000 (easy, yet ten times chemical ISPs) to 40,000 (requires specialized nuclear explosives) are possible, with thrusts in the millions of tons.

This is possible because Orion uses nuclear power to make thrust without requiring the power to be held within a rocket chamber. Thus, very high temperatures, exhaust velocities and efficiencies are possible.

An Orion drive is the only known method of performing manned interstellar exploration with current technology. It would be slow, requiring several generations to get to Alpha Centauri (the closest known solar system other than our own), but it would arrive, assuming it had no accidents.

The most likely real requirement for an Orion craft is to deflect an earth-crossing asteroid from impacting the Earth. Such craft could be unmanned, and inexpensive, launched from orbits outside the magnetosphere to minimize radioactives in the biosphere. Simply impacting with the asteroid would be enough to deflect it.

Carrying through the mass ratios, Orion could be built of steel, without special fittings, and carry crews of hundreds. In 1960, the proposed contractor was Electric Boat, the maker of nuclear submarines.

The design reference model proposed by General Atomic could be built today, and land a thousand tons on Mars in several weeks. If reaction mass such as water were gathered from a local moon, the same design could explore the moons of Jupiter or Saturn with a human crew.

Design

In the 1954 explosion 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 bomb. After the explosion, they were found intact some distance away, proving that engineered structures could survive a nuclear fireball.

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:

	"satellite" Orion	"midrange" Orion	"super" Orion
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.

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.

The extreme design was buildable with materials and techniques that could be obtained or anticipated in 1958. The real upper limit is probably larger now. In interviews, the designers contemplated the large ship as a possible interstellar ark.

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.

Calculations and experiments indicate that a steel pusher plate would ablate less than a millimeter if unprotected. If sprayed with an oil, it need not ablate at all. The absorption spectra of carbon and hydrogen were good to minimize heating. The design temperature of the shockwave, 120,000 degrees F, emits ultraviolet. Most materials and elements are opaque to ultraviolet, especially at the 50,000PSI pressures the plate experiences. This prevents the plate from melting or ablating.

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 alitudes, 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 berillium oxide "channel filler", which was surrounded by a uranium radiation mirror. The mirror and channel filler opened out to an open end. In the 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 degrees as it traversed the 75-foot distance to the pusher plate, and then reheat to 120,000 degrees 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 air, there might be problems from harm of the crew by gamma scattering.

Stability was thought to be a problem, but it developed that random placement errors of the bombs would cancel.

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 schock absorber was at first merely a ring-shaped airbag. However, if an explosion should fail, the one-thousand-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 20-30m 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 expense of the fissionables was thought high, until Ted Taylor proved that with the right designs for explosives, the amount of fissionsables 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. The large size bombs used more explosives to super-compress the fissionables (reducing the falloout). The extra explosives simply served as propulsion mass. The expense of launch for the largest size of Orion was 5 cents per pound to Earth orbit in 1958 dollars.

Problems

The unsolved problem for a lanch 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. To keep this in perspective, roughly 600 people die of cancer each year form eating spices.

However, the fallout for the entire launch of a 6000-ton Orion was only equal to a ten-megaton 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 fission 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.

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 might be solved by launching from very remote areas. EMP footprints are only a few hundred miles wide. The Earth is well-shielded form the Van Allen belts.

True egineering 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, but adverse public reaction to any use of nuclear explosives is likely to remain a hinderance even if all practical and legal difficulties are overcome.

Reference:

"Project Orion", George Dyson, 2002

"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.)