Project Orion (nuclear Propulsion) - Vehicle Architecture

Vehicle Architecture

From 1957 until 1964 this information was used to design a spacecraft propulsion system called "Orion", in which nuclear explosives would be thrown behind a pusher-plate mounted on the bottom of a spacecraft and exploded. The shock wave and radiation from the detonation would impact against the underside of the pusher plate, giving it a powerful "kick". The pusher plate would be mounted on large two-stage shock absorbers that would smoothly transmit acceleration to the rest of the spacecraft.

During take-off, there were concerns of danger from fluidic shrapnel being reflected from the ground. One proposed solution was to use a flat plate of conventional explosives spread over the pusher plate, and detonate this to lift the ship from the ground before going nuclear. This would lift the ship far enough into the air that the first focused nuclear blast would not create debris capable of harming the ship.

A preliminary design for the explosives was produced. It used a shaped-charge 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 built into a can with a diameter no larger than 6 inches (15 cm) and weighed just over 300 lb (140 kg) so it could be handled by machinery scaled-up from a soft-drink vending machine (indeed, Coca-Cola was consulted on the design).

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 vaporized. The flat plate of propellant formed a cigar-shaped explosion aimed at the pusher plate.

The plasma would cool to 14,000 °C, as it traversed the 25 m distance to the pusher plate, and then reheat to 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 shaped distribution profile and low density of the plasma reduces the instantaneous shock to the pusher plate.

The pusher plate's thickness would decrease 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.

At low altitudes where the surrounding air is dense, gamma scattering could potentially harm the crew and a radiation refuge would be necessary anyway on long missions to survive solar flares. Radiation shielding effectiveness increases exponentially with shield thickness (see gamma ray for a discussion of shielding), so on ships with mass greater than a thousand tons, the structural bulk of the ship, its stores, and the mass of the bombs and propellant would provide more than adequate shielding for the crew.

Stability was initially thought to be a problem due to inaccuracies in the placement of the bombs, but it was later shown that the effects would tend to cancel out.

Numerous model flight tests (using conventional explosives) were conducted at Point Loma in 1959. On November 14, the one-meter model, called "Hot Rod" (or "putt-putt"), first flew using RDX (chemical explosives) in a controlled flight for 23 seconds to a height of 56 meters. Film of the tests has been transcribed to video shown on the BBC TV program "To Mars by A-Bomb" in 2003 with comments by Freeman Dyson and Arthur C. Clarke. The model landed by parachute undamaged and is in the collection of the Smithsonian National Air and Space Museum.

The first proposed shock absorber was merely a ring-shaped airbag. However, it was soon realized that, should an explosion fail, the 500 to 1000 ton pusher plate would tear away the airbag on the rebound. So a two-stage, detuned spring/piston shock absorber design was developed. On the reference design, the first stage mechanical absorber was tuned 4.5 times the pulse frequency whilst the second stage gas piston was tuned to 1/2 times the pulse frequency. This permitted timing tolerances of 10 ms in each explosion.

The final design coped with bomb failure by overshooting and rebounding into a 'center' position. Thus, following a failure (and on initial ground launch) it would be necessary to start (or restart) the sequence with a lower yield device. In the 1950s methods of adjusting bomb yield were in their infancy and considerable thought was given to providing a means of 'swapping out' a standard yield bomb for a smaller yield one in a 2 or 3 second time frame (or to provide an alternative means of firing low yield bombs). These days the yield of a standard device would be 'tuned down', as needed, 'on the fly'.

The bombs had to be launched behind the pusher plate fast enough to explode 20 to 30 m beyond it every 1.1 seconds or so. Numerous proposals were investigated, from multiple guns poking over the edge of the pusher plate to rocket propelled bombs launched from 'roller coaster' tracks, however the final reference design used a simple gas gun to shoot the devices through a hole in the center of the pusher plate.