Interstellar missions
Freeman Dyson performed the first analysis of what kinds of Orion missions were possible to reach Alpha Centauri, the nearest star system to the Sun.[12] His 1968 paper "Interstellar Transport"[13] (Physics Today, October 1968, p. 41–45) retained the concept of large nuclear explosions but Dyson moved away from the use of fission bombs and considered the use of one megaton deuterium fusion explosions instead. His conclusions were simple: the debris velocity of fusion explosions was probably in the 3000–30,000 km/s range and the reflecting geometry of Orion's hemispherical pusher plate would reduce that range to 750–15,000 km/s.[14]
To estimate the upper and lower limits of what could be done using contemporary technology (in 1968), Dyson considered two starship designs. The more conservative energy limited pusher plate design simply had to absorb all the thermal energy of each impinging explosion (4×1015 joules, half of which would be absorbed by the pusher plate) without melting. Dyson estimated that if the exposed surface consisted of copper with a thickness of 1 mm, then the diameter and mass of the hemispherical pusher plate would have to be 20 kilometers and 5 million metric tons, respectively. 100 seconds would be required to allow the copper to radiatively cool before the next explosion. It would then take on the order of 1000 years for the energy-limited heat sink Orion design to reach Alpha Centauri.
In order to improve on this performance while reducing size and cost, Dyson also considered an alternative momentum limited pusher plate design where an ablation coating of the exposed surface is substituted to get rid of the excess heat. The limitation is then set by the capacity of shock absorbers to transfer momentum from the impulsively accelerated pusher plate to the smoothly accelerated vehicle. Dyson calculated that the properties of available materials limited the velocity transferred by each explosion to ~30 meters per second independent of the size and nature of the explosion. If the vehicle is to be accelerated at 1 Earth gravity (9.81 m/s2) with this velocity transfer, then the pulse rate is one explosion every three seconds.[15] The dimensions and performance of Dyson's vehicles are given in the table below
"Energy Limited"
Orion "Momentum Limited"
Orion
Ship diameter (meters) 20,000 m 100 m
Mass of empty ship (metric tons) 10,000,000 t (incl.5,000,000 t copper hemisphere) 100,000 t (incl. 50,000 t structure+payload)
+Number of bombs = total bomb mass (each 1 Mt bomb weighs 1 metric ton) 30,000,000 300,000
=Departure mass (metric tons) 40,000,000 t 400,000 t
Maximum velocity (kilometers per second) 1000 km/s (=0.33% of the speed of light) 10,000 km/s (=3.3% of the speed of light)
Mean acceleration (Earth gravities) 0.00003 g (accelerate for 100 years) 1 g (accelerate for 10 days)
Time to Alpha Centauri (one way, no slow down) 1330 years 133 years
Estimated cost 1 year of U.S. GNP (1968), $3.67 Trillion 0.1 year of U.S. GNP $0.367 Trillion
Later studies indicate that the top cruise velocity that can theoretically be achieved by a Teller-Ulam thermonuclear unit powered Orion starship, assuming no fuel is saved for slowing back down, is about 8% to 10% of the speed of light (0.08-0.1c).[2] An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the max. speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.[16]
At 0.1c, Orion thermonuclear starships would require a flight time of at least 44 years to reach Alpha Centauri, not counting time needed to reach that speed (about 36 days at constant acceleration of 1g or 9.8 m/s2). At 0.1c, an Orion starship would require 100 years to travel 10 light years. The astronomer Carl Sagan suggested that this would be an excellent use for current stockpiles of nuclear weapons.[17]
