From Dr. Zubrin's book Entering Space, pg. 88 we find that one kilogram of helium 3 burned in a 60% efficient MHD system could yield 100 million kW-hours of electricty. That's 3.6E14 or 360 trillion joules. From the Artemis Data Book we find that one kilogram of he3 burned with 0.67 kilos of deuterium yeilds 19 megawatt years or 1.66E11 watt hours, the same as 166 million kW hours which would yield 100 million kW-hrs. from a 60% efficient conversion system.
So let's say we have a 60% efficient fusion rocket drive and we want to propell a 1000 ton ship up to 50 kps and reach Uranus in roughly a year. We will need:
0.5(1,000,000 kg.)(50,000 m/s)^2 = 1.25E15 joules!
We will need about 3.5 kilos of helium 3 worth $3 million to $6 million a kilo or $10.5 million to $21 million worth of the stuff. We will also have to brake upon reaching Uranus and use fusion drives to return to Earth where we could brake with mag-sails for efficiency.
A 1000 tons ship is not that huge. We might see 10,000 ton liners in the future. If we are to have hundreds, even thousands of high speed interplanetary flights between Earth, Mars, the Moon, perhaps Venus and Mercury, solar orbital space colonies, main belt asteroids and some of the outer planets too, we can see that substantial tonnages of helium 3 would be necessary at a great cost. If we want to hot rod around the solar system with average delta Vs per flight of 50 kps, fast enough to get us to Mars in a matter of weeks, in 10,000 ton carbon composite spaceships equivalent to 60,000 ton steel ships ( carbon composites 0.050 lb./cubic in; aluminum 0.10 lb./cu. in.; steel 0.30 lb./cu. in.) and we want to see 1,000 voyages a year and a few million space travelers, then we need:
3.5kilos * 10 * 1,000 = 35,000 kilos or 35 tons of helium 3 worth $105 billion to $210 billion!
That's enough to generate 3.5 trillion kilowatt hours with a 60% efficient conversion system. The USA used about 28 trillion kW hours altogether in all forms of energy and about 10 trillion kW-hrs. in electricty alone in 2000 CE. If we start talking about even larger and/or faster space liners and more flights per year for a vibrant interplanetary travel industry of the future then we could use up enough helium 3 to power the world's richest nation for a year every year! Although there are vast supplies of helium 3 in the atmospheres of the outer planets, mining the stuff will require a large investment.
We can see that producing helium 3 for routine interplanetary flight could even someday be comparable to producing enough to power the world when large numbers of cities emerge on the Moon, Mars and other worlds of the solar system and lots of solar and planet orbiting space colonies too! So the use of cycling stations and solar powered interplanetary beam riders makes sense from an economic standpoint. Interplanetary cargo ships could use mag-sails to ride the solar wind. Small couriers and VIP ships like the President's Space Ship One could use fusion power for rapid interplanetary transit. Cyclers will use mag-sails and the solar wind to adjust their orbits as well as solar sails that can be rolled up and unfurled as needed and catch some sunlight for tweaking trajectories without recharging the mag-sail. The solar sails could also get thrust from solar powered microwave beaming stations at Lagrange points in solar orbit. Nuclear powered ion drives might also be used to adjust cycler orbits, but it might be worthwhile to sacrifice a little helium 3 for small fusion thrusters on cyclers that are far more efficient and have higher specific impulses than NEP systems. Finally, cyclers can make use of gravity assists. Taxis will use substantial amounts of propellant. At first, they might use lunar aluminum and LOX. When Mars is colonized and ice mining goes on there and mass drivers are built on top of the great shield volcanoes LH2 and LOX will become available. Robotic miners of carbonaceous asteroids and old short period comets could also supply LH2 and LOX as well as organic chemicals. Some taxis might use NTR and LH2 or LANTR-LOX Augmented Nuclear Thermal Propulsion to rendesvouz with cyclers flying by planets. High thrust like that which can be obtained from LH2/LOX; LH2/NTR; and LANTR is desirable for taxis so that jaunts to and from cyclers are not very time consuming.
For an interstellar flight in a 10,000 ton ship at 0.05c, a rather small and slow moving star ship, we would need:
0.5(10,000,000 kg.)(15,000,000 m/s)^2 = 1.125E21 joules
With a 60% efficient drive we'd need 3125 tons of helium 3 or enough to generate 312.5 trillion kW-hrs. That's enough to power the USA at its 2000 CE rate of consumption for eleven years! So interstellar flight with fusion drives would really put a strain on the helium 3 miners, especially when we look at arks in the 100,000 to 1,000,000 tons range at 10% light speed and faster. Makes a case for solar powered mass beam propelled mag-sailing star ships.
It can also be said that with replicating AI robots we could build all the helium 3 mining infrastructure we need and meet the demand for high speed fusion space liners to convey the masses of humanity between the planets. Economic factors will utlimately decide what kinds of ships and propulsion systems are actually used in the future. The fast fusion liner does have time in its favor because it can make more trips every year than a cycling station can. It can also travel from planetary orbit to planetary orbit without the need for taxis. That will be more convenient. Even with fusion, some LH2 for reaction mass will also be needed, but probably not much more than would be needed for chemically propelled or nuclear thermal propelled taxis.
As Yogi Berra said, "It's hard to make predictions, especially about the future."
