Jupiter was touted as the major source of He3 fusion fuel for Daedalus, some 30,000 tons of it, but Jupiter’s gravity well is HUGE and far beyond the abilities of solid-core fission rockets, stretching the capabilities of gas-core rockets in terms of thrust-to-weight ratios. So what about the other gas-giants?
|Planet||mass (Earths)||radius (km)||P-mag (sec)||P-hydro (sec)||Eq. velocity (km/s)||Orbital vee (km/s)||delta vee (km/s)|
As you can see Uranus has the most forgiving gravity field, but Neptune and Saturn aren’t out of reach either, and Saturn has proximity to the Sun and Jupiter, for gravity assists, in its favour as well as Titan, a moon with a dense atmosphere and decent gravity. The Daedalus study assumed floating factories at the 0.1 bar level in Jupiter’s atmosphere serviced by gas-core automated shuttles, but if there’s enough need for them volatiles from the big planet atmosphere can be scooped and shipped up for processing at an off-world base.
Scoop-ships could also allow starships to be self-fuelling. I have just received an issue of the October 1973 “Analog” – the one with a gorgeous Rick Sternbach cover of two Enzmann starships and the Cover article by G. Harry Stine, “A Program for Star Flight”. It’s quite a memorable article as Stine was arguing for a star flight program to begin c.1990, and the development of a massive in-space industrial base to support the effort. His initial phase would study the nearby stars with Lunar interferometers, then launch million-ton space-probes at 0.9c, and finally launch ten-ship fleets of Enzmann starships (roughly 12,000,000 tons each, mostly deuterium fuel.) Quite a major effort, but he optimistically costs it at $100 billion (in 1973$.)
A few problems arise – 0.9c from Orion-style pulse drives is a touch unlikely, even with mass-ratios over 1,000 – but over all the concept is sound. Magnetic sails might change the approach, but the basic idea of attaching starships to huge masses of propellant, rather than big tanks, is a good one. However I have read that hydrogen and deuterium ice are mechanically like Jello and thus utterly useless as envisaged. Lithium-6 is a fusion fuel and pretty strong at cryogenic temperatures, so it might be the fuel of choice. Either that or carbon nanotubes might allow very, very light weight tanks to keep deuterium Jello in. The 12,000,000 ton starships probably mass just 120,000 tons empty (the design needs BIG mass ratios for speed), but the size Stine quotes is all wrong. Deuterium’s density is 0.16 relative to water, yet the fuel sphere is described as 1000′ across meaning a density of ~ 0.8, some five times denser than deuterium Jello. A sphere 1,710′ across will do nicely.
Refining out 12,000,000 tons of deuterium from a gas giant will be quite a task. Since deuterium is about 1/2000th of the abundance of protium in Jupiter some 24 BILLION tons of hydrogen will be sifted through to collect the fuel. Bit of a tall order, but inevitable when you’re trying to fit 2,000 people on a starship that’s over 2,000′ long and push it through a delta-vee of 0.3 c.
Actually I don’t know the empty mass of the Enzmann starship is 120,000 tons which is a touch frustrating since I’m paying attention to details here. What Stine does say is that the probes will hit 0.9 c with a mass ratio of ~ 1,000, and the starships will hit ~ 0.3 c (thus a delta-vee of ~0.6 c.) A bit of non-relativistic maths, and assuming the same exhaust velocity, means the starships have a mass-ratio of 100 (=1000^(0.6/0.9).) Now fusion reactions don’t make enough particles with sufficient energy to get that sort of exhaust velocity (about 0.13 c), nor are Orion pulse-drives 100% efficient (~25%?) More realistically an Enzmann starship will hit 0.08-0.15 c which is respectable for a fusion-drive.