For serious interplanetary operations we need fusion propulsion – plain nukes aren’t much better than chemical rockets performance wise. Outer Planet access with trip times under a year are probably vital on biomedical grounds due to the nastiness of high-energy Cosmic-rays. Thus the necessity of fusion propulsion.
But before we shoot off to Jupiter, what can we do about Mars and a little bit beyond?
Assume three FH Tankers (52 tonnes fuel, 3 tonnes dry-mass) and a payload massing 55 tonnes. Arrange two Tankers as First Stage and one as the Second Stage to push the payload. What delta-vee do we get? Over-all mass ratio is (220/(220-104))*(110/(110-52)) = 3.6, thus with the Merlin Vacuum engine we get 1.28 x 342s x 9.80665 = 4,293 m/s – enough to put our cargo on a Hohmann transfer to Mars, with a bit of a reserve.
For unmanned vehicles carrying cargo the 258 day Hohmann orbit is preferrable, but punitive for a manned mission. With a bit of extra delta-vee – such as the above figure – a manned mission can save on supplies and cosmic-ray exposure. Gerald Nordley discusses the issue in his on-line essay…
…indicating trip-times of 130-180 days are reasonably feasible. Thus crew can travel quicker than freight. The canonical Mars Semi-Direct would require delivery to Mars of a Habitat, and Earth Return Vehicle and a Mars Ascent Vehicle, all in the roughly 55-60 tonne mass range. Thus a total of 12 Falcon Heavy launches to deliver a crew of six to Mars. A launch cost of just $1.5 billion for a Mars mission is a dream! But eminently practical with Falcon Heavies available.
Going to Mars lets us save propellant via aerobraking – aerocapture into a highly elliptical Mars orbit – which isn’t available if we go beyond Mars to the Asteroid Belt. Trip-times rapidly go up as we move further away from the Sun, especially for tricky fuel-saving orbits with higher aphelia than the destination. Another speed-bump is the non-zero inclinations of the asteroids, which makes them even trickier to reach.
So what do we do? Personally I think this is where we have to start getting out of the rocket straight-jacket and start getting serious about solar-sails – as recently successfully demonstrated by IKAROS and Nanosail-D. There’s a certain elegance – and zero-fuel budget – which has an immense appeal.