Re: Planetary Space Travel

 Let us break down the requirements into their components; it can be shown 
that fuel for the return trip is far and away biggest mass that needs to be 
taken to Mars-so much so, that, if it could be produced on site - eg by 
catalytic extraction and storage from the atmosphere by a previously 
established lander, the size and cost of a Mars mission could be reduced by 
a factor of 10 or more (R Zubrin, Mars Direct)
 Assuming a conventional model of 6 people for a 900 days return trip, 
we have water requirements- 1 litre per person per day totals 4.5 tons. 
With 80 % recycling possible, this falls to 1.1 tons.
Breathable oxygen - consumption 360 litres (0.5 kg) per day per person, 
totalling 3 tons. Food - 1 kilogram per person per day totals 5 tons. This 
makes no allowance for the well-established reduced requirements in low 
gravity due to reduced muscular stress, and the fact that space food is 
often markedly dehydrated, and made up to scratch with part of the water 
ration. The possibility of raising crops on board, or perhaps using bean 
sprouts as rapidly growing sources of protein make the food stores required 
even lighter.
 These figures show a total of less than 10 tons for 6 people for the whole 
3 years, or, if only the outgoing journey (whose supplies must be on board!) 
is counted, less than 8 tons. Structural mass of spaceship and lander dwarf 
this figure many-fold. I think it is fair to say that the return fuel is by 
far the biggest consumable weight load, and that we can effectively ignore 
the food, water and oxygen components.
 All the above being said, however, there is a substantial gain in sending 
a lander/return craft to Mars during one launch window, and having it 
refuelled locally during the 2 years awaiting the arrival of the crewed 
ship. This is the essence of the Robert Zubrin's Mars direct proposal, and 
has been shown to result in much smaller space craft, a longer stay on Mars 
itself, and cost reductions of from 10-20 times (ie down to 20-25 billion 
dollars over 8 years) over the traditional von Braun/NASA approach. Tyhe 
idea of using Martian resources to explore and settle Mars, is surely the 
key to rendering Mars settlement in the next century not only economically 
feasible, but highly probable, as the true implications for costs become 
understood.
 An alternative route is to reduce transit times by nuclear propulsion. In 
the 1960's fission powered third stage were developed for Saturn 5 which 
could double the performance of Lox/LH2 engines, while the potential for 
far greater improvements was well under way (NERVA).
 The technology for local refuelling is under consideration for a demo on 
one of the later Mars lander rover missions, eg 2003 or 2005, and could 
reduce the costs for a sample return mission to within the envelope for two 
Discovery class missions. 
 The newly formed Mars Society (Colorado, August 1998) will provide 
increasing pressure for the Mars option, and is heavily influenced by 
Zubrin's ideas.
 He can be contacted at zubrin@ix.netcom.com. I have no doubt he would be 
delighted to hear from you!