Hi Raoul -

I like Bob, but there's so much nonsense in Zubrin's mistatements of architectures that it is not worth trying to unravel it...

But let's start at the beginning...

First off, as Zubrin points out, there's structural economies in large vehicles. BUT there's no volume demand, and thus there are no economies of scale in production... and that's why heavy lift is so expensive, and not cost effective for manned flights...

Now if you want to go to Mars, a single launch does not provide enough mass up to allow for in-transit repair - you're stuck...

If you go to a larger Mars ship, where the crew doesn't immediately die if something goes wrong, then re-usability becomes the economical way to go - a cycler, which is what Dr. Aldrin has described. The economics of cyclers were demonstrated by Soviet manned Mars analysis in the early 1970's, and again with studies using the Energia.

(There's a heavy lift launcher for you. The possibility of simply mounting a Mir type station on a nuclear rocket and using this to get to Mars orbit ended when the roof collapsed on the Energia storage building.)

The economies favoring cyclers are so gross that even Soviet analysts were able to identify them. Since a heavy lifter can not launch a cycler whole, use commercial launchers to build your cycler for the Earth-Mars transit, then use in-situ fuel production for the going between the surface of Mars and the cycler.

Any manned Mars flight assumes that back-contamination turns out not to be a problem, which is by no means certain at this time. That means that focusing your nation's space program on manned Mars flight may mean you're focusing it on a dead end.

Back to the Moon... Zubrin assumes that cryogenics are used for the up-down at the Moon. Not necessarily so. (Sorry I don't use all Zubrin's acronyms. I want to keep this as simple as possible.)

Next Zubrin assumes that the manned space ship is launched first. Not necessarily so.

Next Zubrin assumes that the ascent and descent module is launched. Definitely not so.

What you do first is transfer your fuel to a lunar orbiting station, and then transfer it there to a
re-usable landing frame. Then you send your manned ship and payload. Manned ship and heavy re-entry heat shield stays in lunar obit, landing frame carries crew and payload to surface to do real work on planetary defense system. Landing frame returns crew to fueling station, and crew takes ship back to Earth.

Now if you split the medium launchers among participating nations, then your total cost per mission drops again... and without participating nations, any manned space flight beyond LEO becomes so expensive that it will not be done...

That's because Mars is simply not an Earth-like planet that could be easily settled.

If it was, we'd be there now.

Finally, that's why Apollo came to an end - the images from Mariner IV.

ep