The hardest part by far is propulsion. Everything else is easy by
comparison - but only by comparison. Nothing about space travel is
easy.

I tend to favor a radical departure in this area - namely MEMs engines
(Micro-electromechanical system engines) Here, the same technology
that is used to make integrated circuits the size of microbes, is used
to make mechanical devices the size of microbes. The easiest way to
think about how this is accomplished is to consider a microscope.
With a microscope its easy to see a microbe. With the same optics as
a microscope, you can also project an image the size of a microbe.
With that ability, and a healthy knowledge of photo-chemical
reactions, you can see how its possible to etch things on a tiny tiny
scale.

The other cool part is that fifty years of micro-circuit design has
given us a tremendous capability to convert any micro-machine design
into large scale production at very low cost. Micro machines are
already in common use as air bag accelerometers, and actuators.
Already in common use in HDTV plasma screens where millions of tiny
nozzles glow in varying colors changing hundreds of times per second.
Already in common use in ink jet printheads printing millions and
millions of dots in seconds creating photographs on demand.

Change the ink in an inkjet printer to rocket fuel, change the nozzles
in a plasma screen to rocket nozzles, and you have a propulsive skin!

http://www.youtube.com/watch?v=mzXwctPXT4c

Which can be attached to conventional tankage and feed systems to
create a stage stack that is quite capable.

http://www.scribd.com/doc/20053585/M...space-Overview

The cool part is that propulsive skins once developed can be mass
produced at very low cost. The other cool part is that these skins
will have 1,000 to 1 thrust to weight ratios and produce 3.6 tons per
square foot.

Cryogenic propellants - using hydrogen and oxygen - have higher
structural fraction than non-cryogenic propellants.

In a SSTO configuration;

H2/O2: 455 sec Isp, 3% structural fraction 87% propellant
10% payload
H2O2/RP1: 375 sec Isp, 2% structural fraction 92% propellant 6%
payload

A lunar landing system requires a total delta vee of 15 km/sec -
divided into 3 stages, that's 5 km/sec each, which using the rocket
equation gives the sizing of the stack needed to reach the moon.