In article ,
says...
(Henry Spencer) wrote in message ...
In article ,
Vincent Cate wrote:
If you use an ion-drive to get to lunar orbit and back and a tether to
collect samples, you don't need to be so mass-limited in your design
and you could bring back much more lunar mass. The difficulty of having
the end of the tether pickup some samples seems much less than having
a couple more rocket stages...

However, the tether deployment, spin-up, and control are basically research
projects, whereas rocket stages are fairly well understood.

On the other hand there is not so much room for improvement in rockets.

Just because NASA has a hard time deploying a tether does not
mean a private venture would. If you start spinning a bit before
you start deployment you could have a steady tension, and things
might work out well.

Oh, yeah -- I'm going to invest many millions of dollars because a
technique someone thought of and posted on Usenet "might" work out well?

I want to point out that tether dynamics are NOT that well understood.
Just try and get any company to pony up large amounts of capital for
something that requires the amount of R&D that's yet to be done before
*any* tether system can be understood well enough to be deployed.

You're right, the results probably would be better, but it's a
longer-term project with higher risk.

From a venture capital standpoint, a system that had a bit more risk
in the R&D but then lower operating costs and produced 100 times as
much product seems better. If a company like spacex or spacedev
were developing a tether system, I don't think it would really take
too long. You could test scoop systems on Earth and tether deployment
in LEO.

And ion from LEO to lunar orbit has bad problems with the Van Allen belts.
Rad-hard electronics and solar arrays are very hard on the budget (and on
the schedule, because of availability problems).

Ouch. I can't buy an off-the-shelf rad-hard module that does
my computation, guidance, and communications? Do ion drives
tolerate radiation ok?

Nope, you can't. It doesn't exist.

An easy mass margin design should be much easier on R&D money.

Only if it doesn't incur major new R&D problems of its own. Much the best
way to provide generous mass margins is just to buy a bigger launch.

The difference in initial launch mass between an all-chemical-rocket mission
and a tether/ion/regolith-thruster mission, for a given payload returned,
seems to be something like a factor of 100 to 1000. As long as launch
costs are high, this seems like an overwhelming advantage. Both testing
and operations have something like 1/100th the launch cost for the
same payload size.

Where are you getting your cost estimates? Off the top of your head?
You have no real idea what the costs would be on either side. Good luck
getting capital based on your gut feelings as to what the costs ought to
be...

Doug