In article ,
Vincent Cate wrote:
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.

Actually a debatable point, but one wouldn't undertake rocket R&D in a
context like this. (The whole point of the rocket approach would be to
reduce risk and shorten the time before revenue starts flowing, by using
at least an *approach*, if not actual hardware, that is reasonably proven.)

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...

Maybe, and maybe not. Venture capital tends to have limited planning
horizons, and to weight risks heavily. This is already a somewhat iffy
venture, with technical risk (a lot of new hardware to develop), political
risk (nobody knows how the government would react), and market risk (will
the stuff *sell*, and how quickly?). That's a bad combination; VCs would
prefer to see one or two of those categories, not all three. Anything
that reduces any of those risks will be attractive; anything that
increases any of them will be very much Not Wanted.

The tether would be very interesting as a *second generation* system,
after a minimum-innovation rocket system paves the way politically and
proves that there is a lucrative market there. In fact, for the initial
system, you might want to forget the new design and pay the Russians to
revive the Luna sample-return system, despite the need for a Proton launch
and the very small return payload.

If a company like spacex or spacedev
were developing a tether system, I don't think it would really take
too long...

The control needed for precise operations in low orbit around an irregular
Moon strikes me as a non-trivial issue. This isn't just a simple rotating
tether, it's a highly dynamic variable-length rotating tether... and the
dynamics are the biggest question mark in such systems already.

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?

Not as such, no. You can buy some, perhaps all, of the necessary pieces
to put one together yourself. But on closer inspection, those pieces
typically are not really "off the shelf": the first thing you get to do
is to negotiate a price (high) and a delivery time (not soon), because
they don't keep an inventory of the things.

Also, there's rad-hard and there's rad-hard. Electronic gear that can
take modest amounts of radiation is not hard to find. But a slow passage
through the inner Van Allen belt is a whole new order of magnitude. That
gets you into territory populated -- rather thinly -- by cost-is-no-object
hardware designed for fighting nuclear wars.

(Which brings in the ugly topic of arms control... Rad-hard electronics
is somewhat sensitive. Ultra-rad-hard stuff is very much so, I believe,
given its traditional primary application.)

Do ion drives tolerate radiation ok?

The thrusters themselves generally aren't bothered, but there are
typically plenty of semiconductors in the power-processing electronics box
right behind the thrusters. (Ion thrusters need multiple well-controlled
voltages, with current limiting and some other precautions, so the power
supply is not a simple piece of hardware.)

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.

Only when operations costs become the dominant problem. The problems of
first-generation systems, at least, will be dominated by development costs
and risk mitigation.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |