Space News Letter to Editor

--- In spacesettlers@yahoogroups.com, rmenich@m... wrote:
> victoriatangoman wrote:
>
> "
> You're correct about the variety of NEO material, and their delta-
v
> advantage, but the Hohmann orbit launch windows needed to realize
> those advantageous delta-v orbits will be few and far between.
> "
>
> True enough. However, there's a nuance to which I'd like to draw
> attention. It is not possible to get down to the surface of the
Moon
> with a xenon arc thruster, but it is possible to reach some NEOs
with such
> equipment. Electrical propulsion systems are more efficient than
> chemical propulsion systems, and they may be safer too. One
might be
> willling to endure a higher delta-V requirement to reach a NEO
than to
> reach the surface of Luna because to reach the NEO one can utilize
the
> much more efficient propulsion system.

Is this a misquote? I'm assuming that the NEO has lower delta-v
requirements and that is what makes them more attractive from the
delta-v point of view.

You raise a very good point about propulsion efficiency that I've
not yet seen addressed in the debate.

I would say that this point adds to the desirability of the NEO
mission.

Chemical propulsion systems seem
> to explode with alarming frequency (e.g., recent problem with
CONTOUR
> probe).
>
> It is not possible to utilize Hohmann transfer orbits with low-
thrust
> electrical propulsion systems, because Hohmann transfer requires
large
> impulse thrusts that are not achievable by electrical propulsion.
Rather,
> electrical propulsion systems utilize continuous-thrusting
trajectories.

Hohmann orbits are designed to transit two points in the most delta-
v efficient manner. Electrical propulsion systems will have the
advantage of not having to wait for rare launch windows but will
have to expend more reaction mass for their transit than they would
if they had been able to use the Hohmann launch windows.

A related issue is scaling. Thus far chemcial propulsion has been
used to move tons of payload about, but the electrical propulsion
systems have only been used for smaller payloads.

Will we simply take hundreds of Hall thrusters and bundle them
together and ship NEO raw material, or the refined products, back
and forth? Where will the reaction mass come from? Mined on the NEO
or launched from Earth? How many "ships in transit" will we need to
have in order to adjust for the long transit times?

> Of course, as many have pointed out before, getting to a NEO takes
a lot
> more time than getting to Luna, and that's certainly a huge
disadvantage
> to NEO development vis a vis Luna development.
>
> Ron

Quite right. Notwithstanding your excellent points, and their
technical advantages, I think that the economic value of the
proximity issue of the Moon thus far trumps the advantages of the
NEOs for the mining of materials available on both the Moon and NEOs.

TangoMan