Space News Letter to Editor

No, that's not a misquote. The energy required to achieve a Hohmann
transfer trajectory from Earth to a NEO is not a useful delta-V measure
when considering the trajectory that a spacecraft using electrical
propulsion would use. The delta-V associated with the latter
continuous-thrusting trajectory would likely be much higher than the
Hohmann transfer delta-V (and perhaps higher than that of reaching the
surface of Luna). But that's o'k in many instances, because the
efficiency of the ion engine is so much greater than the efficiency of
chemical propulsion.

It just depends....

By the way, I've used the term 'electrical propulsion', but the same
comments would apply to any low-thrust engine. For example,
solar-powered 'steam engines' that expel small amounts of water vapor at
high velocities would also not follow Hohmann transfer trajectories. And
the mass driver transfer envisioned by some in which part of a NEO's mass
would be consumed by spewing it behind the NEO for propulsion --- this
also would not be a candidate for Hohmann transfer, but rather for a
continuous thrusting trajectory with a greater delta-V requirement than
Hohmann transfer.

Ron
*******

"victoriatangoman"
05/12/03 07:18 PM
Please respond to spacesettlers

To: spacesettlers@yahoogroups.com
cc:
Subject: [spacesettlers] Re: 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.