Hohmann transfer, not Hoffmann

In a message dated 2/13/01 10:26:35 AM Eastern Standard Time,
rmenich@... writes:

<< Based on my very limited knowledge of orbital mechanics, I understand the
following...

Minor point: It is the Hohmann transfer, not Hoffmann, that is the minimum
energy transfer ellipse between two circular coplanar orbits that are not
too different in size. If the orbits are not coplanar, then more
complicated (e.g., bi-elliptic) trajectories may achieve lower energy than
a Hohmann transfer followed by a plane change maneuver. >>

Hohmann looks harder to pronounce, that's probably why I mistook it for
Hoffmann which sounds more familiar to me.

<< More major consideration: Launch windows for Hohmann transfers happen
once
per synodic period for the two orbits under consideration. For a Near
Earth Asteroid (NEAs) whose semi-major axis is very close to 1 AU, the
synodic period can be very long, leading to very infrequent opportunities
to exploit the minimum energy transfer. >>

Perhaps the asteroid in question should be sellected with that in mind. For
example an asteroid might be selected for mining because it will reach an
otimum position in relation to Earth for a minimum energy transfer of
material. That means the miners will have 5 years to get the material out of
the asteroid and collect it for minimum energy transfer at the proper time,
afterwards the asteroid is abandoned and the miners seek another asteroid in
a more optimum position for another future transfer. One question that should
be born in mind is do the miners simple mine out material and then transfer
it to cislunar space or do the travel around in factory ships and build
things like SPSs in the vicintity of the asteroid and then transfer the SPS
to Earth orbit. I think tons of rock and material might be hard to move all
at once from an asteroid. Setting up a mass driver might be difficult since
you would have to continually chance the velocity and direction of the ore to
take into account the changing position of Earth in relation to the asteroid.
I think a mobile bernal sphere with a factory attached might be a good way to
use asteroid resources. The Bernal would move from asteroid to asteroid and
make finished products out of the material, sending them to Earth in bunches
when the proper orbital positions are reached.

<<
Another consideration is that, with the success of the Deep Space 1
mission, continuous thrusting (low thrust / high specific impulse)
trajectories will become more popular. The higher specific impulse is
hugely advantageous to mission planners. The Hohmann transfer is an
impulsive transfer, and is not suitable for usage by continuous thrusting
engines. The Hera mission (http://www.uark.edu/misc/hera/) intends to use
such low thrust / high specific impulse technology: check it out.
>>

I guess it depends on what is wanted economically. For interplanetary
transfer it shouldn't matter whether the impulse takes 15 minutes, an hour, a
day or a week so long as the ship is traveling at the right velocity when the
impulse ends. If it takes a year of continuous low thrust that would be
different, but is the object really to save reaction mass at all cost? I
understand that if one had a linear particle accelerator capable of
acceleration elementary particles near the speed of light in space, and you
ran it for 100,000 years or so you could end up traveling very close to the
speed of light, but what good would that do? There will probably be some
happy medium between high thrust and high specific impulse based on the
patience of the customer buying the product.

Tom Kalbfus