From NEO to GEO --- an SPS scenario

O'k to most of your points, Charles. Apologies for mistaking the thrust of
your previous messages; I should have read them more carefully.

One point of clarification. You wrote,

"Anything is possible, but a vehicle travelling on non-Hohmann trajectory
will need a lot more delta-vee,
and even a high SI system will add signicantly to the cost of vehicle.
"

My original suggestion --- build the SPS at the NEO and fly it back to GEO
using electrical propulsion --- would involve a continuous-thrusting
trajectory rather than a Hohman trajectory. Electrical propulsion
essentially necessitates non-Hohman trajectories because of low thrust.
But achievable delta-V is directly proportional to specific impulse, and
electrical propulsion systems can offer specific impulses over 10 times
what the best hydrogen/oxygen chemical engine can provide.

As I understand it, there is some latitude during design of electrical
propulsion systems to **choose** the specific impulse. Choosing higher
specific impulse results in larger mass for solar arrays and power
conditioning equipment, so it is often not optimal to choose the highest
specific impulse attainable. Transporting a completed SPS back to GEO
presents an interesting design problem: given that I have a x hundred
megawatts of available power and that I must transport the y thousand ton
SPS back to GEO in z days, what specific impulse should I choose?

Ron Menich

Charles

07/10/01
10:59 PM
Please
respond to
ssi_list

>
> "In particular, the main problem with a NEO is the lack of launch
> windows, they can be several years apart.
>
> On the other hand, we can shuttle back and forth between LEO, GEO and
> Lunar L-4/5 any time we want to.
> "
>
> There may be NEOs located at the Sun-Earth L4 and L5 libration points
> (so-called "Earth Trojans"). Currently, there are 5 known Mars Trojans
> and 1007 known Jupiter Trojans (c.f.,
> http://cfa-www.harvard.edu/iau/lists/Trojans.html ). If Earth Trojans
> were to exist, then there would exist continuous launch windows to them
> from Earth with constant delta-V.
>

Right.

>
> Even sans Earth Trojans, we need to examine the term "launch window"
> carefully, I think, for the case of NEOs whose orbits are very similar to
> Earth's. Launch windows are periods of time during which launch should
> take place in order to achieve a certain low delta-V value, given an
> assumed mission profile.

This means the economic Hohmann transfer opportunity.

> Launches at other times are not impossible, but
> their delta-V values will be higher. It is conceivable to design a
> spacecraft that would allow a range of possible delta-V values, with more
> cargo or less cargo delivered depending on whether the required delta-V
> were smaller or larger. All of the delta-V required for HEO->NEO->HEO
> transport could be provided with high specific impulse engines. This is

Anything is possible, but a vehicle travelling on non-Hohmann trajectory
will need a lot more delta-vee,
and even a high SI system will add signicantly to the cost of vehicle.

On the other hand it can have the benefit of reducing the trip time,
which will please the investors who want their ROI as soon as
possible. This benefit can be quantified using time value cost of
money equations.
The total cost of debt service grows exponentially with time.

> an important difference vis a vis the Moon, for which descent/ascent
> to/from the surface must be accomplished by chemical propulsion with low
> specific impulse (unless you've got your complex mass driver
infrastructure
> or tether infrastructure already built).
>

We were not discussing landing on the Moon, that is a different matter.

We were discussing whether it is better to put the manufacturing
facility at L-4/5 and
bring the NEO to L-4/5, or whether we should send the manufacturing cell
to the NEO and send the finished product back to L-4/5.

> Thus, the required delta-V for a NEO mission could be much larger than
for
> a Moon mission but yet have more cargo capacity because of the high
> efficiency engines that are usable for NEO missions but not for Moon
> missions.
>

Yes it is expensive to soft land on the Moon, but that is not the issue
I have been dicussing in this thread.