Space Forum / Space Flight / September 2003

I'm no celestial mechanics wizard either, but the higher the orbit, the
_lower_ the velocity.  Escape velocity from the Earth's surface is 11.3
km/sec; it's less from a satellite in high orbit.  But then, you've expended
the energy to get to the satellite, yes?

Basically, _any_ stop along the way from the surface of the Earth to the
Moon (or anywhere else) costs delta-vee.  The only reason to circularize an
orbit to match a satellite would be some indispensable logistics: somehow,
there was something needed at the station that was worth the expense of
rendezvous.  Von Braun et al saw space stations as something analogous to
cache-camps in mountain climbing.

While I understand the importance of the Apollo "two rendezvous" strategy
(one in Earth orbit and the other in lunar orbit; the idea was to stretch
launch-landing windows to practical scales), I'm not entirely sure that I
buy the idea of space stations as cache-camps unless the volume of travel is
large enough to pay back the capital and energy costs.

Jim McCauley

So did I. But after reading some litterature and playing around with
some formulas, I would say that it's quite improbable that such stations
will ever come into existence.

1. Not too high, since it should be possible for small
surface-to-LEO-ships to go there,
2. Not too low, since large structures have significant atmospheric
drag.

..maybe around 6-7-800 km.

You don't want to place structures to be permanently manned higher than
about 1000-1500 km due to the van Allen belts.

Note that, in a circular orbit, the escape velocity is always sqrt(2)
times the orbital velocity. Since orbital velocity drops as the orbit
gets higher, so does the delta-v required to leave Earth orbit.

Since the Moon is in Earth orbit, technically, you don't need to leave
Earth orbit to go there.

But, basically, you don't need to change spaceships halfway. It will
only make space travel more complicated and expensive. So interplanetary
transit space stations will probably never be a reality.

Von Braun had his orbiting at 1075 miles.  This seems to have been
chosen because it gives a 2 hour orbit.  See "Across the Space
Frontier", 1952.  However, he picked this before the discovery of the
Van Allen Belts
in Jan of 1958.  We now know this is not a good place to stay.

I like the idea of a hotel/space-station at GEO.  With this a rotating
space tether in LEO could pickup a payload from a suborbital rocket
and toss it on a GTO every 90 minutes.  At the GEO station another
tether would catch the payload.  The GEO station could collect
things for about 2 weeks and then do a few big tosses to the Moon
so they would get there when the Moon was crossing the orbital plane
of the GEO station.  Another tether around the Moon lowers payloads
and lifts rocks to balance the traffic.

The mass of the hotel/station at GEO gives you a big ballast to work
with for your GEO station tether tosses.

  -- Vince

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Vincent Cate                           Space Tether Enthusiast
vince@offshore.ai                      http://spacetethers.com/
Anguilla, East Caribbean               http://offshore.ai/vince
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You have to take life as it happens, but you should try to make it
happen the way you want to take it.    - German Proverb

There are about two good places to put such a station:  in a low Earth
orbit, and at some very high location in the Earth-Moon system.

The low station is the clear first priority.  It's the changeover point
between launch vehicles optimized for coping with Earth's gravity and
atmosphere, and orbital-transfer vehicles optimized for operation in orbit
only.

The right place to put it is an equatorial orbit as low above the
atmosphere as reasonably feasible.  The equatorial orbit requires an
equatorial launch base, but gives you one launch window per orbit instead
of only one or two per day, and reduces radiation dose.  The low altitude
is so the launch vehicles don't have to waste fuel and other resources
doubling as orbital-transfer vehicles (although tugs based at the station
could go down to meet them instead).

Just what "reasonably feasible" is, depends on how you do the tradeoffs.
Arguably ISS is too high, incurring unnecessarily high radiation dose and
fuel costs.  Arguably it is too low, requiring too much reboosting against
air drag.  The choice may be influenced by what reboost technology you
use, and whether you are interested in an option of operating unmanned at
times (as opposed to always having a maintenance crew on hand, to fix
things promptly when they break).  Certainly the number is hundreds of
kilometers, not thousands.

The high station is the assembly point for deep-space operations, and a
halo orbit around the Earth-Sun L1 point is a possible location for it.

It's just possible that it might be worth having a third "transfer"
station in an equatorial elliptical orbit.  Its perigee is at the orbit of
the low station, and its apogee is above most or all of the Van Allen
belts; the orbital period would be chosen to be a multiple of the low
station's, so at perigee it encounters the low station.  The transfer
station has modest "hotel" facilities, but is mostly a great big hangar
with thick shielded walls.  Its purpose is to carry payloads -- notably
people and electronics -- through the belts.  This lets you put that heavy
shielding into that belt-crossing orbit *once*, instead of requiring every
payload to carry its own.

So did I. But after reading some litterature and playing around with some
formulas, I would say that it's quite improbable that such stations will
ever come into existence.

1. Not too high, since it should be possible for small surface-to-LEO-ships
to go there,

2. Not too low, since large structures have significant atmospheric drag.

..maybe around 6-7-800 km.

You don't want to place structures to be permanently manned higher than
about 1000-1500 km due to the van Allen belts.

Note that, in a circular orbit, the escape velocity is always sqrt(2) times
the orbital velocity. Since orbital velocity drops as the orbit gets higher,
so does the delta-v required to leave Earth orbit.

Since the Moon is in Earth orbit, technically, you don't need to leave Earth
orbit to go there.

But, basically, you don't need to change spaceships halfway. It will only
make space travel more complicated and expensive. So interplanetary transit
space stations will probably never be a reality.

--
Steen Eiler Jørgensen
"No, I don't think I'll ever get over Macho Grande.
Those wounds run...pretty deep."