Low cost access to space via tethers

I tried to post this to sci.space.tech with no success. Our moderator
must be asleep because the last post there is over 5 days old.

I have made some basic calculations of a tether scheme to put a
payload into orbit from Earth.

Say we build a rocket that takes our payload to a 100 km apogee with a
zero or small horizontal velocity. From this point the payload docks
with the substation that is tethered to a heavy space station and is
spinning around in order to cancel out the orbital motion. The
substation and the tether accelerate the payload and raise it higher
to an orbital or faster velocity. This slows the space station down.
The space station uses ion engines to restore its orbital velocity.
This could be done with solar panels or a nuclear reactor. The ion
engines having a high specific impulse will use very little propellant
mass to keep the station's orbit. The space station would have to be
heavy enough to make sure it does not reenter when boosting a payload
to orbit.

The numbers I got are the following:

Rotational period 13 minutes.
Radius 1000 km (diameter 2000 km)
Gravity load 6.4 g

Such a scheme is probably cheaper than the beanstalk. Depending on the
masses of the space station and the substation the tether would have
to be 1000 km to 2000 km long. This scheme could be built up
incrementally, gradually adding mass to the space station and more
tethers.

Zoltan

"Zoltan Szakaly" wrote in message om...
I tried to post this to sci.space.tech with no success. Our moderator
must be asleep because the last post there is over 5 days old.

I have made some basic calculations of a tether scheme to put a
payload into orbit from Earth.

Say we build a rocket that takes our payload to a 100 km apogee with a
zero or small horizontal velocity. From this point the payload docks
with the substation that is tethered to a heavy space station and is
spinning around in order to cancel out the orbital motion. The
substation and the tether accelerate the payload and raise it higher
to an orbital or faster velocity. This slows the space station down.
The space station uses ion engines to restore its orbital velocity.
This could be done with solar panels or a nuclear reactor. The ion
engines having a high specific impulse will use very little propellant
mass to keep the station's orbit. The space station would have to be
heavy enough to make sure it does not reenter when boosting a payload
to orbit.

The numbers I got are the following:

Rotational period 13 minutes.
Radius 1000 km (diameter 2000 km)
Gravity load 6.4 g

Such a scheme is probably cheaper than the beanstalk. Depending on the
masses of the space station and the substation the tether would have
to be 1000 km to 2000 km long. This scheme could be built up
incrementally, gradually adding mass to the space station and more
tethers.

Your claim that this would be cheaper than a beanstalk seems to be based
on the fact that it is shorter. Have you actually worked out how much
fatter it would have to be?

The most important parameter in costing a tether is the tip speed. Your
system seems to have a tip speed of about 8 km/sec. I think you will find
that this tip speed will require a (very) large mass ratio unless you have
some pretty remarkable unobtanium.

Also, I notice that you have your station orbiting in the heart of the
inner van Allen belt. Maybe a problem.

You will find a scheme something like yours, and other information about
tethers at http://www.tethers.com/

I tried to post this to sci.space.tech with no success. Our moderator
must be asleep because the last post there is over 5 days old.

I have made some basic calculations of a tether scheme to put a
payload into orbit from Earth.
snip

How about a different tack on this... say instead of using tethers in the
traditional sense, when a spinning tether catches a payload and slings it
off in the other direction, we loft a tether, payload, and counterweight
into orbit and use the whole setup to send payloads into escape orbits.

More specifically (and bearing in mind, this just occurred to me and I
haven't done any of the calculations yet):

Say you had a Cassini-like payload you want to send into escape. You launch
this payload and a tether system on top of an upper-stage like a centaur.

The tether system itself is simpy a couple reels of "cable" and a center
point basically consisting of a flywheel, some electric motors to spin the
flywheel up, and some solar panels.

payload---------------------(reel)[center](reel)-------------------------[
centaur stage]

The system starts spinning (by spinning the flywheel one way, the rest of
the assembly will spin the other way), unreels the cable, and eventually
when it gets going fast enough at the ends, releases the payload. Said
payload would go off on an escape trajectory, and the upperstage/tether
assembly deorbits rather abruptly. At least in my "thought experiments" the
throw capability is limited only to your tether strength and pointing
ability.

Alternatively, you could unreel the assembly, and instead of spinning it up,
keep the length of the tether always radial to the earth (with the upper
stage at the lower end). Once it gets long enough, the counterweight would
be moving slower than its normal orbital speed, and the payload going
faster. Releasing the payload should let it fly off on an escape
trajectory, and the counterweight should again deorbit itself.

I guess, in the end, it comes down to three things: "Aimability", the
material strength of the tether material, and the mass of such a system (ie,
does it have less mass than the amount of propellant that would be required
to do the same job). Though, since it seems that your throw weight is
limited only by the strength of your tether, I would think a Cassini-sized
mission could be launched directly from orbit on a transfer to Saturn
without requiring four gravity assists on the way there. Or, you could
launch a lot more to Saturn but follow the same trajectory.

This whole system probably would not be feasable for transfers to GEO or
lunar trajectories, only for longer/farther interplanetary ones.

I plan on submitting a short proposal to the NIAC NSVFP sometime during
August.

"Bob Martin" wrote in message ...
How about a different tack on this... say instead of using tethers in the
traditional sense, when a spinning tether catches a payload and slings it
off in the other direction, we loft a tether, payload, and counterweight
into orbit and use the whole setup to send payloads into escape orbits.

More specifically (and bearing in mind, this just occurred to me and I
haven't done any of the calculations yet):

Say you had a Cassini-like payload you want to send into escape. You launch
this payload and a tether system on top of an upper-stage like a centaur.

The tether system itself is simpy a couple reels of "cable" and a center
point basically consisting of a flywheel, some electric motors to spin the
flywheel up, and some solar panels.

payload---------------------(reel)[center](reel)-------------------------[
centaur stage]

The system starts spinning (by spinning the flywheel one way, the rest of
the assembly will spin the other way), unreels the cable, and eventually
when it gets going fast enough at the ends, releases the payload. Said
payload would go off on an escape trajectory, and the upperstage/tether
assembly deorbits rather abruptly. At least in my "thought experiments" the
throw capability is limited only to your tether strength and pointing
ability.

You might also be limited by the angular momentum that the flywheel can
hold. The bearings between flywheel and the counter-rotating tether system
also might require unobtanium.

You might also be limited by the angular momentum that the flywheel can
hold. The bearings between flywheel and the counter-rotating tether
system
also might require unobtanium.

Magnetic bearings?

"Bob Martin" wrote in message ...
You might also be limited by the angular momentum that the flywheel can
hold. The bearings between flywheel and the counter-rotating tether
system
also might require unobtanium.

Magnetic bearings?

OK, superconducting unobtanium.

Bob Martin wrote:

I tried to post this to sci.space.tech with no success. Our moderator
must be asleep because the last post there is over 5 days old.

I have made some basic calculations of a tether scheme to put a
payload into orbit from Earth.
snip

How about a different tack on this... say instead of using tethers in the
traditional sense, when a spinning tether catches a payload and slings it
off in the other direction, we loft a tether, payload, and counterweight
into orbit and use the whole setup to send payloads into escape orbits.

The only useful tethers are those which land on the planet's surface. A guy
with a really big crash helmet grabs the end and ties it to the spaceship
before the tether drags the spaceship up into space.

I can think of two ways to do it with todays materials, ~ 4 kps tip speeds,
and yet another with carbon nanotubes if they get developed into multi-Gpa
ropes.

But why should I tell you?

I plan on submitting a short proposal to the NIAC NSVFP sometime during
August.

Who he?


--
Peter Fairbrother

How about a different tack on this... say instead of using tethers in the
traditional sense, when a spinning tether catches a payload and slings it
off in the other direction, we loft a tether, payload, and counterweight
into orbit and use the whole setup to send payloads into escape orbits.

You cannot spin up a long tether with significant masses on its ends
with an electric motor. The motor needs to be mounted on the unmovable
object that is held in place by the irresistible force.

You can spin up such a system over some time using ion engines or
Lorentz forces or other propulsion systems (chemical, nuclear ...).

Zoltan