During Construction: Climb, Winch, or Toss?
After the payload is moved from the SSTT rocket to the end of the tether,
it needs to move somewhere else, so that the tether can
pick up another payload. The payload end of the tether is moving
very fast and does not have enough strength to hold several payloads.
If you want to send a payload just picked up from the SSTT on to GEO
you can toss it and things are easy. But during construction/bootstrapping
of the tether you want to get the new payload to the ballast end
of the tether.
There are several options:
We will look at these options below.
- Climb the tether with a sort of elevator
- Winching in the whole tether to bring the payload up
- Toss the payload from the payload end of the tether and
have it get to where the ballast end can grab it
Climbing The Tether
To climb the tether, at the end of the tether, we can use an elevator device that attaches to the payload.
The elevator then lifts payload up the tether. It would be natural to take payload to either the center
of mass or the ballast end. If the tether is long, say 500 km, this can take awhile and require a huge
amount of power (see next section). At 50 km/hour it would take 20 hours for a round trip. If you
wanted to do another payload every 90 minutes, you would need a number of elevators that could hand off
payloads between them. But this adds to the loads on your tether, so the tether would have to be stronger. It
could be that to begin with there is only one elevator and you can only do one payload per day, but you can do more after
the tether brings up material to make the tether stronger and more elevators.
If there is something climbing up and down the tether the shape of the
tether needs to be compatible with it. In practice this means some tether
shapes would not be used. In particular a sort of funnal webbing shape
which is good for reliability does not seem compatible with a climber.
A flat ribbon or belt shape would be easier to climb. But a flat
cable could be severely damaged if hit by a micro-meteor at the right angle.
It would be useful to have some device that can climb the tether because it could
also be used to inspect and repair the tether.
Tether Climbing Power Requirements
We will start with a concrete example. In our simulation of a
600 km tether with 50,000 Kg ballast and 2.5 km tip speed we
get about 1.1 Gs of centripital force. Imagine we are climbing
a tether at 100 km/hour, we weigh 200 Kg, and we are climbing
against an average of 1.1 Gs.
The formula for potential energy is mass*acceleration*height.
At 100 km/hour we go 27.778 meters/second.
The energy for each second is 200 Kg * 1.1 * 9.8 m/s^2 * 27.778 m = 59,888 joule.
So if we do this every second we need 59,888 watts or about 60 kw.
At 746 watts per horsepower this is about 80 hp.
If we use solar power at 100 watts/Kg we would need 599 Kg of solar panels.
Of course we then need much more power to lift this mass. So in practice
we could not go 100 km/hour with solar power.
A big winch at the ballast end of the tether could roll up the entire tether and payload.
For a short tether (under 200 km) with a lower tip speed, a winch could work very well.
For a tether with higher tip velocity, the tether mass is much larger than the payload
mass. If this is the case, you are winching in a lot more mass than just the payload.
On the other hand,
you can have really big motors and large solar arrays because mass does not bother you.
Large ballast is good for tethers.
If the cost to LEO is low, large mass is easy to afford.
The winch can take several hours to roll up a long tether. For example, a 500 km tether winched
in at 50 km/hour would take more than 10 hours to wind in and 10 hours to wind out.
This could limit the number of launches to 1 per day. However, if you let out
one end as you winch in the other end, you could do a load every 10 hours. If you
could winch at 100 km/hour you could get it down to every 5 hours. If the tether
were only 100 km long you could get it down to every hour.
Robert Forward  had the idea of having a number of winches along the cable. If you imagine 10 winches
each of which can pull in cable at 50 km/hour then a 500 km cable can be wound up in 1 hour.
You need some large batteries. The batteries can charge up when you let the cable out and
then are used when you pull it in. The problem with this is that you need a lot of mass along
the cable which means the cable has to be a lot stronger.
The multi-winch does not seem practical.
If the tether grapple releases a payload at the right time (shortly after the tether is horizontal, on
the up swing), it will toss the payload into a desired orbit.
The best toss will rendezvous with the other end of the tether, the ballast end, with low
delta-V. A slowly rotating tether can make this kind of a toss. For a slowly rotating tether, the
tangential velocity drops as the payload climbs up the gravity well. This means that the vertical
component of velocity can be as low as 400 meters per second. Since orbital velocity is about 7500
meters per second, this toss is a nearly circular orbit.
(The orbital velocity is almost 20 times the vertical component.)
Unfortunately, as the rotation speeds up, the vertical component of velocity climbs. There are two
reasons; first the velocity is higher but second - since the payload spends less time climbing up the
gravity well, it loses less velocity. In fact, there will be only about 100 - 200 meters per second slow
down. This forces the toss orbit to be more elliptical than we would like. At the 4-ton toss, the orbit
will have a perigee of about 150 km altitude, and an apogee of about 9,000 km altitude.
To make the payload rendezvous with the ballast, the simplest model is to shape the ellipse so that the perigee is about
650 km altitude. If the apogee of this ellipse is about 2500 km, the payload will have the same velocity
as the ballast when at perigee. There is a synchronization problem, the tether needs to be at the right
part of the orbit, and the rotation of the tether needs to put the ballast at the payload perigee.
Since both the payload and the tether are under power, this match up can be calculated and executed.
A small spacetug-rocket could bring the payload from the grapple end of the tether to the
ballast end. If this tug-rocket had an ion-drive and solar power it would
not require much fuel to operate. The tug and payload could release from the tether at a certain point in the rotation of
the tether depending on where it wanted to go. It would be going fast enough to be in orbit but not
exactly the desired orbit. You want to send solar panels and thrusters up to be part of your growing
tether. These can be integrated into a space tug so that much of the mass of the space tug is the payload.
Better than a spacetug is using aerobreaking. When tossed from the payload apogee is too
high. It can be lowered to where it could be picked up at the ballast end by aerobreaking.
What is best?
The toss method can get to launching every 100 minutes faster than the climb or winch method,
so for a LEO tether we think the toss method best.
The toss needs less mass on the tether than a winch or climber.
The tether needs to be larger, stronger and heavier to have a winch on the tether.
Once the tether is built, it will be in the toss business.
The question of climb or toss only applies to the construction process, while
the LEO tether is adding ballast or tether segments.
Later we will look at a LEO tether tossing to a hotel at GEO. The GEO tether tip speed
will be relatively low (1.6 km/s), so it would not be hard to just winch in the end of the GEO tether.
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Copyright (c) 2002, 2003 by Vincent Cate. All rights reserved.