- 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

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.

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.

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.

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.

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.