TETHER

TETHER simulates a rotating space-tether in Earth orbit.

A rotating space tether consists of two "payloads" on the ends of a tether line which rotate about their CoG. In this simulation, they rotate around a central "satellite" which contains a motor. The motor acts by spinning two counterweights in the opposite direction.

A rotating space tetheris useful for dealing with two payloads simultaneously. One payload goes into high Earth orbit (perhaps to the moon). The other returns to Earth. By setting up a system of such rotating thethers, it would be possible to transfer payloads to/from the moon with great efficiency.

In this program, such payload transfers are not simulated. Instead, the whole system would be boosted into orbit then spun-up. Clearly, the total amount of energy required to, say, accelerate a payload to escape velocity, is the same whether a rotating tether is used or not. But much of that energy could come from solar panels operating for many months.

Start the rotation by clicking the torque buttons. When the rotation rate is high enough, release the payloads and see if you can get out of Earth orbit. When the cables are released, one payload should return to earth while the other goes into high elliptical orbit or escapes completely.

The simulation demonstrates:

• gravitational stabilisation: the system likes to hang with the tether pointing towards the Earth
• advantage of rotation vs libration vs nil: if you release the payloads when the tether rotates or swings, it goes further
• different rates of rotation: if the tether rotates faster, the payload goes further
• different cable lengths: if the cable is longer then, for the same rotation rate, the payload goes further
• importance of release timing: it matters when you release the payload
• it matters how the torques are applied: "pumping" the libration uses less energy

A bar shows the total energy used. Can you apply the torque so that the payload reaches escape velocity without the total energy used going to the "red".

Is it more efficient if you shorten or lengthen the cable lengths while applying the torque?

The Simulation

The simulation uses straight Newtonian mechanics (and Newton's method for integration). The tethers are modelled as stiff elastic rods. I spin up the tethers by assuming rockets on the masses (adjusted so both thethers get equal torque) - I think that's equivalent to your motor. Hold down the "Pro" and "Retro" Torque buttons to apply torque.

It works well and shows the expected effects: gravitational stabilisation, libration, a minimum torque to initiate rotation, etc.

Initially, I was surprised that the masses tended to boomerang (i.e. not remain in a straight line). I think it's a genuine effect and not an artefact - presumably the effect arises because the masses have slightly different histories as they are at different distances from the planet. Boomeranging is most severe when the torque is just above the minimum. As boomeranging was such a problem, I decided to change to modelling two dumbells instead of five independent masses (press the More button to change the model). I feel it's a cheat but the simulation still seems to work.

The User Controls

There are controls for

• Torque: motor torque value and forward/backward buttons
• Release: cable release
• Reel: reel-in/reel-out cables

(Other controls for orbit height and eccentricity, system masses, etc. are only revealed for the real enthusiasts.)

Freeware

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Questions or comments ? E-mail me at peterbalch@btinternet.com

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