Rotating Spacecraft slingshot

[Idobox]

It comes down to the strength of the tether. 100km is achievable with current materials like Spectra or Kevlar. 200km isn't, it would snap under its own weight before you put a payload onto it. A lower speed is certainly a possibility, but then you need to carry propellant to get to the moon, which is the only place I can think of we'd need to send that amount of stuff with a tether system.

Zylon has a characteristic length of 450km. I don't feel like doing the math right now, but a 200km tether with a 4g acceleration at the tip is in the right order of magnitude without tapering. With tapering, you can get significantly longer cables.

The main use for a tether system would be send stuff to LEO/GEO. That's where we send almost everything anyway.

For GEO, you grab the payload 3km/s below LEO orbital speed, release it 3km/s above it, and you would only .8km/s to get into orbit.

For LEO, it's a bit trickier. You would have to move the payload from the tip of the cable to the center of mass and release it from there. That can be done by rewinding the cable (far from optimal), or a kind of lift to move the payload along the cable.

Of course, you could also use it to send stuff to the moon, using the same procedure as GEO, but with a different burn after release. But I doubt we will live long enough to see stuff sent to the moon that compares with the mass of satellites and manned missions in LEO/GEO.

[peadar1987]

Zylon has a characteristic length of 450km. I don't feel like doing the math right now, but a 200km tether with a 4g acceleration at the tip is in the right order of magnitude without tapering. With tapering, you can get significantly longer cables.

The main use for a tether system would be send stuff to LEO/GEO. That's where we send almost everything anyway.

For GEO, you grab the payload 3km/s below LEO orbital speed, release it 3km/s above it, and you would only .8km/s to get into orbit.

For LEO, it's a bit trickier. You would have to move the payload from the tip of the cable to the center of mass and release it from there. That can be done by rewinding the cable (far from optimal), or a kind of lift to move the payload along the cable.

Of course, you could also use it to send stuff to the moon, using the same procedure as GEO, but with a different burn after release. But I doubt we will live long enough to see stuff sent to the moon that compares with the mass of satellites and manned missions in LEO/GEO.

Is that characteristic length without a payload and a factor of safety though? My figures are from previous research, and I cited them in my MEng. dissertation, but I didn't actually do the calculations myself.

[Idobox]

All the figures are given without safety factor.

The characteristic length is calculated without payload.

Actually, I did the math, it's a slow day at work

A 200km long untapered Zylon cable spinning at 206.3 rotations per day would have the tip traveling at 3km/s, exerting 45m/s² of acceleration (between 4 and 5g). The tension at the middle would be 6GPa, just over the 5.9Gpa rating of the material.

As a result, a Zylon cable of these dimensions would just be able to support its own mass.

For tapered spinning cables, all that matters is the tip speed and payload mass, not the length, with the formula M/m = sqrt(pi) * Vr * e^(Vr²) * erf (Vr) where M is the payload mass, m the cable mass, erf the gaussian error function and Vr = Vtip/Vc.

For Zylon, Vc is 3km/s, which for a 1t payload gives us 11t of cable, well within the capabilities of large launchers like Delta IV or Ariane 5.

With a SLS, we could send up to 130t of payload. With a 110t cable, we could manage 10t payloads, enough for a Soyuz or a dragon.

And because the formula for tether mass doesn't depend on length, we could make significantly longer ones, reducing the acceleration. The problem then is that you have to spin horizontally to avoid the tip getting inside the atmosphere.

[Idobox]

With a safety factor of 2 in tension, for a 1t payload, I get 24t.

Still possible for a Delta IV.

[peadar1987]

Ah that's the difference between your calculations and the ones I was using. My source was using Spectra with a yield strength of just under 3GPa, and a factor of safety of about 3 IIRC.

[Idobox]

Spectra is inferior for this type of task. M5 is assumed to be even better than Zylon when they figure out the right production process. And then, we can dream of carbon nanotubes and graphene (50 and 130GPa respectively, I'm drooling right now).

A safety factor of 3 seems overtly cautious to me. Rockets wouldn't fly very well if we used that kind of safety ratio on tanks and engines. Of course, they don't have to last for years like a rotovator would.