[mrfusion]

You know I've been researching the very same idea. And guess what none of the math I've tried so far rules it out!

You do need a very big cable though. Think miles.

There are actually many promising technologies to get mass into space with much lower marginal cost. I think the problem is that they all have high capital costs and no one wants to take the risk on a new technology.

[tdb7893]

Wouldn't the problem be air resistance? I thought you generally need to bring fuel with you because if you don't the air will brake you too much

[mrfusion]

Air resistance hurts you for sure. but the prospect of not bringing your fuel with you (see the rocket equation) really saves the most.

In my design I have the ship tethered by a cable and a pipe that's delivering the fuel.

[derekp7]

Orbital velocity is about Mach 25. You would need to be going much faster than that, if your only power is applied in the atmosphere (you'd need enough additional speed to get above the atmosphere, and enough to overcome air resistance. And after the cable lets go, the the orbit will be elliptical, which means that at the end of one orbit you come right back to the launch site. So you need to bring enough fuel with you to flatten out the orbit once you are at the highest altitude. Oh, and then you have the weight and tensile strength of the cable, so similar issues to that faced by building a space elevator.

A better idea is the launch loop, but again you have to deal with cable weight and tensile strength.

[mrfusion]

Yes good points. I envisioned it more as a first stage and partial 2nd stage replacement. You'd still need to bring some fuel like you said.

In my design the vehicle would feed its rockets from an onboard fuel tank which would be constantly replenished from the pipe/tether. At detachment time it would still have a full tank to manage drag, getting up to final speed, and circularizing the orbit.

[vanattab]

I don't think you could build a cable that has the tensile strength to hold a payload spinning at 27+ km/s. Even if it was made of carbon fiber I think it would have to be crazy thick and massive. Probably orders of magnitude heavier then the payload itself.

[mrfusion]

Interesting thought. I was thinking just 1.6km/s for use as a first stage replacement. With a 60km tether that works out to only 4Gs. [1]

(1) http://www.calctool.org/CALC/phys/newtonian/centrifugal

[vanattab]

I don't think you could build a cable that has the tensile strength to hold a payload spinning at 27+ km/s. Even if it was made of carbon fiber I think it would have to be crazy thick and massive. Probably orders of magnitude heavier then the payload itself.