July 20, 2007

Improving on the hypersonic skyhook concept

A tidal stabilized tether is called a "skyhook" since it appears to be "hooked onto the sky". They are also called "hypersonic tethers" because the tip nearest the earth travels about Mach-12 in typical designs. Longer tethers would travel more slowly. A grapple system attached to the tip of the tether can thus reach down below the facility and rendezvous with a payload moving in a slower, suborbital trajectory. The grapple would then capture the payload and pull it into orbit along with the tether system. Later, it could release the payload at the top of the swing, tossing it into a higher orbit

The net benefit of the Tether Launch Assist is that it can significantly reduce ΔV that a launch system such as a reusable launch vehicle (RLV) must provide to the payload. Mach 8-12 instead Mach 25 (orbital velocity), so the energy needed is 4-8 times less.

Electro-Dynamic Orbital Accelerator concept has a large vehicle in orbit with a trailing conductive tether. Instead of trying to meet up with a point hook, a vehicle would meet up with the long trailing tether.

The launch vehicle must provide only enough energy to reach OX altitude, thus allowing 30% or more of lift-off weight to be payload, as compared to ~3% for conventional rockets. The result is an enormous reduction in the cost to reach orbit. Payload increases by ten times.

Instead of a meeting an almost point like hook, a sub-orbital vehicle would connect anywhere along a long tether.

NOTE: the tether is unstable and the design would need to be modeled in detail and means to stabilize it found. I was thinking that small sensors and small segments of the tether could have some means of propulsion OR a small towed could be on the end which also flies to stabilize the tether or to pull it taught before rendezvous.


Snake Oil Baron said...

Interesting. I had always thought that those high efficiency thrust mechanisms like ion propulsion were great for satellites and deep space probes due to their efficiency but limited for human space flight since they don't get you the acceleration needed to achieve orbit but here you can use the efficient thrust on the orbital acceleration thingamajig. It is the best of both worlds if it can be made to work.

It also makes those "x-prize" type sub-orbital space vehicles seem a lot less useless.

bw said...

I agree that this would work really well with the slightly upgrade versions of spaceshipone type vehicles.

Kirk Sorensen said...

Here's the basic problem--horizontal tethers are unstable due to gravity gradient forces. So you can't "drag" a tether out the back of a spacecraft like you would out the back of a car.

Jim Fiske said...

Yes, this type of tether is unstable. That may not be a critical problem. Bicycles are unstable. Airplanes are unstable. The question is, can this type of tether be stabilized at acceptable cost? I think it is likely, but not yet certain.