View Full Version : Skyloop - variant of Skyhook

2009-Jan-25, 07:57 PM
I just thought of an interesting variant of Skyhook. Here's the picture of skyhook which inspired this "Skyloop" idea:


The idea for Skyloop is that you have an orbiting loop instead of a rotating line. Like skyhook, the relative velocity between the "bottom" of the loop and Earth's atmosphere might be hypersonic. Unlike skyhook, this does NOT require a high speed encounter.

Because skyloop is a loop, you have a long time to try and hook onto it. Your ship flies "sideways" to the loop and hooks onto the bottom using a big hook (like a carrier jet's arrestor hook). The hook is covered with a durable low friction sliding bearing material.

After hooking on, the loop starts off sliding past the hook--friction and/or magnetic induction is used to bring the ship up to speed. This causes the ship to be hauled up to orbital speed. The ship releases from the skyloop when it reaches the top, flinging it into LEO.

Periodically, skyloop will need to be "recharged". This may be done by latching a solar electric satellite onto the loop. This respins the loop and brings it back up to altitude.

Compared to skyhook, skyloop is easier to latch onto and requires less velocity.

2009-Jan-25, 08:12 PM
It occurs to me that it is NOT necessary to use friction or magnetic induction to bring the ship up to speed. The skyloop is actually most effective when friction is close to zero. The velocity boost when reaching the "top" of the loop is greater when the friction is lower.

2009-Jan-25, 09:01 PM
The loop has to interact with the payload somehow, or the payload will just continue in a ballistic curve back down to Earth. What do you propose?

2009-Jan-25, 10:10 PM
It's a simple hook. Probably with some sort of sacrificial fluid to provide a low friction bearing.

The skyloop's rotation is completely independent of the device's operation. The rotation is only important for maintaining the circular shape.

Essentially, the skyloop works like a half-tube section of slingatron. The ship enters the "bottom" at a speed of about 8km/s. It leaves the "top" at the same speed--about 8km/s. However, this speed is in the reference frame of the skyloop itself. In Earth's reference frame, the orbital speed of skyloop is added to it. The ship starts with zero velocity, and gets boosted up to 16km/s.

Note this is far above Earth escape velocity. In order to "only" boost up into Earth orbit, you'll want to release earlier than a full 180 degrees and/or use braking friction to slow yourself down. Otherwise, you'll go sailing off into an interplanetary trajectory suitable for Earth-Mars or Earth-Venus transits. With a little extra boost from a rocket, you have a trajectory suitable for Earth-Jupiter or Earth-Saturn transits.

However, I imagine the big economic market will be for boost "only" up to GEO. Skyloop's natural boost will sail past GEO.

Ara Pacis
2009-Jan-26, 12:05 AM
What are the dimensions of this loop? At first I thought you were referring to a circumterra loop. After thinking about it, it sounds like a Lofstrom loop in space.

Ya know, maybe we should have a dedicated "Space Infrastructure" Thread.

2009-Jan-26, 01:20 AM
For launch of manned payloads, the radius would have to be pretty darn large. So let's talk unmanned payloads. Assuming an acceleration of 10 gees, and a speed of 8km/s, we have:

a = v^2/r, so r=v^2/a = 64,000,000m/100 = 640km

With a 640km radius, the circumference is 4000km. That's an impressively long steel cable, to say the least!

Honestly, an Earth orbit skyloop would be a huge long term navigational hazard, more or less ruling out satellites in LEO. The good news is that it makes launching things into GEO pretty darn cheap.

A lunar skyloop might be a more useful device. It only needs to deal with a payload velocity of 1.7km/s. Assuming a 10 gee acceleration, that gives us a radius of only:

r=v^2/a = 29km

This loop would only be 180km in circumference. This lunar skyloop wouldn't be a navigational hazard for Earth orbit, and it could loft payloads straight into elliptical Earth orbit.

2009-Jan-26, 01:59 AM
Note that one difference between a loop and a tether is that the loop can not be tapered. All portions are supporting all the weight of the structure.

This makes analysis easier: For an untapered tether with a given tether material in a given gravity field, there is a characteristic length beyond which the tether can not support its own weight. For a loop, doubling radius while keeping tangental velocity constant doubles weight and halves period and acceleration. In other words, for a loop, your choice of material determines the maximum tangental velocity, independent of radius or amount of material used. For steel, that's about 500 m/s, assuming a tensile strength of 2000 MPa. A steel loop with that 2000 MPa steel with 1g centrifugal force holding it in tension could be no more than 4 km in radius. Of course, you must provide for the mass of the payload as well.

In a sling tether, the portions of the tether closer to the axis of rotation experience lower accelerations and require less material to support. In a loop, all of the material is at the maximum radius, the worst place for it to be in terms of structural strength. An untapered tether can have a tip velocity sqrt(2) times higher than a loop made from the same material. And slings can be tapered to remove material that does nothing but add weight, this can yield vast improvements in tip velocity and in material cost.

2009-Jan-26, 02:07 AM
You don't need to support 1g. The spin on the loop is only there to keep the shape circular. A tangential velocity of 500m/s is fine; a tangential velocity of 50m/s is fine.

The reaction force on the loop is actually very small. This is because the payload is traveling so fast with respect to the loop that only a small amount of force is experienced by any particular meter of it. It's sliding along the loop at 8km/s. Assuming a mass of 1kg per meter, that's a mass of 8 tons per second.

Van Rijn
2009-Jan-26, 09:37 AM
There is the electrowheel idea, which sounds quite similar:


I'd like to see the math worked out a bit though (I want to see more on the stresses).

There's also the orbital loop:


And it isn't explained here very well, but there is Donald Kingsbury's orbital coilgun scheme:


Kingsbury assumed active stabilization to keep it from buckling or changing orbital orientation.

2009-Jan-26, 10:16 AM
The loop idea is interesting; it appears to be just a loop of propellant with pre-existing momentum, which transfers a little bit of it's momentum to the payload. Using friction (and worse, lubricant) in space sounds tricky, to say the least. Perhaps some sort of gas might work; compressed air, perhaps. Hah! Riding a hovercraft into space! What an intriguing idea!

I've put some of these ideas on OA; the Space Fountain (http://eg.orionsarm.com/xcms.php?r=oaeg-view-article&egart_uid=478838bfb7f74), the Lofstrom Loop (http://eg.orionsarm.com/xcms.php?r=oaeg-view-article&egart_uid=4684534627504), and the Rotovator (http://eg.orionsarm.com/xcms.php?r=oaeg-view-article&egart_uid=47768c21bcd86); the Electrowheel and this new 'friction loop' idea (which seem vaguely similar) might be next.

2009-Jan-26, 10:53 AM
A gas would work. The relevant details were worked out by the Slingatron concept. Slingatron had a rod-like projectile sliding inside a curved tube at 10km/s. It used a thin layer of gas to cushion between the projectile and the tube.

This would use that same idea, but in reverse. The "hook" is like a small section of tube (with gently tapered "intake" and "exhaust" for the rod-like wire). The wire takes the place of Slingatron's projectile.

The "intake" of the hook would have a gas injector, which provides a thin gas cushion between the hook and the wire. This is the same basic principle as the way a hard drive head floats off of a platter, but of course hard drives don't operate at such ludicrous speeds. Slingatron, in contrast, was designed for Earth escape velocities.

2009-Jan-26, 11:02 AM
There is the electrowheel idea, which sounds quite similar:


Skyloop is almost exactly like electrowheel, with the exception of not using the "electro" part. I originally proposed using friction and/or induction to produce drag--just like electrowheel. However, I realized that this drag just makes the system less efficient. It's better to minimize friction. That way, you minimize waste of orbital energy into heat, as well as double the maximum speed boost (which is enough of a speed boost for useful interplanetary trajectories).

2009-Jan-26, 05:21 PM
Perhaps a combination of a magsail and electrodynamic tether propulsion. Put a superconducting loop in orbit, set up an electric current in it and inflate the field further with plasma, and let electrodynamic drag pull payloads up to speed. You could re-boost the orbiting magnetosphere by using ground-based particle beams (on airless bodies, at least). I've proposed something similar before for initial braking on airless bodies.

2009-Jan-26, 05:30 PM
Just think of the static charge that would build up on such a thing. Discovery had a show a couple of weeks back where they were lifting logs via a helicopter. The ground crew had a grounded wire they had to throw at the cable end before they would grab it. Otherwise they received a nasty shock.
If you had several miles running through the air at high speed.... Pow! Man made lightning hitting your payload.

2009-Jan-26, 05:35 PM
You wouldn't want this thing dipping into the atmosphere. It doesn't cost much delta-v for the payload to go straight up into LEO vacuum. It's the horizontal orbital velocity which is the main "cost". Dipping into the atmosphere would cause a lot of drag.

2009-Jan-26, 06:28 PM
Interesting... I'm still leaning towards a rotovator.