Comments on: Tethercatcher

By: Pete

Pete — Sun, 26 Jul 2009 22:23:47 +0000

There is another form I have been thinking about of late which consists of basically a tapered bow string with masses at each end with payload in the middle. One can then exchange energy between the kinetic energy of the payload and string and the gravitational potential energy of the end masses. This can be done in a direct or harmonic fashion if desired. It is basically the pendulum form of a rotovator. Tether mass ratios or catch/release speeds are not as good as for a rotovator (yet to properly model it), but some things are simpler.

First example, a mass at a higher orbit with another mass in a lower orbit both connected by a tether. The payload has a hook which connects with the tether (bow string) at mid length. One can actively winch energy in and out of the bow string oscillation at one end. Using an elliptic orbit one can also use tether cranking up and down to pump up the orbit (without expending any propellant).

Another example is the same system on say the moon. Two towers (or hills) with a string between them with two large suspended masses (or like force system) pulling it tight. Pull the string back and use it as a catapult. The non obvious advantage is that very high speeds can be reached using large low speed forces (it is effectively a gearing system) and only flexural elements – no bearings. Launch rates can be very high and payload catching/releasing much easier.

By: john hare

john hare — Sat, 25 Jul 2009 23:31:40 +0000

For asteroids a rotovator would make much more sense than the catcher I am suggesting here. With weak enough gravity, a ferris wheel or horizontal hub mount arraingement could work. It doesn’t seem worth it to have it in such a weak orbit.

By: dave taht

dave taht — Sat, 25 Jul 2009 22:29:20 +0000

I had an exchange (email or usenet, can’t remember) years ago on the subject of using tethers for energy exchange on asteroids. Two points came up:

1) The fast rotators have very (<60 km, buildable today) short tether links to balance out their weak gravitational pull. So you can easily build a zero or +positive energy system for launching from the fast rotator class of asteroids.

2) You can actually apply an economic analysis to momentum exchange via tethers where you can take advantage of the asymmetry of the values of the delta-v change to the object – the cost of the delta-v change to the tether and peg a real price on it.

We called that idea “A Momentum Bank”. A run on that bank would push you out of a desirable orbit….

By: john hare

john hare — Sat, 25 Jul 2009 20:39:45 +0000

Extra thought. Does a fishing line get damaged more often when it is on the reel or when it is stretched completely out.

By: john hare

john hare — Sat, 25 Jul 2009 20:37:36 +0000

One difference between this one and a traditional tether arraingement is that this one can be developed and tested on the ground with military projectiles, possibly even with a market for returning expensive PGMs undamaged. The rotovator can only be tested in space. This one can be further tested and developed in revenue service by deorbiting LEO debris.

When it does go into orbital service, it will be with a good understanding of its’ capabilities and limitations. The investment cost is also far lower than a rotovator which must enter service full size at the start, with little or no test history.

In operational use, this system presents a small target area to damage from or to other orbital objects most of the time. A rotovator presents many kilometers of hazard length at any time, and it sweeps a circle of that diameter constantly. Colision avoidance is far more of an issue with the traditional units than this one.

By: MG

MG — Sat, 25 Jul 2009 19:08:36 +0000

I think I see where we are diverging. Your system envisions a much shorter tether to provide a quick transfer of momentum. The more traditional proposal of (near) circular orbit MEO spacecraft requires a long tether, whose earthside end dangles at LEO height, but is moving at suborbital (for LEO) speeds.

So we are trading one high risk problem (2 km/s intercept and capture) for a different high risk problem (multi-thousand km tether control and low speed intercept and capture).

I confess I am not convinced (but I can be convinced) that your proposal has schedule or investment risk advantage over the more traditional proposals.

By: john hare

john hare — Sat, 25 Jul 2009 17:57:52 +0000

Without the 2 km/s rendezvous, the payload vehicle has to reach orbit on its’ own, same as a rotovator, except that the rotovator is inherently more useful. The advantage here is the ability to start at low investment and with short development times.

The high eccentricity is in the early phase when tethercatcher mass is similar to the payload received. As tether system mass increases, eccentricity is lowered to give more assist to the vehicle coming up.

By: MG

MG — Sat, 25 Jul 2009 17:12:12 +0000

If this system presupposes a high Isp spacecraft that deploys / retrieves / manuevers the tether arrangement, then why not place the retrieving spacecraft in a low eccentricity orbit, with a larger semi-major axis? That broadens the operational utility (you don’t have to worry so much about the location of the perigee), and reduces the velocity differential between the catcher and the payload.

One already has to develop tether based infrastructure, so why add the complexity of a 2 km/s closing rate “rendezvous”?