Why Space Tethers have a Bad Reputation

A number of rather ambitious and far-out tether concepts have been proposed by other people. Many of these concepts require materials and technology well beyond what we have today. These have given tethers in general the reputation of not being practical today.

GEO-stationary Tether / Space Elevator / Beanstalk

One of the ideas is building a cable all the way from Earth to a weight beyond geo-stationary orbit. This space elevator idea gets a lot of press. Promoting this idea are websites like SpaceElevator.com or liftwatch.org. There is also a book on space elevators as well as a yahoo news group This would make a cable fixed relative to the surface of the Earth going all the way into orbit. For example of such systems, see HowStuffWorks.com Space Elevator, or HighLiftSystems. To build this they need cables made from materials stronger than any available material. Some people hope that Carbon nanotubes can be made into cables, which may be strong enough.

A space elevator passes through the atmosphere, unlike a space tether. If it is hit by lightning it will be destroyed. Nanotubes conduct electricity well. A conductor going all the way through the atmosphere would be the ultimate lightning rod. The space elevator people say they will put it where there is not much lightning. The investment in dollars and the risk to humans riding the elevator makes even a chance of lightning a serious issue. A space tether does not have a lightning problem at all.

A geo-stationary tether station is about 36,000 km away, and you have to climb uphill the whole way. If the climber averages 100 km/hour it takes 15 days to climb up. Solar powered cars can do this kind of speed on flat ground, but straight up it is not easy. At this rate it would take a long time to bring up cargo equal to the initial weight of the tether (and this assumes you had materials we don't have so that you could even build it). Some places advocate beaming power to the elevator to get around this. Even with plenty of power, there are limits to how fast wheels can climb a rope. And this beaming power is a futeristic concept that makes it seem like tethers are not practical today. In fact, a NASA study estimates it will be 50 to 100 years before this type is practical. This is not what is called "near term technology".

The reliability of elevator climber is a problem. I doubt that any car I have owned has ever gone 22,000 miles without needed some repair, and I am mostly going on flat ground. To go straight up for 22,000 miles without having to have a mechanic fix something is really high reliability for a wheeled vehicle. So this will be a high priced vehicle since high reliability takes high priced engineering and high priced materials. A rotating tether does not need a climber.

A rotating tether is practical with with existing materials and technology. It needs much less mass and so it is cheaper to build and launch. A rotating tether can also lift payloads much faster (like 10 minutes) and more often. Using cheap existing ropes, less mass in space to get started, and more frequent payloads, makes them far more financially interesting.

If it takes 15 days to climb, you have a radiation risk. You are passing through the Van Allen belts. This is not a good place to stay for days even with a couple centimeters of shielding. Your GEO hotel/space-station is going to have plenty of shielding and is only on the far edge of the Van Allen belts. Taking a long time to get there also increases your chance of getting caught by a a solar partical event (aka solar flare). A rotovator can toss to GEO in 4 hours, which greatly reduces the radiation risk. The space elevator approach would require a lot more shielding to get the radiation risk down to the level of the rotovator. The shielding is dead weight. It could take several times the mass of the people to shield the people, which raises the cost of the elavator approach.

The Van Allen belts are also going to be hard on the nanotubes used to make the Space Elevator. The radiation will weaken them rapidly. A rotating space tether does not need to be in the Van Allen belts.

The kind of rotating tether SpaceTethers.com advocates can grab a payload and then toss it about 10 minutes later. With one launch site we could do a payload every orbit, or about every 100 minutes. This is like 14 times per day, instead of every few days as a Space Elevator would be.

The main advantage of space elevators over rotating tethers seems to be that it is easier to explain to someone.

Tether Failures

There have been a few widely publicized tether experiment failures. This gives tethers the image of not working. The experiments that work are not as newsworthy. In the early days of airplanes, where we did not understand so much about flying, there were many newsworthy failures too. It is still the early days of tethers, but understanding and reliability are not far off.

Not All Tethers Are Bad

There are some types of tethers that could be built today. The design proposed by SpaceTethers is small enough to be sent up in one commercial rocket launch. This is a very practical design because it starts out small and uses only off the shelf materials.

Note that at least one tether has actually already been in orbit for years without breaking.

Links

A good paper on the costs/Kg of launching with a Space Elevator is Launch Systems Cost by Jordin T. Kare . The development cost of the Space Elevator is so high that if you take capital costs into account your costs/Kg may not be cheaper than current launch vehicles, let alone cheaper than what they can be in 20 years. The super strong carbon-nanotube composite that is needed for space elevators would make pressure tanks for rockets much lighter. Probably years before it is possible to make enough for a space elevator it will be possible to make enough for some rocket tanks.

More on the Space Elevator at spaceelevator.com.