December 11th, 2008
In ‘Part 2′ of this series on Space Elevator tether strength, I referred to Ben Shelef’s document (The Space Elevator Feasibility Equation) on how strong a Space Elevator tether really has to be and explained how stronger power systems can increase the amount of material a Space Elevator can carry at any one time.
Are these stronger power systems a luxury? Can we start out with a weaker power system and then, at some point in the future, increase its power? After all, isn’t everything a Space Elevator is going to carry to space useful payload – i.e., satellites, tourists, manufacturing plants/capabilities, etc.? Wouldn’t a weaker power system mean that we carry just less useful stuff into space?
The answer to that is a resounding “No”. In addition to being able to support its own weight, there are other ‘housekeeping’ requirements which a Space Elevator tether HAS to satisfy before it can start hauling useful payload.
The Space Elevator tether is going to degrade over time. Micro-meteorites, atomic oxygen, radiation, physical wear and tear from Climber runs, etc., are all going to cause damage to the tether. This means that the tether material is going to have to be periodically replaced. How quickly this has to happen is one of the key, unanswered questions which must be addressed before a Space Elevator is built, but obviously the material has to be replaced faster than it wears out. So, in addition to holding its own weight, the first housekeeping requirement is that a space elevator must be able to replace itself faster than it wears out.
In addition, to being able to periodically replace itself, the space elevator must also be able to increase its carrying capacity. When the initial space elevator is launched and deployed, it will not be strong enough to carry much payload; launching a fully-fledged space elevator capable of supporting, say, a 20 ton climber, would be prohibitively heavy and prohibitively expensive. So the current plan is to launch a ‘seed ribbon’ and, once that is in place, to send up climbers on that seed ribbon with additional material which is spliced to the tether. As more climbers add more material, the more payload the space elevator can carry. Incidentally, this highlights the chief advantage of a Space Elevator over other methods of carrying payload to/from space, its scalability. There really is no practical limit on how big a Space Elevator can be. You can make a Space Elevator which will carry hundreds, even thousands of tons of cargo into space every day; all you have to do is just keep adding tether material to the space elevator ribbon. Of course there’s a price to pay for that; payload dedicated to additional tether material is payload capacity which cannot be used for other things. You do not have to continually increase the capability of the Space Elevator, of course, you can stop when it can handle, say, a one hundred ton climber, but it will take many years for this to happen. The upshot of this is that this second housekeeping requirement says that a space elevator must also be able to carry a ‘doubling capacity’ in addition to carrying normal payload.
Finally, prudence dictates that a spare seed ribbon should be carried by the space elevator up to and stored at GEO (or perhaps beyond, to act as counterweight material) to facilitate making a new space elevator in the case of a ribbon break (or other catastrophic failure). Yes, one could hope that there would be rocket capability to launch another seed ribbon if necessary, but it would be foolish to plan on it. So, this third housekeeping requirement says that the Space Elevator must also be able to carry this spare seed ribbon into space.
So, we have three housekeeping functions that a space elevator MUST be able to perform before it can carry useful payload into space; replacement material to handle tether degradation, additional material to increase the tether’s carrying capacity and a spare seed ribbon.
If the Space Elevator cannot, at a minimum, carry all of this material within the required time frame, then there is no point in even trying to build one – it HAS to be able to do this to succeed. As I discussed in the previous post, the stronger your power system is, the faster, in general, the climbers will be able to ascend the ribbon. And they have to be able to ascend fast enough to allow the Space Elevator to handle BOTH the housekeeping requirements and hauling useful payload into space.
In his paper The Space Elevator Feasibility Condition, Spaceward CEO Ben Shelef converts these housekeeping requirements into mathematical variables which he then matches against possible tether strength scenarios and possible power system scenarios.
There is one other Space Elevator requirement which must be discussed and that is its safety margin. How ‘safe’ does a Space Elevator have to be? Nothing is 100% safe, everything can fail under certain conditions. So how safe do we want a Space Elevator to be?
In my next post, I’ll discuss this in more detail.
(Picture of competition tether from 2006 Space Elevator Games - click on it for a larger version)
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