Q: I think the idea of stopping a spaceship with a cable is absurd.
A: Einstein said ‘If at first the idea is not absurd, then there is no hope for it.’ Also, that wasn’t a question.
Q: All right, then: Wouldn’t stopping a spaceship with a cable require a really, really LONG cable?
However, the real issue is in creating a Carmot-like material in a reliable manner. Once (if) that hurdle is breached, manufacturing it in large amounts should be a relatively trivial matter.
Q: I don’t mean making the stuff, I mean wouldn’t millions of meters of ultrathin cable be impossible to manage?
A: You mean like a cheap, overlong garden hose or all the cables you’ve stashed away from your old electronics? Well, the fact that all that cable would be unwound in space is an advantage, as show-stopping tangles are less likely there. The cable would more or less float along where it was left, until it’s reeled back in - an operation that would be overseen mainly by ‘bots.
More to the point, you have to compare the practicality of this method with the only other methods of braking we have employed (so far) in space:
• Atmospheric friction, which transforms momentum energy (inertia) into heat. This method is always dangerous (your craft could simply burn up) and in any event it is completely impractical for large craft such as those Patriots envisions. It’s also impractical for use with the space elevator, since the Patriots crafts are hauled up in the elevator in pieces and assembled in space. (Meaning that the entire, assembled, craft is too big to haul into orbit via the elevator.)
Friction braking only works if your craft is going to have an extended ride through the atmosphere - that is, if you’re going to land on the planet. Braking with a Carmot cable would be much more efficient in that your craft could remain in space (where it belongs, if you think about it), while only cargo and personnel would be transferred down (via elevator, or by shuttle if there is an emergency or an elevator is not available).
• Retro rockets, which again are insufficient to slow a large craft. Retro rockets are inefficient and clumsy, because you must accelerate the very same fuel (or at least propellant - the Patriots ships require only propellant, not fuel) you will later use to stop the craft. This means your travel is slower because you are carrying a great deal of extra weight (in fuel) that you need just for braking. And if fuel is lost along the way or miscalculated - well, you could be in for a very long ride!
The ships in ‘Patriots’, on the other hand, could be almost completely out of propellant and still brake successfully at their destination. They need carry only a modest amount of extra propellant (ordinary water, used in the form of IceXII pellets).
Keep in mind, I’m not including the ‘magic’ methods used in Star Trek where the ship simply stops because ‘we’re out of warp’, or vessels such as those in Star Wars which are similarly unconcerned about trivial matters such as the laws of physics.