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Comment by hinkley | Hacker News Reader

Tethers shift the economics of landing ships or materials on earth, since they can trade delta v with things outbound.

But it seems like you have to trade power for accuracy; if you can hit a tiny window moving in three dimensions inside of a 3 body problem, then you don’t need to be able to manage as much delta V (increasing to leave, or decreasing to land). Smaller rockets, less need for ablative materials.

But if you miss, boy are you fucked.

One solution for this I’ve heard is you take enough fuel to get where you’re going with plenty to spare. If everything goes right, you offload your emergency supplies (reaction mass, fuel) and get a credit.

I’m not sure how much handwaving goes along with moving liquids and gasses in vacuum, at zero g... burst a line you need a lot of reaction mass to reorient yourself.

Maybe there’s an emergency venting design where all of the forces cancel out...

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> But if you miss, boy are you fucked.

Imagine closing in on Mars knowing that if you fuck up a super-orbital speed, split second maneuver by more than a couple of meters you will drift in space until asphyxiated. It makes a carrier landing seem bland.

If you carry enough fuel to decelerate without the tether it defeats the purpose of the tether.

The comparison between tethers and an aircraft carrier catch cable seems particularly apt. Thanks you. And yes, most times, you can initiate a go around if you fail a carrier landing. Not really an option with an orbital tether. Some sort of cable pair with a targeted coupler would probably work, each side with a heat emitter an heat seeker RCS system. Obviously, there would need to be slack in the system, (nearly local) winches a ways up-cable in the system, may be an answer.

Maybe don't open the tether until you have tugboat sufficient to retrieve a bolter.

The relative motion drops to zero though, at the moment of capture. So it's more like two cars on the highway that pull alongside each other just as they match speed. Might still be too precise for a human, but automated systems should have little trouble.

And it might be easier to store the emergency retrieval system on the tether's hub. That way it doesn't have to be hauled around to be used (hopefully) infrequently.

You have to get the distance and velocities to line up, and it’s challenging to change only one of those in a three body situation.

Because if you arrive with a full tank, it's useless and you vent it?

Maybe there could be uses for the fuel, if you succeed in getting it there.

You keep it for the next tether or you sell it at destination.

How is that different from aerocapture, which is what Starship is going to do?

You are coming in at hyperbolic velocity and have to use the thin and very variable atmosphere to break into an orbit.

At least the tether is completely predictable.

Hard to miss an atmosphere. If you're off to the side a bit, no big deal.

Actually it's completely plausible to skip on the surface of it. Bit easier to hit sideways, yes.

Bringing a few maneuvering thrusters to adjust the trajectory would still helpful and doesn't require tons of fuel.

If it works in KSP, it will probably work in reality.

I think the idea is brilliant, since you don't need rocket fuel to to generate the force you would transfer to the craft.

I’ve seen some proposals where there’s a low-mass extension to the cable that can change delta-v quickly and then reel in the real tether. Moral equivalent of bridging a ravine by tying a thread to an arrow, pulling a string across with the thread, pulling a rope across with the string, pulling a cable or two across with the rope, and pulling or building a simple bridge across using the cable.

But with the bridge, it’s just distance you’re crossing. With a tether you’re also dealing with velocity. And changing velocity changes distance...

In theory I suppose the tether could be equipped with a large-ish rocket that's capable of going out and grabbing anything that misses and putting it back on a correct course. If you had to use it all the time, it wouldn't be worth constantly refueling it -- you might as well not use the tether at all and just use bigger rockets. But if it only gets used occasionally, it's probably a good insurance policy to have on hand and would just be a cost that gets amortized across all the successful launches.

There will be a lot of factors constantly perturbing the trajectories, so lacking the ability to take corrective actions should be out of the question. Unless something happens too close to the point of contact, I expect the situations to be calculated and compensated for. Also, that tiny window that you talk about doesn't actually have to be that tiny. There can be engineered a string of potential contacts with multiple tethers on slightly different orbital altitudes to lower the risk, and for fungible cargo like fuel or other kind of supplies, that may fall be within the acceptable risk margin. Of course, people and other high value cargo can be serviced by other means if necessary.

>> you take enough fuel to get where you’re going with plenty to spare.

Then what is the point? 90+% of spacelaunch is lifting fuel, and the fuel you need to lift that fuel, and and and... all fuel. The whole thing is an exercise in moving reaction mass around using other reaction mass. Hauling around an extra 2km/s of deltaV would require a rocket on the pad double or triple the size of not bringing it. Spacelaunch equipment has to work every time or else it isn't worth the investment.

Even if we treat the tether as a potential 'bonus' rather than a necessity, think of all the fuel spent on the intercept with the tether. Your launch time would have to coincide with the tether, meaning you launch at a non-ideal time. And you would only every launch into the tether's orbital inclination/plane, radically reducing the number of potential destinations.

Tethers are about not treating launches as isolated events. So why are you still thinking about the rocket equation as a single interval?

You could imagine one company having a more reliable but expensive launch platform and another having a cheaper one that occasionally explodes, or has a smaller launch window. You'd fly up fuel on the cheap ones, and your deep space probe on the other. And any fuel left from previous missions can be resold or spent on good will.

Essentially instead of 3 stages to high orbit, or 4 stages to another planet, you have 7 smaller stages (2 + 2 + 3) to another planet.

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