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Let's imagine we have finally developed buckytube rope. A couple hundred kilometers of rope, able to tug a 100-ton craft at 6g acceleration, in a package packable on said craft.

A near-earth asteroid is passing by. A craft is launched on a nearby flyby trajectory. When the flyby point approaches, the craft launches a "missile" - either an impactor one with a harpoon style hook, or some kind of net, or one that could create a loop... with the rope trailing. It catches the asteroid.

The craft unrolls the rope from a spool, while strongly braking the spool spin, so that the acceleration of the craft pulled by the rope is survivable for the crew/payload and doesn't break anything. This is until the spool stops or the whole rope unrolls (in which case it's allowed to fly loose with the asteroid, disengaging from the craft).

After the spool came to a standstill, the craft is anchored to the asteroid, having gained good several km/s essentially free.

What (beyond inventing a buckytube rope) obstacles could stand behind such a method of propulsion? Would heat dissipation of the brake be manageable? (say, some kind of ablator/sublimator, that's a one-off affair). Would that kind of propulsion make sense?

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  • $\begingroup$ The biggest problem with such a system, provided the engineering is solid, would probably be timing and finding places to go. To hitch a ride, you need to locate, intercept, and capture the asteroid. Then, the asteroid also needs to be going somewhere you want to go as ending up in some oblique orbit would be counterproductive. It may require large amounts of fuel to intercept and match velocities to survivable speeds that it's not worth it $\endgroup$
    – Dragongeek
    Jul 17 '19 at 0:18
  • $\begingroup$ @Dragongeek: not necessarily - you can "let go" of the asteroid before matching velocity 100% - treat this like a gravitational assist except using the strong tether instead of the asteroid's weak gravity. $\endgroup$
    – SF.
    Jul 17 '19 at 6:00
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Let's say the comet passes by at 10 km/s and your vehicle speed is 0. Then your propulsion system has to deliver a delta-V of 10 km/s. You can do this either by burning rocket fuel, or by your harpoon/winch/brake system. The amount of kinetic energy you have to generate/dissipate is the same in either case.

First step: guesstimate

A 100-ton craft requires something like 2000 tons of rocket fuel to accelerate to 10 km/s (roughly the performance of a Saturn V).

If we limit ourselves to the temperatures found in chemical rockets, I suspect your winch brake will need to dissipate enough energy to evaporate 2000 tons of water.

If you can use higher temperatures, the system becomes more mass-efficient, but you'd have to add an intermediary step: convert the brake energy to electricity, and use that to drive an ion engine or thermal rocket.

Second step: calculate

The kinetic energy of 100 tons traveling at 10 km/s is $1/2 *m * v^2$ is $5 * 10^{12}$ J is $1.38 * 10^9$ Wh. That's the amount of energy you have to put in to accelerate to 10 km/s, so that's the amount of energy your brake will have to dissipate.
At 6 G, that $1.38 * 10^9$ Wh is generated in 166 (10,000 / 60) seconds, for an average of 30 GW. On Earth, that kind of power dissipation requires a river and dozens of giant cooling towers.

Water requires 2.2 MJ/kg to evaporate, so my guesstimate of 2000 tons was accurate within 10%.

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  • $\begingroup$ With launch, we are limited by heat of combustion of the fuel - its chemical energy density. With braking, only with structural/thermal durability under heat - we can use any method of heat dissipation and I believe many should be quite a bit more efficient than rocket engines. If we have some way of dumping the heat into creation of 50,000K plasma without damaging the ship, it's all fine. $\endgroup$
    – SF.
    Feb 15 '16 at 11:27
  • $\begingroup$ @SF. Heat dissipation is the most difficult problem on a space craft right now. In chemical rockets the heat dissipation is built into the rocket as a sublimator. Your comment on the 50000K plasma handwaves away thermodynamics. $\endgroup$
    – Aron
    Feb 21 '16 at 23:35
  • $\begingroup$ @Aron: Not thermodynamics but engineering. The device would need to be almost entirely built from magnetic fields not to evaporate. The energy could be transferred by "cold" means like electricity in superconductors or kinetic energy (say, rotary) but sooner or later it would need to be dissipated and then serious problems begin. $\endgroup$
    – SF.
    Feb 22 '16 at 2:25
  • $\begingroup$ @SF. Yes thermodynamics. Its called the 2nd Law of Thermodynamics. Basically hot things can only be made hotter with even hotter things (or work). But if you have work, its more efficient to propel your fuel using work, rather than thermal expansion. $\endgroup$
    – Aron
    Feb 22 '16 at 2:29
  • $\begingroup$ @Aron: What fuel? We don't have any spare propellant to eject. The whole idea is a propulsion that doesn't require a reaction mass. The 50000K plasma is a byproduct we want to dispose of safely! (and we have a plenty of work: braking force of whatever brakes, moving against the ribbon.) $\endgroup$
    – SF.
    Feb 22 '16 at 2:42
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Instead of having only one harpoon with a single wire which has to deal with all of the enormous stress, maybe one could use a whole spiderweb of threads. Each of which snaps in a series of relatively moderate chocks over some time period during the fast flyby.

A very close flyby would lower the required length of wires. A big problem is how to attach the harpoons in the comet's surface. But anchoring points or nets could be pre placed by soft landers which are lighter, slower and launched to meet the comet when it is more accessible than when the spacecraft that later uses this installation for deflection and acceleration passes by on its way to target X.

Maybe a pre placed soft landed asset could melt its surrounding on the comet and blow gasses in a long stream towards the trajectory of the flyby spacecraft, providing aerobraking for it as it approaches.

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  • $\begingroup$ I pictured a body the shape of Chury, two rockets drawing a fork of rope, catching it around the "neck" and then twisting around it (and each other) several times. I wonder how viable that is - as for sharp rocks, epoxy style foam can do wonders. $\endgroup$
    – SF.
    Feb 15 '16 at 14:24
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    $\begingroup$ Comets seem to be like rotten nuts. Hard shells but only mush inside. A lasso might decapitate the comet. I think (or guess) that some bubble gum like concept would better spread out the structural stress on spacecraft and comet over area and time. But the objects out there are individual, some are probably made out of solid metal. $\endgroup$
    – LocalFluff
    Feb 15 '16 at 14:30
  • $\begingroup$ Chury's neck diameter is about 2km. 100 tons * 6g - 600 tons spread over 3.14 km surface of the "weight-bearing" half of the neck circumference - about 200kg per meter of length and I definitely imagine the "rope" to be rather like a ribbon (say, 50cm wide), not a string, so 400kg per m^2 on a 2km thick shaft - I seriously doubt decapitation of Chury would be possible. $\endgroup$
    – SF.
    Feb 15 '16 at 14:44
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    $\begingroup$ @SF I cannot estimate the magnitude of the forces involved here. But at 400 kg per m², about like the load of a shipping container for which hardened ground is constructed to handle, I'm afraid that the thin hard crust of a comet could rupture and let out the instantly sublimating subsurface volatiles in an explosive ring around the "neck" of the lasso. And displacing the almost gravitationless loose rubble and dust inside of it into a cometquake. Maybe some comets would, others not. I suspect they are individuals with histories of all kinds. $\endgroup$
    – LocalFluff
    Feb 15 '16 at 19:30
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There is an alternative that entirely removes the heat dissipation issue. Although, it does have some issues of its own. It does this by removing the braking mechanism and spool. The rope starts out completely deployed and perpendicular to the velocity vector of the target. Now when the harpoon anchors in, the rope defines the radius of a circle, with all of the velocity being tangential. The acceleration experienced can be calculated in the same way it is for artificial spin gravity. Now the craft is "orbiting" the target.

Pros

  • If it waits till it has rotated through half a turn and then releases, it will be moving ahead of the target with the same velocity that the target was originally moving ahead of it. Twice the delta V that is possible with the braking method. This is analogous to a gravity slingshot.

  • The energy is stored in the rotation of the whole system. If the target is the destination, the energy will still need to be dissipated, but there is no issue of the target getting away, so dissipation can be slow. The easiest energy sink is probably dumping rotational energy into the target.

  • The release can be timed to have a component of the velocity out of line with the targets trajectory making the system more versatile.

Cons

  • For a fixed initial velocity difference and acceleration, the required rope length to brake to a full stop is half that needed to swing around the target.

  • The anchor will likely need to be more complicated since the rope needs to pivot around the anchor instead of just pulling against it. This is especially true in the case where the end goal is to stop at the target. in this case the anchor needs to be set up to allow continuous rotation.

  • The braking system can be too short for a full stop and still give some delta V, while the swing around system cannot function at all if the rope isn't long enough.

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