You have a mass driver which throws a 1 kg slug of iron at your spaceship. The slug reaches your spaceship and at that time it is going 30 km/s relative to the spaceship. You intend for the spaceship to harvest some of the momentum of the slug, allowing the spaceship to accelerate somewhat without spending propellant. The slug will then continue on, somewhat slower than before. What is the best way to harvest momentum from the slug?

The slug must not be melted or otherwise destroyed because it is aimed so that it will eventually orbit back to where it can be caught and re-used.

The catcher can't be too close to the slug because of the danger of a collision. We can suppose that the spaceship tracks the slug and maneuvers very accurately to catch it, but for safety no part of the catcher mechanism should be within 1m of the slug.

A large eddy current brake might harvest some momentum from an iron slug. However, this is somewhat limited for this scenario because it turns the slug's kinetic energy into heat in the slug. Braking an iron slug from 2 km/s to 0 with an eddy current brake would unfortunately melt it.

The slug has to be iron.

A good solution should harvest a lot of momentum from the slug, and also should not weigh too much. We might say the entire spaceship should weigh no more than 1000 tons.

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    $\begingroup$ Why not use a conductive material with a higher melting point, like tungsten ? $\endgroup$
    – CodeJunkie
    Oct 7 at 7:30
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    $\begingroup$ Why not use a laser and sail? It's much more accurate, more efficient, and you can fire the laser continuously, which takes a lot of pressure off your materials and design. And it's not like you have to worry about running out of photons. $\endgroup$
    – Cadence
    Oct 7 at 9:14
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    $\begingroup$ Answer? It's proprietary. Can't tell ya. Though it definitely isn't a shipboard laser battery that blows the slug into atoms, catches the ion wind with a magnetic sail, then launches an identical slug on the right trajectory. $\endgroup$
    – BMF
    Oct 7 at 9:32
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    $\begingroup$ @Cadence photons are a rubbish way to transfer momentum, and lasers are significantly less efficient than linear electromagnetic motors. Charged particle beams or pellet streams are a plausible way of giving good accelerations without the need for stupendous death ray grids and awkward sails. $\endgroup$ Oct 7 at 13:45
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    $\begingroup$ Is the slug thrown in anticipation of some of its kinetic energy being harvested? Because any interaction that changes its kinetic energy will change its orbit, so the only way it can be aimed to orbit back to any particular point is if all the details of the energy transfer are accounted for at the time it is thrown. $\endgroup$ Oct 8 at 1:06

3 Answers 3


If your mass driver used electromagntic acceleration systems (eg. it was a coilgun) you can use the exact same techniques to decelerate a slug as you used to accelerate it in the first place. You're effectively treating the slug as the moving bit in a giant electric motor, and electrical motors can be run in reverse during dynamic (or regenerative) braking, which is not the same as simple eddy-current braking, as most of the heat will be generated in the spacecraft rather than the slug. Some heating will occur in the slug suring both acceleration and deceleration, but you can always just brake less aggressively and throw more slugs to make up for it. In the limit, this starts looking a bit like a space fountain. You can reduce heating further by using superconducting projectiles instead of dumb iron slugs, but that adds its own issues and increases the required tech-level.

It is worth remembering that this sort of technique does not seem to have been considered for real-world spacecraft propulsion, and that should raise a red flag or two. If nothing else, 30km/s is fairly modest as far as fancy interplanetary travel goes, and it seems likely that a science-fictionally effective nuclear-electric rocket could outperform it (in duration, if not necessarily initial acceleration).

You can see similarities with pellet-beam propulsion... NIAC just awarded a phase 1 grant to some research on this: Pellet-Beam Propulsion for Breakthrough Space Exploration. I'm more interested in stuff like MagBeam myself, but it is a somewhat more sophisticated system with lower technological readiness

  • $\begingroup$ (1) Yes, I started off wondering, "what if a space fountain didn't stop at the edge of space?" Specifically a fountain from Earth orbit to Mars orbit and back. (2) Nuclear rockets use up mass, whereas a fountain like that could keep shuttling ships back and forth indefinitely on just solar power (plus a minor amount of mass spent on maneuvering thrusters). (3) An electric motor design sounds good, and the slug can have windings, but I wonder, how big would the catcher have to be? $\endgroup$
    – causative
    Oct 7 at 15:40
  • $\begingroup$ No need to infer any "red flags" from lack of serious consideration. This design has some fairly obvious challenges. You need to aim the slugs with extremely good accuracy, and if you're off by very little it could instantly destroy the spaceship... $\endgroup$ Oct 8 at 22:28
  • $\begingroup$ "Using up mass" is a funny complaint... there's a lot of dumb mass out there. You might not be "using it up", but you're creating a giant hazard to spaceflight... think how much mass you need to have in-flight, and have a think about how tiny speed-and-direction errors result in wide error margins for pellets at a moderately distant target and colossal catcher requirements on the return trip. Your proposal seems slooooooow and awkward and hazardous in a way that eg. magbeam or nuclear rocketry or various kinds of sails aren't. Still, its nice to see new and different ideas popping up! $\endgroup$ Oct 9 at 8:23
  • $\begingroup$ The slugs can be very hazardous. But that makes the ingredients for a good story right? $\endgroup$ Oct 9 at 13:10
  • $\begingroup$ If you really manage to catch those, you will end up with free reaction mass for another acceleration phase: just fire them out of the same... barrel, that was used to catch them. But then you would need to start thinking about what at the other end of the iron slug assisted internal/external propulsion system sits (in short, who fires those slugs, and what does it do to not experience the same acceleration by itself?) $\endgroup$ Nov 27 at 11:18

Suppose you have a space elevator cable suitable for visiting small planets with no atmosphere. String is always useful. Let us assume this is buckytube cable which can lift 1000 Kg on Earth gravity. Under this tension, the cable will accelerate a 1000 ton ship at about 1 cm/s2, which would be gentle enough not to spill your tea. At the other end, this will decelerate your 1Kg slug at 1000 m/s2, which is 100 x Earth's gravity. Deceleration would take 300 seconds, or 5 minutes.

With these figures, you will need 450 Km of cable. You will also have to be very careful about jerking: climbers' ropes have breaking strengths in tons force because they have to handle the deceleration when the rope suddenly goes tight. Buckytube cables are not springy so they will be vulnerable to this. The cable also has to take the strain for accelerating itself as well as the slug, and I haven't included this. I have chucked your precaution of saying no part of the catcher mechanism is to be within 1m of the slug, because I don't think it should apply here.

450 KM is a lot of cable, but not absurd if you were carrying one anyway as a space elevator. If the slug was going less fast (say 10 KM/s) you could make do with 50 Km, as this goes as the square of the velocity.

Your captain can probably figure out how to do this if they look at a history website for how they cleared Earth's orbit of old, junk satellites back in the 2060's.

I added a bit on avoiding the jerk in a comment, and I am now copying it here. To avoid the jerk, see 'Moby Dick': you chuck the harpoon and then let the line run through a tight grip with thick gloves. This pulls back on the target you have attached to, but not so suddenly that you break the line or pull out the harpoon.

I think this proposal lies somewhere between 'pretty hard' and 'just about possible'. I think it might lose out to other schemes unless you had the apparatus for other purposes, such as an elevator, or hunting space whales.

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    $\begingroup$ How do you attach the cable to the slug? It comes preattached? Like you're holding the cord to someone's harpoon gun? These launch mechanisms are getting out of hand. $\endgroup$
    – BMF
    Oct 7 at 10:27
  • $\begingroup$ @BMF I had imagined some sort of net to catch the slug. I had assumed the slug was just a random lump of stuff, and nothing fancy. $\endgroup$ Oct 7 at 12:25
  • $\begingroup$ A net to catch something moving 30 km/s? That's some net. $\endgroup$
    – BMF
    Oct 7 at 13:49
  • $\begingroup$ If you are catching these slugs and bringing them. on board, they could be part of your cargo. In which case, the ship starts off lighter and can accelerate more. What you are shipping would have to be pretty shock-resistant. No, the figures aren't exactly encouraging, I'll admit. $\endgroup$ Oct 7 at 16:07
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    $\begingroup$ 1000 ton ship accelerating at 1 cm/s^2 = (1e6 kg) (1e-2 m/s^2) = 1e4 kg m/s^2 = 1e4 N force. 1 kg slug decelerating at 1000 m/s^2 = 1e3 kg m/s^2 = 1e3 N force. You have a factor of 10 discrepancy here between forces that should be equal and opposite, possibly from mixing up "kilograms force" with newtons. This then affects the rest of your calculations. That issue aside, it's not at all clear to me how you're spacing this momentum transfer out into a nice gradual process. If the cable is not springy, surely we should expect a sudden jerk when it runs out of slack. $\endgroup$ Oct 7 at 22:16

Heat can be a source of energy, also, as it is in Stirling engines.

Make your tube very long, curved if need be, and alternate your tube between eddy current braking, and "cooling" sections in which the excess heat is also captured as a source of energy. The melting point of iron is 1538C, arrange your sections to only heat the slug to 1200C, and then capture that heat long enough to reduce it to perhaps 500C, and repeat the eddy brake, then the Stirling heat capture, etc.

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    $\begingroup$ How are you transferring the heat from the slug to the Stirling engine? If I've done the physics correctly, it should take approx. 5 minutes for a near-spherical 1kg iron slug to cool from 1200C to 500C by radiative loss. At a relative speed of initially 30 km/s, this means just the first cooling section would need to be on the order of 9000 km long. This creates some practical difficulties. Increasing its surface area e.g. elongating the slug will reduce that time proportionally, but it also makes it more fragile, which could be a problem for something undergoing huge mechanical forces. $\endgroup$ Oct 7 at 23:01
  • $\begingroup$ @GBsupportsthemodstrike I'd say elongate the slug. Make it long rod. I did say a very long tube, perhaps the tube can be filled with a thin gas or transfer agent, for direct contact. $\endgroup$
    – Amadeus
    Oct 8 at 0:46
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    $\begingroup$ The thinner the gas, the less useful it is as a heat transfer medium, and at high temperatures radiative loss dominates over conduction/convection. (Radiation scales with the fourth power of temperature, whereas conductive loss scales linearly.) Even at 1 atm, I think gas would only make a very marginal improvement to cooling time. Meanwhile, you're dealing with some very exciting kinetic effects as the 30km/s slug hits that gas. Larger area will again improve cooling but will also exacerbate those effects. $\endgroup$ Oct 8 at 1:21
  • $\begingroup$ @GBsupportsthemodstrike Okay. stick with the very long slug. This is science fiction, we don't need to explain every detail of the mechanism. We just need plausibility. $\endgroup$
    – Amadeus
    Oct 8 at 11:36

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