i'm in the early stages of designing a scifi game and want to use as little handwavium as possible (without ruining the fun). The first part of the game takes place in the asteroid belt and includes handwavium in the form of being able to break down just about anything into individual atoms and using a 3d printers make everything from an ingot to a fighter craft.

The intended reaction mass is blocks made of ~11.75% Invar (FeNi 64/36% 8,100kg/m³ specific heat capacity peaks at 545j/kg K at 200C Curie temp of 230C) ~88.25% silica harvested from asteroids. Depending on the ship the blocks may be as small as 0.01m³ ~30kg or as large as 1m³ ~3,000kg.

I had the thought of hitting the pellets with high powered lasers and feeding the resulting plasma through a VASIMR thruster.

Or lunching the pellets with a coilgun and hitting them with lasers in something akin to a rocket nozzle to direct the expanding gas while avoiding the whole chunks of very high speed material flying around the solar system. Would the expanding gas cause more thrust than just the momentum from the magnetic acceleration?

The smallest drone has a single 5m long coil, followed by quad 20m, quad 40m, and eight 60m for the largest thrusters.

Backstory Edit: Your ship was sent in advance to build the infrastructure for our first colony at Proxima b. The follow on ship with the colonists was scheduled to arrive 1 year after you but after 2 years the captain decided to head back and find out what was going on. FTL comms don't exist and the experimental FTL drives that we were testing maxes out at 2c and only works in interstellar space so the crew was all in cryo sleep for the trip. When you wake up you find the ship is badly damaged and floating in the asteroid belt.

Edit 2: FeNi 64/36% means the alloy is 64% iron 36% nickel. 8,100kg/m³ means a cube that is 1 meter on each side has a mass of 8,100 kg. Specific heat capacity is how much energy you can pump into the material to heat it 1C (or K). The higher it is the more you can heat it can absorb without losing necessary properties like... Curie temp is the temperature that a magnetic metal rapidly loses it's magnetism.

  • $\begingroup$ Unless both my and googles' math is wrong 0.01m3 is 10,000cm3 which is 10 liters. $\endgroup$
    – Rasip
    May 10 at 3:28
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    $\begingroup$ Both maths are right, mine went astray making a schoolboy error. $\endgroup$ May 10 at 4:47
  • $\begingroup$ Why exactly aren't we using water or hydrogen as reaction mass? The outer parts of the belt are beyond the original solar frost-line, so it's c-type asteroids should be rich in volatiles. Just import volitiles via mass drivers. $\endgroup$ May 10 at 9:16
  • $\begingroup$ As a suggestion, might ion drives fit the bill? They spew out something incredibly tiny at ridiculous speeds, achieving thrust with practically no 'waste'. No clogging of routes or solar systems, nor can it effectively be weaponised. $\endgroup$
    – Trioxidane
    May 10 at 14:32
  • $\begingroup$ About the only difference bewteen the coilgun and an ion drive is the ion drive throws out individually atoms instead of multi kilo chunks. $\endgroup$
    – Rasip
    May 10 at 15:21

2 Answers 2


Induction furnace / coilgun hybrid.

I wince at lasers and fast moving pellets. Much waste, much danger. The horror of moving parts!

I propose instead that you melt your reaction mass using an induction furnace.

An induction furnace consists of a nonconductive crucible holding the charge of metal to be melted, surrounded by a coil of copper wire. A powerful alternating current flows through the wire. The coil creates a rapidly reversing magnetic field that penetrates the metal. The magnetic field induces eddy currents, circular electric currents, inside the metal, by electromagnetic induction.[9] The eddy currents, flowing through the electrical resistance of the bulk metal, heat it by Joule heating.

induction forge


No waste. Nothing moves. Electricity makes molten metal.

I thought maybe the induction furnace and coilgun could share the same coil which would be elegant. But the furnace is AC and AC is no go for a coilgun, or if go, go back and forth very fast.

So they would have to be closely approximated coils. And that is fine because the electrical needs are different.

Here is a nifty thing. As the heated mass turns to liquid, glowing drops are pulled off the main mass and into the coilgun, then fired out the back of the ship.


By having the projectile pulled towards or levitated within the center of the coils as it is accelerated, no physical friction with the walls of the bore occurs. If the bore is a total vacuum (such as a tube with a plasma window), there is no friction at all, which helps prolong the period of reusability

Hopefully the stuff stays mixed so the conductive metal components can pull along entrained silicates and their mass.

This setup lends itself to a cutscene showing the ingot in the coil as it begins to glow, then shed glowing globules into the coilgun and out the back.

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    $\begingroup$ @Rasip - if you use the induction furnace it will not be plasma because I don't think induction heating will work on gas. It would be throwing molten liquids. $\endgroup$
    – Willk
    May 10 at 15:21
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    $\begingroup$ If you throw molten drop, they're still going to be relativistic solid particles in a matter of seconds to minutes (depending on exact composition); I'm also not sure a drop of lava at 0.9 C would be any better than a bead of obsidian at the same velocity... $\endgroup$
    – Zeiss Ikon
    May 10 at 18:02
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    $\begingroup$ @Willk You are missing the last and nearly most important part. We don't want the solar system filled with thousands of tonnes of tiny chunks flying around at a significant chunk of light speed. Kessler Syndrome is bad. On a solar system scale it is fatal to any space travel unless you handwave in some kind of shields which makes weapons a whole lot more complex. $\endgroup$
    – Rasip
    May 10 at 18:16
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    $\begingroup$ @rasip Even at only 0.1 C, they'll be out of the parts of the Solar System that matter in a few days. In some ways, a pellet driver is less dangerous than a fusion rocket. Still a bad day if one of those pellets hits you, though. $\endgroup$
    – Zeiss Ikon
    May 10 at 19:11
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    $\begingroup$ Shooting metal backwards seems like a solid basis for videogame locomotion, fits with the improvisational nature of the mission, and "pellets gone in a few days" seems like an adequate handwave for a game, especially if the idea is that the player eventually hooks up with their lost comrades and then they all return to this place to commence colonization. Your game only has to be concerned with Kessler Syndrome if you insist that it be; will that make it more fun? $\endgroup$
    – Tom
    May 11 at 5:29

Vasimir might be the better option in the sense that on paper at least easily tunable when it comes to your rocket thrust/efficiency ratio. My (lay) opinion would be that assuming the two competing drives are more or less equal in all important metrics the issue with pellet drive would be varying the trust efficiently. Varying the size of the pellets is impractical which leaves varying the power output of the laser, pellet ejection rate and/or perhaps the strength of the magnetic field in the nozzle.

Which is not to say it any or all of the above couldn't be done. Just that engineering that aspect of the drive might be 'simpler' with Vasimir

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    $\begingroup$ Sorry, I didn't mean to imply the size of the pellets would change in a single ship just that the smaller ships would launch smaller pellets. Something that in 20x10x5m launching 1m3 pellets would probably be ripped apart the first time it tried to move. On the other hand a ship that is 105x50x5m would take all day to get some thrust firing 30kg weights. $\endgroup$
    – Rasip
    May 10 at 4:13
  • $\begingroup$ Not really a problem. I was actually thinking more in terms of varying thrust by varying the size of the pellets any one ship carried i.e large ones for heavy acceleration tiny ones for gentle acceleration. With others in between. As I said, to my mind that's not a practical solution. $\endgroup$
    – Mon
    May 10 at 5:21

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