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In one of my favorite books, Tom Swift in The Race to the Moon he uses a spacecraft which is propelled by devices called repelatrons which are attached to the hull of the craft. They can be programmed to repel any substance as such forcing the craft in the opposite direction, they can also be used to stop projectiles, acting as a shield.

My question is whether this could be a feasible form of sub-light propulsion?

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  • $\begingroup$ I’m quite confused by the question. Is the repelatron repelling something (like fuel) in order to make the ship move, is a repelatron fixed to something else repelling the spacecraft, or is a repelatron attached to the spacecraft somehow making the spacecraft move? The first two can happen, the last not so much. $\endgroup$
    – Joe Bloggs
    Commented Nov 3, 2021 at 12:29
  • $\begingroup$ But surely Newton’s third law applies. $\endgroup$
    – Prince Thomas the 42nd
    Commented Nov 3, 2021 at 12:46
  • $\begingroup$ Still need to better understand this device. What does "programmed" mean? It can be programmed to repel Earth, but then we can flip a switch, and it will stop repelling it, and can start repelling Moon? $\endgroup$
    – Alexander
    Commented Nov 3, 2021 at 18:15

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The functioning of this engine might lead to some contradictions.

  • The repulsive force doesn't scale with distance: your engine will end up being repelled by the entire universe, and if we assume the universe is isotropic, the spaceship won't move
  • The repulsive force does scale with distance: your engine will need some mass closeby to keep operating. This is no much better than a rocket.
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If your device works by altering space/time, then yes.

Maybe the devices you refer to are artificial gravity generators. Maybe they work by altering space/time, since that is how gravity does what it does..

https://physics.stackexchange.com/questions/116249/does-gravity-actually-contract-space-time

Usually artificial gravity is handwaved in the interest of telling the story and having your actors all seated on the bridge with a minimum of special effects. But if you want to get into the weeds, altering the local curvature of space/time would be a fine way to get artificial gravity as well as repel or deflect objects.

If you are in a solar system you are feeling the effects of gravity from the star if nothing else. You could propel yourself by repelling the star. The start will not notice but you will be thrown in the opposite direction. If there are closer local gravity wells from planets etc you could throw yourself away from those.

It gets trickier in interstellar space because gravitational bodies are few and far between. That could make for a fine story - your space travelers must slingshot around a solar system, gathering speed for the interstellar trip. They have no brakes until they are again in the vicinity of a gravitational mass.

The Alcubierre drive is a different method to bend space time and propel a space craft. This method can achieve FTL travel without breaking physics. https://en.wikipedia.org/wiki/Alcubierre_drive and it requires a local bending of spacetime. If your device bends spacetime you could use it to make an Alcubierre drive, and that would also let you read up on more specifics on how that would be achieved if that interests you. There is a lot right here on the WB stack. Here is a question from this stack on subluminal travel using an Alcubierre drive. Alcubierre Drive without FTL?

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