It's not Rocket (limited super)Science

[jeff_wilson]

Originally Posted by Fred Brackin

Maybe, maybe not, unless slingshot maneuvers work the way they do in Star Trek. If they did you probably wouldn't need reaction mass.

Some technically don't need reaction mass, but a little is often used to adjust the trajectory for maximum effect.

It's the physics of conservation of energy and momentum. The spaceship loses gravitational potential energy as it gains velocity and momentum by falling closer to the planet. Assuming it avoids an inelastic collision with the planet itself or its atmosphere or any satellites, it will then continue around and fall away from the planet, trading back velocity and momentum for potential energy as it slows due to the planet's gravity. It arrives and leaves with the same speed at the same distance, measured relative to the planet.

However, it is not the direction or the same speed relative to the sun, since the planet is orbiting the sun at numerous miles per second itself, and it is not the same speed relative to the other planets which have their own speeds and orbits of course, so the slingshot maneuver can effectively add or subtract as much as twice the difference in speeds between the slinging planet and the other planets, or the sun. The velocity change of the spaceship can be considerable, but since the mass is so much smaller than the planet, conservation of momentum makes some invisibly small alteration in the planet's orbit that will be lost in the noise of everything else zipping past it.

[jacobmuller]

ROFL - bodies as reaction mass...
We don't have water meters here (yet) so thank you for the numbers.
The PCs would like to keep breathing and vac-suits just won't last long enough to reach anywhere.

Fuel was sabotaged, along with comms, etc. Their priority will be repairs for getting past the planetoid. Survive that and then fix the rest.

I reckon my numbers are off by at least a factor of ten - possibly in a couple of places: 1.5 or 0.15tons water per habitat unit? But, at a high rate of use, 15tons of water is possible.
Draining the hydroponics would be a good idea - but that'd mean they end up not breathing...
But it does give them 2 options - recycled water only for small boost and eventually die of thirst or recycled water plus hydroponics water for bigger boost and die not so slowly from CO2 build-up.

Not going to tell mention the body-as-reaction-mass option (and less people = more water/air). Will wait and see how selfless/ish they are.

[Fred Brackin]

Originally Posted by jeff_wilson

However, it is not the direction or the same speed relative to the sun, since the planet is orbiting the sun at numerous miles per second itself, and it is not the same speed relative to the other planets which have their own speeds and orbits of course, so the slingshot maneuver can effectively add or subtract as much as twice the difference in speeds between the slinging planet and the other planets, or the sun. The velocity change of the spaceship can be considerable, but since the mass is so much smaller than the planet, conservation of momentum makes some invisibly small alteration in the planet's orbit that will be lost in the noise of everything else zipping past it.

Yes, this is most of the stuff I didn't have the time to explain before going to watch Dr Who last night. :)

There's also stealing some of the planet's angular momentum by traveling in the same plane and direction as its' rotation and getting pulled along by its' gravity as it spins. With planets like Jupiter (angular momentum king of the solar system) this can be significant.

Still, I stand by my statement of "singshots don't work the way they do in Star Trek". The possible benefits are limited and specific circumstance only. It's not a general purpose way to gain velocity.

If you're in danger of hitting a planetoid you need to change your vector enough to not hit and this may or may not result in a graceful looking arc around the planetoid. Particularly from a planetoid you're unlikely to gain any significant velocity by stealing some of its' orbital or angular momentum. There's too little gravity involved.