Langrange Calvert and making FTL in a gravity well

[jack]

(1) I've just reread Reality Disfunction. There's the standard some-FTL-drive which doesn't work in gravity. I don't think it's really a spoiler to let on that at one point a ship is trapped close to a planet, and manages to jump out by hitting a lagrange point dead on.

Cool idea. But it is wrong, isn't it? A planet and moon would cancel gravity at some point on a line directly between them, but L1 is closer in than that, since something in L1 orbits, so must have acting on it a net gravitational force inward.

(2) The idea of an FTL drive of some sort which can only work outside strong gravity seems to be an old and common one (Niven, startrek for starters). Anyone know who first invented it?

Comments

[saraphale.livejournal.com]

If you don't get an answer here, have you checked rec.arts.sf.science?

[cartesiandaemon.livejournal.com]

I don't really care :) But I might do, thanks.

[theinquisitor.livejournal.com]

But, conversely, when you consider three bodies in a plane, there must be *a* point with no net gravity, right? Just by continuity. Hell, I think there are even theorems about stationary points which prove it analytically.

Now, one might ask why 'galexy background' gravity is okay, but 'star-system' background isn't, but...

Anyway, it was cool. That's the main thing.

[cartesiandaemon.livejournal.com]

"there must be *a* point with no net gravity, right?"

I thought I said that!

"Now, one might ask why 'galexy background' gravity is okay, but 'star-system' background isn't, but..."

Well, in some canons, star-system is ok, but near-a-planet isn't, but same thing, I guess. Am I confused? Isn't the closer thing going to exert more gravity? Alternatively, perhaps it's the *change* in gravity which is a problem, so being near something is always worse..? Though that might nerf the null-gravity point idea.

why 'galaxy background' gravity is okay

[ewx.livejournal.com]

Perhaps the gravitational force at each endpoint of the jump has to be almost equal; e.g. because there's an energy cost proportional to some function of the vector difference.