On 9 May 2020 at 18:23, Richard Aiken wrote:

> I think what might be happening here is that the air/raft can climb to
> orbit using "simple" cancellation of felt gravity, plus it's modest
> acceleration to overcome the friction of the atmosphere. The
> atmosphere thins and felt gravity drops off as altitude increases, so
> the air/raft's acceleration *could* potentially increase. Yet the rule
> says that even though it can reach orbit it remains incapable of
> interplanetary travel.

It doesn't *have* to increase. Something that can maintain 60 mph
against wind resistance can provide a constant fractional g thrust.
It'd take a while, but not as long as you might think.

Since most cars can accelerate fast enough to push you back in your
seat noticeably, that means a fair fraction of a g.

But let's be conservative and limit things to 1/10th g.

Orbital velocity is 8 km/sec
1/10th g is 1 m/s^2

V=A*T
8e3 = 1 * t
t = 8000 sec or abour 2.2 hours.

At half a gee, it'll take a bit under 27 minutes.

> I take this to mean that the air/raft's propulsion is somehow
> inextricably linked to felt gravity. Propulsion efficiency drops in
> close proportion to the felt gravity. Once the latter reaches
> microgravity strength, the former essentially ceases to function.

Not felt gravity as that has zero depenence of distance from the
planet. It is mostly affected by your velocity vector with respect to
the planet.

That's how a carnival can have you at zero g at the top of the loop
and 2 g at the bottom. Heck, some get up to 3g at the bottom and
*negative* 1 gee at the top.

What it would more likely depend on is the mass of the body it was
pushing on and the relative velocity between them.

Somewhere, I've got a formula someone worked out for me for a drive
that *literally* pushed on a planet/moon/whatever. Aceleration drops
*horribly* as the relative velocities get higher.

That's because the drive is using a constant amount of energy, but
the kinetic energy of the ship goes up as the *square* of the
velocity. So your accel drops like a rock as the speed goes up.

Rockets get around this by carrying the mass they push on *with*
them, so the relative velocity of the fuel starts at zero as it goes
into the engine and increases by a constant amount ass it leaves the
engine. And the mass of the ship drops as it uses fuel, so the
acceleration increases until you run out of fuel (or throttle the
thrust down).

--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com