Re: World building: implications of counter gravity

On 08/11/2017 08:54, Hugh Fisher wrote:
> I am doing some world building for a space game setting, and want to 
> run an idea past people.
>
> My setting is not too distant future. I want easy surface to orbit 
> launch to explain why people are in space, which means using up a lot 
> of energy on each launch.
>
> BUT once in space I want engines to be rather limited, so it isn't 
> easy to, say, divert asteroids into planets.
>
> My idea is counter gravity, an updated version of HG Wells Cavorite, 
> or the liftwood in Space 1889. Not artificial gravity, but some kind 
> of field that INSERT HANDWAVING HERE creates an equal and opposite 
> thrust reaction. So within the gravity of a planet you get lots of 
> thrust, near an asteroid very little, and from a spaceship hull 
> something only measurable in nanometres per hour.
>
> What am I missing? Would this make space travel economical? What else 
> would it be good for?
>
If all your Handwavium Drive does is produce an equal and opposite force

to the local gravity vector, then that just means that you've negated 
gravity -- all gravity -- on your ship. So you won't stay in orbit, for 
instance, which is great for leaving orbit (but not so good for getting 
into it), but tricky if you actually want to go somewhere, because 
you'll have to be pointing in exactly the right direction when you turn 
it on, and it won't speed you up (or slow you down) at all. This means 
that your flight path, for want of a better word, is going to be a 
straight line rather than the usual conic section, so it's shorter but 
has its own problems. Left to itself, your ship will travel with the 
velocity it had when the HD was activated, which isn't going to be very 
fast in interplanetary terms. You also won't be able to go from place A 
to place B unless they're in the correct positions, so there will be 
small launch windows for the trip and large periods when "you can't get 
there from here" -- not least of which will be the times when there's 
something else in the way (like the Sun)! You're really going to need a 
manoeuvring drive to turn the ship and alter its velocity vector until 
it's pointing at the desired destination (and maybe to boost the speed a

bit); kinda reminds me of /BattleFleet/ /Mars/, where you could expend 
manoeuvre fuel to shorten interplanetary trips that were predominantly 
powered by low-thrust ion drives.

Unless, of course, you can throttle the thing and/or vector it. If what 
the magnitude of the local gravity field does is control the magnitude 
of the thrust that you can produce without putting restrictions on where

it's pointing, and the thrust can be greater or lesser than the local 
gravity, then you've got something that's not unlike a rocket with 
endless fuel -- just one where the thrust produced is smaller out in 
space than near a mass. This is great for manoeuvres like slingshots 
because the thrust is at its maximum right where you want it to be -- at

and near "perigee" of the body you're slingshotting around! So you do 
most of your manoeuvring near planetary masses, where you have the 
thrust, and you don't have much delta-V out in the black, but may have 
enough for course corrections en route. Of course, any thrust at all can

build up over the sort of distances we're talking about, so your trip 
time will be reduced from a pure Hohmann orbit, and with enough thrust 
at the ends (near the big masses), you can go almost any time you like.

That works fine for planets, but you'd have to run some numbers to see 
how the HD would work with smaller masses like, say, the asteroids. 
Jerry Pournelle once worked out that the semi-traditional idea of 
Belters zipping around the asteroids but never entering planetary 
gravity wells ("holes", as Larry Niven called them) doesn't really make 
sense because the delta-V needed to move between the bigger asteroids is

pretty much the same as is involved in planetary launches and landings. 
With the HD, though, you wouldn't have the thrust (and hence delta-V) 
near an asteroid as you would around a planet, so Belters might be out 
of luck. Of course, he also came up with the idea of replacing a Belt 
civilsation with one based on the moons of Jupiter, where the delta-V 
requirements are much lower, so the HD might well suit that setting 
rather neatly.