[ft] Orbits in Vector

Does anyone have a system for doing on-table planets and orbits with
vector movement?

I'd kind of like to be able to do slingshot maneuvers and similar as well, but
they don't work in the attempts at orbital rules I've been playing
with...

[quoted original message omitted]

> Does anyone have a system for doing on-table planets and orbits with

Here's what I use:

I place the planet on the table. Under that I have 2 cloth circles. One is 12"
dia. and the other is 24" dia.

Any ship ending its movement on the outer circle gets 1" added to the
vector in the direction of the ship-to-planet. The inner circle does the
same thing, but for 2"

If you want to simulate a stronger field, just change to numbers to your
taste.

> On Sat, 29 Jul 2000, Sean Bayan Schoonmaker wrote:

> >Does anyone have a system for doing on-table planets and orbits with
One is
> 12" dia. and the other is 24" dia.

This is kind of what I was doing, only with much shorter distances - 6"
or so. Expanding it should make it work better...

How big is your planet? Mine are quite small - the biggest foam sphere
the local shop sells is 6" across. Cutting it in half gave me a nice
terrestrial planet and a very nice Mars-class planet, though. (pictures
soon, with luck.)

> How big is your planet? Mine are quite small - the biggest foam sphere

Same. That's the biggest styrofoam sphere they sell at the local hobby shop.

Good enough for my purposes.

On Sat, 29 Jul 2000 20:28:49 -0400, Laserlight <laserlight@quixnet.net>
wrote:

> If I understand it correctly, a slingshot starts with the

There are two reasons for a "slingshot". One is to allow a ship to increase
its velocity without using up its fuel. The problem is that whole conservation
of momentum thing. If you increase velocity dropping into a gravity well, you
lose it coming out of the gravity well. By, as you point out, you only gain
from a gravity well because you are burning up fuel mass on the way out.

The other reason for a slingshot is to make abrupt course changes. You drop
into the gravity well and let the gravity alter your course. This saves having
to burn a lot of fuel to make a course change. This, in fact, is a very handy
thing, something NASA uses all the time with their deep space probes.

I suggest if you want to handle fuel mass consumption that dropping into a
gravity well could give you a +1 to velocity or something like that on
the way out to abstractly represent that. If you want to assume that fuel mass
isn't an issue, don't give the bonus.

Either way, going into a gravity well should give you extra thrust points
above that rated by the ship's drive, but when you exit you have to subtract
those thrust points.

At the same time, the gravity well should have a vector towards the planetary
body. The trick, of course, is how much of a vector would you need? I answer
this in a reply to Sean.

On Sat, 29 Jul 2000 21:43:18 -0700, Sean Bayan Schoonmaker
> <s_schoon@pacbell.net> wrote:

> Here's what I use:

That's a nice, simple way to do it. But, I decided, for the heck of it, to do
some calculations.

I took the "unofficial" scale for FT (1 Movement Unit or MU = 1,000 km, 1 turn
= 15 minutes) and used 1 gravity (g) = 10 metres per second per second. The
result was a calculation of 8.1 Movement Units towards a planet at it's
surface.

Now, the surface of the earth is roughly, in this scale, 6" in diameter.
Gravity is an inverse square law. 3" out from the surface, the vector would be
2 MUs towards the planet. 6" from the surface, it's approximately 1 MU. This
is the same as what Sean came up with.

I just did some additional calculations. If you wanted to be more accurate, an
Earth style planet on an FT table would be 6" in diameter. You would then have
concentric circles out from that. The circles would be 7", 8", 9" 10", and 12"
in diameter, and would have a vector towards the centre of 6, 5, 4, 3, and 2
MUs respectively. Another circle, 18" in diameter, would have a vector of 1 MU
(actually, if you're rounding off vectors to the nearest half, the second last
circle really should be 14" in diameter, and the last circle should be 24" in
diameter). This assumes, of course, you want to keep MUs to whole numbers.

Gravity wells for larger planets aren't really practical at this scale, unless
you use a table edge (although doing some reading, I found that while Uranus
would have a diameter of 26" on the table, it actually has slightly LESS
surface gravity than Earth, so you could actually have a half a gas giant on
your table top, as long as it isn't Jupiter...).

> Does anyone have a system for doing on-table planets and orbits with

Actually, the so-called "slingshot" maneuver doesn't actually add much
_speed_ to an object.  It relies on the distinction between speed and
velocity, in which velocity is a speed in a particular direction, whereas
speed just measures distance travelled over time.

Any two-dimensional motion in space can be described as two separate
motions with respect to the sun: one motion towards or away from the sun, and
another "left" or "right" of the sun. When NASA uses a slingshot around, say,
Jupiter, to "speed up" a probe, what they're doing is using Jupiter's mass
(and angular momentum) to alter the directional vector of the probe, so that
more of its "speed" points away from (or towards) the sun, instead of at a
right angle to it. Although the actual "speed" doesn't change much, the radial
velocity (the important part when trying to escape the sun's gravity) changes
alot, as does the direction of motion.

All that said, what I'd probably do to simulate this in FT using Vector Thrust
rules is allow a close brush with a planet to alter your direction of motion
while retaining all speed. (The amount of "speed" gained in falling towards
the planet is mostly lost again when leaving it, so it essentially cancels out
for game purposes). To make it interesting, you might limit the amount of
direction change based on how close you brush the planet; those who dare to
fly closer get more of a benefit, but get too close and...

Just my nickel ninety-five.

bart

> (actually, if you're rounding off vectors to the nearest half, the

Inspired guess on my part for the 24 MU thing. I WISH that I could claim it
was intelligence... ;-)

From: Allan Goodall <awg@sympatico.ca>

> I took the "unofficial" scale for FT (1 Movement Unit or MU =

You meant 6mu RADIUS, perhaps? Diameter is more like 12.75mu.

> >Does anyone have a system for doing on-table planets and orbits with

Hi,

I found this link
[http://www.go.ednet.ns.ca/~larry/orbits/gravasst/gravasst.html] that
describes the orbital mechanics in detail. The end result is that the craft
doesn't change it relative speed with respect to the planet, but dramatically
increases its total kinetic energy with respect to the sun. In game turns, the
gravitational assist won't do much to the spacecraft's speed unless the planet
is moving during the game. In his example, the spacecraft increases its speed
relative to
the sun from 9.36 km/s to 16.17 km/s (173% as fast!), without the
use of any fuel.

However, it can be used to change the direction of the velocity vector almost
arbitrarily. Having never played FT, I can't really go into much more detail
than that.

> Just my nickel ninety-five.

TTYL,

G'day Laserlight,

We use a simple system whereby the gravilty well of the planet is based on its
diameter. For instance, say you've got a planet that's 6" then if you end up
within 6" of the planet's edge then you get pulled in 3", if you end
up 6 - 12" away then you get pulled in 2" and if you end up 12 - 18"
away you get pulled in 3". Works well and I've used it quite a few times to
swing me around a planet... its a bugger when you lose your engines mid sling
though!;)

Cheers

Beth

On Sun, 30 Jul 2000 18:36:46 -0400, Laserlight <laserlight@quixnet.net>
wrote:

> You meant 6mu RADIUS, perhaps? Diameter is more like 12.75mu.

D'oh!!! You are, of course, correct!!!

That makes the circles I mentioned 12" in diameter for the planet, and
14",
16", 18", 20", 28", 36" for 6 MU, 5 MU, 4 MU, 3 MU, 2 MU, and 1 MU pull
respectively...

Huh?

OK,let me get this straight... so the Gravity is strong up close...not so
strong a little further out, and then strong again even further out???

I *assume* you mean '...and if you end up 12-18" away you get pulled in
1".'
1", right? Not 3"? ;-)

Christopher

[quoted original message omitted]

G'day,

> OK,let me get this straight... so the Gravity is strong up close...not

Yes that'd be right thanks... says she who obviously can't do two things as
once (even if its only type and talk!!!! sigh).

Thanks

Beth