Jumping near gravity sources.

A thread of 2 messages, 13 February 1991 to 14 February 1991, in the order they were sent.

#2146 Jumping near gravity sources.

Dan Corrin · 13 February 1991

In regards to the 100 diameter jump guide that I brought up:

First of all I will clarify the situation again.

	1) The whole point of the discussion is not to compe up with a
	   rationaliztion for the rules, rather to figure out an accurate
	   way to caluclate jump distance, so one can use it in unusual
	   circumstances. (eg. Jump distance from an unusually dense world,
	   gas giant, neutron star, or even a Battleship?)

	2) The diameter is a approximation of the mass assuming that
	   all worlds have the same density. The small ones, usually
	   without an iron-nickel core are less dense however.

	3) using 1/r^2 for a large planet (size 8-9, std. density), the
	   attraction of gravity at "100 diameters", is 1/20th the
	   gravity felt by the sun at 1 AU.

The first suggestion proposed by Marty Jarrio, that we use a 1/r^3 method
is reasonable but is handwaving as "according to modern knowledge" gravity
follows a 1/r^2 method.

Calculating the gravity felt by the craft by summing gavity vectors of
significant bodies as Jim Baranski mentioned, has a nice appeal.

First, there will always be points regardless of the masses involved that
the net effect of gravity is less than a certain threshold, by adjusting
the threshold accoringly, we should be able to arrive at a value that will
make safe jumps best at around 100 diameters of the planet.

Second, There will be points, especially in a system with a large moon,
where a jump can be safely made much much closer to the planet, than 100
diameters. These would be small (perhaps even smaller than some ships),
and uncertain to use, which would limit their use. Ships could only use
these on an outbound leg as they would effectively be "islands" of
acceptable gravity level.

Third, in a talk given by Mark Miller, he always intended there to be a
limited (but large) number of points that a ship could jump from/to,
otherwise piracy couldn't exist (qv my other article in this digest).

The only bad part is that the points of low attraction between a habitable
planet and the sun, may also be "islands". Due to the fact that an incoming
ship precipitates out of jump space when it gets to "100 diameters" this
might make leaving quicker than arriving.


				-Dan Corrin

#2148 Re: Lagrange-jumps, Velocities and 1/r^3

Bertil Jonell · 14 February 1991 · reply to #2146

Dan Corrin [email removed] writes:

> Second, There will be points, especially in a system with a large moon,
> where a jump can be safely made much much closer to the planet, than 100
> diameters. These would be small (perhaps even smaller than some ships),
> and uncertain to use, which would limit their use. Ships could only use
> these on an outbound leg as they would effectively be "islands" of
> acceptable gravity level.
  [..]
> The only bad part is that the points of low attraction between a habitable
> planet and the sun, may also be "islands". Due to the fact that an incoming
> ship precipitates out of jump space when it gets to "100 diameters" this
> might make leaving quicker than arriving.

  I agree with the second paragraph. There are numerous references that no
matter how bad one misjumps, one cannot appear inside the 100 diameter sphere.
Since it is possible to enter jump within this sphere, travel in jumpspace to
another system or planet and appear at their 100 diameter sphere, I interpret
this as meaning that when a ship cross the limit from the outside and in it will
be thrown out of jump.
  That would be a nasty surprise for anyone who tries to use the Lagrange points
for jumping. They reappear again just outside the same Lagrange point.

  One might argue that the ship should be immune to how normal space looks in a
line from jump entry to jump exit, and I agree with that, but ships are
obvioulsy affected by large bodies when they are "close to normal space" and
about to exit jump, the same should be true if they've just entered jumpspace.

  A problem with using Lagrange points and basing the 100 diameters on local
gravitation is that it opens the possibility of jumping from a antigrav area
on the surface. (Hmm, and if one mounts the jumpnet and jumpdrive on the
outside of the ship, and leave them behind, one has got the jump equivalent
of a massdriver for transport purposes. Imagine the savings if one transported
ore between two systems using just *one* jumpdrive but several hundred
unmanned hulls and with a launch rate of one per 5 minutes or something:)

> Third, in a talk given by Mark Miller, he always intended there to be a
> limited (but large) number of points that a ship could jump from/to,
> otherwise piracy couldn't exist (qv my other article in this digest).

  There were an article by him in JTAS about this. Jumpdrives got very limited.
essentially, for each jump-number you could jump to the 100d of the largest mass
in the system. So to jump directly to Earth, you'd need jump-6.
  I don't imagine the system ever became a hit. Not only did it take lots of
time for the characters to get to the main world if they had a standard ship
like a A or an S, but it required that you knew exactly how the system looked.

  It would also put a crimp on the very popular "jump to the middle of nothing"
to refuel or mothball a ship or whatever unless you happened to know the
location of a suitable cometary nucleus above some minimum size.

> This next part is based on poor knowledge, but presumably, one would want to
> travel retrograde (behind the planet in its orbit) in order to get to safe
> jump distance quickly.

  I don't think so, When you start to accelerate from the planet you already
have the velocity vector of the planet, so it won't make any difference if you
accelerate ahead of it or retrograde.
  This is assuming that you have a drive that accelerate using normal physics.
If you use thrusters all bets are off:)

  [on vector preservation in jump]
> that they are normalized w.r.t. each sun, as I have done in the past, then
> why, considering that you are moving to a point where gravity and thus
> the effects of the sun are inconsequential on jumping, are they normalized?

  Perhaps the solution to this and the 100diameter problem is that it isn't
gravitation that is the deciding factor, and that the unknown deciding factor
(that most physicists ascribe to the properties of jumpspace) decrease with
1/r^3, and that velocities *are* normalized by the gravitation of the local
star (which decrease with 1/r^2)

> 				-Dan Corrin

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