Re: Just Like 2 - Relative Velocity
- From: "nuny@xxxxxxx" <alien8752@xxxxxxxxx>
- Date: Thu, 15 Oct 2009 11:40:18 -0700 (PDT)
On Oct 15, 9:13 am, Frank <fpfrankpal...@xxxxxxxxx> wrote:
When she was a teenager, my wife learned to water ski. When she'd had
enough time up on the skis, she decided to return to the dock from
which they'd started. As the boat took another turn past thedock, she
skied right up to it -- and broke two ribs.
What she learned so painfully was that it is not enough -- even on
Earth -- to arrive at the place you want to reach at the time you want
to reach it. You also have to arrive with a relative velocity very
near zero.
I say "even on Earth." On Earth, depite my wife's painfull lesson and
all the cars which crash into parts of the landscape, the main
challenge of transportation is covering distance. You have to expend
energy to have a velocity relative to the ground, and -- when you stop
expending that energy -- you soon stop.
Space is altogether different. Planets have been traveling around the
Sun for millions of years without expending energy to do so. On the
other hand, the problems of relative velocity and potential energy are
monumental.
Several years ago, I had an argument in this very newsgroup with a
person who suggested supplying ores to space stations from an asteroid
by shooting bundles of ore in the proper orbit. "You don't
understand," he said in effect, "it will get there." I understood very
well. (I even understood that he was assuming the optimum relationship
between the asteroid and Earth, a relationship which would only occur
periodically with a period significantly greater than a year.) But it
would get there with a speed many times greater than any artillery
round ever fired on Earth. It might come quite close before shooting
off again. It might actually arrive, with consequent damage.
SF writers have frequently written as though distance were the problem
in space. "The War of the
Worlds," in which Earth and Mars are at their nearest approach when
the Martians fire manned artillery shells at the Earth, may have been
he first book to assume that. (They land within a day or two.) It
certainly was not the last. I've seen serious suggestions that we
should plan to visit one or another comet "when it gets closest to
Earth."
Neglecting -- or rejecting -- the possibility of magic drives, it will
take enormous effort and quite a lot of time to reach another planet
-- however advanced our technology gets. The Hohmann orbit, which
nearly minimizes the effort, consists of half an ellipse with the sun
at one focus of the ellipse. It ends up half-way around the sun from
where it starts out. The distance of the target at the starting time
is irrelevant (except as it determines the location of the target when
we arrive). The travel time to a planet or asteroid more distant from
the sun than the Earth is is more than half a year. The travel time to
any target closer to the sun than the Earth is is more than 129 days.
(There are paths which take somewhat less energy and lots more time
than the Hohmann orbit does; there are orbits which take somewhat less
time and lots more energy.)
Taking a ship and crew along when you are moving freight might turn
out to be terribly wasteful. From some starting places, you could use
the O'Neil-Heinlein electric launch tube to start matter on its
journey. You would always need a rocket on the freight to change the
relative velocity to match the target. (Yes, even if the target is a
Lagrange point.) One problem with the launch tube is that it would
have to be both very long and anchored to something solid along its
entire length. The "something solid" would experience an equal-and-
opposite force to what the freight did, That's fine, if the mass ratio
is astronomical; it's not so fine if you are planning to mine most of
the mass of an asteroid eventually and use the asteroid as your launch
platform. By the time that you had sent off any significant fraction
of the mass, the orbit would be significantly different.
So building an orbital station from lunar materials poses the
question: "what do we use for the braking rockets, and where do we get
that?"
One idea comes from the "movie in reverse" rule. Newtonian physics is
such that if you take a movie of something starting out and moving in
an orbit then run the movie in reverse, this is also a possible orbit.
(It works with any experiment in physics which does not involve
friction or the General Relativity effects you get near a black hole.)
Okay, you have an object at L5. You give it a whack. It leaves L5 and
starts on an orbit. For the right sort of whack, the orbit ends in a
collision with the Moon. A relatively soft whack would work. So far,
so good; run the movie in reverse. That means that there are some
orbits which start with a launch from the Moon and ends with the
object passing through L5 with a relatively low speed. That speed can
be reduced to 0 (relative to L5) with comparatively low expenditure of
fuel. One problem is that the time between launch and arrival would
probably be quite long. Another problem is the window of error
involved in the lunar launch. A third is how long a period of the
lunar day would be appropriate for the launch. Hey! I do BOTE
calculations. I don't program mainframes.
http://en.wikipedia.org/wiki/Interplanetary_Transport_Network
If the arrival velocity is small enough, i. e. very close to
"captured" orbital velocity, a small sacrificial mass can be
jettisoned from the main cargo mass by an appropriate explosive
charge. The cargo mass will enter a "kidney-bean" orbit around the L-5
point, the sacrificial mass will have to be aimed on a "safe"
trajectory somewhere.
Mark L. Fergerson
.
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