flight dynamics of Model Year 2100 Audi (slim Audi content)

[Mike Arman <armanmik@n-jcenter.com>]

The comment about no noise in space generated a startling amount of
speculative e-mail, so I figured I'd summarize the flight characteristics
of the model year 2100 Audi S-500 interstellar runabout.

For the sake of simplicity, we will assume a vehicle with a front, a back,
a top, and a bottom (none of which are really necessary in space) moving
"north", which I have chosen as an arbitrary direction.

It is desired to turn "east", or change the direction by 90 degrees, and
maintain the same velocity as we already had.

Even though there is no air in space (hence no noise), the vehicle and its
contents still have inertia. We have to roll the vehicle along its
longitudinal axis so that when we change direction, the inertial forces act
"downward" in the vehicle. Otherwise, anything and anyone not fastened in
place will slide across the floor as the vehicle changes direction.
Therefore, the first part of the maneuver is to roll the vehicle so the
roof faces the new direction, and then pull the nose "up".

(Fighter pilots do this now: They call it "bank and yank", but this only
works in AIR-planes. There's no air in space, remember?)

Unfortunately, we are not done. The vehicle continues to travel "north" at
the same speed as it was previously, but now it is in a "belly first"
attitude instead of a "nose first" attitude.

We need to cancel out all our "northward" velocity - if we originally ran
the engines for two hours to get to 100,000 mph (for instance), we would
need to reverse them for two hours to get back to 0 mph. Simply running the
engines for two more hours with the spaceship pointed in the new direction
we want to go ("east") would result in us moving northeast instead of east,
since we are still moving 100,000 mph to the north, and now are also moving
100,000 mph to the east. The result is a 45 degree change from our original
course, not a 90 degree change. Two more hours would result in us now
moving east at 200,000 mph, but we are STILL moving north at 100,000 mph,
so our path is NNE (60 degree change). We MUST cancel all velocity in the
old direction, or we will continue to go that way, no matter where the
"nose" is pointed.

(Comparison: A ball rolling on a frictionless pool table. It rolls "north"
at two inches per second. If we apply a thrust pushing it to the "east" at
two inches per second, the new direction is "northeast", at 2.83 inches per
second. Draw a diagram - In one second, the ball travels two inches north,
and also two inches east, and thus traverses the hypotenuse of the
triangle, which is 2.83 inches long. A squared plus b squared equals c
squared.)

This means burning a LOT of fuel, or it means carefully planning where we
want to go before we hit the "launch" button, and sticking to that. In
solar systems, it is possible to "slingshot" around moons or planets (or
even a sun, but you better get that EXACTLY right) to change direction, but
in deep space, changes in direction or velocity can only be accomplished
with fuel.

And that's why the new 2100 Audi s-500 has a hundred thousand gallon fuel
tank! And with 100,000 gallons of fuel, you better believe you'll need
cupholders - you'll be out there for a LONG time.

Best Regards,

Mike Arman