Re: landings (SG/DS)

On Mon, Jun 10, 2002 at 12:41:54PM -0400, Tomb wrote:
> John S continued the discussion thus:
> > 3. I still think dropcaps will be faster time-
> > to-ground than a lander. Let's say you want
> > to insert your force on top of an enemy 
> > outpost to hit them before they can really 
> > figure out what is happening. That's one 
> > case. 

>From a purely physics point of view:

1. Things do not "drop" out of orbit - the dropship sequence from
Aliens is utter utter crap. You need to deorbit the object. This is
not trivial. Somewhere along the deorbit you have to shed about
10000kmh^-1 of kinetic energy. When the Sulaco lets go of it, the
dropship falls vertically. Erm, so what's holding the Sulaco up?
Superman?

Its orbital velocity is holding it up. And, would therefire hold up the
dropship when it was let go of...

Shedding that orbital velocity is no easier than gaining it: the
shuttle does it by slowing slightly (expending fuel) until its orbital
velocity is that of an orbit which is within the atmosphere. At which
point air friction will kick in and it loses the rest of the energy by
converting it to quite a lot of heat. From first deorbit burn to
touchdown the shuttle completes a couple of orbits - far from a
vertical fall.

Now you /could/ do it by simply firing a thruster sideways until your
orbit speed is zero, and which point you'd ... well. Drop like a
stone. That's a lot of thrust tho - to /gain/ that velocity, the
shuttle makes a lot of noise and goes through, what is it? 2 gallons a
foot of liquid fuel and a two solid boosters.


2. Unpowered vehicles are not the fastest route to the ground. They
can, at best, fall at g ms^-2. Powered vehicles can (theoretically)
fall faster. You can't catch something you drop out of a plane by
diving after it (modulo air friction) because you can't fall faster
than 1g - the speed the object is falling at. Powered aircraft can
exceed a 1g fall, (although I'm led to believe it induces nausea. Very
quickly.) This is complicated by the aforementioned need to shed
orbital velocity. Powerdiving into a gravity well is /doable/, but