Re: landings (SG/DS)

>
> From a purely physics point of view:

Without mucking around the physics in sci-fi arguement, and sticking 
strickly to the rules and fluff as already put out...

>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...

You are assuming a ship is orbiting a planet as opposed to a fly by and 
drop...  Orbiting a planet is kinda like calling your enemy on the 
telephone to tell him you're coming for him...	He may not know exactly 
where or when, but knows you're there and coming.  Not likely to get you

surprise.

Other than that, you're dead on.

>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.

Given 1. fuel-less systems for thrust (For practicle purposes here at 
least) and 2. a system whose thrust "points" makes the shuttle launch
like 
a stroll in the park...  Dumping V. to drop out of orbit at point X
within 
1 orbit isn't too outrageous.  Not something we can truly comprehend
right 
now, but not physically impossible.

>
>
>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
>you'd have to be mad and have a lot of fuel to waste.

There's another bit here too...  Unpowered vehicles need to slow down, 
unless you don't mind squashed contents.  They usually either need to 
retard thier speed the whole way, or do multiple chute-type drops and 
sudden decel's.

Now, powered craft need not do so until the absolute last minute, and 
depending on the G strain they're willing and able to take, that last 
moment can be pretty short.  When you then take gravity plates into 
account, thus keeping contents from going squish, you can do as high G
as 
you're dropship is capable of physically withstanding...

"Powerdiving into a gravity well is /doable/, but you'd have to be mad
and 
have a lot of fuel to waste."

Frankly, two things I've found the military to consistently excel 
at...  Being mad, and wasting lots of fuel.  Seriously.  Very serious.