Re: superhero space navigation

On Mon, 07 Apr 2008 04:41:05 -0500, Tim Little
<tim@[EMAIL PROTECTED]
> wrote:

>On 2008-04-07, Leonard Erickson <shadow@[EMAIL PROTECTED]
> wrote:
>> Well, if nothing else, I'd like an idea as to what a normal handheld
>> GPS unit would *do* on a trip to low orbit.
>
>I suspect it would depend a lot on how much "sanity checking" the GPS
>was designed with.  I wouldn't be surprised if most of them just
>indicated an inability to determine position if they calculated an
>altitude of a few hundred kilometres or more.

Well, from what I read somewhere, most cheap units are deliberately
designed to fail if they discovered they are moving above 200 mph or
so.  

Supposedly it's to prevent them being used as a cheap missile guidance
system.

>> I was rather surprised when I looked up the specs and discovered how
>> high the GPS satellites are.
>
>If they were in low orbit, almost all of them would be below the
>horizon almost all the time.  Even at 5000 km altitude, the surface in
>line-of-sight would be only about 20% vs ~40%, and about twice as many
>satellites would be needed to ensure sufficient satellites in view
>everywhere at all times.

I seem to recall a figure like 17,000 km (or was that miles?)

Whoops. Just checked the Wiki article and it says 20,200 km.

>> I'm suddenly picturing an old laptop running one of the spaceflight
>> simulator programs. :-)
>
>Yes, you can pick up an old Pentium laptop for pretty close to nothing
>these days.  I've still got one that runs XEphem and Celestia.
>
>
>> Call it 10 gees.
>
>Definitely enough to ignore gravitational corrections.  Hmm, I have an
>add-on script for Celestia that does a reasonable approximation of
>moving the viewpoint under acceleration and gravity.  I'll see what it
>looks like :-)

Well, I'm figuring that the actual limit may be surge rather than
acceleration. On a short trip there's not enough time to ramp up the
accel to the really outrageous values. :-)

This also avoids the problems of low level, high mach number flight
(like trashing the landscape)

Don't suppose there are any formulas for distance as a function of
surge, are there? 

A = S*T 
V= 0.5*S*T^2
D= k*s*T^3  ???

Of course I'd probably run into *nasty* relativistic effects on
flights beyond Mars or so if I go that route.