Newsgroups: sci.space.tech
From: eder@hsvaic.hv.boeing.com (Dani Eder)
Subject: Re: Gas giant atmospheric habitats?
Date: 25 May 94 21:03:44 GMT

pickrell@netcom.com (Brian Pickrell) writes:
>Marcus Lindroos INF (MLINDROOS@FINABO.ABO.FI) wrote:
>: "only" about 17km/s. Launch a Voyager or Pioneer from a "cloud
>: city" and it would just be able to escape to orbit.
>:  
>Even if you postulate some sort of nuclear rocket, the fact remains
>that your launch vehicle will be very, very heavy and will have to be
>supported by the balloon's buoyancy.
>
>That's one big balloon.
>
>Brian Pickrell
>
>p.s. If we're bringing along a nuclear air-breathing
>rocket anyway, perhaps we
>should use it as an airplane and forget about the lighter-than-air 
>idea.  But that wasn't what the original poster proposed.

Circular orbit velocity for Uranus is about 15.6 km/s.  Uranus' equator
moves at 3.8 km/s as a result of the planet's rotation.  Thus the 
velocity required by a launch vehicle is 11.8 km/s.  Now, consider a
nuclear rocket.  What would it like to use as fuel?  Hydrogen.  What
is most of Uranus' atmosphere made of? Hydrogen.  So the vehicle
can arrive empty and tank up while at the planet.  The orbital velocity
is only 1.25 times the exhaust velocity of a decent nuclear rocket,
so the mass ratio is 3.4:1, which means it is reasonable to leave
Uranus with a single stage.  The size of a manned vehicle will be set
more by the minimum shielding required for a human crew.

A nuclear reactor on idle can easily provide a few MW of heat to
keep a hot atmosphere balloon hot and to drive refrigerators to
liquefy the hydrogen out of the atmosphere (the atmosphere is pretty
cold to start with, so the refrigeration is not too inefficient.

This stays with known nuclear rocket capability.  If we postulate mre
more advanced concepts, we can imagine a nuclear atmosphere-breating
engine up to Mach 6 or so (in that atmosphere the speed of sound is higher than
in air) and then switch to internal fuel the rest of the way.  The 
mass ratio would be good enough in that case to orbit Uranus with net
fuel left over.  Then you could design a mission that arrives in
Uranus orbit, the 'lander' goes up and down a few times to refuel the
transit vehicle, and then the transit vehicle escapes from Uranus.

The mass ratio for this mission is not bad at all.

Dani Eder

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