Cool... thanks for looking those up!

Back of the envelope:

Vehicle mass: Let's just arbitrarily add three Apollo capsules (http://en.wikipedia.org/wiki/Apollo_Lunar_Module), that should contain enough gear and personnel for.. something:

45000 kg

At the 50 km height in venusian atmosphere in 75 degrees celsius this would need a volume of normal air mixture of 45000 kg / (0.336 kg/m3) = 133928 m3

for neutral buoyancy.

Which, in a ball shape, would mean a ball of radius of:

((133928 m3) * 3 / (4 * 3.14))^-3 = 31,7m

So a sphere with a diameter of 60 meters filled with normal air mixture would roughly compare with a vehicle encompassing enough gear to fill three Apollo capsules.

Which is really an arbitrary measure but means that the idea about bubble vehicles filled with fresh air bouncing in the Venusian atmosphere is actually feasible at least in the science fiction if not necessarily engineering sense of the word :)

I wonder if these numbers could be improved by considering an oxygen-helium atmosphere rather than an oxygen-nitrogen atmosphere. I'm not sure how feasible this would be in a large enclosed space, my initial concern would be the development of pockets of high helium and low oxygen concentration but perhaps a simple ventilation system could keep the air mixed at an acceptable ratio. I believe oxygen-helium was considered for the Apollo missions but not used.

According to some quick calculations a mixture of 22% oxygen and 78% helium would have a gas density of 0.423 kg/m3 at 20 deg C. I used (density1vol1 + density2vol2) / (vol1 + vol2) with density1 = 1.331 and density2 = 0.1664 and a total volume of 1000m3.

The fact that Venus' gravitation is only 90% of Earth's and further (though slightly) modified due to the height of 50 km above the surface would mean that the gravitational attraction would be somewhere around 8.722 m/s2 which is about 88.9% Earth standard.

A Venusian (Venerian?) habitat also has the benefit of less disastrous consequences of developing a small leak in the wall of the habitat since the pressure differential would not be so great though an all out failure of atmospheric containment would unfortunately not be at all survivable even if protective equipment were being worn at the time. The only chance would be immediate activation of the ascent vehicle. Although I would be curious to know if the entire habitat could be constructed in such a way as to remain a rigid shape once it had been deployed, possibly in the form of a Hybrid Airship or Dynastat[1] This could allow a longer grace period on evacuation should it be necessary and might have added benefits to maneuverability during the mission. Take off and landing concerns could be ignored since there would be none other than the initial mid-descent inflation and fuel capacity would only be limited by the available sunlight.

[1] http://en.wikipedia.org/wiki/Hybrid_airship#Dynastats

Airships are fairly robust. It took the British a couple years in WWI before they managed to get enough firepower on a Zeppelin to kill it. Which is to say: they handily survive light machine gun fire. Which would give me a little bit of comfort were I strolling about inside one on Venus.

" they handily survive light machine gun fire"

While I consider war abhorrent, after this comment I cannot help myself but think about WW1 style aerial combat in Venus amidst spherical balloon habitats.

I like it. That'd be a great visual for some pulp-retro-scifi story.