While doing research previous in preparation for the Venus ISRU series, one of the questions that I knew needed a good answer was “how do you actually send vehicles to/from a floating cloud colony?” Unlike the any other near-term manned spaceflight destination, there isn’t a fixed point of land that you can touch-down on. Also, with its thicker atmosphere and only slightly lower gravity, launch from Venus will likely take two stages. How do you recover stages if they can’t return directly to launch site? If you can’t come up with a good answer to these questions that doesn’t require crazy advanced technology, it could be a showstopper. Because a flying cloud city isn’t very useful if you can’t get to it.
This morning, I had an epiphany. One of the papers I had read over the past year or two about Venus missions was a paper by Geoffrey Landis on low-altitude Venus balloons 1. One of the mind-blowing conclusions from this paper was that you could make a 1mm thick titanium spherical pressure vessel about 3.8m in diameter that could both survive reentry, and function as a “balloon” that would hover at around 5-10km altitude. This got me thinking…
Rocket stages are relatively low density when empty… Could you get a rocket stage post-burnout to float in the Venusian atmosphere? If so, could you do it at an altitude high enough that the temperature wouldn’t destroy the stage?
Short answers: Yes, and maybe.
In order to figure this out, I needed a few pieces of information. First, I needed a good estimate for the atmospheric density on Venus with respect to altitude. It took some digging, but I eventually found this table in another paper by Geoffrey Landis2:
In case you’re wondering, the best curve fit I could get (R=.99991) for the first 60km was rho = -0.000340*H^3 + 0.055606*H^2 – 3.184604*H + 64.563149
Because the floatation altitude is the altitude at which the density of the stage equals the atmospheric density, we need to estimate the density of the empty stage. For this we need the inert mass of the stage and the approximate external volume of the stage. To simplify the volume calculation, we’re assuming that the only volume in the stage is the tanks themselves. This is a bit conservative, as the volume of engines and other structures also helps a tiny bit, but it’s much easier to get an estimate of the tank volumes than any of the other relevant volume numbers. Even tank volumes typically aren’t published, so we estimated them by taking the propellant load, estimating the tank mixture ratio (unless we knew it), estimating the propellant bulk density at launch, and estimating the amount of ullage space. I created a spreadsheet to calculate these density numbers, and to then estimate the resulting “flotation altitude” and the temperature at that flotation altitude3.
A couple of key takeaways:
- All of the stages could float at altitudes >5km
- LOX/LH2 stages tend to float higher than LOX/Kero stages (fluffier tanks)
- More mass efficient stages (higher pmf stages) tended to float higher
- The pressures at this altitude are in the 30-40bar range, so you’d want to keep the tanks themselves pressurized enough that the tank was always a little bit higher pressure than the outside atmosphere. This could be done by letting the residual cryogens boil, and using a relief valve set to some nominal say 15-20psid setting.
- The temperatures are all still a bit on the high side. No metal parts would likely fail, but this is hot enough that unless cooled (either via active refrigeration or by boiling-off a coolant), any electronics or plastic components would fail.
Anyhow it’s an interesting idea in many ways similar to ocean recovery, but without the abrupt interface issues with ocean recovery. You’d probably want to use a vehicle purposely designed for this application, both with temperature control for all sensitive hardware, with optimized bouyancy, and with corrosion resistant external coatings.
But isn’t it cool to think of a disembodied Centaur tank flying around like a dirigible at 30,000ft?