Venusian Rocket Floaties

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.

The end result was:

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?

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
  1. Landis, G., “Low-altitude Exploration of the Venus Atmosphere by Balloon.” AIAA-2010-268
  2. Landis, G., LaMarre, C., and Colozza, A., “Atmospheric Flight on Venus.” AIAA-2002-0819
  3. Venus Rocket Floaties Spreadsheet
Jonathan Goff

About Jonathan Goff

Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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31 Responses to Venusian Rocket Floaties

  1. Andrew W says:

    I’d go with HASTOL.

  2. ken anthony says:

    I suspect atmospheric density would defeat HASTROL.

    What altitude gives us room temperature?

  3. Ken,

    The spreadsheet has that table in it, and it’s got the curve-fit equation for temperature in the 0-60km range. I think the right altitude is somewhere close to 52km or so. I haven’t really done the math yet (though I’m sure someone else has). You want to do it, and report in the comments?


  4. George Turner says:

    Add deployable airbags, shading into balloons, and your tanks can float at almost any altitude you want, with your listed depths being where they end up if there is an airbag or balloon failure. This will have to be done for the capsule anyway (as part of the abort system) because capsules are much denser than tanks – and a crew couldn’t long survive the temperatures and pressures deeper down. You’ll also want a balloon attached to the tank so you can float it back up to the altitude of your occupied structures, which both gets it out of the heat and high sulfuric acid exposure and allows it to collect the more intense Venus sunlight, potentially powering an electric motor and prop so it can mosey over to a recovery airship or dodge storms.

    Also of interest is the launch method. Either you use a dedicated and potentially expendable balloon to allow the fueled rocket to drift a safe distance away from the occupied airship/city, or you just drop it like an X-15 and let it light up on the way down. And odder option would be using a rocket-pump turbine to power a ducted fan from a commercial high bypass jet engine, which would give you low exhaust temperatures and a much easier job of recovering the stage zero section, since the fan stage could maneuver and return to the launch platform.

    I think about this problem every now and then because at some point we really need a radioisotope date for rocks on the surface of Venus to figure out when and how it completely resurfaced itself, which it seems to have done fairly recently based on the crater patterns, as if it had a global melt down. Then we’ll want to search for any remaining evidence of what Venus surface and atmosphere might have been like before the great melt, if any such evidence is left. If the necessary tests can’t be done at surface temperature and pressures, that would require a surface sample mission, with a return either to orbit or to an airship equipped with the necessary scientific instruments.

  5. Lars says:

    Getting stuff down into the Venusian atmosphere is the easy part – at least compared to getting out of it.

    And while your empty rocket stage may float at 5km, you want your floating base to be MUCH higher (you mentioned 50km in an earlier post) – so their float-ability down there is not particularly useful in practice. (Just like the fact that rocket tanks float on our oceans doesn’t help them to get to orbit) 😉

  6. Paul451 says:

    Launching from Venus is no harder than launching from Earth. Provided you have a launch platform at a manageable altitude (and hence pressure/temperature.) The hard part is the chicken/egg of getting the infrastructure in place when you don’t already have the infrastructure in place.

    [This is a problem on Mars too. Mars is harder than Venus for reentry. So landing an entire launch vehicle, plus the ISRU refueling infrastructure…]

    Presumably you could deploy an ballute in place of a parachute. Filled by an enlarged version of the pressurisation system necessary for the tank itself. Since the volume/altitude equation is so kind on Venus, the ballute would not have to be as large as anything used in Earth reentry, in order to keep the float-height reasonable for recovery. Even better, an inflatable reentry shield will likewise reduce the density of the rocket (or capsule). (Subsequent recovery could be with a simple blimp.)

  7. Lars,
    I’ve been thinking that a solar powered aircraft first stage is probably the right approach. I don’t know how high you could get such an aircraft, but it would help with lowering drag losses, and if you have a two stage rocket, it can get your first stage back closer to an RTLS maneuver.

    As for the 5km vs 50km (with a high efficiency LOX/LH2 stage you could get as good as 20-25km floating altitude), so long as its in a state that you can easily send a dirigible, you *might* have a recovery method there. I’ve been shying away from big inflatable balloon bags, but I guess if you’re using helium as pressurant, a small “zero-pressure” balloon doesn’t have to be that heavy to get you enough buoyancy to float higher while something comes to pick you up…not sure. Just wanted to present this analysis and see where the conversation went.


  8. Karl Hallowell says:

    I wonder how important corrosion protection would be? Perhaps a vitreous enamel layer of some sort of high temperature glass would be sufficient?

  9. Lars says:

    One way of getting off Venus could be a “Skyhook” (also would work on Earth) – you could use high altitude aircraft to rendezvous with the tip to transfer cargo.

    It is a lot of infrastructure to put in place at LVO – but it would reduce the infrastructure needed in the floating Venus base, where many materials and volatiles would be scarce.

  10. David Summers says:

    Just to throw something silly out there, how about this: You deploy a parachute to slow the stage down on entry, then once you have gotten down to a manageable speed you bleed nitrogen into the parachute. Nitrogen is lighter than the CO2, so you float back up to your floating base.

    You just need a very large parachute!

  11. Nels says:

    Assuming the residual gases in the tanks are at ambient temperature, and approximately atmospheric pressure, then the ratios of their densities to the ambient density will be just the ratio the the molecular weights. O2 is only a little lighter than CO2 at the same pressure. Hence I suspect that were really looking only at H2/O2 stages — and the O2 tank probably needs to be filled with gaseous H2. Unless, of course, a separate purge tank filled with H2 or He is carried for the Falcon 9 stage.

  12. Paul451 says:

    While you are at it, why not use the last of your fuel & LOx and fire a burner. Hot “air” balloon.

  13. ken anthony says:

    294 K = 70 F.
    (294-736)/-7.84 is about 56 miles up.

  14. ken anthony says:

    …or is that km? anyway, you’d be comfortable somewhere in that range. Thanks.

  15. Andrew W says:

    By slowing the city relative to the atmosphere around it you can use the HASTOL concept I mentioned earlier, your winged rocket powered vehicle sits on a runway with the air moving over it at high speeds (think carrier take-offs) it’ll only need a delta V of maybe 3km/s to rendezvous with an orbiting rotovator.

  16. George Turner says:


    I’m not sure hot air is going to buy you that much at depth because the atmosphere is already very hot, and the extra buoancy is proportional to the difference in absolute temperature between your gas and the atmosphere. If the balloon material is already operating close to its thermal maximum, you won’t have much of a temperature range to work with. At higher altitudes, however, it might make a lot of sense, but since the gas will continuously loose heat to the outside atmosphere and since it may take quite a while for rendezvous and pickup, it might just be a waste of fuel.

  17. Bernard says:

    While not a floating city, this concept could support early floating robotic science stations. Something akin to NASA’s LCROSS mission where Centaur was guided into a collision with the Moon to determine the composition of the Moon’s crust to a few meters. For Venus, a similar inexpensive bus could guide a spent Centaur stage to enter Venus’s atmosphere. Ballutes or similar technology would be required to survive atmospheric entry. I think a floating science station could provide fantastic insight into Venus.

  18. Karl Hallowell says:

    Bernard, something like that probably is a necessary precursor for any colonization attempt as well. For example, I’d want to know what my chances are of getting dropped a few dozen lethal kilometers by bad weather events. And is it worth pumping atmosphere from well below in order to harvest certain elements that aren’t available at altitude?

  19. George Turner says:

    Since Venus is a giant atmospheric disaster anyway, would a nuclear thermal rocket stage using trivially available nitrogen as a propellant be beneficial for getting in and out of the atmosphere?

  20. George,
    LN2 isn’t a very good propellant for NTRs. The Isp drops down into the (if I’m doing the math right) ~250s range, which quite frankly sucks. You’d be much better off with LOX/Methane or LOX/Propane.

    Now Ammonia I think still gets you in the 460-520s range in an NTR (if I’m doing my BOTE calcs right), and gets you most of the density advantages…but I’m not really sure if it’s a win over just plain chemical.

    Probably something for a future blog post.


  21. George Turner says:

    Yes, at 3500K I’m calculating about 250 second ISP, too. Bummer. I figured it could do better than that. Well, there’s hydrogen in the H2SO4, and a nuclear SSTO might work for Venus, where nobody is going to complain about it, but getting it there might be a problem.

    A low tech choice would be PBAN solids, since you’ve got N, O, H, and C in the atmosphere, along with some sulfur, and making ammonium nitrate in the lower atmosphere should be fairly easy compared to Earth. The motor casing might present some ISRU issues, but if you can build a floating city you can certainly build a big steel pipe, and the atmosphere is awash in carbon for making a graphite nozzle, and if need be, epoxy/graphite motor casings. You could also synthesize CL-20 (China Lake 20) from atmospheric gases, a promising new solid with a higher ISP.

    A quite different approach would be to use the floating cities as penal colonies and then wait for some enterprising criminal genius to escape from the planet, and then figure out how he did it. It would probably make the greatest prison movie of all time.

  22. born01930 says:

    Why not have a donut shaped dirigible…the recoverable stages rather than having to land go under the donut and then rise up into the hole for capture. For launch you ignite then the donut splits in half with each side departing to minimize damage from the exhaust.

  23. On the VEXAG decadal survey website there is a inflatable plane I will have to see if I can find it. I am thinking of a dream-chaser with a deployed ballut air-frame

    So Jon your excel spreadsheet needs to model the Dual Thrust Axis lander (ULA) and a pressurized SpaceX Dragon both of these are heaver then empty tanks however,
    perhaps ballast and venting?

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  26. priusmaniac says:

    On Venus you are in the same situation as when you want to land on a ship at sea with a plane or a helicopter. You must learn to land on a moving vessel or you are not going to make it.
    In this case you can have a Falcon 9 reusable rocket that land on a pad at the floating city. Exactly the same procedure as the one envisioned here on Earth except that you always have to plan the return exactly on the pad. Of course you will have to bring the first Falcon 9 to Venus to start with. Once landed on the pad you can refuel it with oxygen and methane obtained from processing the atmosphere. Another possibility would be to use a ducted rocket space plane since a skylon type wouldn’t work due to the absence oxygen in the atmosphere. Nevertheless the carbon dioxide would provide a usefull gas to supplement the rocket propulsion in a ducted rocket configuration. On the other end you would need a landing path under the floating city to be able to land and take off with you space plane. Therefore the Falcon 9 approach would probably be more realistic and economic. To simplify the operations you would be better off with a single stage rocket instead of two stage rocket. You can do that by looking at past proposals like the resusable nova launchers of mm.

  27. Adam says:

    Just tanking up on the ambient air would be easier in energy terms than extracting hydrogen. Sure the mass-ratio is terrible, but the tanks can hold ship-loads more CO2 than LH2 for a given volume. Stop thinking “performance” is measured by *just* the Isp.

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