Venus ISRU: Introduction

While the Moon still has my “heart” when it comes to offworld destinations, I’ve long been interested in the idea of Venus Cloud Colonies (some references here and here and this article from earlier today). As several others have pointed out in these articles, at about 50km altitude, the atmospheric density on Venus is close to “sea level” density on earth, and temperatures are basically Mediterranean, you get plenty of sunlight, and the CO2 atmosphere is sufficiently denser than air on earth that a breathable air mix provides about half the buoyancy on Venus as Helium does on Earth. Basically, at 50km you could build multiple-km-scale flying cities that would be extremely roomy since more air space means you can support more mass. Or in other words, Lando Calrissian, eat your heart out.


But a question that commonly comes up is that while sure you can make a super large city like that float in the Venusian atmosphere, how do you get it there in the first place? There’s also the question of why you’d want to, but I want to focus this series on how you might build your castles in the sky. What I’d like to suggest in this blog post series is that the Venusian atmosphere may provide most of the raw materials needed to build such flying cities using in-situ resources, and many of those resources may be readily extractable. I’m not a chemical engineer, so consider the following blog posts to be more of a starting point than a finished product, but I wanted to at least spur the conversation.

First Topic: The Benefits of Atmospheric-Feedstock ISRU

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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 is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. 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.
This entry was posted in ISRU, Space Development, Space Settlement, Venus. Bookmark the permalink.

15 Responses to Venus ISRU: Introduction

  1. Paul451 says:

    50km may not be the ideal height. Although the air temp might be 20-50 deg C, remember that the insolation is double, any enclosed space is going to heat up much more and faster than it would on Earth’s surface. Therefore you’d be better to operate higher, colder, and use your insolation and waste heat to manage your internal hab temperatures.

    So, particularly for the first generation habs, I suspect that approx. 60km is better. And the outside temperature is around -10 to 0 deg C, but the sun will add 2.5kW/m^2. Also, natural pressure at 60km is 1/5th Earth sea-level, which means you can run a pure O2 atmosphere inside the habs/balloons, simplifying the air systems.

  2. Karl Hallowell says:

    You can vary to some degree the density of the structure. That could allow you to rise when the sun is shining and fall when it isn’t, allowing even more control over your structure (and some ability to navigate in the atmosphere since winds probably change direction to some degree as altitude changes). There might also be some reason to drop altitude while ISRU harvesting since higher density means more material to harvest/process.

  3. Paul,
    As I understand it, 50km is still fairly low in the cloud level, so I heard that the insolation was closer to earth-like. But shouldn’t it be possible to also control the heating effect by varying the radiative properties of the habitat? That said, I wouldn’t be surprised if a detailed trade suggested a different altitude. As for running a pure O2 atmosphere…that sounds like Apollo 1 levels of “danger Will Robinson”. It would be fun to work with someone like Geoff Landis who has access to much more detailed info than you or I to try and really tease out the optimum.


  4. Karl,
    I like your idea of having a way of varying your density enough to vary your altitude when it makes sense. I agree that lower altitudes will have denser ISRU feedstocks. I wasn’t able to get good data on water vapor density vs. altitude, but I’m sure the info exists somewhere. At least for H2SO4, it drops off fairly quickly from 50km going to 60km.

    It may turn out that you do lower altitudes on the night side, to keep warmer, and that’s when you harvest your ISRU feedstocks, and then during the day you rise up higher, and do energy intensive processing? Not a fan of “batch & que” proceses, but it might make sense, since you’ll be a lot more power constrained at night (unless you were at a latitude where the winds brought you around Venus on a close to 24hr cycle.


  5. Paul451 says:

    “As for running a pure O2 atmosphere…that sounds like Apollo 1 levels of “danger Will Robinson”.”

    Apollo 1 fire was caused by 100% O2 at 1 bar. (Probably over 1 bar, since it was a leak test.)

    100% O2 at 0.21 bar is no more dangerous than 21% O2 at 1 bar.

  6. Paul,
    I’m not so sure that’s the case. It’s not just the partial pressure of O2, it’s also the presence or lack of a buffer gas (N2 or He). The buffer gas helps absorb heat, making it harder to ignite things. Now admittedly, you could probably do a .21bar atmosphere that was mostly oxygen with say helium (which is a better buffer gas than nitrogen), but the people are going to be talking funny.

    No, I think I’d control things thermally using albedo, not trying to go up that high of altitude.


  7. Ed says:

    Just wondering whether anyone has thought about remote (robotic or teleoperated) mining on Venus’s surface to get metals, etc. to support the floating cities. Would be an interesting challenge to design machines that can operate at the high temperatures on the surface, and tethers or balloons that can fly ore up to the cloud cities for processing.

    Might even be a few human adventurers that visit the surface temporarily, at great risk…22nd Century reality shows.

  8. George Turner says:

    Looking at the table of density, temperatures, and pressures, it might be better to go deep. Assuming the craft uses air for lift (because oxygen and nitrogen are readily available in the atmosphere), if you fly it at 55 km (84 degrees F, 0.9207 kg/m^3, 0.5314 bar) you could lift 0.31 kg/m^3, while at 50 km (171 F, 1.594 kg/m^3, 1.066 bar) you could lift 0.54 kg/m^3. But if you drop down to 30 km (435 F, 10.51 kg/m^3, 9.851 bar) you could lift 3.46 kg/m^3.

    Or, if you built an airship 500 feet long and 80 feet in diameter with hemispherical endcaps, so it had a volume of 67,373 cubic meters, it could mass:
    Height (km) Temp (F) mass (lbs)
    0 km 864 F 3,274,300 lbs
    5 km 795 F 2,520,295 lbs
    10 km 725 F 1,906,267 lbs
    15 km 658 F 1,412,517 lbs
    20 km 586 F 1,030,455 lbs
    25 km 511 F 736,328 lbs
    30 km 435 F 512,953 lbs
    35 km 360 F 345,220 lbs
    40 km 293 F 222,566 lbs
    45 km 234 F 136,097 lbs
    50 km 171 F 80,556 lbs
    55 km 84 F 46,530 lbs
    60 km 14 F 23,722 lbs

    This brings up some obvious trade offs. At 55 km you don’t need air conditioning but the vehicle can only mass 46,530 lbs. At 30 km you have to insulate the heck out of the crew or plant compartment (probably making it spherical) and provide a huge cooling unit, but you’ve got 512,000 lbs to play with and can thus make a much, much stronger structure or have a much higher floor load or floor area. But as you go deeper you have to add mass for insulation and air conditioning, and you start having to shift from aluminum and composites over to steel, while losing a lot of sunlight. You don’t have to have a pressure vessel at those depths if you breathe a trimix, and you may not even want to crew such vessels most of the time, just using them as floating farms with a much higher payload per structural size, depending on the sunlight issues. So somewhere in there is probably a few local optimums, depending on materials and power sources.

  9. priusmaniac says:

    This is very interesting to discuss since i have some ideas of my own as well.
    One of those it to go down on the ground not just anywhere but at the highest ground elevations. That is Mounth Maxwell, where the pressure is not 90 bars but 60 bars and the temperature not 450 C but 320 C. That may not sound like a big difference but it actually is a huge one. First the pressure of 60 bars makes it possible for humans to work using special gases like demonstrated by the Franch research center Ifremer. This means that you can travel on the ground at the actual 60 bar pressure without mechanical resistance required. Second the drop in temperature of 100 C now mean that silicones can survive there. It is also somewhat easier to have a refrigeration unit working. These two elements mean that you can have people inside a non pressure protected building breathing an adapted gas and living in an environment at 22 C thanks to active cooling. Residence time can be days not hours. In other words you now have a place on the surface where you can actually process raw materials like ores to turn them into metals instead of hauling the raw material up in large quantities. You can actually make big energy saving on the processing exactly because you can still use the closeby 320 C temperature.

  10. Jeff Wilson says:

    How could you even land on a floating city on Venus? How could you go from orbital speed, to a much lower, flying speed, and then an even slower, landing speed? You’d have to have enough fuel to fire retro-rockets to safely enter the atmosphere, plus enough fuel to fly or glide to the city, plus enough fuel for a controlled landing. And once you approached the city, where would you land? On the roof? In a net? On a hook? If we are to consider floating cities on Venus, then shouldn’t we practice with one here on Earth? How about a floating city for Earth?

  11. Jeff,
    I haven’t worked out all the details yet, but have some ideas. The deorbit burn is relatively cheap (likely less than 100m/s delta-V, which is nothing in rocket world). You’d use the atmosphere to do most of the slowing on the way down. With the slightly lower gravity, and significantly higher air density, terminal velocity for a rocket ship should be about 30% lower than on earth (at Venus cloudtop level). That said, with the cross winds, doing a powered landing like on earth is probably way too iffy.

    Most likely you’re going to want to either do some sort of winged/helicopter system (probably combined with some sort of stand-off snare mechanism), or some sort of deployable balloon system. I haven’t had the bandwidth to study this out to the point where I have a clear favorite yet.


  12. Jeff Wilson says:

    John, I’m not an expert, but I suppose that you’re right, that it wouldn’t take that much fuel, to slow the spaceship down just enough for gravity to take over and bring the ship down. But then, we’d have to have a vehicle that could withstand the heat of re-entry, plus have wings or some shape that provides lift, if we are to use gravity and the atmosphere to glide us to our destination. I don’t know what the g forces are on re-entry, but whatever they are, they would be imposed on astronauts that had been in weightless flight for at least a few months, which would be quite a shock to the body. If we had a helicopter-type of space ship, for landing on the floating city, then the rotors would have to be in a heat shield on re-entry, or else they’d be incinerated, and then they would have to be extracted from the shield, opened up and put into operation, all the while the ship is descending into increasing atmospheric heat and pressure. The helicopter mechanism would have to pop out and work quickly, which would be a terrifyingly stressful operation, knowing that you can only fall to your death, unless everything goes perfectly. So, I think the helicopter idea won’t work. I think that for landing on the floating city, we may or may not need a lifting body shape for approaching the city, but we will definitely need a powered landing, using thrusters like on the Apollo, lunar landers. It occured to me that an interplanetary space ship might need to be caught by a blimp in flight, and then the blimp can slowly and safely approach the city. Or, of course, the landing vehicle need not be an interplanetary ship, but it can be merely a shuttle, designed to operate between a space station in orbit and the floating city. But to me, the idea of floating in the sky in Venus is scary as hell, knowing that a meteor or space junk might collapse your balloon, or that the acid atmosphere will dissolve and collapse it. And then you fall, knowing that you will probably be crushed to death by air pressure. Or imagine your city vehicle surviving the fall intact, and when you hit the ground, you would be simultaneously crushed and burned to death. The idea of a floating city on Venus is so technically difficult and SCARY, that I think that it is not realistic. I think that a better idea is a manned space station in permanent orbit around Venus. The purpose of such a station would be to learn what interplanetary travel is really like, to get experience for a Mars mission, and to study Venus and to prepare to terraform Venus.

  13. Jeff,
    A couple of quick comments:

    1- It’s not slowing it down so that gravity takes over, it’s lowering the perigee of the orbit enough that the atmosphere takes over. Gravity is always in effect. On earth going from ISS to an entry trajectory takes as little as 50-60m/s of delta-V. On Venus I’m not sure how high up you have to orbit in order to not have air drag pull you down quickly, because it has a lot thicker of an atmosphere and a little less gravity. One of these days I’ll have a good simulation/analysis tool for things like this, but the day job interferes…
    2- Regarding heat shields, yes you would need some form of TPS, but you need that to reenter any real planet with an atmosphere. But the heat shield will always be far lighter than rocket propellant to slow down from orbit. Atmospheres are your friends.
    3- As for G-loads after long trips, that’s always an issue that has to be dealt with for manned flights anywhere beyond the moon. But there’s no reason with a lifting reentry you could keep things down around 3Gs or less. There are things you could do to prepare the crew before arrival if it turns out to be a concern.
    4- Regarding the helicopter option, people are looking at that for earth entry too. It’s not as hard as you think. You keep the blades stowed on the lee-side of the vehicle (the side facing away from the brunt of the reentry heating), and have them hinge outward at the top. There’s several ways of making sure they deploy correctly. NASA and private industry have looked at this in the past.
    5- My personal preference leans toward having the landing vehicle use a deployable blimp–at least enough of one to allow it to be tugged in slowly to the cloud city. Lots of trades and little time, since this is a hobby for me. My day job is more focused on practical things like grabbing a boulder off an asteroid.
    6- Meteors or space junk are as unlikely to reach that altitude (53km) as they are to reach the surface of the Earth (where the air density is similar), and there are we think fewer asteroids that get in to Venus’s orbit than earth’s. The odds of a meteorite impact even hitting the colony, much less destroying it, are vanishingly small. The balloon for the colony can be at the same pressure as the outside air, meaning that a hole will result in a slow leak not a pop. And there’s tons of things you can do to design the balloon to minimize failure probability (multiple compartments, rip-stop materials, etc).
    7- Regarding the sulfuric acid, since we know it’s there, you just have to pick structural materials that resist sulfuric acid attack (there are plenty of them to chose from). Known problems are the easiest kind to deal with.
    8- Venus orbital facilities aren’t a bad idea too (in fact you’d definitely want them as well), since you need ways of refueling orbit-to-cloud colony shuttles, as you’re right that they’d probably want to be different from the ships that go between Venus and Earth.
    9- The colony itself doesn’t scare me very much at all. But I do agree the entry and landing (and launch back to orbit) seem sporty right now. I give a pretty high probability though (>80%) that if we ever decide to venture in that direction, we’ll find a safe and reliable way of doing that. I’m just not 100% convinced I know what that will look like yet.


  14. Jeff Wilson says:

    1- Thanks for the correction on what actually brings a ship down from orbit. I assumed that whenever you are in orbit around a body, that all you have to do to hit the body, is to sufficiently lower your orbital speed, so that gravity overwhelms your forward speed and brings you down. Lower speed means a closer orbit, until the orbit collapses, with or without an atmosphere, or so I thought.

    3- This is a problem that I have never heard adequately addressed by anyone, whether astronauts can even function on their own, when they are subjected to g forces for the first time, after several months of weightlessness. I’ve seen the pictures of cosmonauts returning after several months in Earth orbit, and they are utterly helpless.

    5- Yes, a deployable blimp may work. The Apollo missions used parachutes, so I suppose that perhaps parachutes could be converted into inflatable blimps. Still, I’d personally feel more secure if I was in an aerodynamic, flying craft, with fixed, solid wings, than depending on helicopter blades or gas bags to work properly.

    6- OK, you made me stop worrying about meteors. And yes, compartmentalization would help protect the overall survivability of the balloon. But now, in 2014, I can’t ever imagine feeling secure, floating in the sky, knowing that I could never bail out and land safely. You didn’t mention it, but one website did suggest that the Venusian colonies would be tethered to the ground, which I can’t imagine as being realistic, due to the weight of the cables.

    9- I see the Venusian, floating colony as serving only one purpose, and that is to be IN the atmosphere, in order to terraform it, for eventual human habitation of the surface. Just hanging out in the sky, seems like such a limited and tenuous existence, with little to gain from it in itself. As of now, I am annoyed that NASA has not put small, floating, mechanized balloons, on Mars and Venus already, to study the atmospheres with.

    I see Venus as being THE FIRST target of manned, interplanetary travel. Just merely going there, to our nearest planetary neighbor, and being in orbit, and then leaving, would be practice for the eventual Mars mission.

    Thanks for taking the time to write such long and thoughtful replies. Such concern and politeness is very rare. I don’t mean to turn your blog into a chat room, so if you know of a better forum for me to discuss my endless stream of ideas in, please let me know.

  15. Shevek23 says:

    Hi, I forget if I’ve ever posted here or not. Anyway I’ve spent the last day of my vacation fascinated with this site! And this topic really grabs me; I’ve given it some thought myself.

    I should be going to bed soon and have not yet read the other Venus posts on what is available in situ. But I’m pretty sure that the acid traces in the atmosphere contain hydrogen and hydrogen will be mentioned there. True, high up at 55 km the traces will be relatively lower; it’s thin stuff. (But still present enough even there that engineering surfaces exposed to the outer atmosphere must take into account the risk of corrosion, I’d think).

    I believe a Franco-Russian project has in fact deployed balloon probes in Venus’s atmosphere, and these balloons used helium. I was flabbergasted to learn that; hydrogen seemed to be the obvious candidate. To be sure there will be little difference between the lift either provides in an essentially CO2 atmosphere; both would provide fantastic performances even at pressures similar to Earth’s sea level or substantially lower. On the surface or even at highlands, they’d be pretty amazing. The issue I had with using helium versus hydrogen had to do with the unwisdom of shipping it all the way from Earth; to displace a given volume, the helium stored presumably as a liquid during the many months of transit would have twice the mass. The name of the game in space probes is of course to get the most utility out of the least mass, every extra gram costs somewhere else. Also of course, while liquid hydrogen is beastly enough to deal with, requiring insanely low temperatures for storage at reasonable pressures and being quite annoyingly undense, liquid helium is even worse in both respects. Also, both light gases have a tendency to seep away and be lost; again helium is even worse than hydrogen.

    In terms of a long-term exploration/scientific colonization project such as we are speculating on, hydrogen also has the virtue that with infrastructural plant installed somewhere, it can be replenished from the trace acids. We can pretty much forget the prospect of usefully extracting helium on Venus! (At any rate we’d have to have orders of magnitude more development and settlement before we could contemplate trying to do so).

    The niftiness of hydrogen is that of course it is the lightest buoyant lift gas bar none. This is true on Earth as well as anywhere else. But on Earth it has the liability of being flammable in our atmosphere. Not so on Venus! Not only the CO2 but all other components are non-oxidizing; hydrogen is as safely neutral as helium is there.

    It’s pretty cool to observe that a typical Terran atmospheric mix is a lift gas in the carbon dioxide atmosphere of Venus; half the lift we expect from helium at a given pressure is pretty nice. But the airships we have generally managed to make here on Earth require roughly half the lifting force the volumes of helium (nowadays) they enclose for basic structure. This is true of rigid, semirigid or pure pressure (aka “blimp”) designs. Using ultramodern materials aggressively we can probably improve on that considerably, get it down to a third or even a quarter. Maybe.

    But consider how fantastic the results of using hydrogen instead would be on Venus!

    Let’s take the biggest rigid airships of the 1930s–that would be the last two Zeppelins, the Hindenburg and Graf Zeppelin “II”. (Scare quotes because after the loss of Hindenburg, they scrapped the original Graf Zeppelin and simply used the old name with no numbers for the final one; they never called it “II.” But everyone else does). Let’s also consider the final two rigids commissioned in the US Navy, the Goodyear-Zeppelin constructed sister ships Akron and Macon, ZRS-4 and -5. They were very nearly as big though they used helium instead of hydrogen, and I want to mention them because they also used a somewhat different structural strategy, involving three longitudinal keels spaced nearly at 120 degrees around the hull instead of a single one on the bottom. (Actually the German ships also had a rigid keel running lengthwise along the center line). The “keels” aren’t there to reinforce the hull against bending mainly actually–they are there to provide locations where concentrated weights are cantilevered smoothly over sufficient lengths of the ship, and provide access ways.

    Ok, conceptually speaking all these airships were over 200 meters long and over 30 in diameter–the German ones being bigger at 240 and 40 max respectively, the American ones were 235 or so IIRC and the max diameter in the same proportion. Let’s just say they were 240 by 40, and at typical operating altitudes displaced about 200 metric tons of air net, and massed about 100 tons, for a net useful lift of 100 tons–fuel and other stuff.

    Now, let’s move them to Venus and get rid of their numerous lift gas cells, installing instead a couple ballonets forward and aft, to allow some range of altitude variation while maintaining a modest relative pressure; the outer skin is made airtight and buoyant lift acts directly on the hull rigid members that the skin is bonded to, via differential tensions on the panels of hull material. This simplification will lighten their structure, as will replacing 1930s metals and fabrics with modern stuff, so let’s say the structural mass is now 60 tonnes.

    First we fill one with air. The air has molar mass of approximately 30, being an average of nitrogen and oxygen with the former predominating 4:1. (I’m working with round numbers here). The carbon dioxide it displaces, presuming similar temperatures inside and out and a very modest pressure difference stiffening the hull that we can ignore (blimp gauge pressures are a few percent at most of atmospheric gross pressure; making the materials strong enough to endure an entire bar would make the airship impossibly heavy, and they are plenty strong at mere percentages of bars–inches of water were the traditional units of USN LTA operations) the density difference is proportional to the molar mass–carbon dioxide being an oxygen molecule plus a carbon, we have 44/30. So the air lifts 14/44 or 7/22, a bit under one third of the mass of a cubic meter of CO2, at whatever that is at whatever height we are at. Let’s say we are at the same pressure where Earth’s air is just one kg per cubic meter, which is a modest height above sea level. Then outside a cubic meter of the carbon dioxide atmosphere masses 44/30 kg, and each cubic meter inside contributes to 14/30 times the gravity level. (Note that a given amount of buoyant lift gas lifts the same mass in a given atmospheric mix, in any gravity field–the higher the gravity, the stronger the density gradients of different gases, which is the basis of gaseous buoyancy, so while the lift force produced by a given volume on Venus will be less, the mass that force lifts will be same as on Earth, if we had the same sets of gases involved at the same temperatures and pressures). This is indeed half of what we’d get from pure helium on Earth, which would be 26/30, so we do a little bit better–the same as half the lift we get from hydrogen in fact.

    Sound good? Well, I roughly said we get 200 tons of lift on Earth with hydrogen inside, and here we’re getting half that, which is to say just 100 tons. The air filled airship then can lift a mere 40 tons of payload, including any fuels, reserve ballasts, etc. And that all came from me handwaving away 40 of 100 historical tons of structure on the assumption that modern materials are stronger and lighter and we’ve simpled up the interior a bit.

    It strikes me as pretty marginal.

    But now look, suppose we take two other such hulls, just the same, and fill then up with hydrogen instead. Now the 44/30 kg of carbon dioxide per cubic meter is displaced by hydrogen massing just 2/30. The lift per cubic meter displaced is now 42/30 instead of just 14/30; we have multiplied it by 3! Each of these two hulls now lift 300 tons instead of one, and each masses 60 like the other, so we have 480 to add to the air hull’s net 40. If we bundle these three hulls together with the two hydrogen ones on top, we can now proceed to load up the bottom one with 520 tons of non-buoyant stuff.

    Even if we divide by three and deduct 40 tons, that’s still a useful lift of 133, substantially better than the airships of history did.

    And that’s good, because the casual acceptance of the much weaker lift we get from mere air here on this thread, which is only half as effective as hydrogen or helium is here on Earth, made me wonder if anyone here has ever looked seriously into airships. They are an interest of mine you see. And much as I’d like to see them used more, I have to admit that a craft that is by definition no more massive than the same volume of air it displaces would be is a pretty marginal proposition.

    Someone remarked on crosswinds for instance. What crosswinds? The winds of Venus’s upper atmosphere blow at terrible speeds of course. But an airship, like any balloon, moves along with the winds it is immersed in, unless it is under power and driving itself with propeller thrust. Now winds do vary over distance, and airships are so large and fragile that they do indeed get torn apart by relative winds. I’d think this would not be much of a problem on Venus though, as I doubt the factors that make for local turbulence in our atmosphere, especially at the low levels airships are generally confined to, would apply there. Therefore just about the only winds one would observe, if sitting inside such an airship, would be the headwind created by the craft’s own propulsion. Shut that down, you are free ballooning.

    So while there would be many problematic aspects involved in a space craft approaching and being captured by an aerostat in Venus’s upper atmosphere, stormy winds would not be among them, unless the aerostat were to be anchored by lines running all the way to the surface so as to remain in one spot. Then the winds would be effective.

    The reason I’ve been writing in terms not of free balloons but of dirigible airships is that I don’t think you’d want your cloud stations to be randomly blown about the planet with no control at all. Letting the winds take them where they will has some advantages, especially in early exploration; why not let the atmosphere give you a free tour? But you might wind up being stuck on the night side forever; I suspect upper atmosphere air sinks there in the middle of the night zone, and rises in the middle of of the day zone. An opposite circulation, at much slower speeds because of much greater densities, is happening at the surface I suppose, night side air slowly migrating to the middle of the day zone where it rises to close the circuit.

    Since solar power is the most obvious energy source, though, I think the craft is going to want to strategically navigate the upper winds to keep from being blown over to the night side, because you’d never drift out again if I am correct in my guess. Expeditions to the night side will require some kind of power storage to come back, and the crafts will generally steer to stay in daylight.

    Why d0 I suggest using two hydrogen hulls to lift an air-filled hull? Well, I want to keep the hydrogen separated from the air. It is no hazard in the carbon dioxide atmosphere but it won’t do to bring the hydrogen into direct contact with air-filled volumes. Hence the triangular structure; having one hull above the other struck me as unfortunately unsymmetrical for an airship, which should be prepared to maneuver at any angle, yaw or pitch. Obviously you can’t have them side by side either, so 2 of one and 1 of the other it was, and I favored the hydrogen, to allow the huge volume of the air-filled hull to be filled up with lots of stuff.

    As for how to recover heavier than air vehicles such as incoming landing craft from space or vehicles coming up from the surface, consider this. The US Navy ZRS airships used to operate biplane fighters. They’d be recovered by flying up to the airship’s belly; with the airship being able to reach 60 knots airspeed and the airplanes having a lower stall speed than that, they could sidle up and engage hooks built in on top of their wings to trapezes hanging from the bottom. Then they were hauled up, the pilot got off and rested or was relieved, the plane was refueled and rearmed, and it could fly off again with great ease, mainly being dropped.

    Now when we have aircraft that have much higher stall speeds than the cruise speed of the airship, we would seem to have a problem. It occurred to people early on that the very large upper hull might be long enough for a runway, but it sure does alarm me to think of an airplane coming down on the fragile back of such a sky giant.

    No, here’s my suggestion. If the trapeze the plane is aiming for is hanging a great distance down, and the pilot, perhaps assisted by automated equipment, is good enough to hook it on the fly, then the plane will swing on the line, and the potential difference between the low hanging trapeze and the airship bottom deck level will absorb considerable kinetic energy. Combine this with active air braking and, provided the hook-on operation can be managed at all, we have a pretty fail-safe system for airplanes coming in considerably faster than the airship can cruise to be tethered and brought aboard.

    Say we wanted to have an airship that could land a modern high-performance jet fighter that has a stall speed of say 150 knots. But the airship can make 60 knots. The difference between them is 90, which is about 45 meters/sec. That corresponds to about a kilojoule per kilogram of airplane mass in kinetic energy; say the mass is 15 tonnes, so 15 megajoules. But gravity on Earth makes for a potential difference of nearly 10 joules per kg per meter of height, so if the line is 100 meters long, about the length of an American football field, by the time it swings all the way up to airship height it will be pretty much arrested. It’s a lot gentler than the yank a Naval fighter experiences if it catches the arresting cable on a carrier deck; just twice the prevailing gravity force yanks it up when the cable is brought to full tension. Briefly the airship gets that 30 ton yank; the stresses better be well distributed on the hull! But I think this is clearly doable. Remember, if daunted by a 100 meter long–the airship is probably twice that long or more. And yet there is ample separation between the fast airplane and the fragile mother ship in case anything goes wrong; the trapeze structure can be designed to be low impact if the pilot flies too high and hits it wrong (too much height though and it just might shred his airplane). Missing it below just means another go-around, much safer than a landing on a carrier deck.

    So-if we have airship habitats flying around on Venus, I do think spacecraft, or vehicles flying up from the surface, can also hook on to them from below in this fashion.

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