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Imagine a very long bridge (Visbi Bridge - named after the designer) across an immense canyon - really almost a wide deep gorge -use on an earth-like world. My concept is that part of the support of the bridge would be a ballon-like, lighter-than-air technology of some kind. The ends of the bridge would use a suspension set-up, with piers and cables. Out toward the middle, the bridge would need additional support

1.I am wondering if this might be feasible, assuming a somewhat more advanced materials technology than we presently have? Gravity and weather would be the similar to our earth.

2.How long could such a bridge be? 1 mile? 2 miles? 5 miles? Assume advanced materials that are at least conceivable today.

3.Would this concept make sense in opposition to flight? In other words it would have more capacity for the energy consumed.

4.Are there other technologies that would realistically allow very long bridge spans?

On this world, the technology is close to what we have. The civilization on this world is less energy intensive than ours, is not car-based and is more sparsely populated. I am ruling out anti-gravity technology or energy bridges, etc. This bridge would not have to bear enormous loads, mainly people in some sort of vehicles and valuable/light cargo. I assume provisions for closing the bridge and/or reconfiguring the bridge structure in unfavorable weather and wind conditions.

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    $\begingroup$ What happens in crosswinds? Your balloon-supported structure has to be strong enough to support the load of the wind blowing the balloons around. $\endgroup$
    – jamesqf
    Jan 26, 2015 at 6:50
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    $\begingroup$ I don't know your world but i believe "immense canyon" could very well lead to "huge amount of wind", at least often enough that a LTA bridge might not be the best option... $\endgroup$
    – Sheraff
    Jan 26, 2015 at 10:55
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    $\begingroup$ @jamesqf That's a very valid point. But here's a solution: tether your balloon to the canyon floor, to the left and right of the bridge (as seen by someone walking across it.) That should make a more rigid "tension A frame" or "tension pyramid" structure. $\endgroup$
    – Level River St
    Jan 26, 2015 at 12:38
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    $\begingroup$ There's already a bridge with a ~2km span (1¼ miles). Composites are starting to be used in (small)bridge decks, which would massively decrease the load on the cables of a suspension bridge. If you assumed composite cables as well you might hit the desired distance without the need for extra support -- especially if you believe some of the hype about the potential of graphene. $\endgroup$
    – Chris H
    Jan 26, 2015 at 14:22
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    $\begingroup$ A competing solution would be just running heavy (read: most advanced material allowable, perhaps kevlar or carbon nano-tube based) cables and then attaching gondolas to the cables - if there is a large storm/etc. then the gondolas can be removed, leaving you with just cables. For comparison some current gondolas have spans of over 3km between their support posts, and as long as you aren't transporting anything super heavy have many advantages over bridges. $\endgroup$
    – user2813274
    Jan 26, 2015 at 21:08

8 Answers 8

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From an engineering standpoint, an LTA bridge is similar to a pontoon bridge. You'd basically have a continuous line of balloons (or multiple lines, if you're willing to pay more for increased redundancy) inflated with the lifting gas of your choice. Groups of balloons would support bridge segments, and each segment would be tethered to the ground below with a number of anchor cables.

In unusually severe weather (say, a once-in-a-century hurricane), the bridge can be "landed" by separating the segments, releasing some lifting gas, and using the tether cables to guide the pieces down to the ground. You wouldn't do this except as a last resort, since re-launching the bridge is almost as much work as assembling it in the first place. Landing can be done in less than a day, since each piece can be landed independently, but re-launching needs to be done one segment at a time, from the ends to the middle, so that everything lines up.

  1. Yes, it is feasible. You'd need to avoid building such a bridge in areas affected by severe weather (where "severe" depends on the available technology, eg. if you can produce huge sheets of Kevlar fabric, you can handle far worse storms than if you're using gas-tight silk and hemp ropes).

  2. Like with a pontoon bridge, there is essentially no limit to how long you can make the bridge: each segment is self-supporting and self-stabilizing, and the connection to the adjoining segments is only to maintain alignment.

  3. You can't really compare a bridge and an aircraft. An LTA bridge has a high up-front energy cost, but once it's in place, the energy cost is mainly that of replacing lifting gas lost to leakage or deliberate releases. An aircraft, on the other hand, has a high per-trip energy cost.

  4. A suspension bridge can go at least 2000 meters between towers; the approach spans can easily triple this. If you are willing to build a massive support structure, a trestle or viaduct bridge can cross a virtually unlimited distance.

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  • $\begingroup$ The biggest issue I have with this is that the buoyancy of a pontoon is related to the volumetric density difference between the materials. So, for a vacuum-filled pontoon to be as buoyant in air as an air-filled pontoon is in water, it would have to be 783 times as large. The other issue is that a pontoon in water isn't at peak buoyancy - as you put load on it, it sinks and increases the vertical force, thereby creating a stable asymptote. If your balloon is in the air, it won't do that - it'll be [nearly] 100% buoyant [nearly] all the time, at an unstable asymptote. $\endgroup$
    – iAdjunct
    Oct 7, 2016 at 22:09
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A significant problem with suspending a bridge from balloons is how the load of traffic is supported.

Let's say that your bridge is in equilibrium under its own weight. When you add the vehicle(s) going across the bridge, the amount of gas in your balloons isn't going to increase, therefore they provide no additional lift. So the weight of the vehicle(s) must be carried solely by conventional means, i.e. by bending of the bridge deck. With a very long span you would need a very stiff bridge in order not to get excessive deflection.

Note that this problem doesn't arise from pontoon style bridges (which, as Mark pointed out, is probably the closest any real bridge gets to your "balloon bridge"). This is because when you add vehicle(s) over the pontoon supports, the vertical reaction at the supports increases, i.e. the contact pressure between the pontoon and the water increases.

Your best bet is to stick to conventional bridge styles, but to use advanced materials. The current records for bridge spans are (approximately) 1km for cable-stayed and 2km for suspension. The main restriction to increasing these spans is the weight of the cables - each cable has to carry its own weight as well as some of the weight of the bridge deck and vehicle traffic. The cables on the current longest bridges are mainly carrying their own weight. You could solve this by making the cables out of something lighter than steel, e.g. carbon nanotubes. This is obviously not done currently because of the excessive cost of carbon nanotubes when such a large quantity would be needed.

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    $\begingroup$ Good answer, however pontoon bridges do sink down into the water until they displace enough extra water to compensate for the weight of the vehicles on them. $\endgroup$
    – Tim B
    Jan 26, 2015 at 13:34
  • $\begingroup$ If there's more than enough lift from the balloons, the tension in the bridge cables will be reduced with essentially no dimensional change. The tension will increase as the bridge is loaded. You could mock up a model even without helium by using a pulley for the lift component. $\endgroup$
    – Chris H
    Jan 26, 2015 at 14:29
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    $\begingroup$ The deflection issue is why my bridge has tethers anchoring it to the ground: with an unloaded bridge, tension in the cables provides the equilibrium force for the balloon lift. As the load increases, tension on the cables is reduced; as long as the bridge is not overloaded, deflection should be minimal. $\endgroup$
    – Mark
    Jan 26, 2015 at 19:44
  • $\begingroup$ The bigger issue is that a pontoon bridge segment is supported EQUALLY across it's entire length by the water. A balloon supported segment would be supported ONLY at the points the balloon cable is attached to the deck. That places huge amounts of forces on small points. (However, this could be remedied if the balloon cables were more akin to a standard suspension bridge, but now we're again adding a lot of mass just for the cable supports) $\endgroup$
    – DA.
    Jan 26, 2015 at 22:19
  • $\begingroup$ @DA. Many balloons anchored at many points would be similar to a pontoon bridge and mean that a small number of failures would affect only the carrying capacity and not the structural integrity. This may offset the extra cable weight would add each cable would be thinner. $\endgroup$
    – Chris H
    Jan 28, 2015 at 9:51
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Air is incredibly light. It's hard to conceive of any materials that are actually lighter than air and sufficiently so to keep lift while under load.

However, it might be possible to use balloons as a lifting device. The gas is a subject of some contention. Helium has a lifting force of 1 gram per litre, which is not very much. Hydrogen is about 8% better but it's very flammable which is why airships no longer use it. Search Hindenburg. Methane can also be used and doesn't escape balloons as easily but it's a much worse lifting gas. So let's go with helium.

Let's also say that we're using nice advanced graphene to build this bridge; although not currently developed enough to do this with, it might well be soon. Graphene has a mass of 0.77mg per square metre, or 21.36 milligrams per cubic metre.

We're building a bridge. How much material does that need? Assuming a span of 1km, width of 10m and average depth just 50cm (which should support enough for your needs), that gives:

$$ 1000 \times 10 \times 0.5 = 5000 \text{ m}^{2} $$ $$ 5000 \times 21.36 = 106800 \text{ milligrams} $$ $$ = 106.8 \text{ grams} $$

Therefore, to lift the bridge alone you need 106.8 litres of helium. You will need extra helium equivalent to the weight of the vehicle in grams when a vehicle crosses (or a person for that matter). Let's say your heaviest vehicle is 1 tonne. That's an extra 1,000,000 litres of helium...

In short, you're going to need a lot of helium. It's fairly impractical.

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  • $\begingroup$ From a mathematicall and logical perspective this post terrible. 1) The density of graphene is about 2 g/cubic cm or 2000 kg/ cubic meter. 2) 1 lifting gram per liter of helium, AT WHAT PRESSURE? Helium in balloons is much more dense than helium in the atmosphere so what are you talking about? 3) Why would you assume that an average depth of 50cm of graphene would be needed? Or that graphene would even be an appropriate building material? $\endgroup$
    – Myles
    Jan 26, 2015 at 13:44
  • $\begingroup$ If you have graphene with a density of $21mg /m^3$, use that as your lifting material! It's about 2% of the density of air at STP. $\endgroup$
    – frodoskywalker
    Jan 26, 2015 at 16:11
  • $\begingroup$ @Myles Can you cite me your resources? I'd like to take a look. For your second point, the lifting force of helium is equal at all pressures - while it is indeed more concentrated/dense in balloons, that just means you can fit more into a smaller space - the lifting capacity is exactly the same. $\endgroup$
    – ArtOfCode
    Jan 26, 2015 at 17:29
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    $\begingroup$ @ArtOfCode appliednanotech.net/tech/graphene_films.php As you increase the density of a gas the mass of gas/liter increases and decreases the lifting capacity. Think of it this way, a nearly empty helium tank weighs much less than a full one in spit of the fact that they both contain the same volume of helium. $\endgroup$
    – Myles
    Jan 26, 2015 at 17:36
  • $\begingroup$ @Myles See my 0.77mg/m^2 came from the IET, so now I don't know who to believe :) I get your point about the gas, so either at STP, or 22.72 grams per mole $\endgroup$
    – ArtOfCode
    Jan 26, 2015 at 17:45
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There is a great article put out from MIT:

http://cba.mit.edu/docs/theses/15.09.Carney.pdf

In it, the authors describe the feasible application of lightweight cellular lattices made from a crystal-like array of struts:

enter image description here

It's certainly not "lighter than air," but there is a lot of empty space, and it's realistically achievable.

enter image description here

The bridge could be as long as possible using a solar powered, gantry-style assembler, like so:

https://www.youtube.com/watch?v=7MssR1zmlpU

Or using the bipedal robots shown here:

https://www.youtube.com/watch?v=ytRJHtg_jJw&t=1m9s

The cells (?triangular bipyramids?), presumably, would need to be made in bulk first before their use in assembly.

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    $\begingroup$ Welcome to Worldbuilding! +1 For such a good find, although you might want to elaborate on details such as how feasible it is for a bridge (quoting the thesis would be useful) and how long you could make it $\endgroup$
    – Mithrandir24601
    Aug 29, 2017 at 8:44
  • $\begingroup$ The robots could also feasibly be equipped with a KUKA-like welding apparatus. $\endgroup$
    – user42036
    Sep 1, 2017 at 22:51
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As other's have mentioned, your idea is akin to floating bridges. Seattle has some of the biggest ones:

enter image description here

They are kept stable via angled anchor cables on the sea (or lake) floor.

So at a very basic level, one could treat a floating air bridge the same as a floating water bridge by having anchor cables attached to the surface below.

However, there are some significant differences:

  • a floating bridge has some lateral 'twist' due to tides and waves and such. A floating air bridge would have the same but at a much greater scale given the resistance to the air would be much less (think of the rope bridge scene in Temple of Doom). Your anchoring system would have to be fairly complex to prevent the twisting of the bridge deck in mid-air.
  • a floating water bridge is supported equally along it's entire length. A floating air bridge, using balloons, would only be supported at the point of connection between the balloon and deck. This would mean the bridge structure would have to be reinforced to handle the point-loads much more so than a floating water bridge. (this could perhaps be remedied by having the balloons replace a standard suspension bridge's main towers...you'd essentially have a suspension bridge held up via balloons rather than towers).

I am not an engineer, but perhaps some of these concerns could be handled via a different recipe for the atmosphere. Perhaps the atmosphere on this planet is much more dense than on earth making these ideas a bit more practical.

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  • $\begingroup$ The connection point between the balloon and the deck is going to be very similar to that between a suspension bridge cable and a deck, no problem there. $\endgroup$
    – AndyT
    Jan 30, 2015 at 13:26
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Mark's excellent answer covers most of this, there is one last thing it missed though.

Lifting gas escapes from balloons, for example Helium Balloons a few days after a party have already shrunk.

No matter what you try you will gradually have lifting gas escape from the pontoons, so part of the servicing schedule for the bridge will be for people to go through and refill them. The viability of the bridge will to a large extent depend on the availability of the lifting gas and how fast (or slow) the leaking can be made.

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  • $\begingroup$ Perhaps if hydrogen were used for the lift gas moisture could be captured from the air and electrolysed using solar/wind power to keep it topped up. The same power source could be used to handle some weather. $\endgroup$
    – Chris H
    Jan 26, 2015 at 12:27
  • $\begingroup$ @ChrisH there is a major reason why hydrogen isn't use in blimps anymore, and I imagine this would avoid highly reactive gasses for the same reason: it burns! $\endgroup$
    – user2813274
    Jan 26, 2015 at 21:02
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    $\begingroup$ @user2813274 the OP said weather and gravity were similar. They didn't say anything about attitude to risk. Hydrogen doesn't spontaneously combust even if it escapes. The supply of Hydrogen is essentially unlimited. Helium is in rather short supply. $\endgroup$
    – Chris H
    Jan 26, 2015 at 21:56
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    $\begingroup$ Note, further, that helium is specifically the one gas above all others that is best at escaping from otherwise excellent seals, so hydrogen would leak a lot slower. $\endgroup$
    – Nathan Tuggy
    Jan 27, 2015 at 3:49
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How about kites?

You'd need horizontal-ish guy ropes to counteract the lateral component of the force and computer-control of the kites' angles relative to the wind - perhaps you could assume a steady katabatic wind guided by the canyon. Wind is stronger at higher altitude. If the wind was constant enough and strong enough you might be able to use the bridge itself as a lifting body.

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  • $\begingroup$ This is a creative solution, so +1, but it seems to be rather infeasible on Earth. This will be an interesting way to keep a bridge on a planet with constant winds aloft, though. $\endgroup$
    – March Ho
    Jan 26, 2015 at 13:00
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    $\begingroup$ @MarchHo I'm sure I remember reading about near-permanent katabatic winds on mars but I can't find a real science reference. Here's an interesting read about some projects that helped give me the idea. $\endgroup$
    – Chris H
    Jan 26, 2015 at 14:14
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As discussed in comments and other answers there are several issues:

-Adding bouyant balloons to a bridge would reduce the weight load, do nothing to reduce the wind load (in fact it would make it worse.)

-Lift gas would leak over time.

Here's my solution

The neutrally buoyant cable car / ski lift!

This is basically just like a normal cable car / ski lift, except that the weight of the cars / chairs is reduced by a buoyant balloon. This enables the spans to be much larger. During high winds, all cars have to be towed in to the base stations to prevent damage.

One issue is ensuring the cars are indeed neutrally buoyant before they are released, otherwise an unloaded car would pull up on the cable just as much as an overloaded one would pull down. Varying the amount of bulky lift gas seems inconvenient, so it would probably be done with ballast weights.

Finally, if an intermediate support were needed part way across the span, as mentioned in my comments on the question, you could make an A-frame from 2 cables tethered to the canyon floor to left and right of the direction of travel, held in tension by a balloon. It is unlikely but conceivable that this would be more economic than a steel pylon on some worlds (i.e. those with a very dense atmosphere.)

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