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Technicalities of Airships

Gryphos

Auror
Very early on in the world-building process of my story I decided that airships would be a very prominent aspect of my world. The 'Aerial Revolution', well, revolutionised how the world worked, with fast and reliable transportation across any terrain and a new dimension in which to wage war.

When I originally decided to have airships I also decided that they were going to be very different from real life zeppelins. I wanted them to be smaller and have the gondola be suspended beneath the balloon, and the ship be propelled and steered by propellers.

Basically, I wanted my airships to be like this:
dUDHBiB.png


Only recently have I been forced to really come up with the details of how airships operate, and consider all kinds of crazy things.

So, with the balloon so small in proportion to the hull compared to real life, I decided that I wasn't going to use any real world gas as the lifting agent. So I made up a gas which for the time being I'm calling aerithium. Aerithium is a non-combustible gas with tremendous amounts of lift, allowing the balloons to be a lot smaller. The balloons would also be compartmented so if there's a hole the entire ship won't go down.

So that's how the ship stays up, sounds good. But how does it come back down again? Well, I thought that, so long as the lift of the gas and the weight of the ship's load are in equilibrium, the ship could be made to rise and fall through use of vertical mounted propellers on the sides of the ship. Meanwhile horizontal mounted propellers on the stern move the ship horizontally. (Yes, I know how zeppelins corrected altitude in real life, with air intake and whatnot, but because of the reduced size of my airship balloons, that won't work here)

But then I realised, for that to work the ship's load weight must be kept constant, but with passengers boarding and leaving and cargo being loaded and unloaded, the weight's going to be changing a lot. So perhaps then it's possible for the crew to alter the amount of gas in the balloon. Letting gas loose to account for loss of weight would be simple enough, but what about when you're gaining weight? There would definitely be pumping stations at docks, but when not docked I'm not entirely sure what they'd do. One option is to just jettison stuff of equal weight to what you've gained, but that might not always be possible.

So I had another idea, something I'm calling aerithium bricks. On ships they would have stores of aerithium bricks which, when burned inside a special compartment, will break down into aerithium and fill the balloon. The brick doesn't need to be pure solid aerithium or anything like that, probably just some kind of compound. But even so, I don't know a huge amount about physics, so it's possible that might not be a sound idea.

So what have I missed? What have I failed to consider? Also, how equal must the lift and weight be in order for the propellers to be able to control altitude? Will it need to come down to a single kilogram or is there a bit of leeway?
 

CupofJoe

Myth Weaver
There is a thing called the hybrid airship. Roughly it is a huge aerofoil and the airship is flown up and down with the usual aircraft controls. From what I remember, they are very slightly negatively buoyant, so if left to their own will sink gently to the ground, but if you drive them forward the aerofoil generates lift and the thing takes off and you don't have to be going too fast.
You would still have to be somewhat careful about the weight but it wouldn't supercritical unless you were at the extremes of what the aerofoil could cope with.
Here is a company SkyCat that is trying to sell the idea.
 

Devor

Fiery Keeper of the Hat
Moderator
Physics is by far my worst subject, so I hope someone will correct me if I'm wrong. But it's my impression that a stable weight, and a stable amount of gas, would result in a specific altitude where the two equalize with the atmospheric pressure. So to go up ten feet, you need to add a little gas. To go down ten feet, add a little weight or remove a little gas. There's no level where it hovers, and you can just pull it up or down and have it hover at that altitude.

That said, simply having the balloon, and using your gas bricks as fuel, should be fine. And there's absolutely no reason small balloons means you can't release gases and have it work just like a zeppelin - especially as, realistically, it could be several small balloons inside a larger one, so that if the outer one popped, you still have the inner ones. The outer balloon might even be mostly a protective frame, but it might still have mechanisms for releasing the gases. Also, the lift might be dependent on the heat of the gas, giving pilots yet another mechanisms for adjusting the ship's movements.

If you're looking at any kind of combat or fast maneuvering, I think propellers could still be pretty cool. You need to get out of the way, or ram another ship, then turn on the propellers for that quick boost to one side. I don't think I would use propellers for regular movements, though. Too many propellers and you might question the need for a balloon.

Come to think of it, you could also consider the use of a sail, especially if it was attached to that outer frame of the balloons I mentioned or to the sides and underbelly of the ship. Sails don't get you up, but they do look cool and move you forward. I have no idea how the speed of a sail might compare to the air release mechanisms of a zeppelin. But if they're not faster, they could work as a backup if those fuel bricks are scarce or expensive.
 

Penpilot

Staff
Article Team
I think your basic idea is sound. I have a background in science. It's computer science, but I had to take a lot of chemistry, and physics etc, and it doesn't ring false to me. I'm no expert, but I think that's the point. Unless, an expert picks your idea apart, I don't think the average reader is going to pause during reading and say to themselves, "Hey that doesn't make sense at all."

I think you've thought out the idea well enough and taken it to a point to where the rest can be handwaved away.
 
Haha brings back memories of Castle in the Sky Laputa - gotta love those big flying ships! Wow, sails on an airship what a great idea Devor. Devor's description sounds right - its about the buoyancy you have that determines your altitude (think of the smoke of forest fires that plateaus/mushrooms out at a certain altitude) and you can control that solely with the amount of gas, its similar to a hot air balloon or to scuba diving, in each case you have set weights.

Another thing you could consider Gryphos would be exotic/magical hull materials that are light weight, and the use of jet propulsion for fast manoeuvres - depending on how far you want to extend your technology.
 

Gryphos

Auror
Alright, so to be clear, my idea of there being a lift/weight equilibrium and the ship being made to rise or fall by propellers will not work, and you can only change altitude by changing gas levels?
 
Well you can get propellers generating lift if they spin on a vertical axis (like a helicopter's) so yes it could work with a lift/weight equilibrium and using propellers to rise/fall - but you'd need to still control the gas to keep the buoyancy equal to the air density as you go up or down by a substantial distance and the air density changes - ie you need to add more 'light' gas the higher you go up - otherwise the propellers would need to work much harder to keep the airship up. It would be like being weighed down with stacks of weights while you are scuba diving - yes you can stay at your position by paddling down with your flippers but its harder work, you're using mechanical power rather than bouyancy to remain at your elevation. Modern helium blimps do use propellers that can rotate so they can point upwards to give lift, but I understand they also use gas/weights so that the propellers are used mostly for manoeuvring rather than a major lift force.
 

Gryphos

Auror
Devor said:
Come to think of it, you could also consider the use of a sail, especially if it was attached to that outer frame of the balloons I mentioned or to the sides and underbelly of the ship. Sails don't get you up, but they do look cool and move you forward. I have no idea how the speed of a sail might compare to the air release mechanisms of a zeppelin. But if they're not faster, they could work as a backup if those fuel bricks are scarce or expensive.

Sail-powered airships do sound pretty call, and in a way I could make that work as a kind of emergency protocol. Say an airships runs out of fuel mid-flight, if they had sail mechanisms stored in their ship somehow they could set some sails up and fly back to civilisation using the wind.

BronzeOracle said:
Another thing you could consider Gryphos would be exotic/magical hull materials that are light weight, and the use of jet propulsion for fast manoeuvres - depending on how far you want to extend your technology.

I was thinking the ships' hulls would mainly be made out of wood with some kind of structural metal skeleton, at least for civilian ships. For military ones they'd probably be made out of a fairly lightweight metal and maybe even have an armour-plated balloon. As for the jets, I can't really see that working in the kind of steampunkesque setting I've created.

BronzeOracle said:
Well you can get propellers generating lift if they spin on a vertical axis (like a helicopter's) so yes it could work with a lift/weight equilibrium and using propellers to rise/fall - but you'd need to still control the gas to keep the buoyancy equal to the air density as you go up or down by a substantial distance and the air density changes - ie you need to add more 'light' gas the higher you go up - otherwise the propellers would need to work much harder to keep the airship up. It would be like being weighed down with stacks of weights while you are scuba diving - yes you can stay at your position by paddling down with your flippers but its harder work, you're using mechanical power rather than bouyancy to remain at your elevation. Modern helium blimps do use propellers that can rotate so they can point upwards to give lift, but I understand they also use gas/weights so that the propellers are used mostly for manoeuvring rather than a major lift force.

Ah, I see, that does make sense. So basically my airships could operate altitude using only the propellers, but upon rising or falling a great enough distance for there to be a considerable change in air density, they would need to be permanently spinning to maintain that altitude. That's where the aeronithium (I'm now calling it aeronithium) bricks and letting loose of gas comes in, finding that equilibrium again so that the vertical propellers don't have to stay on.

So I'm imagining the propellers could still be entirely relied upon up until there's a noticeable difference in air density. What kind of bounds are we talking about here? And I'm still keen on knowing how precise the equilibrium would have to be. For example, would a new passenger on board mean having to adjust the gas levels?
 
These are tempting questions for the engineer in me, I'm fighting the impulse to make a spreadsheet!

Yes, the propellers would primarily be for horizontal movement and some vertical manoeuvring, with the gas/weights used for vertical elevation changes. The use of the propellers for vertical movement outside of the buoyancy equilibrium elevation (ugh I'm not sure that's even a correct terminology... my old lecturer could be turning in his grave here) would increase depending on how high or low you went from the equilibrium elevation. Basically if you go above it there's a force pulling you down, if you go below it there's a force pushing you up. It would get more noticeable the further you go. In terms of how noticeable, how fast the propellers would need to go, I really don't have an idea without doing calculations - probably using Stokes law see Stokes' law - Wikipedia, the free encyclopedia

The buoyancy of the airship will change for any change in weight - including a new passenger. So a new passenger would make the airship start to drop, but its just the velocity of the drop would be slight - depending of course on the proportion of the passenger's weight to the total weight of the airship. I'm guessing the weight of your ship but with wood and steel it would be many tonnes, so a person at 70 kg is what only 1% of that? So it wouldn't drop fast (you could again estimate the drop speed using Stokes law) but yes you'd need something to compensate, either more gas or using the propellers pointed upward. Or accept the drop in altitude. I'm guessing that using a dash of aeronithium brick would be the way to go and a skilled airship pilot would know roughly how much to burn for an average passenger??

BTW helium airships typically don't go that far up in altitude partly because it requires more helium, which is expensive as it ultimately needs to be vented in order to drop back down again.
 

Gryphos

Auror
BronzeOracle said:
I'm guessing that using a dash of aeronithium brick would be the way to go and a skilled airship pilot would know roughly how much to burn for an average passenger??

Yeah, that's exactly what I was thinking. I'm imagining little dialogues happening when a new passenger comes aboard.
"How much do you weigh?"
"Like 70 kilograms."
"Alright, so that's, what, half a brick?"
"Sounds about right."

BTW helium airships typically don't go that far up in altitude partly because it requires more helium, which is expensive as it ultimately needs to be vented in order to drop back down again.

I imagine my airships probably won't go particularly high either. They'll probably still need to go around mountain ranges rather than over, unless of course the captain's particularly daring and/or insane.
 
I imagine my airships probably won't go particularly high either. They'll probably still need to go around mountain ranges rather than over, unless of course the captain's particularly daring and/or insane.

Yeah I can imagine a captain whingeing when requested to ascend to a height to get to some hero's destination "That's not going to be cheap... these bricks don't grow on trees!!" unless of course they do grow on trees? ;o)
 
Hi,

I'm actually writing a steampunk at the moment which has an airship, but it's actually a hot air balloon with a propellor and tail assembly.

However there is a way you can decrease altitude in a helium airship without venting gas. In essence it's about density. So if you have a compressor in your balloon that you can turn on and off, you can decrease your boyancy simply by sucking up a little of the gas into cylinders. Sucking the gas out of the balloon decreases the volume of the balloon. The total weight of the airship remains the same, therefore the overall density of the airship increases a little, and the airship sinks.

Sails on an airship sound cool, but practically speaking they would be useless. The airship sails along at the speed and direction of whatever wind current it's in. Sails can't increase this speed. Nor can the help the ship move at an angle to the wind, because an airship has no keel. In a yacht the sails take the wind pressure which tries to push it in one direction. The hull and the keel of the boat are held by the water, which assuming no current, acts as resistence. And the boat moves in a direction determined by the product of these two forces. In an airship there is no resistence. So there is only one force acting on the ship and that's the air current. The way a balloon changes direction or speeds up or slows down is by changing alititude so that it finds a more suitable air current. If you added sails to an airship what you'd see was nothing save the sails hanging flat. The ships moving at speed x in direction y and so is the wind.

Sorry to be a kill joy.

Cheers, Greg.
 

Gryphos

Auror
psychotick said:
However there is a way you can decrease altitude in a helium airship without venting gas. In essence it's about density. So if you have a compressor in your balloon that you can turn on and off, you can decrease your boyancy simply by sucking up a little of the gas into cylinders. Sucking the gas out of the balloon decreases the volume of the balloon. The total weight of the airship remains the same, therefore the overall density of the airship increases a little, and the airship sinks.

That is pretty nifty, but thinking about it, I'm really liking the idea of balancing venting gas and burning bricks, and all the interesting situations and stuff it can lead to.

Sails on an airship sound cool, but practically speaking they would be useless. The airship sails along at the speed and direction of whatever wind current it's in. Sails can't increase this speed. Nor can the help the ship move at an angle to the wind, because an airship has no keel. In a yacht the sails take the wind pressure which tries to push it in one direction. The hull and the keel of the boat are held by the water, which assuming no current, acts as resistence. And the boat moves in a direction determined by the product of these two forces. In an airship there is no resistence. So there is only one force acting on the ship and that's the air current. The way a balloon changes direction or speeds up or slows down is by changing alititude so that it finds a more suitable air current. If you added sails to an airship what you'd see was nothing save the sails hanging flat. The ships moving at speed x in direction y and so is the wind.

Huh, learn something new every day. Ah well, I was never particularly set on the idea of sails anyway. I was always thinking in terms of engine-powered propellers.


One thing that there is to also consider is the docking of these airships. The way I have it now they dock at massive mooring towers. The way I'm picturing these mooring towers is kind of like a more cylindrical Eiffel Tower, each one being abel to hold a varied amount of airships, but usually about a dozen or so I'm imagining (keep in mind my airships are much smaller than real-world zeppelins). Each docking point on the tower would have a pier for the airship to dock alongside and support struts for it to rest on. Upon docking the airship would be attached to the pier via docking cables that hook onto the ship's railing. The tower might even have a kind of crane built into it to help with the loading of goods onto ships.

Of course there are exceptions. One city that features prominently in my story is situated on the top of a plateau. There they don't bother with mooring towers and instead just put the piers and support struts on the edge of the cliff.

And of course some ships I imagine would be specially designed to be able to land on the ground, built to have flat bellies and retractable stands or something like that. Explorer ships would definitely be made like this, as they'll be having to stop in uncharted territory and be able to be loaded with exotic animals and whatnot.

As to ships having to make long journeys that last several days. Some may choose to fly through the night, but when they really need to stop what would they do? One option is for them to stop at a random port they come across and wait there for the night, which would probably be what most would do. But what about military ships on campaign, who perhaps have to wait at a rendevouz point or something like that? Military ship designs, which would likely have a gun or two sticking out from the bottom, wouldn't be able to land on the ground. So I'm thinking they would bring the ship down low and attach it to a set of stakes or a tree or something, just to stop it blowing away. Then the crew could either sleep on the ship over night or perhaps set up a rope ladder and sleep on the ground.
 

Cerberus

Dreamer
Hi,

I'm actually writing a steampunk at the moment which has an airship, but it's actually a hot air balloon with a propellor and tail assembly.

However there is a way you can decrease altitude in a helium airship without venting gas. In essence it's about density. So if you have a compressor in your balloon that you can turn on and off, you can decrease your boyancy simply by sucking up a little of the gas into cylinders. Sucking the gas out of the balloon decreases the volume of the balloon. The total weight of the airship remains the same, therefore the overall density of the airship increases a little, and the airship sinks.

Sails on an airship sound cool, but practically speaking they would be useless. The airship sails along at the speed and direction of whatever wind current it's in. Sails can't increase this speed. Nor can the help the ship move at an angle to the wind, because an airship has no keel. In a yacht the sails take the wind pressure which tries to push it in one direction. The hull and the keel of the boat are held by the water, which assuming no current, acts as resistence. And the boat moves in a direction determined by the product of these two forces. In an airship there is no resistence. So there is only one force acting on the ship and that's the air current. The way a balloon changes direction or speeds up or slows down is by changing alititude so that it finds a more suitable air current. If you added sails to an airship what you'd see was nothing save the sails hanging flat. The ships moving at speed x in direction y and so is the wind.

Sorry to be a kill joy.

Cheers, Greg.

In my fantasy story I also use lifting gas canisters to pump gas in and out of the balloons as the main means of ascent and descent. I also have the ship built out of extremely light materials so it's somewhat flimsy but doesn't require as much to keep in the air, and has double the balloons necessary to keep it aloft in case of malfunction or so they can bring on extra cargo.

This is a good subject since IMO the most important questions for an airship are weight and propulsion. For propulsion my airship uses a massive engine to turn two propellers on the sides of the ship. Its maneuverability comes from lessening or even reversing the speed of the propellers in relation to each other, as well as limited ability to rotate the direction the propellers are pointing to also give it some vertical maneuverability without increasing or decreasing the lifting gas in the balloons.
 
I have airships in my novels. I've stuck with hydrogen as I want the combustability and it's a gas that can be produced by a living creature from water (which is what my airships are).
The buoyancy is increased however by my having the world one with a higher atmospheric pressure - so I get a lot more lift per unit volume.

In theory the best 'realistic' lift would be from a vacuum airship with no gas in it at all i.e. a vacuum - the problem is however making that strong enough to not crumple - so I guess that won't ever happen. Your fantasy gas sounds like it beats a vacuum by several orders of magnitude - nothing wrong with that in a fantasy setting.
 

CupofJoe

Myth Weaver
In theory the best 'realistic' lift would be from a vacuum airship with no gas in it at all i.e. a vacuum - the problem is however making that strong enough to not crumple - so I guess that won't ever happen. Your fantasy gas sounds like it beats a vacuum by several orders of magnitude - nothing wrong with that in a fantasy setting.
Oh... Vacuum ships.... I like the sound of that... [reaches for Quill and Ink...]
 

Svrtnsse

Staff
Article Team
I too have airships in my setting. One of my characters is working as import manager at a small company that specializes in selling small luxury vessels to the very rich.

I have admittedly not spent very much time thinking about the technicalities of how the ships actually work - they just do. It's a big balloon thingy with some kind of gondola beneath. For my story, the important part isn't how the airships stay flying or how they're piloted, but how they fill a role as a means of transportation within the society.

Basically, they function in much the same way as passenger ferries do in the real world. They're not very fast, but they are (or can be) quite comfortable and can carry a lot of passengers etc.
 

Gryphos

Auror
Svrtnsse said:
Basically, they function in much the same way as passenger ferries do in the real world. They're not very fast, but they are (or can be) quite comfortable and can carry a lot of passengers etc.

That's one of the ways my airships are used too. Airships have basically become the go-to method of transportation, at least between cities and major towns. Horse-drawn carriages are still obviously used within cities and travelling between towns without mooring towers, but any long-distance travel is done via airship. This leads to a variety of specially designed airships for certain needs, whether they be passenger ships with a below-deck area to sit in comfort, or huge cargo ships with deep hulls in which to store things.
 
However there is a way you can decrease altitude in a helium airship without venting gas. In essence it's about density. So if you have a compressor in your balloon that you can turn on and off, you can decrease your boyancy simply by sucking up a little of the gas into cylinders. Sucking the gas out of the balloon decreases the volume of the balloon. The total weight of the airship remains the same, therefore the overall density of the airship increases a little, and the airship sinks.

A great option, if they have the machinery to suck the gas back in that way; lower-tech ships might not.

In a lot of ways it comes down to weight, and cost. Is the fuel (and compressors and tanks) to recycle 1 "brick" of gas lighter than just dumping the gas and having another brick ready for later? Or is the heavier way still cheaper, so only the more expensive ships take the really weight-efficient option to stretch out their long runs?

In theory the best 'realistic' lift would be from a vacuum airship with no gas in it at all i.e. a vacuum - the problem is however making that strong enough to not crumple - so I guess that won't ever happen. Your fantasy gas sounds like it beats a vacuum by several orders of magnitude - nothing wrong with that in a fantasy setting.

This would be a fun twist on things, if you have reinforcing spells that can make a ship with a partial or even total vacuum in the gas envelope. (They'd have to be a highly specialized magic or superscience, or else you'd have a lot of supermetal available and better airships would be the least of your world's quirks to keep up with.) I guess the difference between these and other airships would be that they might be smaller (thus more maneuverable?), and they might have more (or less?) endurance because the material or the reinforcing power adds a different amount of weight than the usual amount of ordinary fuel. Or if the ship was really tough you might finally have an airship that didn't "pop like a balloon" in a fight-- until someone cancelled the reinforcing spells. :p
 
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