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Setting: In my world I've introduced some science-defying material. I use it to grant the ability of flight to airships that are less Hindenburg-y and more ship-y.
They are not literally ships that fly, but in order to comply with the aesthetics that I have in mind and still stay afloat, real lifting gases just don't cut it.

These Airships, most of the time, consist of two or more parallelly arranged lifting bodies with most of the mass of the ship hung between them, the center of mass being below the lifting bodies. This arrangement is intended to give them a sort of stability in the air similarly to a catamaran.
There are different classes of ships that refer to different sizes, armaments and tasks of the ship, the largest of these reaching widths of up to 160m and lengths of up to 400m. Aspect ratios vary from smaller ships @7:2 to larger ships @5:3.

Armaments differ between classes, but are similar to WW1/WW2 ship armaments. Similar goes for the ship classes and engagement styles.
E.g. cruisers/battleships will fire heavy shells over vast distances (mostly for land bombardment), while smaller ships feature lighter weaponry to engage in ship-to-ship fighting.
Additionally most ships, down to corvette-sized ships carry a compliment of fighter/bomber biplanes (smaller ships may carry one or two fighter-craft, while larger ships may carry multiple squadrons of fighter/bomber-craft).

Technology: The world is set technologically somewhere between the industrial revolution and the invention of internal-combustion-engines. There's been heavy development and optimizations regarding steam-driven-engines as there is, at least in parts of the world, little to no oil available (also because I like steamengines).

Electricity is something fairly new and so far doesn't go further than being used for creating light aboard airships and being used for telegraphing (little to no electric infrastructure).

While oil is found and burned in other parts of the world, in this part of it the most commonly used fuels burned for heat are coal and peat.
Additionally natural-gas, coal-water-slurry and fishoil are most commonly burned in lanterns to provide light.

Situation: I got the associated lifecycle of the mentioned phlebotinum to a point where I am quite happy with it regarding the occurrence of the element and its behavior(s).

But: The lifting power dictates the element to display either of the following two properties:
A) The element displays antigrav properties, thus allowing it to deflect/repulse/dilute gravitational forces acting upon it or a sufficiently concentrated mass of it.
B) The element displays negative mass, thus gravity does not apply a pull on it but rather a push forcing it away continuously

Both of these explanations are, to my understanding, far beyond anything explainable/discovered in our current understanding of physics. They are themselves not the topic of this question but mainly a fact regarding the environment.

Making use of the gas to lift objects off the ground, applying the laws of buoyancy, I would aim to provide sufficient negative mass to reduce the average weight per m3 of the object-to-be-lifted to the average weight of air at the altitude I'm aiming to lift the object to.

Again regarding airships I want them to be able to lose/gain 'weight' while afloat without these 'weight-changes' having huge influence on their altitude/flying-capabilities (think e.g. of a fleet of aircraft launching from a carrier / returning to it; or a complement of several hundred soldiers and landing boats leaving the baseship / returning to it).

Question: What means are there to regulate lift (overall weight) without having to rely excessively on external means?

I would like to focus on means that are within the technology constraints (they must not necessarily match the time-constraint; e.g. electromagnets can likely be a thing, same goes for electric engines - alas they make little sense when we need to drive steamengines/-turbines in order to provide the electricity anyways).

I would further like to focus on ways that do allow a ship to float at least for a week or two before needing refueling (that means a ship need to be able to carry enough fuel for floating a week or two).

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  • $\begingroup$ Switched accepted answers, as the now accepted answer is independent of how the lifting gas works - universally more useful $\endgroup$
    – dot_Sp0T
    Commented Feb 26, 2019 at 7:35

3 Answers 3

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Use a method similar to that used on some submarines: filling tanks with seawater when you need to go down (get heavier), and emptying them when you need to go up (get lighter).

Just like with submarines, air is all around you, and in compressed form it has quite a high density of 870 kg/m^3, pretty close to that of water. Equipment for liquefying air appeared at the start of 20th century in our world, so it should be quite possible with your level of technology. So just have air tanks distributed around your ship, and fill them with liquid air when you need to compensate for mass loss. In a normal state you will probably want them about half-full, to allow for both gain and loss.

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  • $\begingroup$ is it necessary to use liquid air? I mean, what would happen if you fill up such a tank with air a double normal pressure? shouldn't this make the airship decent? *confused* $\endgroup$
    – Confused Merlin
    Commented May 17, 2016 at 8:17
  • $\begingroup$ It is not necessary, but it's the easiest way. If you simply fill with air at double pressure, you will simply have a bit more air, not enough to really matter. Or did you mean "with liquid air at double pressure"? That wouldn't change much, because liquid air, like most liquids, is almost non-compressible. That's why it's easy to deal with: you just fill the volume proportional to mass you want gained. $\endgroup$
    – Alice
    Commented May 17, 2016 at 11:56
  • $\begingroup$ I have to admit a slight leak in knowledge about this topic... so you have a tank - empty - with 1atm... then you stuff as much air as possible (not liquid, gas) inside this tank. until no more compression can be archived by the technology in use. And now: does this tank gain weight (not mass, WEIGHT)? From nothing than air compressed at... say... 5atm? $\endgroup$
    – Confused Merlin
    Commented May 17, 2016 at 13:36
  • $\begingroup$ The liquid air is made by compressing air and removing heat generated in the process. The method doesn't really need achieving pressures much larger than 5 atm - the colder it gets, the more mass — and weight, they are same in a fixed point in a gravity well — you can put in the same volume at the same pressure. Of course, you don't want hot atmosphere air going directly into your tanks, first you condense it, then add to the tanks. As for the last question, well, air has some mass as well, that's why 1atm is not zero pressure, you just need a lot of it to notice. $\endgroup$
    – Alice
    Commented May 17, 2016 at 15:51
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Have your phlebotinum's effect depend on it's density. I'll assume it gets stronger with higher density/compression (the interaction between particles is more intense), but it could be the other way around just as easily.

Either way, you'd deploy it in large cylinders/pistons (potentially running the length of the lifting bodies) that can be mechanically compressed by use of a steam engine or similar.

Changes in altitude or weight could be controlled simply by the pistons. (for these examples, the phlebotinum increases its antigrav effect when compressed)

  • Launching a fighter wing: "Bridge to Engineering: Extend the pistons by 20% on my mark".
  • Landing: "Bridge to Engineering: Extend the pistons to 80%, standby for final approach"
  • Emergency climb: "Retract the pistons! Full retract! Get us out of range of those flak cannons!"
  • Always: "Engineering to Bridge: She canna take any more, captain. She's gonna blow!"

Concerning longevity, your ships could theoretically stay airborne indefinitely without using fuel. In practice, the ship would always have a steam engine running on standby to maintain enough pressure for sudden maneuvers, as well as to power onboard electrical devices. In an emergency, some fancy clockwork might allow the sailors to manually extend/retract the pistons, but too slow to be acceptable under normal conditions. Eventually, most ships will need to land to take on coal/peat and water, though some could have it delivered by smaller ships and never land.

Note: upon re-reading your question, I realized that your catamaran designs would need some way to synchronize lift between the hulls or risk being torn apart. Piping between them to equalize steam pressure would probably solve this. A backup of two trained engineering teams and good signaling between them is advisable regardless.

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  • $\begingroup$ Upvote for "Engineering to Bridge: She's gonna blow!" $\endgroup$
    – Oleg Lobachev
    Commented Jan 4, 2018 at 0:05
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I found an interesting answer by searching for patents on airship ballasts. This patent describes a method for generating ballast using engine exaust. The basic idea is that hot exaust exiting the engine (the engine drives the propellers) is funneled through a long tube. As the hot air passes through the tube, it cools enough for the water in the air to condense. The water is then pumped into a ballast tank, and it weighs the tank out. If the ship needs to ascend, water can be dumped out.

Benefits of this system:

  • As long as the engine is running, water can be collected for ballast

  • Collection of the water is low cost, little to no energy is needed to transport the water.

  • Water is heavy, so not much will be needed. Air, on the other hand, is much lighter, so it needs to be collected in large amounts.

  • Water can be collected during flight, which is essential. Using other weights, like sandbags, once the sand is dropped the ship will not be able to ascend.

  • If the water is purified to remove exaust by-products, it could be used for drinking. this could extending the time ships can travel before having to land for supplies.

  • Evaporation will not be a problem as long as the ballast tank is enclosed (water vapor can't escape), so the ship can hover as long as needed.

  • Water is relatively cheap, so it can be loaded onto the ship at port in proportion to the weight of cargo.

  • Water is harmless, so dumping it out won't hurt anyone, in the way that dropping a sandbag from the airship might hurt someone.

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  • $\begingroup$ I may be reading this wrong but if you are only collecting water from burnt fuel won't you still lose mass because the water in the exhausts will weigh less than the mass of the entire exhaust. Also you'll lose mass when things like fighters take off. $\endgroup$
    – Bellerophon
    Commented Mar 3, 2019 at 9:38
  • $\begingroup$ @Bellerophon Yes, collecting air as a ballast is an option, but it is so light that it needs to be compressed, which takes energy. This system is more passive. As long as the engine is running, it can collect ballast. $\endgroup$
    – John Locke
    Commented Mar 3, 2019 at 13:46
  • $\begingroup$ @Bellerophon When things with mass leave the ship, the ship will get lighter and start to ascend. Then more ballast would need to be collected to stop the ship from ascending. Usually though, the weight of the ship isn't changing mid-flight. That would be hard for any system to accommodate for. If the ship needs to descend, collecting air will lower the ship faster than releasing water will make the ship ascend faster than releasing air. $\endgroup$
    – John Locke
    Commented Mar 3, 2019 at 13:52
  • $\begingroup$ @Bellerophon Assuming the ship runs on hydrocarbon, it should more or less generate as much ballast as fuel is consumed, maybe generate slightly more ballast than fuel is consumed, mass-wise: the carbon goes away as CO2, but each two hydrogen will bring an oxygen back as H2O. Oxygen has a higher atomic weight, and at least as many oxygen atoms are brought than carbons are lost (at best twice more, if you are using methane). If some water vapour escapes, this could at least compensate for fuel consumption. And now I wonder how much moisture you can extract from air... $\endgroup$
    – Eth
    Commented Jun 3, 2019 at 15:26
  • $\begingroup$ @Eth Depends on the outside temperature. You can extract all of the moisture provided you make the air cold enough. $\endgroup$
    – John Locke
    Commented Jun 3, 2019 at 16:14

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