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This is a self-contained follow-up to a different previous question. Based on the answers I got there I've taken a different approach to my original idea, and now want to try using antigravity on the inside of the balloon instead of the whole ship. Two new questions came up while I was thinking through the concept.


Background: Airships are a staple of fantasy fiction, but in the real world airship design is drastically limited by the amount of lift you can get out of a balloon. I'd like to create a world where compact and elegant airships of steampunk and fantasy are made possible with the addition of just one magical element: an engine that produces a limited kind of anti-gravity. After that, I'd like to keep the physics science-based, with the goal of producing an internally consistent world that still has room for awesome airships.


The Question: I have a handwavium anti-gravity engine. When I put this engine inside a large balloon, and start it, it reduces the effect of gravitational pull (weight) on all the air in the balloon to the point that it becomes functionally as buoyant as a vacuum balloon.

  • Q1: There is mass inside the balloon, but it now has no weight. Will the air pressure outside still cause the balloon to collapse?

  • Q2: If the balloon is breached, such as by a bullet, what will happen? There's no lift gas to escape; new air entering the chamber loses weight. So will the balloon keep flying until I rule the magic "reaction chamber" no longer works? Would there be any other effects?

My physics background is confined to Google, so if anything here is muddled let me know in the comments and I'll fix it.

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    $\begingroup$ The gas in a balloon is just about at atmospheric pressure; a bullet hole is not immediately dangerous for a balloon, because the gas will mix with air very slowly. And you do realise that the lifting power of hydrogen is only about 10% less that that of a weightless gas, and it neess to fancy anti-grav engine? $\endgroup$ – AlexP Aug 26 '17 at 22:11
  • $\begingroup$ @AlexP: Yes, but that 10% seemed to go a surprising way when I started calculating rough balloon sizes. (Plus, air is substantially less explosive.) But I'll be sure and check my work and make sure I'm not barking up the wrong tree. I should note that anti-gravity magic is going to be canonical to the setting in other ways no matter what, so it would be almost strange not to use it for the airships. $\endgroup$ – Era Aug 26 '17 at 22:14
  • $\begingroup$ The cubic root of 1.1 is 1.03... Just saying. $\endgroup$ – AlexP Aug 26 '17 at 22:35
  • $\begingroup$ @AlexP: Worst comes to worst I can experiment with "charging" parts of the airship with antigravity to reduce the weight ... unless that leads to it being blown about like a kite of course. o_O I better go and google wind resistance... $\endgroup$ – Era Aug 26 '17 at 22:42
  • $\begingroup$ You are not gaining much by reducing the mass of the gas if however you reduce the gas of the ship you can make them bigger. Say you had a fantasy wood that is as light as balsa but as strong as hickory. In the game dwarf fortress the fantasy metal mithril was as strong as steel but had the same density as styrofoam. with those you could build a huge airship, or even better a huge rigid airship. $\endgroup$ – John Aug 27 '17 at 15:53
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Let's have a look at what you're actually doing here and see if that helps.

Your 'anti-gravity' engine is lowering the weight of your airship by working on the gas in the balloon. This in turn alters the density of the airship as a whole, making it 'lighter than air', so it floats.

So the answer to question 1 is pretty simple: The gas inside the balloon still obeys the ideal gas law, which doesn't really care about weight, only pressure, volume, the number of molecules of gas and the temperature. The dynamics of your balloon are the same as they ever were. Please note that this means you'll have to watch out for your balloon exploding as you get to higher altitudes and the difference between internal and external gas pressures becomes greater. I'll say more about this later.

Question 2 depends upon the specs of your airship. If you have a blimp (with a non-rigid balloon) then a bullet hole will lead to your lift gas rushing from the inside of the balloon to the outside and your balloon collapsing, along with a commensurate drop in lift. If you have a zeppelin (with one or more balloons inside a rigid structure) then it doesn't matter if your balloons are already open to the rest of the atmosphere: the density of your airship as a whole remains the same regardless of what's actually in the balloons.

Which brings me back to point 1: If your ship isn't rigid and your balloon is sealed it will start to expand as you go higher into the atmosphere. This will lead to a greater volume of 0 weight gas, or to put it another way: a decrease in the density of your airship as a whole. This would continue up to the physical limitations of your balloon (like a weather balloon).

If your ship (more specifically the anti-gravity field) is rigid then just make sure that air can't flow completely freely. Air from below will push up into the 0 weight zone and be accelerated upwards by more air entering the 0 weight zone from below. Your airship won't function if air is allowed to enter from below the balloon and escape from the top of the balloon, but would work just fine even if the bottom of your 'balloon' were reduced to the same consistency as a string vest. It might get a bit windy depending on the exact boundary effects of your anti gravity generator, but if you really wanted to this scheme of generating lift could work with a parachute.

Which is counterintuitive and trippy as hell.

ADDENDUM: The density of gas inside the zero gravity envelope will still have an effect on the 'jerk' or rate of change of acceleration of the ship. This is worth considering if you plan to turn the generator on and off as a method of altitude control.

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I hate to say it, but this is a lot of effort to get high-altitude airships.

Anti-gravity (assuming it wasn't fictional) works on mass. Where usually two balls attract, now one or both repels because the nature of the mass has changed. Gravity is (from a certain point of view) directional. So something as ethereal as air would be repelling all the air around it and you probably wouldn't get any lift. Worse, what would keep the engine from working on the infrastructure of the cabin? Why use a balloon at all?

Lift in airships is created by density. The density of the gas inside the balloon is less than the density of the air around it, so it rises. It will rise until the density of the outside air is equal to the density of the gas inside the balloon. What you need is something that makes the gas inside the baloon less dense.

You might be getting to the point where you're overfocusing. The effort you're putting into making the airship work will become a distraction in your story. What I'd suggest is that the magical element be the gas. Gas so low in density that it can achieve the altitudes you're looking for.

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    $\begingroup$ What matters is density only because everything responds to gravity the same way. If the equivalence principle no longer holds true, and you can alter the gravitational "charge" of some material then that simplification no longer works, and you have to fall back on calculating buoyancy directly from the difference in the weight of the balloon vs. the weight of an equal volume of air. $\endgroup$ – Logan R. Kearsley Aug 26 '17 at 23:59
  • $\begingroup$ You can't reduce the weight of the gasmix much more without going into the negatives (at which point a denser mix provides more uplift again) - alas that's what I'm doing in my world, preventing too dense a mix by not having any suitable compression technologies $\endgroup$ – dot_Sp0T Aug 27 '17 at 8:07
  • $\begingroup$ @dot_Sp0T: Hi, your comment made me curious - are you talking about negative weight? (Not negative mass.) Is there any physics for how that would work? $\endgroup$ – Era Aug 27 '17 at 13:42
  • $\begingroup$ @Era why not join me in the Worldbuilding Chat and we can converse on the subject? $\endgroup$ – dot_Sp0T Aug 27 '17 at 14:02
  • $\begingroup$ @dot_Sp0T: Be glad to, never used the chat room before. Should I create a new room? $\endgroup$ – Era Aug 27 '17 at 14:09

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