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IE, atmosphere density, gravity, chemical composition of land, air and sea...

Somewhat of a counterpart to my own Are there *plausible* planetary conditions where jet aircraft just don't have much advantage over propeller-driven aircraft? I've seen How would modern naval warfare have to have developed differently for battleships to still be relevant in the 21st century? which doesn't cover this and Delaying the development of aircraft which partly covers it, but answers are unfocused for my purposes because many of them are more alternate-history stuff that fits the previous question.

No frame-challenge questions. The planet has to have an atmosphere of sufficient density to support fixed-wing aircraft and to hold liquids on the surface. Suitable materials have to exist to build and power warships and aircraft in the first place. IMPORTANT EDIT: Forgot to specify the more basic assumption: The boats and planes are being built by a land-dwelling species, so along with air and sea, the planet must have dry land.

Tristan Klassen
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    I am not knowledgeable enough for a full answer but: Can there be highly advanced jets without GPS (or anything similar)? Attempting precise navigation at Mach 3 with just simple instruments, maps and sight sounds rather reckless to me... – SRMM Jun 05 '20 at 13:58
  • I would say yes (for at least some definitions of "highly advanced jets"), given that the SR-71 first flew in the 60s, but work on GPS didn't even start until the 70s – shay Jun 05 '20 at 14:11
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    Not to challenge the situation or change your mind, but can I ask *why*? Is it just for stylistic purposes or is this a plot device? – Tom J Nowell Jun 05 '20 at 19:18
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    At the moment, this is more an exploratory question than anything tied to a specific work. Developing anything that depended on this before I knew if it was possible would be... backward. – Tristan Klassen Jun 06 '20 at 03:16

6 Answers6

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Water World

On a water world, everything of importance could be under water instead of on top of it. This means you can convert your whole navy into submarines where they can better access and control these under water territories. Your ocean can still have an atmosphere above it where planes could fly, and planes would still be an important technology because they can travel so much faster than submarines, but they would have very limited means to interact with what is happening below the waves. Because airplanes can't see what is under the waves without some kind of sonar device in the water to transmit them information, they would be flying blind more often than not. In fact, the fleets could be so deep that airplanes may not have a good way to engage them even if they did know where they are.

The advantage of ships would be further enhanced if we are talking about an aquatic species instead of humans. Their airplanes will need to carry a lot of water so the pilots can survive out of the ocean. Putting just a single cubic meter of water onboard to fill a small cockpit would add an entire metric ton of weight. That is the weight of the entire explosive payload of many WWII era light bombers; so, meeting the engineering needs of both breathing out of water, and packing weapons that could sink a ship would be much harder.

IMPORTANT EDIT: Forgot to specify the more basic assumption: The boats and planes are being built by a land-dwelling species, so along with air and sea, the planet must have dry land.

Water-Like World

Well this edit derails my original answer... so here is another option along the same vein of thought that should still satisfy your needs. Make the atmosphere much thicker than Earth so that it is LIKE moving in water. Airplanes experience drag in proportion to lift; so, on a planet with a much thicker atmosphere, planes would still fly just fine but much slower. Instead of being able to move in at hundreds-to-thousands of miles per hour, planes would be restricted to speeds that are not much better than ships in the sea, or cars on land.

By taking away their mobility advantage, they would be much easier to pick off with AA weapons, and much harder to deploy when and where you need them. Deployment ranges would be drastically nerfed as well since it would take more fuel and time to cover any given distance.

Apart from slowing planes down, it would also make them more maneuverable. This would make interceptors far more effective such that fighters escorting your fleet could more effectively intercept inbound bombers before they can reach their mark.

The last and perhaps most important point about planes is engagement ranges. In general, larger weapons platforms can support longer ranged weapons. In the current model of air superiority, planes often have less range than ships, but they can use their speed to close into their own kill range very quickly. In contrast, if planes were much slower, then they would have to spend a lot longer between coming into the range of the ship's heavy cannons/ long-range-missiles and being in range to launch their own much smaller, more range limited torpedoes.

Perhaps a better way to visualize this is to picture any modern conflict between a naval bombardment and tanks. Tanks should be able to harm a warship if it could get close enough to shoot back, but in general it can't so the tanks just get wiped out.

Nosajimiki
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    In regards to water in the plane -- Wouldn't a normal scuba system nearly work with just a small amount of water? -- I think the gaseous oxygen just need to be dissolved into the water, with the water being like a medium for oxygen transfer. I'm thinking about something like what Abe Sapien from Hellboy wears. – Aww_Geez Jun 05 '20 at 14:03
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    Organisms designed to exclusively live in the water have all sorts of complications when you take them out of it beyond just breathing, like not being strong enough to lift thier arms or they dehydrate very quickly. Even dolphins die rather quickly if you put them on land and they breath normal air by design. The best you could probably do to make an aquatic life form not need a water tank is a robotically assisted full body water suit, but robotic exosuits alone weigh in at 1500lbs; so, for purposes of a cockpit, you are not necessarily doing your-self any favors. – Nosajimiki Jun 05 '20 at 15:05
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    The most famous submarine-using war - WWII - saw the development of Sonobuoys. Airplanes can just drop a (temporary) sonar array near where they think a sub is operating. Also, what makes torpedoes so effective is the fact that water is incompressible - if your entire sub is filled with water, a hit by a torpedo is likely to instantly kill everybody aboard due to the shockwave. – Clockwork-Muse Jun 05 '20 at 16:00
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    That is a good point about the torpedoes, but frankly sonobuoys were only dropped when planes had a reason to. With all the action happening under the waves, when would a plane ever have a reason to know when to drop one? – Nosajimiki Jun 05 '20 at 18:12
  • I like this - until / unless you have a submarine ship with a very long, flat, unobstructed top deck spending a lot of time on the surface there is no opportunity to even start experimenting with low powered fixed wing flight. (Seaplanes and flying boats need higher power to weight in order to overcome drag on takeoff.) – KerrAvon2055 Jun 06 '20 at 00:07
  • Ouch. This is a frame challenge I forgot to specify against. The assumption / requirement was that boats and planes be built by a terrestrial species. – Tristan Klassen Jun 06 '20 at 03:06
  • @TristanKlassen no worries, it just gives me a reason to post the other idea I had. – Nosajimiki Jun 06 '20 at 03:59
  • Curious. You say a much higher air density will do it, but two other answers say a significantly lower air density will do it. That's what I was puzzling over when I asked my question in the first place: low or high density? I don't know how to choose between the answers. – Tristan Klassen Jun 06 '20 at 20:13
  • Re: your note about flak blast radius: Yeah, but that blast amplification also applies to bombs, making near misses more damaging to ships. And the same drag that slows aircraft also slows bullets, reducing the range for AA guns. It makes it easier for planes to attack above the guns' ceiling. But denser air also means bombs reach a lower terminal speed, less effective at penetrating armor, and giving more time for a ship to move out of the way... – Tristan Klassen Jun 06 '20 at 20:19
  • @TristanKlassen ships are already in water which already maximizes the blast problem, but ships can be made with thick multilayered hulls to resist indirect hits, planes are stuck with very minimal armor/structural options because of weight issues. If you go low pressure, you have to go VERY low before it becomes a significant problem, but then you can not breath. Any thinner atmosphere with enough Oxygen for people or people like aliens to breath in will improve flight advantages, not detract from it. – Nosajimiki Jun 07 '20 at 15:39
  • "planes are stuck with very minimal armor/structural options because of weight issues" In a denser atmosphere, planes can't fly as fast, but they can be built heavier. IIRC, lift is (roughly) proportional to wing area x speed x air density, drag is proportional to frontal area x speed squared. So, take 100atm, comparable to Venus. With the same thrust, a plane can reach 1/10 the speed it does on Earth (this sounds like the range you're thinking of in "not much better than ships in the sea or cars on land"). It also can be 10x as heavy for the same wing area. – Tristan Klassen Jun 07 '20 at 22:24
  • That is a solid point, aircraft could be better armored (or at least get away with much smaller wings). So, perhaps flack would not balance out as I first thought. Even with much thicker armor though, planes vs battleships would still be no different than tanks or gunboats vs battleships. – Nosajimiki Jun 08 '20 at 03:52
  • Wrong. The problem is that planes have the advantage of gravity. With denser atmosphere, they can lift heavier bombs. – Tristan Klassen Jun 08 '20 at 15:56
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Depending on the tech level I would suggest intense weather conditions making flying hazardous, e.g semi-permanent dense fogs or high winds/storms.

Note that any conditions that make early/primitive flight difficult or next to impossible of the above would impact the development of more modern, advanced flight technology unless that is introduced from/by external sources.

However in your 'world' zeppelins might still be a plausible option.

Binyomin
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Mon
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    Under dense fogs please include huge swarms of large bugs. – Willk Jun 05 '20 at 02:28
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    Intense weather conditions would also render ships unlikely to be used. – infinitezero Jun 05 '20 at 17:37
  • This was more or less what I would suggest. Before radar, foggy or cloudy conditions made airstrikes against naval units a no go. Often the target could not be found at all, other times it forced the aircraft into easy AA altitudes. A ship can also handle much worse weather than a plane. A moderate storm will bring down a plane, but it's just another day at sea for a ship. – UrQuan3 Jun 05 '20 at 18:08
  • Zeppelins are actually much more sensitive to weather than planes. Historically, the majority of airship crashes were caused by weather. – Kofthefens Jun 06 '20 at 02:07
  • Yes but it also depends on whether the planet has extensive high mountain ranges and the types of storms encounterd - and the lifting gas used. Ignore hydrogen because helium is more plentiful than it is here on Earth or go with hot air or even at a higher tech level vacuum balloons and Zeppelins become a lot more reliable/safer. – Mon Jun 06 '20 at 09:30
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I'd suggest a low-density but high-oxygen atmosphere would make it hard to develop safe flight, never mind weaponise it.

In order to attack a ship with an aircraft, you need to do two things: first you need to fly a plane to the target, then you need to drop (or propel) something explosive onto it. Explosives are heavy, and they're also dangerous to handle. A thin atmosphere relative to the surface gravity will make it hard to develop planes with high carrying capacity, and those planes that do exist will have to be larger and more ponderous, making them easier targets for AA weaponry.

Meanwhile a high-oxygen atmosphere makes it very dangerous to use rocket-propelled missiles, as well as making the whole development of flight itself much more prone to accident. This will hold back specifically the development of long range air offence, which was what really did for naval power, and keep the belligerents in the position of trying to overfly the ships with bomber aircraft and drop gravity-propelled (maybe guided, but still free-falling) bombs on them, while being exposed to AA fire. All in all a much more balanced confrontation.

Stephen
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    This was my first thought too, but early anti-ship weapons were not rocket propelled missiles, but propeller driven torpedoes that could be dropped into the water and would then "drive" into the ship. Also, reducing atmospheric density reduces drag meaning your airplanes can fly faster while still getting the same lift. If you want to slow down an airplane you need to increase the total density of the atmosphere. – Nosajimiki Jun 05 '20 at 13:46
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    @Nosajimiki-ReinstateMonica The airplane flys faster in less dense air if it can fly at all. Less dense air makes it harder to fly in the first place. Not much drag on the moon, but you won't see many airplanes there. – gmatht Jun 05 '20 at 15:46
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    .... rockets had been used safely (relatively) on wooden ships for multiple centuries by the time anybody attached them to an airplane. Part of the whole weapon development process is making sure that your weapon is safe for handling and storage. A sufficiently oxygen-rich environment doesn't prevent development of airplane-launched rockets, it mucks with all development of explosives/incendiaries. It might provide more issues with the engines for planes, since the most likely initial fuel is gasoline (and early planes were firetraps due to construction and maintenance). – Clockwork-Muse Jun 05 '20 at 16:08
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    @gmatht Not only that, the same plane must fly faster in less dense air to generate the same amount of lift. Airflight will be much harder in this world, but what planes exist will be faster, not slower as claimed in the answer. – Charles Staats Jun 05 '20 at 16:13
  • @gmatht Stephen's answer suggests an Oxygen rich environment; so, there must by that assertion be enough air to fly reasonably well in. As for lift, it does not take any more energy to maintain a lifting speed in a thin atmosphere since the drag and lift scale proportionally. All these problems can be solved by just scaling up your flap and propeller surfaces areas. – Nosajimiki Jun 05 '20 at 18:07
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    I would recommend against this one, though it is an interesting idea. At very high oxygen levels, many things burn that do not burn otherwise. Steel wool, humans, etc. I'm having trouble finding hard numbers for safe percentages. – UrQuan3 Jun 05 '20 at 18:28
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    It's also worth pointing out that, while you will need to fly faster for the same lift, the speed of sound will be lower. Mach 1 in pure oxygen is about 90% of Mach 1 in normal Earth atmosphere for equal temperature. The increase in required flying speed and decrease in Mach 1 will result in a significantly smaller flight envelope for subsonic aircraft. You will hit the Drag-Divergence Mach Number at a lower airspeed, due to Mach 1 being lower. – reirab Jun 05 '20 at 21:18
  • @Nosajimiki-ReinstateMonica Note that, while drag and lift scale proportionately, less dense atmosphere usually also means taking a hit in the propulsion category. If you've already canceled out drag losses with lift losses, then you don't have it left to cancel out with thrust losses, so you still need a bigger plane with bigger engines to get the same weight off the ground, even if it does fly faster in the process. – Matthew Najmon Jun 06 '20 at 00:17
  • Also, being fast is good for a military ground/water-assault aircraft, up to a point, but that "up to a point" part is important. If the speed necessary to maintain lift in your thinner atmosphere is too high, it'll start requiring some pretty sophisticated targeting systems to hit the ship you aim at, rather than hitting the ocean a dozen meters, a mile, or even many miles (depending how low the atmosphere density, and thus how fast the flight) before or after overflying that ship. – Matthew Najmon Jun 06 '20 at 00:24
  • @reirab You seem to be missing my point. A plane's cruising speed is where air resistance is equal to thrust. If the air only has half as much resistance, then you can get up to moving twice as fast with the same amount of thrust before those two forces equalize. It is being able to move faster off of the same thrust that gives you the needed boost to your lift. – Nosajimiki Jun 06 '20 at 07:53
  • @Nosajimiki-ReinstateMonica Not sure if you meant to tag me or not, but I understood your point. I was just pointing out that the flight envelope (at least for subsonic aircraft) is being constrained on both sides by this situation. In practice, Drag-Divergence Mach Number is the limiting factor for, say, airliners, especially in the thinner parts of the atmosphere. While the speed you need to maintain flight goes up, DDMN goes down. Of course, the other issue is that you won't actually have the same thrust. Thrust drops with decreasing density for air-breathing engines. – reirab Jun 06 '20 at 08:09
  • @Nosajimiki-ReinstateMonica This also means that you will need very long runways and landing gear/wheels capable of very, very fast speeds, but that's a solvable problem. – reirab Jun 06 '20 at 08:14
  • I agree with that in a general since, but in a high-oxygen, low density atmosphere, that problem is solved. A purely low density, low oxygen atmosphere has all sorts of issues, my argument is just against the atmosphere specifically as described here. – Nosajimiki Jun 06 '20 at 14:16
7

Less dense air, for two reasons:

  • It would make flight harder, similar to Mars
  • It would make laser weapons more viable. They would probably be good at targeting fast-moving aircraft, but would need such power supplies that they can used on ships, but not on aircraft.
Mark
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  • I was starting to get disappointed as I read through answer after answer and didn't see this xkcd linked. Thank you for not letting me down. :) – reirab Jun 05 '20 at 20:45
  • Shouldn't less dense air automatically also mean less gravity or a completely different composition of the air? Otherwise I don't see how you would be able to get less dense air. – D.J. Klomp Jun 05 '20 at 21:53
  • @D.J.Klomp, less-dense air just means there's less air filling up the same amount of planet. Earth and Venus have similar gravity, and not-too-different atmospheric composition, but wildly different air pressures. – Mark Jun 05 '20 at 22:10
  • @D.J.Klomp Besides less gravity or a different air composition, the other major way to get less dense air is to have a less-tall stack of air above it. The pressure of an atmosphere at a given place is a roughly function of the weight of the column of air directly above it. Weaker gravity will make that column weigh less, but so will the column having less stuff in it. So, you could just have a planet with less atmospheric gas per square mile of surface. – Matthew Najmon Jun 06 '20 at 00:13
  • You say "low density", but another answer says "high density". Low air density limits a plane's payload. High air density limits a plane's speed. Which is more important? – Tristan Klassen Jun 06 '20 at 20:25
3

Other guys put decent answers, so I do some less decent:

  • Your people could have low G tolerance, making maneuvers in jet speeds above 1MACH very stressful for body, maybe only few people in your world can be trained to handle those Gs so jet pilots would be extremely rare and most of pilots would fly classics. This could be done perhaps by lowering gravity of your planet and your people are less sturdy.
  • Big sea monsters which would allow only big ships to pass - only battleships and carriers could pass without being attacked, smaller ships are being often attacked by the sea monsters or cannot carry effective defense against them. I know this is not about flying, but I had idea to force the big ships as only way to fight on sea.
Prahara
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No fossil fuels. A ship could run on wood (with reduced range /speed). But without readily available fossil fuels, the development of internal combustion engines would be greatly hampered, and without those aviation wouldn't really exist.

loreson
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    Short-range aviation works just fine off alcohol fuels, and even short-range flights are considerably longer than naval artillery range. – Mark Jun 05 '20 at 22:08