I have a setting where the surface of a planet is slowly being terraformed by a hostile force. The ground is slowly changing into a swampy/sludgy liquid like consistency. Stepping or driving through it is extremely hazardous and practically impossible beyond a few meters.

To fight on this new battlefield, hover tanks were created using some of the hostile force's own technology. By compressing a chamber of special particles and sludge, the chamber itself rises/levitates off the ground. Effectively counteracting the heavy weight of an object. By replacing the tank treads with a multitude of these chambers, tanks can effectively hover. Pitch and angles can be controlled by reducing the compression of certain chambers in certain areas where tracks used to be. This allows them to cross over the sludgy areas and normal areas without issues seamlessly. The tanks themselves are casemate/assault guns and use rocket thrusters to maneuver around.

If the compression chambers essentially replicate anti gravity/hovering, then why don't my tanks simply fly or hover too high off the ground?

In effect I want the hover tanks to follow the terrain. I don't want helicopters or a situation where the hover tanks for example can cross a bridge with a giant hole in it. What should happen is that it should sink like a rock since there's no ground underneath it.

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    $\begingroup$ Just never fire your tanks main gun, otherwise you get a spectacular demonstration of the newtons third law. "Hey Bob did that tank just flip over and smash in to the ground upside down?" "a-yep". $\endgroup$
    – John
    Commented May 23, 2023 at 20:45
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    $\begingroup$ Websearch "ground effect flight" $\endgroup$
    – keshlam
    Commented May 24, 2023 at 16:43
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    $\begingroup$ @John Instead you just have to have to fire an equivalent shot in the opposite direction at the same time to counteract it, and then you have all of the fun of trying not to shoot your allies! $\endgroup$
    – Kevin
    Commented May 24, 2023 at 16:58
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    $\begingroup$ Because then they'd be called "fly tanks" $\endgroup$
    – Richard
    Commented May 24, 2023 at 18:33
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    $\begingroup$ "Recoil-less Rifles" have been a thing since at least WWII - and no, I'm not talking about a small-bore, hand-held weapon, but a tank-killer type. The rest of this sounds exactly like a Hovercraft. $\endgroup$
    – MikeB
    Commented May 25, 2023 at 7:20

12 Answers 12


I actually worked on a contract developing a hover tank for the US military, and it was produced and worked. I have never seen it in production, but it is possible.

Basically something like a jet engine in the tank, very powerful, with skirts that can be deployed around the tank and treads. So it works just a like a hover boat.

Hovercraft, Wikipedia

The purpose of this tank was to cross swamps, rivers and relatively short distances across lakes or whatever. Except the deployable and retractable skirt is steel, not fabric.

It doesn't fly for the same reason hovercraft don't fly; the ground effect of the fan isn't providing enough thrust to lift the boat/tank through the air on its own. When focused and contained by the skirt, it can lift it a few inches off the ground/water, but any higher than that the force is dispersed too widely in the air to lift any further.

So it's a balancing act; and the boat/tank doesn't have any wings or other structures to stabilize or guide flight anyway, it would immediately tip and fall. You produce just enough power to lift, no more.

Compare your hover tanks to hovercraft on swamps and water, readers will intuitively understand and accept that hovercraft cannot fly any higher than a few inches. You don't need a lot of exposition and invention to do this, working hover tanks were built decades ago.

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    $\begingroup$ +1 for ground effect, and for pointing out that to any reader, if it don't have wings, it ain't gonna fly. This may be less true in these days of drones, but nobody expects heavily-armored drones, because lift ratios. $\endgroup$ Commented May 23, 2023 at 18:30
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    $\begingroup$ @DewiMorgan Thanks. Drone rotors and propellers are basically rotating wings! But like a helicopter, you need at least two rotors to be stable; without the back rotor countering the rotational force of the main rotor, the helicopter is just going to spin, it isn't going to fly. $\endgroup$
    – Amadeus
    Commented May 23, 2023 at 19:10
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    $\begingroup$ @DewiMorgan The Apache attack helicopter has better armor than most armored fighting vehicles (AFVs). It is for all practical purposes a flying tank. While it's armor is ofcourse not as good as a main battle tank, it can still stop all sorts of small arms and a wide range of anti-material riffles and heavy machineguns. $\endgroup$
    – Nosajimiki
    Commented May 23, 2023 at 19:20
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    $\begingroup$ Apaches were shot down multiple times in Afghanistan and Iraq from machine gun fire. $\endgroup$ Commented May 24, 2023 at 7:27
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    $\begingroup$ @Nosajimiki Not that the Apache isn't a fine vehicle but when one talks of a flying tank I think most would think of a Hind (which to make the ridiculous possible, costs twice as much as an Apache). Back to the OPs question, I'm glad this answer is here because I was thinking isn't what the OP is describing a hover craft? I have a hard time with suspension of disbelief if someone uses some amazing technology to produce what is most definitely inferior to modern technology without explanation as to why it wouldn't work. $\endgroup$
    – Quaternion
    Commented May 24, 2023 at 14:17

Each chamber creates a conical force field that repulses matter under the tank allowing it to hover over the ground/sludge/water. The field quickly declines with range so the tank has to hug the terrain. The field cone is wide enough to keep the tank from sinking into the sludge.

It is basically an advanced version of air cushion. I suppose alien tech is just more convenient and effective. Rocket thrusters seems to be unnecessary.

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    $\begingroup$ The strength of an active radar drops a x^4 rate. Because the emitted signal is conical and drop quadratic with distance. But so is the reflected signal. Creating a double quadratic drop-off. If the "special particles" bounce of the ground and generate a force that way the effect will be very sensitive to distance. $\endgroup$ Commented May 23, 2023 at 14:39

In addition to the technical reasons given in other answers (about the tanks being more like present day hovercraft that require a skirt), there's a practical reason: your tanks are still slow.

LEGS: Do you know what we call flying soldiers on the battlefield?

TINO: Air support?

LEGS: Skeet.

-- Schlock Mercenary

Tanks are relatively slow, even yours - they might be able to reach 60 or 70 miles an hour (~95-115 km/h), but that's well slow enough for a person with a shoulder-mount RPG to take aim and score a hit fairly often. Another tank, which might have computer-assisted aiming, would have no trouble in shooting it out of the sky at those speeds. Compare to current military jets that can hit ~1,300 mph (~2,100 km/h) or even civilian jets that fly around 500 mph (~800 km/h).

But: your tanks can "fly" over terrain previously though impassable, opening up new attack vectors. They can also evade some fire by moving in "impossible" directions (ie., sideways, without having to turn the body).

Tanks are relatively slow because they're heavy: they carry heavy armor and weapons. Lightening the armor or armament enough to make the tank faster would mean that it was no longer useful in the niche that tanks fill: heavily armored, mobile artillery. They would very quickly become planes - quite useful in their own right, but filling a wholly different niche.

Since tanks are heavy, they can hide in bogs and swamps, possibly even up to the turret. Being semi-submerged can aid in stealth and can reduce fuel consumption (tanks don't float, but they're more buoyant in a bog than in the air).

Compounding the "skeet" problem is that flying higher would require additional life support systems (even just supplementary oxygen), adding even more weight. It'd also require adding more weight in the form of crash-mitigation safety systems (ejection seats, etc.). And, of course, don't forget the tyranny of rocketry (and flight in general): as you add weight, you need to add fuel to carry that weight, which is even more weight you need to carry, so you need more fuel to carry the fuel.

By the time you've converted your tanks into high-flying machines, you've either turned them into fighter jets or highly-explosive skeet. Since the former already exist and the latter is bad for morale, hover tanks are simply not used for flight above a couple of feet off the ground (and, thus, aren't designed to be used in that way, completing the circle).

  • $\begingroup$ If the tanks are slow then this answer makes some sense. But it moves the problem onto why the tanks would be slow. Once they are hovering they (presumably) don't get friction from the ground (this could be handwaved in as a feature of the hover-tech). So for a hovering thing heaviness doesn't automatically imply slowness. $\endgroup$
    – Dast
    Commented May 24, 2023 at 12:25
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    $\begingroup$ @Dast: tanks in the real world aren't slow because of friction with the ground so much as they're slow because they're heavy. Changing a tank's velocity - even if it's hovering - requires an amount of energy proportional to mass (F=ma), and energy = mass (be it as fuel or as batteries). The faster they get going, the more energy they have to have on board to change their direction. Their use also doesn't require them to be particularly fast: they just need to get into place then fire over the horizon. And, of course, they don't want to run into a tree-shaped bollard at 500 mph. $\endgroup$
    – minnmass
    Commented May 24, 2023 at 13:58
  • $\begingroup$ Good comment. But I still think that friction (be it from the ground, or just the internal friction of the treads against themselves and the wheels driving them) is still obviously the limiting issue for the speed of real tanks today. If getting a modern tank to 200mph was simply a matter of putting one more gear in the gearbox and burning a little extra diesel to get up to speed then they would obviously just do that (they would do the same for cars by the way). Not for the battlefield, but if they could power down motorways at 200mph they could move between battles so so fast. $\endgroup$
    – Dast
    Commented May 25, 2023 at 9:11
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    $\begingroup$ I did the calculation for fun. The kinetic energy of a 48,000kg tank moving at 500mph is equivalent to the energy provided by 242 litres of petrol burned at 15% efficiency. This means that in an ideal zero friction environment the M1 Abrahams would only need to burn 10% of its fuel to reach this 500mpn speed. I think this makes it clear that friction really is the limiting factor, not the weight. $\endgroup$
    – Dast
    Commented May 25, 2023 at 9:20
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    $\begingroup$ The most effective countermeasure against a 500 mph tank would just be to make it fly -- build a ramp in some unexpected yet convenient place and watch the tank rise, tumble and crash $\endgroup$
    – syck
    Commented May 26, 2023 at 9:58

They emit powerful microwaves

Microwave ovens are fascinating things. If you leave one empty and run it for several minutes (which you shouldn't do), you'll notice the inside remains relatively cool to the touch, but you'll notice that if you put in a cup of water, it becomes scalding hot pretty quickly. Microwaves cause water to resonate and heat up very quickly; so, by using your hover chambers, you are blasting the sludge/water under you to heat it up creating steam which provides a reaction force pushing up into the underside of your tank.

What makes this fundamentally different than other hovercraft is that the engines in a traditional hovercraft can be scaled up a bit to give you a helicopter and thus flight because the reaction mass comes from the air or your burning fuel, but your microwave generators can't heat up or create a reaction force with the air; so, just making a bigger one will not help you fly. This means you can in theory have engines that are practical for carrying a heavy tank long distances over water and mud that is way to heavily armored to be able to fly using traditional propulsion, but be useless for making any sort of other heavy armored aircraft by sacrificing just a bit of speed, range, or armor as you could with a jet, fan, or rocket powered hovercraft.

enter image description here

Why use an alien Hover Tank instead of existing Human Technology?

Frankly, given your setting, humans have already done a good job of designing a wide range of vehicles for traversing swamps which means that any stollen alien technology has to beat what humans can already do. Many Armored Fighting vehicles are already designed to operate in swamps, but they all have their drawbacks.

Swamp Boats: These flat bottom boats propelled by above water fans are great in swamps because they displace so little water they can ride on just a couple of cm of water and even push their way through mud and water foliage. They are also pretty fast; however, these do not scale well into armored vehicles because they intrinsically need a low weight to surface area ratio to work. They also cant drive at all on land; so, if you have above water banks to contend with you have to go around.

Hovercraft: These are relatively fast on both ground and water, but actually not good in swamps. The soft body air cushions get caught on branches and plants; so, while these are good for driving up onto a sandy beach-head, they have limited utility in a swamp.

ATVs: ATVs are vehicles designed to float when they go in the water. There are various Armored Fighting Vehicles designed to cross rivers and swamps, but their weight and design makes it very hard to make one that can do this quickly. A floatable tank might get you where you are going, but not very fast. They also risk getting stuck in mud more than your other options.

Helicopters: Apart from the OP not WANTING a "flying tank", helicopters dominate their top with a giant propeller that makes it hard to mount a tank like weapon system. Tanks need to be fast for deployment, but their main function is to hold ground. So when you land a helicopter to conserve fuel, it does not leave you anywhere for a heavy top turret.

Quadcopters: Unlike a helicopter, these do have large top spaces you can put a turret on for holding ground, but they are much harder to armor than a helicopter. Armored attack helicopters like the US Apache have small, very well armored cabins designed to protect the cockpit and engine. However, Quadcopters have much more spread out systems which make armoring it much harder. Helicopter's already have limited lift ratios making heavy armor difficult, a quadcopter is even that much harder.

So what does the microwave hover tank bring to the table?

The microwave thing is a limitation to constrain the real features of the engines that we are handwaving into the setting. The feature is that this system allows one to fly at very low altitudes. Because it flies a bit above the surface, it has no ground resistance meaning it can move faster than any boat or tank for highspeed deployment. It will pass just above the land and water, pushing its way through light foliage just like a normal tank, but while moving at helicopter like speeds. But unlike a helicopter, its propulsion is down facing meaning you have room on the top for a big turret so that when you get to your deployment zone, you can set down and hold ground just like a normal tank.

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    $\begingroup$ @JosephSible-ReinstateMonica Now that you mention it, I'd really hate for someone to test this theory and accidently catch thier microwave on fire. Disclaimer added. $\endgroup$
    – Nosajimiki
    Commented May 24, 2023 at 1:56
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    $\begingroup$ Major problem with this proposal: if your tank is forced to stay in position for more than a few minutes (perhaps even a few seconds), you may find your hover tank is now an immobile pillbox because there's insufficient moisture left beneath it to provide pressure. $\endgroup$ Commented May 24, 2023 at 7:32
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    $\begingroup$ I like this answer because a tank should absolutely leave a trail of superheated destruction everywhere it goes. That's what tanks are for! $\endgroup$
    – codeMonkey
    Commented May 24, 2023 at 16:04
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    $\begingroup$ @StarfishPrime Any sufficiently advanced technology is indistinguishable from magic... when dealing with alien tech in particular, this is important to keep in mind. Just because we human don't know how to turn lead into a fissile fuel source that decays in a way the emits microwaves instead of gamma rays does not mean it can't be done... it just means we don't have the foundational science to do it with. $\endgroup$
    – Nosajimiki
    Commented May 25, 2023 at 14:30
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    $\begingroup$ @Katy No offense taken. You were very respectful, and I think it's a valid concern. Generally I would learn towards a more traditional hovercraft myself, but the OP here has a specific problem in a specific setting, and sometimes that takes something a bit outside of the box to meet all of an OP's criteria. $\endgroup$
    – Nosajimiki
    Commented May 30, 2023 at 18:51

Don't explain the effect, because it is basically magic and therefore any explanation is at best technobabble and isn't likely to add anything to your story. Instead, just handwave in some hovertank "laws"... perhaps power required to maintain the hover is proportional to the cube of the height above the ground or something. If you want to get more technobabbly, then maybe hover altitude is proportional to hover cell compression and when compression exceeds X then the hover cell material undergoes some (possibly irreversible) phase change and ceases to hover and you end up flying more like a brick than a bird. That gives you a maximum hover altitude which doesn't even have to be proportional to weight if you didn't want it to be.

I don't want... a situation where the hover tanks... can cross a bridge with a giant hole in it. What should happen is that it should sink like a rock since there's no ground underneath it.

  • your tanks are floating above the ground, which leaves them in a fairly low-friction medium
  • your tanks use some form of jet propulsion, which potentially allows very high speeds
  • if they go fast enough and get a bit of air (maybe you build a little kicker ramp next to the hole) then they're gonna fly clear over a gap determined by their speed and air resistance, because conservation of momentum is a thing.

Designing the things to be a lifting body or some flavor of ground-effect vehicle will incrase the distance they can fly (or at least, glide aggressively). Shooting canyons and rivers will be practical for some or all of your vehicles, as you've currently described them.

There are ways in which you can tweak the setup, for example the "hover" effect is somehow "sticky" to the ground below and so you can't exceed some speed without breaking the hover effect, or it has both an attractive and a repulsive effect so if you try to fly over a hole you get sucked into it (which would also require some inconvenience around turning it off and back on again) and so on. That would also make your hovertanks drive more like wheeled vehicles rather than hovercraft, though I guess its up to you to decide whether thats a benefit to your plans or not.

Helicopters can at least be "fixed" with the aid of commonplace and effective antiaircraft weaponry. This will also make the tanks more practical, too.

  • $\begingroup$ Even some normal tanks can move quite fast and can end up catching some airtime when going over a ridge. $\endgroup$ Commented May 23, 2023 at 20:53

Military hovercraft such as the BH.7 existed, briefly. They would probably have carried rockets rather than guns, because these are lighter and have less recoil. They were useful in flat, estuary terrain where boats and trucks might have to take a much longer path.


A simple and easily overlooked solution that should be fairly elegant: sludge + power = "repulsive sludge" (as if it weren't repulsive enough already). Stick a container of sludge underneath something and run power through it, and it will repel (and be repelled by) non-powered sludge. As long as the effect falls off with distance, bingo: instant SEV (Sludge-Effect Vehicle), with the same basic behavior as a hovercraft or other ground-effect vehicle, or a magnet suspended above a superconductor.

Compression allows "shaping" the sludge, and thus the details of how the repulsive effect is focused, without having to vary the power (which is going to be a concern; non-powered sludge next to powered sludge on the same vehicle frame would try to pull it apart). Note that this only works because the sludge is semi-liquid; you'll end up with a 'wake' outward from the repulsion effect, and shifting where you 'push' against that is one way to get propulsion. Note that if you're doing at least quasi-hard SF you might want to consider speed limitations induced by how liquid the sludge is, and how much of a wake things would be throwing upward / outward.

As others have noted, you're going to have problems if you fire any sort of heavy weapon which is not dramatically out-massed by the vehicle itself, or very carefully aligned, but depending on the exact details of the effect you might be able to 'brace' against the sludge and transfer a fair amount of the recoil force to it through the frame.

An interesting side effect is that if this scales sufficiently well, the only limitation on effective ship size is how much sludge you can "power up" and how much of a ground cushion is required; your actual vehicles would have to deal with an interesting hybrid environment that isn't entirely like land or water based combat, so there might end up being a lot of experimentation to figure out what works and what doesn't, since neither a traditional tank nor a traditional gunboat are likely to be ideal fits.

  • $\begingroup$ Options for dealing with recoil - 1) use a recoilless weapon, with a large backblast. 2) Use a weapon based on LASER or MASER or Alien tech or 3) temporarily increase the Sludge Effect behind the vehicle so the recoil back and SEV forward somewhat cancel each other. 3b) Invert the sludge effect with some probes into the ground so it "pulls" the tank forward at the instant of firing. 4) Rockets, like an RPG that take a small charge to throw themselves up and then ignite a sustainer charge that is the real propulsion. $\endgroup$
    – Criggie
    Commented May 24, 2023 at 5:23
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    $\begingroup$ Indeed. I intended to list both "recoilless" (rockets or anything else that doesn't distribute recoil to the frame) and energy weapons but utterly spaced on it. I did cover the 'brace against the sludge' idea, although I didn't consider the possibility of an attractive force to the sludge — the mental model was mostly based on a self-balancing force equation, which is simplest when it falls off with distance and is strictly repulsive, but it is an interesting notion. $\endgroup$ Commented May 24, 2023 at 16:34

The tanks could use some variation of Magnetic Levitation that is only possible due to the properties of the sludge itself. Since the levitation requires the properties of the sludge to work, the farther away from the sludge, the less effective the levitation would be.

This would require a few different variations of vehicles as sludge travel would require completely different mechanisms than ground travel. A sludge-travel (ST) only tank with internal ST engines would need to be deployed to sludge areas in other ways, likely by other vehicles. A hybrid ground-travel (GT) and ST vehicle might be less maneuverable and/or less equipped due to needing to have both GT and ST engines.

Another possible issue would be sludge displacement. If the ST tanks are too heavy, then they may need to be constantly in motion or else they would start displacing the sludge underneath them causing them to sink, or require them to deploy emergency buoyancy devices that are easily targetable and could cause maneuverability issues. Lighter but less equipped tanks could be light enough to float on sludge while not moving, but would also be more fragile.


Inverse square law: assume that your (whatever) force lifting the tank off the ground drops off as the square in proportion to the distance between the emitter and the surface below it.

Say that for a given power input you have enough force to lift a 60 tonne tank a meter off the ground. At two meters, (twice the distance from the surface), you only have a fourth of the lifting force, meaning you can only lift 15 tonnes. At three meters (three times the distance), you're down to 6.7 tonnes, and at four meters, 3.75 tonnes.

So, to go high, you either significantly increase the rating of the power plant to feed more energy into the lifting system (which means increasing the mass you need to lift), or you lessen the weight, but even then you quickly get into diminishing returns. If the powerplant of your tank puts out enough force to lift it a meter off the ground, at ten meters it will only be able to support 600 kilograms. Even ignoring the mass of the powerplant, that's about half the mass of VW Beetle. Your tanks ain't gonna fly. You can alter the numbers, of course, to get the effect you want.

There's another interesting effect: from your description, propulsion is by redirecting some of the lifting force in a given direction, basically to "push" the vehicle. Well, if the lifting force works as I described above, you also run into the issue that the amount of force pushing at an angle on the ground to move also goes down the further it is off the ground. In other words, the higher the tank the less force can be used to push it along, meaning it has lower acceleration, braking, and turning ability. In essence, it will wallow around in the air, making it a significantly easier target.

The end effect is that you might have a tank that has an optimum cruising height, but enough engine capacity that it can go up slightly to get over some obstacles but at a cost of maneuverability, not much different from current tanks where they have the capacity to gun the engines and put more torque into climbing steep slopes at the cost of speed. And it's not something you want to do all the time due to engine wear and fuel usage.


The sludge may repel the sludge as if it were a magnet. But like a magnet, if the tank turns upside down it falls hard. So hard that sometimes a falling tank sinks in the mud and drowns the crew. Only if they are close to the ground the tanks are stable. One meter above the ground is already risky.

This could give you a plot twist, some daredevils sometimes perform dangerous maneuvres by passing high over some obstacles.


The tanks need a continuous supply of fresh sludge for their sludge ballasts

Since the sludge is on the ground, the tanks must stay close to the ground to collect fresh sludge. Installing large resevoirs of sludge on the tanks would make them too heavy to even hover.

Even if the hostile force can achive flight using sludge ballasts — for instance because they don't need a continuous supply of sludge — the tanks' designers weren't able to reverse engineer or adapt the technology perfectly, so the tanks have limitatiations the hostile forces' vehicles don't.


I have something similar for my world. The answer I came up with is two-fold:

  1. power consumption goes up exponentially with hover height
  2. it's quite difficult to maintain balance between all the 'hover nodes' to keep the craft level, especially when subject to winds at increasing heights

So theoretically, you could 'fly' the craft for some time but you'd burn through your power reserves fast, and you'd need an absolute hotshot pilot who can prevent the craft from tipping over.

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    $\begingroup$ Surely you'd only need a hotshot pilot if it was the 60s, or something. You could theoretically go and buy (or even build!) an octocopter right now and fly it about just fine, because computer control systems are cheap and effective. Even in the 70s, harrier pilots were specialists with experience and training, which isn't really the same as an elite hotshot. $\endgroup$ Commented May 25, 2023 at 9:54

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