What explosive mines discriminate against hovercraft?

Question

Tech level is post WW2 era.

Hovercraft is often loaded with tons of explosives which is then set off within the proximity of an enemy vehicle or infrastructure, they can cover a large distance in a short period of time and completely ignores pressure sensitive anti-tank blast mine. I am wondering what measure at that time can be proven effective in stopping these deadly floating tanks in their track without hampering friendly forces?

Specification

• Main Armament: Browning machine gun and grenade launcher
• Crew: 1
• Payload Capacity: 50 tons
• Max Speed: 70km/hr
• Armor: 16mm

Answer 1

Just so you know: a hovercraft hovers BECAUSE it applies great pressure downwards. If a 50 ton machine is hovering, it is applying a downward force of 50 tons in order to cancel out gravity.

Helicopters and airplanes force the air around their blades and wings, but hovercrafts apply force on the air right underneath the skirt. That air will press against the ground or water. It will spread through the while skirt, so usually the pressure per area will be smaller than for a wheeled vehicle of similar weight but it will still be considerable.

Also consider that many classical mines are not triggered by pressure alone, or by direct vertical pressure at all. Each kind of mine is taylored for a different kind of target, so many different triggers exist. Let me direct you to this answer in skeptics.se. Here are some excerpts:

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Mine trigger/fuze methods

TM-83 mine

If we take as an example the Russian TM-83 mine this has a two-stage trigger, a seismic sensor arms an infra-red sensor that finally triggers the mine. The mine can be set-back up to 5m from the road it is targeting.

Seismic trigger

A hovercraft travelling over ground supports it's weight on a cushion of air, that means it still exerts the force of it's full weight on the ground but spread out over a larger area than a typical wheeled vehicle. Because of that weight and it's movement over the surface it will still produce sound and some vibrations that can be transmitted through the ground.

A hovercraft produces more noise and vibration than many land vehicles (Ref, Ref) as anyone who has travelled in one or stood near one can attest.

It is hard to find relevant studies of hovercraft noise, one that sheds a little light on the subject is Hovercraft Underwater Noise Measurements in Alaska which shows that hovercraft travelling over a frozen river produce more underwater noise than snowmobiles.

As an example, a single event (pass-by) hovercraft sound exposure (LE) of 134.1 dB measured at a distance of 78 feet is approximately equal to 14 snowmobile events at the same distance.

Although both snowmobiles and tanks (the targets of anti-tank mines) are both tracked vehicles, in other respects they are not comparable. It is therefore difficult to apply this information to the subject at hand. At the least, this study does show that hovercraft can transmit significant vibrations through the surface on which they travel.

Infra-red trigger

A hovercraft also has engines that produce heat and therefore can be detected using an infra-red detector.

Type 72 mine

The Chinese Type 72 mine is triggered by a pressure of 300 Kg but is resistant to overpressure produced by anti-mine ordnance.

Pressure trigger

The skirt of a hovercraft might exert a force equivalent to less 100 Kg over the area of a mine. (Ref: "250 lbs/sf". Ref: "245 kg/m²", "202 kg/m²")

M75 Mine

The South Koreans are known to have deployed the M75 anti-tank mine. This has a magnetic trigger.

Magnetic trigger

A magnetic trigger would presumably be set off by any sufficiently large vehicle with a significant amount of steel in it's structure and with typical engine construction.

In "World Naval Weapon Systems, 5 Ed. By Professor Norman Friedman, PH.D.‌​" he says "Seemine anti-invasion … Fuzing is presumably magnetic to destroy landing craft including hovercraft" Which suggests he believes magnetic triggers in mines can, in principle, detect hovercraft.

TM-57

The Russian TM-57 anti-tank mine can be triggered by a tilt-rod fuze.

Tilt-rod trigger

Since the skirt of a hovercraft is usually in contact with the ground, it is possible that it could deflect a tilt-rod by the 25 to 30 degrees needed to trigger the mine.

BLU-92

The BLU-92 is related to the M-75 and can be triggerred by trip-wire.

Each side of the BLU 92 (an anti-personnel mine) has four hair thin trip wires that shoot out as soon as it lands. Ref

Tripwire trigger

The air cushion of a hovercraft, and its skirts, disturb the surface over which it is travelling for at least the whole width of the vehicle (unlike a wheeled or tracked vehicle)

It may be plausible that there is a possibility that the skirt, or debris disturbed by the air cushion, will strike a trip-wire with sufficient force to activate a mine.

Answer 2

Build a dedicated anti-hovercraft fuse.

In the real world there is no need for such a thing. If there was a need, it could be built. I can think of several approaches which should be no more difficult than the sea mines tried during WWII.

• Hovercraft move on an air cushion trapped under a skirt. That means they will cause pretty drastic changes in wind velocity close to the ground.
• Hovercraft engines and fans are noisy. There could be acoustic mines.
• They probably include plenty of steel. Magnetic influence mines.

The logic in the fuse will be more complicated than a simple pressure mine, but sea mines solved that. Wait for a sufficiently strong signal, then blow as it starts to diminish.

Answer 3

Two mechanisms come to mind:

Tilt rod triggers. These are simply wood, plastic or metal rods which are attached to the fuse mechanism, and trigger the mine when they are pushed past a certain angle by a vehicle or person tilting the rod. They can also be placed in other positions - in former Yugoslavia, tilt rod mines were sometimes found in trees with the rods pointed downwards. The passage of a vehicle on the road would activate the tilt rod striking the roof of the vehicle, and 5kg of explosive would then detonate, damaging trucks or killing armoured vehice commanders who were "hatches up".

During the Viet Nam war, some mines were modified to use a propeller like mechanism to deter helicopter landings. The downwash from the helicopter rotors would start the propeller spinning, which would generally be connected to a small generator that would send a current to the fuse and activate the mine. The air cushion of a hovercraft will do the same thing for mines fused in this manner.

Of course, hovercraft are also vulnerable to many types of weapons, so the simple option of shooting at them also exists. The skirt which contains the air cushion is usually made of a flexible rubber material, so automatic cannon or machine gun fire could damage it enough to tear it open and "spill" the air cushion, causing the machine to crash into the ground. Anti tank weapons also could destroy the machine rather easily, even frst generation ATGM's are moving at several hundred miles per hour (the WWII era X-7 was derived from the X-4 Air to Air missile, which was fast enough to be fired from a fighter plane moving at 400 MPH and hit bombers moving at 300 MPH).

You also could have the entertaining notion of having ground attack aircraft hunting the hovercraft, much like WWII era "Typhoon" or P-47 Thunderbolts attacked German tanks with automatic cannon, heavy machine guns, bombs and rockets. No Hovercraft is going to outrun a low level fighter bomber moving at 400 MPH. Of course the ground attack airraft are also going to mess you up by attacking the fuel trucks, transporters, field workshops and other logistical support you need to operate the hovercraft, so they may just end up grounded in a field somewhere without fuel or spare parts to run.

Of course, the simple solution is simply to stay in rough or complex terrain where hovercraft cannot operate anyway.

Answer 4

You can trigger the mine either with a wire broken/pulled at the passage of the hovercraft, which is generic for anything pulling it, or with a trigger mechanism activated by a Venturi tube, which reacts specifically to the air flow produced by the hovercraft air cushion.

I suspect the Venturi might become unreliable after some time, due to the outer environment concurring to obstruct it.

Since you now moved the timeframe to post WW2, you can also have electronic triggering and magnetic detection.

Answer 5

You actually don't need mines for this unless you're operating in a large, mostly flat, area. Hovercraft are rather picky about the ground they're traveling over being mostly flat, which is a large part of why they aren't really used much by the military even in modern times other than occasionally as deployment craft for beach landings (but even then amphibious vehicles are usually preferred as they tend to handle rough weather better).

Standard passive anti-tank defenses (dragon's teeth, Czech hedgehogs, etc) would be more than enough to keep suicide-bombing hovercraft at bay, but even simple historical 'defenses' such as a ha-ha will stop a hovercraft in it's tracks. Unlike tracked vehicles, a hovercraft can't realistically deal with sudden sharp elevation changes (a tank can quite often climb rather steep stairs, but most hovercraft cannot reliably do so).

Beyond that though, some simple active sentry defenses should be sufficient. By definition, a hovercraft has to be light, otherwise the fans couldn't lift it off the ground. Many designs don't use much metal, and when they do it's often lighter stuff like aluminum or titanium. This is directly at odds with it being well armored given your proposed tech level (the earliest composite armor dates to 1950, and was still rather heavy consisting of glass and steel), meaning that it's unlikely that you couldn't damage it with what, given the tech level, would be 'normal' anti-materiel rifles and heavy machine guns (which you should have anyway to deal with other types of attackers).

If, however, you really want to use mines to deal with this, including all the other issues they bring about, then your best bet is probably a wind-speed sensor at ground level. A hovercraft traveling over a specific point in the ground will cause a sudden increase in wind speed at ground level in the direction it's traveling, then highly turbulent (or possibly zero) winds as most of it passes over, and then a sudden increase in wind speed moving in the opposite of the direction it's traveling. The trip point you want to use is right as that reversal in wind speed happens, possibly with some timing requirements as well. While this could be dealt with using something probably looking just as ridiculous as a mine flail, doing so would significantly slow down the attackers (no different from actual mines in actual warfare at the time).

Answer 6

Assuming WWI or 2-ish technology: just use the mechanisms they already used and had for these types of weapons:

Trip Wires or Electronic fuses

Trip wires were commonly used with mines as an activation mechanism. This concept works with a hoover craft as well. The disadvantage of trip wire was it was easier to see than a standard pressure sensor and as the location of the "trip wire" might need to be altered (raised) this disadvantage would increase. However, its still possible as one could disguise them in the barbed wire which littered the "deadman's land" fields that trench warefare used. I would also set this up as a daisy chain of mines to improve the chances of disabling or even just hitting this quick vehicle.

On a similar note, which would be more accurate but also more dangerous for troops. You could set up a electronic fuse (think the old TNT "plunger") which a solider would trigger once the vehicle was over the mines.

Radio Activated Mines

This became more of an issue in modern warefare (and with IDEs) but radio activated mines are a real thing. The radio technology of WWI required pretty big comm equipment which was usually transported by horse or mule¹ but, as trench warefare was largely static, it is conceivable that a few stations could be setup which allow for activating a mine. The range and low development of radio technology would make these much less reliable than fuses but the ability to be farther away and not have to disguise wires themselves certainly makes this an interesting tactical option.

Magnetic Mines

These were actually developed during WWI but did not see much action until WWII (and then only as a Naval weapon)². However, if these hovercraft are seen as a potent threat and weapon it makes sense that this weapons development would be accelerated and a ground-based version developed.

1: "Golden Age of Radio in the US", Radio on the Frontlines exhibition. Digital Public Library of America.

2: Campbell, John, "Naval Weapons of World War Two" (London, UK: Conway Maritime Press, 1985)

Answer 7

An armoured hovercraft is problematic because of the weight. If you can make cheap carbon fibre or other high strength composites then you might have a chance, but with current technology something light enough to hover is going to have very low survivability when hit by an explosion directly underneath it. This suggests that your anti-hovercraft mines are probably going to be not much bigger than an anti-personnel mine, although perhaps designed to fire a concentrated blast or projectile directly upwards to punch through the undershell.

Triggering the mines is straightforward; hovercraft have skirts which touch the ground quite a lot. There are lots of possible sensors that can pick up a skirt passing over them, starting with a plain stick that gets pushed over. In theory you can have a hovercraft which has no skirts (see an early prototype here), but these require huge amounts of air blowing downwards. Note the size of the ducted fan in the prototype. There are lots of possible sensors that can pick up a skirt passing over them, starting with a plain stick that gets pushed over.

Answer 8

Think low tech

If you want to deny hovercraft access to an area, you don't need landmines. I suggest the article "The Most Effective Weapon on the Modern Battlefield is Concrete." Area denial can be more effective than indiscriminate explosives.

Ramps

Much like our commander-in-chief, the hovercraft runs into trouble when faced with a ramp. Tanks, by contrast, are designed to navigate a variety of terrain. Check out this video of an Abrams climbing a steep ramp. No way a hovercraft is going to do that from a standing start. According to the Handbook on Drowning, "Generally the maximum continuous slope that can be tackled is in the order of 1 in 10." A HMMWV has a maximum grade that's six times that steep. So if you engineer a 20% grade, your tanks and trucks will be able to climb it while rendering the area hovercraft-free.

Pointy sticks

The skirt is a weak point in a hovercraft. Install pointy sticks with blades on the ends of them and the hovercraft will not be able to pass over.

Vehicle access gates

I live in Washington DC. We have a lot of obstacles installed across the city to prevent vehicle-based attacks. Fixed barriers like this and moving gates like this will stop your hovercraft. And your machine guns and grenade launchers won't get you through. A tank could blast through many of these barriers or climb over them (depending on the barrier) but a hovercraft with limited weapons will be stuck. Even a series of flower planters will stop your hovercraft.

Answer 9

Could a trigger based on Barometric Pressure be suitable?

A Mercury Barometer, invented in the 17th century, could be used with wires inside the tubes. If the pressure becomes too severe, the mercury could be pushed beyond the reach of the wires, and break an electric circuit.

Quick, BoE calculations: if your 50,000kg hovercraft had a similar dimensions to a Sherman tank:

Weight: 50,000 kg × 9.8 N.kg^-1 = 490,000 N
Area: 6 m × 3 m = 18 m²
Downwards pressure = F/A = 490,000 N / 18 m² = 27.2 kPa, or 166.5 mmHg

Therefore, your wires in the barometer would need to stretch 166.5 mm in the tube.

If you were concerned about daily atmospheric changes triggering your mines as well, you could compare the pressure in mines sufficiently far apart.

Possibly the same sort of thing could be done with aneroid barometers too, which might be smaller. These were invented in 1844.

Answer 10

Vertical springs, with spikes/blades on the ends.

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These are spaced out sufficiently that troops on foot can simply walk past them. When the bumper of a car or the treads of a tank push against them, they cannot stab or cut it - the spring instead bends and moves them out of the way. (Your paintwork might wind up rather scratched though)

When a hovercraft encounters them, however, it catches on and slices up the skirt maintaining the air cushion, soon grounding the vehicle.

Basically, anti-hovercraft caltrops.

Answer 11

Lets make a fuse out of a weather gauge. We need two instruments, the anemometer and a weather vane. When the anemometer spins fast enough it charges something. The charge slowly dissipates. There are multiple things that can hold this charge, the weather vane directs where the charge goes. If two opposing points have a charge the mine goes off.

This will only detonate when subject to a very strong wind that then switches direction quickly. The exact mechanics of what is charged are a matter of engineering.