Hovertanks

[Zarel]

Let's just leave relativity out of this. The E = mc^2 equation is irrelevant when calculating the energy of a projectile, unless the projectile is a nuke or antimatter bomb or something.

[3drts]

... If the barrel is attached to something else (ie mounted, or held against a shoulder), then the masses are coupled, and the energy as a result of the recoil changes ...

This is only the case when the objects are rigid, and in that case, we would also need to apply elastic collision between the gas, rifle, and bullet, instead of 'waste heat'.

#1) What do you mean "instead of waste heat". What does waste heat have to do with anything?
This is conservation of momentum, how fast Mass A recoils after launching mass B at velocity V.
Heats got nothing to do with this.

2) Rigidity helps to simplify the equations, but it is not a requirement.
The energy of the gun that is recoiling is determined by the integral of Force*distance.
The force isn't there for a single instant, but rather an interval of time (if it weren't it couldn't do any work)
As soon as there is any resistance on the recoiling gun, its KE from the recoil is going to be change.
Any mounting on a gun will start providing resistance while the force from the gunpowder is still being applied.
Rigidity only changes the degree to which the guns mass, and what its mounted to, are coupled.
So how one holds a gun itself, can affect how much energy it recoils with.
(Indeed, on many recoil operated handguns, "limp wristing" often results in insufficient energy to cycle the next round in).

The energy of a weapon recoiling as not constant, and KE is a poor way of measuring recoil.

[Rade]

Just to get back to the topic, the Russian navy Zubr and Murena hovercrafts
http://www.youtube.com/watch?v=yeFXhitek4g

[TVR]

... What does waste heat have to do with anything? ...

What else would prevent a bullet from ricocheting off everything all the time?

... This is conservation of momentum, how fast Mass A recoils after launching mass B at velocity V. ...

|V| can't be determined by A or B in this case, otherwise the simultaneous recoil would affect the relative V, which would affect the amount of recoil.

... Any mounting on a gun will start providing resistance while the force from the gunpowder is still being applied. ...

Whether that occurs depends on the size of the object and the speed of sound in that medium.

[3drts]

What else would prevent a bullet from ricocheting off everything all the time?

What does that have to do with recoil?

|V| can't be determined by A or B in this case, otherwise the simultaneous recoil would affect the relative V, which would affect the amount of recoil.

Recoil is simultaneous.
If it is not, then conservation of momentum is violated (if only temporarily)

If I launch a projectile at velocity V (the frame of reference being the projectile's initial position), the recoil in terms of momentum will be the same, every time.

The velocity at which the launcher recoils will vary.
The energy with which the launcher recoils, will vary.

Momentum is the only absolute way to measure recoil.

[Rman Virgil]

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I found the discussion a few months back @ the Physics Forums (HERE) very instructive (sans narcissism too, which was especially refreshing). That discussion also dovetailed beautifully with this thread's subject, Hovertanks, and my first post - to wit:

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This discussion reminded me of the SRN4 which operated for almost 40 years between Dover and Calais (over water), beginning in the early 60's.

It was capable of carrying over 380 passengers and 40 cars, was was 91 feet wide,185 feet in length, weighed 300 tons, could cruise at 70 mph and had props 20 feet in diameter.

With all that in mind, could there be any logical weaponized version ? Over water, some ideas occurred that seem sensible.

Over land ? Too much working against it to make sense, realistically. Stuff already mentioned and things like being every bit as vulnerable to anti-tank land mines as trax or wheels.

Though, hover tanks in com-vid games have been very popular since first introduced as such back in 1991 I think. Enduring for well nigh 20 years as game entertainment does make for a strong impression.
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The SRN4 actually being one of WZ Creators, Pumpkin Studios, main inspirations for Hovers in WZ 2100 (and to some degree, the Unit Transport too.).

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[Aranor]

Wow been ages since I posted here. Back on topic I think the basic argument is to not have this happen on a hover craft when firing http://dsc.discovery.com/videos/mythbus ... speed.html
The truck in the video is moving at the speed the projectile exits the barrel at. Thus when its fired the projectile goes nowhere and falls straight down. Firing a projectile from a non stationary hover craft could result in similar results as the vehicle is forced in the opposite direction of the projectile; the projectile looses velocity much quicker than the normal loss over distance as expected. I can't imagine much worse than pulling out your bfg, firing and having you 50lb bullet land 20 feet in front of you and your enemies fall out of their tank laughing.

This argument also is based on hover vehicles using an air cushion. Was it ever described that's how the hover propulsion systems worked? Though considering their inability to steer very well in the game I guess that's irrelevant.

[3drts]

Just because its called a "physics forum" doesn't make the people there giving answers experts in physics.

Case in point, post 12 of that thread:

One thing I haven't considered here is the huge increase in powder that would be required to maintain a constant muzzle velocity. Since energy is proportional to the square of velocity, you'd need 100 times as much explosive to launch a projectile at the same speed as one 10 times lighter.

That is blatenetly wrong.
It takes 100 times as much energy to launch a projectile 10 times as fast.
It takes 10 times as much energy to launch a projectile at the same sepeed as one 10 times lighter.

Further, the assumption that the amount of explosive/propellant used to launch a projectile exactly scales with amount used, is not neccesarily correct, but we'll go with that for now.

Aranor:
This is not a problem unless one round of ammunition weighs on the same order of magnitude as the vehicle+ gun itself (note I said gun, rockets are another story).
M1V1=M2V2
If M1 is 10x M2, then the velocity difference will be 10x.
So Travelling forward, or travelling backward, firing your weapon normally won't make much difference in its muzzle velocity relative to the earth's surface.
Consider an M1 Abrams:
Muzzle velocity on its cannon is over 5,200 feet per second (5,200-5,700)
Its top speed is 42 mph (and its recoil won't get it anywhere near that speed).
5,200 feet per second is about a mile (5,280 feet= 1 mile).
There are 3,600 seconds in an hour, the muzzle velocity is over 3,600 miles per hour.
Do you really think it makes a big difference if it fires while moving forward or backward, even if it could go 200 miles per hour?
3,400 vs 3,800, either way, its pretty f*king deadly.

Also, the vehicle recoiling backward will not have much effect on the projectile velocity.
M1V1=M2V2
Lets call M1V1 the projectile's momentum, and fix its mass.
All velocities are relative to position zero, with the frame of reference being when m1 and m2 are joined at time point zero.
Lets say M1 is 1/100th the mass of M2 (if 1 hundred projectiles weigh as much as the vehicle, your vehicle is too small)
Lets fix V1 at 1,000 m/s.
V2' will be 10 m/s
Now lets say a hover tank is half the weight of a normal tank, and define M2' as 50*M1.
Now v2 is 20m/s.
Now KE is 1/2 MV^2.
M2= 100M1 M2'=50M1
V2= 1/100*V1 V2'= 1/50*V1
Projectile KE= 1/2 M1V1^2
Lets drop the 1/2 , as they will cancel out later anyway.

Vehicle 1 KE = 100M1*(1/100*V1)^2 = 100* M1* V1^2 / 10,000 = 1/100 M1V1^2
Vehicle 2 KE = 50M1*(1/50*v1)^2= 50*M1*V1^2 / 2,500 = 1/50 M1V1^2

As we can see, in the first case, the vehicles KE is 1/100th that of the projectile's.
In the second case, the vehicles KE is 1/50th that of the projectile.
So the extra energy from propellant you would need is only 102/101 = 1.0099, or 0.99%
Allowing the vehicle to recoil freely, and reducing the mass of the vehicle to 50%, will only require a gun 1% more powerful to propel the same projectile at the same speed, assuming the first vehicle weighed 100 times as much as the projectile it launched.
To a first approximation, if the guns power is not increased, then the velocity will be (101/102)^1/2 = 0.99754 x as much as it was before, ie a 0.246% reduction in velocity.

I could have done this assuming a fixed power gun, but those equations are more complex.
The point is, recoil has negligible effect on the velocity of the projectile.
Bullets are much mch lighter than their guns, and they travel much much faster than the guns would from recoil if allowed to move freely.
consider:
http://en.wikipedia.org/wiki/10.5_cm_leFH_18/40
2,000 kg gun weight, fired a 105 mm shell. r= 50.25mm = 5.3 cm
Lets assume the shell was spherical, and solid lead (not 75:25 steel casing:explosive, as most shells are approximately)
4/3pi*r^3 = 4/3*3.14159265358979323846... (lets jsut call it 4*r^3)
4*5.3cm^3= about 600 cubic centimeters.
Density of lead: 11.34 grams/cc.
Weight of a 105mm solid lead sphere: 6.8 kilograms. (15 pounds approximately)
2000 kg gun, 6.8 kilo solid lead shot.
Well over a 1:100 mass ratio (more like 1:300)
And considering an approximation with iron's density (since the shells it fires are less dense than pure iron), the ratio would be even more skewed (but this is negated by the fact the shell is longer than the sphere approximation).
And howitzers, like mortars, fire large shells rather slowly, for direct fire cannons, the mass difference is significantly higher.
Now mount the gun to a chassis, and you'll find the chassis weighs about 10x that of the gun (or if an armored chassis like a tank, about 30x that of the gun- 60+ ton abrams, the 105mm was only 2 tons.
The 1:300 mass ratio just became 1:3,000 to 1:9,000
Vehicle recoil is insignificant, even for indirect fire artillery lobbing heavy, relatively slow shells.

The effect is known as gravitational lensing (and is actually a prediction of general relativity) that occurs because massive objects warp space time. It is not the photons which bend, but space.

Space bends, and the trajectory of the photon bends as a result, but that distinction is not important.
What is important, is that the vector/momentum of the photon has changed (since we acknowledge a photon has momentum).
So then, for conservation of momentum to hold, the star/blackhole/whatever the gravity source is must also experience a change in momentum, ie a very slight tug towards the photon.
This must mean the photon itself has gravity/bends space.

Light has gravity..... weird thought.
Therefore by only considering mass in orbital equations, we get an incomplete picture of the gravity present in the system.
But it seems to me light must have gravity.

What makes it even weirder, is that gravity waves exist, and that the force of gravity travels at the speed of light.
So we have light, producing gravity, the effect of which spreads at the same speed the photon does.

So if you shine a light in a given direction (in space, where we assume it is close enough to a vacum), the gravity of the photon won't arrive until the photon does.... is that the gravity for the entire integral of the photons path, ie the integral of the force of the photons gravity at each position along the way (due to the exponential nature of gravity, you could ignore most of the calculation based on where the photon was very far away).
As the photon interacts with a stars gavity, presumably its momentum changes, but the photons gravity wave hasn't reached the star yet...
Do we have a temporary break in the conservation of momentum?
Does relativity somehow rectify this?
Its weird....
I like classical physics (which recoil is) much better, I can understand classical physics.

[TVR]

... With all that in mind, could there be any logical weaponized version ? Over water, some ideas occurred that seem sensible. ...

The closest thing to an armed hovertank would probably be the Bora-class Guided missile hover-catamaran.



Its displacement is very shallow, it's very fast by surface standards, the centre of the ship is a very convenient place to launch future auxiliary hovercraft, and it is literally immune to any form of torpedo or naval mine. These qualities make it a great littoral combat ship.

Some of these qualities would apply to a miniaturized terrestrial version, for example, the air cushion would exert very low ground pressure, which would reduce the effectiveness of shaped-charge contact mine, if it were to even trigger, and although hovercraft may not be the best choice for tactical manoeuvring, they have better fuel efficiency, and can cross rivers with no engineering support. This makes them great for strategic movement, much like the M18 Hellcat tank destroyer.

... Rigidity only changes the degree to which the guns mass, and what its mounted to, are coupled. ...

Rigidity only changes the speed of sound in that medium, which is not function of how much the masses are coupled, but how long it takes before they are.

... What does that have to do with recoil? ...

If there wasn't energy lost during impact due to being perfectly rigid, then every projectile would infinitely ricochet instead of stopping.

... the recoil in terms of momentum will be the same, every time. ...

Yes, but not the velocity that the launcher recoils at.

This is conservation of momentum, how fast Mass A recoils after launching mass B at velocity V.

Otherwise energy of recoil would also be constant due to being proportional to v^2, which is not true in Newtonian mechanics.

... the force of gravity travels at the speed of light. ...

The force of gravity travels at the speed of electromagnetic radiation in a vacuum, which can't actually be reached by electromagnetic radiation due to vacuum being perpetually imperfect.

[Aranor]

I think you missed the point I was making. This is a hover vehicle not a tank. Ever tried to compress the suspension on a tank? Doesn't work very well. And as far as moving it there is a lot of friction to overcome. Especially sideways. Lets take this for example http://www.aerogo.com/index.html Those would be kids moving a Ford F-350 with relative ease. And a push on the front causes the rear to move as well.
Using a cushion of air greatly reduces the friction. Apply the force of the explosion causing the recoil to the chasis which your weapon is mounted to in such vehicle allows the chasis to move. Most hover crafts have a couple feet of air cushion below them. While there is air pressure pushing against the weight of the vehicle it is not as stable as a frame attached to or resting on the ground. The recoil from your weapon depending on its power output is going to cause the entire vehicle to shift in any number of ways (Direction of the projectile pending). While the movement of the vehicle is not going to have as drastic an affect on the projectile as the video I posted, it is going to affect the accuracy.

3,400-3,800 mph may be pretty smurfing deadly, but not if you miss your target. Any rapid firing without repositioning the vehicle will only add to its movement from its original position causing need to realign the weapon.

Oh and last I checked this was in the general section, not a physics forum.

[Lancefighter]

unless a similar cannon was shot directly opposing the first cannon!
(no but really, counterweights or similar can be used to mitigate the issue?)

[Aranor]

If your chasis is near frictionless how are counter weights going to help? It already has a resting weight of x amount of tons.

[Lancefighter]

moving counterweights.. dunno the proper term for those, i just mean that for each time a cannon is fired, a pneumatic ram pushes a weight in the opposite direction (obviously not out of the chassis) but this should help reduce recoil, moreso for cannons than rapidfire stuff... but does a machinegun really need recoil reduction? (id think a static acceleration towards the opposite direction of recoil would work for fast-firing stuff)
I digress.
Basically what im saying is that recoil is largely unimportant imo >.>

[Aranor]

For smaller weaponry I would agree with you. Nothing much to worry about. I was going for larger cannons. The way you describe the counterweights makes sense except for one thing. Which way do they move? (I never seen em in action) Counter movement on the X,Y and Z axis would be needed.

[Rman Virgil]

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In the spirit of Applied Phlebotinum, it obvious that WZ creators made use of an undocumented mechanism to account for hover tank effectiveness with weapons generating substantive recoil. It is called RDT aka Recoil Dampener Technology. Perhaps it should be considered, alas, for explicit inclusion in the Research Tech Tree...

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