Hovertanks

[Lancefighter]

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.

physics would state they are required to travel with an acceleration opposite of the direction the cannon is firing. However, in reality, unless something is actually being ejected, there is the problem of deceleration after they reach the other side of the hovercraft.. in which case, we use magical recoil dampers that... .... .... warning, random math ahead.
(assuming a 120mm cannon similar to the m1a1 abrams tank)
120mm cannon firing a DU penetrator round produces about 20kg of mass moving 1600m/s out of the cannon. 32,000kgm/s.
A SRN4 has a weight of 270 tons. thats 270,000kg.

The cannon fires.

The SRN4 moves backwards (relative to bullet direction) at .1185 meters a second. (around 1/3 feet/s, or 12 cm/s.. )

A SRN4 just fired a tank cannon, and was moved backwards by about the same amount as a stiff wind. I see no reason why a hovercraft could not maintain this with a fairly high rate of fire.
A 20oishmm US naval cannon, firing a 120kg round at 850m/s has a kgm/s of 102,00. The SRN4 moves a whopping .3778 meters a second. Rate of fire is around 4 rounds a minute.

I guess looking at that im kinda unimpressed by the numbers im getting. Someone quick, tell me im wrong so this all makes sense.

[3drts]

... 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.

Not what I was referring to.
I was referring to pneumatic recoil dampeners like found on Howitzers. The mass of the gun, and the carriage both startto move backward before the round has left the barrel, however, because the coupling is not rigid, one mass is moving at a different speed to the other. Say 1000 N is being exerted on the gun, and due to the pneumatic recoil system, the carriage only experiences 10 N, and after the round has left the barrel, the carriage is still experiencing a force as it tries to arrest the barrels velocity.

... 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.

..... this has nothing to do with recoil when the gun fires....

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

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

Exactly why you shouldn't use energy or velocity as a measure of recoil

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.

Mass A will recoil backwards at the same speed every time (we assume mass A does not change). If we pick a Mass C that launches mass B at velocity V, its velocity of recoil will only be the same as that of A, if C=A.
The velocity at which mass A will recoil is given by B*V/A

... 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.

Well, define a small enough space, and you do get a perfect vacuum, I'm talking down to the planck length.
But most space is close enough to a vacuum, that the difference in speed is insignificant.
And it is also irrelevant as far as photons having gravity, and if gravity waves cause transient breaks in conservation of momentum.

[Aranor]

Ok, if anyone cares to try lets have some fun with this. I don't have the room or I would. Make a wooden box. Make its surface with a perimeter similar to a tank only small enough to handle. Four borads for the sides, basically a upside down box. Using pvc piping and propane make a potato gun to scale of a cannon or cannons of you choice. Mount he cannon upside down box and fire it. Now close the bottom of the box and put it on water and fire it. See how stable it is in both conditions. Try again with the box moving on water. Obviously it won't move very well on the ground without some sort or wheel.

To further prove my point here is a video of the French Archer system. http://www.youtube.com/watch?v=suRWbVD2Uwk&feature=fvsr Watch the back end of thos trucks when they fire. The action starts at about 8:45. It is in French though. Those trucks, which have a stabilizing system preventing them from moving is already engaged. They still dig in. Another one, watch at 17 second mark http://www.youtube.com/watch?v=u6tYXlH8 ... re=related Those weapons are designed with an impressive recoil system. Here is one of a M1A2 firing while stationary. Turrent turned 90 degrees to the right. Watch the underside of the tank shake after it fires http://www.youtube.com/watch?v=putRj-2hP2Q ThM1A2 weighs 69.54 tons. http://www.tankmastergunner.com/m1a2specs.htm I'm not sure how exactly the suspension works on them or the force required to move it but it did move from one round. And as you saw on the archers, the lighter the rig the move it moved. Depending on its design it is either riding on a balloon or continuous forced air at a constant pressure. And yes you can fire it in the direction you are moving to counter some of that force but how often is your target directly in front of you and how often are you firing on the run?

And here are some videos for fun
Target practice with a tank http://www.youtube.com/watch?v=VQVT0bOPLqg

A real VTOL in the works, check how unstable it is with four jets lifting it http://www.youtube.com/watch?v=ElS9BKSsezw

Hoverbike http://www.youtube.com/watch?v=32JmTBTl ... re=channel

Hovercraft http://www.youtube.com/watch?v=zp9q_2uI ... re=related

Larger hovercraft taking off http://www.youtube.com/watch?v=CppcnFqX ... re=related

Lifter technology which is just about impractical http://www.youtube.com/watch?v=XKi9OOS-e94&feature=fvst

[3drts]

Those things only move a few inches, a hovercraft wouldn't be much lighter than those trucks, if at all.
Considering gyrostabilized sites, and modern fire control that allows tank cannons to turn and fire when on the move over rough terrain, even if a combat hovercraft moved a few feet after each shot instead of a few inches, it wouldn't be much of a problem.

Besides, those trucks were firing artillery, which have much more recoil (as measured in momentum), due to much heavier shells (which are also fairly low velocity relative to tank cannons).

[TVR]

... after the round has left the barrel, the carriage is still experiencing a force as it tries to arrest the barrels velocity. ...

Yes, and as force on each particle is applied at t + (c*d), that particle also doesn't move at all until the compression wave reaches it.

Which relates to the next point, specifically how rigidity affects the impact and launch of a projectile.

... (energy lost during impact due to being perfectly rigid) ... has nothing to do with recoil when the gun fires ...

As the projectile is accelerated by elastic collisions with gas molecules, whether energy can be used for work depends entirely on the rigidity.

If we use perfect rigidity, then there is absolutely no recoil from the perspective of the launcher. Or the target.

With recoil including muzzle climb and the shock-waves from applying a force on a projectile, rather than just the resulting thrust.

... And it is also irrelevant as far as photons having gravity, and if gravity waves cause transient breaks in conservation of momentum. ...

Photons have relativistic mass, thus energy does curve space-time, which affects the relative velocity with an external observer.

But gravity itself does not have momentum, as gravitational waves can 'escape' from a singularity.

[3drts]

that particle also doesn't move at all until the compression wave reaches it.

True.

If we use perfect rigidity, then there is absolutely no recoil from the perspective of the launcher. Or the target.

Nonsense.

... And it is also irrelevant as far as photons having gravity, and if gravity waves cause transient breaks in conservation of momentum. ...

Photons have relativistic mass, thus energy does curve space-time, which affects the relative velocity with an external observer.
But gravity itself does not have momentum, as gravitational waves can 'escape' from a singularity.

Never claimed gravity has momentum.

What I'm saying relates to gravity waves.
ie, lets say a photon interacts with the gravity of a start, the photons momentum changes as it interacts with the gravity of the star.
Now, the stars momentum must change as well, but it will only start to change when the gravity wave of the photon reaches it.
What happens to conservation of momentum inbetween the time a photon interacts with the gravity of a star, and when the photons gravity wave reaches the star.

But according to relativity from the photons point of view(which due to imperfect vacuum, we'll assume is travelling 0.999999 c), the gravity should be travelling forward relative to the photon at c.
Its all very confusing.

[TVR]

... If we use perfect rigidity, then there is absolutely no recoil from the perspective of the launcher. Or the target. ...

Let me rephrase this, there would be no felt recoil, only a change in relative position.

... Now, the stars momentum must change as well, but it will only start to change when the gravity wave of the photon reaches it. ...

The photon's change in momentum isn't observed by the star, therefore it doesn't exist in the star's frame of reference.

This is because time is not universal, these propagations do not occur at the same proper 'time', unlike Newtonian mechanics.

If the sun were to vanish (by moving away at FTL), Earth would still orbit around its barycenter until the change is noticed, which is limited by c.

[Jorzi]

I think the main problem with hovertanks is steering.
It would be entirely possible to make a hovertank that weighs over 60 tonnes (like most MBTs)
Since it has significant weight it could propably fire any direct-fire weapons or mortars without problems
Howizers would propably be a bit inaccurate because of the reasons described by Aranor (the slight movement of the vehicle can alter the trajectory of the shells, which produces notable errors at long range)

Maneuvering and acceleration, however, would be problematic. It is relatively easy to turn a small hovercraft, Bur when you have a 60-ton piece of metal that needs to be redirected by the means of a fan, you get problems, or at least you get design challenges.
Either the fans would need to be huge or they would need to spin extremely fast.
Gas turbines, in one form or another would of course provide more compact thrust, but as has been said, since they are so close to the ground they would get problems with dust and saltwater (even the M1A1 has some problems with dust). However, if dustproof jet engines were invented, they would propably be able to do the job.