Mythbusters: "Tank Fall" - (A-Team Spoilers)

[Trekker4747]

There will be spoilers in this post, and likely in this thread, concerning the "A-Team" movie that recently came out. This is your only warning.


Ok, in the A-Team movie after Hannibal and his team manage to break out of their prisons they get on a C-130 (or similar design) military plane and fly off, they're soon attacked by unmanned predator drones and the plane takes critical hits and is going down, it's quickly blown up by another drone.

The A-Team managed to escape the blown up-plane by hiding in a tank in the cargo hold and either driving out the plane or simply falling out of it after the plane blew up. The tank's parachutes open up and -for now- the team is safe. Soon the predator drones take target on the tank, Face uses the tank's guns to take out some of the drones (as the tank is falling but at breathable atmospheric level) but in the process two of the three chutes are destroyed and the remaining chute isn't sufficient enough to cause it, and the team, to land safely.

Hannibal gets the idea to use the tank's main turret as a "thruster." He has it pointed in a direction, fires it, to "push" the plane in the air so that it's over a large lake on the ground. (This part, obviously, would work "in theory" due to equal and opposite reactions.) When the tank is over the lake he points the gun toward the ground and fires it some more to "slow" the plane to a more suitable "landing" speed in the lake.

"On paper" moving the plane in the air like that, and to slow it, "should work." But in practice would it work? The Mythbusters have to test this. I don't care how, but they have to do it.

Things we need to answer:

The force of the gun being fired as a "thruster" isn't going to more than the force it takes to expel the cannon's shell. Is this enough to push the tank through the air enough to move it any significant distance?

Would the same force be enough to "slow" the tank down before it crashed into the lake?

Would getting the tank "over" the lake be enough? Would it fall straight down or would it float, shift, and shuffle in the air and need constant adjustment to keep it over even a very large lake?

Do tanks have enough shells on-hand to allow all of this to happen?

Would the passengers inside the tank survive the violent movements and the crash into the lake?

Would the tank survive the crash into the lake, the submersion and be able to drive out of the lake (as what happens in the movie and assuming it landed right-side up.)

There's other questions about this scene too, but it's just begging to be tested and it's right up the Mythbusters' alley!

 

[Christopher]

Dang, did you have to choose such a misleading title? You scared me into thinking I'd missed a special episode or something.

And I'd say pretty much the whole thing sounds bogus from a physics standpoint, or at least grossly exaggerated.

 

[Trekker4747]

Dang, did you have to choose such a misleading title? You scared me into thinking I'd missed a special episode or something.

Sorry!

And I'd say pretty much the whole thing sounds bogus from a physics standpoint, or at least grossly exaggerated.

Well, again, I suspect it could work "in theory." I mean as the tank is falling if it fires the cannon the projectiles fires out of the barell with x-amount of force and speed so there's got to be an equal amount of force and speed going in the opposite direction, right? Sort of the same reason why helicopters have a tail rotor, to counteract the opposite-twisting motion the body will experience from the main rotor.

But the question I have is, would the force of the firing cannon be enough to push a tank in a mostly frictionless environment (falling through the air rather than being on the ground) through the air to change it's position? Could firing the cannon "down" slow the tank's fall (like a descent engine on a space-lander does) enough for it and the passengers to survive?

Like I said I've no idea how they'd test this (to do so would require them to at the very-least get ahold of a decomissioned tank, be allowed to use munitions, and to get a heft helicopter to lift the thing) without staying stictly in "small scale" but it seems to me that on a cursory, basic, level the "physics" are sound. In practice? A lot can change. (Like I said, I doubt the cannon would be powerful enough to move the tank that much, and that falling, movement and wind wouldn't whip the tank around a lot and wouldn't just fall straight down.

 

[Nardpuncher]

I'm not anywhere near a physicist, but I think it could work, but would probably need to be fired dozens or even hundreds of times to make a difference. But that's just me guessing.

 

[Christopher]

Of course the theory of action and reaction is sound, but it's a question of reaction mass. I profoundly doubt the mass of an artillery shell is great enough relative to the mass of the tank to produce a substantial enough recoil force to maneuver the tank through the air, and especially not to decelerate it to a safe-ish landing. How long does it take to reload between shots? Any deceleration achieved by firing the cannon downward isn't going to last; the tank will instantaneously start accelerating again as soon as the countervailing force stops, quickly cancelling out any deceleration. So unless the cannon is literally firing continuously, with virtually no interval between shots, it's going to do nothing more than slightly delay a terminal-velocity impact.

Also, don't you think the cannon in a tank would be specifically designed with shock absorbers to minimize the recoil experienced by the tank? Okay, as long as the tank was in midair and not held in place by friction against a solid surface, the shock absorbers might not work as well, but still, the sheer inertia of the tank body's mass would serve to "hold it in place" and let the shock absorbers do at least part of their work. So that would reduce the effectiveness of firing shells as a means of reaction thrust.

Also, is it even possible for the tank to be oriented with the gun pointing downward? A falling object will end up oriented with its center of mass as low as it can get, i.e. with the heavier side down; this is why falling cars on Mythbusters always tilt forward, and why stunt performers doing car leaps weigh down the trunk to prevent that tilt (because a car engine is really heavy). I would assume the heaviest part of a tank is the bottom, so a tank would pretty much fall in an upright orientation, making it impossible to point the gun directly downward.

 

[Scott Butler]

^ In the movie, there's one parachute still attached at the back end of the tank, that's why it falls front-end-down.

I think the first part, where they direct the fall in midair, is plausible because of the velocity of the shells as they leave the barrel. Firing a bullet can provide enough kickback to make person fall over, expelling gas provides enough thrust to launch 100,000 tons into outer space. So I think it's plausible they could adjust their "landing" site to aim for the lake.

The part about firing down to slow their descent? That I'm not buying. In this instance I think the tank has too much inertia to significantly slow its fall. Odds are by the time they could fire another shell any deceleration from the previous shell would already have been canceled out by gravity.

Survive impact and drive the tank out of the lake? Depends on how much that last parachute is slowing them, how well the tank it built, how quickly they're decelerated when they hit the water, etc. I have no idea about that, but I'll say this: It wouldn't be The A-Team if they didn't survive impact and drive the tank out of the lake.

Here's another thought on the firing-straight-down: Apart from trying to decelerate, could this be a good idea to break the surface tension of the water? If you hit the water's unbroken surface at the speed, it's like hitting a wall. By churning up the water first, can they help ensure a "softer" landing? Or does that not really matter as much since they were hitting the water nose-first anyway?

 

[Christopher]

^ In the movie, there's one parachute still attached at the back end of the tank, that's why it falls front-end-down.

Okay, that much makes sense. It also means they don't have to decelerate quite as much to reach a survivable impact velocity, but the points about the mass of the projectiles, the firing speed of the gun, and the shock absorption built into the cannon would still apply.

I think the first part, where they direct the fall in midair, is plausible because of the velocity of the shells as they leave the barrel. Firing a bullet can provide enough kickback to make person fall over, expelling gas provides enough thrust to launch 100,000 tons into outer space. So I think it's plausible they could adjust their "landing" site to aim for the lake.

Well, how does a tank deal with recoil? When tanks on the battlefield fire their guns, do they roll backward? I know I've seen footage of artillery doing that, but that's something where the total mass of the weapon isn't as great as a tank. If tanks don't roll backward from recoil, is it because they brace themselves against the ground, lock the treads, or something? In that case, then it could be plausible that a tank in midair could use the gun as a reaction drive to an extent. But if the gun is designed with shock absorbers built in, so that the gun itself absorbs the recoil rather than transmitting it to the tank's body, then that would seriously diminish its ability to function as a thruster. And I think that is consistent with footage I've seen of modern tanks firing, although it's not a subject I've studied in any detail or would ever want to.

The part about firing down to slow their descent? That I'm not buying. In this instance I think the tank has too much inertia to significantly slow its fall.

Well, that's the same issue in both instances, regardless of the direction of thrust. Inertia is a body's resistance to acceleration, and it's the same regardless of whether the acceleration in question is countering another acceleration such as gravity.

Odds are by the time they could fire another shell any deceleration from the previous shell would already have been canceled out by gravity.

This is the key point that's specific to the question of decreasing the velocity of the fall. Here you're quite right. If your car is careering downhill and you tap the brakes once every ten seconds, that's not going to materially improve your chances of survival.

Survive impact and drive the tank out of the lake? Depends on how much that last parachute is slowing them, how well the tank it built, how quickly they're decelerated when they hit the water, etc.

Yeah, what kind of features do tanks have for protecting their occupants from accelerational stresses? I assume there are seatbelts/harnesses. Do they have airbags? I wouldn't expect a tank to be designed with crumple zones, though. They're pretty rigid, I'd imagine, so there wouldn't be much in the structure of the thing to absorb force before it reached the passenger compartment. Maybe if there were heavy shock absorbers in the seats, or between the passenger compartment and the outer body, then maybe. But if the force of impact is great enough and not enough of it is absorbed before reaching the occupants, then it wouldn't matter how well they were strapped in, those stresses would still -- in Mythbusters parlance -- break every Shock Watch sticker in their bodies.

I have no idea about that, but I'll say this: It wouldn't be The A-Team if they didn't survive impact and drive the tank out of the lake.

Well, yes of course; in the A-Team universe, Rule of Cool trumps all other physical and probabilistic laws. But the question on the table is just how many real-world laws are being violated in this case.

Here's another thought on the firing-straight-down: Apart from trying to decelerate, could this be a good idea to break the surface tension of the water? If you hit the water's unbroken surface at the speed, it's like hitting a wall. By churning up the water first, can they help ensure a "softer" landing? Or does that not really matter as much since they were hitting the water nose-first anyway?

Adam and Jamie busted that one way back in the first season, in the Hammer Drop myth (i.e. could a construction worker falling off a bridge save his life by dropping his hammer first to break the surface tension?). They found that there was a very small reduction of impact force but not enough to make a significant difference. Even with the surface tension broken, the water is still too viscous to move out of the way of the falling person or vehicle quickly enough to be safe. So it's not like hitting a solid concrete wall, it's more like hitting a pile of rubble. Not exactly a meaningful improvement at those speeds.

 

[SG-17]

Well they were firing tank shells into the water and you saw how much water was displaced by them, so the gap in the water caused by the shell impacts could have significantly weakened the impact force.

 

[Christopher]

^Nope. That just means they hit the water slightly later. If anything, that would infinitesimally increase the impact force since they had a split-second longer for gravity to accelerate them (assuming they were below terminal velocity due to the slight deceleration imparted by firing the shells).

To put it another way, moving the water out of the way beforehand accomplishes nothing. What's needed is the absorption of the kinetic energy of the impact. A surface that deforms during an impact will absorb some of that kinetic energy through its deformation; this is why airbags work, why cars have crumple zones, why helmets are filled with styrofoam, etc. But water doesn't deform enough upon impact to absorb the force sufficiently. Water is incompressible, so it can't absorb force by getting squished the way styrofoam or, say, a pile of cardboard boxes can. It can only deform by moving, by being pushed aside by the impacting body. And because of its viscosity, it can only move aside so fast. So something hitting the water at low enough velocity can have that force absorbed safely, but if you have too much velocity, too much kinetic energy, then there's just no way in hell that the water will be pushed aside fast enough to reduce your deceleration to a safe level. It doesn't matter how you change the surface tension or shape or position of the water before you hit it; bottom line, it's the fundamental mechanical properties of the fluid itself that doom you.

 

[Trekker4747]

See? See?!

I'm sending this in to Mythbusters all five of them need to team up and test this!

 

[gh4chiefs]

I haven't seen the movie, so I'm not familiar with the scene in question, but just reading Trekker's description makes me glad I made the decision to pass on this one.

 

[Trekker4747]

I haven't seen the movie, so I'm not familiar with the scene in question, but just reading Trekker's description makes me glad I made the decision to pass on this one.

It's a fun, damn, scene, though. Made very funny with the conversation between BA and Murdoch and the other chaos inside the falling tank.

 

[Scott Butler]

Well, how does a tank deal with recoil? When tanks on the battlefield fire their guns, do they roll backward?

Not appreciably, but I think that's just because they have the brakes on, so to speak. Looking up some videos - http://www.youtube.com/watch?v=3KFCd8zdI-Q&NR=1 http://www.youtube.com/watch?v=r-25BjbKm-8 - there's definitely backward force on the tank. To be sure the gun is designed to minimize recoil, but it's not completely eliminated.

What I meant before is not that inertia changes between trying to move laterally and trying to slow vertically, but that inertia is more of a hindrance in the latter case, so the threshold for "too much inertia" changes. When you're trying to move laterally to aim the landing, inertia makes it hard to get moving, but there's no significant outside force acting on you (air resistance, maybe some wind) so as long as the inertia isn't so huge as to hold you essentially immobile, you can accumulate velocity with successive shots. But when trying to slow the fall, you can't accumulate delta-v unless each individual shot slows you more than 9.8 * reload time. So it takes much less inertia to prevent you from slowing your fall than it takes to prevent you from aiming the fall.

 

[Scott Butler]

I haven't seen the movie, so I'm not familiar with the scene in question, but just reading Trekker's description makes me glad I made the decision to pass on this one.

It's a fun, damn, scene, though. Made very funny with the conversation between BA and Murdoch and the other chaos inside the falling tank.

Yes, definitely. That's one of the things the group I saw it with commented on, unlike a lot of movies where things look great in the trailer and end up falling a bit flat, the tank scene lives up to expectations.

"... ... Recently!"