# Are 60000 RPM electric motors enough to a tennis launcher-like weapon to launch a bullet at same speeds as conventional rifles?

Well, I've made this question before, but it wasn't well written, so I deleted it. I think this time is good enough?

Context:

Semi-futuristic scenario where instead of gunpowder, they use weapons that are electronic launchers that accept several different projectiles. So instead of needing soldiers with different weapons (like shotguns, submachine guns, rifles etc), they use the same weapon with different projectiles and speeds for the current situation, like non-lethal ones. Which lowers the cost of mass production.

The idea is to make an electronic launcher that instead of launching projectiles with raw energy (like a Railgun that uses the electricity of an entire city to launch a projectile at mach 7), they simply use electric motors that rotate continuously during combat in a parallel line. Launching projectiles at a rate greater than any assault rifle.

You know like a tennis ball launcher.

The Question:

Asking around I heard that you would need the driving wheels to rotate as fast as the bullet itself.

So I found this Online RPM (revolutions per minute) to m/s (meters per second) converter, and it says that if I had a wheel with 15 cm of radius (0,15 meters, 30 cm of diameter) with a rotation of 57932 RPM, I would reach 910 m/s at the tip of said wheel. Which is the muzzle velocity of the M4A Carabine Assault rifle.

However, I think I'm missing something. I don't know what, but I feel like this is just too simple of an answer to confidently say it could launch a bullet at that speed. You can easily find electric motors that can achieve this speeds today (an angle grinder achieves 20000 rpm, for example), but I see no electric launchers.

Is 60000 rpm enough to launch something at bullet speeds?

• Should the question be 'what type' or 'is it practical'? Commented Oct 31, 2022 at 21:40
• Any kind of electric motor can be more powerful or less powerful. Any kind of electric motor can be coupled to a transmission which would multiply the rate of rotation by any factor you please. Which makes me believe that the question is not really about "what kind" of electric motor... Commented Oct 31, 2022 at 21:57
• "I see no electric launchers": SpinLaunch says hello. (And, again, gears exist, and were invented thousands of years ago. As long as the motor can provide the required power you can use a gearbox to obtain whatever rpm you want.) Commented Nov 1, 2022 at 13:37
• With a machine gun like weapon you want to aim at different targets within a second or a fraction of a second. Your gun would have a strong momentum and it would be very difficult to change the aim. Enemies would be able to dodge your bullets. Commented Nov 1, 2022 at 13:54
• @Fulano Even two opposing motors would resist the rotation of the spinning axis. Commented Nov 1, 2022 at 14:49

## You are forgetting about slippage

There are all sorts of practical reasons not to do this like weight, gyroscopic forces making it hard to handle, consistency issues with aiming, energy requirements, etc... but these are just things that make it a bad idea, not impossible. For sake of this question, let's assume you can solve all of these problems using appropriate future tech and have narrowed the problem down exclusively to: "Can the flywheels impart the energy you need?" To this, the answer is still a pretty big nope.

In order for your flywheel launcher to work, it would need to transfer energy perfectly to the bullet, but when you spin a wheel against a surface in excess of the wheel's traction it slips. So, even if your wheels are moving at 910 m/s, there is no assurance that the bullet will. In order to make such a weapon even conceivably workable, you need to consider a method to accelerate the bullet over time in a way that allows the bullet to go from slipping to moving with the wheels in the time it takes it to leave the barrel.

A mechanism like a tennis ball launcher might only get 1cm of contact with a normal sized bullet meaning you would need to have enough traction to accelerate the bullet at 91,000 m/s^2 which is an unfathomably high rate of acceleration for what you are doing. In contrast, when firing a tennis ball, such a device only needs to reach speeds of 20m/s and gets closer to 2cm of contact meaning it's only accelerating at about 1,000m/s^2. While I can't put a hard limit on how much traction is possible without evaluating every conceivable material combination, I think it's safe to say that we can design a system to accelerate at 1,000m/s^2 with little difficulty.

So, instead of wheels, consider using rotating tracks ~91cm in length. If these tracks achieve the same traction as the tennis ball launcher, then when you feed the bullet into the system, it will slip against the tracks, but still achieve acceleration as it speeds up to 910 m/s^2 at which point it will stop slipping and begin moving with the tracks just as it leaves the barrel. Without doing too much material analysis, I think we can reasonably assume you can make a track with better friction than a tennis ball launcher, so something ~50cm in length (like a normal assault riffle barrel) would still seem reasonable, just remember as you shorten the barrel length that the traction requirements go up exponentially; so, getting down to handgun lengths is WAY less believable than getting down to a riffle length.

One final note to consider is that your 15cm wheels will make this a pretty big weapon system even if you work out all the engineering hurdles; so, chances are this will either be a vehicle mounted weapon, or you need to make those wheels much smaller.

• I don't know if i should add a new question or just ask here... But how much torque would this system need to have? High speeds means less torque, so... Any electric motor would be enough, or you would need some serious sized electric motor? Commented Nov 4, 2022 at 22:27
• @Fulano The engine can be weaker than you probably think. The bullets are not getting their energy directly from the engine but from the wheels. The wheels are much heavier than the bullet so you're relying on thier angular moment to propel the bullet, not the motor directly. So you could fire a burst of several bullets or maybe one larger projectile like a 30mm grenade before having to wait for them to get back up to speed. Commented Nov 5, 2022 at 2:44

## Engineering

A weapon with a 30cm diameter wheel spinning at an edge speed of 900+ m/sec is not going to be man-portable. The edge speed is much higher than the speed of sound, so it's going to be extremely noisy and will require a lot of power to keep it spinning at that speed. The wheel needs to be enclosed, or careless fingers will get shredded. Overall, it's just too big and heavy to be practical; the gyroscopic effects will also make it hard to move and point.

## Physics

The great advantage of chemical explosives is that they can release energy very quickly and briefly. It comes out as heat, and a conventional gun is a very simple heat engine. It isn't terribly efficient, but it does the job.

Your electric gun essentially uses a flywheel to store energy in a form that can be converted to bullet acceleration via friction. That conversion is very inefficient and hard to control, because the bullet will be flung away from the flywheel as soon as it touches it. Far more energy needs to be put into the flywheel than will end up in the bullets. This really isn't a good idea.

Close Enough

So the perimeter is $$2 \pi r = 0.2 \pi \simeq 0.63$$ metres. 60,000 rotations per minute is 1000 per second. So the rim of the wheel moves at 1000*0.63 = 630 metres per second.

From Wikipedia

Firearm muzzle velocities range from approximately 120 m/s (390 ft/s) to 370 m/s (1,200 ft/s) in black powder muskets,[3] to more than 1,200 m/s (3,900 ft/s)[4] in modern rifles with high-velocity cartridges.

So we are in the same area. If you could somehow get the tennis launcher spinning that fast without overheating or exploding, the ball would move at bullet speed.

It is a different question whether the ball would stay that fast for long, or what happens when the ball hits something.

What your talking about is known as a Centrifugal gun. The idea has been around since the American Civil War. I recalled the Mythbusters episode that attempted to create one. The overriding argument against them seems to always come down to accuracy. However given enough technical wizardry I see no reason the concept could work where a quiet heatless non chemical solution is needed to propel a projectile. In fact it a device like this could be a terrifying anti-personnel landmine type weapon. Several disks stacked on top of eachother popping up in the midst of an advancing enemy line, rapid firing firing 50cal steel balls in a flat 360 radius?

The concept is also in development to launch satellites into LEO.

• Centrifugal guns are slightly different. They load a bullet into a single spinning disc that accelerates as goes down a spiral track to a hole in the outside of the disc where it roles along the outside casing until exiting the firing hole. A similar weapon in that they both use spinning discs, but fire using very different principles. Commented Nov 2, 2022 at 21:21