The DREAD, and why it won’t work.

“No heat, no recoil, no sound, no gunpowder, no flash — just 120,000 rounds per minute of pulverizing power. The next generation of weapons systems has arrived: the DREAD centrifuge-powered weapon system.”

Erm, no.

The DREAD is a weapon system that supposedly offers all sorts of benefits (outlined above). There are many gushing positive articles about it all over the web saying how it could revolutionise warfare ({1}, 2, 3). Unfortunately, they’re all wrong.

How it works

The DREAD consists, as far as I can tell, of a spinning frame that picks up little metal balls, spins them around at 3,000 ft/s (914 m/s), and in later versions, the inventors claim, at 8,000 ft/s (about 2.3 km/s). That’s very fast. Not only that, but it can fire 2,000 of these every second.

Here’s a picture of it.

Lets pick apart the claims shall we?

1. No Recoil

As every secondary (or ‘high’) school physics student knows, and as Mr. Newton so kindly demonstrated for us, there is no action without an equal and opposite reaction. Basically, if you throw something one way, you get pushed the other.

This manifests itself in the recoil of a gun — bullet flies off in one direction, the gun gets kicked back in the other. It’s how propeller, rocket and jet engines work (throwing air/fuel backwards in order to go forwards). Nothing is exempt from the rule. Not even the DREAD.

So how much recoil would a DREAD produce? By my calculations one of the 0.5 calibre balls would have a mass of about 60 g (assuming the density of iron). A simple calculation shows that at 8,000 f/s, 2,000 every second, the recoil would be like a force of 300,000 N. Thats equivalent to 31 tonnes of recoil. You’d have a hard time keeping that steady.

Note: For those of you who are picky, I know recoil is not a force but a series of impulses. What I’m quoting here is an ‘equivalent force’, if you like.

2. No Sound

True, there’s no explosion, which is most of the noise from a gunshot. But, there are metal balls travelling at 7 times the speed of sound, 120,000 a minute, and these will generate one hell of a sonic boom. However, I guess it’s fair to say it will be harder to tell where the shooter is.

3. No Heat

From what I can tell, the spinning bit on it can’t be more than about 30 inches wide. To have a rim speed of 2.3 km/s, it would need to spin at nearly a thousand times a second, or 60,000 rpm. That’s fast, but not outlandishly fast (some turbines can reach 150,000 rpm) so this claim I guess will not be too bad.

However, there is another problem, and that is:

4. No Gyroscopic Effect

Have you ever held a spinning bike wheel by its axle and tried to turn it? You’ll find it doesn’t move the way you want it to. This is called precession, or the gyroscopic effect. It’s what keeps a gyroscope or spinning top upright, and makes it difficult to turn spinning things. And the DREAD is spinning very, very, very fast, and so should experience a massive amount of gyroscopic effect. But it’s alright, because:

“According to Charles St. George, the DREAD’s inventor, the DREAD does NOT create/cause ANY gyroscopic forces or effects, whatsoever. To put it another way, the DREAD does NOT act like a gyroscope. So, the DREAD will NOT have ANY adverse effect on the mounting vehicle’s maneuverability, even while the DREAD is being fired on its highest rpm and velocity settings. NO adverse effect. None, whatsoever. I can’t make it any clearer than that.”

Well, that’s okay then.

4. Totally reliable

Defense Review claims that the weapon cannot stop or jam. No evidence has been produced to back up this claim, but it seems incredible that something with parts moving round at 60,000 rpm won’t experience at least a little wear and tear (especially in a combat environment, where its being knocked about and dust, sand etc. could get inside), and it seems reasonable to assume that this will eventually bring the whole thing to a stop.

Conclusion

I’m sure the concept could be made to work. However, not the way they’re saying it could. What’s worse, the inventors are promoting their device by making outrageous claims based on pseudo-physics and downright lies. The websites linked to above repeat these claims without knowing the first thing about basic physics (even New Scientist), and in doing so mislead the general public.

DREAD homepage (now defunct, 8/05/2008)

Edit: Why the DREAD produces recoil

There has been some doubt expressed as to whether allowing a projectile to leave (rather than forcing it) can generate recoil. I argue that it can, and here’s my why.

Note: secondary (high) school physics required.

In a normal rifle, there is an explosion that exerts a force forwards on the bullet, pushing it forward. There is an equal force on the rifle, pushing it backwards. This means the rifle has a momentum backwards, towards your shoulder.

In order to bring the rifle to a halt, your shoulder has to exert a force on the rifle butt, forwards. This, by Newton’s third law, means that rifle exerts a force on you, into your shoulder (backwards). This is the force that is felt as recoil.

In the DREAD, we have the scenario of a spinning disk with masses arranged equally around the edge. For simplicity, we will assume there are just two. This disk is spinning, but because the centre of mass is not moving and so has no overall momentum. With the disk spinning clockwise:

COM is the centre of mass of the entire system, p denotes the momentum of each ball. Notice how, because the balls are travelling in opposite directions, the momenta cancel out to zero.

However, imagine that the left projectile is now released. It is no longer attatched to the gun, and so can be discounted as not part of the system. We now have this situation:

The ball on the left is detatched from the disk, and now rapidly flying towards the top of the picture, but we can ignore it.

Now, the total momentum of the system is p, backwards into your shoulder. This, like the situaton described above, means that your shoulder pushes against the gun, the gun pushes against your shoulder, and hence: we have recoil.

If your shoulder wasn’t there, the DREAD would continue to fly backwards forever (or until it hit something). Notice that it has momentum p, thus momentum is conserved. I know I’ve only demonstrated it with two balls, but this argument can be generalised to more.

If you’ve managed to follow this, congratulations, hopefully you too can decry the DREAD as pseudoscientific blather.

If you haven’t, please don’t make angry posts.

---