DREAD weapon system, power requirements

[SK]

Depending on the mass of the disk, the angular velocity would decrease correspondingly. Imagine a heavy tank mounted disk, weighing say 200 kilograms, spinning at 100,000 rpm, with a relatively large electric motor that has been accelerating the disk for several hours. The decrease in angular velocity to eject 1000 tungsten spheres would be negligible, although each batch of spheres would travel a little bit slower in full automatic mode. Whenever the weapon stopped firing, the electric motor would kick in, gradually accelerating the disk back up to top speed.

While true, this somehow contradicts the claims of the supposed makers about the weight, portability and size of the DREAD. I edited my program to take disk mass into account, I´ll further extend it if I find the time. Some example outputs:

Disc with Diameter 762 mm has to spin at 20051 RPM to provide 800 m/s
to the outmost bullet on the disk. Energy required to spin the disk up with
accumulated 1000 Bullets in the 20 Feed trays and mass of the disk 20.0 kg
is 3.73 MJ. KE of bullets is 0.51 MJ then.

Disc with Diameter 762 mm has to spin at 20051 RPM to provide 800 m/s
to the outmost bullet on the disk. Energy required to spin the disk up with
accumulated 1000 Bullets in the 20 Feed trays and mass of the disk 200.0 kg
is 32.68 MJ. KE of bullets is 0.51 MJ then.

Note how grossly inefficient this whole thing gets. To accelerate 1000 round balls (@ 800 m/s , 1,48 MJ) towards your target you have to put 32 MJ of energy into the disk, and this is without taking into account any losses in drivetrain etc. , that will naturally occur. We better don´t start considering gyroscopic effect and the like here.
I´m not into material science, but it´s a good guess that you can´t just increase mass of the disk as you want, as all mass will be subject to centrifugal force, eventually leading to a (catastrophic) failure at the weakest link. Pervect went into this in one of the first posts.
The system mentioned in DefRev´s first article is said to weight in at 28 pounds empty, so I think we should go from there.

 

[epepep]

While true, this somehow contradicts the claims of the supposed makers about the weight, portability and size of the DREAD. I edited my program to take disk mass into account, I´ll further extend it if I find the time. Some example outputs:


Note how grossly inefficient this whole thing gets. To accelerate 1000 round balls (@ 800 m/s , 1,48 MJ) towards your target you have to put 32 MJ of energy into the disk, and this is without taking into account any losses in drivetrain etc. , that will naturally occur. We better don´t start considering gyroscopic effect and the like here.
I´m not into material science, but it´s a good guess that you can´t just increase mass of the disk as you want, as all mass will be subject to centrifugal force, eventually leading to a (catastrophic) failure at the weakest link. Pervect went into this in one of the first posts.
The system mentioned in DefRev´s first article is said to weight in at 28 pounds empty, so I think we should go from there.

I've got a bunch of sharp buddies interested in this one. Brought another physicist into the mix. Hope he responds. Another dude and I are still in the "no recoil" camp. Others agree with you. I think the recoil of a linear gun is f = ma. I believe the acceleration in the DREAD occurs as the balls are accelerated from the center of the disk to the outside, and is distributed across several revolutions, and thus applies 360° -- in effect no LINEAR recoil. Acceleration equals zero therefore force equals zero when the ball leaves. The recoil occurs by decreasing the Angular momentum. When the ball leaves the perimeter, it exchanges linear momentum for angular momentum. Total system momentum is conserved. (Hey I might be wrong!)

On the weight bit, this guy is obviously a salesman. I think a soldier would be limited to BB's. But a tank-mounted monster disk, weighing 200 kilograms, spinning up over the course of an hour or two, that MIGHT be able to generate some firepower.

 

[epepep]

Air Drag on a Sphere

The formula (English units -- sorry -- my main modern source is at work) for decrease in velocity of a supersonic sphere is:

Vs = V0 * exp-[12(A/w)*(rho_air)*(Cd)*R]

where A = projected area of sphere in sq in, w = weight of sphere in ounces, rho_air = density of air = 0.00071 oz/in3, Cd = drag coefficient = 0.43, R= distance traveled in feet, V0 = initial velocity ft/sec, Vs = striking velocity ft/sec

so Vs = Vo * exp(-0.0016*R) for a 7.62 mm tungsten sphere (mass = .16 oz, @ density = 19 g/cc)

so at 500 feet distance, if V0 = 8000 ft/sec

Vs = 8000*exp(-.0016*500) = 3600 ft/sec

at 1000 feet, Vs = 1615 ft sec

Still respectable, but 8000 ft/sec is way higher than standard muzzle velocity (~2200 ft/sec).

So I think we need to think way bigger here -- let's go for tank mounted, 120,000 rpm, 200 kilogram disk, 8000 ft/sec initial velocity.

Forget the infantry man and his BB's.

 

[epepep]

"Braniac" speaks

This is from my retired physicist friend. He's fantastically intelligent -- nearly half as smart as he thinks he is -- which is a bunch. But he's still wrong sometimes, just like everybody else...

1. Definitely not "no wear". I suspect the bearings will take quite a beating, but let's assume that the technology is available (jet engines, turbines, etc.) Without seeing the video, I would think the ejection port or whatever it is would definitely be subject to wear from mechanical "abrasion" and probably from heat. Do we grant them the technology for that? And whenever a bullet is ejected from a tray, the remaining bullets are going to move out radially and slam into whatever restrains them.

2. On first thinking, "no recoil" seems reasonable. Until release, the centripetal force required to hold the bullets in "orbit" is provided by the mechanical strength of the "gun" itself. When a bullet is released, it seems that there would just be a lessening of the tensile stresses. A "normal" gun experiences recoil because it suddenly imparts momentum to the projectile and must absorb an equal and oppsite impulse to balance that. In this system, the bullet already has attained its final momentum before it is released. The gun does not suddenly "act" on the bullet. Up until the moment of release, the gun has been acting on the bullet... it has been continuously changing the direction of the bullet's momentum vector. Upon release, it simply quits acting on the bullet... but the change which occurs is a lessening of the tensile stresses and it's not obvious that this would produce a recoil impulse.

3. Imbalance would definitely be a factor. Initially, the arrangement is symmetrical. But when the first bullet is released from the first tray, the symmetry is destroyed and the balance continues to change as each bullet is released.

The effect may well become greater as the ammunition is expended, because the mass of one bullet represents a steadily increasing percentage of the total remaining mass. You would actually have to consider how the moment of inertia is changing to verify/refute this.

It would seem that some sort of harmonic would be generated because the system would momentarily return to balance every time all the trays have released their Nth bullet. Can the materials and bearings handle this? I have no idea.

4. I agree with the guy, that if all the bullets are going to attain the same velocity, then the system has to provide a huge amount of power while firing. Why? Because initial spinup gets only the outermost bullet in each tray up to full speed. Thus the total rotational kinetic energy available to be transformed into linear kinetic energy is (far) less than the total linear kinetic energy of all the bullets would be if they were all ejected with the same velocity as the first bullet. Either the drive mechanism must provide a huge amount of power, or else the velocity of the succeeding bullets released from a particular tray will steadily decrease.

5. Certainly will be a gyroscopic effect. Would probably require a very powerful control system if you wanted to be able to change the plane of rotation during the firing sequence. If no such change were desired, then the gyro effect would be a good stabilizing factor.

In summary, an interesting concept, but it appears that the guys are pretty good "spin" doctors.

 

[Danger]

The engine (probably a gas turbine)

It's already been established that a turbine is pretty much the only (practical?) power source for this thing. Speaking now as a weapons designer; if I had to use a freakin' jet engine to power a weapon, I'd just channel it into a honkin' big blowpipe and hopper-feed the ammo into the blast stream. (More engineering involved than that, of course, and I'd never really design something that silly, but that's the basic principle.) I bet it'd be a lot more efficient and accurate than this monstrosity.

 

[pervect]

This is from my retired physicist friend. He's fantastically intelligent -- nearly half as smart as he thinks he is -- which is a bunch. But he's still wrong sometimes, just like everybody else...


2. On first thinking, "no recoil" seems reasonable.

Actually, it isn't at all reasonable. I'm surprised that someone who is a retired physicist could make this mistake. I was actually surprised that there is so much confusion about this very simple issue even by non-physicisits. Especially after the correct answer was pointed out a number of times.

If you shoot bullets out of a gun, they carry momentum. This causes recoil, because momentum is conserved. That's really all there is too it!

Working out the details in terms of the motion of the body is slightly tricker, but not a lot so, and has already been done by myself in this thread a couple of times.

I think I've already suggested thinking about a rotating rod on a frictionless surface a couple of times already. Imagine the rod initially has a zero average velocity, i.e. it's not moving except the fact that it's spinning. If a small piece of this rod breaks off, the small piece goes forwards, and the large piece goes backwards!

There are a number of ways of seeing this, of which the fact that the center of mass of any closed system stays at the origin is only the simplest.

A more complex way of seeing this may be simpler for some people (?). If one actually draws out the velocity of each piece of the rod as a function of position, one finds that it's a linear function of position, i.e.

v = -5*w at x=-5, v=0 at x=0, v=5*w at x=5

which can be summarized by the simple equation

v = w*x

So what happens if the piece from x=4 to x=5 breaks off? It obviously moves forwards. What happens to the remeaning parts of the rod?

Well, each small piece of the rod maintains its velocity. We can decompose the velocity of the large rod segment which strteches from (x=-5 to x=+4) as the sum of a rotation around it's new center of mass (at x=-.5) and a linear velocity as follows:

v(x) = w*x = (x+.5)*w - .5*w

The first term, (x+.5)*w, represents the velocity due to the rotation around the new center of mass at x=-.5, note that this term is zero at x=-.5 as desired, because x+.5 represents the distance between 'x' and the center of mass which is at -.5. x - (-.5) = x+.5.

You can think of this as

v(x') = w*x' - .5*w, where x' is the distance of a point from the new center of mass, so that x' = x+.5.

The second term, -.5*w, represents a constant linear velocity that the rod segement as a whole has (a negative velocity).

This result, which could have been and was predicted from the conservation of momentum, should not be a surprise, especially after it's been explained so many times.

The small segment of the rod shoots forward, and the large segement recoils backwards. The center of mass of the enitre system stays at the origin. The total linear momentum of the small piece in the forwards direction is equal but opposite to the total linear momentum of the larger piece, because the total momentum of the system is zero.

 

[transit442003]

there are probably millions of ways that we could try to counter act gyroscopic force and tensions but to look at this simply with the little information we have (hey they may invented some top secret material which is indestructible and all that) the costs for this one gun is going to be around the same as sending america to space so is the benefits that good to produce one? judging by the calculations u have made no the gun itself has no aiming capabilities whatsoever so really it is just fire and hope which isn't what u would want when having a barrel of a machine gun down your neck. the materials to be used are either so expensive or short supply so u would probably not get many of these weapons made and also the production costs are going to be huge and probably come out of tax payers pockets so i conclude that the money that will be spent on producing this is not worth the benefits (if any) that this weapon will provide.

 

[Alkatran]

There will be no 'recoil' per say (a sudden force backwards when the weapon is fired), instead there will be a constant (or almost constant, probably oscillate slightly) force pushing back on you.

Because the bullets are 'slowly' accelerated then released you simple have the force spread out over a longer time.

It would definitely require some kind of mount. Because if you're firing continuously all of that inertia is being transferred to whatever's holding the weapon.

 

[pervect]

The timing of when the forces appear depends somewhat on how one measures the forces - the vibration of the device may mask the appearance of a force by some reasonable defintions of "force" for up to half a turn of the rotor. Any such masking effect is eventually lost as the rotor turns, however. When you average over the entire turn of the rotor to "damp out" the vibrational effects and concentrate on the average force, the momentum to propel the projectile(s) has to come from somewhere - and the only place it can come from is from the weapons mount. The average net force will be v*dm/dt, where v is the velocity of the projectiles, and dm/dt is the mass flow rate.

 

[ohwilleke]

As far as weight goes, the biggest omission seems to be the weight of the power supply. This is a huge part of any railgun, and would be a huge part of this as well.

 

[SK]

On the weight bit, this guy is obviously a salesman. I think a soldier would be limited to BB's. But a tank-mounted monster disk, weighing 200 kilograms, spinning up over the course of an hour or two, that MIGHT be able to generate some firepower.

Yes, but there´s stiff competition. Modern 120mm tank guns deliver >10 MJ KE per shot at an instant, repeatedly, without the operator having to wait 2 hours.
For less than lethal applications (flinging lots of rubber balls around at ~100 m/s or so) I give the DREAD the benefit of doubt, for everything else it´s just too cumbersome and impractical.

 

[epepep]

Actually, it isn't at all reasonable. I'm surprised that someone who is a retired physicist could make this mistake. I was actually surprised that there is so much confusion about this very simple issue even by non-physicisits. Especially after the correct answer was pointed out a number of times.

If you shoot bullets out of a gun, they carry momentum. This causes recoil, because momentum is conserved. That's really all there is too it![/COLOR]**************************************

Hey, let's not get wrapped around the axle on this one. I just thought it was a good brain teaser. I'll give you the last comment and promise not to respond -- we'll agree to disagree. The rifle you talk about is very, very different in two important ways, from the spinning disk:

(1) The rifle is rapidly accelerating the bullet, F = ma. So say the bullet weighs 5 grams and the gun weighs 5000 grams; further the bullet is accelerated to 1000 meter/sec, and the rifle barrel is 1 meter long. By using v = at and s = ½at², by algebra, a = v²/(2S), where a = acceleration, t = time, v = velocity, s = distance.
Anyway a = (1000*1000)/(2*1) = 500,000 m/sec²
F = ma = .005 kg * 500,000 m/sec² = 2500 Newtons which is the recoil.

so the acceleration of the rifle
a = f/m = 2500/5 = 500 m/sec²

However, for the disk, the bullet is sitting on the edge of the disk, spinning. The acceleration occurred earlier, perhaps several minutes earlier, when it was accelerated from the center to the outside of the disk. Now it is just spinning at max velocity -- no linear acceleration (ignoring centripetal force). Then it is released.

(2) For the disk, the force does NOT act through the center of mass, as it does for the rifle. It has a lever arm, equal to the radius of the disk. To maintain equilibrium, the forces and moments must equal zero. With the rifle, the moment is zero, so there is a simple backward recoil. For the disk, there must be an opposite moment -- this occurs by a change in the angular momentum of the disk, i.e. the angular velocity. And this occurs when the torque was initially applied -- as the ball was accelerated from the center of the disk to the outside perimeter.

OK, I'll say no more -- you are probably right. That is just the way I, and some other dudes, see it.

 

[epepep]

[the materials to be used are either so expensive or short supply so u would probably not get many of these weapons made and also the production costs are going to be huge and probably come out of tax payers pockets so i conclude that the money that will be spent on producing this is not worth the benefits (if any) that this weapon will provide.[/QUOTE]
**********************************
Such utterly huge amounts are spent on other weapons (B2 Bomber = $2Billion apiece, modern fighter $50Million apiece, etc), that I think this technology deserves a shot -- perhaps $50 Million or so. If it doesn't work out -- it was worth the chance. Like everyone here has mentioned, it has some technical challenges -- but I think it shows enough promise to at least investigate it.

 

[epepep]

Yes, but there´s stiff competition. Modern 120mm tank guns deliver >10 MJ KE per shot at an instant, repeatedly, without the operator having to wait 2 hours.
For less than lethal applications (flinging lots of rubber balls around at ~100 m/s or so) I give the DREAD the benefit of doubt, for everything else it´s just too cumbersome and impractical.

***************************************
True, but the 120 mm gun was developed over several centuries. Perhaps it is worth it to pursue development of this spinning disk thingy for five years or so using modern technology to see what it can do?

 

[Alkatran]

Such utterly huge amounts are spent on other weapons (B2 Bomber = $2Billion apiece, modern fighter $50Million apiece, etc), that I think this technology deserves a shot -- perhaps $50 Million or so. If it doesn't work out -- it was worth the chance. Like everyone here has mentioned, it has some technical challenges -- but I think it shows enough promise to at least investigate it.

That is the stupidest logic I've ever heard.

Why not spend that 50 million solving something more important? We've already shown why the weapon won't work well.

 

[russ_watters]

I tend to take the opposite point of view as well - if the inventor/manufacturer think it has potential, let them front some money to deveop it into a real weapon.

I agree that it has some intriguing possibilities, but also agree that there are significant technical challenges (weight, power, and spin-up time) to overcome to make it viable.

 

[pervect]

My $.02

The idea does not look particularly promising for tanks - you want not only armor penetrating capacity, but accuracy at long range for an anti-tank gun. Accuracy is not going to be a feature of this weapon, with its vibration, gryoscopic effects, and spherical projectiles rotating at 90 degrees to the axis of flight.

It does not look particularly promising for a standard vehcile mount anti-infantry weapon either (IMO). The fact that you can carry a lot of ammo is nice, but the aaccuracy and gyroscopic issues aren't particularly nice.

Where it may have some promise would be where sheer volume of fire power at a relatively short range is important, for instance as a point-defense short-range anti-cruse missile weapon of last result. (The navy has some such system based on conventional high-speed cannon, its name escapes me at the moment). Spin-up time may be a big problem in this application, though.

Possibly it might work as a cannon replacement for aircraft or hellicopters, but I'm not sure about that.

One of the biggest problems is the inventor who is overselling the idea and making elemetary mistakes about physics in the process (elementary to the point where it is apparent he doesn't understand the conservation of momentum!) What the weapon needs is first a better defintion of its role, and then a good mechanical engineer type to take a close look at the system (mainly the rotor, though the pointing and aiming systems would also be an issue) to do a preliminary design and stress analysis - or perhaps to seek a very small grant for a person-year's worth of time to do such a preliminary analysis.

At a guess, this will never happen given the current personality of the inventor, who seems to be wrapped up in his own ideas (to say it politely) or a crank ( to say it not so politely).

 

[ohwilleke]

The name of the system you are thinking of is the Close in Weapons System (CIWS).

 

[SK]

True, but the 120 mm gun was developed over several centuries. Perhaps it is worth it to pursue development of this spinning disk thingy for five years or so using modern technology to see what it can do?

Contrary to what the inventor of the DREAD says about his weapon, everything a 120mm gun does is compliant with the laws of physics. You don´t have to ignore conservation of momentum and energy requirements to make it work in your head, like it´s the case with the overhyped DefRev version of the DREAD.
While we´re at it: During the development of guns ball shaped ammunition was abandoned more then a hundred years ago. With advertising it as accurate ammo Mr. St George is a bit late.

Where it may have some promise would be where sheer volume of fire power at a relatively short range is important, for instance as a point-defense short-range anti-cruse missile weapon of last result. (The navy has some such system based on conventional high-speed cannon, its name escapes me at the moment).

The problem here is that you need some range and high muzzle velocity, two things the DREAD will have big problems with due to energy requirements and ball shaped ammo. The Vulcan Phalanx CIWS is often considered too short ranged, against supersonic missiles reaction time is very low and there´s a chance that a missile, while hit by the system, will still impact the ship due to inertia or damage it in a close-by explosion.


In the "New Scientist" article ( http://www.defensereview.com/modules.php?name=News&file=article&sid=727&mode=&order=0&thold=0 ) the following is said: "St George says the projectiles travel at around 300 metres per second upon release from the weapon, about the same speed as a handgun round.". This sounds at least more realistic than the hype in the DefRev articles and is in line with what others and me have suggested:
If it´s going to be real, it will be short ranged and very limited in it´s applications (no tank or ICBM killing, yadda yadda).

Does anyone have the formula for velocity drop of subsonic spherical projectiles in air?

 

[Danger]

Does anyone have the formula for velocity drop of subsonic spherical projectiles in air?

Not at all, but check the ballistics figures for a .38 plus P handgun round (ME, MV, mid-range trajectory, etc.) and cut the numbers by at least 20%.

 

[extreme_machinations]

HOW 'S THIS DREAD SYSTEM DIFFERERNT FROM A BASEBALL OR TENNIS BALL MACHINE[THE ONE THAT THROWS BALLS AT YOU] ,i bet the inventor got the idea from these ,just changed the balls to golf balls ,hehehehehehehehehehehe
STUPID GIT probably could'nt work out the momentum details .

 

[Danger]

HOW 'S THIS DREAD SYSTEM DIFFERERNT FROM A BASEBALL OR TENNIS BALL MACHINE

As far as I know, all pitching machines use the same system as the Hot Wheels 'supercharger' I had 40 years ago. ie: the ball (or car) is introduced to the rim of a rapidly spinning wheel(s), and almost instantly accelerated to it's launch velocity. This Dread fiasco seems to maintain them in a constant state of acceleration which is confined by the housing until the release mechanism (and how the hell does that work?) unlocks and let's them out.

 

[extreme_machinations]

i think even in a pitching machine the ball is introduced at the centre of the spinning wheel fitted with throwing arms which is then acclerated[rotated] ,the ball then slides up the throwing arms and flies off tangentially at the realease point which is same as the dread sys ,a slight diff is that it just doesn't hold the balls long enough for the machine to accelerate it to a high speed .
yup ,to me they're both same .

 

[Danger]

i think even in a pitching machine the ball is introduced at the centre of the spinning wheel fitted with throwing arms which is then acclerated[rotated

That might be right. I've only seen them from a distance or on TV. Regardless, no one ever claimed that they have no recoil. They also have a barrel of sorts for accuracy.

 

[lynxpilot]

My approach would be to try to 1. not put the cart in front of the horse, and 2. not to reinvent the wheel.

There were some claims by the inventor and the supporters of the inventor that the weapon has "no recoil", emphasis on quotes. I will quote more loosely the other claims because I can't be bothered to look them up and that would be the 8,000 fps projectile velocity, with 120,000 rounds per minute, and a 150 W power supply.

All you need to do is to take the weight of the projectiles, the velocity, the firing rate, and to calculate the power requirement. Use physical constants like conservation of energy and momentum. Take the energy of each projectile divided by the rate of fire and come up with power. It's easy. You can also calculate the force on the weapon which will create recoil. And of course you, as physics students, know that this weapon cannot fire projectiles of ANY mass without recoil. That alone is enought to debunk the whole thing.

This thing is a hoax. Use your basic skills in physics.

 

[JO 753]

I woudnt be so quick to dismiss this, especially since they are claiming a working prototype. I can think of several things rite off the top of my head to counter some of the major problems you are citing. Going by the patent info is useless, since it could be purposely misleading to keep the device from being copied. Same thing could apply to some of the obviously wild claims.

 

[Danger]

Going by the patent info is useless, since it could be purposely misleading to keep the device from being copied.

Not if it's obtained from the Patent Office.

 

[pervect]

I woudnt be so quick to dismiss this,

I guess there's one born every minute, as PT Barnum said.

 

[JO 753]

Maybe you have yet to discover the value of patent 'protection', Danger. The only real way to protect an idea is to keep it out of the heads of anybody who may want to steal it.

 

[Danger]

Maybe you have yet to discover the value of patent 'protection', Danger.

I have a pretty fair exposure to patents, actually. Information obtained from a Patent Office is a copy of the original filing. No one knowingly files false information in a patent application for a practical product, because it doesn't protect the real design. The patent is for legal purposes only, giving you the right to seek compensation through the courts if someone kifes your idea. A false filing would do nothing in that regard, and anyone can still 'reverse engineer' the thing once they see it.

 

[agribusinessman]

1. Velocity 800 ms ~ approximation from literature
2. Diameter 762 mm – assumed from “32 inches wide” in literature
2. Rotational Speed ~ 20051 RPM (335 Hertz) - derived from diameter assumption.
3. Feeder Trays - 20 (from literature).
4. Mass of projectile: 4.633 g - Huh? I would use 3.7 grams. You don't know the exact alloy so the density is +/- 1 percent. A stainless steel dimpled ball of 7.62mm (.308) diameter is 1.7 grams, 12.7 mm (50 caliber) is 8.3 grams (1-2% loss due to dimpling) . Density is 18.8 for tungsten, 15.7 g/cm3 for tungsten carbide, and 8.0 for stainless steel. The steel ball weights are for actual steel balls (commercially available) derated 1% for dimpling.
5. Shot burst: 10. This causes some conflict with the 120000 rounds per minute number since it implies a rotation speed of 12000 rpm (200 hertz). Which implies the production device will be 1.3 meters wide (allowing for outer casing width).

From the above:
1. The device fires 10 rounds per spin (1/2 of trays). As long as the device is programmed to fire in units of 10 it will only experience rotational torques for a fraction (9/10th) of a rotation and will be balanced when not firing.
2. The device fires in a 2 cycle round robin. This gives the 3 ball shot advance mechanism 2 cycles to move the next ball in a set of trays into position.
3. The device operates at the speed of a high performance hard drive (which takes 7 watts and can spin up in several seconds).
4. There is talk of "reloading/rearming" in the patent. Although a gatling gun loading mechanism is possible (dump shot into the center of the disk and let rotational inertia fill the slots) reloading and firing are apparently separate steps.
5. I am familiar with mechanical harvesters (for tree crops) which use 30 hertz rotations, and 100 lb edge weights on 2 ft reaction wheels. The rotational speed of the harvester is 1/10 that of the Dread device resulting in accelerations 1/100 as great on an weight imbalance 10000 times greater. The havester is not destroyed by use. Pneumatic isolation of the disc (perhaps with fluid dampening) will be sufficient to deal with the momentary imbalances. The motor should be connected to the wheel with a flexible coupling.
6. If the weight imbalance acts through the center of mass on the axis the net effect of the forces won't cause precession, i.e. the forces won't have a component perpendicular to the plane of rotation.
7. Start up torque requirement. For the sake of argument we will assume that the weight of the ammunition (308 caliber) is 3700 grams. The weight of the “disk” is assumed to be the same as that of the ammunition. Total weight 7.4 Kg. Radius = .76 m. Moment of inertia ~ 2.14 kg/m2 (solid cylinder approximation). 5 second start up is assumed (same as a computer hard drive).
a = Δv/Δt
acceleration = 2* Π * 335 hertz/5 = 210 rad/s2
t = Ia = 2.14 kg*m2 * 210 /s2 = 449 kg * m2 /s2 = 449 Nm

449 NM is a fairly large electric motor (on par with an engine dynamo). So the system is practical but has a fairly long “arming” period (longer than the 5 seconds used for illustration) for the disk to spin up prior to firing. There will be a loss of rotational speed during continuous fire. Making the inertia of the disk greater will mitigate this.

 

[pervect]

1. Velocity 800 ms ~ approximation from literature
2. Diameter 762 mm – assumed from “32 inches wide” in literature
2. Rotational Speed ~ 20051 RPM (335 Hertz) - derived from diameter assumption.

While you're at it:

centripetal acceleration of disk:

(800 m/s)^2 / .4 m = 1,600,000 m/s^2

(I used 400 mm for the radius, that's rounded up, and closer to 16" than your figure).

For comparison, a typical high speed centrifuge generates "up to 30,000 gravities" (from the www - that's only 300,000 m/s^2), and it has only a static load, not the dynamic load generated by feeding it bullets.

I think if you look back in the thread you'll see calculations that the 800 m/s version will fragment a steel rotor even without any other loads.

 

[SK]

1. Velocity 800 ms ~ approximation from literature
2. Diameter 762 mm – assumed from “32 inches wide” in literature
2. Rotational Speed ~ 20051 RPM (335 Hertz) - derived from diameter assumption.
3. Feeder Trays - 20 (from literature).
4. Mass of projectile: 4.633 g - Huh? I would use 3.7 grams. You don't know the exact alloy so the density is +/- 1 percent. A stainless steel dimpled ball of 7.62mm (.308) diameter is 1.7 grams, 12.7 mm (50 caliber) is 8.3 grams (1-2% loss due to dimpling) . Density is 18.8 for tungsten, 15.7 g/cm3 for tungsten carbide, and 8.0 for stainless steel. The steel ball weights are for actual steel balls (commercially available) derated 1% for dimpling.

These are values I approximated by looking at the patent paper, the DefRev article and by using common values in current weapon tech (bullet speed and diameter) for comparison. I set density of Tungsten to 20 g/ccm, that´s not too far from 19.3g/ccm you commonly find.

3. The device operates at the speed of a high performance hard drive (which takes 7 watts and can spin up in several seconds).

Well a hard drive doesn´t spin that fast (yet), and it´s a closed, light system, so the comparison doesn´t tell much.

I am familiar with mechanical harvesters (for tree crops) which use 30 hertz rotations, and 100 lb edge weights on 2 ft reaction wheels.

Do you got a picture of these? Isn´t there a counterweight so net centrifugal forces amount to zero?

6. If the weight imbalance acts through the center of mass on the axis the net effect of the forces won't cause precession, i.e. the forces won't have a component perpendicular to the plane of rotation.

Well, the thing will still react wildy to changes in direction due to gyroscopic effect. I think that was the main issue presented here.

449 NM is a fairly large electric motor (on par with an engine dynamo). So the system is practical but has a fairly long “arming” period (longer than the 5 seconds used for illustration) for the disk to spin up prior to firing. There will be a loss of rotational speed during continuous fire. Making the inertia of the disk greater will mitigate this.

449NM might doesn´t seem much, but 449NM at 20000 RPM isn´t quite as easy to achieve, IMHO something like this will badly get into the way:

No matter what the torque requirements are, the power requirements I calculated are valid and in the Megawatt range. It might be possible to make the problem sound small by presenting a value like 449NM, but that doesn´t mean you don´t have to put an average of 0.98 MW into the system in my intial example.
Making the inertia of the disk greater might help mitigating the velocity loss of the ammo, but it also means you have to put much more energy into accelerating the whole thing, see my 20kg and 200kg disk examples above.
As Pervect said, just playing around with disk mass isn´t that trivial either, as strain on material is very high, making it a whole topic for itself.

 

[Felix_the_cat]

Almost no recoil

Hey guys,
here comes the next foreigner with bad english.
I'm from Germany

I read your whole discussion, and if you consider the projectile which is shot, is not replaced by another one, there will definitely be a recoil.

But, just imagine two rails spinning round the center of a closed circled box. (check attachments)

If you then put a ball in the middle, it will be draged out as far as possible.

If you now put in 20 on each rail and one ball is released from one of the two rails another ball wil replace it at once.

The difference of weight on the one rail will be almost in the center of mass, and if balls a continously supplied eliminated at once.

 

[3trQN]

It claims to be virtually silent haha, when was the last time you heard (sorry didnt hear) an electrically powered centrifuge that runs at 120,000rpm... This review SoldierTech reads like some trekky has had too much coffee when writing the review...

It may be virtually silent when running in your head.

 

[Crosson]

To the original poster, yes you are absolutely correct. This is the true content of the equivalence principle, that a freefalling observer is in a locally flat spacetime, just like an observer moving at constant velocity. In freefall we do not feel our own weight, this is what is meant by weightlessness i.e. the lack if a force.

 

[DuckPhup]

Recoil? Nonsense! Precession? Nonsense!

A counter-rotating disk of equivalent mass will eliminate precessional effects. Not an issue.

Once the gizmo is up to speed, with fully loaded ammo channels, you've essentially got a flywheel... which wants to stay in motion (according to Newton). The only subsequent energy requirements will be what is required to restore angular momentum lost to frictional forces, and the minimal inertial forces involved in accelerating individual pellets in 1/50th-D. step increments.

Re recoil: When a pellet is released, there will be no recoil. Think about it... a bit of the mass of the flywheel will be lost. Under constant power input, the darn thing would be trying to speed up... not slow down. Of course, a velocity servo control system would tightly control that.

Unbalancing: Not an issue. Remember, the thing has a magazine capacity of 10,000 (.50 cal) or 50,000 (.308 cal) rounds. Suppose that there are around 1,000 pellets (.308) in the flywheel channels (20 x 50). That tells us that there is a central hopper full of pellets, supplying a chamber where there are a whole bunch of pellets, whirling around, desperately trying to get into the feed channels... but being prevented from doing so by virtue of the fact that the feed channels are already full. When a pellet is released, the pellet column instantly advances .308 inches, and the expended pellet is instantly replaced... the channels essentially remain solid. Any purturbance in the system created by the 'release' (NOT 'discharge') of a pellet... a very tiny fraction of the mass of the flywheel system... will be very minute, and very short-lived. So, there might be a tiny bit of vibration, which can be dampened by a sufficiently robust support/drive system. (This is probably the reason that there are 20 channels, rather than 6 or 10 or 12... or 24 or 36. Too few channels... to much vibration. Too many channels, too much additional flywheel bulk required to maintain integrity.)

Re recoil (again)... and conservation of energy: The 'recoil' is pre-loaded into the system, and is felt long BEFORE a pellet is expelled. The energy expended in bringing the flywheel (including its full load of pellets) up to its designated rotational speed is the equivalent of the 'recoil' experienced by conventional munitions.

Re Spinning projectiles: Get a golf ball, and drill a hole through it. Attach about 3-feet of kite string. Twirl it around your head, as fast as you can, then let go of the string. Does it spin? (Hint: No.)

It seems to me that such a system will operate pretty much as claimed by the inventor, and the only real impediments would be materials-related... i.e., building it from something that won't come totally unglued at the desired angular velocities. Also, diven the spherical projectiles, it probably only sufficiently accurate over relatively short ranges, compared to conventional munitions. Of course, the sheer volume of fire might mitigate that deficiency, to an extent.

 

[Mech_Engineer]

Your examples are ill-conceived at best, and misinformation at worst. Several people have gone through intensive calculations analyzing the power requirements and physics involved. Unless you have some ACTUAL MATH to prove you point, your claims are no better than the mud-slinging inventor's.

 

[WellDuh]

Easy Peasy mate

Took me maybe 10 minutes figure this one in the rough.

counter-rotating room-temp superconductor electromagnetic centrifuges with center feeds.
https://www.physicsforums.com/images/smilies/cool.gif


This balances effects of lost angular momentum by paired synchronized fire from centrifuges with opposite spin. No mechanical rotor.
https://www.physicsforums.com/images/smilies/love.gif
:!)

Bullets are magnetically levitated so as not to touch the interior once feed in - so frictionless. Magnetic gating of projectiles. Gee doesn't this sound like a pair of Cyclotrons scaled up to macroscopic particles via the miracle of superconductors?

No moving parts past the feed mechanism...and probably not there if you are clever (magnetically siphon out ammo from ammo tank).

Power arguments? Really moot. It is common for weapons to quote fire rates that cannot be sustained. So yes you are limited to the energy available during a given firing period. https://www.physicsforums.com/images/smilies/rolleyes.gif


But then again lasers commonly store up energy in capacitor banks for discharge over a short period of time. So DREAD can meet its specifications for short burst fire. Very short bursts with long recharge times for man portable. Several or longer bursts with HUMVEEs helicopters etc with short recharge periods.
https://www.physicsforums.com/images/smilies/blushing.gif


Of course several other weapon systems have started needing higher energy power plants with HUMVEEs. Not that I am endorsing them as practical or wise for the battlefield -- but reports exist of multi-megawatt power sources to accompany HUMVEE and small truck lasers coupled with large quick discharge power banks. Hmmm I wonder what a stray penetrating round would do there? Similarly what happens if those superconductors in the weapon breakdown? I am betting you don't want to be the gunner in a HUMVEE or man portable -- crispy critter time.
https://www.physicsforums.com/images/smilies/surprised.gif


Accuracy...hmmm. Well further electro mechanical gyroscopic gun stabilization would likely help (sort of like Abrams) sink minor vibrations. Plus nothing says you can't add a follow on magnetic guass gun barrel to correct aim point velocity after the ammo is "charged" with KE directed in approximately the right direction.
https://www.physicsforums.com/images/smilies/angel.gif


Plus the anti-tank guys are missing the fact that the decreased spacing of arriving ammo is even like hydraulic drilling to armor. This gun can just afford to paint more square inches of armor with effect. Effectively the same weight of ammo is delivered per square inch even spread out a bit. That is IF there is increased drift over more stable guns, it is countered more total arriving ammo. And I am not really sure we have reason to believe there there will be more drift over something like the A10 30mm gun - after all that vibrates the whole plane in space. You need a real weapons tests or full engineering data and simulation to even speculate on this.
https://www.physicsforums.com/images/smilies/redface.gif

 

[WellDuh]

It claims to be virtually silent haha, when was the last time you heard (sorry didnt hear) an electrically powered centrifuge that runs at 120,000rpm... This review SoldierTech reads like some trekky has had too much coffee when writing the review...

It may be virtually silent when running in your head.

But what if the bullets are spun by a pair of counter-rotating superconductor electromagnets? https://www.physicsforums.com/images/smilies/love.gif
:!)
Then the only moving part is the bullets. And I suspect the chamber quickly becomes a vacuum during fire. Basically this is a Guass gun circularized to solve the recoil and barrel length problems.
https://www.physicsforums.com/images/smilies/biggrin.gif


However, the gun could still be located by stereoscopic acoustic analysis of the sonic cracks. Easily performed by a PDA and a couple of sound guns at known baseline locations a few hundred meters apart -- or militarily a radio linked system of two or more GPS locatable mobile parabolic dishes.

The real point being that an unaided human spotter couldn't locate such a superconductor gun. I forget the precise length of the base line for triangulation required. Something like it has to be longer than a halfwave length of the shockwave at the speed of sound at the observation points.

 

[WellDuh]

Your examples are ill-conceived at best, and misinformation at worst. Several people have gone through intensive calculations analyzing the power requirements and physics involved. Unless you have some ACTUAL MATH to prove you point, your claims are no better than the mud-slinging inventor's.

Weapons almost never sustain peak fire rates. Most mechanical weapons systems must stop after a few hundred rounds at peak rates due to heating effects - anything from thermal expansion effects to incipient barrel damage (phase changes in metals). Think firing in bursts.

Here the limit is likely energy drawn from a storage bank and the recharge rate of the generating power source. Oh and you can probably "pre-charge" a number of bullets in the centrifuge itself at just below release speed. Plus if needed you might be able to plug in a second or third HUMVEEs generating system to your storage (capacitor?) bank -- especially useful as not all HUMVEEs would mount such weapons.

Also remember two things. Those bursts allow you to beat through armor or break an enemy charge. And if you need sustained fire on this weapon you can drop back to as low a rate as your power source can sustain.

 

[WellDuh]

1. Velocity 800 ms ~ approximation from literature
2. Diameter 762 mm – assumed from “32 inches wide” in literature
2. Rotational Speed ~ 20051 RPM (335 Hertz) - derived from diameter assumption.

While you're at it:

centripetal acceleration of disk:

(800 m/s)^2 / .4 m = 1,600,000 m/s^2

(I used 400 mm for the radius, that's rounded up, and closer to 16" than your figure).

For comparison, a typical high speed centrifuge generates "up to 30,000 gravities" (from the www - that's only 300,000 m/s^2), and it has only a static load, not the dynamic load generated by feeding it bullets.

I think if you look back in the thread you'll see calculations that the 800 m/s version will fragment a steel rotor even without any other loads.

And there is your problem -- you assume a mechanical rotor. Is there a rotor in a Cyclotron? Nope.

So what if you applied some room-temperature superconductors to levitate and spin only the bullets themselves?

 

[Danger]

So what if you applied some room-temperature superconductors to levitate and spin only the bullets themselves?

If you can find any, I want to buy one.

 

[WellDuh]

My approach would be to try to 1. not put the cart in front of the horse, and 2. not to reinvent the wheel.

There were some claims by the inventor and the supporters of the inventor that the weapon has "no recoil", emphasis on quotes. I will quote more loosely the other claims because I can't be bothered to look them up and that would be the 8,000 fps projectile velocity, with 120,000 rounds per minute, and a 150 W power supply.

All you need to do is to take the weight of the projectiles, the velocity, the firing rate, and to calculate the power requirement. Use physical constants like conservation of energy and momentum. Take the energy of each projectile divided by the rate of fire and come up with power. It's easy. You can also calculate the force on the weapon which will create recoil. And of course you, as physics students, know that this weapon cannot fire projectiles of ANY mass without recoil. That alone is enought to debunk the whole thing.

This thing is a hoax. Use your basic skills in physics.

Recoil is a tricky question in that they are obviously talking subjective recoil. Further for physics we are asking a question of momentum and not kinetic energy.

So given two counter-rotating centrifuge systems which release projectiles with opposite momentum essentially in synchronization --what is the net effect? Total system momentum is still in balance for each pair, is it not?
https://www.physicsforums.com/images/smilies/eek.gif


Duh 120k rounds per minute is a burst RATE. It does not say that burst rate can be sustained for a minute. In fact there are references that say a burst is like 10 rounds (or 1/200 of a second). Quoting fire rates to round per minute is merely for convenience in comparing different weapons systems.
https://www.physicsforums.com/images/smilies/grumpy.gif


I suspect the time between bursts varies with what is providing main power -- obvious quite different for man carried or HUMVEE or Jet engined vehicle. And sustained fire rate is going to be much lower for all but the largest powered vehicles. https://www.physicsforums.com/images/smilies/frown.gif


Hmmm...many devices have more than one power supply. I would suspect that the quoted 150W power supply primarily refers to powering the common control mechanisms. They did say the info given was incomplete.

 

[russ_watters]

If you can find any, I want to buy one.

You can buy it - what I want is the patent!

 

[WellDuh]

While we´re at it: During the development of guns ball shaped ammunition was abandoned more then a hundred years ago. With advertising it as accurate ammo Mr. St George is a bit late.

You probably need to readdress whether the old reasons for cylindrical ammo over ball ammo is still valid.

(1) Primary reason -- Cylindrical ammo gave a much better gas seal than spherical but irrelevant in this case

(2) The aerodynamic lift effects of spinning cylinder is more easily fixed than sphere spinning in random direction -- relevant for subsonic although spherical ammo doesn't bleed energy like tumbling cylindrical ammo; But I highly suspect this is not relevant for supersonic spherical ammo as the shockwave in front of the sphere will bury rotational effects.

Also dimpling addresses many of the subsonic issues.

"St George says the projectiles travel at around 300 metres per second upon release from the weapon, about the same speed as a handgun round."...

Does anyone have the formula for velocity drop of subsonic spherical projectiles in air?

Maybe the prototype does only fire at 300 m/s. Whatever makes you feel good about your old blackpowder guns. https://www.physicsforums.com/images/smilies/biggrin.gif


However, if DREAD really does use superconductors to speed bullets similar to how a Cyclotron speeds atomic sized particles...then supersonics is easily in range given enough impulse power (as in capacitor high rate flash discharge).

I don't believe the unmodified Stokes Law applies to supersonic objects. But the rate of velocity loss is probably similar for any two simple objects presenting the same frontal surface area and without any designed supersonic aerodynamics such as wasp waisting. That would be an advantage for conventional projectiles - they can be designed to have supersonic aerodynamics for a price and at risk of complicating and jamming ammo feeds. However I've vaguely recall that once you breach the hypersonic range most of those supersonic aerodynamics once more go out of play. So unless someone can enlighten us with some links to equations --

I suspect that spherical and cylindrical ammo are just about equal in performance near and above the speed of sound. Further for anti-tank/anti-missile uses you are really at the limit of your range once your ammo drops below sonic. https://www.physicsforums.com/images/smilies/rolleyes.gif


So cylindrical ammo has a measurable range advantage for anti-personal work and unarmored vehicles once at subsonic speeds. On the other hand if the ammo starts as supersonic you are likely talking miles of range...which might mean extra range is a disadvantage because you can't see exactly who you are hitting (civilians or allies trying to flank the enemy). https://www.physicsforums.com/images/smilies/eek.gif

 

[WellDuh]

Room Temperature - maybe maybe not

If you can find any, I want to buy one.

Can you afford military prices?

Well actually technically high temperature superconductors are what is known proven state of the art. And they would be usable with a good liquid nitrogen supply.

On the other hand you did notice this might be a Bell labs guy didn't you?

 

[WellDuh]

Practical Superconductors 101

You can buy it - what I want is the patent!

Well do you think it is practical with Type IIs?

Feed me some detailed knowledge on something I am not that familiar with.

 

[WellDuh]

Practical Superconductors 101

You can buy it - what I want is the patent!

Well do you think it is practical with Type IIs?

Feed me some detailed knowledge on something I am not that familiar with.

Really I would think price would be what would make this a weapon of the distant future (20+ years).

 

[Danger]

Will somebody please lock this thread before my head explodes?
Or, at least, teach Duh how to either use smileys properly or not at all.