Bullshit Bingo, and DREAD
May. 12th, 2005 11:47 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
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djfiggy, this New Scientist story about 'DREAD', a supposed new wonder-weapon. It's worth watching the promotional video, which is amusingly cheesy - with all the cut-out animation, I was half expecting a giant foot to come down and squish the bad guys.
The basic idea: take a bunch of ball bearings, spin them up to high speed in a circular chamber, and then release them so they fly towards the target. Certainly possible in theory, whether you can make it practical is another matter. The NewScientist article is somewhat sceptical, but not nearly sceptical enough. I've collected some passages from the video and related articles... you can all just read along and press your buzzer every time you see flagrant bullshit.
A couple of choice passages from the video:
"The DREAD offers an extraordinary rate of fire, up to 120,000 rounds per minute in controlled bursts, with no recoil... the DREAD's lack of recoil means there is no need for special mounting devices."
(Accompanied by dodgy animation of satellite firing DREAD at something. BTW, to avoid confusion, when a weapon's described as firing "X rounds per minute", this isn't usually intended as a sustained rate for a whole minute - it's like a bullet that travels at 700 km/hr even though it's only in flight for a second or two.)
"The DREAD is the first small-arms weapons [sic] to offer real stealth characteristics. It has no blast, flash, or heat signature, and can therefore operate in a sustained fire mode without detection in all combat situations."
"...totally eliminate the multiple failure modes that conventional weapons exhibit due to their complexity, thermal characteristics, and gas discharge. In addition to reliability, safety has been a major consideration in the DREAD design. The lack of powder propellant means ammunition storage may be accomplished without the substantial precautions currently required."
The brochure claims it to be "totally jam-proof".
Also available: a related patent.
And from a Defense Review Article: "The DREAD depicted in the video is a functional prototype that operates on a less-than-lethal mode. [I suspect this means "the prototype isn't actually powerful enough to match the hype".] This prototype was dismantled for security purposes to protect the technology, after the making of this video..."
"The DREAD is both heatless and frictionless, and doesn’t generate any high pressures. So, there’s virtually no wear and tear on the system, no matter how many rounds are fired through it back-to-back, even if it's run constantly on full-auto at 120,000 rpm, the whole time."
"Imagine a gun with no recoil, no sound, no heat, no gunpowder, no visible firing signature (muzzle flash), and no stoppages or jams of any kind. Now imagine that this gun could fire .308 caliber and .50 caliber metal projectiles accurately at up to 8,000 fps (feet-per-second), featured an infinitely variable/programmable cyclic rate-of-fire (as high as 120,000 rounds-per-minute)..."
"According to its inventor, not only is it possible, it’s already happened. An updated version of the weapon will be available soon. It will arrive in the form of a tactically-configured pre-production anti-personnel weapon firing .308 caliber projectiles (accurately) at 2,500-3000 fps, at a variable/programmable cyclic rate of 5,000-120,000 rpm (rounds-per-minute). The weapon's designer/inventor has informed DefRev that future versions of the weapon will be capable of achieving projectile velocities in the 5,000-8,000 fps range with no difficulty..."
"The DREAD won't jam because, according to its inventor, it can’t jam. The DREAD's operating and feeding mechanisms simply don’t allow for stoppages or jams to occur. It thus follows that the DREAD Centrifuge Weapon will be the most reliable metallic projectile launcher/ballistic device on the planet. DefRev is not at liberty to publish exactly why the DREAD can’t jam, since Mr. St. George hasn’t given us permission to describe the gun’s operating and feeding mechanisms in any detail... Even if the DREAD-equipped vehicle does get identified and fired upon by the enemy, the risk of a catastrophic explosion from a bullet strike on the ammunition supply is zero, because the DREAD’s ammunition doesn’t contain any propellant. There’s no gunpowder onboard to blow up. That just leaves the gas tank (vehicle’s)."
Big claims. Flagrantly violates Newton's Third Law. I smell DoD funding!
Newton's Third Law: for every action, there is an equal and opposite reaction. This also tells us that momentum is conserved. It doesn't matter whether you get your propulsion by gas pressure, mechanical action, or the invisible magic of electromagnetism; when you push on something, it pushes back.
A .30 caliber projectile is 7.62mm in diameter. For a ball, that makes a volume of about 1.85 cubic centimeters. Make this out of lead, and it weighs ~ 20 grams.
The patent talks about firing these things in bursts of 20-40. Let's suppose we fire off a burst of twenty balls (400 grams), at a speed of 3000 fps, which I'm going to round to 1000 m/s.
Before firing, the system (gun + ammo) wasn't moving anywhere (except round and round - I've read through the patent but I'm still not sure how the handwaving away of gyroscopic problems is supposed to work), so it had a net momentum of zero.
After firing, those twenty balls have a combined momentum of 400 kg m/s in a forwards direction, so the rest of the system - the gun - picks up a momentum of 400 kg m/s in a backwards direction. This is unavoidable; the momentum of the ammunition *has* to be matched by the momentum of the gun, unless you're dumping mass backwards, which doesn't seem to be the idea here. (If it is, the gunner standing directly behind the weapon in those pictures is in a whole lot of trouble.)
Let's suppose the gun weighs 40 kg (~ 100 pounds), roughly the same as a Browning M2 heavy machinegun; that translates into a backwards velocity of 10 m/s. Something - either the gunner, or a solid mounting secured to a vehicle or heavy object - has to stop that, unless they really want the gun flying backwards as fast as an Olympic sprinter can run. Recoil, ladies and gentlemen. A new design may make recoil easier to manage by spreading it out a little, making it less jerky and thus easier to compensate for, but at the end of the day you have to soak up that momentum. No way around it.
(Incidentally, if you're wondering what 'pushes' the gun backwards: at the moment of release, it's *already* moving backwards. It's as if you're spinning a set of barbells around the middle - one moves back as the other moves forwards, they pull on one another to stop the system from separating, and that connection breaks *both* of them fly away.)
Now let's suppose, as the Defense Review hack above does, that we actually *did* sustain a rate of fire of 120 000 rounds per minute, as he implies would be possible... for just one second. That's 2000 rounds (40 kg of ammo), for a recoil of 40,000 kg m/s. This translates to a recoil force of 40,000 newtons - as much as it takes to lift a two-ton truck off the ground!
The "impossible to jam" and "silent" claims look dodgy to me, but they don't in themselves violate the laws of physics, so I'll let them alone. (Supersonic projectiles will still 'zip' in flight, but this isn't such a big deal; as long as the firing itself is quiet, it's pretty hard to locate the shooter from the sound of the projectile, for reasons I won't go into here.)
Instead, let's shift from momentum to energy. A 20-gram round moving at 1000 m/s has ten thousand joules' worth of kinetic energy; if you're firing a 20-shot burst, you need to come up with 200 kilojoules of energy. Again, there is no way around this without breaking the understood laws of physics.
An M1 Abrams tank has a 1500 horsepower gas turbine engine; this translates to about 1000 kJ. In other words, if you devoted the *entire* engine output of a main battle tank to powering one of these guns, ignoring losses in mechanical-electrical-mechanical conversion, you could get just enough power for five bursts a second. Clearly this is not a feasible approach, so you're looking at stored energy.
Incidentally, the fact that it takes as much power to run this gun as it does to drive a 60-ton tank at 45 mph should suggest something about the plausibility of the "no heat" bit. Electrical might be more heat-efficient than a gas turbine engine, but not that much better. Conventional machineguns also generate a lot of waste heat, but much of it is shed in the propellant gases and in the brass cartridge cases they eject. (One reason why 'caseless' designs haven't come good as yet - serious heat issues.)
OK, back to storage. There are several ways you can store energy - flywheel, battery, capacitor, or even as fuel to be converted to electricity via a fuel cell or engine-driven generator. (Indeed, the propellant in a bullet is another form of energy storage.) Whichever one you choose, the storage requirements for such a system are far from trivial - not only does it need a lot of energy, but it needs it *fast*. I won't get into that here, because I'm not well enough versed in power & energy storage densities, but there is one thing that all these methods have in common:
They all store energy.
Well, duh. But it's important to remember that a power system is not just a black box that makes things go when you plug them in. And it doesn't make energy, it just contains and transforms the energy you put in it. If you want to fire 5000 rounds from DREAD, somehow you have to store fifty megajoules of energy in your 'black box'... and if a stray bullet breaks your 'black box', that energy doesn't just vanish. The only things you can really destroy that way is the containment and controlled-release system, and when you destroy the containment you have a very good chance of releasing large amounts of energy in an uncontrolled fashion - a process that scientists refer to as "fire and/or explosion". So let's not have any of this "no risk of an ammo explosion" rot, eh?
djfiggy, this New Scientist story about 'DREAD', a supposed new wonder-weapon. It's worth watching the promotional video, which is amusingly cheesy - with all the cut-out animation, I was half expecting a giant foot to come down and squish the bad guys.The basic idea: take a bunch of ball bearings, spin them up to high speed in a circular chamber, and then release them so they fly towards the target. Certainly possible in theory, whether you can make it practical is another matter. The NewScientist article is somewhat sceptical, but not nearly sceptical enough. I've collected some passages from the video and related articles... you can all just read along and press your buzzer every time you see flagrant bullshit.
A couple of choice passages from the video:
"The DREAD offers an extraordinary rate of fire, up to 120,000 rounds per minute in controlled bursts, with no recoil... the DREAD's lack of recoil means there is no need for special mounting devices."
(Accompanied by dodgy animation of satellite firing DREAD at something. BTW, to avoid confusion, when a weapon's described as firing "X rounds per minute", this isn't usually intended as a sustained rate for a whole minute - it's like a bullet that travels at 700 km/hr even though it's only in flight for a second or two.)
"The DREAD is the first small-arms weapons [sic] to offer real stealth characteristics. It has no blast, flash, or heat signature, and can therefore operate in a sustained fire mode without detection in all combat situations."
"...totally eliminate the multiple failure modes that conventional weapons exhibit due to their complexity, thermal characteristics, and gas discharge. In addition to reliability, safety has been a major consideration in the DREAD design. The lack of powder propellant means ammunition storage may be accomplished without the substantial precautions currently required."
The brochure claims it to be "totally jam-proof".
Also available: a related patent.
And from a Defense Review Article: "The DREAD depicted in the video is a functional prototype that operates on a less-than-lethal mode. [I suspect this means "the prototype isn't actually powerful enough to match the hype".] This prototype was dismantled for security purposes to protect the technology, after the making of this video..."
"The DREAD is both heatless and frictionless, and doesn’t generate any high pressures. So, there’s virtually no wear and tear on the system, no matter how many rounds are fired through it back-to-back, even if it's run constantly on full-auto at 120,000 rpm, the whole time."
"Imagine a gun with no recoil, no sound, no heat, no gunpowder, no visible firing signature (muzzle flash), and no stoppages or jams of any kind. Now imagine that this gun could fire .308 caliber and .50 caliber metal projectiles accurately at up to 8,000 fps (feet-per-second), featured an infinitely variable/programmable cyclic rate-of-fire (as high as 120,000 rounds-per-minute)..."
"According to its inventor, not only is it possible, it’s already happened. An updated version of the weapon will be available soon. It will arrive in the form of a tactically-configured pre-production anti-personnel weapon firing .308 caliber projectiles (accurately) at 2,500-3000 fps, at a variable/programmable cyclic rate of 5,000-120,000 rpm (rounds-per-minute). The weapon's designer/inventor has informed DefRev that future versions of the weapon will be capable of achieving projectile velocities in the 5,000-8,000 fps range with no difficulty..."
"The DREAD won't jam because, according to its inventor, it can’t jam. The DREAD's operating and feeding mechanisms simply don’t allow for stoppages or jams to occur. It thus follows that the DREAD Centrifuge Weapon will be the most reliable metallic projectile launcher/ballistic device on the planet. DefRev is not at liberty to publish exactly why the DREAD can’t jam, since Mr. St. George hasn’t given us permission to describe the gun’s operating and feeding mechanisms in any detail... Even if the DREAD-equipped vehicle does get identified and fired upon by the enemy, the risk of a catastrophic explosion from a bullet strike on the ammunition supply is zero, because the DREAD’s ammunition doesn’t contain any propellant. There’s no gunpowder onboard to blow up. That just leaves the gas tank (vehicle’s)."
Big claims. Flagrantly violates Newton's Third Law. I smell DoD funding!
Newton's Third Law: for every action, there is an equal and opposite reaction. This also tells us that momentum is conserved. It doesn't matter whether you get your propulsion by gas pressure, mechanical action, or the invisible magic of electromagnetism; when you push on something, it pushes back.
A .30 caliber projectile is 7.62mm in diameter. For a ball, that makes a volume of about 1.85 cubic centimeters. Make this out of lead, and it weighs ~ 20 grams.
The patent talks about firing these things in bursts of 20-40. Let's suppose we fire off a burst of twenty balls (400 grams), at a speed of 3000 fps, which I'm going to round to 1000 m/s.
Before firing, the system (gun + ammo) wasn't moving anywhere (except round and round - I've read through the patent but I'm still not sure how the handwaving away of gyroscopic problems is supposed to work), so it had a net momentum of zero.
After firing, those twenty balls have a combined momentum of 400 kg m/s in a forwards direction, so the rest of the system - the gun - picks up a momentum of 400 kg m/s in a backwards direction. This is unavoidable; the momentum of the ammunition *has* to be matched by the momentum of the gun, unless you're dumping mass backwards, which doesn't seem to be the idea here. (If it is, the gunner standing directly behind the weapon in those pictures is in a whole lot of trouble.)
Let's suppose the gun weighs 40 kg (~ 100 pounds), roughly the same as a Browning M2 heavy machinegun; that translates into a backwards velocity of 10 m/s. Something - either the gunner, or a solid mounting secured to a vehicle or heavy object - has to stop that, unless they really want the gun flying backwards as fast as an Olympic sprinter can run. Recoil, ladies and gentlemen. A new design may make recoil easier to manage by spreading it out a little, making it less jerky and thus easier to compensate for, but at the end of the day you have to soak up that momentum. No way around it.
(Incidentally, if you're wondering what 'pushes' the gun backwards: at the moment of release, it's *already* moving backwards. It's as if you're spinning a set of barbells around the middle - one moves back as the other moves forwards, they pull on one another to stop the system from separating, and that connection breaks *both* of them fly away.)
Now let's suppose, as the Defense Review hack above does, that we actually *did* sustain a rate of fire of 120 000 rounds per minute, as he implies would be possible... for just one second. That's 2000 rounds (40 kg of ammo), for a recoil of 40,000 kg m/s. This translates to a recoil force of 40,000 newtons - as much as it takes to lift a two-ton truck off the ground!
The "impossible to jam" and "silent" claims look dodgy to me, but they don't in themselves violate the laws of physics, so I'll let them alone. (Supersonic projectiles will still 'zip' in flight, but this isn't such a big deal; as long as the firing itself is quiet, it's pretty hard to locate the shooter from the sound of the projectile, for reasons I won't go into here.)
Instead, let's shift from momentum to energy. A 20-gram round moving at 1000 m/s has ten thousand joules' worth of kinetic energy; if you're firing a 20-shot burst, you need to come up with 200 kilojoules of energy. Again, there is no way around this without breaking the understood laws of physics.
An M1 Abrams tank has a 1500 horsepower gas turbine engine; this translates to about 1000 kJ. In other words, if you devoted the *entire* engine output of a main battle tank to powering one of these guns, ignoring losses in mechanical-electrical-mechanical conversion, you could get just enough power for five bursts a second. Clearly this is not a feasible approach, so you're looking at stored energy.
Incidentally, the fact that it takes as much power to run this gun as it does to drive a 60-ton tank at 45 mph should suggest something about the plausibility of the "no heat" bit. Electrical might be more heat-efficient than a gas turbine engine, but not that much better. Conventional machineguns also generate a lot of waste heat, but much of it is shed in the propellant gases and in the brass cartridge cases they eject. (One reason why 'caseless' designs haven't come good as yet - serious heat issues.)
OK, back to storage. There are several ways you can store energy - flywheel, battery, capacitor, or even as fuel to be converted to electricity via a fuel cell or engine-driven generator. (Indeed, the propellant in a bullet is another form of energy storage.) Whichever one you choose, the storage requirements for such a system are far from trivial - not only does it need a lot of energy, but it needs it *fast*. I won't get into that here, because I'm not well enough versed in power & energy storage densities, but there is one thing that all these methods have in common:
They all store energy.
Well, duh. But it's important to remember that a power system is not just a black box that makes things go when you plug them in. And it doesn't make energy, it just contains and transforms the energy you put in it. If you want to fire 5000 rounds from DREAD, somehow you have to store fifty megajoules of energy in your 'black box'... and if a stray bullet breaks your 'black box', that energy doesn't just vanish. The only things you can really destroy that way is the containment and controlled-release system, and when you destroy the containment you have a very good chance of releasing large amounts of energy in an uncontrolled fashion - a process that scientists refer to as "fire and/or explosion". So let's not have any of this "no risk of an ammo explosion" rot, eh?
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no subject
Date: 2005-05-12 04:07 am (UTC)And you could solve the recoil problem by simply firing pellets wildly in all directions. }:)
no subject
Date: 2005-05-12 04:22 am (UTC)Explosion is a serious issue with flywheels. The main limitation on flywheel storage is the tensile strength of the material, and if you exceed that limit (by running too fast, or if the wheel's damaged or flawed) they'll throw bits of the wheel like heavy-duty shrapnel. See here for some flywheel-explosion stories from the old days. Some modern flywheels are designed so that if they fail they go into very small pieces, which won't travel too far, but there's still a massive release of energy.
no subject
Date: 2005-05-12 04:34 am (UTC)no subject
Date: 2005-05-12 05:04 am (UTC)Um, excuse me, but I was promised a bingo board. And I have yet to be given the layout of the board, or how I would arrange the bullshit into rows. *nod*
I would also like my stapler...
no subject
Date: 2005-05-12 05:58 am (UTC)sol.
.
no subject
Date: 2005-05-12 03:23 pm (UTC)After firing, those twenty balls have a combined momentum of 400 kg m/s in a forwards direction, so the rest of the system - the gun - picks up a momentum of 400 kg m/s in a backwards direction. This is unavoidable; the momentum of the ammunition *has* to be matched by the momentum of the gun, unless you're dumping mass backwards, which doesn't seem to be the idea here.
Hate to correct a better scientist than I, paisano, but I think you're wrong. :)
As I understand this thing, it's accelerating the projectiles in a centrifugal chamber and then just letting them escape at a tangential vector. That will not cause an equal reaction vector backward from the direction of fire. What it will do is cause an angular reaction -- the angular momentum of the rotating chamber will decrease every time you throw a projectile out. That just means you will have to keep pumping energy into the chamber to keep it up to speed.
If it's not using a rotating chamber but some kind of spinwheel accelerator like a baseball pitching machine, then it will have recoil. It will also be prone to jams. So I don't think they're using this method...but then I don't see how they can claim projectiles moving faster than Mach 1. To throw things from a spinning chamber faster than sound, the chamber is going to have to rotate faster than the speed of sound, and unless it's made of unobtanium the whole thing is going to fly apart.
As for heat, I'm sure it will have heat due to the running of the engine (and as you mention, it's going to have to be some whopping big engine). But it's not going to have a muzzle flash. And unless the engine runs red hot (which it might) it's not going to have any signature in visible light. It'll be lit up like a christmas tree in infrared, of course, and every modern army uses IR sensors anyway.
This thing is basically a slug-thrower, straight out of science fiction novels from the 1950's. If it works. From what I know so far I'm skeptical, but greater engineering challenges have been conquered before...
no subject
Date: 2005-05-12 10:51 pm (UTC)Momentum has to be conserved. One of the 'no recoil' scenarios they use is firing this thing from a satellite, which is a nice case because there's no transmission of force to the ground to confuse things. Let's suppose we have a 100 kg satellite, with that mass including one DREAD. For the moment I'll ignore the angular component and assume it's somehow counteracted (e.g. by a gyro spinning the other way - the patent seems to be saying he has some way to ignore all the angular issues without a gyro, which looks like more bullshit to me).
Stage 1: The satellite, gun, and ammo are sitting there in space, dormant. What is the total linear momentum of the system? (Ignore orbital movement - take an inertial frame that zeros the initial velocity of the system, and assume steps 2 & 3 take place quickly enough that gravitational effects can be ignored.)
Stage 2: The gun spins up (as does the countergyro or whatever) but doesn't fire; it's spinning but not going anywhere. What is the total linear momentum of the system now?
Stage 3: The gun releases a 20-round burst, moving in the positive-x direction. What is the linear momentum of that burst? What is the momentum of (satellite + gun)? What is the momentum of (burst + satellite + gun)? And if it's not zero, how did it change?
There's a trick to this, which I'll explain in a separate comment.
no subject
Date: 2005-05-13 01:14 am (UTC)The tricky part here, which makes it *look* like there should be no reaction, goes something like this: "At t < 0, we're spinning it around. It's not going anywhere. And then at t=0, we just release the round - we're not giving it any more 'push' at t=0, just letting it continue in the direction it was moving, so there's no reaction force."
This is true as far as it goes. But the reason there's recoil is that before t=0, there is a reaction force, as required to keep the round moving in a circular path. Before firing, this reaction force has to be balanced by equal and opposite forces arising elsewhere in the system*. Once the round is release, those are no longer balanced, resulting in a net force on the gun.
*There are several ways these forces can arise; the details depend on just how the spin-up and feed systems work.
For an example, suppose the gun is rotating around its center of symmetry, with two rounds on opposite sides of the chamber - at any given moment, their velocities are equal and opposite. At t = -epsilon, round A is travelling at 1000 m/s in the positive-x direction; round B is travelling at 1000 m/s in the negative-x direction; and the chamber is rotating with no net movement except angular. Since this is circular motion, A is being accelerated by reaction forces in the positive-y direction, and B in the negative-y direction (with corresponding forces on the chamber in the negative-y direction where it contacts A, and positive-y where it contacts B).
At t=0, we release round A. We're left with a chamber (zero net linear momentum) and round B (still travelling at 1000 m/s in the negative-x direction). The initial kick is actually *sideways*, positive-y direction, because we've just lost the contact force between A and the chamber; half a revolution later, assuming no reloading, it'll be kicking in the negative-y direction because B will now be over that side. But in between, we have to arrest and reverse B's motion in the negative-x direction, which means pushing the gun backwards via reaction forces. (Viewed from outside, the apparatus will no longer be rotating around the chamber's geometrical axis of symmetry, because that's no longer the center of mass. The net result is that it moves backwards, kicking from side to side.)
If, at t = 0+epsilon, we load another round in to replace A, we can kill the sideways wobble (and that would probably be a very good idea), but getting it up to speed still requires force, which generates a reaction. More complicated setups make the analysis trickier, but don't provide any answer to the fact that the system of "unfired gun + unfired round" has the same net momentum as "gun post-firing" + "fired round"; if A = B+C, and C is non-zero, A ~= B.
no subject
Date: 2005-05-12 06:44 pm (UTC)no subject
Date: 2005-05-12 08:20 pm (UTC)no subject
Date: 2005-05-12 11:24 pm (UTC)no subject
Date: 2005-05-13 01:16 am (UTC)