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Thread: Bullets and stuff

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    Bullets and stuff

    I've read that an SR-71 is faster than an M-16 rifle bullet. Is it easier for an object with more mass to go faster than an object with less mass?

    Could a projectile reach hypersonic velocities with chemical propellant?

    Are man portable electromagnetic projectile weapons possible?

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    Quote Originally Posted by cjackson View Post
    I've read that an SR-71 is faster than an M-16 rifle bullet. Is it easier for an object with more mass to go faster than an object with less mass?
    Well, in the sense that adding propulsion systems to an object means it is easier to get it to high speeds - yes, I guess. Or you could just build a system that accelerates something over time then lets it go. The energy needed increases with mass, but so can the complexity. There is no real way to answer this one. For example if you let the launching mechanism be arbitrarily large then it is easiest to gets something small up to speed. It you impose size/time/acceleration limits then the question becomes highly dependent on the limits you have set. A bullet is all about getting mass to a point with enough energy to do damage. Giving it propellant means you get less mass hitting the target - so you have to trade off speed versus mass against damage caused - which is not a simple relationship because larger, slower bullets tend to fragment when they (which is far more lethal), reduce collateral and so on.

    Quote Originally Posted by cjackson View Post
    Could a projectile reach hypersonic velocities with chemical propellant?
    Yes, although you might need a large calibre round to pack in the fuel and stuff if you are still thinking weapons. Otherwise here is a testbed that did just that.

    Quote Originally Posted by cjackson View Post
    Are man portable electromagnetic projectile weapons possible?
    Yes, but they require large power supplies. The limit right now is on the number of shots you get and projectile speed. Better power densities and they might even become practical! The US Navy is actively researching and procuring ship-based systems, but that is because they have a ship sized generator to back it up.

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    Quote Originally Posted by cjackson View Post
    I've read that an SR-71 is faster than an M-16 rifle bullet. Is it easier for an object with more mass to go faster than an object with less mass?
    One thing you need to remember is that it takes an SR-71 much, much, much longer to reach its top speed than a bullet. And the bullet has no engine. If you could put an engine on a bullet I'm sure you could get much faster. As a rule, it takes less energy to get a lighter object to go fast than a heavy one. However, one additional issue is that as you go higher in the sky, the air gets thinner so it gets easier to go fast. When an SR-71 hits those top speeds, it is very high in the sky. Whereas an M-16 bullet is at ground level, where the air is very thick.
    As above, so below

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    I expect that if you doubled the charge in the M-16 shell,
    doubled the length of the gun barrel, and mounted it on a
    solid base, you'd approximately double the bullet's speed.
    You couldn't do anything like that with the SR-71.

    -- Jeff, in Minneapolis
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    Quote Originally Posted by cjackson View Post
    I've read that an SR-71 is faster than an M-16 rifle bullet. Is it easier for an object with more mass to go faster than an object with less mass?
    There isn't a simple relationship. Jet engines don't easily scale down to the size of chemical propellant guns, and such guns don't easily scale up to the size of jet aircraft, or down to the size of particle accelerators which can easily beat both in terms of speed.


    Quote Originally Posted by cjackson View Post
    Could a projectile reach hypersonic velocities with chemical propellant?
    http://www.spacex.com/assets/img/201...update/027.jpg
    http://en.wikipedia.org/wiki/Sprint_missile

    If you mean to restrict things to guns, it's doable, but takes some extra complexity:
    http://en.wikipedia.org/wiki/Light_gas_gun


    Quote Originally Posted by cjackson View Post
    Are man portable electromagnetic projectile weapons possible?
    They're not impossible, but practicality is another issue. You've got a lot of equipment needed to accelerate the projectile, compared to a tube closed at one end and bit of propellant. An electromagnetic weapon is going to be pretty fundamentally bulkier and heavier than a directly equivalent chemical weapon. It may have capabilities that chemical weapons can't easily achieve, though. An electromagnetic mortar running from batteries charged in the field by a vehicle's power system might add as much weight in batteries as it saves in propellant, but still increase the amount of ammunition that can be delivered to the front by removing the need for propellant.

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    There is a maximum muzzle velocity of about 2 000 m/s for projectiles fired from guns with conventional propellants: tank guns firing armor piercing, fin-stabilized, discarding sabot (APFSDS) get pretty close that value. In other words, taking that M-16 bullet and using the propellant charge from one of the New Jersey's 16 in guns and a very long barrel won't result in 50,000 m/s muzzle velocity -- it will result in about 2,000 m/s muzzle velocity and a lot of wasted energy.
    Information about American English usage here and here. Floating point issues? Please read this before posting.

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    I have seen medical lasers that use power supplies the size of a very small refrigerator. They are not exactly quiet and the noise they make sounds nothing like a Sci-fi laser. It sounds like two metal bars being slammed together.

    I suspect you might want one for an electromagnetic projectile weapon.
    Solfe

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    I haven't done much reloading in a long while, but my (possibly faulty) memory says your standard 5.56 NATO round has about 3,000 fps muzzle velocity.

    I know there are a lot of newer designed rounds, but a couple decades back, the .220 Swift was considered speed king, and hot loaded, you could get a bit over 4,000 fps muzzle velocity. I'd have to research what is considered the top speed round now. That said, in the rifle I had, best accuracy was better if I loaded down a bit, in the 3,800 range. (Measured by chronometer).

    I seem to remember the legendary P.O. Ackley having made up some wildcat rounds that were impractical at best, but very very fast.

    What it takes to make a plane that fast, I can't comment on. For a bullet, its a lot about powder capacity and barrel length.

    Wasn't there just some sort of test of a plane/drone/missile that was doing Mach 20? Considerably faster than an M-16, or any other bullet I am familiar with.

    TJ

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    I read recently that there is a fundamental limit to gas velocity. When a compressed gas is released to a vacuum, the exit velocity has a maximum achievable value of four times the speed of sound in the compressed gas.

    I don't know where I read it or if it is true.

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    As far as conventional bullets go, somewhere between 4500 and 5000 FPS is about the hard limit. If a standard barrel has a 1 in 12 rifling twist, the bullet makes 1 revolution per foot traveled. At 4000 feet per second, that's (60*4000) 240,000 RPM. Not too far above that, the bullets no longer hold together and you get a gray smoke trail that ends in a puff about 60 yards away. This was either with a .223 (same round as an M-16) or a 6mm. I don't recall which one my uncle was firing.

    M-16's also have a much faster rifling twists. 1:9 or 1:7.
    I'm Not Evil.
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    Quote Originally Posted by Tog View Post
    As far as conventional bullets go, somewhere between 4500 and 5000 FPS is about the hard limit. If a standard barrel has a 1 in 12 rifling twist, the bullet makes 1 revolution per foot traveled. At 4000 feet per second, that's (60*4000) 240,000 RPM. Not too far above that, the bullets no longer hold together and you get a gray smoke trail that ends in a puff about 60 yards away. This was either with a .223 (same round as an M-16) or a 6mm. I don't recall which one my uncle was firing.
    The limit has nothing to do with rifling...you can always just use less twist or a smoothbore. It's a matter of the dynamics of the expanding gases.

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    I have to interject a chuckle in this thread.

    I used to have black powder pistols and muzzle loaders. The numbers listed in this thread are so far beyond what black powder can do, they seem scary-fast to me. You can increase the amount of black powder to increase speed, but there is a magic "do not exceed" number that you do not want to hit.

    The changes in the technology are incredible.
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    A projectile, not considering rockets/missiles, would go faster depending on the type of gun and propellant. A ram-air gun might be able to get to those speeds, but it's a different type of gun system.

    you can make an electromagnetic gun manportable, but you'll be limited in power an projectile mass and velocity, but it be done within those constraints. Whether it would be deadly, would depend on velocity and distance from target, like any other gun.
    Et tu BAUT? Quantum mutatus ab illo.

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    Quote Originally Posted by cjameshuff View Post
    The limit has nothing to do with rifling...you can always just use less twist or a smoothbore. It's a matter of the dynamics of the expanding gases.
    The maximum upper limit, maybe. But the physical limit on the bullet is far lower. The smoothbore option would stop it from shedding the jacket (but accuracy would suffer greatly), and a solid brass bullet may not have the issue at all, but those aren't really standard.

    The only three ways I've ever seen a gun fire, and still fail were:
    Too little powder, which sticks the bullet in the barrel.
    A little too much powder, which does anything from flattening the primers, to blowing the case in half, to causing the bullets to vaporize (not literally, but it's the common term).
    Way too much powder, or the wrong type, which causes the gas to expand faster than the bullet can get out of the way (barrel obstructions fit this as well), which causes the action to release the pressure with variable degrees of explosive force.

    Solfe:
    It's not really the technology as much as the chemistry. Smokeless powder burns more slowly, and different types burn at different rates. If you put the powder used in a .44 Magnum into a .50 BMG, the .50 would have a very good chance of blowing up.

    Pistol powder needs to release its energy as fast as possible. It burns quickly to provide a very short term "kick".
    Large rifle powders burn slower which lets the expanding gas maintain a more even pressure over a longer time, even though there is far more of it. A .50 BMG is more of a really hard push than a kick. The maximum pressure is all about the same.

    Black powder is the fastest of the lot so it's very easy to get too much and create an "overpressure situation." Those are bad.
    I'm Not Evil.
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    Quote Originally Posted by cjackson View Post
    I've read that an SR-71 is faster than an M-16 rifle bullet.
    All depends. If you fire forward an M-16 in the cockpit, the bullet will catch up to, and contact, the plane.

    Forty years ago, there were some .22 caliber bullets that were manufactured with a kind-of caseless propellant. Let's see if I can find a wiki page on it.

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    Quote Originally Posted by grapes View Post
    All depends. If you fire forward an M-16 in the cockpit, the bullet will catch up to, and contact, the plane.

    Forty years ago, there were some .22 caliber bullets that were manufactured with a kind-of caseless propellant. Let's see if I can find a wiki page on it.
    It was the Daisy V/L rifle, I think.

    Heckler and Koch also made an assault rifle called the G-11 that used caseless ammunition, but it never went into production.

    At one point, the US Navy (I think) was looking at a 25mm caseless gun to replace the 20mm cannon on fighter aircraft.
    I'm Not Evil.
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    Basically the upper theoretical limit for a gun, i.e. something which accelerates a projectile through gas pressure, is limited to the speed of sound in the pushing gas.
    Hypervelocity guns achieve very high velocities by using a two stages system were stage one is a classical gunpowder gun which fires a large massive projectile in a large bore barrel which is filled with hydrogen or helium, then using the energy of that projectile to compress that second-stage gas and pushing it through a smaller bore barrel for the real projectile.
    As the speed of sound in those gases are >3 times that of regular air, it's possible to get velocities far faster that the speed of sound in air.
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    Quote Originally Posted by Tog View Post
    The maximum upper limit, maybe. But the physical limit on the bullet is far lower. The smoothbore option would stop it from shedding the jacket (but accuracy would suffer greatly), and a solid brass bullet may not have the issue at all, but those aren't really standard.
    No. The limitation is due to gas dynamics, not the bullet. We're not talking about how fast you can push a specific weapon designed for lower velocity projectiles. Bullet velocity is not limited by the bullet's structural integrity, even with conventional materials, and brass is hardly an exotic material...high velocity rounds are often materials like tungsten or depleted uranium. And once again, it is entirely possible to use less twist to the rifling or a smoothbore, and a smoothbore is actually preferable in many cases due to the high aspect ratio of typical high velocity rounds...rifling is less effective if not actively counterproductive for such rounds. These rounds are often fin stabilized instead, and together with the high velocity get quite reasonable accuracy...not that accuracy is actually relevant to the physical limits on bullet velocity. The Steyr IWS 2000 sniper "rifle" is actually a smoothbore gun firing tungsten carbide or depleted uranium APFSDS rounds.

    As HenrikOlsen stated, it's related to the speed of sound in the propellant gases. The two-stage light gas guns he mentioned can achieve far higher muzzle velocities, and are used in research into protecting spacecraft from micrometeorite impacts. Guns using hydrogen-rich propellants or hydrogen itself are also being researched, due to the higher velocities that can be achieved with these propellants.

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    I'm interested in the speed of sound in helium, hydrogen. Why is it quicker when the gas molecules are further apart ? I can see why a projectile might move faster (less friction) but a sound wave requires a medium to travel through and exist in.
    Last edited by headrush; 2012-Jun-04 at 05:16 PM. Reason: added "and exist in"

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    The speed of sound, c, in any medium is (∂p/∂ρ)0.5 or the square root of the partial derivative of pressure with respect to density. For a perfect gas -- a very good approximation for hydrogen or helium at pressures under a few megapascals -- this reduces to c=sqrt(γ R T/M) where γ is the ratio of specific heats (1.67 for helium, 1.4 for diatomic molecules, 1.3 for triatomic molecules), R is the gas constant (about 8.3), T is temperature (in kelvins -- celcius and fahrenheit are useless for thermodynamics) and M is molecular weight.

    For helium, at 288 K, c is about 1000 m/s vs about 340 m/s for air.

    Pop over to the Hyperphysics site. It's a great place for this sort of stuff.
    Information about American English usage here and here. Floating point issues? Please read this before posting.

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    Quote Originally Posted by HenrikOlsen View Post
    Basically the upper theoretical limit for a gun, i.e. something which accelerates a projectile through gas pressure, is limited to the speed of sound in the pushing gas.
    Except ram accelerator uses similar principles to work around it.
    Et tu BAUT? Quantum mutatus ab illo.

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    Quote Originally Posted by swampyankee View Post
    The speed of sound, c, in any medium is (∂p/∂ρ)0.5 or the square root of the partial derivative of pressure with respect to density. For a perfect gas -- a very good approximation for hydrogen or helium at pressures under a few megapascals -- this reduces to c=sqrt(γ R T/M) where γ is the ratio of specific heats (1.67 for helium, 1.4 for diatomic molecules, 1.3 for triatomic molecules), R is the gas constant (about 8.3), T is temperature (in kelvins -- celcius and fahrenheit are useless for thermodynamics) and M is molecular weight.

    For helium, at 288 K, c is about 1000 m/s vs about 340 m/s for air.

    Pop over to the Hyperphysics site. It's a great place for this sort of stuff.
    Thanks swampyankee, I'll pop over there later

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    Quote Originally Posted by cjameshuff View Post
    We're not talking about how fast you can push a specific weapon designed for lower velocity projectiles.
    Ahh, that's the problem. I was.

    Specifically, the types similar to the M-16 mentioned in the first post. Everything I was saying was based on that, mainly in response to JeffRoot's post and partially to TJMac, though I guess I forgot to quote them. With a conventional copper jacketed, lead bullet, you'd never hit the gas speed limit if it was over about 4500 to 5000 FPS, and with some thin jacketed bullets, 4000 isn't possible.

    I'll go back to lurking now.
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    Quote Originally Posted by headrush View Post
    I'm interested in the speed of sound in helium, hydrogen. Why is it quicker when the gas molecules are further apart ? I can see why a projectile might move faster (less friction) but a sound wave requires a medium to travel through and exist in.
    They aren't, it's lighter because each individual molecule is lighter, not because they're farther apart.
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    Quote Originally Posted by HenrikOlsen View Post
    As the speed of sound in those gases are >3 times that of regular air, it's possible to get velocities far faster that the speed of sound in air.
    Don't forget that the speed of sound in a gas is mostly dependent on temperature, and the propellants used in firearms turn into very hot gases when combusted.

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    Quote Originally Posted by DoggerDan View Post
    Don't forget that the speed of sound in a gas is mostly dependent on temperature, and the propellants used in firearms turn into very hot gases when combusted.
    They're also fairly dense.
    Et tu BAUT? Quantum mutatus ab illo.

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    The helium/hydrogen secondary gas undergoes adiabatic heating as they're compressed, it's definitely not cold by the time the pressure disk ruptures to let it into the thin barrel.
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    Quote Originally Posted by Ara Pacis View Post
    They're also fairly dense.
    Somewhat counter-intuitive, but the speed of sound in gases is "nearly independent of pressure or density for a given gas."
    http://en.wikipedia.org/wiki/Speed_of_sound

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    Quote Originally Posted by BioSci View Post
    Somewhat counter-intuitive, but the speed of sound in
    gases is "nearly independent of pressure or density for
    a given gas."
    http://en.wikipedia.org/wiki/Speed_of_sound
    Ha! I must have known that at one time, but if so,
    it is gone from my ability to recall, and while reading
    this thread I was thinking about whether the speed
    would increase or decrease with density of a given
    gas, and couldn't come up with a good reason for it
    to go either way.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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