All I know is that any propelled object loses it's punch if the originating object is floating free. i.e. shooting a gun will propel the shooter backward. I imagine it would be the same thing for a crossbow.
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Providing the mass of the firing body is significantly greater than the projectile, projectile weapons will still be generally effective in zero gravity; Newtonian mechanics will give a decent approximation of what the recoil from a given weapon will be in a free-floating scenario.
If I were to try and dream up advantages of a crossbow in zero-gravity, they would probably be the same for any spring-driven weapon system: independent of air pressure (some explosives/firearms may not operate in low pressure environments), relatively independent of temperature (airguns and firearms may have issues in substantial cold/heat), no pollution (even smokeless powder leaves trace chemicals which may have undesirable consequences in a small volume airspace).
I've not heard of sawdust bullets and a blindingly fast google search didn't help me out, however I would suggest that depending on the structure of your fictional space habitat, normal subsonic hollowpoint ammunition may in fact work fine; if a spacecraft cabin is designed to withstand the impact of micrometeorites I would have no issues with plausibility - at least on that count - were your characters to start shooting at one another.
One advantage of a cross bow, is that one can attach just about anything to the bolt. Gas cannisters, explosives, hypodermic needles. One can also have armor piercing tips or tips that go splat when they hit something harder than a person. The bit disadvantage of a cross bow is the size. They are actually big compared to the size of the hallways the ship would have.
Dart guns might be even better. they might use CO2 or some safe gas to propel the darts. They may even use ship air, filling the chamber between rounds. The actual technology for gas propelled projectiles is well within the technology of any technician on board a ship, since it was first being tested befor Napolion came to power. Technologies to eliminate recoil is a little more tough, but recoiless weapons are in existence and would not be sending you backwards. very hard.
quote:All I know is that any propelled object loses it's punch if the originating object is floating free. i.e. shooting a gun will propel the shooter backward. I imagine it would be the same thing for a crossbow.
I'm not quite sure what you are saying here. Shooting a gun will propel the shooter backwards, yes, but that will not in any way reduce the velocity of the bullet. Shooting a gun on earth propels the shooter backwards, but you normally brace yourself against the recoil (if you don't you can still be knocked over, though obviously that's a combination of gravity as well as the momentum from the recoil). Firing something in zero-g qould require different form of bracing as gravity wouldn't hold you to the ground so you wouldn't get traction in the same way (though if you were firing slightly upwards you'd probably be fine; firing slightly downwards and it's a different matter).
The recoil is a simple function of mass times velocity. If you are shooting a mass of x at a velocity of y, then the recoil would move a mass of 100x at a velocity of y/100. A crossbow bolt has a significantly lower muzzle velocity than a bullet, even given the higher weight of the bolt in comparison, so lower recoil.
I have never come across sawdust bullets. Are you assuming a thin shell that will break up on impact, or actually just a wad of sawdust? The latter would pretty much be the feeblest shotgun ever so I assume you are talking about shelled ammunition - presumably so it doesn't risk punching through walls of a spaceship, but would still injure a human? Clarify what your question is and you'll probably get clearer answers.
Recoilless weapons, IIRC, mostly work by blowing hot gases backwards at high velocity, and that sounds highly inadvisable in a spaceship
Sorry about the delay. The Czechs used to produce a Medium Density Fibre (MDF) bullet for fighting within aircraft. The idea was to kill the person but if you missed it would disintegrate rather than pierce the skin of the aircraft. They were produced for use in small semi-automatic machine guns.
I just wanted to know if anyone had used them or seen them in use, or any ammunition remotely like that.
I have the Janes Military Journal article (circa 1995) on them, but it is just a list of stats.
It is for the interior of a spacecraft. I find laser-blasters both twee and cliched and want a realistic alternative.
[This message has been edited by Andrew_McGown (edited November 15, 2009).]
Bullets, whether metal or sawdust, propelled with gunpowder have fairly long ranges, even on Earth. The velocity of the bullet in gravity allows it to travel farther and more accurately according to aim without having as much arc. This is one of the main reasons why guns are more effective than bows. Arrows are larger and do not receive as much propellant force, so in gravity they have more pronounced arcs and are less accurate.
This is not the case in zero gravity. Even though an arrow would not be released with as much force, it would be equally as accurate as a gun (no arcs). I'm not sure how the one shooting the arrow would be affected. The propellant is not pushing, but pulling. I would be tempted to say the one shooting the arrow would be propelled forward slightly.
I would be tempted to say the one shooting the arrow would be propelled forward slightly.
The reaction to having the mass of the arrow be accelerated one way is for the remaining mass in the system (your body) to move the other way. It's the law of conservation of momentum (you impart momentum on the arrow by trading it from your own, which is why there's that whole equal and opposite reaction phrase)
To think about it from a speculative angle: If firing an arrow moved you forward, then we wouldn't need rockets for propulsion - we'd just fire an arrow attached to a string, reel it in (which would also pull us forward), and fire it again.
If I roll a bowling ball, I do not go backwards - even if I am standing still and swinging my arm.
My argument is that the arrow is propelled by the forward motion of the bowstring - the energy has already been created by pulling the string back. The forward release of energy would have a greater effect than the opposite force of the arrow leaving the bow. A gun is a different weapon. The energy is chemically stored. It is a stationary object that has a propellant - thus you would be forced backward.
Regarding the rocket arrow, this would go against the law of conservation of energy. However, I promise you that if you tie string around your neck and the other end to an arrow, and you shoot it, it will pull you forward - please don't try to prove me wrong.
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I hope it doesn't appear we're trying to derail the thread here, but this is really (really) interesting to me. Not least because the short story I'm currently working on is hard-SF and I wonder how much explanation will be required for (what I believe to be) the 'real' physics/chemistry to make sense for the average reader, since our experience of physics on Earth sometimes makes zero-g behaviour unintuitive.
If I roll a bowling ball, I do not go backwards - even if I am standing still and swinging my arm. Conservation of momentum only works in a closed system. Bowling is not a closed system, your feet are touching the ground and so can apply an additional force to counter the momentum of your body that you've used to accelerate the ball. Yes, in this case you will continue moving forward. If you were floating in space and bowled the ball, your body would move (as a whole) in the opposite direction.
My argument is that the arrow is propelled by the forward motion of the bowstring - the energy has already been created by pulling the string back.
The bowstring's forward movement is countered by the tension in the bow itself. As you pull on the string, you are 'opening' the gap between the string and bow, so that when it is released, that gap snaps shut. Once again, it's not a closed system: Your feet are on the ground, and your hand is on one side of that gap in the bow mechanism. So planet Earth (indirectly) holds one side of the bow for you and the rest of the bow propels forward. Thus on Earth, the action can appear to yield a forward motion. If floating in space unsupported, there is a small backwards motion made by the bow that counteracts the forward motion of the string, and the law of conservation of momentum applies.
Conservation of momentum does apply when grounded on Earth, of course, but since the mass of the Earth is a squillion times larger than that of the projectile (arrow, bullet, bowling ball), the net effect of firing it is relatively zero.
My argument is that the reaction depends on how the energy is stored.
Chemical would obviously push someone backwards. We both agree on this.
I also agree that the bowling ball would not work without gravity. Swinging the ball would rotate one's body, but not as much as the ball due to it's lesser mass. The release would not push one backward but could cause a spin.
However, if one tied a string to the bowling ball and oneself and did the same experiment, the bowling ball would pull one slightly forward.
I understand what you are saying about the arrow. However, there is a slight forward pull as the string enters and breaks the plane of the bow ends. Since the energy of the bow is created by released tension, it is not subject to the lack of gravity. It is my argument that the force of the bowstring exceeding this plane is greater than the opposite propellant force of the arrow.
I am trying to see what you are saying about a closed system; however, the bow jerks forward, not backward.
Imagine you are floating free, not touching anything. If you are holding a basket ball and push it away in a chest pass the ball will fly forward and you will fly backward. Equal and opposite reactions. The distance you fly backwards is less than the distance the ball flies forwards because you have different amounts of mass. Force = mass x acceleration.
About the bowstring, though... I'm not sure. I'm having a lot of trouble picturing it. Maybe if you draw a freebody diagram it will become clear.
Edited to add: Sorry, I don't know anything about MDF bullets. Have you considered using rubber bullets? Is it necessary to kill your target or just put them out of commission?
[This message has been edited by ScardeyDog (edited November 16, 2009).]
philocinemas - if ou are tied to a bowling ball in zero gravity, and you throw the ball forwards, it will not pull you along. For a start, you won't be able to throw the ball anywhere near as fast a you can under gravity. When you bowl a ball normally, you actually start moving forwards (using friction against the ground as the opposition to your action) and then transfer much of YOUR momentum to the ball. If you have nothing to gain traction against/momentum from, you can't impart that momentum to a ball.
The arrow thing is a bit trickier. The arrow is taking its momentum from the string. So where is the string getting ITS momentum? Well, drawing the bow creates potential energy in the system (you can see this most effectively in modern, pulley-equipped hunting/sporting bows that you can hold at full draw for MUCH longer, because the energy is almost all stored inside the system and not all pulling on your arms: only when you release the small amount of resistance does the rest of the system come into play. The power curves of modern bows are an art form in themselves).
To create this energy, however, energy still has to be put into the system in some way. There's simply no other possibility unless Newtonian conservation is flawed (and there's no evidence it is, on a macro level). As it's the draw that creates the energy held in the system, it's tempting to presume that the act of drawing requires bracing against the ground... but clearly that isn't the case. One can draw, and release, a bow in mid-air (as anyone whose hobbies are parachuting and archery could doubtless attest). Therefore there must remain an action/reaction component. It isn't noticeable (unlike firing a gun) simply because of the speed, mass, and acceleration time of an arrow compared to those of a bullet from a gun, but it's still there. So if you fire an arrow in zero gravity, you'll go backwards.
Photons have zero mass. They have to, in order to be able to travel at the speed of light. Mass at velocity v = mass at rest divided by the square root of (1 - (v-squared over c-squared)) is the equation that matters here. As v approaches c, that divisor approaches zero. The general assumption is that anything divided by zero is infinite. This is not true - though any drawn graph approaches zero asymptotically, it never reaches it. The truth is, no-one knows in physical-world terms what the result of that calculation would be if rest mass was anything other than 0, and it isn't wise to try.
However light does have momentum, hence "light-sails" and other such space travel ideas. The amount of light (or the area it works on) would have to be vast. In practice, the recoil from a laser is infinitesimally tiny if you are dealing with anything weighing grams (orders of magnitude more massive than an individual atom, let alone a theoretical photonic mass). Remember a laer is not necessarily throwing out any more phtons than a light-bulb; it's the organisation of those photons that gives it all its curious powers. Anyway, check out "radiation pressure" in Wikipedia and wander the links from there.
All right, it looks like I'm losing this argument. Let me see if I can redeem myself (at all).
My bowling ball comment was originally just meant to be an example of acting upon an object without it causing an opposite reaction. However, I realize it is the gravity that gives the bowling ball its kinetic energy. In the zero gravity case I pictured the right shoulder joint is a fulcrum. The right arm already pulled back, creating stored energy. The arm swings forward and releases. Two things happen: I begin to spin heals over head, in accordance with the arc of my swing, and the bowling ball travels away from my spinning body in a straight trajectory. As the ball reaches the end of its rope, it should exert a force on me. My thoughts were that this force, being of different direction than of my spin, would pull me.
Oddly enough, I've done this experiment in water with a diving brick. Granted, I was still under the influence of some gravity (but no intoxicating substances) and the brick didn't go very far, but this pretty much describes what happened, except that I didn't have the brick tied to me. I would think that since the brick (or bowling ball) in space has no resistence, it would behave as I described).
Regarding the bow and arrow. If you shoot a bow, without dampers, there is a forward motion recoil. I do not know how that force could not move you forward in space. Granted, I know you're sending out a projectile in zero-G, but it is starting from a point beyond the escape plane, the verticle plane that lies between the two fulcrums. I would think stored energy set in forward motion, would create forward motion from inertia.
I suppose I'm having a hard time grasping closed systems and why this force wouldn't move you forward in space like it does on Earth. And I don't mean to hijack this thread, but it appears to me to be relevant to the original question.
You are basically saying that if you fire a bow in space, the arrow moves forward, and the bow moves forward.
Something has to move backward at some point during the whole operation. As noted, I don't believe that the drawing is involved per se. I therefore think it's at the release. I think you notice the forward motion of the bow (remember, the bowstring will move forwards past its stable point, and back, and forwards again) simple because you are pushing against it already, so don't notice the push back, but do notice the pull component which is only generated by the undamped string oscilating until it reaches equilibrium.
I live near the New Mexico spaceport, I'll see if I can get aboard a flight when they start up and try the bow-in-space thing. I'm sure if I tell them it's for science, they'll be understanding and waive the normal $200,000 fee... I'll let you know how it goes.
"Sorry to be a nuisance, but why don't 'laser blasters' send the shooter backwards? Is it because of the infinitesimal mass of light particles?"
Lasers do most of their damage due to the energy of their beams, not their momentum. An equivalent bullet would be one with an explosive shell. Most of the damage is not cause by it hitting the target. It is cause by what happens after.