Welcome back all!  In last week’s episode we talked about external ballistics, and the forces that affect a projectile in flight.  We also talked about those same projectiles flying around on Mars, in outer space, and on planets made of bubblegum.  This week we’ll discuss stability of a projectile, and the things we can do to limit the effect those forces have on the projectile’s flight.

So what can we do to help them out? obviously we want to hit our target and Mother Nature (or other resident natural deity on whatever sci-fi world you’re writing) is doing her damnedest to make sure we miss.  We have to find some way stabilizing our projectile, to make it less susceptible to outside forces.  Here’s what would happen if we fired a projectile with no means of improving stability, as it flies, minute disturbances and inequalities in forces applied cause it to tumble.  This video comes from YouTube user vidaday, and is awesome.

The easiest way to do this is to increase the mass of a projectile.  The heavier it is, the

less effect wind and atmospheric effects will have on the projectile.  This comes at a serious price however.  A heavier projectile requires a heavier propellant charge, which requires a bigger gun, etc.  We could also increase the speed of the projectile, so it is exposed to the outside elements for a smaller period of time, lessening their effects.  Of course, making something faster requires more power, or making it lighter.  We’ve reached an impasse.

So how else can we increase stability of a projectile?  The two most popular methods are rotational stability, and Fin stability, or a combination of both.

Instructional Cutaway of a tank barrel, showing rifling. From Wikipedia by baku13

Rotational – This one is easy to visualize.  Turn on your TV on any Sunday and watch a football game.  That football being thrown by my man, Quentin Barnes (GO KRAKENS!), is being thrown with rotational stability.  Making the ball spin along its axis prevents it from tumbling end over end, and keeps it going where it was pointed.  This is accomplished through a really complicated combination of forces, brought to you by another of Newton’s friends, Magnus.  Rotational stability is usually accomplished by rifling the bore of the barrel.  Rifling is a series of Helical grooves which engrave the surface of the bullet, and impart spin on the projectile as it travels up the bore.  Generally speaking, lighter and faster projectiles will require less spin than heavier, slower projectiles, so the rate of twist of the rifling becomes important if the type of bullet is prone to change.  Rate of twist is also dependant on the length of the projectline, not just the weight.  Hunters and target shooters have to be conscious of the rate of twist as they select their bullets.  Too fast a rate of twist, and the bullet may be over stabilized, causing it to not nose over at the top of the ballistic trajectory, and then causing it to tumble.  It could also spin so fast that the jacket separates from the softer core of the bullet, causing the bullet to fragment in mid-air.  Too slow, and the bullet will be destabilized and it might tumble from the get go.

Fletches on an Arrow, from Wikipedia by Dfrg.msc

Fin – adding fins along the body of a projectile has been a tried and true method of adding stability for millenia.  Just think of an arrow.  Without the fletches at the end, there would be nothing keeping that arrow going straight. The drawback to fin stability is packaging.  Due to their size and bulk it makes it hard to create a weapon that can shoot such a projectile.  Of course, adding folding fins can take away that problem, but then it adds the problems of increased weight and complexity.  Increased weight means reduced range unless we up the charge, and now we need a bigger gun to hold the increased charge.  And a bigger gun comes with all kinds of added problems.  See what kind of mess we open up here?  The added perk of using Fin stabilization is that the bore does not need to be rifled, as the stability does not need to come from rotation.

Now let’s see the difference on a projectile fired with both Rotational and Fin stability. From YouTube user 269armor:

Of course, both of these methods of stability assume that we are travelling through a medium like the air in the Earth atmosphere.  If we remove the atmosphere, there is very little need for stability, as there is very little outside atmospheric force acting on the projectile.  The bullet will merely keep going in whatever direction is was last aimed.  Stability of aim, barrel whip, lock time and hundreds of other factors become more and more important than stability in a reduced atmosphere.  More on those other factors later when we get into weapons design.

RECOIL IN REDUCED GRAVITY

This is where our old pal Newton comes back to make our life complicated again.  He taught us that for every action there is an equal and opposite reaction.  If the propellant charge causes a pressure build up which sends the projectile in one direction, it will send the gun in the other direction.  In the case of most rifles, that’s straight back into your shoulder.  The larger the charge, the heavier the recoil force.  There are many methods of controlling the recoil force, the simplest being to increase the weight of the weapon being used.  There are various muzzle attachments that can be put onto the barrel, controlling the expanding propellant gasses, but that’s back into Intermediate ballistics.  We’ll deal with that later.

So what happens if you Sci-Fi writers put a gun into outer space? or on some other planet?(bubblegum scented perhaps?)

Well, shooting in microgravity would have our old pal Newton rear his ugly head again.  Maybe not ugly.  That powdered wig though? I dunno, I guess it’s a choice.  Without anything to brace yourself against, the recoil forces of the gun would send you spinning like a top. Conventional Terran firearms on other planets would have severe problems.  Most firearms use methods of recoil compensation that bank on a few constants, namely gravity and atmospheric pressure.  (I know, I know, atmospheric pressure is not exactly constant, but it does tend to stay relatively stable enough to design around it).  For example, a Heavy Sniper rifle uses its weight and a muzzle brake to absorb the recoil impulse.  Some of these heavy sniper rifles weigh in excess of 35 pounds!  With reduced gravity, the weight of the firearm would be less able to absorb the recoil impulse, imparting more of that force to the shooter.  Shooting a large calibre rifle on Mars would be horrible.  That being said, if we have the technology to send guys to mars with guns, something tells me we can design a recoil compensation system for the Martian atmosphere.  Weapons systems using spring or hydraulic dampener to absorb the recoil energy would need to be calibrated for the reduced gravity of another world.  Also, if using hydraulics, they would have to use a fluid designed to cope with the massive temperature extremes found outside of earth.  Watch this YouTube Video to see a trained sniper handling a .50 Cal Barrett Rifle.  This rifle weighs around 30 pounds, and is shooting one of the most powerful cartridges capable of being shoulder fired by a person.  Brought to you by user dsmluck, notice how the heavy weight and the muzzle brake effectively tame this monster of a round, and make it possible to shoot comfortably and quickly.

Now, let’s simulate what happens if we go to another planet. What if we reduced the gravity, and that rifle only weighed 10 pounds? Here’s a video montage of people shooting the .577 T-Rex round, brought to you by A-Square from South Africa. This is a vicious round, made for up close Safari hunting against dangerous game. The recoil forces of the .50 cal and the .577 T-rex are similar, in the 200 pounds range. From user boxgig:

Let’s keep in mind though that in this case the rifle’s weight was reduced, but the user’s mass remained the same.  I’m sure you can imagine what would happen if that rifle were given to someone weighing 60 pounds or so.

Another alternative is a recoilless rifle.  Newton teaches us that forces act and react equally.  If the bullet is propelled in one direction with a certain force, then the gun is propelled in the opposite direction just as hard.  Well, what if the gun absorbed none of the force, and it was free to travel on its own?  Enter the venturi, and the recoilless rifle.  Invented as a means of having infantry soldiers carrying anti tank and anti fortification weaponry in a light and simple package without the need for huge weights and complicated recoil systems.  Their principle is simple.  Instead of the explosion being contained inside the gun, it is controlled by a cone-shaped device known as a venturi.  This system does have a few drawbacks though.  For one, the blast of the explosion being propelled rearwards makes it impossible for anything to be directly behind the gun for fear of damage or injury.  Placement becomes critical.  inattention can lead to severe injury or death.  Secondly, the large blast can kick up a large cloud of dust and smoke, which would act as an obscurant to the vision of the crew, and as an indicator to the enemy.  Check out this YouTube video of troops firing an 84mm Carl Gustav Recoilless rifle, from user NalteX.

Because a single soldier does not have to absorb the recoil of this weapon, the size of the projectile can be increased dramatically.  The firepower that they give an infantry platoon makes up for its drawbacks, and employed smartly they can be a horrifying weapon to face.  For our Sci-Fi writers, recoilless weapons of this type are my bang-for-your-buck choice for shooting on an alien planet with reduced gravity (the pun was a happy accident, I assure you).  Sure energy weapons have a lot of advantages, but they can’t carry a payload.  A laser is just light, and this could be anything from a flare, to smoke, to chemical weapons, to armour-piercing explosives or anti personnel fragmentation.

Tune in next week for Terminal Ballistics!  Let’s actually start hitting stuff!