Hard Science Fiction Weapons; Kinetic

Yes, this is what happens when a 2.4g aluminium ball hits a
aluminium plate with 6.8 km/s relative velocity,
~ 14 kJ impact energy

Weapons of Choice: Kinetic

     As I have noted before major warships are unlikely to have a place in a hard SF military constellation( I prefer, when I remember, to use constellation instead of fleet), but this does not mean that warships are non-existent, and those that do exist, will have to be armed.  But in a world where lasers are most commonly found in DVD players, and military units have difficulty taking out fragile missiles, the laser fire that fills the space battles of our imaginations seem no more than dreams.  Then to the technology used in your setting might be such that a spacecraft cannot provide power for a weaponised laser.  Orion drive spacecraft, for example, are fast and powerful, but need separate electric generators as the drive does not produce power like a fission thermal unit can.  Thus if the warcraft are Orion drives or similar, electricity will be at a premium.  Other difficulties include the mass and fragility of radiators needed for large amounts of power generation.
   The solution is both simple and compacted, but has an unexpected advantage for anyone attempting to portray a realistic space battle in a visual medium.  Unpowered kinetic weapons are a must, fuelled by chemical explosives, and self contained missiles and rockets.  While such weapons are the staple of ground warfare in the real world, the designs used are unsuitable for space-based applications, although those that can be used in space often can also be used on the ground.  Due to the shear size of a space conflict the velocity of a projectile must be as high as possible to increase the chance of a hit, and thus lasers, travelling at the speed of light, are preferred.  Fighting with slower kinetic weapons the waring spacecraft must come much closer, making for a more visually dramatic combat. I will look at several systems, and some examples, both real and otherwise, of weapons that can be used without power supplies in a SF military, both in space and on the ground.

The friction trail and impact flash of a HV
kinetic projectile similar to the one in the previous picture.

Light Gas Guns

   One of many factors limiting the muzzle velocity, and thus effective range, of any conventional gun is the speed of sound in the propellant.  A light gas gun, or LGG, bypasses this problem by using a gas with low molecular density, such as hydrogen or helium with a speed of sound several times greater than in air.    This can give muzzle velocities of up to 8 km/s as opposed to the 1-2 km/s of a conventional gun.  Because the hydrogen or helium working fluid cannot itself provide the energy to fire a small amount of conventional propellant is used, although an electric arc is another possibility, and offers weight savings if the power is available.  In a third variant the hydrogen is combusted with a small amount of oxygen, and this type is the combustion light gas gun.    In the most common type the conventional explosive is detonated behind a piston, super-compressing the working fluid until the pressure ruptures a membrane between it and the projectile.  As the barrel has a narrower bore than the piston the working fluid is accelerated, propelling the projectile out of the barrel.

Image from Wikipedia page on Light Gas Guns

      A LGG could easily be built at any scale, as a personal weapon, or as the main armament of a tank, spacecraft, or warship.  A sniper rifle utilising the technology would be especially effective, and so would anti-armour weapons.  Some prototypes of combustion LGGs have been built, and NASA uses small conventional LGGs in hypervelocity impact research, so it is a proven technology, although with some engineering hurdles like weight, reliability, and accuracy before militarisation could be achieved.

The Ram Accelerator

   Like the LGG the Ram Accelerator is used for research purposes, and is being looked at as a method of non-rocket space launch, shooting one ton cargoes into LEO several times per day at a very low cost.  In operation it acts like a scramjet, as shown in the picture to the left, allowing for continuous acceleration along the length of the barrel.  Unlike the LGG the muzzle velocity is not limited by the speed of sound, although a hydrogen/oxygen gas mixture is used. Velocities stained are likely to be similar to a LGG, but there is a possibility of higher speeds than the LGG can ever reach.
   Although the LGG can be scaled down, the Ram Accelerator is likely to have a fairly large minimum size, offset by the fact it is easier to build a large Accelerator as the barrel is a simple metal tube with relatively low stresses combated to a LGG.  A Ram Accelerator is thus suited as the main weapon of a vehicle, perhaps as a spinal mount, or a long ranged bombardment weapon on the surface of a planet.  Like a Combustion LGG the fuel is hydrogen and oxygen, easily extracted from water, and so likely to be available in any location throughout the solar system.  Payloads can be up to a ton, allowing rounds to carry loads such as guided missiles, nuclear warheads, or others.

Missiles

   Missiles, self targeting, self powered, long ranged and deadly, have a lot to offer in space combat.  While laser point defence may render them useless in a purely kinetic battlefield they are are force to be reckoned with.  Longer ranged missiles may be field by liquid propellants, and carry submunitions or smaller missiles, and be used to open an engagement from long range, while smaller rockets are used as point defines and close quarters weapons.  They are far to varied a field of weaponry to do justice to without a full blogpost dedicate to it, so I will only make a few points.  The first is that in the majority of cases, as I have said before, no warhead will be needed.  The second is that bigger is not necessarily better.  The Sprint Anti-Ballistic Missile, built during the cold war, massed out at 3500 kg, could accelerate at 100g, and had a top speed of mach 5 in atmosphere.  A space  based version would be deadly as a point defines system, or as a close quarters weapon.  At this level of technology the only plausible torch missiles are stripped down orion drive spacecraft, basically just and engine unit.  These could accelerate at thousands of g, and have impact energies of gigatons, acting as cross system ship-killers.
   As an aside, the fabled Gyrojet weapon may have been an failure in its original iteration, but advanced designs or more research could turn it into an effective futuristic weapon, either as a sidearm or a mounted weapon.  It has the advantage of almost zero recoil, which is a obvious advantage when fighting in free fall, and also masses less than a conventional small-arm.


Other Options

   Although not strictly falling under the heading of this blog are nuclear weapons.  Although conventional nuclear explosives are virtually useless in space, nuclear shaped charges or bomb pumped lasers may well rule the battlefield.  These will form the subject of a future post, after I have completed the Kelvin deigns, and several other posts of a non-military nature.




Note from the author 
   This is the first post for a while, and my schedule has been coming apart at the seems, so sorry to anyone who was expecting new posts sooner.  They should be more regular from now on, and I have made Friday the update day, starting next week.  I also intend to start posting my own SF writing on the blog, both short stories, and analysis of settings, to illustrate the concepts that play into world-building.  That should start in a few weeks.  Another possible addition is the inclusion of free Blender  3D meshes of spacecraft parts and components for downloading, if I can get blogger to allow it.  If not, I will find some other place to put them, and include links.  Meshes will mostly be small components like airlocks, docking rings, engines, truss sections, and radiators.