Friday, 6 March 2015

Fight Or Flight: the Kzinti Lesson & Combat Spacecraft

The Kzinti Lesson

Postulated by the SF author Larry Niven the Kzinti Lesson states simply that 'A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive.'  Or as TV Tropes calls it, the weaponised exhaust.  Would you want to be standing behind an Orion drive spacecraft when it started a burn?  How about a mass drive system, or a Beam Core Antimatter Drive with a exhaust of pure gamma rays?  And photon drives have 300MW of power per newton of thrust.  Given the energies involved with fast interplanetary flight, or even slow interstellar, the most powerful weapon a future space warcraft may have at its command is the main drive unit.

   Of course, the drive plume of most spacecraft is not well collimated, and so it would be a short ranged weapon at best.  With a plasma exhaust, however, it might make an effective 'soft-kill' weapon.  The direct effects of the particles, along with secondary radiation, could damage or destroy sensors, weapons, and any other equipment mounted on the exterior of the craft and making a boarding action feasible.  Most photon drives, mass drives, and a few others have collimated exhausts, and are in essence gigantic DEW weapons.

   The connection between weapons and drives is much more closely intertwined however.  In Footfall, one of the greatest SF invasion stories ever written, the battleship Michael uses an Orion pulse drive.  It carries a secondary armament of battleship gun turrets, but the primary weapons are the so called 'spurt bombs'.  These are nuclear pumped lasers, primed by the same detonations that accelerate the spacecraft.

   This got me thinking.  Of all the different spacecraft drive systems so far invented or imagined, which lend themselves most readily to a particular weapons system?   An externally powered craft like the Michael has to generate power if lasers and EM guns are used, so unpowered weapons like Bomb Pumped Lasers become ideal, as well as dovetailing nicely with the existing logistical needs.

   The other kind of nuclear pulse propulsion, that of Inertial Confinement Fusion has a battery of powerful lasers as part of the drive system.  Surely a spacecraft could be designed with the lasers deep within the hull, routing beams either to the weapons or drive as needed.  This also introduces and interesting tactical element when a spacecraft's commander must decide between firing his laser cannon, or accelerating.  Electron beams and particle beams can also be used in the same way, although with greater difficulty re-routing the beams.

   In the designs for the ~250 km long Bond/Martin worldship design the drive is a scaled up Deadelus style pulse system.  But each 'pellet' weighs in at several tonnes of deuterium/hydrogen ice, and is ignited by a kinetic slug accelerated to a blistering 1,000 km/s, or 0.33% of the speed of light.  At that speed one kilogram has 5*10E11 Joules of energy, about the same as three ATBIP bombs, basically scaled up MOABs.  To put it into perspective the Project Thor Rods from God kinetic impactors have ~5*10E10 J.  Nothing could survive a hit from something of that calibre.  It might be hard to aim a many hundreds of kilometres long spinal mount, but nothing would need a second shot.  Given, accelerating it to that speed is a massive challenge.  It might end up being a beam powered spacecraft in its own right.  Do the math on the impactors acceleration, its a little scary.

   On a smaller scale this means that many if not all spacecraft have at least a short ranged weapons system, which is good for the plot.  On the other hand it reduces the number of people who are actually allowed to have a spaceship, which is a bad thing from a SF authors point of view.  On a large scale it means that habitats with the ability to move under power through a solar system could easily wipe out every other habitat, making for a very balkanised setting.  And if your 'Verse contains forcefields which allow fusion drives with star-core conditions...  Best to keep an eye on the brake lights of that tramp freighter up ahead.


  1. I believe the use of engine as weapon is unlikely and not due to range problems.
    Space battles will be conduct across staggering distances.
    The environment of space is really sensor friendly. There no real obstacles/horizon in the path between ships blocking sight & firing, no medium to absorb radiation or slow projectiles and background of 3 Kelvin makes every heat signature appear clearly.
    According to Atomic Rockets: "The Space Shuttle's … main engines could be detected past the orbit of Pluto". Fire exchanges in distances of merely few light sec. will be influenced by the time lag takes the heat radiation left enemy ship till it reach the sensor & time the beam/projectile travel back to enemy ship.
    In that time lag enemy ship will do evasive maneuvers. The best method to avoid been hit will be to change rapidly & randomly the main engine thrust resulting in unexpected acceleration and unexpected future position from enemy POV.
    A torch ship with cruising acceleration of 1g could randomly change its acceleration between 0g to 1g if not above 1g.
    For every time lag of 't' sec. for detection & firing there will be 5*t*t meters of potential displacement.
    Laser beam travels at 1c but requires dwelling time on a specific point on the target, the time lag coupled with the changing in target acceleration will prevent that.
    EM projectiles don’t require dwelling time but since they are much slower the chances of them hitting maneuvering target are slim.
    A duel between two warships will be very similar to 'crossing the T' maneuvering from old sea navies' tactics. The uncertainty about the location of the enemy ship could be reduced if the enemy ship move toward or from your ship. When such a thing happen the proper motion of enemy ship from yours POV become minimal and hitting it becomes easy.
    In this new battle field the optimal 'high ground' is that:
    1. Your ship appears from enemy POV in their 'poles' either in front of their nose or behind their engine.
    2. Enemy ship is in your 'equator' zone of POV.
    Such positions increase your ship proper motion relative to the enemy and reduce their proper motion relative to you.
    Using ship engine as spinal cannon will be suicidal; it will be difficult to constantly re-aim it toward a maneuvering target and will reduce the ship proper movement (from enemy POV) to zero; making the ship a "sitting duck".

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