Friday, 22 January 2016

Space Combat Part 1: Tactical Manoeuvres


   I originally wrote this post as a guest post for the Future War Stories blog( link), where it generated a lot of very interesting discussion in the comments.  Since then, and mainly as a result of the comments, I've decided to expand on the theme of tactical manoeuvres.  I'm posting this so that anyone reading either part will be able to find the other; I do encourage reading the comments on Future War Stories though, they have almost as much stuff as the post itself.

   Hopefully this will develop into an extended series of posts, some of which will be design related like my  Building a Space Battleship, and some will be looking at tactics, strategy, etc.

  SF Worldbuilding has been on a bit of a hiatus, but I'm going to be posting again, hopefully.  I remember saying that before though, so don't get to excited.  If anyone has any ideas for non-military future tech they'd like a post on please leave a comment; my main interest is military SF, so I find other things hard to think of, but want the blog to have a broader scope than that.

"I am a leaf on the wind" - Wash
The hand can't hit what the eye can't see

   As both 'Wash' Hoban(Firefly) and Han Solo(Star Wars) have demonstrated on numerous occasions firepower is not the only asset that can win a fight.  Quite often in movie SF the heroes of the story will be aboard a smaller spacecraft than their opponents, their only hope of survival lying in their superior abilities.  While this is largely due to dramatic reasons, it does draw attention to the importance of manoeuvrability in space combat.  When dealing with hard SF - no handwavium forcefields or technobabble shields - one shot kills are very probable: nukes, mass drivers, particle beams, lasers, all posses more than enough potential to negate any form of armour we know about today.  And while no real spaceship will every fly with the grace of a X-wing starfighter this does mean that the ability to avoid hits may be more important than surviving them(structurally, the crew is still a concern), much like the situation in arial combat today.

   For SF writers this is a boon.  A battle that requires manoeuvres is intrinsically better suited to one in which humans might play a role.  Randomness and intuition could be vital, and so far computers don't offer that.  Even if the ship can fly and fight itself this leaves room for a human tactician, negating Burnside's Zeroth Law of Space Combat - SF fans relate more to humans than they do to silicon chips.  However, it can also pose difficulties.  Space is not a familiar environment, and movement in it is counterintuitive at best.  It is also radically different for a spacecraft in orbit around a single planet, in a planetary system, or in deep space.  And for those of us who try to avoid the dreaded 'Space is a Ocean' trope this can be very...frustrating.

   So, I'll look at four basic situations; deep space with low relative velocity, deep space with high relative velocity, single planet, and planetary system.  For each I'll also take a look at the changes in the situation that different tech will have.  This post is not so much about manoeuvring itself, but about how different situations shape it.  An in depth discussion of tactical manoeuvring down to the level of orbital physics or specific technologies would make the article far to long.  In the future I'll attempt to do follow up articles that look at manoeuvring in the context of a specific spacecraft, but for now this should provide an indication of what a spaceship would be doing.  For simplicity's sake I'm only going to consider one-on-one battles in detail, not constellation engagements.  Fleet actions are a whole separate ball game, and will warrant a separate post.

Deep Space - low relative velocity

   Just what is 'deep space'?  For the purposes of a story it is that area of space which only the bigger spacecraft can reach, so interplanetary or interstellar, depending on tech levels.  From a navigational perspective it could be defined as 'flat' space.  That is, space in which the gravitational acceleration is insignificant.  Insignificant is defined by the power of the drives your spacecraft is using, so this adjusts itself to match the setting.
   Manoeuvres here are closest they will get to those found in Space Opera.  The lack of a gravitational source means that movement in any direction is equally easy, and the fight becomes truly 3D.
   For high tech - multi-gee acceleration and big delta-V - the fights will be 'dogfights' to some degree.  This will be more marked if the craft use spinal mounted weapons, or if they have large blind spots in offensive or defensive weaponry.  If kinetics are the main weapon then the fight could become quite interesting, with KE rounds restricting the possible choices for manoeuvring, a possible tactic for the adept captain to employ.  Missiles will be very effective, with s straight line of flight to the target, as will beam weapons.  Particle beams will benefit, as they are degraded in accuracy and rage in the presence of a planet's gravity or magnetic field.  If lasers are the primary weapon then the fight will be less of a dogfight, and more of random 'drunk-walking' to throw off targeting.
   For low tech - milligee acceleration and limited delta-V - visually this would be quite boring.  The ships cannot perform elaborate manoeuvres to get in each other's blind spots, nor can they expect to dodge beams and kinetic weapons at short ranges(ranges dependant on velocity of the weapon).  Instead orientation and sensor data is the most vital.  The spaceship must bring the most weapons to bear, while at the same time keeping a small target profile, and reducing signals that might give its opponent an effective targeting solution.  The ships orient themselves, enter weapons range, fire a few salvoes, and the battle is decided.  In this case missiles are very effective, as they can come in at an angle to the primary attack vector, distracting sensors and absorbing point defence capacity.  Kinetic rounds are also more effective, not only can the score a hit from longer range, but they can be more easily used to 'box in' an opponent than if accelerations were high.  As before, 'drunk-walk' will be used to throw off targeting.

Deep Space - high relative velocity

   The chances are that spaceships will rarely intercept each other in deep space.  It is simply to large, and too easy to see someone coming.  When they do it is likely to be a head-on pass at high relative velocity for two spacecraft following the same or similar orbit in opposite directions.  Note that once unrealistically powerful torch-drives become common, interception is possible, if still unlikely unless both parties wish it, or one slips up.
   It turns out that for both high and low tech the manoeuvres are much the same in this situation.  Any reasonably fast orbit will result in the two ships passing with Rv of tens if not hundreds of km/s. At this speed there is not time to dogfight.  Even a torch ship, which will have a much higher intercept velocity, will take so long to cancel its Rv and return to the battle it would be considered as a separate engagement, rather than a second pass.  For a ship with foreseeable tech it would be nearly impossible.  If anything it will resemble a joust between two medieval knights on horseback.  Unlike a joust, however, they might not be a winner.
   The longest commonly accepted range for a laser weapon to target effectively is about one light second, or 3*10^8 meters.  At a very low end relative velocity - I randomly chose 40 km/s, which means that each ship has ~half solar escape velocity, which is not unrealistic, nor is it that high for a advanced ship.  At this range and closing speed the time for targeting the incoming ships and its projectiles is ~2 hours.  Plenty of time to shoot down incoming projectiles, you say.  But at this speed one kilogram of inert matter has an energy of 8*10^8 J.  And how many of those is the opposing ship going to throw out in your path?  You can make considerable sideways movement relative to direction of travel in an effort to avoid the projectiles, but the opposing ship can easily see any move you make, and at charter ranges dodging will become impossible.  Pretty much any kinetic hit at this speed will be fatal, so it will be the ship with the best point defence, sensors, and emergency manoeuvring that will survive.
   Durin the approach both ships fill space with inert projectiles, possible with last ditch terminal guidance.  They will be hard to spot at long range, tiny, inert, and possibly cooled down so that they have no discernible thermal signature.  So it will be only in the last stage of the pass that the combatants can begin to dodge the projectiles.  High lateral acceleration and powerful attitude control will help to weave through the incoming fire like a skier on a slalom course.  Good sensors will be needed to sport the incoming, and good PD to shoot those that can't be avoided.  However, it is my personal opinion that this sort of situation would be 'two men go in, half a man comes out'.  If energy wagons are primarily used, them this is even more so the case, as dodging becomes effectively impossible.

Orbital Space - single planet

   Most space battles in SF take place in orbit around a planet.  This makes sense in both hard and soft SF 'Verse's for several reasons.  Primarily it is the place where hostile spacecraft are most likely to meet.  It also adds a new layer of complexity to the fight, introducing 'terrain' to the tactical considerations.  The planet can hide opponents, restricts manoeuvres, sucks up delta-V, and provides something to crash into.
   Aside from hiding spacecraft who are on the other side a planet can slo provide some cover for combatants.  Picking up a spacecraft against the disk of a planet is significantly harder than spotting one against the backdrop of space after all.  A low orbit that brushed the atmosphere prevents opponents from attacking from most of one hemisphere, a great advantage.  For a craft equipped to reenter the atmosphere it also offers the possibility of manoeuvres not possible with the amount of delta-V they posses.  From reading Atomic Rockets kinetic weapons seem to hold the advantage shooting from a higher orbit at a lower.  A DEW is not effected so much, and so the orbit used is less of an advantage or disadvantage aside from the detection aspects.  Lasers also posses the potential to be 'bounced' around the horizon by remote drones, meaning that the attacker can shoot without exposing themselves.
   So the aim of any manoeuvres is pretty simple.  Orientation to bring weapons to bear, and the standard 'drunk-walk' are a given.  The opposing captains will try to gain the better position in an orbit underneath the enemy ship, or perhaps between the enemy ship and the sun, which might help to blind sensors.  This will be complicated by the fact that change orbital inclination is very hard compared to other manoeuvres, restricting the spacecraft to a 3D layer of space, although not  2D plane shown in so many soft SF works.  Forcing the ship into a lower orbit will decrease its orbital period, and vice versa.  Combined with changing the orbit from circular to the elliptic and back this gives spacecraft commanders the ability to surprise their opponents by appearing around the planet at a different place or time than expected.  There will also be a large amount of 'minelaying' of a kind, seeding or its will kinetic projectiles in order to herd the enemy into a bad position.
   But while the aim of the manoeuvres is simple, execution is not.  Trying to explain it is beyond me, so I suggest that anyone serious about grasping orbital mechanics begins by playing the Kerbal Space Program game, or browsing youtube for anything helpful.  It makes a lot more sense visually than it ever will in writing.
   High tech - for advanced ships a planet is a much smaller piece of terrain, a hill rather than a mountain.  They can more easily afford to change orbits, to drop below minimum orbit al velocity or go over the maximum, and can perform delta-V heavy manoeuvres such as change the orbital inclination.  The ultimate of course is a ship that has drives powerful enough to reverse its orbit completely, surprising its opponent when it emerges around the opposite side of the planet to what was expected.  With higher acceleration and delta-V the seeding of orbits becomes less effective, much easier to dodge than with a low powered spacecraft.
   Low tech - with low levels of acceleration, even if the spacecraft has a high delta-V, changing orbits can take days if not weeks.  The position of the enemy will be highly predictable, and so kinetic weapons become very important.  The advantage converted by different orbits will be much more apparent, as it is harder for anyone to turn the tables on their opponent.  Most tactics would be a combination of manoeuvring into a good position, and using kinetics to force the enemy into a bad one.  Low tech ships would also gain a large advantage by being able to dip into the atmosphere, as this provides essentially free deceleration, saving reaction mass.

Orbital  spaghetti
Planetary Systems

   Adding more heavenly bodies to the mix vastly increases the tactical possibilities.  While 'planets' per se do not do much, moons do.  A gas giant with seven or eight moons is a extremely complicated system, and has travel times of only hours or days as opposed to years between planets, and that is with Hohmann orbits.  High acceleration, low delta-V spacecraft could follow complicated routes, sling-shoting themselves around the moons to gain an unexpected position. For much of the time they could be out of sight of the enemy, making it a scenario reminiscent of The Hunt for Red October.  The fact that moons often have lower gravity than planets also means that the manoeuvres in proximity to them can be more extreme given the same tech level.  It even brings up the possibility of landing on a moon, camouflaging the spacecraft, waiting for the enemy to pass by, and then launching and taking them by surprise.  The changes imposed by tech levels are the same as those for a single planet, so I won't both to go into detail.  This kind of setting will be the most complicated for a SF aficionado to get right, and I would suggest finding a solar system simulator to model the setting before attempting to figure out a complicated battle.  It does lend itself to far more interesting scenarios, however, and will be far more rewarding.