If you want to get an intuitive feel for how interplanetary combat is likely to be, there are a few simulation games that can help. Otherwise, read on:

For supplementary information, please read Rick Robinson's Rocketpunk Manifesto. Specifically:

What Will It Be Like?

What's its going to be when space combat finally arrives?

Is it going to be like WWI aircraft? That is, rickety ships with a few crude weapons bolted on as afterthoughts, flown by a few aces who are familiar with the eccentricities of their craft? (Imagine a Space 1999 Eagle Transporter as a futuristic "Sopwith Camel") Or will it be more sophisticated?

Watching the evolution of space warships will be interesting as well. In the movie THE ENEMY BELOW (the movie that the ST:TOS episode "Balance of Terror" was based on) the German U-Boat commander was reminiscing. He said that in WWI, when you submerged in a U-Boat, you were never quite sure that the cantankerous submarine would surface again. The captain would eyeball the target through the periscope with no gauges, do some arithmetic in his head, and order the torpedo fired verbally. If you were lucky, it would make it out of the tube.

But now, the captain moans, it is all mechanized. He looks through the periscope with cross-hairs, which relays the settings to the plotting table and the automatic firing calculator. The captain thinks it is terrible that they've taken the men out of war.

So in the future one can imagine a Belter pilot, crying over her beer-bulb at Ceres Bar. She'll bend your ear about the good-old-days during the Asteroid War of Independence, figuring vectors and delta-vs by the seat of your pants, early mornings on the Cosmodrome with your leather jacket and anti-nuke goggles, flying for Duquesne's Flying Circus.

Nowadays, she'll complain that pilots just zip up into the acceleration tank and let the computer fight the ship. They've taken the men out of war...

Try to imagine what would it be like on the deck of an escort class interplanetary craft, shepherding a convoy of logistic hulls and on the lookout for convoy raiders. The signals officer will be alert on his ladar scope, trying to burn through the stealth of the wolf packs.

But then there is scientific reality to consider. Unfortunately, it seems that the more accurate you make it, the less interesting it becomes. Having said that, keep in mind that much of the following is speculative and controversial. If you don't like it, ignore it. Just try to be self-consistent and work out the ramifications of anything you postulate. Go to The Tough Guide to the Known Galaxy and read the entries "SPACE WARFARE" and "WARFARE"

When it comes to scientifically accurate space war simulations that are NOT ultra-top secret classified military studies, the two best that I've found are the tabletop boardgame Attack Vector: Tactical and the computer game Children of a Dead Earth.

Yes, they make conflicting predictions about how space combat will be. This is because the designers cannot know what are realistic technological assumptions to use. They did their best, but the only way to be sure is if this is part of an ultra-top secret classified military study. In which case your clearance is probably not high enough to read it.

But either is light-years better than just making crap up, as does most media science fiction.


Who doesn’t love giant space battles with lasers and missiles and shields, oh my? Communists, that’s who. From Battlestar Galactica to Deep Space Nine’s Dominion War, some of our most riveting science fiction has come from spaceships trying very hard to destroy each other. Unfortunately, things get complicated when you introduce real science into the equation, and not just because there’s no sound in a vacuum. Space is an alien environment, and it’s difficult for us puny humans to understand all the potential difficulties of fighting there. The issues are so numerous that it’s no wonder filmmakers keep going back to a futuristic World War II.

1. Space Is Really Big

To quote the great Douglas Adams, “You just won’t believe how vastly hugely mindbogglingly big it is…” Models of the solar system don’t really do it justice. The distance between Earth and Pluto is about 53 times the distance between Earth and Mars. Objects in the asteroid belt are often millions of miles apart, and that’s considered densely packed in space. Instead of a Romulan warbird filling the viewscreen, enemy ships would appear as little more than blips on a radar display. Long range missiles and laser weapons would mean that the opposing sides would never get close enough to see each other.

While this can work pretty well in novels, it’s a major problem for visual mediums that depend on action packed dog fights to keep things interesting. What’s more, because space is so big, travel time can become a serious factor. In a lot of scifi franchises, travel is near-instantaneous due to fancy warp drives or what-have-you. In a more realistic world, getting from where you are to where the enemy is can take days or weeks, even with a relatively high level of technology.

2. Weapons Advance Faster Than Armor

The iconic image of two capital ships slugging it out broadside to broadside simply does not work in space, and that’s because neither vessel would be able to take more than one or two hits from the other. In real life, we are already at the point where a single missile can sink a large warship, and that curve is only going to get more pronounced with new technology. It turns out that destroying something is much easier than keeping it intact.

Throughout history, weapons have gotten progressively more destructive, and armor has been unable to keep up. Modern body armor can usually stop small arms fire, but it is absolutely useless against anything more powerful. Even heavily armored tanks can be easily destroyed by much less sophisticated weaponry. The reasons for this are pretty simple. Machines rely on moving parts – be they a heart and lungs, or a fuel injection system. A weapon doesn’t have to destroy the entire target. Just mess up a vital system, and suddenly the whole thing stops working. Armor has to protect an entire object, while a weapon only has to apply destructive energy to one part of it.

In space, no one will think twice about deploying nuclear missiles and all the power they contain, to say nothing of all the new and interesting ways humans will surely think up to kill each other. Even energy shields don’t solve the problem in a realistic universe, because the amount of energy needed to run them would pale in comparison to how much energy a laser would need to punch through.

There is one possible method of effectively armoring space ships, but it almost never features in stories: make your ship out of an asteroid. It turns out that a thick layer of rock and dirt is very effective at stopping explosions, even nuclear ones. There’s a reason bunkers are built underground. Of course, lumpy space rocks with engines welded on aren’t nearly as majestic as the USS Enterprise, which is probably why they don’t show up in science fiction very often.

3. Planetary Bombardment Is Really Easy

We all know how unrealistic the Death Star is, but what if I told you it was also complete overkill? Wiping out all advanced life on a planet, while a monstrous act, isn’t that hard if you have some handy space rocks floating around. All the would-be mass murderer has to do is grab a good sized asteroid and accelerate it toward the planet in question, then lean back and let physics do the work. It turns out that humans, like dinosaurs, are highly susceptible to asteroid impacts.

Stopping an incoming asteroid from the ground is really hard, and there’s no reason the attackers couldn’t launch a few dozen at once just to keep things interesting. The same principle applies to space stations or any other object that can’t significantly alter its trajectory. Rather than massive fleets laying siege to defiant worlds, all it would take is a single ship threatening armageddon. By the same token, actually conquering a planet with ground troops would be insanely hard, but that’ll have to wait for another day.

4. Everything Happens Super Fast

X-wings and Vipers might look like they’re speeding along, but it turns out they’ve got nothing on NASA. Back in 1969, the Apollo 10 spacecraft reached 24,791 mph, or about 32 times the speed of sound. Presumably, spaceships of the future will be even faster. They’ll have to be, because as was previously pointed out, space is really big. The problem is that humans simply cannot react at those speeds. Imagine yourself as a future gunner, trying to shoot at an enemy ship zipping by so fast that it’s gone before your brain has registered it was there.

To put this in perspective, the Komet rocket fighter flew at about 700 mph, and that was considered too fast for any practical fighting. The Komet would zoom past its targets so quickly that the pilot didn’t have time to shoot anything. Lasers go even faster; the speed of light, to be exact. Any ship under attack by laser weaponry would be hit at exactly the same time they realized they were being shot at. There would be no time for any dodging or weaving, no matter how good the pilot was.

5. Maneuvering Will Kill Your Crew

It turns out that going really fast in space isn’t a problem. Without air resistance, the speed of light is the only real limit out there. The problem is in how fast you accelerate. When a ship’s engines go to full burn, the crew are going to be pushed really hard into their seats. Human beings can only withstand so many gs, and it gets worse the longer we have to sustain the pressure.

Combat maneuvers don’t have the luxury of slowly building up speed. When enemy missiles come burning in, you have to get out of the way fast, and that sort of violent movement will pose a real danger to your crew. Without magical devices like Star Trek’s inertial dampeners, sudden acceleration or deceleration can turn us fragile humans into pink smears on the back wall. The incoming ordinance doesn’t have the same biological restriction as a crewed ship, so avoiding it will be next to impossible. Sorry, Starbuck, but it turns out that Cylon missiles don’t have your fragile meat body problems.

6. Everything Should Be Done by Robots

Computers can process information much faster than humans, and that gap is only going to get wider as technology advances. Even the modern military is relying more and more on computers when human reaction time just isn’t fast enough. The Phalanx missile defense system, for example, uses automatic radar targeting, with no direct input from its human operator.

Even without the development of fully sapient artificial intelligence, computers are getting better and better at independent operation. What’s more, they can be built to withstand far greater strain than the human body. Also, computers don’t need to breathe. A hull breach matters a lot less when there’s no air to leak out, to say nothing of the space saved from not having toilets.

Fully automated warships would be able to react faster and be much better at surviving than any ship crewed by flesh and blood humans. Not only would realistic space battles consist of radar blips shooting at each other, humans would be confined to watching those blips from even farther away. Both sides would send out robot fleets, and then have to hope that their version of iTactics was more advanced than the other guy’s. Perhaps not as exciting as the Battle of Endor.


None of this is to say that realism can’t or shouldn’t be applied to space battles, just that it must be done carefully, because it will drastically affect the story. Battlestar Galactica solved the problems of distance and speed with a teleportation-style FTL drive that put opposing ships within spitting distance of each other. The Honor Harrington series builds drama around the long wait between firing off a salvo of missiles and when they finally hit their targets. Leviathan Wakes absolutely revels in the crushing gs caused by violent combat maneuvers. What’s important is that authors who want to write hard science fiction be aware of these problems, and either turn them into advantages, or make a conscious choice to ignore them. Believe it or not, you can still write hard scifi even if you have to fudge a detail here and there. You just have to know you’re doing it.

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All that Glitters is not Gold

    One of the biggest problems in creating a SF story in any medium is the concessions that have to be made to the medium in which the story is told.  This is especially prevalent in visual media such as movies, games, and comics/animation.  The problem is acerbated for those how prefer SF that falls onto the 'harder' end of the spectrum, as they lose many inaccuracies usually used because they make the telling of the story easier.  While someone unconcerned with how a space battle might actually look given the technology concerned it is a simple matter to add some glory lasers and explosions, copying what has worked in other franchises like Star Wars or Star Trek.  For those of you that do worry over these details, read on..

   I'm going to discuss some of the more common elements of space battles in visual SF, although by no means all of them.  For each I'll try to look at why they are shown the way they are, why they are inaccurate, and how they can be changed for greater realism.  A lost of this sort of thing comes down to the exact details of the setting so I will focus on fairly broad issues, things that are unlikely or impossible given our current understanding of physics.

 Who, What, When, Where, Why?

   While you might think that a discussion of physics or technology would be the most important part of this blogpost, there are other aspects to creating a space battle equally important.  If the battle is part of a story then it has to show the story or it is ultimately pointless.

   Say you are producing something along the lines of Star Wars or Battlestar Galactica with main characters who fly fighters, and with a story that focuses on the personal element that they bring into the tale, rather than the grand strategy of admirals and kings.  In this situation frantic dogfights with wheeling fighters, explosions, and hair-raising manoeuvres will probably serve the story quite well.

   If, on the other hand, your story is about a nation, a war, or a leader rather than a simple soldier it has different requirements.  Tactics will let you explore the leader's abilities, strategy will convey the importance of certain actions or individuals in the grand scheme of things.  In this case your might want to set your scene on the bridge of a starship, with huge tactical displays, and hasty conferences between the officers.

   The answers to the above questions will help you to decide on the details of your setting that facilitate the decision; things like the technology used, the strategies and goals, etc.  Only then can you begin to work on the visual aspects of the space battle and know that it will support and compliment the story you are trying to tell, rather than distracting the viewer from it.

   The two examples I gave are by no means the only choices, and you can even switch between the two within the course of a single battle.  There are also other similar considerations; the design or a ship can be used to reflect on the nature of those who built them.  Dull colours and blocky shapes can suggest lower level technology when contrasted with organic curves and polished metal.  The scenario can also be expanded visually; chaotic formations during an ambush, or elegant fleet manoeuvres during the siege of a planet.

Space is Big

   Space is called Space for a reason, mostly because there is a lot of it.  Yet in a almost any work of visual SF - Star Wars, Star Trek, Farscape, Babylon 5, etc. - ships huddle together as though running out of space; as they are, in a way.  The situation is even more ludicrous when you delve into the lore of the various settings and find out that the weapons involved are apparently capable of engaging the enemy at beyond visual rages.  In anything remotely 'realistic' this is an almost inexcusable error.  The reason it is so widespread is quite simple; screens have only so much space.  To show a fleet or a battle in as exciting a way as possible, or to convey the course of a battle, the artist needs to crowd the spacecraft together.

   Fortunately the cure is almost as simple as the explanation.  The first is a technique occasionally used in Babylon 5; it involves showing one ship firing, and then cutting to its target and showing the impact of its weapons.  Having a planet or other large astronomical feature in the background can help to give a sense of the distance between the opposing spacecraft, and if visible energy weapons are being used colour can differentiate between enemy and friendly fire.  The second technique, and the one that better conveys a fleet, is to frequently show the tactical displays being used by the officers directing the battle.  Since such a display should be designed to provide information in a clear manner it should not be too hard to make it legible to the audience.  It also has the advantage that the display can be used to show planned manoeuvres before they are carried out.

Visible Drive Trails & Weapons Fire

   Whenever missiles are used in visual media they are almost always have a neat trail of smoke or vapour to mark their progress, something occasionally produced by manned craft as well.  Likewise energy weapons will glow brightly in all the colours of the rainbow, and even kinetic rounds will glow brightly.  Once again the reasoning behind this is simple, it makes it easy to see what is happening and it looks cool.  Movies and games both use this tactic, games often going on to give fighters long exhaust trails that make their trajectory say to determine visually.  The Homeworld strategy games are notable in this regard with most small ships producing visible trails.

   Glowing kinetic rounds, while they may seem the most unlikely, are in fact the most realistic - to an extent.  Gauss cannon or railguns could heat up the projectile they are firing; simple iron shots might be glowing red or white hot by the time they leave the barrel.  But while they might emit some light it is likely to be so little that combined with the small size and high speed of the projectiles they would be invisible to the human eye.

   Missiles with smoke trails also seem to be quite plausible at first.  Aircraft can leave contrails behind them, and chemical fuelled rockets will most likely produce some smoke, although it is minimised in missiles to make them harder to spot with the Mk.1 Eyeball.  In space however a vapour or smoke trail will never form; with no air to slow and support particulate matter from the rocket engine the trail will disperse far to fast for the human eye to catch.

   Energy weapons glowing seems like a pretty rational thing.  They are after all composed of energy and in the case of plasma weapons are utilising a state of matter that does usually glow.  Lasers will not be visible without particles to scatter the beam, the only reason that we can see them on Earth.  The component particles of a particle beam do not emit energy unless they are slowed or deflected, and when this does occur the radiation emitted is probably not going to be visible given the energies involved.  Plasma weapons might be visible as glowing projectiles, if they ever prove to be feasible in the first place; but given the tennis nature of fusion plasmas, and the relatively small volume of a plasma bolt, they might be very hard to see even then.  The weapons that will be very visible would be things like nuclear shaped charges, although they would also be very brief.

   Aside from the fact that this means space battles will never look like how Hollywood thinks they will there are a few other result of this.  You will almost never see incoming fire.  In the case of particle beams and lasers even sensors will not, in the first case because there is no radiation emitted, in the latter because the beam is travelling at the speed of light.  Missiles and projectiles might be seen, but not by the human eye due to their probable velocities.  In a written work this is easy to accommodate and while a visual work might have more trouble many weapons can be given a muzzle flash or other effect to show that they have fired, and energy weapons can have radiators that begin to glow(although visible glowing radiators are in themselves unrealistic).  The bigger problem is that without visible weapons fire it may be difficult to convey to the viewer what the individual spacecraft are shooting at.

Explosions & Damage

   One of the most dramatic moments in a space battle is when a spaceship with 'the good guys' aboard takes a hit.  In most franchises this results in a big gout of orange fire and then, if we're lucky, a shot of the hull plating torn away from the support structure underneath, preferably glowing from the energy of the shot.  This same ball of improbable fire makes its appearance when a spaceship, missile, or asteroid is blown up, and often on the detonation of nuclear weapons.

   The problem with this is that high energy explosives use in warheads do not produce an orange fireball like an exploding car.  Normal high explosives usually produce a nearly invisible blast since it is thermal energy that creates the visible fireball, and most explosive warheads are optimised to produce mechanical damage.  A thermobaric warhead would create a nice gout of flame, but they are fundamentally unless in space.  The explosion of fuel stores, the ignition of leaking atmosphere, and the release of superheated coolant can be used to explain these gouts of flame to some extent, but even then the effect will be far more rapid than inside an atmosphere, becoming more of a flash than a ball of fire.  And even if something aboard the targeted vessel is ignited there will be no billowing smoke clouds, since these too will expand extremely rapidly in the vacuum of space.

   High energy weapons such as lasers, particle beams, nuclear warheads, and possible plasma cannon, might leave glowing areas on the armour of a spaceship.  I don't have the knowledge to judge on that.  I do know that they will not produce big orange fireballs, though.  High powered weapons strikes are going to be a bright flash of light, potentially coloured depending on the materials and weapons involved, with mechanical deformation affecting the surrounding area to some extent.  The extent of the damage and its appearance is quite a complex question, and the best place to get answers is over on Atomic Rockets.

   Related to this is the explosive end of every ship damaged in battle, usually with a nice fireball.  Given the penetrative qualities of kinetic weapons, and the soft kill potential of radiation based weapons, it seems likely that many ships could be put out of action with very little exterior damage, at least in the case of larger ships.  Given that real fusion and fission reactors are likely to explode the only explanation is that it is a result of high energy reactors of other kinds loosing containment.  Certainly in Star Trek with their antimatter warp cores even an small but penetrating hit could cause a massive explosion.  From a historical perspective it was easier to add a fireball back when CGI was in its infancy that it was to do complex battle damage, so that is the reason why it is so common in older works; newer works just continuing the trend for the most part.

   Anything nuclear is usually surrounded by a lot of inaccuracies in any fiction, so it is no surprise that many works do not show nuclear explosions the way they should.  In space, without an atmosphere to produce a fireball nuclear explosions will be very very brief, albeit of great intensity.  Of course the afterimage would last much longer if you did see it in person.  Often in movies and games nuclear explosions, even when they do not produce fireballs, persist for quite a while; in the real world you would blink and miss it.

   The final inaccuracy commonly found alongside dramatic explosions is that nearby spacecraft will shudder as though hit by a blast wave; yet this is impossible without an atmosphere.  The total mass left behind by an exploding missile, or even a ship, is insignificant compared to the volume of space, and unless the other spacecraft is ridiculously close, or shrapnel is involved, this is not going to happen.

   None of the above are significant problems.  They are mostly matters of convenience, of limitations in the medium used to depict that battle, or of convention.  There is no particular trick to incorporating the 'correct' version of space battle visuals, but it does seem to be somewhat uncommon.  I suspect that it is largely a result of the saturation of Star Wars and Star Trek; people are inspired by these franchises to create their own SF but often end up with a variation, rather than something that makes more sense on the world-building side.  This is kind of like the ridiculous size of many spacecraft in SF, everyone else is doing it, so there is a temptation to use the exist metric rather than try to make the viewer use your universes internal scale or logic.

Details, Details...

And SF authors should use Heinlein's technique of adding an odd detail or two in order to remind the reader that this is taking place in the future (his favorite example: "The door dilated"). For instance, World War I aircraft pilots wore silk scarves, everybody who enjoys Snoopy knows that. What most people don't know is the reason behind the scarves. The early rotary engines would spew a steady mist of castor oil lubricant into the pilot's faces. The scarves were a handy towel for the pilot to clean their goggles, and to keep the castor oil from running down their neck.

SF authors are advised to do their own thinking about the day-to-day life of their star pilots, and attempt to identify odd practical habits that would turn into identifying hallmarks. In his Known Space novels, Larry Niven's asteroid miners have a habit of not making hand gestures when they talk. In the cramped control cabins a gesture might accidentally hit a switch, with dire results.

Master Artist Rhys Taylor is working on some images of hypothetical but scientifically accurate Orion drive warships battling it out around the Jovian moon Callisto.

The Targeting Problem

Frank Chadwick of Game Designer's Workshop created a starship combat game called Star Cruiser. In his analysis, developments in tactical combat can largely be viewed as attempts at better solutions to the targeting problem. That is, the trouble is not with the destructive potential of the weapons, they are quite potent enough. The trouble is getting the weapons to reliably hit the target.

This can be done two ways: increase the precision of each shot (precision of fire), or keep the same precision but increase the number of shots fired (volume of fire). Obviously it is preferable to increase the precision of fire. For starters a volume fire version of a weapon will generally be much larger than a precision fire version.

There are three main elements to precision of fire:

Enemy positionThe location of the enemy when your shot arrives.
Weapon performanceThe actual flight path of your shot as affected by the physical characteristics of the weapon itself and the environment through which the shot passes.
Weapon controlThe degree to which you can precisely control the aiming of the weapon.

Note that you can trade precision for increased range, that is, if you can increase the precision of your weapons, you can chose to target a hostile spacecraft at a greater range at the old precision.

Naturally your target is going to be trying to decrease your chance of hitting. They will be trying to decrease your precision of fire and decrease your effective volume of fire.

Precision of fire is decreased by interfering with the three factors listed above (obviously). The easiest is their position, by evasive maneuvers, by interfering with your targeting sensors, and by reducing their target signature.

Volume of fire is decreased by rendering harmless shots that actually hit. This is done by armor, point defense, and science-fictional force fields.

Cherrypicking for Effect

Much of the details the space combat for your science fiction novel or game are driven by the initial assumptions. So if the author desires to write about a certain style of interplanetary warfare, they can cherry-pick the initial assumptions to allow that style.

For example: if your initial assumptions include incredibly powerful lasers and incredibly precise aiming technology, you going to have a problem if the desired warfare style includes a pitched battle between spacecraft of the Lunar Revolutionary Navy and the Terran Royal Fleet occurring halfway between Terra and Luna. The two fleets will be shooting their opponents to pieces while in orbit around their respective worlds. If they try to fly to your desired battleground at midway point, the battle will be effectively over long before any of the surviving ships reach it.

When Ken Burnside was creating his starship combat game Attack Vector: Tactical (AV:T) he cherry-picked the laser ranges in order to allow the warships to maneuver. If the range is too huge, maneuver is pointless. In the following quote, it is interesting to see the various implications and trade offs.

One AV:T armor point is approximately 35 mm RHA. One AV:T damage point is ~50 Mega-Joules (MJ) delivered to a spot size of 8 cm square. Most AV:T lasers have conversion efficiencies of 20% to about 5%, with efficiencies dropping as wavelengths decrease. The waste heat in a laser is dissipated/transferred at about 600 MJ/game-segment; when the waste heat is dissipated, the laser can fire again. Flash coolers allow you to expend coolant to do this faster, heat exchangers do this faster, but without expending a limited resource.

AV:T lasers were cherrypicked to make interesting game ranges; a 200 nm laser table does exist for AV:T, but weapons that are that long ranged tend to minimize the tactical decisions of maneuver, which wasn't the game I wanted to write.

One AV:T energy point is one GJ in a superconducting adductor. AV:T batteries range in generation from 1st (1 Giga-Joule per battery) to 6th (6 GJ per battery) with the most common being 2nd generation batteries.

One AV:T defensive damage point is roughly 4.167 MJ, or about a kilo of TNT in round numbers.

I don't worry about dwell time for the lasers, because it happens in time frames of less than one segment, which is my atomic unit of time in the game.

Some things you can change to make a radically different setting:

  1. Lengthen laser ranges (by shorter wavelengths). AV:T lasers start in the near infrared.
  2. Improve laser cycle times by improving their conversion efficiencies — if you could make a laser with about 40-50% conversion efficiency, you'd have one that would operate continuously for around 2-5 points of damage per segment for as long as you could power it. Hook it to a dedicated reactor system, and you get a very different feel.
  3. Improve the efficiency of the missile propulsion systems; this one gets tricky and works better for a novel series than a game. AV:T missiles have Isp in the low 300s and are conventional solid fuel rockets; this is done for maintenance reasons. You want something that can be stored and handled with the minimum of hazards for an oops. Hypergolics, cryogenic fuels and nuclear fuels all increase the hazards of routine operations, and in my universe, that outweighs their performance benefit; it may not in yours. (I generally try to keep in mind that the majority of the missile techs you'll meet are 19-20 year old kids who joined the Navy because their grades or finances wouldn't take them to college. Klutz and "E1-proofing" are at a premium.)

The better question about your ships is this:

Who's paying for them, and why?

Naval architecture can be seen as a tax on shipping; the total naval budget will be seen as some percentage of the total monetary value of the commerce they're protecting.

Who are they protecting it from?

This sort of question is one of the thorniest ones in making a space travel setting. In order to determine the size of a navy, you need to determine launch costs, shipping costs and shipping volumes of a pair of blue jeans from point A to point B.

We've done this for Ten Worlds, and it sorta kinda works...but Ten Worlds, with 10 planets and a carefully undefined number of systems with can cities (orbital habitats), has a total population of ~67 million as of 2267.

Even then, there are things carefully not mentioned in the background (heavily automated factories and the like) to keep industrial civilization running; we deliberately set up the planets for economic specialization, and there are STILL things that nobody makes that people just do without.

For comparisons on your ranges:

Gunboats = 625 to roughly 1250 tonnes. Dedicated rider craft. The (somewhat blurry) categorization between "Gunboat" and "System Defense Corvette" is "Does it generate its own onboard power?"

Corvettes = 2250 to about 3250 tonnes. 2500 tonnes or below, they're usually dedicated rider craft.

Frigates = Roughly 4750 to 5500 tonnes. This is usually the smallest range of ship that is fully FTL capable with substantial cruise endurance. In many ways, the parlance of frigates is that they're the smallest (and thus, cheapest) ship that can be sent independantly somewhere to do something useful. Naval treaties and general force projections refer to "frigate equivilants".

Light Cruiser = 6750 to about 8300 tonnes. This, at the lower range, is a "super frigate" — something that does a frigate mission, but with better armor and bigger guns, possibly at the cost of maximum thrust. At the higher end, there's a definite opportunity cost in terms of maximum thrust, but it's usually made up for with increased endurance, increased armor, and increased firepower. At least until it has to dodge incoming kinetics...

Heavy Cruisers = 8750 to 10500 tons. It's possible to make a light cruiser that has comparable thrust rates to a frigate — past about 8500 tons or so, you can't do that, and that's the usual demarcation point for a heavy cruiser. These tend to have facilities for command and control, and firepower that's meant to kill frigates in short order. The problem is you can't have them everywhere, they're expensive to build and expensive to maintain.

Within this size range, we have missions. Common nomenclature is that a frigate designed to work within a battlegroup is called a destroyer. One optimized for longer independant patrol missions is called a frigate or patrol frigate. Corvettes can be FTL capable, but most are limited FTL, and some are lugged around by freighters to guard important systems.

Doctrinally, some nations put freighter style cargo pod attachment points on their ships to extend cruising range by adding fuel or cargo or both; these get dropped off before you enter the fight. Some also put gunboat docking ports on (which are fairly similar, all things told) Some don't like the idea that dropping a cargo pod means that the enemy COULD make a spoiler attack by killing it when you're retreating. Some nations eschew offensive gunboats, because the life expectancy of a gunboat crew is fairly short. Others do it because it's the best way to get an overwhelming throw weight of missiles on a target. Still others do a mix and match approach...

Freighters don't carry weapons; they can replace a "life support rated" pod with a gunboat or two. This is used by pirates to conduct piracy; it's also used by ships worried about piracy to defend themselves...and sometimes it's rather difficult to tell the difference between the two. (This also has the benefit that a gunboat crew is small enough to make an interesting Role-Playing-Game party...)

In AV:T, there's an attribute for a drive called "Generation", which is a measure of drive efficiency. Lower generation drives have less fuel efficiency than higher generation ones; there is another attribute called "Maximum Drive Output" — for the same MDO, a lower efficiency drive will generate more thrust, but require a larger fraction of the ship's mass be fuel to get the same total delta V.

From thread in sfconsim-l forum (5/14/2006)

Miscellaneous Notes

"Vesta acquisition."

In response to the verbal from the autopilot, Dieter Ulans flipped his datavisor in front of his eyes and prepared to take direct command of the massive ring of lasers and reaction engines that was Hercules. He hit the juicer button and felt the rush as the drugs began to wash into his veins. "Com'monn jockey juice!" he whispered and then began to croon: "All my thoughts of you, you, you -- all that I've sought is you, you, you." The tiny green symbols on the datavisor began to zip past his eyes at an increasing speed.

His subconscious easily absorbed and processed the information even as his conscious mind took in the blue numbers and symbols on the main screen that showed the gross situation as Hercules and five other ships of the Martian battlefleet began their final approach to Vesta Main Station. "Joey Kolnichok, I know you're here and I'm going to personally fry your tender little parts." The ship thrummed as the main three o'clock engine cut in and changed vector in response to a movement of Dieter Ulan's right ring finger. It was his former classmate he sought -- Josip V. Kolnichok - the one who had beaten him out his bid for a cushy transport command and who had also cast aspersions on his loyalty to the company. This had cost Ulans two points on his profit sharing plan and that was a deficit he intended to make up by turning J.V. Kolnichok and the Des Jardin into a bright, glowing gas.

"80-80. Ready track. Ready main. On my mark FC to you and...mark!"

A second green line began streaming across the datavisor as Ulans took control of the main laser fire control systems. Every time he blinked, the little green symbols paused. Every time he squinted his eyelids, a bright blue bullseye magically appeared where he looked on the main screen. Just tap your foot when your buddy shows, he thought, and you'll make him a star. He began to click his teeth together. His finger tips sweated in the close-fitting control caps. Only eighteen k-k's from Vesta and still no Company. What had they done -- written the station off? The entire ship reached into his heightened awareness. The awesome engines designed to hurl inert cargo on multi-million-kilometer tracks through space. The heavy mining laser converted into a terrifying main weapon now slung in the cargo grapples. The thousands of bits of information from the ship's computers and sensing radars. Where the hell were they? "Come on, you Company fish, swim out into the pan."

Violently the ship executed a maximum burn maneuver with her nine and twelve o'clock engines. Some of the datastream elements were now glowing red. "Damage report: two mike hit on plates 1023/24 negative critical. Integrity 80-80."

"Beautiful, Dee, You saved our jewels with that cut."

Ulans tapped his foot reflexively. On the blue cross hair showing on the main screen, a yellow dot bloomed. Six thousand kilometers distant, several people died.

"Gotum! Gotum!" Jacklin screamed in the interphone. "You're writin' the book, Dee baby! Writin' the book!"

What would you know about books Jacklin, thought Dieter -- besides which I do not feel well and have not the faintest idea of what I'm doing. The rest of the crew was shouting over the phones, estatic with victory. Bystanders he thought. Goddamned passengers. Me -- I'm doing it and I don't know it until I've done it.

"17 to main power...16" Jacklin began to recite the seconds remaining until the main laser had built itself a new charge. No sound, mused Ulans, no bang. They should put sound effects on these things so that you could hear a bang when you made a shot. The slight vibration and the glow on the screen wasn't enough. No real way to relate to that. Should be some noise.

Once again, the crew was slammed by heavy G-force as the ship responded to the almost automatic commands of Ulans' fingers. A loud whanging started up in some distant part of the Hercules. He could actually feel the deck rippling under his feet. Not so lucky this time.

"Damage report. Nine mike frontal hit on super B. Partial dislocation on six o'clock. Six isolated. Explosive decom in cell four, five, six, and F-2. Randalls and Chung do not respond. Losing power on three o'clock and associated vanes. Looks bad, Dee."

"What's happening with the rest of the task force?"

"They're 180 on the other side of Vesta doing a job on the remaining Company element. We're being tracked by a triple-A Company police ship -- most likely the transport Des Jardin."

"Can we run for it, Dee?"

Ulans looked through the datavisor at the blue figures on the main screen even though he already knew the answer.

"No chance. We've got maybe one shot as Kolnichok closes. Maybe I'll get fancy and burn him off. At worst, we'll wind up in the Company Can -- after all they don't want to blow up this ship -- they think they own it!" he lied. He knew he was going to die, but the rest of the crew was even more helpless than he so why burden them with reality?

Three little orange dots appeared on the screen. Look at him -- blowing out decoys even though he knows we're out of maneuver -- that Kolnichok, grinned Dieter. So which one is you, Joey, and which are the aluminum balloons? (Seven dots grew on the screen, all had slightly different vectors.) Now you know my heater can take you in one flash and you also know that one zap is all I'm going to get. And if I take it you've got a perfect excuse to blow me up for the honor of the company rather than recapture valuable property for the accountants. So what's it going to be? I think you shot off too many balloons too early Joey -- cause the other ones aren't making the course correction you just did. Ain't that you, Joe?

Ulans squinted and tapped his foot.

From the introduction to the wargame BATTLEFLEET MARS by Redmond Simonsen

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