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In the game "Stelaris" torpedoes and missiles are different categories of weapon. Presumably they wanted to include numerous kinds of weapons for the player to chose from. These weapons are all launched from spaceships in deep space. (They only ever travel through a vacuum; never through air or water.)

What explanation could provide a distinction between these two interchangeable concepts within the genre of space opera/sci-fi?

Please try to keep it relatively grounded within real world physics and engineering. In the future, science does not invent a magic McGuffin machine...

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    $\begingroup$ I've always assumed when it comes to space combat that missiles have better guidance and can turn and follow targets, they are small and fast. When it comes to torpedo, I imagine them bigger, slower, way more power and not so much turn power. So you would use missiles against smaller and faster targets, you would use torpedo against large and slow target. $\endgroup$ – Miroslav Saracevic Jan 20 '17 at 9:58
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    $\begingroup$ Torpedoes are self-propelled? $\endgroup$ – colmde Jan 20 '17 at 11:05
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    $\begingroup$ Submarines can launch both missiles and torpedos. One key difference between the two is the type of propulsion system: missiles are rocket-propelled while torpedos are propeller or water-jet propelled. $\endgroup$ – Eric Jan 20 '17 at 14:02
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    $\begingroup$ In the Wing Commander franchise, in WCII, torpedoes existed as anti-spaceship munitions to penetrate capital ship "phase shields", which were impervious to missiles and starfighter guns. This made attacking a capital ship that was equipped with "phase shields" much more a difficult, slow process that made the destruction of a capital ship a bigger achievement. In other games in the series, a starfighter could use missiles or ships guns to attach a capital ship, making it much more routine and less satisfying. $\endgroup$ – PhasedOut Jan 20 '17 at 16:52
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    $\begingroup$ @Devsman What makes you think real-world engineering doesn't have space missiles? ICBMs spend most of their flight time outside of the atmosphere. Indeed, the Mercury missions used ICBM boosters as their launch vehicles. Of course, you're right that there aren't any space torpedoes in real-world engineering, as space isn't filled with water, making submerged travel rather difficult there. Missiles would be one of the few types of modern weapons that actually would be effective in space, though. Successful tests have already been performed with missiles shooting down satellites. $\endgroup$ – reirab Jan 20 '17 at 21:16

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In a purely terminological sense, there is no difference between a torpedo and a missile if they inhabit the same space - a missile is a guided, self-propelled weapon that travels through the air and a torpedo is a self-propelled weapon that travels through the water (in modern terms, inevitably guided as well).

My first thought though could be to do with the method of damage. In the anti-shipping mode, torpedoes are fuzed to detonate below the ship and use the effects of pressure and cavitation to effect damage on the target.

Missiles have a very wide range of destructive modes, but they do include direct strikes against targets (admittedly, some anti-submarine torpedoes also use direct contact and shaped charges, but never mind...).

My thought would be that the difference in space could be that torpedoes are heavyweight weapons with large warheads that can damage enemy ships even without a direct hit, whilst a missile is a smaller, more manoeuvrable weapon that would need a direct hit.

Another possibility is the difference in the contemporary world between ballistic and cruise missiles - a cruise missile's engine works for the entire flight, whereas a ballistic missile boosts in the early stages of its flight and then follows a trajectory towards its target. A torpedo could be a very long-ranged weapon that has a long burn time for the greatest propulsive efficiency (possibly for attacking stationary or non-manoeuvring targets) whilst a missile has a fast boost-only engine for getting it up to high speed as fast as possible, in combat against targets directly threatening the launch platform.

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Send your torpedos through sub space. They go "underneath" so they're torpedos. Missiles travel through normal space. Different tactics and countermeasures apply to the distinct types of weapons.

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    $\begingroup$ Normally this always seems like a nonsense distinction. Effectively they're interchangeable and the author uses whatever they prefer. Your answer works in cheeky and funny ways. Plus one for humour and lateral thinking. $\endgroup$ – a4android Jan 20 '17 at 4:47
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    $\begingroup$ And then, you can have "submarines" that travel in that subspace. And after that, you get accused of ripping off Space Battleship Yamato. $\endgroup$ – Euphoric Jan 20 '17 at 8:30
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    $\begingroup$ @Euphorix, yeah, I was about to post an anwser using Space Battleship Yamato. The idea that space submarine navigates throught sub-space (just like "underwater" but with dimension) and still uses periscope and torpedo is fantastic. (I've only watch the SBY 2199 tho, it's episode 13) $\endgroup$ – Asoub Jan 20 '17 at 9:00
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    $\begingroup$ @Euphoric What is this "Space Battleship Yamato" you speak of, and why are they ripping off Star Trek? (Romulans and their cloaked vessels were explicitly designed as "submarines in space".) $\endgroup$ – Mason Wheeler Jan 20 '17 at 15:34
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    $\begingroup$ @jorfus I think that space is the thing moving, and not the torpedo. $\endgroup$ – PhasedOut Jan 20 '17 at 16:54
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Hi and welcome on board the star cruiser Pedant's Dream! On board this state-of-the-art warship we carry plenty of weaponry- railguns, lasers, missiles, torpedoes-

What's the difference between those last two, you ask? Good question! It's really a matter of semantics. In general, lighter munitions used for antifighter and self-defense purposes are classified as "missiles," while heavier weapons intended for use against capital ships are "torpedoes."

To many, the distinction seems rather pointless, but the higher-ups in the UN Astro Navy bureaucracy insist on it. Some think that it's because they get kickbacks from the weapons companies, but I don't really care. All that matters to me is whether you can load it into the right launcher and fire it into the right direction. Welcome aboard, rookie!

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    $\begingroup$ Pedants' Dream, surely? ;) $\endgroup$ – Matt Bowyer Jan 20 '17 at 2:38
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    $\begingroup$ A single pedant may suffice. $\endgroup$ – Adrian Colomitchi Jan 20 '17 at 2:41
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    $\begingroup$ It may, but the apostrophe wasn't the thing about which I was being pedantic! $\endgroup$ – Matt Bowyer Jan 20 '17 at 2:46
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    $\begingroup$ The GSV Torpedoes have more Gravitas agrees with this. $\endgroup$ – pjc50 Jan 20 '17 at 10:29
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    $\begingroup$ @Byte56 Oh, are you one of the original cryosleep colonists? I didn't realize we had one! Let's see, that's a pretty archaic term, but if I remember from my history class, you're either thinking of the old unguided missiles or the old pre-space-elevator space launch systems. We haven't used those in a few hundred years. Still, great to have you here! $\endgroup$ – Timpanus Jan 21 '17 at 2:50
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Mass, thus manoeuvrability:

  • missiles - lightweight, fast to accelerate (changing direction involves acceleration, right?), more manoeuvrable - but pay attention to cruising speed - the faster they go, the harder veering becomes; more usually than not they have homing devices. Lower damage.
    Use against fighters, bombers

  • torpedoes - pack a massive punch, thus they are massive. Hard to accelerate, many models (like photon torpedoes) won't even change direction once launched.
    Use against capital vessels

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  • $\begingroup$ This is the distinction in the "Freelancer" game, missiles have solid homing capabilities but don't pack much of a punch. Torpedos have only a tiny amount of homing ability making them difficult to use against mobile targets but deal far more damage. $\endgroup$ – Blake Walsh Jan 20 '17 at 12:51
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No need to discuss tactics or engineering qualities. The definitions are enough to show that ...

Missiles are projectiles in general, and torpedoes are self-propelled.

According to dictionary.com,

Missile describes "an object that is forcibly propelled at a target, either by hand or from a mechanical weapon" and can range from everything to a pencil to a nuclear warhead. It's broad enough to cover space-based weaponry as long as it is thrown or launched.

Torpedo describes "a ... self-propelled ... missile designed to be fired from a ship or submarine or dropped into the water from an aircraft and to explode on reaching a target". While torpedo is usually associated with water, you can take the self-propelled aspect away instead if you so desire.

Missiles are any of the things that you launch (explosives, shells, bursts of plasma), and torpedoes are things with their own fuel that don't simply rely on momentum (like self-guided rockets).

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    $\begingroup$ Mostly right, however "cigar shaped" is not a part of their definition. A missile is anything that is thrown. Torpedo is slang for an underwater missile that takes its name from an animal... An argument can be made, etymologically that a torpedo is a missle meant to stun used underwater. $\endgroup$ – Durakken Jan 20 '17 at 4:39
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    $\begingroup$ @Zxyrra I didn't ask for which word to use. I asked for you to INVENT a fictional distinction that could explain why BOTH terms could be used in a space opera to refer to different types of space weapons. $\endgroup$ – Lorry Laurence mcLarry Jan 20 '17 at 5:40
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    $\begingroup$ Please explain photon torpedoes in Star Trek. $\endgroup$ – Adrian Colomitchi Jan 20 '17 at 8:05
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    $\begingroup$ @AdrianColomitchi it's the same as the word "Starship": historical reasons. There is no reason they couldn't call them "Starplanes" or something like that, but most language transferred from the navy, thus the use of "Torpedo" as a mean to describe a ship-to-ship missile. $\endgroup$ – BgrWorker Jan 20 '17 at 8:34
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    $\begingroup$ @LorryLaurencemcLarry The fact that are used in Sci-fi today is enough of a reason to justify a difference in the language. Ships use torpedoes, fighters use missiles. They're the same thing technically, but we call them different things just because. $\endgroup$ – Durakken Jan 20 '17 at 14:41
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Considering both weapons got similar propulsion and operates in the same environment the only difference is how they kill.

Torpedoes are made to target huge vessels and explode it's thick hull. A big military vessel can need multiple shots to sink.

Missiles are intent to target small and fast targets. They seek the target and detonates a fragmentation warhead when they get close. They actually don't hit the target and that's specially true with anti-air missiles. A missile is intended to open hundreds of small holes with hot metal scraps.

There are dedicated anti-ship missiles like the legendary Excocet. They are made to take advantage of the kinetic energy to penetrate the hull above the water line (where the hull is more slim) or even try to target the command center. You can say they are rocket flying torpedoes.

There's of course dedicated anti-runway, anti-tank missiles actually explodes things like a small torpedo.

When I play Stellaris I also try to imagine torpedoes as more massive as missiles and intended to penetrate and do huge explosions inside the hull.

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    $\begingroup$ This! Don't know why no one mentioned this. Each Sci-Fi game or TV show makes this pretty clear. Torpedoes are heavy hitters, slow, poor maneuverability but you don't want to get hit by one of them. Usually used to take out other capital ships. Missiles can be almost anything that explodes, they even come in swarms. $\endgroup$ – r41n Jan 20 '17 at 10:29
  • $\begingroup$ Yes! although it feels like most anti-ship missiles, like the Block3-Excocet, behave more like cruise missiles until they reach arming distance (inertial guidance, ground hugging, not using a rocket engine the whole time, because they got a jet engine). $\endgroup$ – Doomed Mind Jan 20 '17 at 13:29
  • $\begingroup$ The Exocet is a cruise missile, although most versions actually use a rocket motor. It's a cruise missile because the engine runs all the way to impact. $\endgroup$ – Matt Bowyer Jan 20 '17 at 14:27
  • $\begingroup$ At the end of the day, in a military context, the origin of a word is not as important as its immediate use. In such a setting, using the word "missile" to refer to fast highly manueverable self-propelled projectiles, meant to target faster ships, vs "torpedo" for high yield capital-ship killers, would make sense when you have a few seconds to relay to your troops what kind of attack the enemy just fired off. I mean compare "Torpedo fired at our flagship!" vs "High yield self propelled missile fired at our flagship!" XD $\endgroup$ – Oskuro Jan 23 '17 at 9:33
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A missile chooses targets automatically and homes in on them, a torpedo requires target selection and has less maneuverability.

the TE24 (Terran Enterprises model 24) template is the Galactic standard for all mass-produced spacebound projectiles. This standardisation allows ships across the Milky Way to rearm at any friendly station or salvage any unfired ammo from both friendly and hostile derelicts.

Part of the standardisation is a modular approach. Each template-based projectile has a number of slots that can be fitted with any combination of modules, ranging from a variety of payloads, engines and advanced guidance features to integrated IFF and advanced automated target selection modules.

Missiles have an engine optimised for speed and maneuverability, a close-range payload, an automated target acquisition and tracking system and a guidance system that works in tandem with the TATS. When fired, they choose their target manually based on a number of parameters, then automatically follow the target until they explode or the target is dead. These missiles can be fired multiple at the same time, at which point each missile will choose a different target. if a target is destroyed before the missile can reach it, it chooses a different target.

Torpedoes have an engine optimized for speed, 2 highly damaging payloads and a guidance system that required selecting a target in advance. These deal much more damage than a missile, but are not capable of acquiring targets automatically. A target must be configured in advance while loading the torpedo. A target cannot be changed after launch, and a missile targeting a destroyed target will still attempt to detonate against any remaining pieces of the target.

Because of this distinction, missiles are most often used against smaller targets where manual targeting and a potential loss of target are inefficient. Torpedoes are most often used against immobile or slow moving targets: capital ships, space stations, automated defenses,...

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    $\begingroup$ usually, the distinction between guided and unguided missiles is not "missile" and "torpedo", but "missile" and "rocket", which is also the distinction used in todays military. See Hydra Rocket and Hellfire Missile as examples for this in the US Army. This is also reflected in most Sci-Fi universes. $\endgroup$ – Doomed Mind Jan 20 '17 at 13:17
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    $\begingroup$ It is hard to see why a civilisation that can build spacecraft like those would employ unguided weapons. Unguided weapons are becoming rare in contemporary Western militaries, let alone future ones. $\endgroup$ – Matt Bowyer Jan 20 '17 at 14:29
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    $\begingroup$ @MattBowyer One reason I could think of is that guidance can often be jammed or disturbed in some way. Generally, it's easier to make the opponent miss you than to dodge the opponent, especially with really big capital ships for which torpedoes are ideal. $\endgroup$ – Nzall Jan 20 '17 at 14:42
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    $\begingroup$ It's quite interesting to claim that space-torpedoes wouldn't have fancy guidance, as water-torpedoes have used fancy guidance for more than a century. $\endgroup$ – Peteris Jan 20 '17 at 14:43
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    $\begingroup$ @MattBowyer In modern military terms an unguided self-propelled warhead would be a rocket, not a torpedo. An unguided NON self propelled warhead is a bomb (which can also be guided). $\endgroup$ – Jason K Jan 20 '17 at 14:53
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Torpedos make use of the enemy ship's electric and magnetic fields.

Background: Some SciFi settings feature propulsion systems based on expulsion of ions or plasma. Yet others feature a strong magnetic field around the ship, a field which may contain plasma. This field is "pushed" by the solar wind. These are not farfetched, but mimick designs in use today or on the drawing board at NASA.

In any case, there's a "medium" consisting of particles or magnetic fields, in close proximity to enemy crafts, that a missile could "push against" using electric or magnetic fields. This creates an analogy with torpedos that push against water. A missile with this propulsion would be able to "sneak onto" enemy vessels by showing zero exhaust and be able to "latch onto" the fields of large vessels, like a missile following the wake of a ship.

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There's only a few things missiles / topedoes can do. So, we can make a very short list of how you might distinguish them.

How they're stored

Presumably they're stored as they are today, in internal explosion-resistant lockers, that aren't going to be much different for space missiles vs. space torpedoes. But there is another option: one or the other might be carried around permanently on external structures, like how bombs are carried on the wing pylons or conformal tanks of present-day jet fighters. Although a distinction based solely on how they are carried alone doesn't really seem to justify having the two different names.

How they're sent

This might work alongside storage. The name torpedo has its association with stealth (from submarines and all), while a missile might take more preparation to launch. So, torpedoes might be able to be launched from concealed openings on a space-ship's hull. While a missile might first need to be assembled, mounted and fueled on an external fixture, that is either in plain sight or more easily detected by enemy ship's equipment. The distinction would be most important when the intentions of an approaching ship are ambiguous. So, a torpedo could be fired in a surprise attack from close range, while a missile might not be suitable for this, as it needs the extra time and obvious preparation of being fueled up on the pad.

How they're guided

See Matt Bowyer's answer

How they're received

eg. missiles = bigger holes. If an adversary needs to spend more time setting up a missile and fueling it while it's hanging off a structure on the side of the ship, it might be expected that this extra investment comes with a payoff. Probably that's greater damage to an enemy ship. Alternatively, if missiles are no more destuctive than torpedoes, then the need to assemble them externally may simply be a reflection of cheaper or less advanced technology.

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A potentially suitable distinction in a space combat setting could be that missiles are designed to overwhelm enemy defences through speed, numbers and decoys, whereas torpedoes are more like small drones delivered to carry an explosive payload, complete with armouring, shielding and defensive systems of their own.

This draws a hard line between what is a missile and what is a torpedo (systems installed), as well as accounting for size, cost, speed and yield differences. It makes the most sense if sensors are assumed to be perfect and there is some form of 'instantaneous' (in that the weapon can't react before it's hit) weaponry available to use as point defence.

What makes this work particularly well is it draws a distinction between the use cases for missiles and torpedoes straight away. Missiles are for dealing with fast combatants that are light on point defence: torpedoes are for use against larger combatants that have the ability to destroy lightly armoured missiles; as they will sustain multiple point defence impacts before being destroyed.

This also opens up possibilities for arms races, where each side creates swarms of torpedoes that are designed not only to attack the enemy ships but also other ordinance. This may lead to a more 'drone V drone' style of space combat depending upon resources and construction capability available.

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If you're talking of realistic space combat in an universe with realistic near-future technology, I'd consider the acceleration capabilities and available delta V as the main criteria for the distinction. Missiles have lots of thrust and acceleration -- much more than the usual spacecraft -- but low delta-v, so they're more effective in short range combat. Torpedoes have low thrust and acceleration, but lots of delta-v. They can be launched from long range and perform more complex orbital maneuvers to reach the target.

If you want to be more technical, missiles are cheap, small, ready-to fire, solid-fueled guided rockets. They can be very cheap, launched with short notice and in large quantities. After launch they calculate the vector for an intercept, point and burn all the fuel, and use some secondary propulsion for fine-tuning the intercept. If they miss the target, they're out of the battle.

In contrast, torpedoes are essentially suicide drone ships. Large, expensive, with variable thrust and lots of delta-v and payload. They might have to be fueled before being launched depending on their size and kind of fuel being used. They might have their own power generator or solar panels instead of batteries so they can remain in an orbit for a long time if needed. If they miss a target they can try another intercept, look for a target of opportunity, etc.

Tactically speaking, missiles are more effective against targets roughly in the same orbital plane. If the target orbit has a completely different inclination, the low delta-v limits the tactical options and usefulness. Torpedoes can be used against targets in a completely different orbit, or even against targets orbiting another body. Also, combat spacecraft are likely to have strong point-defense capabilities, so missiles will be launched in salvos intended to overwhelm the target defenses, while a torpedo is more likely to rely on its own defenses, armor, electronic warfare, etc. They might have much more sophisticated computers and AI so they can make tactical decisions on their own to adapt to new conditions.

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  • $\begingroup$ nice, best answer so far $\endgroup$ – MolbOrg Jan 20 '17 at 21:15
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    $\begingroup$ @MolbOrg Thanks. I added a more detailed explanation. Let me know if that's an improvement. $\endgroup$ – Pedro Werneck Jan 20 '17 at 22:46
  • $\begingroup$ I would consider "so missiles will be launched in salvos intended to overwhelm the target defenses" as a spec of inconsistency with the answer. But probably mostly because I'm not a big fan of close combat wall of useless text how I would like them to fight. But in some scenarios and technologies, it may have a place. Overall I think your edit is useful for the possible reader. Like that for sure "torpedoes are essentially suicide drone ships". So still best answer so far as for me, but my views on the problem are not popular)) $\endgroup$ – MolbOrg Jan 20 '17 at 23:03
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Well, modern missiles work on the rocket principle, force of expelled gas propels the missile forward, while torpedoes move using propellers, which is the only real distinction, based on the environment where each operates.

In space, of course, there is no air, so no real need for torpedoes, you'd only ever use missiles, so conventional distinctions are out. (see Zxyrra's answer)

So what could be the difference? Matt Bowyer, Adrian Colomitchi and Timpanus have mentioned all the answers I though of immediately, so I won't go into them again.
The only things not mentioned yet are type of payload and fuel.

One of the two uses antimatter(or some other exotic)/nuclear warheads, the other doesn't.

Alternatively, one has limited conventional fuel in stock, and can thus only travel a short distance (astronomically), while the other has a small fusion core, which allows it to travel further, and also adds to the explosion on contact.

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    $\begingroup$ Not quite. Propellers are not the only propulsion system anymore. There are supercavitating torpedoes like the russian Shkval or the German "Barracuda". These use rocket engines for propulsion. I also think that neither payload nor propulsion principle should be the difference, but how you use those things. $\endgroup$ – Doomed Mind Jan 20 '17 at 13:23
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    $\begingroup$ Also, a hypothetical cruise missile that used a turboprop engine would almost certainly still be termed a missile. $\endgroup$ – Matt Bowyer Jan 20 '17 at 14:30
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The best way to think about it is like this...

The reason most sci-fi calls spacecraft "ships" and fleets of the craft as "navy" is because of the fact that these craft most mimic earth bound ships and the way the manoeuvre/move through water. Ships in space is roughly equal to how a submarine moves UNDER water.

This is accurate for the bigger ones, ie. something along the lines of a Patrol or Corvette class and larger, smaller attack craft would be more likened to fighter craft (tighter turning circles etc, but similar controls like thrusters facing in all directions for control).

So if you're to take into account the munitions that they use; torpedo's and missiles, we'll forget about lasers for now as its not in your question but you must also forget about relative earth sizes for what we consider conventional torpedo's and missiles as in space both could be any shape and size theoretically.

A torpedo would be mainly for large ship to ship combat, designed for the range between laser and missile range, so basically, minimal guidance but to provide a lot of damage, possibly cheaper too.

A missile would again be for mainly ship to ship combat, could vary a lot more in size and power between smaller spacecraft battles to large ship battles and even planet destroyers. These would generally be more sophisticated too.

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  • $\begingroup$ Welcome to worldbuilding! Nice answer $\endgroup$ – EveryBitHelps Jan 20 '17 at 16:52
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From my experience, Missles are self-propelled projectiles fired through the air, and Torpedos are self-propelled projectiles fired through the water. But that doesn't actually mean anything here.

Traditionally, craft that travel through space are considered ships, or vessels, rather than aircraft, or planes, due to their functional similarity to submarines. As missles and torpedoes can both be fired from submarines, and spacecraft travel through neither air nor water, the decision of whether the weapon is a missile or torpedo is up to the discretion of the creator of the weapon, be that the author or the inventor.

Also, there are rocket propelled torpedoes. https://en.wikipedia.org/wiki/VA-111_Shkval

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Simply make Torpedoes a class of missile.

For example, the T1269 'Harbinger' is a Torpedo-class Missile system.

A similar example to this is the idea of Destroyers and Battleships. Fundamentally, both are ships that carry weapons and are used in wartime roles. However, they fit different billings and as such are classified as Destroyers and Battleships.

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  • $\begingroup$ Is the Harbinger from Volition's game Descent/Conflict Freespace: The Great War? $\endgroup$ – Crowley Jan 23 '17 at 16:31
  • $\begingroup$ @Crowley Not intentionally, just a made up name that sounded aggressive enough to be a large weapon. $\endgroup$ – SGR Jan 24 '17 at 8:02
  • $\begingroup$ hard-light.net/wiki/index.php/GTM_Harbinger $\endgroup$ – Crowley Jan 24 '17 at 12:23
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For some reason Sci Fi always comes back to navy terminology, so I will follow a very simple, Navy-Esque, bits of reasoning.

Torpedoes are used for destroying big ships, and are designed with the goal of destroying big ships in mind. Their targeting, payload, propulsion, etc. are all built with the idea of destroying big enemy ships.

Missiles are designed to destroy everything else, from small attack craft to huge planetary cities.

Finally, Marines are there to handle anything that you can't just blow up. Have fun with your spaceships. I'd like to hear more of the story

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Size and Use

Missiles are for fast moving targets, like fighters, or light freighters. They are small enough to fit on a fighter and are typically homing missiles, with good capacity to make spontaneous turns and "drifts" in space.

Torpedoes, in the contrary are much bigger and much stronger, they are built to damage capital ships and space stations. They have are fired at a longer distance and have less control over themselves. They can be mounted on bombers class ship, gunships and other "siege" spaceship.

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500 years ago, the Air Forces used missiles, and the Navy used torpedoes. At the founding of the Space Forces, combined crews were brought together into Spaceships, but flatly refused to abandon centuries-old prerogatives.

Even today, with identical weaponry and training, the two groups continually antagonize each other. All systems are in pairs, one blue, the other light blue, and God help you if you touch the wrong side.

c.f. the "breastplates" of the modern British army

This is a scientific answer, but the science is sociology, not physics.

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Look at how missiles and torpedoes work these days:

Missile: Fast, maneuverable but subject to defenses.

Torpedo: Slow, big boom, not very maneuverable, almost invulnerable other than to being spoofed.

Now, for space use:

Missiles: High energy devices, they leave an obvious trail--in the absence of electronic warfare the enemy will always know exactly where a missile is unless it's been drifting for a while. They rely on speed and electronic warfare to get through to their targets. In general the warheads will damage but not destroy the target. They are highly maneuverable, able to engage fighter craft.

Torpedoes: Low energy devices. They come in at speeds little more than the vessels speed, relying on stealth to get through. Purely passive target seeking, although they can get target updates from the ship that fired them. They're shipkillers, if one gets through it's likely to destroy the target ship. Because they're slow the enemy can choose to turn tail and run and will generally get away, albeit at the cost of being forced out of the battle. They are not maneuverable enough to have any chance against fighter craft.

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No one else has specifically mentioned this, so I'm going to give my answer.

In the real world, the term missile has two major connotations: a guided rocket and an object that is forcibly propelled at a target.

Under both definitions, a torpedo is simply an underwater missile, with one exception... Unlike most air missiles which have a ballistic trajectory (a clear exception being cruise missiles), torpedoes cruise to their target. That is, torpedoes are powered from launch to impact.

Extrapolating this into space warfare we can come up with a distinction

Trajectory

  • Missiles are ballistic, being powered during a small fraction of their flight time and coasting the rest of the trajectory to their target.

  • Torpedoes are non-ballistic and cruise all the way to their target, meaning that their engines are functioning until impact.

Guidance

  • Missiles have a very high maneuverability while they still have fuel. Once their fuel runs out they become dumb projectiles. They might be equipped with maneuvering systems, but they will probably only be used for small adjustments rather than large trajectory changes.

  • Torpedoes have their engines working during the entire trajectory, meaning that their maneuvering capabilities are constant.

Engines

  • Since missiles need to accelerate very quickly, they are probably equipped with solid fuel engines or high thrust liquid fuel engines. Either way, their propulsion is most likely chemical

  • Torpedoes, on the other hand, need huge amounts of Delta-V but they need to use it sparingly in order to reach their target. Given this, they are probably equipped with some form of ion propulsion. It might not provide large amounts of thrust, but it compensates by working for much longer periods.

Dimensions

  • Missiles, given their outlines characteristics, probably have a small size in order to be able to stack a lot of them in a craft

  • Torpedoes, given their large propulsion requirements, are likely of a large size, many times larger than missiles.

Situations

  • Missiles are likely to be used for close range encounters since that where they shine the most given their thrust output. They can also be used for engaging stationary targets given their fine-tuning long range capabilities. Missile warheads are probably armor piercing or high explosive.

  • Torpedoes are pretty much useless for close combat since they can easily be dodged or destroyed thanks to their low thrust. However, torpedoes SHINE in extremely long range engagements. They can accelerate leisurely to enormous speeds given enough distance. Their size and distance may make them easy targets for interception, so they'll probably be equipped with cluster or multiple warheads which would be deployed if interception is imminent, overwhelming most countermeasures.

In other words, missiles are used for active engagements while torpedoes are for bombardments and interception.

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Missile: Self-propelled, heat-seeking, small-medium projectiles. They are lanuched "en-masse", and their objective is disrupting enemy tactics and small vehicles rather than attacking big objects.In space, they are meant to scatter the enemy's fighters.enter image description here

Topedo: fast, self-propelled, medium to big, uni-directional (they don't follow targets), highly penetrating and exploding after contact ( not on-contact ) capital ship attacker projectile. they are launched individually, usually produce a light and hard to detect signature, and they tend to blow up various levels of the ship, opening gaps on the hull.

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  • $\begingroup$ we've had guided torpedoes since the end of WW2, roughly the same time we got guided missiles. Torpedoes also typically are not contact fused any more but proximity fused (though they tend to have a contact fuse backup). $\endgroup$ – jwenting Jan 25 '17 at 8:23
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This all depends on context, specially since most Sci-Fi deals with spaceships as being in the Navy, probably due to similarities to sea-based battleships (one large vessel with lots of personnel, each with different functions).

Specifically for Stellaris, there's this from the Stellaris Wiki:

Missile weapons are rather different from other weapon types. They have a 100% accuracy and varying tracking values, but in turn can be shot down by Point Defenses (equipped only by destroyers and above).

  • Missiles have the best base range of all weapons. They also have equally high tracking for all slot sizes, giving any ship a good counter to corvettes.
  • Conventional Torpedoes have the ability to skip right past shields, making them especially dangerous to shield-reliant foes, but their weakness is that they lack tracking and are only available for the special torpedo slot.
  • Swarmer missiles are medium slot, shorter ranged missiles that split up with the goal of overwhelming enemy defenses in a saturation attack. Their evasion is so high they can not be shot down.

You may also find some discussion on tactics like this one on Reddit.

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Star Trek has your answer. Species 8472(encountered in primarily in St. Voyager), the undine(according to star trek Online), lives in fluidic space. There maybe other species that also live there. If you need to fight species living in fludic space clearly torpedoes are you answer.

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I'd have a differentiation between rockets, torpedos and missiles in terms of how they are flown.

Rockets would be unguided weapons - cheap, short range and fired en-masse, deathblossom style.

Missiles would be guided systems with propulsion throughout their flight time, and active thrust controls. They'd be roughly equivilent to a modern day air to air or surface to air missile. I'd see these as weapons used to take down fighter to "gunship"/corvette class ships or fun emplacements.

One of the interesting things about space combat is distance. A torpedo makes a ton of sense as a long range missile, that's launched from railguns or with a booster, coasts into range then goes into active attack mode. They'd need less propellant for their weight, generally get deployed in the early stages of an attack, and tend to be massive shipkillers. Something like the missile systems in the honorverse comes to mind, though they're an evolution of single stage missles as I've described here.

How they're used would be the interesting thing here - rockets would be used in massive numbers to overwhem mount defences or shields. Missiles would be tailored for dogfights and short range combat.Torpedos would be used tactically - predeployed, coasting towards a planned engagement area, and then getting activated later. Combat with torpedos as such would revolve around avoiding or destroying enemy torpedos before they can do any damage, while trying to herd the enemy into your kill zone.

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The most likely answer revolves around target, and diverges from there. If it can target anything, then there's little need for two words in most cases. The case where there is use for both words is interesting though and makes a good base case, so I'll bring it up.

In that case, missiles and torpedoes are the same thing. That is, until they leave their ship and are tasked either to hit the other ship at which point they become torpedoes, or they are tasked to intercept torpedoes and become missiles. That way it makes it easier to discuss whether each ship has more interceptors than weapons incoming, and other such relevant metrics. This comes from a universe where ships are very fragile and delta-v is somewhat limited, so the same warhead that allows a decent probability of a kill on an incoming torpedo would be devastating to an enemy ship. The other factor that makes that all work is that terminal defense means that torpedoes want to be accelerating rapidly toward their target when in close to make the missiles' time as hard as possible so both sides want a high acceleration engine and torpedoes work best when launched dead until they see the enemy ship in their intercept basket.

On the other hand, you may find that the weapons to take on a ship are very different from the weapons to intercept incoming targets on a ship (torpedoes, small craft, other things). In that case, the torpedoes as anti-ship weapons and missiles as defensive weapons split still makes sense. It doesn't really matter if torpedoes are actually small ships with nuke-pumped lasers and composite armor arrays with aerogel to dissipate incoming lasers and missiles are small KE vehicles launched in large numbers to overwhelm torpedoes' lasers and armor. What matters is that the terminology remains consistent and the reader can understand what's going on.

Lastly comes the case of attacking ground targets, in which case I'd say that bomb would be a reasonable term for a weapon that gains most of its KE from the target's gravity well. If you're using pure KE munitions, then something like rod would probably be suitable.

Finally, torpedo is an old word for mine that came to shift fully away from describing mines as it came to describe the new self-powered underwater warheads. So there's historical precedent that you can do what you want and shift meanings to match new realities in warfare, but the language is likely to solidify on something that makes what you say clear (especially during the early 1900s, the distinction between a mine and a torpedo was very important, see the Live Bait squadron for an example of how bad getting it wrong could be). So as long as you're clear and consistent, you can't be too wrong.

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Suppose a sci-fi universe with the following constraints:

  1. Space dust and rocks etc. are deflected with some sort of strong force field; these are your standard "deflector shields". Possibly there has been an escalation of arms between deflector shields and plasma cannons, which will not be hugely material here except that a "deflector shield bank" might be a valid target for trying to selectively disable.
  2. Lasers work much as they do in the real-world: invisible, they travel at speed c, they can't be deflected by deflector shields really. They have, however, been neutralized by dissipative hull technology, which requires a lot of mass but basically means that you need to focus a laser on a point for several seconds before you can melt anything.
  3. This is a problem because our mastery of some non-Einsteinian notion of gravity has led to inertial dampeners which allow massive ships to have a sort of characteristic "jitter" which distributes the laser over larger areas. It also makes it harder to target small fighters or large cruisers with projectiles, so even though we have some sort of mass drivers (maybe railguns, maybe not) firing masses with high speeds, this is not enough to hit a ship with a precision payload.
  4. You have some ability to see "hot" projectiles coming, whereas you would not be able to see the normal bullets from a mass driver.
  5. Electromagnetic pulse weapons can disable electronics at close range.
  6. There is some distinction between small, agile "fighters" and medium-size "destroyers" and large "cruisers".

Then it might make sense to have about three different sorts of powered projectiles.

A missile would have an explosive payload, an internal engine, sensors, and a tracking computer. These would be useful for a cruiser trying to get rid of small fighters. The idea is that these could be like "really fast computer-guided kamikaze fighters", you neutralize the EMP threat by always firing a cloud of 20 of them per fighter, so that a fighter gets overwhelmed by trying to dodge these ones while EMPing those ones. With a couple big cuts you can imagine that a small fighter's engines or weapons could be compromised and they're basically out of combat.

A rocket would not be guided at all and would be analogous to a "space bullet." They would be fired cold with a mass driver, then would rapidly accelerate while 'hot,' perhaps by detonating a big nuclear explosion in a blast chamber and riding out the shock wave as it destroys the chamber. They would basically be a big, heavy slug of matter, not deflected much by a deflector shield, targeted only by being fired "cold" one way and then after a timer expires, being accelerated in another (possibly different) way. They probably come in two varieties: "light" rockets are best for fighter-on-fighter dog-fighting combat; they are fast straight-line projectiles not unlike guns, but probably fired in "banks" so that a fighter needs to simultaneously dodge several of them at once. However at larger distances these are pretty easily melted by lasers before they hit a deflector shield which will deflect the vaporized forms, so for mid-range cruiser-on-cruiser combat you instead see bigger slugs, basically a big chunk of dissipative hull, so that lasers do not melt enough of them before they pass through the deflector (and then any further laser-ing would just add heat to the remainder of the slug, increasing hull damage).

Finally, maybe torpedos are rockets which are partially guided by the firing ship, therefore they behave a little less like "bullets" and more like they live in some "fluid" medium which they can push against. Imagine that we configure this blast chamber to push slightly off-center from our rocket payload, then as it gets accelerated to high velocity it also spins wildly. It's possible that, since we know the trajectory it's going to follow ahead-of-time, we track that trajectory with a carefully pointed laser. That laser then evaporates some external "casing" on the spinning payload, which gets ejected in some preferential direction. The result is that torpedos appear as "curveballs" for a period of time after they go "hot". They are maneuverable enough to vaporize destroyers but overkill for (and perhaps unable to target) fighters. They have a slight accuracy benefit over rockets for doing precision strikes on those jittery cruisers, since they can be guided somewhat "en route", but probably there is a tradeoff here. Presumably once they are detected and the target's lasers are reoriented to fire at them, they can be deflected from their intended target by laser defense just the same way that they are guided. Therefore they probably are designed to "run out" of this fuel right around the time that an enemy laser is able to target them, after which point they basically become an inferior version of a rocket (i.e. they are stuck on their straight-line trajectory and they have inherently less mass and velocity). So they are only better at targeting than a rocket over short ranges, but when they are available they are decisive.

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As I commented, one example is how David Weber uses the terms in his Honor Harrington series. Missiles are long range, semi autonomous, weapons with very high speed used to degrade a target vessel's defenses (and possibly disable or destroy it). Torpedoes are shorter ranged, slower, weapons used by mostly smaller ships when they "move in for the kill". As they're smaller (in his universe) a smaller ship can still carry a large short range punch when they would only be able to carry a very small number of missiles which would have little effect against the defensive systems of a larger vessel.

There's also a difference in warheads. His missiles tend to have a variety of warheads, mostly designed to overload shields and other defenses and render the target vessel incapable of defending itself. The second strike with torpedoes or laser fire at shorter range will then finish him off (the torpedoes having large thermonuclear warheads).

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The term torpedo is not universally defined for space weapons

Torpedo is a naval term and refers to a self-propelled weapon that travels trough water. Note that there have been guided and unguided torpedos and the mode of propulsion also varies quite a lot. Aerial torpedo is used for air-dropped torpedos that still travel through water after being dropped.

Missile is commonly used for self-propelled weapons that travel through air or space, regardless of where they are launched from (including submarine launched, which do travel a short distance through water at launch).

So the distinction, as far as existing weapon systems go, is the medium in which the weapon travels while in its self-propelled phase.

For space, where all weapons travel through, well space, the distinction does not have a defined meaning. You could either just call every self-propelled weapon a missile (and that would certainly not be wrong) or you can make up your own distinction criteria, since no universally accepted definition exists.

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  • $\begingroup$ OP is aware of this, he is asking which criteria would be best for defining a missile vs a torpedo assuming both are used in space and only space. $\endgroup$ – Miguel Bartelsman Jan 25 '17 at 3:56

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