Stanley Steamer Spaceship

Speculative fiction spacecraft seem prone to steam leakage. These vessels have a tendency to vent dense streams of steam-like gas every time they do much of anything, outside and in.

When an engineer is confined in the bowels of a ship, he's almost guaranteed to run into a cloud of the stuff. Maybe it's hot, like a steam pipe opening up, maybe it's cold, like a liquid oxygen pipe opening to atmosphere. It's usually dangerous, and always cool-looking on film.

Because Space Is an Ocean, this may have started as an homage to classic cinema depicting naval life. On a submarine or steamship, streams of thick steam were ubiquitous. It's an easy and cheap way to make an area look industrial, damaged, dangerous, gritty, or any combination thereof.

On the outside of the ship, ventings usually accompany an atmospheric landing, power-up, lift-off, or other such event. This comes from footage of real spacecraft, which often sever lines and conduits with explosive charges during launch.

Steam venting seems out of place in a high-tech setting, but never forget that the most advanced modern naval vessels are driven by steam engines. The fact that the boiler is nuclear powered doesn't enter into it, the guts of the engine are old-school. Even the most advanced real-world theoretical engine designs are built around liquid or gaseous reaction mass (in the form of hydrogen, oxygen or water) for propulsion. Despite this, steam does not regularly vent into engineering spaces, which is good, because getting hit by a jet of steam at 600-1250 PSI and 400 deg F would ruin your entire day.

Also note that unlike in internal combustion engines, in steam engines, the gas doing the pushing does not normally get vented during operation. The water is boiled, the steam pushes the piston, and then cools and condenses back into the boiler. If the working fluid escapes, that is a problem with the engine, not normal operation.

Many real rockets using cryogenic (really really cold) fuels have water vapour condensing around their tanks, just like you can breathe out white clouds on a cold and dry day. You can also sometimes get this effect from a large refrigerator or freezer. Note, though, that this happens only because the atmosphere we see them in (Earth) has water vapour; you should not see this happening on asteroids, in space, on the Moon, etc. Unless space life-support is meant to replicate earth atmosphere (would make sense, considering pure oxygen is quite flammable - which complicates things like hot steam engines).

Compare Steampunk and Smoke and Fire Factory.

Anime and Manga

 * While not a spaceship, Raising Heart from Magical Girl Lyrical Nanoha usually discharges some kind of gas after any particularly impressive attack.
 * Considering the sheer amounts of energy being thrown around, it's probably coolant, or at least ambient gas that gets superheated by accident. It's apparently a design feature, since the vents have caps that pop off to let it happen.
 * Not just RH; Bardiche and really, most other Intelligent Devices and Armed Devices do this. Especially with a cartridge system. This kind of supports the theory that Magi-Link Cartridges generate a lot of waste heat, if not for the AI system decompiling the Powers as Programs attacks in split second and then cooling down in-between. In other words, AI split-second overclocking in weapon forms.
 * It even gets supported by the side materials: Nanoha mentions her new Raising Heart jury-rigged with a cartridge system is a total maintenance nightmare.

Film

 * The Alien films and their spin-offs feature tons of steam blasts from leaky pipes, perfect for cheap scares when fighting xenomorph infestations.
 * Subverted a bit in the first film when a steam burst that is annoying Ripley is actually shown to be under the control of another crew member.
 * Though he doesn't realize it until after she leaves. "Son of a bitch." *STEAM!*
 * Justified after Ripley sets the Self-Destruct Mechanism, which involves turning off the cooling units of the ship's reactor. The ship's systems are automatically venting in an effort to cool itself.
 * The same can also be said of, though not on a spaceship, the converter on LBV-426 towards the end of Aliens when she goes to look for Newt. Notice those pipes she passes that are glowing a dull red, yeah, thats the coolant pipes trying to compensate a reactor thats about to pop. The light you see on the bottom floors is the residual radiation of the core...neat sound effect too!
 * And how can anyone forget the scenes in Star Wars, where Darth Vader walks dramatically through steam exhausts that for some reason are set around the ship's main entrance. Made even weirder, though more Badass, in the novelizations, which claim that the steam is burning hot and that normal people won't exit until it's evaporated. Would it have hurt to put the exhausts in some more people-friendly place of the ship?
 * This is actually lampshaded as a security feature to prevent assaults or sabotage when docking.
 * No, actually it's smoke, because they breached the entry rather than just opening it (presumably because the smoke would give them a bit of cover from the defenders.
 * I believe the above was referring to the Imperial Lambda-class shuttle in Episode VI, not the door breach into the Tantive IV in Episode IV. The Millennium Falcon also vents steam/coolant/whatever after setting down on the Cloud City landing platform on Bespin. Nobody tries to walk through it.
 * In the Star Trek reboot movie, when Enterprise is getting sucked into a black hole near the end and they're running the warp engines at maximum power, Scotty is running around in Engineering trying to hold the ship together. A pipe cracks overhead and steam comes venting out.

Literature

 * In The Amtrak Wars books the Wagon Trains do vent steam; it's used as a close-range defense system, and capable of blasting the flesh right off your bones.
 * The Soviet officers in the novel The Hunt for Red October mention a cook who tried cleaning his pots and pans with steam from the primary coolant loop (read: radioactive steam) for the ship's reactor and ended up killing himself and irradiating the entire engine compartment. "At least he cleaned his pans though. They should be safe to use after several hundred years."
 * Chasm City features a space station that is cooled by pumping ice through pipes, turning to steam in the process before being ejected into space; this is justified, as the station's heat radiators were broken off by space debris.
 * Jerry Pournelle and Larry Niven's alien invasion story Footfall includes a spaceship with an Orion-class nuclear engine that is indeed cooled by and powered by steam.
 * Possibly exaggerated in a story by Brazilian author Luis Fernando Verissimo, where an Insufficiently Advanced Alien race built a wood-powered spaceship, with chimney and such, when they were trying to build a nautical ship instead.

Live Action TV

 * Star Trek, all incarnations, most notably in Voyager's landing sequence.
 * Thunderbirds made extensive use of steam, smoke, and zero-thrust rocket motors to depict takeoffs and landings in miniature. Rockets in flight were filmed inverted, so the smoke would rise away from the rocket instead of climbing after it.
 * In the Mystery Science Theater 3000 episode The Phantom Planet, Tom Servo explains this: "Look, there's the problem, their dryer's leaking."
 * Several ships in Farscape had this, especially when they were malfunctioning. One particularly bad-ass sequence that was used in the opening had John and D'Argo walking in slow-motion through a steaming corridor. Near the end of Season 3, an imploding space-ship has steam going off all over, leading to some prime horror, as one strikes
 * In Stargate Universe, for some as of yet unexplained reason, there are steam CO2 vents on the floor of the gate room of the Destiny. They fire every time the wormhole closes.
 * The original Battlestar Galactica Classic pilot does this. Starbuck and Cassiopeia are seen kissing in the hangar bay, while Starbuck's other love interest catches them by surveillance camera. Cue the push of a "Steam Vent" button.
 * The pilot of the re-imagined version had Ragnar Station. Justified, as Leoben had just ripped a steam pipe. The rest of the station wasn't really steamy at all.

Real Life

 * During the Apollo 13 disaster, the crew could see vapors venting from their damaged spacecraft. And even before launch, the rocket had plumes of gas coming off of it because the liquid oxygen and such was so cold. The movie relayed this faithfully.
 * Of course, the Apollo spacecraft wasn't supposed to do this...
 * As mentioned above, getting hit with a jet of steam in Real Life usually invokes a particularly hideous and gruesome version of the Chunky Salsa Rule: it literally cooks you alive like a giant, ambulatory shrimp.
 * Very high pressure steam leaks are dangerous in another way - one procedure for finding them involves waving a broom handle around until it's neatly cut by the invisible knife.
 * Water makes an excellent heatsink, as it can absorb remarkable amount of energy in the ice to water and water to steam phase changes. This property makes it exceedingly valuable in a realistic space warship which would cook itself to death otherwise (as space is not cold in real life). Open cycle cooling where coolant is vented into space is useful in some circumstances, but leaking 600 degree steam into engineering or living spaces would only happen in the result of catastrophic damage.
 * Or, potentially, to deal with either mutinying crew or intruders.
 * Some engineers theorize that a solar powered steam-propellant spaceship could be used for economical Mars travel, which just goes to show we could easily go full circle and start traversing the solar system in steam ships....

Tabletop Games

 * Warhammer 40,000 has Stanley Steamer Spaceships and Stanley Steamer tanks.
 * It should be noted that the primary STC tank of the Imperium is of extremely basic design, so much so that the things can be adapted to run on wood with minimal adjustments. Steam is a relatively popular form of power in many tanks, especially considering the availabilities of particular fuel types can vary wildly in an empire that's lost count of how many planets it contains.
 * Justified for Space 1889 where ships use solar boilers to power their "aether propellers" between planets. The boilers, which consist of a large parabolic mirror and a boiler on a turntable vent their safety valves directly into the engine room on a space ship, in order to preserve as much water as possible from being lost.

Video Games

 * The mining barges from EVE Online discharge smoke and flames from ports on their flanks. Averted, since it is most likely the barge ejecting the excess material from extracting the ore.
 * Do remember that these exhaust pipes are always venting, even when docked in a station.
 * In the Fallout 3 add on mothership zeta, the featured ship has an area called 'steamworks' which is mostly filed with steam releasing pipes.

Western Animation

 * Teen Titans, though that was when the Titans were Trapped in TV Land. All other interstellar ships were steam-free.