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When in space, the amount of force required to move goes down drastically, due to the virtual lack of drag. In this environment, would be feasible for engines in space (more specifically belonging to generation ships) to use steam?

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    $\begingroup$ Just to clarify, this system will not use the steam engine to climb out of atmosphere right? $\endgroup$ – Cem Kalyoncu Oct 13 '16 at 17:06
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    $\begingroup$ What stream engine you imagined? Obviously not the one used to turn wheels, so what's your idea? What would you call a steam engine? $\endgroup$ – Mołot Oct 13 '16 at 17:13
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    $\begingroup$ I would use a steam engine in space to make espresso. $\endgroup$ – kingledion Oct 13 '16 at 17:42
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    $\begingroup$ Technically speaking, modern rocket engines that use liquid hydrogen and liquid oxygen as the fuel are steam engines. The combustion reaction between the oxygen and hydrogen flash boils the product (which is water) and ejects the steam from the nozzle at ludicrous speeds. So, high-pressure steam is what provides thrust, the only difference is from where the steam gets its high temperature/pressure. $\endgroup$ – MozerShmozer Oct 13 '16 at 18:08
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    $\begingroup$ @MozerShmozer: technically speaking, a steam engine is specifically defined as an external combustion engine, which a LH/LOx rocket engine is not. But you're right, it does make some sense to call such a rocket “steam engine”. $\endgroup$ – leftaroundabout Oct 13 '16 at 19:28

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Using steam to propel yourself in space obviously requires that you:

1) Have a steady supply of water

2) Have energy to boil said water

You might be able to scavenge water from comets, etc., but this is still a very wasteful process, and water doesn't simply grow on trees ... in space.

Furthermore, water and large, complex systems don't typically play nice together. The last thing you want is pipes rusting out in your engine, not to mention that any time you shut the system down said water would start freezing, and burst pipes could really ruin your sojourn to another star system. Here on Earth we have the luxury of being able to constantly manufacture new pipes, and fix/maintain them, but on a generation ship it would be a lot more difficult.

All in all, there would be a lot of practical challenges to implementing that sort of propulsion system.

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    $\begingroup$ Burst pipes, tanks, corrosion and overheating are very real problems in all rocket engines. Some propellant is so cold you need to pre-chill the engine so it doesn't flash boil when used. The LM engine propellants did so much corrosive damage that the engine was effectivly a run-once device. Most engines would melt apart but for the fancy coolant systems in use. Soyuz can only be fueled for a short time before the fuel corrodes the tanks. A bit of rust on your plumbing seems pretty mild in comparison to real rocket engines. $\endgroup$ – Innovine Oct 13 '16 at 17:04
  • $\begingroup$ Also you don't need constant supply of materials. A generation ship requires minimal amount of course correction. It will run during launch and descent. $\endgroup$ – Cem Kalyoncu Oct 13 '16 at 17:27
  • $\begingroup$ @Innovine - I didn't know all that. All the more reason to come up with a system that won't rust when on a space ship meant to last hundreds of years, don't you think? $\endgroup$ – AndreiROM Oct 13 '16 at 18:57
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    $\begingroup$ "Have a steady supply of water" - not necessarily. You can have a supply of Hydrogen and Oxygen. Many modern rocket engines do it. Or, you can have a closed loop where the water circulates and turns a turbine, generating electricity. $\endgroup$ – vsz Oct 13 '16 at 21:14
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    $\begingroup$ @vsz but where, sir, will the electricity propel the ship? $\endgroup$ – The Great Duck Oct 14 '16 at 5:31
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Depends on what you mean by "steam engine". A NERVA-type engine using a nuclear reactor with water as the reaction mass would be a steam engine, no? Likewise if water was heated with a concentrating solar reflector.

Indeed, comets do change their orbits that way, as water (or other volatiles) is boiled by sunlight.

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    $\begingroup$ Nuclear reactors in general are steam engines. They turn water to steam and use it to spin turbines, doing work (most commonly, generating electricity). $\endgroup$ – JAB Oct 14 '16 at 1:57
  • $\begingroup$ @JAB: Good point, though turning a turbine doesn't seem to do much for spacecraft propulsion. Though maybe you use a reactor to turn a turbine to generate electricity for your ion drive. That's keeping with known science: I suppose something like a reactionless drive could be powered with steam-generated electricity. $\endgroup$ – jamesqf Oct 14 '16 at 4:28
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A steam engine in space (assuming you mean a steam rocket engine) is not only feasible, but under some circumstances even desirable. The Neofuel site makes a strong case that using water directly from comets, asteroids etc. rather than breaking it down to hydrogen and oxygen, mostly because the extra mass of equipment needed overwhelms the advantages of higher ISP.

All you really need is a source of energy to heat the water into steam, and away you go. This can range from solar mirrors (a solar moth), beaming laser or microwave energy to the ship or using an on board nuclear rector. So long as the water is receiving enough heat energy, you have steam. Throttling the engine is as simple as adjusting the flow rate of water through the heat source.

enter image description here

The NEOfuel proposal for a steam rocket

enter image description here

Simplified schematic of the NTR steam rocket. The water bladder(s) would be far, far larger in real life

The down side of a steam rocket is truly terrible ISP of 195, compared to @450 for the ideal H2/O2 liquid fuelled engines or between 800-1200 for hydrogen NERVA rocket engines. You would be using enormous masses of water to get from point A to point B, and getting from Earth to the outer planets would take many years at best. The NEOfuel site calculate a water tanker carrying a payload of 10,000 tons would carry 325,000 tons of water as reaction mass.

Final Configuration: H2O NTR Option

10,000 tons Net payload

358,000 tons water propellant

104 tons water bladder

208 tons water bladder tank armouring

656 tons Nuclear Thermal Rocket engines

122 tons water extractor

By contrast, the LH2 option works out like this:

10,000 tons useful payload

(138,000 to 453,000 tons water for LH2) 15,400 tons to 50,300 tons LH2 propellant

154 tons to 503 tons armored LH2 tanks

472 to 1536 tons of LH2 NTR engines

48 - 156 tons to extract water

559 to 10248 tons for electricity for electrolysis

70 to 5027 tons for cryolizer hardware

372 to 10609 tons for electricity for cryolizer

So steam is a means of getting low cost bulk payloads around the solar system cheaply, but you will have to accept low ISP, massive amounts of water being used as reaction mass and long trip times.

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  • $\begingroup$ I think OP said "generations ship" to point us more to the interstellar travel. Is there enough water (comets) outside the Oort cloud? Anyway could you edit to delete the weird picture, because predicting thousands of tons of ice available on Moon surface severely hurts the credibility of the idea (which seems suitable to me for kidnapping whole ice comets). $\endgroup$ – kubanczyk Oct 14 '16 at 9:01
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    $\begingroup$ If you read the NEOfuel site, the design is predicted on the availability of large quantities of water on the Moon's poles, as discovered by the Clementine mission in 1994. NEOfuel is based on the idea that there is water virtually everywhere in the Solar System. $\endgroup$ – Thucydides Oct 14 '16 at 14:18
  • $\begingroup$ I'm not clear why a NERVA-type engine using water would be different than an H2/O2 rocket. They're both ejecting steam out the back, no? So assuming the nuclear engine can reach the same temperature & pressure as the chemical reaction, the result should be the same. And you don't have to fool around with cryogenic liquids. $\endgroup$ – jamesqf Oct 14 '16 at 17:10
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    $\begingroup$ A Nerva type rocket is heating and accelerating water, with a higher molecular mass than the LH2 of a conventional NERVA. A LH2/LOX rocket runs at a much higher pressure and temperature than the nuclear reactor of a NERVA. It is possible to run the NERVA at higher temperatures and pressures, but the increased stress could be detrimental to the reactor. Breaking water down exposes the reactor to molecular oxygen as well. $\endgroup$ – Thucydides Oct 15 '16 at 2:42
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    $\begingroup$ Read the Atomic Rockets link. Lower mass=higher ISP, all other factors being equal. Xenon is preferred for other technical reasons rather than molecular mass. The LH2 NERVA vs a LH2/LOX rocket should make that point clear (800-1200 ISP vs 450 ISP). And pushing the temperature to improve the ISP of a steam rocket will have very detrimental effects on the reactor. $\endgroup$ – Thucydides Oct 16 '16 at 3:43
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Yes, you could boil water to build up lots of pressure, and vent the steam as a propellant, and the spaceship would move. But it would move very slowly, and then you'd be out of water.

It's not a very effective propulsion mechanism.

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    $\begingroup$ how slow only depends on the speed of the exhaust. it does not have to be that slow. $\endgroup$ – njzk2 Oct 13 '16 at 19:49
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    $\begingroup$ How would you suggest making it faster? Boiling more water? That'll make it heavier... $\endgroup$ – Innovine Oct 13 '16 at 20:04
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    $\begingroup$ increase the pressure. the only important factor is how fast the exhaust is $\endgroup$ – njzk2 Oct 13 '16 at 20:17
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    $\begingroup$ Heating water to the point it becomes plasma will dramatically increase ISP, but at that point it no longer is a "steam rocket". $\endgroup$ – Thucydides Oct 14 '16 at 14:19
  • $\begingroup$ @njzk2 the mass of the vessel is also important. You need more water to make higher steam pressure. Tyranny of the rocket equation. Steam powered spaceship doesn't scale well, at all. $\endgroup$ – Innovine Oct 14 '16 at 14:24
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The question didn't state that the steam engine needs to be used to propel the ship.

So yes, if the ship is large enough and needs a lot of electricity, it is feasible to use a steam engine, or more specifically, a steam turbine. The water is boiled not by burning coal, but by nuclear fission. There are modern ships and submarines which do this. Of course, they also use it for propulsion, as electrical motors turn propellers. You obviously can't use propellers in space, but if your internal electrical needs are high enough, a nuclear reactor can be warranted on such a spaceship. And, nuclear reactors are actually a form of steam engine.

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  • $\begingroup$ This is interesting. Makes me wonder about using other kinds of liquids to drive turbines in a closed loop system (no loss), but that don't require any additional heating (like water). For example a liquid that can be stored and kept cold in space and away from living quarters, but will start to boil and drive turbines as it is moved to areas that are being kept climate controlled for human comfort anyway. Perhaps the collection and compression there might negate any savings from not having to heat it. Anyway, nice lateral interpretation of the question. $\endgroup$ – coblr Oct 14 '16 at 0:06
  • $\begingroup$ @coblr, using the heat energy from the living quarters to heat up the liquid will cool down the living quarters—probably far too much. Even if it is only a small amount of liquid you are heating, that energy has to come from somewhere. On the other hand, you could limit the flow of liquid through the living areas and use it for free air conditioning. ;) $\endgroup$ – Wildcard Oct 14 '16 at 3:14
  • $\begingroup$ @Wildcard: ya, damnit. I always forget the second part. I'd be curious to see the numbers but far to lazy (and ignorant) to try and ballpark it. Cooling for all those computers and bodies would be a great idea. $\endgroup$ – coblr Oct 14 '16 at 16:56
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It depends on what do you call a steam engine. A steam locomotive obviously can't be used, since in space we have no external support, so we need a jet engine. But can a jet engine use steam to propel itself? A rocket engine might work using liquid hydrogen and liquid oxygen, producing hot water (=steam) in the process. But can we still call it "a steam engine" though?

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  • $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$ – Azuaron Oct 14 '16 at 11:24
  • $\begingroup$ @Azuaron you're right, but the question wasn't clarified, unfortunately. It's still unclear, was OP asking about using engines to propel the ship, or are they just a part of the ship's infrastructure. $\endgroup$ – enkryptor Oct 14 '16 at 11:31
  • $\begingroup$ And that's what comments are for, not answers. $\endgroup$ – Azuaron Oct 14 '16 at 11:43
  • $\begingroup$ @Azuaron I agree with you. How can I improve my answer? The answer was "yes, you can use steam to propel the ship, but that will be a jet engine, not a steam engine". $\endgroup$ – enkryptor Oct 14 '16 at 11:49
  • $\begingroup$ Well, first, it seems like you have some questions you want clarified. If that's the case, you should put a comment on the question and wait for a response. After you get your clarification, you'll be able to write a better answer. $\endgroup$ – Azuaron Oct 14 '16 at 11:57
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I haven't done the math on this but it seems to me a steam engine would be incredible dumb since the H and O that you would be mixing and then boiling just to be released into space would take more energy to produce similar or less thrust than just jettisoning oxygen/? like most RCS thrusters currently do. So possible, but not feasible.

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The Neal Stephenson book Seveneves shows characters embedding a nuclear reactor into the center of the ice core of a comet, then melting the ice of the comet into steam to use for thrust, turning the entire comet into a spaceship and rocket engine.

So if you have a piece of ice 2km in diameter to use as a water source, it's certainly possible.

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  • $\begingroup$ This is the example I was thinking of. They didn't seem to have much in the way of water left when they reached their destination, but they started with so much that what they had left over was still central to the plot from there on. $\endgroup$ – Scott Whitlock Jan 22 '17 at 17:11
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No, not by any sensible definition of steam engine, at least not for propulsion (since you mentioned drag, I will focus on propulsion first).

A rocket works by throwing out propellant to the back. This provides forward thrust. Now, in order to gain more forward speed, you need to either throw out more propellant, or throw the propellant out faster.

In orbital mechanics, there is no drag, yes. But you are still subject to gravity. You are nit weightless, you are in free fall, which means objects and people inside the spaceship appear to be weightless. Being in orbit means actually falling sideways fast enough that you miss the ground and swing around.

In order to change your orbit, you need to change your velocity. This is called delta-v (which literally means "change in velocity").

You need to change velocities quite drastically. In order to get fom the ground to low earth orbit (LEO) you need to have ~9400m/s delta-v alone.

So, back to what I said before, you can either throw out more propelannt, or throw it out faster. In your spaceship, the supply of propellant is limited. Thus, maximizing the speed at which you throw propellant out is the key factor. This is called the exhaust velocity.

But, you can't boil water to arbitrary temperatures, so the exhaust velocity (which is achieved by expanding the exhaust in a de laval nozzle) is still limited. And even if you could boil it to arbitrary temperatures, you still need fuel to boil it in the first place, which is limited, too.

It turns out, you can build a steam rocket. but its horribly inefficient. According to this Wikipedia site, you can achieve an Isp of 195s with a steam rocket - well below the modern Hydrolox rocket which achieve Isp greater then 450s (Isp and exhaust velocity are closely related, ve = Isp * g0).

So your efficiency is horrible. This means the only way you can get anywhere is by increasing propellant mass. The concept of "mass fraction" is actually quite important for modern rockets. To give you an example, the STS external tank was 96% fuel and only 4% structure. A soda can has 94% soda and 6% structure (by weight). So the ET held more percentage of fuel then a soda can. Modern rockets like the Ariane V or upcoming Ariane 6 have even more propellant.

You can easily see that its not really feasible to miracuosly improve the fuel fraction by a great margin (or we would have already done it, while still using more efficient propellants).


This leaves you with using steam engines for electrical systems. You could - in theory - use a nuclear reactor, which boils water and then turns propellers.

Buts thats not as exciting and steampunky as using it for propulsion.

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$F=ma$

Newton's first law (correct me if I am wrong). Second law, for every action there is an equal and opposite reaction. Basically, in order to move forward, you need to push a lot of something behind you at a low speed or very little of something at a high speed.

When we walk, we apply a small amount of force, acceleration, opposite to where we walk. It just is not noticeable, because the earth is very big and we are very very small.

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    $\begingroup$ Actually, those are his second and third laws. $\endgroup$ – Peregrine Rook Oct 13 '16 at 18:03
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    $\begingroup$ Yes but how does it work in space? $\endgroup$ – Vincent Oct 13 '16 at 19:16
  • $\begingroup$ what you really want is the rocket equation, and figure the exhaust velocity as a function of the pressure you can mount (which in turn in a function of how hot you can get the steam) $\endgroup$ – njzk2 Oct 13 '16 at 19:50
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Let's find out!

In space, you use conservation of momentum to propel yourself. Basically, you throw mass very quickly at the back to push yourself forward.

The most efficient systems use very small particles with very very high velocities. The more simple systems use combustion to create hot gas that creates pressure that creates exhaust velocity.

The thrust of such an engine is the exhaust speed multiplied by the exhaust rate (how much mass you send * how fast you send it).

How much mass can be variable on the size of the exhaust, how many engines you have... so let's focus on the exhaust speed:

Locomotives steam boilers have been know to hold up to 1,500 psi (10.34 MPa), or about 100 atm.

Unfortunately, I have no idea where to go from here. My understanding of that topic is too limited to make the computations.

I fear that would be disappointing, though, as methalox rocket engines have exhaust velocities of 5000m/s and more.

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I suspect that lurking behind the question is a steampunk motivation.

In that case it is useful to recognize that what the 19th century called "steam power" was really a code word for coal powered via a steam turbine.

While the Earth bound practice with steam engines was to vent the steam and refuel with water along the way, you could have some internal functions of a space ship that were powered by coal and rather than venting the steam, sent it to a cooling area exposed to the chill of space and then recycled it.

This would be useless for propulsion purposes, and would take a lot of weight per energy density, and you probably would want to vent the particulate exhaust of the coal combustion into empty space, but it could run life support systems on board and the heat generated by the engine could also heat the ship.

For propulsion into orbit, from a 19th century steampunk kind of perspective, you might want to launch the space ship with coal powered internal systems from a huge cannon a la Jules Verne, or some sort of giant catapult, or a hydrogen gas explosion in a long glass tube, possibly from a high altitude attained via a balloon or an airship.

To return its occupants to Earth, the ship might break into two pieces like a one stage rocket that would spring apart from each other perhaps with a highly coiled metal spring that could be released when it was time to re-enter. Neither the launching method nor the return method would provide all that much thrust, so you'd need to limit space voyages to near Earth orbit (and if the landing capsule were pointed the wrong way when activated everyone would die, while if someone was not in the landing capsule when it was activated, that person would die, in either case in deep space after life support supplies eventually run out). After the return spring operation, an Apollo style parachute (or better yet a parachute modeled after a huge Victorian umbrella) could be used to protect the folks in the landing capsule.

While you couldn't make it to the Moon, let alone anywhere else that way, you could get a super high altitude view of the world (for purposes from spying to weather prediction) without having to have a working aircraft wing (something not developed until the early 20th century by the Wright Brothers), and you could also go up and then land elsewhere on Earth to go around the world in a matter of hours rather than Verne's 80 days by balloon. You could also use the vantage point of space for an early space-telescope for precision astronomy impossible from Earth at the time.

As long as you stayed inside the strong, airtight ship (perhaps lined with lead to prevent cosmic rays from doing too much harm), there would be no need for space suits. You could travel in a coat and tie.

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    $\begingroup$ There is no "chill of space". Instead you have a near-perfect isolation, which means it's hard to release any heat away from the ship. For real space vessels the overheating is a major design problem. $\endgroup$ – kubanczyk Oct 14 '16 at 8:12
  • $\begingroup$ A fair point. Still some amount of infrared radiation must escape the ship, somehow - perhaps that would have to be incorporated with a heat exchanger and the soot removal system. $\endgroup$ – ohwilleke Oct 14 '16 at 21:04
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You can use steam engine heated by a very high energy density source of heat to produce steam to turn turbines to produce electricity (such system are known as 'nuclear power plants' here on Earth). Recycle your steam back into the system otherwise you will need to hunt for comet ice to refill your tanks. Use that electricity to power pure laser/microwave propulsion which doesn't require any additional propellant (well, almost, you'll be losing mass carried out by photons leaving the business end of your rocket). Your propellant is the light, which is very lightweight but flies away at the maximum possible velocity, can't get faster than a speed of light in this reality. Since steam engines are kind of inefficient, significant portion of the energy will be escaping as heat and will heat up the ship and will eventually be radiated away from it, which may work for or against your desired acceleration vector (depends on which side of your ship heats up higher). You may need to build a large array of radiators to get rid of that waste heat, for efficiency reasons keep them on the same side of your ship as the laser engine so the infrared photons from the waste heat also accelerate your ship. Note that you may need a generation ship to get from Earth orbit to Moon orbit using this set up.

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Yes you could

But that wouldn't be the best way, the question is how are you heating up your liquid and how exaclty do you wan't to probell, if you would use solar panels and then use the electricity to heat your liquid up, you could propell yourself forward, however it won't work on a "generation ship" since you can't use it mechanicly "propeller or something like it" and so your only way to use the energy would be to throw the liquid in gasform out, witch wouldn't last that long.

Also would it be an unneccesary step between, since you could use the electricity directly on photon engines or even simpler, light engines, which propell very slowly but in an environment where you have basicly infinite energy and nothing to slow you down these engines can archive far bigger speeds than rockets or similar.

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Yes you can use steam effectively, whatever steam comes out will instantly freeze. This frozen water can be collected in a device like a solid parachute at the back and then recycled again and again. Nuclear fission can be used for heating the water, and as space is already super cold the necessary cooling effect for the nuclear device can be regulated. Solar energy can be stored for other things. This way you have a cheap and endless supply of fuel to make you move in any direction.

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  • $\begingroup$ Welcome to Worldbuilding! I'm not sure where you get the steam instantly freezing from - inside a generation ship, you need reasonable temperatures, so it's too warm inside. Meanwhile, outside the ship, the pressure's so low that it would probably just dissipate off. Similar for the nuclear fission cooling - if you want to cool something quickly, you'll probably need some way for the heat to dissipate as I suspect that radiation cooling won't be fast enough $\endgroup$ – Mithrandir24601 Mar 18 '18 at 16:53

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