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Post by apophys on Aug 12, 2018 12:01:06 GMT
If the vapor pressure of your chosen propellant is low at its storage temperature, it shouldn't be too much of a problem to mold your propellant tanks to efficiently fill the internal space under the armor. Using the armor as the propellant tank directly may cause issues with heat (e.g. propellant freezing if the armor doesn't insulate sufficiently from the external radiating surface, or armor shattering due to being cryo-cooled too far), but as long as there's even a little vacuum gap, it's fine. There may be some mass inefficiencies caused by the larger tankage surface area and odd structural distribution and weird plumbing, but that shouldn't be too bad. The benefit of having propellant work as extra armor, and the volume savings requiring less armor to begin with, should be able to cover it. I am assuming inert propellant like water or methane, of course; you probably wouldn't want to put an explosive monopropellant (or liquid fluorine) in tankage with a lot more surface area than it absolutely has to have.
While water-based NTRs, Resistojets, and MPDs lack the thrust of more energy-dense hydrocarbon-based propellants No, water has less exhaust velocity than common hydrocarbons due to dissociation issues and higher average atomic mass; so, the thrust is higher for the same engine power. The differences aren't really that big anyway. Inert propellants aren't "energy-dense," since you aren't getting your energy from them. Energy density is used to compare chemical and nuclear fuels and energy storage options like batteries or flywheels. I know that my description specifically applied to combustion rockets. I thought that was clearly implied by the reference to real-life spacecraft, ALL of which to date have used chemical combustion rockets. I didn't mention NTRs or MPDTs because I have only a pretty vague knowledge of how they work. [...] I also know that separating fuel and reaction mass is bad in terms of spacecraft efficiency. [...] To your last point, both chemical and nuclear fuel shouldn't really be bolted to the outside of your ship if you can help it, at least not if you're expecting to ever get shot at. I'm aware that nuclear fuel is probably less hazardous in this context, but you still don't want it floating away when somebody shoots a hole in your armour. Chemical rockets heat up propellant with a reaction, and throw it out the back through an expansion nozzle. NTRs heat it with a nuclear reactor, and do the same. Laser thermal rockets heat it with an offboard laser, and do the same. Resistojets/arcjets heat it with electricity, and do the same. Ion drives like MPDs ionize propellant and throw it out at high velocity with electromagnetism. Electric drives (both general types) do get used on current, real spacecraft once they get to space. So if you refer specifically to chemical rockets, you must say so. Propellant for everything else is chemically inert, so inertness tends to be the default assumption.
Moreover, chemical propellant is only explosive in space if it is a monopropellant (undesirable due to lower exhaust velocities, not just volatility). Bipropellant stores fuel and oxidizer separately; neither is explosive on its own (down on Earth, we have an atmosphere of oxidizer). Liquid fluorine will set nearly anything on fire if it spills, though.
Separating fuel and propellant is part of the design for basically anything that isn't a conventional chemical rocket, nuclear fusion, or the highly-theoretical nuclear salt water rocket. Chemical fuel is very poor in energy density, so you definitely wouldn't want to separate fuel and propellant if your fuel is chemical (you'd have tons of wasted mass for no apparent benefit), but otherwise it is not a problem. Note that for solar thermal or solar electric craft, the "fuel" is the sun.
Nuclear fuel is generally very compact (as in UO 2 or UC); you don't have to worry about the volume it takes unless you use it in a very diluted form like a NSWR. I see no reason you'd even consider bolting it to the outside of your ship.
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Post by anotherfirefox on Aug 14, 2018 11:05:24 GMT
I want to remind you guys that M1 tank actually use its propellant tank to protect their crew. It's not a nonsense, rather a common sense.
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Post by AtomHeartDragon on Aug 14, 2018 11:13:45 GMT
I already tend to wrap small propellant tanks around crew modules and don't jettison droptanks unless pressed for delta-v.
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Post by bigbombr on Aug 14, 2018 11:17:13 GMT
I already tend to wrap small propellant tanks around crew modules and don't jettison droptanks unless pressed for delta-v. Especially with flattened armor, you can have propellant tanks next to your crew module without increasing cross section.
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Post by Rocket Witch on Aug 26, 2018 16:12:35 GMT
Coal bunkers were a thing on warships of circa 1890–1920, placed between bulkheads around the outer rooms as an extra buffer zone. Some spacecraft today use solid propellants, like PTFE pellets for ion engines, so the same concept could be applied to reduce the propellant loss upon damage that would occur with liquids as there wouldn't be any internal pressure pushing everything toward the hole. Linking the pellets together with wires or adhesive could prevent loss entirely, though given the necessarily small size of the pellets these additions would also need to be suitable for expenditure as reaction mass.
If a magnetic propellant were used, I guess it might be possible to contain it around the hull without any tanks at all, depending on the power needs.
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Post by Dhan on Sept 19, 2018 17:25:16 GMT
It doesn't seem practical at all. Why go through the trouble when an external drop tank can provide almost all of the benefits without the significant downsides.
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Post by AtomHeartDragon on Sept 19, 2018 19:12:40 GMT
It doesn't seem practical at all. Why go through the trouble when an external drop tank can provide almost all of the benefits without the significant downsides. You either waste no mass for tanks themselves or can afford much flimsier ones that would get popped by micrometeorites or energetic particles when mounted outside.
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Post by vegemeister on Sept 21, 2018 23:25:21 GMT
The big win here isn't that the armor spacing makes a good fuel tank. It's that a fuel tank makes *fantastic* armor, because fuel is far denser than anything you could reasonably use as whipple stuffing without compromising the mass ratio of the ship. A meter of ethane has much greater sectional density than a meter of graphogel. And if you make it a self-sealing fuel tank, the fuel doesn't even leak away if you get shot up a bit.
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Post by treptoplax on Sept 22, 2018 0:38:17 GMT
The big win here isn't that the armor spacing makes a good fuel tank. It's that a fuel tank makes *fantastic* armor, because fuel is far denser than anything you could reasonably use as whipple stuffing without compromising the mass ratio of the ship. A meter of ethane has much greater sectional density than a meter of graphogel. And if you make it a self-sealing fuel tank, the fuel doesn't even leak away if you get shot up a bit. I'm not sure you want liquid fill between your Whipple shields and inner armor belts, though; that might conduct shock waves cleanly and defeat the whole purpose of a Whipple shield.
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Post by AdmiralObvious on Sept 22, 2018 4:27:05 GMT
The big win here isn't that the armor spacing makes a good fuel tank. It's that a fuel tank makes *fantastic* armor, because fuel is far denser than anything you could reasonably use as whipple stuffing without compromising the mass ratio of the ship. A meter of ethane has much greater sectional density than a meter of graphogel. And if you make it a self-sealing fuel tank, the fuel doesn't even leak away if you get shot up a bit. I'm not sure you want liquid fill between your Whipple shields and inner armor belts, though; that might conduct shock waves cleanly and defeat the whole purpose of a Whipple shield. Probably. Most whipples wouldn't be thick or strong enough to contain most fuels though.
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Post by Dhan on Sept 22, 2018 16:16:26 GMT
I'm not sure you want liquid fill between your Whipple shields and inner armor belts, though; that might conduct shock waves cleanly and defeat the whole purpose of a Whipple shield. Probably. Most whipples wouldn't be thick or strong enough to contain most fuels though. I think that it would just be implemented as a ring of fuel tanks between the whipple and the main armor. I don't see the entire space being a fuel tank.
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Post by AtomHeartDragon on Sept 23, 2018 10:36:50 GMT
Probably. Most whipples wouldn't be thick or strong enough to contain most fuels though. I think that it would just be implemented as a ring of fuel tanks between the whipple and the main armor. I don't see the entire space being a fuel tank. With no need for structural rigidity or thermal and micrometeorite resistance, such tanks could be extremely thin-walled.
Or you could effectively fill this space with hermetically sealed blocks of lightweight foam material, that would help reinforce that, provide whipple stuffing and function as part of surface tension drainage system.
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Post by jageriv on Oct 3, 2018 1:26:18 GMT
This does remind me a bit of how airplane wings work with their fuel: jetliners from what I've seen don't really have "tanks" in their wings, but simply the internal space of the wing serves as a tank, with some walls to prevent the fuel from sloshing too uncontrollably and throwing the plane out of balance. The structure itself however is what is used for the tanks "wall".
So, the big question I think would be "how much space is normally used for this armor". The more "empty space" in the armor, the more useful using that space as a fuel tank would be. Especially since the outer width would be a signifigant part of the overall internal volume.
For example, let's say the ship overall was 10 m wide (assuming a cylinder). Lets say the armor layer is 1 meter wide. So, you have a "cargo" area 8 meters wide, and an armor ring 1 meter wide. The overall cross section is about 78.5 m^2. The area of the cargo area is about 50.24 m^2, while the 1-meter wide armor layer has an area of 28.26 m^2.
This means that that final meter of thickness taken up by the armor represents some 36% of the overall cross-section. So, if that armor layer some small share of that overall thickness, with most of it being spacing, then depending on the exact nature of your layout, you might be able to increase fuel capacity by some 30-50%, depending on where your internals are.
As others have already mentioned, many of us already do use fuel tanks as armor where possible, such as to protect crew quarters. Its just that such protection isn't integrated into each other: the armor shell has to be distinct from the fuel tanks, same as structural reinforcement: its just the way things are designed in this game, but not something you necessarily have to do: certain real-world spaceships I know integrate those functions together, such as that extremely thin walled ballon rockets, like on the Atlas rocket. So you could also potentially use the fuel to provide structure and support to the armor, maybe starting with a liquid and allowing enough gas to boil off to keep some pressure to keep the armor "inflated", or simply design it so that once the main fuel has been burned off, the structural support of the armor/ship is no longer necesary.
So, on the more basic question of "is it possible", yeah, it should be very possible to do, and wouldn't require all that much design difficulty. Wether its something you "should" do is a much more difficult question. Your looking there at a "balancing of trade offs" question rather than a "can we do this" question, where we don't know most of the trade-offs.
I guess the first question is, just how much empty space are you likely to have. I haven't been keeping up with game meta. Can someone point to some current, reasonable armor designs so we have an idea of at least what currently makes sense in the game? I know this isn't real life of course, but it could give an idea of whats likely, and how much room there is to play with without compromising the armor integrity, at least in the game.
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Post by phoenixzix on Oct 10, 2018 14:06:16 GMT
My doctrine currently is to use the empty space created by my modules as a sort of "storage area", and role-play it as something akin to a ship's hold.
I have actually thought about this once, and dismissed it, primarily because, I believe, to do certain maintenance (i.e. changing out reactor fuel rods, plumbing and electrical-rewiring, battle-damage repair, access to certain modules,) you will have to remove a large portion of the armoring. In which case having part of the fuel tank in them would greatly increase the complexity and man-hour of such activity.Thus i have dismissed such an idea. However i believe this is an engineering trade-off that i made in keeping with my fleet doctrine. As mentioned by others as long as you are storing inert there is little reason why it can;t be done.
And i can see it been a great design feature if your the intention was to have a drone vehicle, or if your design philosophy prioritize defense over maintainability(or manuverability). In fact i think the ship in Tau Zero (a Bussard-Ramjet ship) had its fuel tank as armor (primarily because mass limitation of a near-c vehicle)(correct me if i am wrong)
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Post by airc777 on Oct 13, 2018 1:35:05 GMT
This sounds like a great idea, although I might build my ships in a weird way that might make this more helpful to me then other people. Potential problem I see is that if the entire armor profile is considered one fuel tank without any option to add bulkheads sub dividing it then you will have no fuel reserve redundancy on a ship that uses fuel for armor and you intend to put it somewhere where it's going to get shot at.
The reasons this would be of great help to my specific fleet design are as follows: *Currently all of my gunships use a 9.60Km/s hydrogen deuteride nuclear thermal engine making 1.7 mega newton, so I don't use explosive fuels. *My gunships have I think relatively thin armor, I rely mostly on having a very pointed front and pointing directly at the enemy at all times. Should my armor fail my internal compartments are in order of from front to back fuel, reactors, radiation shield, crew, radiation shield again, and then the engines. (I mount ammo and lasers radially around the crew) I've already decided I'd rather lose the ability to move then lose the ability to shoot.
Additional reasons to do this is that it might improve the packing efficiency of the internal volume of the ship, reducing it's overall size and thus reducing the weight of the armor.
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