[Spaceships] multi-fuel fuel tanks?

[acrosome]

The various Spaceships books mention various types of fuel (actually remass), like hydrogen, water, methane, or ammonia. Are the standard fuel tanks assumed to be multi-fuel and thus capable of holding any of these, so that you can decide to top up with hydrogen most of the time but then load water when you need the acceleration? I ask because IRL spacecraft fuel tanks are pretty specific to what they can hold, so I assume not.

Assuming that fuel tanks are fuel-specific, what might be a reasonable statistic for a specialized multi-fuel tank capable of accepting any of those four fuel types? I was thinking double cost.

[Anthony]

Spaceships doesn't really discuss whether things are multi fuel, and realistically you'll have issues because there are huge differences in density between different fuels.

[ericbsmith]

Realistically fuel tanks can only store one type of fuel and Spaceships doesn't go into the type of detail to deal with that.

[the-red-scare]

Presumably a multi-fuel tank would basically be a hydrogen tank (since that’s the least dense) with some sort of fancy modifications to accommodate the more dense fuels in a microgravity environment. At Spaceships resolution a simple cost increase should do it.

The real trouble would be in the engine, where you’d need either some crazy plumbing or just multiple sets to accommodate the different fuels. Maybe reduce thrust by 1/2 to account for all the extra equipment taking up boom boom room.

[Kale]

As Anthony says, there are different densities involved, which means your pumping systems have to be radically different comparing something like a gas to a liquid. I'd suggest ignoring this (as Spaceships does by default) otherwise you will have to have a mix of dedicated tanks.

Another possibility is 'bladder' tanks, that allow you to convert empty cargo bays to fuel tanks. This is mentioned in one of the supplements I think, but I can't recall which one. It is meant as short term storage and may be considered volatile depending on the fuel. If you were just carrying water though it would be quite safe, and would let you occasionally use some spare cargo space to load up on high-thrust fuel while your regular fuel tanks hold hydrogen or whatever your efficient cruising drive uses.

[Anthony]

I would also point out that the rules for varying thrust and performance by fuel type are really just maximums for thermal rockets; you can increase thrust by just increasing the amount of reaction mass you're feeding into the system (you can't do the reverse because the existing numbers are the max temperature the drive can handle). You'll need some additional drive plumbing, but nothing on the complexity of what you'd need for multiple fuel types.

[Ulzgoroth]

My impression is that Spaceships assumes that a rocket has its remass (and high thrust/regular thrust option) set at design time. A rocket that can switch would strictly be a Reconfigurable System.

Whether the fuel tanks can take multiple contents by default...doesn't seem like the kind of question that it speaks to, but as others have said it's rather technically demanding. And fuel tanks don't have a lot of mass available for technically demanding (given that they effectively are allotted zero dry mass).

Originally Posted by Anthony

I would also point out that the rules for varying thrust and performance by fuel type are really just maximums for thermal rockets; you can increase thrust by just increasing the amount of reaction mass you're feeding into the system (you can't do the reverse because the existing numbers are the max temperature the drive can handle). You'll need some additional drive plumbing, but nothing on the complexity of what you'd need for multiple fuel types.

That assumes that your system is designed with the capacity to operate at multiple flow levels (besides 'off' and 'on'). I don't think that's a given, though it's surely less of a problem than a multi-fuel rocket.

[Anthony]

Originally Posted by Ulzgoroth

That assumes that your system is designed with the capacity to operate at multiple flow levels (besides 'off' and 'on'). I don't think that's a given, though it's surely less of a problem than a multi-fuel rocket.

It's not a given, but it's along the lines of adding an afterburner to a jet engine.

[Kale]

Originally Posted by Ulzgoroth

My impression is that Spaceships assumes that a rocket has its remass (and high thrust/regular thrust option) set at design time. A rocket that can switch would strictly be a Reconfigurable System.

Originally Posted by Anthony

It's not a given, but it's along the lines of adding an afterburner to a jet engine.

This is a thing that already exists. The VASIMIR plasma rocket has a 'high' and 'low' setting that trades thrust for specific impulse (fuel efficiency).
https://en.wikipedia.org/wiki/Variab...oplasma_Rocket
In my own games I allow torch drives and fusion rockets to be Reconfigurable Systems. The fusion rocket is good for efficient cruising and the torch is good for breaking orbit, combat, etc.

[Say, it isn't that bad!]

The "t" spec for a fuel tank might be regarded as an abstract unit, if you're looking into more realistic fuel systems.

*Certainly, one cubic metre of liquid hydrogen, does not weigh the same as one cubic metre of Orion drive bombs.

* Since I haven't actually looked this up, in the unlikely event the weights are equivalent, I guess I'll just shrug and move on. :)