[Spaceships] Perpetual Motion?

[acrosome]

The Chemical Refinery (SS1 p.19) requires one power point and produces rocket fuel (e.g. 0.5 tons/hour at SM+5). This is specified as LOX and H2.

The Fuel Cell (SS1 p.20) runs on rocket fuel and produces one power point for an arbitrary amount of time, which is 3 hours at TL7 on integral tankage

A Fuel Tank (SS1 p.17) of rocket fuel will power a fuel cell for 4x as long as integral tankage, strongly implying that the integral tankage is 1/4 of whatever a system mass is (e.g. 1/4 of 5 tons at SM+5 is 1.25 tons).

Since the TL7 Fuel Cell runs for 3 hours on 1.25 tons of rocket fuel, it consumes 0.4167 tons of rocket fuel per hour. If it powers the refinery this makes 0.5 tons of rocket fuel per hour. Thus, this combination can produce a net 0.0833 tons of rocket fuel per hour forever.

It's even worse at higher TLs and larger systems. At TL10+ for SM+5 it's a net 0.495 tons/hour. I'm thinking that Chemical refinery production should be divided by 10- that would be just enough to keep from breaking even.

Errata? Or just a rounding error/below granularity?

Also, since the chemical refinery makes rocket fuel, is there a rule to divide production by 9 if you're only keeping the hydrogen? This seems reasonable- an oxygen molecule is mass 32, and two hydrogen molecules are mass 4, so the mass fraction is 4/36 = 1/9.

Another question: For rockets that have the option of using water as reaction mass- can one switch between water and hydrogen at will, or must the rockets and tankage be fuel-specific? Clearly in the Real World it is the latter, but I don't see how making a multifuel rocket would be beyond the capabilities of a society that can make a ram-rocket. Perhaps double cost (since a ram-rocket is 5x cost)? There are also rules for other reactions masses, like ammonia; same question.

Related question: Clearly, one uses the Chemical refinery to crack water into LOX/H2 or to make hydrogen. I assume that one can specify a version that isolates ammonia or methane as well. But what about water? It only costs $20/ton, but I assume that's at the starport and is mostly infrastructure and handing costs. So if you have to do a true frontier refuel someplace that liquid water is available can I assume that one can just filter it and pump it into the tanks, and that any ship capable of using water will have such a filter/pump? If so, what is a fair pumping rate? One fuel tank per hour? Is it different if the water is ice?

Final question: Are there stats for what one power point is, somewhere?

[Flyndaran]

I think much of problem comes from how many different types of power production are made equal.
Fuel cells would really produce only 1/10 PPs, and solar arrays 1/100.
There are also a few types of fusion plants with most not producing more power than fission, just requiring easier to get/make fuel.

Spaceships may be the only thing we have unless and until the Vehicle Design System gets published, but it's compriseD of very "chunky" units.

[Mailanka]

Originally Posted by acrosome

Since the TL7 Fuel Cell runs for 3 hours on 1.25 tons of rocket fuel, it consumes 0.4167 tons of rocket fuel per hour. If it powers the refinery this makes 0.5 tons of rocket fuel per hour. Thus, this combination can produce a net 0.0833 tons of rocket fuel per hour forever.

It's even worse at higher TLs and larger systems. At TL10+ for SM+5 it's a net 0.495 tons/hour. I'm thinking that Chemical refinery production should be divided by 10- that would be just enough to keep from breaking even.

Errata? Or just a rounding error/below granularity?

Also, since the chemical refinery makes rocket fuel, is there a rule to divide production by 9 if you're only keeping the hydrogen? This seems reasonable- an oxygen molecule is mass 32, and two hydrogen molecules are mass 4, so the mass fraction is 4/36 = 1/9.

You mean other than the fact that it requires ice to process?

The industrial systems are probably too efficient. SS7 addresses this:

Slower Industrial Systems
The production capacity for industrial systems can be too
high for economic realism or game balance (especially if they
are owned by player characters). This design switch reduces
the production or refining rate of mining, refinery, fabricator,
robofac, nanofac, and replicator systems from “per hour” to
“per day.” Systems at TL5 drop to “per month” and at TL6, to
“per week.”

[acrosome]

Originally Posted by Mailanka

You mean other than the fact that it requires ice to process?

I'm missing your point, there. Can you dumb it down?

Originally Posted by Mailanka

The industrial systems are probably too efficient. SS7 addresses this:

Ah! I hadn't noticed that. Thx.

[Mailanka]

Originally Posted by acrosome

I'm missing your point, there. Can you dumb it down?

Sure!

Chemical Refinery (TL7): Different types are possible, but
the most common spacecraft type processes ice or water into
hydrogen and oxygen for rocket fuel or reaction mass.

You're not creating rocket fuel out of nothing. You're taking ice or water, and refining it into rocket fuel. So you couldn't strictly have a perpetual motion machine, because you'll run out of (usable) water or ice. I suppose in your specific case, you might be able to trap the water being burned by the fuel cell for the purposes of making fuel-cell fuel, and then feed it to the refinery to make more fuel-cell fuel. I suppose that's what you mean by a perpetual motion machine?

[Ulzgoroth]

I'm not actually sure whether the error is that refineries are to productive (suggested by the Slower Industrial Systems) bit, or that Fuel Cells are too efficient as well as being vastly too powerful (as part of the design choice to make them unrealistic so they can be playable alongside reactors).

Of course, it could easily be both.

[VariousRen]

I don't see any problem at all, we do the same thing on earth. Oil refineries require some amount of energy (say 1 unit) to produce fuel. That fuel provides enough energy to power the refinery and have some extra left over (for example, the fuel provides 3 units of energy). This means infinite power, up until the point where you run out of raw materials.

For your spaceship you need access to raw fuel components to generate fuel, so really you are turning cargo space and a factory into an inefficent fuel tank.

[Anthony]

Originally Posted by Mailanka

I suppose in your specific case, you might be able to trap the water being burned by the fuel cell for the purposes of making fuel-cell fuel, and then feed it to the refinery to make more fuel-cell fuel. I suppose that's what you mean by a perpetual motion machine?

Yes, you can. It's one of a general category where certain components are made somewhat cinematically good so they're actually an interesting option for PCs, plus the way power accounting is done usually tracks fuel use based on average rather than peak power use.

Looking at this realistically, water has a heat of formation of about -16 MJ/kg, so converting a short ton of water into hydrogen and oxygen takes 14.4 GJ or 4 MWh, meaning the SM+5 fuel processor requires a minimum of 2 MW (plus additional for liquefying). 1 EP, based on other items, is somewhere in the 50-100 kW/ton (of ship) range, or 1.5-3 MW. That's pretty high given its size, but the actual efficiency isn't absurd if we go with the higher estimate for an EP.

However, a fuel cell is grossly unrealistic. A 1 EP fuel cell producing 3 MW and weighing 1.125 tons has a power density of 2.7 kW/kg, which is order of magnitude higher than any realistic device. At realistic TL 7 efficiencies, it should produce 2-3 MWh per ton of fuel; with 0.375 tons of internal fuel, it should run out of fuel in 20-30 minutes, not 3 hours, and even TL 10 reactors should last less than an hour.

[Ulzgoroth]

Originally Posted by VariousRen

I don't see any problem at all, we do the same thing on earth. Oil refineries require some amount of energy (say 1 unit) to produce fuel. That fuel provides enough energy to power the refinery and have some extra left over (for example, the fuel provides 3 units of energy). This means infinite power, up until the point where you run out of raw materials.

For your spaceship you need access to raw fuel components to generate fuel, so really you are turning cargo space and a factory into an inefficent fuel tank.

The raw materials in this case don't contribute any energy. A fuel cell runs by combining hydrogen and oxygen to water, and a refinery produces fuel for it by cracking water to hydrogen and oxygen.

You can quite easily run this as a closed cycle with the same water being cracked, recombined, and cracked again. Doing so will require a net energy input, though.

[acrosome]

Originally Posted by Mailanka

You're not creating rocket fuel out of nothing. You're taking ice or water, and refining it into rocket fuel. So you couldn't strictly have a perpetual motion machine, because you'll run out of (usable) water or ice. I suppose in your specific case, you might be able to trap the water being burned by the fuel cell for the purposes of making fuel-cell fuel, and then feed it to the refinery to make more fuel-cell fuel. I suppose that's what you mean by a perpetual motion machine?

Oh, yes, I was being rhetorical in my use of the phrase "perpetual motion."

Originally Posted by VariousRen

I don't see any problem at all, we do the same thing on earth. Oil refineries require some amount of energy (say 1 unit) to produce fuel. That fuel provides enough energy to power the refinery and have some extra left over (for example, the fuel provides 3 units of energy). This means infinite power, up until the point where you run out of raw materials.

You are missing an important point; oil is not a valid comparison.

You have to think of tanks of O2 and H2 as sort of like a battery. It is a way to store energy. You start with the ground state (water) and pump energy into it to electrolyse it into O2 and H2. Later, you burn that in the fuel cell to get that energy back, producing water again. And the Second Law of Thermodynamics says what? That you will lose some energy in this process, so you should never be able to use a fuel cell to produce more fuel than it consumes.

Oil is different because what you are doing is mining a resource that is already rich in chemical energy, and all you have to do is burn it to release it. You aren't making the oil, or converting it into a higher energy state- you're only extracting it. The apt fuel cell comparison would be if we started with graphite and hydrogen, and ran it through a chemical reactor to make gasoline, then burned the gasoline for power. I guarantee you that you would never have a net energy gain in that scenario.

(Also, trapping the water that they produce is exactly what most fuel cells do. They usually include a electrolysis setup to re-crack the water when you give them some power, from a solar cell for instance. I'd bet this is what the ISS does so that it has power when it's orbit takes it to the night side of Earth. Most fuel cells really are a sort of a battery.)