New ion craft using fuel cell

[nholzric]

Effective Isp for a fuel cell powered ion engine = 2000N/(0.15765 * 9.81) = Isp of 1293 seconds

Nice. So I can use an ISP of 1293 for a LF+O->fuel cell->electricity->Ion Engine ship and calculate it's dV based on the weight of the ship and the weight of all the Xenon+FL+O?

Except I have to make sure the Xenon to FL+O ration is correct. Or at least usable if I have another power source and extra Xenon (in which case the overall ISP would be somewhere between 1293 and 4200).

Thanks for running those numbers, at first glance the units look good.

[DancesWithSquirrels]

Side note: I'm thinking that fuel cell/ion combinations almost require an action group tied to both the fuel cells and the ion engines, plus an additional power source (solar/RTG), so that when you're not thrusting the fuel cell isn't burning LF+O to power your probe core, science transmissions, or lights.

Otherwise you might get to the moons of Jool and realize your LF+O supply is gone just from running your Stayputnik...

[Foxster]

This is your basic ion/fuelcell craft:

You need a ratio of 100:2 units of Xenon to LF.

[DancesWithSquirrels]

Foxster - Don't forget a secondary power supply or extra LF+O to run the probe core.

Edited May 6, 2015 by DancesWithSquirrels

[Foxster]

Foxter - Don't forget a secondary power supply or extra LF+O to run the probe core.

Yup, thanks. This is just the basic propulsion bits for a long range probe with PB-NUK and other stuff.

[FleshJeb]

They should be. Also remember they don't push to 100% battery capacity, so if you have solar it will take priority and the fuel cell will kick on and properly throttle only when battery levels drop below 95%

That's a nice feature. It would be amazing if it were a tweakable percentage.

[DancesWithSquirrels]

Fuel ratios and scaling: 100 units of xenon should require the power from running 4.495 units of LF+O through a fuel cell.

100 xenon == 4.495 LF+O, or 2.023 units of fuel and 2.472 units of oxidizer.

To add up to one ton of fuel (when calculating deltaV ratios) you will need 3079 units of xenon, 62.29 units of LF, and 76.12 units of oxidizer.

Edited May 6, 2015 by DancesWithSquirrels

[polle]

I designed a Fuel Cell Tank for my interplanetary SSTO.

The Fuel Cell has a total mass of 48.7 T and is worth 12800 units of L+OF generating Fuel out of ORE in time.

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It is more efficient then bringing all the fuel up in tanks, but you will still have a bottleneck.

[panzer1b]

Ive actually done some experiments and i found it to be not worth the hassle.

1st of all the 6x fuel cell is almost as heavy as a gigantor panel, which means if you plan on powering anything semi heavy you will need ALOT of fuel cells as well as fuel to run them. basically from my own experience, the best and only reliable way to make ion drives if batter spam. Now that massless batteries arent massless, its more challenging, but its the same basic idea, have enough batteries to allow you to do a full burn to wherever you need to go in one shot, and then have a few RTGs or solars to recharge you over the long term. Most of my ion craft (such as superlight fighters ect) utilize 1-3 tons of batteries to function, since you dont need a constant supply but just enough to do a burn, there is no reason to carry the super heavy generation equip when you still end up relying on batteries anyways.

While i have found benefits of fuel cells over pure nukes, and that you do get way better dV, it is not worth it over a battery bank supplemented by some lightweight charging over a longer time (RTGs, even some micro solars will do).

the only place right now fuel cells make any sense is with resource converters (you can pull a net gain from them), or with earlier career and thats not because they are better then other options, its because you have no choice as RTGs are one of the last things you can unlock.

[quantumpion]

Solar panels don't change the Isp, they don't shed any mass. So the Isp for a solar powered ion craft of any size/dV capability is 4200s.

Sorry, allow me to clarify: since large solar panels are dead weight, how does the delta-v for a probe of equal total mass compare between solar panels and fuel cells+fuel?

- - - Updated - - -

the only place right now fuel cells make any sense is with resource converters (you can pull a net gain from them), or with earlier career and thats not because they are better then other options, its because you have no choice as RTGs are one of the last things you can unlock.

Wait, you can get a net gain of energy by running a resource converter on fuel cells powered by its own fuel output? That should be physically impossible - it violates the 2nd law of thermodynamics.

[Red Iron Crown]

Sorry, allow me to clarify: since large solar panels are dead weight, how does the delta-v for a probe of equal total mass compare between solar panels and fuel cells+fuel?

That would depend on how much xenon is meant to be consumed.

Wait, you can get a net gain of energy by running a resource converter on fuel cells powered by its own fuel output? That should be physically impossible - it violates the 2nd law of thermodynamics.

Not at all. Witness how we use petroleum today.

[Deustodo]

AFter spending a good time just staring at the screen, doind some calcs, thinking how the larger fuel cell is not just 6 smaller ones combined, since, 6 small ones produce 9 electricity/sec while the array doubles that. Altho the difference between the array and the small batteries are how fast they consume fuel. if you slapped 12 fuel cells in your craft, then you would have the same power output of the array. I launched a test ion probe to minmus, and with 6 small fuel cells, I could operate the ion engine without losing any power. From experience, from Dres onwards, fuel cells are better than the solar panels.

[mikegarrison]

Wait, you can get a net gain of energy by running a resource converter on fuel cells powered by its own fuel output? That should be physically impossible - it violates the 2nd law of thermodynamics.

No it doesn't. We do this all the time. We extract oil from the Earth using energy that comes from oil. If the resource you are extracting has more energy in it than it costs to extract it, then you can run the extractor using energy from the extraction itself and still have some left over.

this doesn't violate the second law because it's not a closed system. You have energy coming into the system (via the resource you are extracting).

[quantumpion]

Oh that's true. I guess I was thinking that the processor was just extracting water, I forgot to consider that "ore" could be anything, including hydrocarbons.

[Brotoro]

No it doesn't. We do this all the time. We extract oil from the Earth using energy that comes from oil. If the resource you are extracting has more energy in it than it costs to extract it, then you can run the extractor using energy from the extraction itself and still have some left over.

this doesn't violate the second law because it's not a closed system. You have energy coming into the system (via the resource you are extracting).

We extract oil from the earth using energy that comes from oil AND THE OXYGEN IN EARTH'S ATMOSPHERE.

[mikegarrison]

We extract oil from the earth using energy that comes from oil AND THE OXYGEN IN EARTH'S ATMOSPHERE.

Well whatever "ore" is, it obviously includes oxygen, because an ISRU can turn it into oxygen.

[Foxster]

Good call on making sure to take some extra power generation and to remember to turn off the fuel cells when not running the engines.

I ended up out of LF+O because I used some running science experiments and while transiting to Jool.

Gonna try to limp home on the remaining Xenon and the power from two PB-NUK. Could be a looooong drive home tonight.

Update...

Or maybe I'll just have to sacrifice this little craft. A close encounter of the Tylo kind has nobbled things...

Edited May 6, 2015 by Foxster

[Brotoro]

Well whatever "ore" is, it obviously includes oxygen, because an ISRU can turn it into oxygen.

Yes. And if you have to chemically separate the ore into fuel and oxidizer, that will require energy. You can get the same amount of energy back when you burn the fuel and oxidizer. But there will always be net losses in the system.

Look, I haven't played with the resources system yet, but this sounds seriously goofy.

[mikegarrison]

So I've barely played with the fuel cell. Does it use up fuel constantly whether there is demand or not? Or does it only use fuel on demand, based on the EC required?

Really it seems like it should have the same functionality as a battery, and only use fuel when the EC load is applied. But that may have been too much bother to implement.

- - - Updated - - -

Yes. And if you have to chemically separate the ore into fuel and oxidizer, that will require energy. You can get the same amount of energy back when you burn the fuel and oxidizer. But there will always be net losses in the system.

Look, I haven't played with the resources system yet, but this sounds seriously goofy.

No. You are missing the point. If the fuel cell combined with the ISRU used only fuel/oxy to make more fuel/oxy than it started with, that would be a violation of the second law. But it doesn't. It uses fuel/oxy+ore to make more fuel/oxy than it started with, but less ore than it started with. Ore represents a source of energy that you add to the system. So it's not free energy from nowhere. The energy comes from using up the ore.

[Foxster]

So I've barely played with the fuel cell. Does it use up fuel constantly whether there is demand or not? Or does it only use fuel on demand, based on the EC required?

Really it seems like it should have the same functionality as a battery, and only use fuel when the EC load is applied. But that may have been too much bother to implement.

Kinda. If your craft is sat doing nothing then you won't use fuel even with the fuel cells turned on. You should also not start to use the fuel cells if there is other power available (solar or PB-NUK) but that doesn't seem to work quite right.

[DancesWithSquirrels]

As I understand it the fuel cells will kick in whenever battery charge goes below 95%. Given that, I suspect that the fuel cells just cycle on/off at full power, at whatever rate (whatever percentage of 'on' time) keeps the charge around 95% based on other demands. IRL I would hope for a few percent of hysteresis (if battery>90%, charge until 95%, or the like), but that's just me.

But given any battery, fuel cell, and demand, that would be equivalent to running at whatever partial power is needed.

[Brotoro]

No. You are missing the point. If the fuel cell combined with the ISRU used only fuel/oxy to make more fuel/oxy than it started with, that would be a violation of the second law. But it doesn't. It uses fuel/oxy+ore to make more fuel/oxy than it started with, but less ore than it started with. Ore represents a source of energy that you add to the system. So it's not free energy from nowhere. The energy comes from using up the ore.

I don't think I am missing the point. If you have some LFO to start with, you can use the energy from reacting it in a fuel cell to chemically separate ore into LFO. You should NOT get more LFO than you started with (you should get LESS because no process can be perfect).

The reason we can do this with oil is because we are reacting the oil we extract with oxygen in the air. And that oxygen is given to us from hard-working photosynthetic lifeforms that captured the solar energy in the first place. You don't have that free input with the ISRU situation.

Edited May 6, 2015 by Brotoro

[Red Iron Crown]

I don't think I am missing the point. If you have some LFO to start with, you can use the energy from reacting it in a fuel cell to chemically separate ore into LFO. You should NOT get more LFO than you started with (you should get LESS because no process can be perfect).

The reason we can do this with oil is because we are reacting the oil we extract with oxygen in the air. And that oxygen is given to us from hard-working photosynthetic lifeforms that captured the solar energy in the first place. You don't have that free input with the ISRU situation.

You're assuming the LF and O exist as the end product of their usual combustion in the ore, in which case it would mean that using the produced LF+O to make more would be a losing proposition. But...if the LF and O are simply mixed in with other elements or combined in compounds that take less energy to break down than LF + O produces, then a net energy win is possible.

[problemecium]

Hmmm...

This thread makes me think the Fuel Cell ought to be adjusted so that it can take in any fuel type, either by automatically picking based on some sort of priority system or via an action group switch.

Then you could make your ion craft feed the Fuel Cell with Xenon, effectively making a self-powering (and somewhat less efficient) ion engine.

[Brotoro]

You're assuming the LF and O exist as the end product of their usual combustion in the ore, in which case it would mean that using the produced LF+O to make more would be a losing proposition. But...if the LF and O are simply mixed in with other elements or combined in compounds that take less energy to break down than LF + O produces, then a net energy win is possible.

Correct. Do you have an example of such a compound that we can find out in space?