Fuel cell consumption

[soulsource]

I'm wondering if I get this correctly:

A trip to Jool takes about 2.43e7 seconds. The octo probe core uses 1.2 electricity per minute, or 0.02 electricity/second. At full conversion rate the fuel cell would output 1.5 charge/second. This means, with only the probe core using electricity, the fuel cell would run at a fraction of 1/75. Now, again at full conversion rate, the fuel cell would use 6.08 units liquid fuel per hour, or 1.689e-3 liquid fuel per second.

To only supply the octo, 1/75th of this is required, yielding 2.252e-5 units of liquid fuel per second.

To supply the octo for the full trip it would therefore need nearly 550 units of liquid fuel (and the corresponding amount of oxidizer). That's more than a full FL-T800 tank!?!

With that huge fuel amount necessary, it'd probably pay off to just mount Gigantor arrays instead (not that they'd be needed)...

Did I miss or miscalculate something?

(I am aware that there is no energy consumption if the vessel isn't currently active, and that there is also the option to just lock some energy storage and let the probe deplete of energy until it arrives at its destination, but at least the first option sounds like an exploit to me...)

Edited October 8, 2015 by soulsource

[FancyMouse]

The magnitude sounds right to me. After all, the fuel tank is used in a time span of years. Nothing strange to me.

And btw, you don't have to spend 2.43e7 seconds to go to Jool. Just a few extra dV will give you a wide range of possible travel time. Look at what transfer window planner shows and it's actually quite interesting.

[Ripper2900]

You could add a few fixed solar panels at various angles to ensure ones is pointed at the sun at all times. Then when you've done your transfer burn, shut the fuel cell off.

Then you arrive at jool, turn it on again. You can't turn it on without a little power, so you'll need those extra solar panels. There's no reason to run the fuel cells for the whole trip.

[Kryxal]

Also of note here is the "turn off a battery" trick for some reserve power, and probably point your craft to solar north or south to make things easier for the solar panel placement.

[tbarcello]

I think the purpose of liquid fuel cells isn't powering octos. Imho. You can use mainsail to get to jool instead of using nukes, but that's not their intended purpose.

[soulsource]

Thanks for all the answers. That confirms what I've been thinking about, and I'll probably - for the first probes - just use solar panels, as inefficient as they might be. As long as they keep the probe core cosy (I like to imagine that it doesn't like being cooled to near absolute zero) they should suffice.

For later missions that actually have higher power demands (I'm using SCANsat) I'll bring along fuel cells, but only to power the demanding equipment once it's switched on.

[KerikBalm]

note that when the craft isn't in focus, no power is drained...

Send it on a trajectory to jool, switch to some other craft, come back to it just as it arrives at jool.

Think of it as putting your craft into hibernation

[FancyMouse]

(I like to imagine that it doesn't like being cooled to near absolute zero)

Interestingly, you might want to read another thread - if your sole purpose is to keep something warm, you don't need those energy at all because it hardly lose any.

[soulsource]

Losses might be low, but for a journey of 280 (earth) days they do become relevant. A rough estimation I just did yields for a probe with 1m² surface area and 1kg weight, made of a material with heat capacity of 1 J/(kg*K) and starting at 300 K temperature, that it would be at about 3 Kelvin when it reaches Jool. That of course neglecting heat input from the sun, which would keep it much warmer.

[Brainlord Mesomorph]

note that when the craft isn't in focus, no power is drained...

Send it on a trajectory to jool, switch to some other craft, come back to it just as it arrives at jool.

Think of it as putting your craft into hibernation

Can’t you actually put it into hibernation?

Disable all the batteries? Including the probe core? And let it coast?

As long as it has one small solar panel, you’ll have enough power to turn everything back on when you get there.

[monstah]

and that there is also the option to just lock some energy storage and let the probe deplete of energy until it arrives at its destination, but at least the first option sounds like an exploit to me...

Can’t you actually put it into hibernation?

Disable all the batteries? Including the probe core? And let it coast?

As long as it has one small solar panel, you’ll have enough power to turn everything back on when you get there.

That's what I do, and I don't feel it is an exploit. In a real life setting I find it very plausible that a) there'd be backup power and there'd be some very low-power system allowing for hybernation, and that's what the toggle resource button does, to me.

[moogoob]

I wish I'd read this thread before my last career Jool mission. Arrived at Jool with just enough LF+O to send science from low Jool orbit - not enough to continue on to the outer moons like I planned. Had to do a second launch for that.

Hibernation mode by turning off the batteries... makes so much sense I can't believe I didn't think of it.

[Foxster]

Just stick an RTG on and call it good.

[moogoob]

Just stick an RTG on and call it good.

There's a bit of a gap in Career mode between when you get fuel cells and when you get RTGs. My later missions are always RTG-powered.

[DancesWithSquirrels]

Hibernation, definitely hibernation. Shut down the probe core until you get in the vicinity of Jool by turning off the batteries.

[Stoney3K]

There's a bit of a gap in Career mode between when you get fuel cells and when you get RTGs.

Isn't that a bit in reverse? I mean, in the real space flight history, fuel cells were developed after RTGs which were damn simple (but really dangerous) to construct.

[moogoob]

Isn't that a bit in reverse? I mean, in the real space flight history, fuel cells were developed after RTGs which were damn simple (but really dangerous) to construct.

Career also starts with manned pods, where the real space programs started with Sputnik & co, IE probes and sounding rockets.

Kerbals are a bit backwards, it seems.

[technion]

Isn't that a bit in reverse? I mean, in the real space flight history, fuel cells were developed after RTGs which were damn simple (but really dangerous) to construct.

With the exception of powering power-hungry ion engines, there is no reason in the game to use a cell over an RTG. If you got RTGs early, there'd be no point the cells existing.

[Foxster]

With the exception of powering power-hungry ion engines, there is no reason in the game to use a cell over an RTG. If you got RTGs early, there'd be no point the cells existing.

Umm, I've used them to power a mining operation in dim and distant places.

[FyunchClick]

Umm, I've used them to power a mining operation in dim and distant places.

Exactly. RTGs are nice to power a probe core or as an emergency backup, but they lack the output needed for mining and ISRU where you can't use solar pannels (Dres and beyond basically).

[technion]

Umm, I've used them to power a mining operation in dim and distant places.

Well, and the IRSU, which is another power hungry device.

My point is, general flight operations and keeping probe cores alive isn't a useful case.

[Stoney3K]

If you use a cell to keep your mining rig and ISRU up, at least you have the advantage that it can keep going forever since the ISRU can supply the fuel cell with a continuous flow of fresh fuel.

[Kryxal]

That's assuming the presence of an engineer ... it doesn't work so well if you use more fuel than you make.