New ion craft using fuel cell

[DancesWithSquirrels]

In real life fuel cells are very picky about what they eat - and since the power comes from an electrochemical and energy releasing "burn", single component LF, oxidizer, let alone an unreactive noble gas like xenon, cannot provide power through a fuel cell.

[magnemoe]

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.

Yes this is the huge error in KSP ISRU, you will use more energy generate the fuel than you get from it. As other say it does not apply on earth because of life. Same is true for Kerbin and perhaps Laythe. Nowhere else, or does anybody know of settings who give chemical energy outside of life?

However this does not matter much as solar is free, nuclear is almost so, the fuel you produce let you break the rocket equation, you insert an fixed mass instead of more fuel so if you mission weight is large enough ISRU win.

Not saying fuel cells are pointless, Apollo and the space shuttle used them. on an ISRU base they would be an option to run life support during night.

[Venusgate]

Skimmed the thread.

Am I the only one to think that fuel cell generation really doesnt have to match electricity consumption? As long as you know how much deltaV your batteries are going to give you in a single burn, you should only need one fuel cell to rebuild your supply until the next burn (either kicker or sling or mid course correction).

Also, solar panels are great but at a distance, this will be the only ion toy pre-end game.

[Pthigrivi]

Another thing I haven't quite gotten a full grasp on is transfer and capture burn efficiency as it relates to night-side burns. My sense has always been oberth means burning as close to a given world is optimal for raising or lowering the apoapsis which often puts you in the shadow of that world. Have we been considering the break-even point for the weight of a fuel cell on these types of burns?

[mikegarrison]

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.

Dude, there is no easy way to say this. You are just wrong.

Nothing about the ISRU and fuel cell combination breaks the laws of physics or thermodynamics, as long as you allow that this near-magical "ore" exists.

Edited May 7, 2015 by mikegarrison

[polle]

Ore is lighter than its components

[Jarin]

Ore is lighter than its components

Ore is mass-neutral, specifically, and by design. You basically assume that all "slag" is discarded automatically by the drilling unit before it is stored in tanks.

[FleshJeb]

Another thing I haven't quite gotten a full grasp on is transfer and capture burn efficiency as it relates to night-side burns. My sense has always been oberth means burning as close to a given world is optimal for raising or lowering the apoapsis which often puts you in the shadow of that world. Have we been considering the break-even point for the weight of a fuel cell on these types of burns?

The closer you are to a world, the shorter the shadow-time: Same distance, higher speed. Unless you mean doing the burn early or late while still outside the shadow. You'd pay a dV price for off-axis, and lower Oberth, but an ion craft designed for it might have a better mass fraction than one capable of night-side ops. Personally, I always carry enough batteries to do the longest night-side burn I expect to encounter on my mission. BurnDV/TWR = seconds of battery required

http://wiki.kerbalspaceprogram.com/wiki/Orbit_darkness_time

http://www.prism.gatech.edu/~bnichols8/projects/kspdarkness/main.shtml

[Pthigrivi]

^Thats fantastic, thanks FJ. Now off to find the break-even weight for fuel-cells v batteries....

Oh and PS... how closely have people considered the trade-offs between night-side and day-side-only Ions? I tend to just load up just enough solar for max thrust and put a z-200 on it so it doesn't die, then I just burn when the sun is shining. My thinking has been xenon is so light Im still getting a launch-weight savings even with the dV loss.

Edited May 7, 2015 by Pthigrivi

[Renegrade]

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

Yes. Ore.

Seriously though, you're complaining of a thermodynamics violation in a game where little green men (and women) can live in tiny cans without any power in deep space forever. And I have heard of things like unreacted oxygen and hydrogen and methane appearing in various places in the real solar system (keep in mind that the solar wind itself is basically hydrogen ions, which could easily liberate oxidizers in stuff it hits, at the very least). Ore could very well be some sort of dangerous slurry of oxidizer and fuel that only requires low-energy mechanical separation to turn into LF/O.

(I say dangerous as if it's just mechanically mixed in some way, it could probably burn or explode very easily when exposed to a spark or heat etc)

Thermodynamics is a load of hooey. u mad, science?

I'm just thankful we have inverse-square solar radiation finally.. not gonna rock the boat on the Ore front.

[Foxster]

Guys, I am interested in the talk about KSP mechanics and parts here. Think you could save the science discussion that is not game-pertinent for the science sub-forum?

[Renegrade]

Guys, I am interested in the talk about KSP mechanics and parts here. Think you could save the science discussion that is not game-pertinent for the science sub-forum?

Sure thing, my bad.

[Spartwo]

Better than expected...

[juanml82]

This is your basic ion/fuelcell craft:

http://i.imgur.com/iOVDgV8.jpg

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

By 2 units of LF you mean the sum of both fuel and oxidizer?

Anyway, I got this building a manned ion lander. The good thing, it weights 3.7 tons and fits a landing bay. The bad thing is, well, delta-v is good, but it's possible to build light (but not so light) landers with higher TWR using chemical rockets. Removing the material lab and the goo increases the TWR, which could be specially useful in the Mun and Eeloo. It can probably be tweaked and optimized a little more, so a bit more of twr gets squeezed out of it

[Foxster]

By 2 units of LF you mean the sum of both fuel and oxidizer?

I meant 100 Xenon to 2 LF plus matching O.

Its not a hard or well researched rule though. It just happened to be what worked for my first craft. Not a bad starting point though.

[Landarin]

Not sure if anyone else has done this yet, but I just calculated the "equivalent" Isp of a 6 fuel cell per ion engine setup.

1 ion engine require 8.74 charge / second, and each fuel cell produces 1.5 charge / second. So you need 6 fuel cells.

6 fuel cells (which in total weigh 0.3 tons, btw) use 0.01 liquid fuel / second and 0.0124 oxidizer / second, or 0.112 kg / second of reaction mass.

The ion engine uses 0.486 xenon / second, or 0.0486 kg / second of reaction mass.

Add the two up and plug it into the Isp formula, you get 2kN/(0.1606 kg/s * 9.81 m/s^2) = 1,269.4 s of Isp.

It comes out to around 1,269 s, which is a big drop from the 4200 s of just an ion engine, but still higher than a nuclear engine. So it might be more worth it to just spam solar panels / batteries / RTGs.

Thoughts?

[cryogen]

I confirm Landarin's maths.

Fuel Cells, and Fuel Cell Arrays (the bigger one), output 400 and 450 units of electricity respectively, per unit of combined (fuel + oxidizer). (E.g.: the Array produces 18 electricity units/sec, out of 0.02/sec fuel and 0.02/sec oxidizer, for a total of (18/sec) / (0.04/sec) = 450). Since both fuel and oxidizer weigh 5 kg/unit, their mass efficiencies are 80 electric units/kg and 90 electric units/kg resp.

The IX-6315 "Dawn" produces 2.016 kN of vacuum thrust, while consuming 8.81 electricity units/sec, and 0.489 units/sec of xenon gas. Xenon weighs 0.1 kg/unit, so that's 0.0489 kg/sec of propellant flow. And the fuel cell (Array) consumption is 8.81 units/sec / (90 units/kg) = 0.0979 kg/sec. (So, the (fuel+oxidizer) / xenon ratio is exactly 2:1, neat). The overall I_sp is

2,016 N / (0.0489 kg/s + 0.0979 kg/s) / g = 1400.0 seconds

Repeating the math for the less efficient (smaller) Fuel Cell, it's

2,016 N / (0.0489 kg/s + 0.1101 kg/s) / g = 1267.7 seconds

- - - Updated - - -

By the way, this is totally unrealistic. In the real world, burning chemical fuel for electricity, and using that to power an ion thruster, would be way worse than the plain chemical engine. Like I_sp < 80 s.

The energy density of stoichiometric (H₂+ 1/2 O₂) is 15.9 megajoules/kg. (Derivation: ÃŽâ€_fH° of H₂O (l) is 285.8 kJ/mol; a mole of water weighs 18 grams; 285.8 kJ/(18 g) = 15.88 MJ/kg). And, a real ion thruster with a 4,200 s I_sp would have a power demand of, at minimum, KE / p = [(1/2) (42 km/s)² / kg] / [42 kN s / kg] = 21 kW / N. So with a 100% efficient H2/O2 fuel cell, the specific impulse (H2/O2 only) would be

(15.9 MJ / kg) / (21 kW / N) / g = 77 seconds

It really is that bad! Reminder, LH2/LOX combustion engines go up to 465 s (see ESA's Vinci engine).

The root problem is, I think (?), the mismatch between the energy density of H2/O2, and the much higher energy density of 40 km/s exhaust. Higher v_e means both less propellant mass, and higher specific energy, which means more fuel cell mass. We're way above the "knee" of the curve: we're increasing fuel cell mass much faster than we're saving xenon mass.

I won't second-guess why Squad made fuel cells so powerful. They're much better at balancing gameplay than I am

Edited May 26, 2015 by cryogen
got distracted by a rattlesnake D:

[DancesWithSquirrels]

According to the wiki pages the basic fuel cell and fuel cell array have the same efficiency: 0.02475 O and 0.02025 LF (0.045 total) reacted to provide 18e, which is sufficient to propel 1 unit of Xenon. The Isp from those numbers comes out at 1292.3 seconds for either power source.

Now, if the actual .cfg files for these have 2.016 kN thrust and differing fuel consumption numbers than listed on the wiki pages (which I thought were auto-generated from the .cfg files) show (fuel cell, fuel cell array, ion engine respectively), then these numbers are off base.

[cryogen]

According to the wiki pages the basic fuel cell and fuel cell array have the same efficiency:

You're right! Huh, interesting. I got my figures from the in-game screen; I think it loses precision through rounding. The Array is described as having 0.02/sec for both fuel and oxidizer.

Here's the relevant parameters from the .cfg's -- you're totally right:

ResourceName = LiquidFuel

Ratio = 0.0016875

...

ResourceName = Oxidizer

Ratio = 0.0020625

ResourceName = LiquidFuel

Ratio = 0.02025

...

ResourceName = Oxidizer

Ratio = 0.02475

Edited May 26, 2015 by cryogen

[DancesWithSquirrels]

Yeah, rounding is always a pain.

I have to say (as a complete aside) that the Fuel Cell Array product description is a bit odd - "Why have just one fuel cell when you can have six?...", as the power production of a single is 1.5e/s and the array is 18e/s, or an even dozen times larger, not six.

[Red Iron Crown]

Looking for sense in KSP part descriptions is a bit of a fool's errand. They're mostly jokes.

[cryogen]

Looking for sense in KSP part descriptions is a bit of a fool's errand. They're mostly jokes.

I have learned something

[tater]

I don't use ions much.

What about the giant battery part? It holds 4000 EC, which is a 457.7 second burn with the ion according to the wiki. That would consume ~1/3 of a pbx150 for all of 0.2 tons, right? Chuck something on it to trickle charge the battery, or just put 3 on there if you need to use 700 xenon in 1 burn. You can lock them when not in use, too.

Why are batteries not a good choice?

[juanml82]

I don't use ions much.

What about the giant battery part? It holds 4000 EC, which is a 457.7 second burn with the ion according to the wiki. That would consume ~1/3 of a pbx150 for all of 0.2 tons, right? Chuck something on it to trickle charge the battery, or just put 3 on there if you need to use 700 xenon in 1 burn. You can lock them when not in use, too.

Why are batteries not a good choice?

They are. I made two ion landers for low gravity bodies, but I haven't tested them extensively. The one with the fuel cell has lower dV (still above 2,500 m/s) but better TWR because, at the end of the day, it's too much of a hassle to keep adding oscar tanks plus the correspondent amount of xenon, so I've just settled to an FT-100 plus three (I think, maybe they are four) 0.625 inline tanks plus one radial tank.

I also designed one with three 2.5 m batteries (or was it four?) and four rtgs to recharge it. With four ion engines, that can run for some 6 minutes, I think, which should be enough. Since I can just toss as much xenon as I want without keeping it in proportion to LF, it has better dV but lower TWR. I haven't tested it extensively, though.

The main drawback is that, since they have to be so lightweight, I've chose to skip monoprop, which can be a problem for docking and landing in a slope (then, I've landed on 20° slopes on Ike and settled down the thing with the pod's reaction wheels)

[Jett_Quasar]

I've actually seen a video featuring a small VASIMR rocket engine that used water a it's propellant (real world). I wonder if anyone has thought of using the output of a fuel cell for the propellant of an ion engine. Interesting discussion.

JR