how much LFO per unit of xenon in fuel cell ion crafts?

[Screeno]

Just as the title says, is there a perfect ratio of LF and O units to Xenon gas units when you use fuel cells to power ion engines? For example, I'm planning a test craft, just to send to minimus and back to see how much fuel it uses, it would use 4 ion engines and 4 fuel cell arrays, one per engine. I thought of putting the large 1.25 meter xenon tank in conjunction with a fl t200 tank, I wonders if that would be enough LFO for the 5000+ xenon? Oh yeah it is a manned lander btw

[Streetwind]

This thread should have the answer somewhere in it, if I recall correctly: http://forum.kerbalspaceprogram.com/threads/116611

[Screeno]

Thanks! Found out that the ratio is about 100:2 (xenon / lf) ill take that into consideration when I'm building this, +rep!

[Geschosskopf]

EDIT: I was sober and not thinking clearly.

Edited May 19, 2015 by Geschosskopf

[DancesWithSquirrels]

From the previously mentioned thread, see Isp and TWR here, with fuel ratios here;

For energy balance you need one fuel cell array for every two ion engines (or six small fuel cells for one engine, the mass ratio remains the same). Turn your fuel cells off when not running the ion engines, or you'll find your probe core and SAS have wiped your LF/O.

Stats:

Isp 1293

TWR 5.4

Fuel ratio 100 units xenon requires 2.023 units LF, 2.472 units of oxidizer

Balanced tanks with a bit of extra LF/O in case you forget to turn off the fuel cells:

Two PB-X50R radial tanks (400 units each) to one Oscar-B (needs 16 LF)

Three PB-X150 (700 units each) to a FL-T100 (needs 42.5 LF)

On the high of the scale, one PB-X750 (5250 units of xenon) to four ROUND-8 tanks, or a FL-T200 plus an Oscar-B (needs 106.2 LF)

If you're going really big, three PB-X750 can be run from one FL-T800 with LF/O to spare.

For delta-V calculations, figure one ton of fuel adds up to: 3079 units of xenon, 62.29 LF, 76.12 oxidizer.

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Edit: As per some discussions on the previous thread, it may be far more efficient to have some steady low power supply (solar panels or PB-NUKs) with sufficient batteries to supply a 300-400m/s dV kick, keeping the higher Isp of pure xenon. Just recharge near apoapsis and boost at each periapsis. I think I can build a one-kerbal ion lander capable of anything short of Laythe, Tylo, and Kerbin that way, although I'm going to need a bunch of Z-4K batteries on it...

Edited May 19, 2015 by DancesWithSquirrels
Added battery comment

[Geschosskopf]

From the previously mentioned thread, see Isp and TWR here, with fuel ratios here;

Yeah, my bad on the LF:O ratio. I blame being at work and sober instead of at home and buzzed. From checking various cfg files, everything in mthe game seems to burn 1 LF to 1.2... O. Also, I seem to have misread the wiki on how much Xenon was in the tanks. I shall go back and edit.

Edit: As per some discussions on the previous thread, it may be far more efficient to have some steady low power supply (solar panels or PB-NUKs) with sufficient batteries to supply a 300-400m/s dV kick, keeping the higher Isp of pure xenon. Just recharge near apoapsis and boost at each periapsis. I think I can build a one-kerbal ion lander capable of anything short of Laythe, Tylo, and Kerbin that way, although I'm going to need a bunch of Z-4K batteries on it...

Problem is, only outward-bound and homeward-bound transfer burns are really amenable to the multi-orbit thing. When you're capturing at an airless world or making some other big burn, it's all or nothing in 1 go. So if you're planning on doing any of that, you need enough batteries to last perhaps an hour or burning. And this gets into heavy batteries.

And FWIW, a Gigantor in Laythe orbit with 100% sun exposure makes 0.92 EC/s. Lesser solar panels need not apply.

[Xannari Ferrows]

Why not just get some official sources?

For every 18 units of charge, the resource intake reads:

Liquid fuel ratio = 0.02025

Oxidizer ratio = 0.02475

This means that Electric charge ratio = 18

For every one unit of Xenon, 18 units of Electric charge are used.

For the 5250 Xenon tank, this means that you will need approximately:

107 units of Liquid fuel

130 units of Oxidizer

[DancesWithSquirrels]

"...only outward-bound and homeward-bound transfer burns are really amenable to the multi-orbit thing..."

Actually, you can manage pretty well without massive burns. Approach an SOI without too much excess velocity (burn to match the planetary orbit before entering the SOI, enter at low velocity relative to the planet), then it's only a few hundred mm/s to reduce a close approach to an eccentric orbit. From there you can repeatedly bump at periapsis to drop to a circular orbit at whatever altitude you like. It's just the inverse of a multi-kick departure.

---

"For every one unit of Xenon, 18 units of Electric charge are used."

Duh. Headslap - that would have simplified many things. I suspect my 2.023 rather than 2.025 LF to 100 units of xenon ratio was just from rounding. I think my Isp calculation (1293 for a fuel cell powered ion ship, compared to 4200 for solar/RTG/battery ion engines)) is still reasonable, though.

Edited May 20, 2015 by DancesWithSquirrels

[Geschosskopf]

Actually, you can manage pretty well without massive burns. Approach an SOI without too much excess velocity (burn to match the planetary orbit before entering the SOI, enter at low velocity relative to the planet), then it's only a few hundred mm/s to reduce a close approach to an eccentric orbit. From there you can repeatedly bump at periapsis to drop to a circular orbit at whatever altitude you like. It's just the inverse of a multi-kick departure.

But that's only possible in certain situations, such as there's no game time (as opposed to real time) constraint. It might take years, even decades, of game time to make that work. Sometimes you can't wait that long.

Really, I don't see much use for fuel cell-ion ship except for the very special case of flying an aircraft on Duna, as demonstrated by Brotoro recently. And maybe a lander for Bop or Pol. If you're working from Duna inwards, solar panels suffice. Otherwise, if you're at Dres and beyond, use LV-Ns to move and RTGs to run the electronics, which will cover the ore scanner and SCANsat stuff. The main use I see for fuel cells in stock is to run the ore drills and ISRU in the outer planets. Fuel cells seem to run through LFO way too fast to handle life support needs (if you're into that).

[DancesWithSquirrels]

"But that's only possible in certain situations..."

Say you're going to Eeloo. Find the transfer orbit that will take you to an intercept with an apoptosis equal to Eeloos orbit, but leading it slightly. When you get close, burn until your periapsis is only slightly below Eeloos (while Eeloo catches up with you), with an intercept somewhat further along the orbit - you will approach the SOi at near zero relative velocity. That orbital change can be very slow, driven only by your RTG's, as long as you mostly negate your relative velocity prior to entering the SOI.

From there, once you've entered the SOI, burn with repeated periapsis kicks to first stay within the SOI and then lower your orbit eccentricity. Pretty straightforward with ion engines, a battery bank, and some RTGs to recharge it - it doesn't take a lot to change a barely approach/exit pass into an eccentric orbit.

If travelling to Moho, do the same but have your intercept lag Moho. You will have a higher velocity than Moho at periapsis, and as you slow at periapsis you will catch up at a reducing relative velocity, with a target of near-zero relative when entering the Moho SOI. The difference there is you'll be using solar panels rather than RTGs.

[Laie]

Actually, you can manage pretty well without massive burns.

But that's only possible in certain situations, such as there's no game time (as opposed to real time) constraint. It might take years, even decades, of game time to make that work. Sometimes you can't wait that long.

It's hardly ever necessary. The probe I sent to scan all Joolian moons in 0.90 could do ~600m/s from batteries and that was way more than enough for every maneuver out there. I think even 300m/s would have sufficed, but I'm not sure of that. The only truly massive burn in that mission was the one taking me out to Jool in the first place.

Edited May 20, 2015 by Laie

[DancesWithSquirrels]

"The only truly massive burn in that mission was the one taking me out to Jool in the first place"

And even that could have been accomplished in either a sequence of short battery powered burns or in a slow burn at the continuous rate your panels/RTGs could power, once you left the Kerbin SOI.

[Laie]

Truth be told, it was accomplished with a 48-7S and some rocket fuel. Vessel was four RTGs / 4k battery capacity / two xenon tanks and turned out to be totally overengineered for the mission. Eventually it could proceed to Eeloo (800m/s transfer from Jool, no problem). I didn't take notes and can't really tell you by how much, so take this with a grain of salt... but I think two RTGs and one Xenon tank would have sufficed to map all Joolian moons. And if the battery lasts for 400m/s in one go, that should likewise be enough.