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Analysis of rebalanced electricity sources [1.0.2]


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With physicsless part mass now counting and solar panels getting a balance pass in 1.0.2, electricity generation is now a whole new ballgame.

Let's compare the efficiencies of all the electricity generating parts. My main metric for efficiency will be est (electricity per second per ton), with mass a second metric. All panel efficiencies are given for Kerbin distance from the sun-- they drop off very quickly as you move outwards.

OX-STAT panels: 70 est, 0.005 t

OX-4L/W panels: 91.4 est, 0.0175 t (non-retractable)

SP-L/W panels: 64 est, 0.025 t

Gigantor panels: 81.3 est, 0.3 t

RTG: 9.4 est, 0.08 t

Fuel cell: 30 est (+0.125t LF/OX per 10,000 units electricity), 0.05 t

Fuel cell array: 75 est (+0.125t LF/OX per 10,000 units electricity), 0.24 t

So, what to make of this? Obviously, if retractability is not an issue, the OX-4L/W panels are going to be your best inner solar system electricity source for everything except the smallest probes, where the OX-STAT will be better because of its lower total weight. If you need retractability, and have a large-ish ship, go Gigantor.

The outer solar system (where solar panels suck) is more difficult, and I will need to play and think more before making any definitive comments. I will say that the fuel cells can run an ion engine for ~19 minutes on 0.125 t of fuel, which is pretty darn good. Fuel cell ion propulsion might be feasible for small-medium ships, maybe even OP. Does anyone have the math skills to calculate ion engine Isp with the fuel cell LF/OX consumption counted as reaction mass? I reckon it's excellent. Fuel cells even work for probe missions, with a trick. On long-duration probe missions with a fuel cell, turn off the fuel cell and the fuel cell's internal battery before warping. The probe will drain the rest of the batteries during warp and the craft will die. When coming out of warp, turn to fuel cell's battery back on, use that power to activate the fuel cell and voila! You have power with no wasted LF/OX!

The RTG seems a bit rubbish, especially given the fuel cell trick just outlined. The fuel cell requires so little LF/OX that only the smallest ships would benefit from trading it for the far lower est of the RTG. But the fact that the RTG weighs 60% more than the standard fuel cell makes it worse for small ships, not better. So is the RTG useless? I'm not sure. Perhaps there is a narrow range of conditions where using it is optimal, someone with more math skills than me can do the calculations. In any case, the RTG will always be the best choice when ease and convenience matter more than every last drop of efficiency, and that is an important role.

So, what do I think of all this? I'm mostly really happy that OX-STAT spam is no longer the objectively correct answer to everything. My life will be harder, but it's the good sort of challenge. If I could change one thing, it would be to make the Gigantor's efficiency equal to that of the OX-4L/W panels. It's too awesome of a part to be relegated to the "retractable panels on a medium-to-large ship" role-- it should be the go-to for any sufficiently large inner system craft.

All in all, I'm extremely happy with the changes :D

What do you guys think?

Edited by a2soup
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What do you guys think?

I pretty much concur with everything you wrote there (although I'd write EC/t rather than 'est', that makes me think of the timezone hehe).

As I mentioned before, my own changes in the Horrible Nerf prior to 1.0 was as follows:


// Stock HorribleNerf
// Panel EC/ton Cost/EC EC/ton Cost/EC Size
// OX-STAT 150 133 40 2000 45x32=1440
// SP-* 80 200 60 1750 (roughly same as OX-4*)
// OX-4* 114.286 190 60 1750 222x36=7992
// Gigantor 51.429 166 80 1500 548x200=109600
// RTG 9.375 4400

I think that was a better solution (naturally), but what they've done isn't too far different.

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*Maths incoming*

Ok, the ION engine consumes

0.486 xenon/s = 48.6E-6 t/s

8.74 ec/s

and produces a thrust of 2kN

With fuel cells, the ec costs

8.74 ec/s * 12.5E-6 t(lfo)/ec = 109.3E-6 t/s

so the total mass flow rate is

48.3E-6 + 109.3E-6 = 157.9E-6 t/s

Specific impulse is

thrust/(mass flow rate) = 2kN/(157.9 t/s) = 12.63 E3 m/s which is equivalent to ~1300 s

So it's a big performance hit, but still better than any other engine, including the LV-N.

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Does solar panels energy generation depend on distance to kerbol now or not?

It depends on distance to Kerbol and varies by the square of that distance rather than linearly, as was the case before. So they are stronger near Kerbol than they used to be and weaker far away (almost useless at Jool, instead of ~half power).

Specific impulse is

thrust/(mass flow rate) = 2kN/(157.9 t/s) = 12.63 E3 m/s which is equivalent to ~1300 s

Thanks so much for the math! So fuel cell ion propulsion is likely the optimal method for most outer solar system craft, especially given the 0.25 t of the ion engine vs. 3 t for the nuke. And I don't know how much they toned down the nuke heating, but that can only help the ion in the comparison. Of course, the use of the ion is usually limited by tech tree position and patience.

Edited by a2soup
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an ion engine can run for about 3 hours with the biggest xenon tank. a single 6-pack fuel cell can run two engines for that long with about 215 LF + 263 Ox. a simple satellite built this way with no payload has 5100 dv and .09 twr in the VAB. but that doesn't account for the LF/OX usage... time to scale it up!

Edited by gavman420SSJ
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*Maths incoming*

With fuel cells, the ec costs

8.74 ec/s * 12.5E-6 t(lfo)/ec = 109.3E-6 t/s

so the total mass flow rate is

48.3E-6 + 109.3E-6 = 157.9E-6 t/s

Specific impulse is

thrust/(mass flow rate) = 2kN/(157.9 t/s) = 12.63 E3 m/s which is equivalent to ~1300 s

So it's a big performance hit, but still better than any other engine, including the LV-N.

How do you end up with a mass flow rate for the LFO which is twice as high as the xenon? The fuel cell uses a tiny amount of LF and Ox per second, even when you calculate how much it would need for 8.74 ec/sec.

When I did the math myself, and mind you, I'm by no means 100% confident that I did this correctly (I'm no rocket scientist), I got 0.50785 for the total mass flow rate (combining xenon, lf, and ox), and using that, 4019 for the Isp (2041.2 / 0.50785).

My process was this:

I re-calculated the ion engine's thrust first from its listed Isp and mass flow rate in vacuum to get 2041.2, since "2.0" was lacking in precision. (Possibly more accurate values are in data files somewhere?)

I also converted the fuel cell's consumption values from per-hour into per-second and then compared them to the fuel cell array (and determined that they likely had the same per-ec consumption). That appeared to be 0.0011259259259 lf/sec/ec and 0.00137407407 ox/sec/ec, although those are really not as precise as the number of digits would indicate, since they were calculated from the fuel cell's listed stats by calculating 6.08 * 3600 / 1.5, and 7.42 * 3600 / 1.5.

Then, multiplying the lf/sec/ec and ox/sec/ec by 8.74 each, the mass flow rate is 0.486 xenon/sec + 0.00984059259 lf/sec + 0.0120094074 ox/sec = 0.50785 total/sec.

(If I went wrong somewhere, hopefully someone will point out where and how.)

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Sorry, too early for me and maths.

Can anyone tell me what's the bottom line for an ion/fuel-cell craft in term to Xenon to LFO ratio? I.e. how much LFO do I have to carry to match a 700 unit Xenon gas tank?

=====

+Caffeine update...

The fuel cells sizes are little odd:

You need 8.74 charge to run an ion engine flat out. The basic fuel cell makes 1.5, so you'd need 6 that weigh 0.05, total weight 0.3t.

But for a mass of 0.24t you can use the fuel cell array and that kicks out 18 charge, enough for at least two engines or three if not run flat out.

Anyway, for a one engine craft the fuel cell array looks good.

zm4KUgw.png

The craft above used 2.06 LF for 100 Xenon.

So a 700 unit Xenon tank should need 14.42 LF or an Oscar-B with a bit removed.

MfM3cm4.png

This means you will have about 0.425t for the LFO and fuel cell array. You could use 12 PB-NUKs at 0.96t. Or one Gigantor at 0.3t for the lowest weight but then you have to find the room for it and make sure its always in sunlight.

Edited by Foxster
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I love these changes, and with the new tank the world of solar sailing just got much more interesting. I've been playing with a hybrid system of gigantors, a fuel cell and a small amount of LFO for burning in shadow. Seems pretty great.

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Then, multiplying the lf/sec/ec and ox/sec/ec by 8.74 each, the mass flow rate is 0.486 xenon/sec + 0.00984059259 lf/sec + 0.0120094074 ox/sec = 0.50785 total/sec.

(If I went wrong somewhere, hopefully someone will point out where and how.)

What units are you adding up here, is this kg or KSP units? KSP resource units are a mess, so 1 xenon has a different mass (and tank volume) than 1lf / 1ox.

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How do you end up with a mass flow rate for the LFO which is twice as high as the xenon? The fuel cell uses a tiny amount of LF and Ox per second, even when you calculate how much it would need for 8.74 ec/sec.

When I did the math myself, and mind you, I'm by no means 100% confident that I did this correctly (I'm no rocket scientist), I got 0.50785 for the total mass flow rate (combining xenon, lf, and ox), and using that, 4019 for the Isp (2041.2 / 0.50785).

My process was this:

I re-calculated the ion engine's thrust first from its listed Isp and mass flow rate in vacuum to get 2041.2, since "2.0" was lacking in precision. (Possibly more accurate values are in data files somewhere?)

I also converted the fuel cell's consumption values from per-hour into per-second and then compared them to the fuel cell array (and determined that they likely had the same per-ec consumption). That appeared to be 0.0011259259259 lf/sec/ec and 0.00137407407 ox/sec/ec, although those are really not as precise as the number of digits would indicate, since they were calculated from the fuel cell's listed stats by calculating 6.08 * 3600 / 1.5, and 7.42 * 3600 / 1.5.

Then, multiplying the lf/sec/ec and ox/sec/ec by 8.74 each, the mass flow rate is 0.486 xenon/sec + 0.00984059259 lf/sec + 0.0120094074 ox/sec = 0.50785 total/sec.

(If I went wrong somewhere, hopefully someone will point out where and how.)

What units are you adding up here, is this kg or KSP units? KSP resource units are a mess, so 1 xenon has a different mass (and tank volume) than 1lf / 1ox.

Yes it looks like you forgot to convert KSP "units" for the fuel to mass. Both LF and Ox are 1 tonne / 200 units, and xenon is 1 tonne / 10,000 units. I never have quite understood why Squad did things that way, instead of displaying everything in mass units. Maybe at some point they were thinking about having different density fuels, i.e. the same tank can hold less hydrogen than kerosene?

I also looked in the config and the ion engine's vacuum thrust is exactly 2, and vacuum Isp is exactly 4200s. Btw, you have to be a bit careful dealing with Isp in KSP, since apparently the game uses a value that's slightly off from g0 (9.82 according to the wiki) to convert from thrust-specific impulse (measured in s) to mass-specific impulse (measured in m/s, aka effective exhaust velocity), which is what gets used in the Tsiolkovsky rocket equation (the delta-v eqn.), Dv = Ve * ln(m0/m1).

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I'm not sold in the fuel cell beating the RTG. The weight difference is so little in only matters in small probes. The RTG produces little electricity, but you don't often need a lot of electricity at once unless you're transmitting data or using ion engines. Of course, I'm also coming from 0.90 and it's massless batteries, so electricity storage was weightless. But even then, if you're flying a 40 ton vessel, the weight of the electric equipment is negible. In a lightweight lander, this is another matter, though, but it boils down to the same: if you aren't transmitting reports, a few dozen kilograms in equipment aren't going to be making a difference. And the RTG is a hands-off equipment.

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I'm not sold in the fuel cell beating the RTG.

The 0.03 t weight difference between the RTG and small fuel cell represents the weight of LF/O required to generate 2400 electricity. So, very roughly, if you plan on using less than 2400 electricity on your mission, you are probably better off with the fuel cell. You will have to balance this with the ∆v lost to the fuel cell's LF/O, but that shouldn't be very much except on very small probes. Unless you are using ions (in which case your requirement for high electricity generation rate tips the scales towards fuel cells anyways), you will almost certainly use much less than 2400 electricity during your mission. Naturally, if you have a probe core, you will have to micromanage your batteries by shutting them all off (along with your fuel cell) when warping so that the probe doesn't suck power during warp.

And the RTG is a hands-off equipment.

This point definitely stands. Unless you are an efficiency-squeezing, battery-micromanaging mass pedant (like me :sticktongue:) the RTG is probably the preferable choice for any mission with a probe core.

Note: All of this discussion applies only to missions past Dres. Inside of Dres's orbit, solar panels are usually the best choice.

Edited by a2soup
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Yes it looks like you forgot to convert KSP "units" for the fuel to mass. Both LF and Ox are 1 tonne / 200 units, and xenon is 1 tonne / 10,000 units. I never have quite understood why Squad did things that way, instead of displaying everything in mass units. Maybe at some point they were thinking about having different density fuels, i.e. the same tank can hold less hydrogen than kerosene?

I also looked in the config and the ion engine's vacuum thrust is exactly 2, and vacuum Isp is exactly 4200s. Btw, you have to be a bit careful dealing with Isp in KSP, since apparently the game uses a value that's slightly off from g0 (9.82 according to the wiki) to convert from thrust-specific impulse (measured in s) to mass-specific impulse (measured in m/s, aka effective exhaust velocity), which is what gets used in the Tsiolkovsky rocket equation (the delta-v eqn.), Dv = Ve * ln(m0/m1).

Ah, thanks. I did indeed think that every fuel had 1 mass per unit, among other things that you pointed out.

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