Joules/EC

[spudman2]

To start, I want to know how many RL units of energy are in an ElectricCharge. Last time I tried this (didn't post it) I tried to figure it out by using specs from the Ion engine. I intend to do that again, but I will be looking at the propellant this time, not how much energy 2kN of thrust adds to a craft. The answer to that is that it gives quadradically more energy the longer it runs. (work = force x distance, and kinetic energy = 0.5 x mass x velocity^2) Let's start with some specs from the wiki.

Ion engine consumes xenon at a rate of 0.04849 kg/s.

Ion engine consumes EC at a rate of 8.7285 EC/s.

Ion engine has an Isp of 4200 seconds.

Wikipedia tells me that I can convert from Isp to m/s by multiplying Isp by 9.81 m/s^2 (why g? somebody explain?).

That, if correct, produces a speed of 41202 m/s.

One kilogram of mass will have 848802402 Joules of kinetic energy at that speed (0.5mv^2).

That mass will take 1/0.04849 seconds to expel and use 8.7285/0.04849 EC in the process.

That is 180 EC.

Divide 848802402 by 180 to get to Joules/EC.

4715568 J/EC? Somebody tell me what I did wrong.

[TMS]

Somebody tell me what I did wrong.

Should have used pirate-ninjas.

How come you want to work this out? Pretty sure EC won't have been scaled on something sensible.

[UmbralRaptor]

You can convert Isp to m/s by multiplying by g because of the unit weirdness involved in how Isp is defined. For rocketry, effective exhaust velocity is more intuitive/natural, but KSP went with a wacky unit that existed in part because people thought lb*s of impulse per lb of propellant was a good idea.

I would not trust any EC to Joule conversions, since last I checked, they were all over the place, and would lead to things like vehicle lights consuming kilowatts.

[Sharpy]

I believe you'd be much more successful with solar panel efficiency vs Kerbol luminosity. There are far too many variables in construction details of a ion engine to convert its ISp to power usage.

[AbacusWizard]

Rocket science would be so much easier if all the engineers used metric.

[Guest]

eheheheh

Good luck with that one. Any conversion you make will be completely arbitrary. If you want accurate measurements switch to RO and never look back. KSP is terrible for anything beyond suspending your disbelief. It may be fun but there is no direct real world analogue you can make.

[cantab]

Yeah, I agree that Ec isn't very consistent. To come close to a decent figure you'd want to consider all sources of information. Some have obvious problems though. Ion engines have been tinkered with for gameplay reasons. Reaction wheels are likewise strong for gameplay reasons, though if you treat them as control moment gyroscopes they may behave more sensibly. Wheels are outright unphysical in the way they work.

A commonly used value is 1 Ec = 1 kJ. Taking that you get some reasonable values. Probes consume 10-100 Watts, not including their reaction wheels. The RTG makes 750 Watts, which is about five or ten times too high. The 3x2 solar arrays are about 1.5 kW, which I think is within an order of magnitude of what it should be. The lights are 40 Watts.

[NathanKell]

Yep, arbitrary unit is arbitrary.

Though on the solar panels--if you take 1EC = 1 kJ their W/m^2 is totally screwy (the surface area is too small), but their watts per kilogram is actually pretty spot-on.

Voyager's RTG started at 470W so 750 isn't that far off...

[mikegarrison]

Yep, arbitrary unit is arbitrary.

Though on the solar panels--if you take 1EC = 1 kJ their W/m^2 is totally screwy (the surface area is too small), but their watts per kilogram is actually pretty spot-on.

Voyager's RTG started at 470W so 750 isn't that far off...

I think it's fair to say that RTGs are overpowered in KSP, but not as outrageously as some other things are. New Horizon is 200-300W (it was close to 300 when it launched, making about 200 at Pluto). http://www.boulder.swri.edu/~tcase/Ottman-Hersman_IECEC_paper.pdf

[Signo]

I think it's fair to say that RTGs are overpowered in KSP, but not as outrageously as some other things are. New Horizon is 200-300W (it was close to 300 when it launched, making about 200 at Pluto). "snip"

I heard it was 225W. So being arbitrary, and since I tried to make the same conversion I thought that New Horizons got 2 RTGs (provided they did not clip anything) for 225 that gives us a raw calculation of 112.5 per. In KSP a single RTG can provide 45U per minute, so here you are with your arbitrary power U value.

BTW, I had this idea because I wanted to model a ram air turbine and an average one produces 400W that I did not know how to compare to kerbal power units.

[Jesrad]

Should have used pirate-ninjas.

Today I discovered that Google does not know pirate-ninja as a unit

[AbacusWizard]

Easy; it's just one unit of the area under a curve on a graph whose axes are pirates and ninjas.

[Coga19000]

Well, the Ion Engine is a bad example for measuremen, as it is probably the worst in a chain of inconsistencies between KSP engine and RL engine performance. Actually, now that I think about it, every single part of the game is an inconsistency... Except for pirates and ninjas, obviously.

And the thing with exhaust Velocity being specific impulse time Earth Gravity is that it is actually the reverse ;specific impulse is exhaust Velocity divided by Earth's gravity. In reality, specific impulse is a totally rredundantterm that was implemented in rocket science for the sole purpose of facilitating research in a workspace with users of both the metric and imperial system. By multiplying the same unit (Isp) with an easily remembered and science -related number (go, or Earth's gravity) rocket scientists can have the exhaust Velocity in either m/s or f/s, depending on their preference.

[NecroBones]

And the thing with exhaust Velocity being specific impulse time Earth Gravity is that it is actually the reverse ;specific impulse is exhaust Velocity divided by Earth's gravity. In reality, specific impulse is a totally rredundantterm that was implemented in rocket science for the sole purpose of facilitating research in a workspace with users of both the metric and imperial system. By multiplying the same unit (Isp) with an easily remembered and science -related number (go, or Earth's gravity) rocket scientists can have the exhaust Velocity in either m/s or f/s, depending on their preference.

This is the important part to remember. Isp being based on "G-seconds", so to speak, wasn't an arbitrary choice. Rather, it made the unit of measure universal, and could be used in conjunction with any other system of measure.

[cantab]

BTW, I had this idea because I wanted to model a ram air turbine and an average one produces 400W that I did not know how to compare to kerbal power units.

For what it's worth you could just do like the stock game largely does and choose a value based on gameplay. Planes in my view shouldn't be using reaction wheels, so you just need enough power to run a probe core, and maybe whatever a life support mod wants.

[Keldor]

You could always use the torque the reaction wheels are able to produce against their power consumption to get a estimate for power conversion. This would have the advantage of being physically accurate, no matter what strange amounts of power various sources produce. Bonus points for then determining the luminosity of Kerbol from this and the size of the solar panels. Don't forget to factor in heat loss on each stage! Use heat dissipation, surface area, and the thermometer. Oh, and you should be able to figure out the halflife of the isotope used in the RTGs from their power production and mass.

It would be interesting to see exactly how ridiculous the in game numbers are.

Edited July 26, 2015 by Keldor

[Jesrad]

Easy; it's just one unit of the area under a curve on a graph whose axes are pirates and ninjas.

I thought it was a KWh/sol ?

[spudman2]

In reality, specific impulse is a totally redundant term that was implemented in rocket science for the sole purpose of facilitating research in a workspace with users of both the metric and imperial system. By multiplying the same unit (Isp) with an easily remembered and science -related number (go, or Earth's gravity) rocket scientists can have the exhaust Velocity in either m/s or f/s, depending on their preference.

Ohhhhhhhhhh

+rep

[Xannari Ferrows]

Well, I have calculated that 1EC may equal 119 Joules, and solar panels are 41.65 watts. It looks not totally insane.