First of all, thanks for this awesome mod!
On the balance side of things, I don't think it is really bad at the moment.
Here is how I look at it:
Resources: Argon should be darn cheap and Xenon expensive (as it is in the real world). Lithium somewhere in between.
Xenon is quite rare and costs as much as $800/kg in the real world
Lithium ~$20/kg (battery grade $20,000/ton)
Argon probably <$1/kg (the air is almost 1% Argon!).
Basically:
Lithium (low ISP) - if thrust is more important than ISP
Argon (medium ISP) - solid middle choice, cheap, reasonable thrust and power, get more expensive if you want higher ISP (with higher power and/or lower thrust)
Xenon (high ISP) - if you need very high ISP, but you get lower thrust and need more power (and are also more expensive)
Legend for the tables below
Choice/dilemma of electric satellite engines
Argon (Hall): small, cheap electric engine for satellites but with low ISP (for electric) and reasonable thrust and reasonable power usage. This is how it is now:
Xenon (ion): small, more expensive with high ISP but low thrust and higher power usage.
Seems pretty ok to me as it is . Gyro-1 maybe should get a little ISP boost so it closer to Dawn/Gyro-2. The Xenon engines cost about as much as their Argon counterparts, but the Xenon fuel is of course more expensive. And if you want higher ISP, you need more power, so you need more solar panels/batteries/nuclear power, which adds mass to the vehicle.
Medium-Large sized craft engines:
MPDT(Lithium): moderately expensive, low ISP, high thrust, high power usage engines
-If the lithium fuel was made a little bit more expensive these engines could get a price reduction.
PIT(Argon): moderately expensive, higher ISP than MPDT and Ar(Hall), reasonable thrust and reasonable-to-high power usage (I like the power/ISP slider)
-These should probably be priced a bit higher for their ability to get better thrust and ISP than the Ar(Hall thrusters).
KP-01 has more (19.5kN, ISP 6000, power/ISP slider) for 7002 than the GW3 'Triplet' (with static 4200 ISP) has for 8120.
The KP-XL and KX-XK have (too) high ISP in high power mode, I think they should be lowered to 7500-8500 so they don't get into the Xenon efficiency territory.
Now the in my eyes most advanced, most expensive, electric engines known to kerbalkind:
VASIMR (Argon mode): should be very expensive engines, in Argon mode low-thrust mode they should be able to provide better ISP than PITs at lower thrust with the (customizable thrust/ISP slider) and they should be able to provide almost as much thrust as MPDT in high-thrust mode (but with worse ISP than MPDTs).
VASIMR (Xenon mode): Very expensive engines, in Xenon mode very high ISP should be possible but with very low thrust (customizable thrust/ISP slider)
- This seems all wrong yo me. Xenon mode should have higher ISP/lower thrust than Argon. They also need to be more expensive as they are the 'most complex' engines.
VW-200 compared to VX-100 to is 4-5x thrust for only 2x the price. Same goes for VW-200 to VW-10K. I'm not saying it should be completely linear as you also need to pay for the energy infrastructure needed (batteries, capacitors, nuclear power plants etc).
VX-100 on argon on a craft (8190, 0.4t ,6.2kN,) vs. a KP-01 PIT ( 7002, 0.25t , 19.5kN). The KP-01 is cheaper, has a lower mas, 3x higher max thrust. Yes, it uses 2-4x the power, but you can run it with 25-50% thrust limiter and still get a better deal.
Anyway, thanks again for developing these mods!