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Can't help thinking this is an area the game could use an overhaul. Switchable tank contents would greatly reduce the number of parts in game, as would being able to have engines run off different fuels. However, which fuels should be in game, what's the best compromise between realism and needless complexity? Stock As it is we have just 3 liquid resources - monopropellant, liquid fuel, and oxidizer. Due to it's lower ISP, there is no reason to ever use mono. Real Fuels This is a fun mod. With Boiloff and density to consider, cryogens are not always the best choice. It also means new fuels could be unlocked in career mode, giving a reason to spend tech points in this area of the tree. On the other hand, the sheer number of fuel choices would make the game hard to maintain and balance between patches. Also unless the number of engines was vastly increased you'd frequently find yourself in situations without an engine to match to your tank of choice, or vice versa. Semi Realism (compromise) Increase to four liquids. Two oxidizers, one storable, one cryo, and two fuels, one storable, one cryo. If one half of the propellant mix is cryogenic (eg. storable fuel with cryo ox or vice versa) you get a 15% boost to ISP. If both are cryo, 30% more ISP. However cryo tanks suffer boiloff. This would at least force some choice on the player, depending on mission type. Unlocking cryogenics could be something to put points in the tech tree.

As you've probably heard by now, liquid fuels used in spacecraft fall into one of two categories 1) cryogenics - those relying on hydrogen or methane as the fuel, and oxygen as oxidizer. Best ISP but can only be used for launch due to boiloff. Mid course corrections, return journeys must be fuelled by 2) storable propellants - stuff that's liquid at about room temperature. Apart from the Kerosine/Hydrogen Peroxide combo used by the UK in the 1960s, this generally means really nasty stuff. Hydrazine, fuming nitric acid and derivatives - corrosive, carcinogenic, toxic - and don't forget to put "highly" in front of each of those adjectives. Technicians working with the stuff are more or less dressed in a full spacesuit down on earth, it's that bad. Along comes the https://en.wikipedia.org/wiki/Green_Propellant_Infusion_Mission which uses https://en.wikipedia.org/wiki/Hydroxylammonium_nitrate instead of hydrazine. ISP is a percent or two higher, and it's significantly (40-50%) denser. It's also less volatile and far less toxic - either this woman is very brave or it really is ok to swill about in an open beaker Now, in KSP we have just 3 fuels Liquid Fuel, Oxidizer, and Monoprop. Given that there is no boiloff, and given that ISPs given seem a low compared with state of the art, and the high densities, i am guessing that LF/O are both meant to be storables. LF cannot be Kerosine, because it wouldn't work in a NERV long term (after a while, soot would clog the heat exchanger). Hydrazine or this new HAN fuel must be what we are dealing with. Then again, RAPIER engines require a cryogenic to work, so that's a bit of a mess. Anyway, second question. Anyone who knows a bit more about chemistry able to speculate if this new HAN can be used with an oxidizer rather than just as a monoprop for better ISP, and are there any oxidizer candidates with similarly friendly properties? Are there any house rules you can use to try make KSP more realistic with regards to real world fuels? 1. treat LF/O as hypergolics. That means no use of the RAPIER engine as a house rule. 2. treat LF/O as as cryo. That means, any LF/O used to power a rocket engine or RAPIER must not be stored in a wing/strake tank and any fuel in that tank must be jettisoned shortly after making orbit to simulate boiloff. LF stored in wings or used to power whiplash, panther or LV-N is assumed to be Hydrazine or HAN.