Initial post copied from another issue:

'Should I add a 0.5l volume override for LiquidHydrogen?'. (this would essentially mimic the CryoTanks setup)

I'm a bit torn on that, as I think that physics are well.. physics; double density-LH2 is a bit cheaty in my mind, and breaking fundamental laws of the universe (the real one anyway).

On the other hand, from a gameplay/enhancement/balance/fun perspective, I can absolutely see the point in it. Even at double density you still get the point that LH2/Hydrolox is a very light fuel (comparatively, to LFO), but you keep the size of the rockets needed to use the fuels at 'reasonable' sizes (unity physics really doesn't like 200m long ships).

The good and the bad as I see it regarding doubling up on LH2:
Bad:

• Not physically possible in our universe; you can get slightly more dense LH2 by chilling it even further, but I know that it is nowhere near double.
• Really, I think that is probably the main downside, but its a pretty big one one for me from a physics standpoint.
• Could potentially break existing craft; the amount of resources would not be adjusted, but dry mass of the tanks would (I think; would need to look at the code closer).

Good:

• Consistency and compatibility between CryoTanks
• Reduce the size of tanks needed for Hydrolox based rockets (?%), even larger reduction in size for pure LH2 tanks (50%). Important from both a craft design perspective and a physics stability standpoint. So double points there :)
• Fairly consistent with the 'Realism-Light' balance that I'm aiming for. Still allows for weighing of the various design choices regarding fuels without punishing LH2 due to game-physics problems (super-tall wobbly rockets are no fun).
• Existing code already determines tank dry mass from total filled resource mass, so would automatically adjust to the altered volume and maintain the existing mass ratios. Not really a 'benefit', but greatly eases the pain of the transition; only a couple of parts would need manual updating.

So, with KSP being a game and all, and with RO being around for much greater realism, I may just make the move to double-density LH2. I'll give it some thought, probably try it out on a few rocket designs and see how it feels. Perhaps get some opinions from others on the forum (though I'm sure most non-RO users would be all for it).

In my current career game I've hardly used the LH2/Hydrolox based engines mostly because of the size of rocket needed for them was prohibitive and made it difficult to incorporate with the LFO/Hypergolic stages... which generally points to a gameplay/balance problem that needs to be looked at (even if its not 100% realistic) .