Advanced Lightweight Fuel Tanks

[0something0]

(This is basic rocket science groundwork. If you are already familiar with the maths, feel free to skip below)

The rocket equation tells us that the sacred number, Delta-V (dV) can be found with the following. 

dV = I_sp * g * ln(m_full/m_empty)

Given KSP's non-infinite selection of parts, there are absolute maximum values that can be had for each of the variables-  Isp and the mass ratio (m_full/m_empty):

The maximum Isp attainable with chemical engines are 350 s (Poodle), or 380 s with Making History (Wolfhound), 800s with the NERV, and 4200s with the Dawn. But I think the stock parts do give a decent range of Isp's to go along with tech progression. 

The same cannot be said for the mass ratio. Liquid fuel (and LOX) tanks have a maximum mass ratio of 9, which gives us an absolute max delta-v of 7536, 8182 and 17226 m/s with the Poodle, Wolfhound, and NERV respectively, as the number of fuel tanks approach infinity. With xenon, the maximum is attained with the X150 Xenon Container (the 0.625 diameter one), giving us a max delta-v of 78081 m/s. 

(Now for the actual suggestion)

These delta-v caps are enough to get you most places, especially with advanced orbital mechanics techniques, and are basically removed with ISRU. However, the mass ratio values don't really fit in with the progression - they don't get better with technology or with the square-cube law, and the normal cylindrical form factor that you get from the start has better mass ratios than the fancy aircraft parts (which might be justified since those need more dry mass to do aircraft things). And these hard caps do encourage players to use aforementioned workarounds such as ISRU, gravity assists, etc. But I still think it would be beneficial to progression to reward research into more advanced fuel tanks by giving greater mass ratios to larger radius tanks, and to perhaps add a special supermaterial 1.25 m/0.625m  tank to bring the smaller radiuses in line with the larger ones. 

[Bill Phil]

Yeah that would be pretty cool. Maybe you could argue that the normal tanks have separate structural elements from the tanks themselves. Combining the two creates much better tanks.

Keep in mind that delta-v limitation is only for one stage. Multi-stage vehicles or vehicles with drop tanks are much less limited.

[Incarnation of Chaos]

So I actually kinda do this with a bit of Roleplaying in my Career games. About half way through I'll install SMURFF to reflect advances in tank construction.