Liquid Hydrogen tanks are too heavy.

[Marcafk]

Liquid hydrogen tanks weigh almost as much or slightly more than the weight of the fuel they contain, this makes rockets equipped with the nuclear engines less efficient than regular LF/O rockets.
Assuming cryogenic storage at -250 degrees and pressure of 220 kPa which is what the space shuttle's main fuel tank used we obtain a density of 68.52 kg per cubic meter.
The 5 meter tank has a volume of 65.45 cubic meters, it should contain 4.484 tons of liquid fuel . It contains 5.314 Tons. The tank weighs 6 tons, however the space shuttle's tank weighed only 16% as much when empty, with 4.484 tons of fuel which would be 63,285 units, this would be a tank weight of .854 tons.
The 10 meter tank has a volume of 523.6 cubic meter and should contain 35.877 tons of liquid fuel. It contains 46.053 tons. It weighs 30 tons but assuming a similar 16% dry weight it should then weigh 8.77 tons and contain 506374 units of fuel.
Noteworthy is that the space shuttle's tanks originally weighed 40% more but were lightened significantly over time, also they were designed to carry the significantly heavier liquid oxygen as well.
For balance it would be a good idea to have the jumbo tank have a smaller empty fraction, perhaps 11% like stock fuel tanks, the triple tank should probably have somewhere between the small and jumbo.
Suggested values are:
Small spherical tank: 64,000 fuel. Dry weight .85 tons. Dry fraction 15.78%
Triple small tank: 128,000 fuel. Dry weight 2.1 tons. Dry fraction 13.19 %
Large spherical tank: 500,000 fuel. Dry weight 4.5 tons. Dry fraction 11.27%

[futrtrubl]

Other than recommending you use CRP density figures since that isn't going to change I have nothing to add.
LqdHydrogen density is 0.00007085 t/liter so 0.07085 t/m3 or 70.85 kg/m

[Marcafk]

That's fine, a mere 3% increase, I actually based the suggested values on the apparent ingame weight, it never occurred to me why I was multiplying everything by 3%. So everything there is already based on what the CRP uses for hydrogen density.