The thickness is set by MMOD protection requirements.

Simplified calculation for the wall thickness required by the pressure vessel:

$ \sigma_\theta = \dfrac{Pr}{t} $

$\sigma$ = Young's modulus: 70 GPa
p = pressure: 10⁵ Pa
r = radius: 2200 mm
t = thickness

$ t = \dfrac{Pr}{\sigma_\theta} $

t = 0.003 mm which seems too low to me. Haven't found a better calculation though.

Other elements of the ISS have a pressure shell of 1.4 mm (FGB) with thicker Whipple shields in front.

The Whipple shield config is designed to protect against debris with these specs:

• 1.3 mm diameter aluminium sphere
• impact speed 7 km/s

This requirement dictates a thickness of 4.8 mm.

Rule of thumb: at 7km/s, aluminum sphere can penetrate completely through an aluminum plate with thickness 4 times the sphere’s diameter

A multi-layer spaced shield provides more effective protection from hypervelocity impact than single layer (total shield thickness < projectile diameter)

The thickness is set by MMOD protection requirements.

Simplified calculation for the wall thickness required by the pressure vessel:

$ \sigma_\theta = \dfrac{Pr}{t} $

$\sigma$ = Young's modulus: 70 GPa
p = pressure: 10⁵ Pa
r = radius: 2200 mm
t = thickness

$ t = \dfrac{Pr}{\sigma_\theta} $

t = 0.003 mm which seems too low to me. Apply a safety factor of 10 (far higher than used in practice) and you still get a paper-thin skin. Haven't found a better calculation though. Falcon 9 first stage tanks are something like 6 mm thick for 6 bar pressure plus the weight of the upper stage+payload.

Other elements of the ISS have a pressure shell of 1.4 mm (FGB) with thicker Whipple shields in front. So 4.8 mm is far thicker than necessary just to contain the pressure.

The Whipple shield config is designed to protect against debris with these specs:

• 1.3 mm diameter aluminium sphere
• impact speed 7 km/s

This requirement dictates a thickness of 4.8 mm.

Rule of thumb: at 7km/s, aluminum sphere can penetrate completely through an aluminum plate with thickness 4 times the sphere’s diameter

A multi-layer spaced shield provides more effective protection from hypervelocity impact than single layer (total shield thickness < projectile diameter)