W = p Δ V
The Sub had a Volume of the 142 cm (56 in) internal diameter, 2.4-meter-long (7.9 ft) carbon fiber-wound cylinder
so 3.8m³ of air, should be about 3.8m³ x 4x10⁷ Pa = 152x10⁶ J, 152MJ

TNT has about 4.2x10⁶J/kg, so I get about 36kg of TNT.
But I ignored the front and back Domes, which will add a bit, and possibly have other issues as well, so the ballpark should be right.