With a 2.5" OD?.... Tubing in 7075-T6 is almost impossible to find.... 2024-T3 is easy.... Below at 3500 psi....With 7075, you need a 0.175" wall for 3.1:1 to Yield and 3.5:1 to Burst.... That is 1071 cc for 18" reservoir, and 3 lbs. for 24" LOA.... at 3/16" wall it is still lighter than Titanium....With 2024, you need a 0.205" wall for 2.5:1 to Yield and 3.5:1 to Burst.... for 3.0:1 to Yield that goes up to 0.240".... At 1/4" wall it is 926 cc for an 18" reservoir and weighs 4 lbs. for 24" LOA.... Bob
That is because the axial stress on the tube wall is half the hoop stress