new_contents: import unittest
  import tank_structure as ts
  from units import inch2meter, psi2pascal
  
  
  class TestStringMethods(unittest.TestCase):
  
      def test_sample_8_3(self):
          # Do sample problem 8-3 from Huzel and Huang.
          stress = psi2pascal(38e3)
          a = inch2meter(41.0)
          l_c = inch2meter(46.9)
          E = psi2pascal(10.4e6)
          v = 0.36
          weld_eff = 1.0
          p_to = psi2pascal(180) # Oxidizer max pressure 180 psi
          p_tf = psi2pascal(170) # fuel max pressure 170 psi
  
          # Ox tank crown 0.135 inch thick.
          self.assertAlmostEqual(inch2meter(0.135), ts.crown_thickness(
              p_to, 1.395*a, stress, weld_eff), delta=inch2meter(0.005))
          # Fuel tank cylinder 0.183 inch thick.
          self.assertAlmostEqual(inch2meter(0.183), ts.cylinder_thickness(
              p_tf, a, stress, weld_eff), delta=inch2meter(0.005))
  
          # Critical external loading for fuel tank cylinder 10.8 psi
          self.assertAlmostEqual(psi2pascal(10.8), ts.cr_ex_press_cylinder(
              a, inch2meter(0.183), l_c, E, v), delta=1e3)
  
  
  if __name__ == '__main__':
      unittest.main()
subject: Add limited unit test for tank structure.