- Updated design method (added obelisk geometry information)

docs/documentation/sizing/tank_design/tank_design_method.md

@@ -43,6 +43,15 @@ The obelisk method simplifies the wing by dividing it into several volumes. Depe
 
 ![](figures/01_tank_locations.png)
 
+##### Obelisk geometry
+
+The geometry of the obelisks is obtained based on the wing geometry.
+Knowing the chord length $l_\text{chord}$ and the thickness-to-chord ratio, the maximum profile thickness $h_\text{max}$ can be obtained.
+The actual thickness $h_1$ is calculated using the a-to-d factor (user input).
+The width of the obelisk $w_1$ is defined as the distance between the front $p_\text{fs}$ and the rear spar $p_\text{rs}$ of the wing.
+
+![](figures/03_wing_box.png)
+
 The obelisk volume can be determined using two different approaches that are described in the following.
 The user can select the desired method via the following node in the `program_settings` section of the _confXML_:
 `configuration[@ID="tube_and_wing"]/specific/kerosene_tank_design_parameter/obelisk_calculation_method`.