ARCA is using a more aerodynamic cylindrical shape for Helen vehicle because of the following two main reasons:

1. Helen will be launched from the solar balloon at 14000m, comparing with Haas - from 18000m and Stabilo – from 22000m. This decision, to lower the launch altitude was taken because of the launch site restrictions. For example Haas will be launched from a much bigger launch site, but the expenses are important. ARCA didn’t want to assume these expenses for now.

2. Helen is a lighter rocket comparing with Stabilo and the lighter rockets are more affected by drag compared with heavier rockets. In other words, the drag/thrust ratio is higher on lighter rockets, than on heavier rockets.

Cumulating these two elements, we can see why Helen is cylindrical and Haas/Stabilo has spherical shape.