Comprehensive wind tunnel testing informs design for prelaunch stability
As part of the continuing development of its Delta IV launch vehicle system in the late 1990s, the Boeing Co. asked us to determine what wind loads the launch vehicles were likely to experience over the several weeks they are on the launch pad prior to launch.
Various launch vehicle configurations were under consideration, each consisting of multiple tall cylinders assembled side by side or stacked on one another, but of different sizes and geometries. Therefore, multiple investigations were required to ensure all configurations would have the integrity to withstand high winds should they occur prior to launch. Three representative launch vehicle configurations were selected for detailed testing and evaluation: medium, medium plus, and heavy.
Our task was to study wind effects for the three selected vehicle configurations, both in the loaded and empty conditions, on each of two different launch sites - Cape Canaveral Air Force Station, Florida, and Vandenberg Air Force Base, California. We were thus charged with evaluating a total of 12 different vehicle/launch site configurations. We also tested additional configurations to include the possible aerodynamic effects of nearby structures, which were configured differently on each launch site.
Because of the launch vehicles’ low slenderness, in aerodynamic terms, we anticipated that there would be significant three-dimensional airflow effects, with the most complex flow patterns around the conical tops and also in the slots between the main booster and the strap-ons. Various protuberances on the surface of the vehicles, such as wire tunnels and liquid oxygen lines, further affected the already complex wind patterns on these vehicles; none of the launch vehicle configurations presented a purely cylindrical cross-sectional shape to the wind for most wind directions.
We were primarily concerned about three wind effects: vortex-shedding, wake-interference, and wind buffeting.
Vortex shedding occurs when wind crosses a cylinder at approximately right angles to its longitudinal axis – a very common occurrence at both launch sites. As the wind passes by, it produces forces normal to the wind direction. This alternate shedding of vortices causes oscillation, and, for lightweight structures with low damping, such as the cylinders used in the launch system, the amplitude of vibration can potentially be high enough to cause concern.
Wake interferences come from nearby structures, such as components of the launch tower. Any time the wind blows within a certain range of directions, this wake impinges on the launch vehicle. This range of wind directions depends on the shape and positioning of the structure relative to the vehicle.
Wind buffeting is a phenomenon all flexible structures submerged in the earth’s boundary layer experience. Flexible structures typically move significantly in strong winds, simply due to the random buffeting action of wind turbulence caused by friction with the ground approaching the structure. The launch vehicle will have its lower modes of vibration excited by this effect.
We performed an extensive analysis of local wind conditions for all wind directions at both launch pads based on historical records. Our statistical analysis of the wind climate served as the basis for a comprehensive wind tunnel test program we conducted at different model scales.
The design loads derived from the wind tunnel testing informed Boeing’s design and the Delta IV launch vehicle system continues its successful operations today, maintaining an active launch schedule. Since the retirement of the Space Shuttle, the Delta IV in its heavy configuration has been the largest vehicle available for launching payloads.