Enclosure Venting and Pressure Equalization
As a launch vehicle ascends, air contained within compartments and bays must be vented overboard to a decreasing atmospheric pressure. At front-facing openings, the increasing vehicle speed means hotter air entering the compartments at those points.
Sometimes the design concern is to provide adequate (but not excessive) pressure equalization paths, and sometimes the concern is to keep avionics from getting too hot or too cold (for example, below the dew point). Such thermal design concerns can be complicated by expansion cooling and compression heating of air, which can include a strong dependency on adjacent compartments. For example, compression heating of a compartment can be exaggerated when the air entering that compartment is itself being warmed by compression of an upstream compartment.
Other times the goal is simply to calculate the pressures in the bays particularly if there are multiple holes around the vehicle that air can escape or enter. If the vehicle is at a high Mach number, the external pressure can vary significantly from front to back and top to bottom. This can cause the differential pressure across the external to be quite large which is a particular concern if there are doors, hatches, or other moveable components that have to seal.
Another purpose in such analyses could be to satisfy a general ventilation requirement (for example, 10 air changes per minute in each compartment) so that any gas (such as leaking fuel vapor) will be exhausted quickly.
Similar problems face scientific balloons, aircraft bays and cabins, instrument pods, and other flight vehicles.
An intentionally generic example problem of bay venting and refilling has been developed to illustrate key modeling concepts. This example covers a vehicle that ascends from sea level to 40,000 feet and then returns, simultaneously accelerating from Mach=0.1 to 1.4 and decelerating again to Mach=0.1 as it lands. The entire flight takes 6 minutes (tf = 0.1 hours).
Four bays are arranged as follows (the openings are flush and sharp-edged, whereas in the drawing they are exaggerated in order to make them more visible):
A sketch-pad style FloCAD® model is built. When limits on positive and negative (vacuum) pressure differential and caps on internal temperatures are imposed, the initial design fails to meet the requirements. Bay 3 overheated, and the pressures were too high (relative to the external static pressure on the sides of the vehicle) in several bays.
The SINDA/FLUINT Solver (an optimization and tasking module) is then set up to find new sizes for the intercompartmental openings, inlet, and exhausts (6 design variables in total) that will meet the design requirements (3 constraints) for the flight profile while minimizing the inlet size (the opening at the leading edge to Bay #1). The design that was found decreased total flow, but increased flow through Bays 1 through 3 while reducing flow to Bay 4:
Inlet to Bay 1: 0.316 in2 (minimized) (was 1.7 in2)
Bay 1 to 2: 3.16 in2 (was 0.6 in2)
Bay 2 to 3: 1.35 in2 (was 0.2 in2)
Bay 3 exhaust: 0.646 in2 (was 0.1 in2)
Bay 2 to 4: 0.0223 in2 (was 0.1 in2)
Bay 4 exhaust: 0.0586 in2 (was 0.02 in2)