Ascent Venting Analysis

As a launch vehicle ascends through the atmosphere, gases that were initially at on-pad conditions escape through vents, structural seams, joints, and other orifices. The rate at which the gases escape depends on the differential between the external local pressure and the interior, or compartment, pressure. If the gases do not escape fast enough, the pressure inside the compartments causes the walls of the launch vehicle to bulge outward, leading to a potential “burst” case. If the gases escape too quickly, the external local pressures may cause sections of the exterior to buckle, leading to a potential “crush” case. Proper analysis of the venting environment requires definition of the launch vehicle’s trajectory, external aerodynamics, vent design(s), internal layout, and system/subsystem/payload contributions to the compartment thermal and pressure environments (see NASA SP-8060, Compartment Venting). The data that define each of these elements may be provided from a variety of sources:

Trajectory

3-DOF/6-DOF

Nominal/dispersed

External Aerodynamics

Wind tunnel testing

CFD

Semi-empirical/theoretical

Vent Design

Government/contractor-provided

Manufacturer-provided

Internal Layout & Systems

Manufacturer

Requirements-based

The trajectory data defines the ambient pressure, density, and temperature as well as the vehicle’s speed and orientation. In conjunction with the trajectory data, the external aerodynamics provides the external local pressures as a function of the flight Mach number and orientation. The vent design defines the modeling approach used to simulate the vent fluid dynamic environment and the discharge coefficient. The internal layout and systems provide information on the connectivity between internal compartments, potential leak sources, and structural components that may contribute to the compartment flow via outgassing. Starting from the on-pad conditions, specialized software is used to compute a mass and energy balance throughout each trajectory of interest, which results in a time-dependent profile of the compartment pressures and the differential of the compartment pressure and local external pressure for each vent location. These profiles are compared against design limits to ensure that neither a “burst” nor a “crush” occurs.

Discharge Coefficients are Determined for Honeycomb Vent During Ascent

CFD Venting Analysis Showing Air Ingestion into Internal Volume

CFD Modeling of Innovative Vent Design

Produce Burst/Crush Curves Used for Primary Structure Skin dP Determination