Seal Design Feature for Redundancy Verification
- Sunday, 01 June 2014
The technology may be of interest to designers of high-altitude aircraft and submarine vessels.
Lyndon B. Johnson Space Center, Houston, Texas
NASA has requirements for redundant seals to protect human-occupied cabin atmospheres, as well as fluid and gas systems in space vehicles exposed to the harsh environments. Comparable requirements have been passed down to the International Space Station (ISS) Program, and are now levied on the Orion Multi-Purpose Crew Vehicle (MPCV).
The purpose of this invention is to allow independent verification of redundant critical seals, which are seals through which leakage would constitute a catastrophic or critical failure. The problem, in the case of the Orion Crew Module (CM) side hatch, is access to the seals. In most cases with other hatches, access to the seals does not present the same problem because the area where the seals are located can easily be bagged with plastic material and tape. Bagging the area creates a containment volume for a trace gas around the entire perimeter (inside and outside) that allows ease of seal leakage verification using a mass spectrometer leak detector (MSLD) per requirements.
In the case of the CM side hatch, seals are not accessible from either side, and are buried deep in the configuration when the hatch is closed. In the case of the Shuttle Orbiter, side hatch seal leakage verification was performed by pressurizing between the redundant seals, and performing a leak decay test. This verified that both seals together met the leakage requirement; however, it did not meet the requirement to verify each redundant seal path independently as required by the Orion requirement version levied to the program by NASA and thus was the driver for this invention. The leak rate specification is much smaller for the Orion vehicle than the Shuttle Orbiter or any other manned vehicle in NASA’s history, and cannot be verified accurately by performing a leak decay test.
In this invention, a two-beaded silicone rubber molded seal with a metal substrate retainer is used and the interstitial area between the two seal beads is connected to the leak detector (MSLD) via the test port connector on the CM Hatch Service Panel. The outer area of each seal bulb is shrouded by a helium environment that is contained by the following features designed into the seal retainer, which is the core of the invention:
- A series of machined through holes called conductance passageways in the metal seal retainer that are connect ed to one another by conductance grooves encompassing the entire outer perimeter of each seal bulb.
- An elastomeric material is permanently adhered to the outer corners of the metallic seal retainer. When the hatch is closed, the elastomeric material is compressed together creating a containment dam and thus a containment area for the helium trace gas.
This containment volume is evacuated and then back-filled with helium to facilitate the test of each seal bead. The volume between the seal beads is evacuated by the MSLD, which quantifies any leakage across that seal bead. The same process is then repeated for the other seal bead.
Similar type seals (with redundant seal beads mounted in metal retainers) are currently used in other humanrated space vehicles, and do include design features such as leak test ports and conductance grooves within the seal interstitial area to accommodate redundant seal verification. Now with the added features that this invention brings, a common leak test method known as the Helium Leak Test, Vacuum Method – Hood Test can be employed to not only verify the redundancy of the seal, but actually quickly quantify the leakage rate of each seal path independently.
This work was done by Doug Harrison of Lockheed Martin Space Systems Co. for Johnson Space Center For further information, contact the JSC Technology Transfer Office at (281) 483-3809. MSC-24680-1