Space Station Freedom Airlock Depress/Repress System Design and Performance 921378

This paper will discuss the airlock depress/repress (D/R) system under design by McDonnell Douglas Space Systems Company as part of the Work Package 2 (WP-02) portion of the NASA Space Station Freedom (SSF) program. Requirements for the airlock to support 52 extravehicular activities (EVA) per year have driven several design features which make this system unique from any used on Orbiter, Skylab, or Mir. Use of the airlock to support 52 EVAs per year is a part of the program's overall attempt to make life and work in space a routine endeavor. As a result of this goal, system design must minimize time and resources required to support EVA while maximizing system maintainability, safety, reliability, and usability.

Requirements aimed at realizing this design goal are found in JSC 31000 Volume 3, “Space Station Projects Description and Requirements Document,” Revision H (Reference 3). The D/R System must minimize EVA time by depressurizing the airlock within 10 minutes. At the same time, provisions must be made to recover 90% of the airlock gas prior to opening overboard vent lines. In addition to being operated from inside the airlock, the system must be controllable from inside SSF. Provisions for depressurization from space must also be made. Hardware must support two pressure change rates: a 0.34 kPa/sec (0.05 psi/sec) nominal and a 6.89 kPa/sec (1.00 psi/sec) emergency rate. Above all, the system must allow for safe and reliable use over the entire 30-year planned life of SSF.

The paper will describe how the current system has evolved from the above requirements. The resulting configuration utilizes an extensive network of interfaces with most of SSF' s various distributed systems. Of particular interest is the use of software and sensor feedback through the station data management system (DMS) to allow for partially automated control of the system from a number of remote locations including the ground. At the same time, the system is equipped with fully manual override capability to meet safety requirements and to accommodate user preferences.

Specific subjects addressed will include the overall system architecture, system interfaces, software control methods, analytical system performance models, intended system use scenarios, human factors considerations, and specific hardware design details. Major hardware components described will include the airlock depress pump assembly (ADPA), the airlock D/R system equalization valve assemblies (D/REVA), and the airlock D/R control consoles. Reference will also be made to airlock designs and lessons learned from earlier programs.