The International Space Station microgravity experiment

Abstract

A broad class of scientific experiments have evolved which utilize extreme low acceleration environments. The International Space Station will provide such a environment, in conjunction with an unparalleled combination of quiescent period duration, payload volume and power, and manned or telescience interaction. The International Space Station is the world's first manned space vehicle with These place limits on the acceleration levels within the pressurized laboratories and affect everything from flight altitude and attitude to the mechanical and acoustic energies emitted by an air circulation fan. To achieve such performance within the program's resource constraints, a control approach has been adopted which balances both source and receiver disturbance mitigation. The Active Rack Isolation System (ARIS) provides acceleration attenuation at the payload rack level, and tall pole sources have been reduced either by isolation or design modifications. Assessments indicate that the vehicle is capable of meeting the requirements, although some current marginal non-compliances do exist. Assessment refinements will continue through the verification phase with greater reliance on test and on-orbit measured data as part of a long term effort to clearly define and understand the constitution of the acceleration environment. This process will assure that the design and operation of the International Space Station will support significant science during both its assembly and, more importantly, its long term mature operational phase. Copyright O1996 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. REQUIREMENTS A primary mission of the International Space Station (ISS) is to provide an extremely low acceleration environment to enable a broad class of scientific research which exploits conditions. To ensure that such low acceleration levels are achieved the International Space Station is designed to a set of microgravity requirements. The ISS requirements stipulate the duration, location, and allowable acceleration levels which are associated with the ISS mode. With regard to duration, the requirement is for the specified acceleration levels to be sustained for at least 180 days per year. These acceleration levels are to be achieved at 50% of the International Standard Payload Rack (ISPR) locations. There are 12 ISPRs in the U.S. Laboratory, 10 ISPRs in the European Attached Pressurized Module (APM), and 10 ISPRs in the Japanese Experiment Module. The acceleration levels themselves are addressed separately for quasi-steady and accelerations. Quasi-steady accelerations represent a class of accelerations which are continuous for a relatively long time, and hence may be characterized by very low frequencies. For ISS, the acceleration from the disturbance source must be continuous for over 100 seconds for it to be considered a quasi-steady disturbance. This criteria may be subsequently related to a frequency criteria of less than 0.01 Hz. Gravity gradient, drag, and orbit rotation accelerations are the quasi-steady disturbance sources traditionally considered in low earth orbit vehicles. However, given the ISS definition, accelerations induced by vehicle control rate error, and potentially from vent and appendage slew events, also fall within the quasi-steady definition bounds. Vibratory accelerations are then defined as those acceleration with frequency content above the quasi-steady criteria through 300 Hz. Fan and pump imbalances, valve movements, rotary joint articulation, and exercise equipment operations are examples of typical vibratory disturbance sources. Note that science payloads and crew locomotion are not subject to these vehicle The quasi-steady acceleration requirements are specified in the time domain and limit the instantaneous peak magnitude to 1 fig. In addition, the perpendicular component to the orbital average quasi-steady vector must be equal to or less than 0.20 jig. This latter criteria implicitly places a stability requirement on the vehicle's quasi-steady performance. In the regime, there are two primary First, the vehicle's induced acceleration environment due to all disturbance sources in any 100 second mode interval is limited to the one-third octave band root-mean-square acceleration levels defined in Figure 1. This frequency domain requirement is supplemented by a second requirement hi the time domain. The time domain requirement applies to individual transient sources to offset the effects of the 100 second averaging associated with the combined requirement criteria. Individual transient disturbances are limited to 1000 fig at any instant, and 10 |ig-seconds hi any 10 second interval on a per axis basis.