NASA SP-105 VACUUM TECHNOLOGY AND SPACE SIMULATION Donald J. Santeler Donald W. Jones David H. Holkeboer Frank Pagano Prepared under contract NASw-680 by Aero Vac Corporation Scientific and Technical Information Division 1-9 6 6 NATIONAL AERONAUTICS AND SPACE ADMINISTRATION Washington, D.C. For Sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D. C. 20402 Library of Congress Catalog Card Number 66.60047 \ Foreword LTHOUGH SPACE VEHICLES encounter many environments during their various missions, vacuum is the most persistent con- dition affecting design and operation. The study of vacuum effects on materials, components, or systems, alone or in combination with other environments, has grown into a distinct technology during the past several years to the point where vacuum test chambers of many shapes, sizes, and capabilities are now in use. The research and development activities carried out in such vacuum facilities are highly important to progress in the space exploration program. As this program advances, the sophistication and number of vacuum systems in our Nation's laboratories steadily increase and more and more individuals are exposed to the subject of vacuum, as designers, operators, or experimenters. As the utilization of vacuum technology has increased, so has the need for an authoritative technical manual which will introduce interested personnel to the subject, allow an engineer or scientist skilled in vacuum to probe the field in depth, or furnish the technician with information on equipment characteristics and practical vacuum techniques. Although there are many excellent reference books, reports, and papers dealing with the vacuum field, no specific text is available which combines material on space vacuum simulation, technology, and the related problem of pressure measurement in space chambers. Therefore, to fill this information gap, the National Aeronautics and Space Administration has had a comprehensive vacuum manual prepared under contract. Its content and format are the result of consultation between the authors and NASA technical personnel and review by NASA staff members at Headquarters and at several Research Centers. This final document should serve as a useful reference volume on the state-of- the-art of vacuum technology applied to space simulation. Mac C. Adams Associate Administrator .for Advanced Research and Technology National Aeronautics and Space Administration iii Prelace PACE-SIMULATION FACILITIES have proven their worth in several technological areas: basic research, development, and engineering. Countless hours of testing have been and will be under- taken to advance our knowledge of environmental effects on space vehicle systems and components. As our space flight capabilities increase, our needs for more definitive preflight and post flight test information will also grow. Both preflight and postflight studies in vehicle and compo- nent reliability, vacuum heat transfer effects, materials degradation, and bearing and metal fatigue can be performed, under simulated conditions, in the laboratory. The special facilities and vacuum levels required for these studies created a host of new technical problems in a variety of scientific and engineering areas. To solve the difficulties which arose in vacuum tech- nology, new and ingenious solutions were developed. Many of these developments have reached the general scientific community through technical papers, delivered at symposia and published in journals, dealing with space simulation versus space environment duplication, level of vacuum required, test procedures, and evaluation techniques. However, there is no single source which consolidates the numerous interacting vacuum techniques required for space simulation. Since 1960, a number of authoritative books have been published which update the published state of the art of vacuum technology and which cover a wide range of interests. For example, Dushman discusses the scientific aspects of vacuum, while Guthrie proves useful for the training of vacuum-laboratory technicians. Still another volume, Pirani and Yarwood, is concerned primarily with the engineer. However, the range of information covered in most texts tends toward a general, rather than a specific, interest. Therefore, any author will provide descriptions of various types of vacuum gages, but the specific problem of pressure measurement in space chambers is rarely discussed. It is the intention of the following chapters, therefore, to examine to •_ reasonable extent the essential specifics of vacuum technology. As a result, many general aspects of vacuum technology have been omitted, and the reader is referenced to texts which treat these aspects. An attempt has been made to present new material on each topic, or a new treatment of the subject. In conclusion, the engineer who needs to purchase, operate, maintain, or otherwise use vacuum-space-simulation equipment and who must have a thorough understanding of the principles of vacuum technology, as related to space simulation, will find "Vacuum Technology and Space Simulation" specifically designed for his particular requirements. Contents chapter page 1 VACUUM NOMENCLATURE ........................... _ 1 Molecular Fundamentals ............................ 2 Gas Flow ......................................... 6 Directional Effects .................................. 7 Glossary .......................................... 8 Symbols .......................................... 15 2 PRESSURE ............................................ 19 Definitions of Pressure .............................. 19 Kinetic Theory of Gases ............................. 23 Molecular Kinetics .................................. 28 Application to Space Simulation ..................... 33 3 PRESSURE MEASUREMENT 41 Types of Gages ..................................... 41 Ionization-Gage Characteristics ...................... 52 Vacuum-Gage Calibration .......................... 55 Space-Simulation Gages ............................. 63 4 PUMPING SPEED ...................................... 71 Steady-State Case .................................. 71 Transient Case, Pumpdown, and Repressurization ...... 77 5 GAS FLOW IN COMPONENTS AND SYSTEMS .......... 83 Molecular Flow Through an Orifice .................. 84 Molecular Flow Through a Tube of Circular Cross Section .................................... 85 Laminar Viscous Flow in a Long Circular Tube ........ 88 Flow Characteristics of Complex Shapes--Molecular Flow ............................................ 94 Flow Characteristics of Complex Shapes---Transition Flow ............................................ 105 Flow Characteristics of Complex Shapes--Laminar Viscous Flow ................. .:.................. 105 Flow Characteristics of Complex Shapes--Choked or Critical Flow ..................................... 106 Flow of Gas Through Cylindrical Tubes ............... 108 Traps and Baffles .................................. 116 vii CONTENTS chapter page 6 PUMPING SYSTEMS ................................... 123 Rotary Oil-Sealed Pumps ........................... 124 Positive Displacement Blowers ....................... 134 Diffusion Pumps ................................... 135 Sputter Ion Pumps .................................. 142 Sorption Pumping .................................. 143 7 CRYOGENIC PUMPING ................................ 151 Perfect Cryogenic Pumping Surfaces ................. 151 Extended Cryogenic Pumping Surfaces .............. 154 Liquid Nitrogen Traps .............................. 166 8 GAS LOAD ........................................... 173 Definition of QL, Gas Load on Systems ................ 173 Sources of the Gas Load ............................ 175 9 OUTGASSING OF MATERIALS ......................... 197 Metals ............................................ 197 Cleaning Chemicals ................................ 208 High-Temperature Degas ............................ 210 Low Temperature .................................. 213 Exposure .......................................... 213 Polymers .......................................... 213 10 SYSTEMS ............................................. 223 Space Simulation .................................. 223 Types of VacuUm Systems .......................... 229 High-Vacuum Systems--General Considerations ....... 234 System Design ..................................... 248 Troubleshooting Diffusion-Pumped Systems ........... 267 11 LEAK DETECTION ..................................... 275 Gas Flow Through Leaks ........................... 277 Basic Equations of Steady-State Leak Detection ........ 279 Basic Equations of Transient Leak Detection ........... 283 Other Aspects of Leak Detection ...................... 288 Vacuum-Gage Methods OfLeak Detection ............. 291 Leak-Detection Techniques for Systems ............... 300 °°o Vlll 1 Vacuum Nomenclature INTRODUCTION HE FIRST STEPS into any new venture generally require a guide to insure that the correct path is being followed. Thus, an engineer assuming new responsibilities in activities concerned with vacuum tech- nology and space simulation will need aid in becoming acclimatized to the language, terminology, and units peculiar to vacuum. It is the intent of chapter 1 to serve as a map to the charted courses of vacuum technology. Terms and units defined in this chapter will be elaborated upon in later chapters. All symbols used in the text, their descriptions and units have been alphabetically