NASA/SP-2005-4540

Journey Into Space Research Continuation of a Career at NASA Langley Research Center W. Hewitt Phillips

Monographs in Aerospace History Number 40 July 2005

Journey Into Space Research Continuation of a Career at NASA Langley Research Center

by W. Hewitt Phillips

NASA History Office Office of Policy and Plans NASA Headquarters Washington DC 20546

Monographs in Aerospace History Number 40 July 2005

Cover photograph (L-05136): Little Joe rocket being launched from Wallops Island. Several of these rockets were launched to study separation of the escape capsule from the Mercury capsule.

The use of trademarks or names of manufacturers in this monograph is for accurate reporting and does not constitute an official endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics and Space Administration.

L-63-3268

Phillips in 1963 at the age of 45. At this time, he was chief of the Space Mechanics Division.

Table of Contents

List of Figures vii

CHAPTER 1 Introduction 1

CHAPTER 2 Blast Off Into Space 3 Education in New Fields of Research 4 Initial Space Research at the Flight Research Division 6 Initial Space Research at Langley 13

CHAPTER 3 Stability and Control of Space Vehicles 17 Rotating Vehicles in Free Space 18 Moments Acting on a Satellite 22 Gravitational and Centrifugal Moments 22 Magnetic Moments 24 Effect of Radiation Pressure 24 Effect of Aerodynamic Forces 25

CHAPTER 4 Rendezvous 29 Effect of Orbital Mechanics 30 Simulation Experiments 31 Implementation of Rendezvous in Apollo Program 32

CHAPTER 5 Space Simulation Studies 35 Lunar Landing Research Facility 36 Lunar Orbit and Landing Approach (LOLA) Simulator 45

Monographs in Aerospace History Number 40—Journey Into Space Research v

Table of Contents

Docking Simulator 47 Aids for Extravehicular Maneuvering 48

CHAPTER 6 My Work With Johnson Space Center 51 Design of Shuttle Control System 51 Studies of Shuttle Control System 53 Problem With Pilot-Induced Oscillations 55 Flight Control Review Group 56

CHAPTER 7 Continuation of Research Following Decline of Space Program 59 Variable Sweep Wing Supersonic Transport 59 Differential Maneuvering Simulator (DMS) 61 Circulation Control 64 Control Configured Vehicles (CCV) 65 Propulsive Effects Due to Flight Through Turbulence 66 Decoupled Controls 67 Soaring Gliders 68 Complementary Filters 70 Turbulence Problems 71

CHAPTER 8 Some Nonaerodynamic Studies 75 Some Studies of Fireplaces 75 Control System for a Small Hydrofoil Boat 77

CHAPTER 9 Concluding Remarks 79

Appendix 83

References 87

Index 91

About the Author 95

vi Monographs in Aerospace History Number 40—Journey Into Space Research

List of Figures

FIGURE 2.1. List of topics covered in Henry A. Pearson’s lecture series. 5

FIGURE 2.2. Drawing of concept for entry vehicle studied by members of Flight Research Division. 8

FIGURE 2.3. Effect of entry angle on variations of deceleration, velocity, and flight-path angle, g, with altitude. Wing loading, 20 lb/ ft2; angle of attack, 90°. 10

FIGURE 2.4. Time histories showing effect of reduction of angle of attack on deceleration and other trajectory variables. Initial flight-path angle –0.5°, wing loading 20 lb/ft2. 10

γ FIGURE 2.5. Effect of entry angle, o, on heat-transfer rates for entries at 90° angle of attack. Wing loading, 20 lb/ft2; entry altitude, 350,000 ft. 11

FIGURE 2.6. Large model of entry vehicle being tested in Langley Spin Tunnel to study control at or near 90° angle of attack. 12

Monographs in Aerospace History Number 40—Journey Into Space Research vii

List of Figures

FIGURE 3.1. Illustration of motion of ellipsoid of inertia during free rotation of body in space. (Taken from ref. 2.1.) 19

FIGURE 3.2. Model used to illustrate stability of rotation of body in space. 21

FIGURE 3.3. Sketch of drag device proposed for causing an orbiting spacecraft to enter Earth’s atmosphere in case of failure of retro-rocket. Many small vanes could produce required drag. 26

FIGURE 5.1-1. Pilot model for LLRF being flown by NASA test pilot Jack Reeder. 39

FIGURE 5.1-2. Lunar Landing Research Vehicle showing original cab for pilot. 40

FIGURE 5.1-3. Lunar Landing Research Vehicle hovering with stand-up cab for pilot. 42

FIGURE 5.1-4. Lunar Landing Research Facility at NASA Langley Research Center, Hampton, Virginia. 42

FIGURE 5.1-5. Simulated craters on terrain in landing area of LLRF. Shadow of vehicle with lighting simulates low Sun angle. 43

FIGURE 5.1-6. Lunar Landing Research Facility in use at crash test facility. 44

FIGURE 5.3-1. Docking simulator. 48

viii Monographs in Aerospace History Number 40—Journey Into Space Research FIGURE 6.1. Comparison of height response of several airplanes: F-104 and B-52—conventional aft tail, YF-12, B-58, and Shuttle—delta wing. Note that the Shuttle requires 2.15 s for the c.g. to return to its original altitude. 57

FIGURE 7.1. Figure 7.1. Glider model of supersonic transport with variable-sweep wing and retractable canards. Length, 30 in., span (swept) 15 in., span (unswept) 26 in. 60

FIGURE 7.2-1. Pictorial view of Differential Maneuvering Simulator (DMS). The large spheres are projection screens, and they contain the pilots’ cockpits, projectors, and servomechanisms to move the projected displays. Each pilot sees a view of the opposing airplane. 62

FIGURE 7.2-2. More detailed sketch of Differential Maneuvering Simulator. 62

FIGURE 7.3-1. Use of Coanda effect to deflect airstream at trailing edge of wing. Illustration shows effect of oscillating cylinder that closes one side of the opening or the other. A slightly smaller rotating cylinder could alternately close and open gaps at a high frequency, producing a rapidly oscillating airflow. 64

FIGURE 7.4-1. Control-configured vehicle (CCV) chart. 66

FIGURE 7.4-2. Damping and stability control chart. 66

Monographs in Aerospace History Number 40—Journey Into Space Research ix List of Figures

FIGURE 7.5. Heavily weighted glider (40-in. wingspan) gliding slowly (about 3 ft/s) underwater in large tank. Fluorescent dye is emitted from wing tips. When glider made complete circles, trails from previous circle were always well above glider. 69

FIGURE 7.6. Comparison of attenuation of lift due to spanwise averaging on an elliptical wing with that due to unsteady lift effects for wings of aspect ratios 3 and 6. 73

x Monographs in Aerospace History Number 40—Journey Into Space Research CHAPTER 1 Introduction

Space flight has long been a subject of field until the nation was startled by the interest both to scientists and to the Russian launching of Sputnik. The last general public. Science fiction became chapter of the preceding volume on the popular with the works of Jules Verne, history of my work at Langley (ref. 1.1) whose fanciful stories of space exploits describes how the nation was galva- inspired many later science fiction publi- nized into action and started a national cations. These stories were usually not space program. These developments based on valid science and technology are described in more detail in the book or they were ahead of the developments Spaceflight Revolution (ref. 1.2). that might have made them possible. These works, however, served to stimu- The advent of the space program was a late thought on space flight for many welcome event to many of the research years. Some groups, such as the British groups at Langley. One reason for this Interplanetary Society, made serious attitude was that aeronautical research studies of the requirements for space had reached a plateau at this time. flight. These efforts failed to lead to Many of the research contributions of practical developments because of lack Langley and other National Advisory of financial support or interest from gov- Committee for Aeronautics (NACA) cen- ernmental organizations or from the ters had reached fruition in the design public. These early studies had little and production of advanced airplanes. effect on actual developments in the These airplanes included jet bombers space program because, with greater and transports, and supersonic fighter support and larger numbers of investi- airplanes. Research and design work gators, the results were quickly redis- on the supersonic transports, the covered and not until later was it found British Concorde and the Russian that some important results had been TU144, had progressed to a point that worked out previously. construction could proceed with some assurance of success. No really revolu- Studies of the possible military applica- tionary advances for atmospheric air- tions of space flight were started by mili- craft were envisioned at that time or tary organizations in the United States have occurred in the ensuing years. about 1950, but these studies were Some of the wind-tunnel organizations, classified secret. I, like the general pub- however, expressed concern that their lic, was unaware of any activity in this work might be cut back or otherwise

Monographs in Aerospace History Number 40—Journey Into Space Research 1 Introduction

affected by the emphasis on space be done at Langley. All future flight research. research on such airplanes was to be In the case of my work and that of the done at the Edwards Air Force Base in Flight Research Division, another event California (now called the Dryden Flight occurred that required a change in Research Center). The engineers in my direction. A NACA Headquarters edict, division therefore were available for published in 1958, stated that no further assignments in the field of space testing of high-speed airplanes would research.

2 Monographs in Aerospace History Number 40—Journey Into Space Research CHAPTER 2 Blast Off Into Space

With the start of the space program, a direction in the Guidance and Control rapid transition occurred in the types of Branch of the Flight Research Division research performed at Langley. At (1959–1962), the Aerospace Mechanics first, the emphasis was on educating Division (1962–1963) and the Space research personnel so that they could Mechanics Division (1963–1970). No become acquainted with the new disci- attempt has been made to present plines and skills required in space research. Second, there was a diverse the dates or time periods of different effort in space research, applying the research programs. Because over newly acquired knowledge to the solu- 40 years has passed since the start of tion of problems that were recognized the national space program, however, as being important in this field. Soon, many readers today may be unfamiliar however, a national space program was with the progress of space flight and the established, involving both unmanned times at which certain goals were and manned satellites. Space flight cen- accomplished. To assist the reader in ters were established to take the lead relating the work described to the in specific types of work, such as scien- tific satellites, interplanetary probes, progress in space flight, an abbreviated and manned space flight. The various chronology of space launches and mis- groups at Langley generally initiated sions, both Russian and American, is work that would contribute to a specific presented in appendix I. These data phase of the national space program, were obtained from the TRW Space Log and general research programs were (ref. 2.1). Much more detailed discus- phased out. In some cases, research sions of the various NASA space pro- programs were stopped by manage- grams are available in the NASA ment because they did not fit in the Historical Publications that have been national space program or because the prepared to describe each major pro- work being done had been assigned to other centers. gram. Some of these publications are given in references 2.2, 2.3, and 2.4. The discussion that follows applies Others are given in the lists of reference mainly to the work being done under my works included in these publications.

Monographs in Aerospace History Number 40—Journey Into Space Research 3 Blast Off Into Space

Education in New Fields of which various engineers would look up some subject involved in space Research research and give a lecture on it to the whole division. The notes on these The start of the space program was a lectures were collected in a volume time of rapid transition for most aero- (ref. 2.5), the table of contents of which nautical research organizations in the is given in figure 2.1. These notes were country. The Russian feats of orbiting widely distributed in the NASA centers Sputnik satellites had captured the inter- but were never published. A copy of this est of the general public, first from a volume is available in the Langley sense of wonder that space vehicles Research Center Library. As stated in actually existed and second from a the preface of the volume, the initial sense of fear that these vehicles, with demand for the notes was so great that, unknown capabilities, might signal the “for the sake of expediency, this goal (of start of an era in which the enemy would rapid distribution) is best achieved by have technical expertise exceeding our making the material available in its own. The engineering community was present unedited form instead of follow- perhaps less alarmed but nevertheless ing the usual NACA editing procedure.” realized that a great deal of study and Later, most of the engineers involved research was necessary to become had become involved in specific space familiar with the disciplines involved in projects and therefore had no time for this relatively unknown field. the work of preparing the volume for more formal publication. The effect on the administrators in Washington, as is well known, was to Similar studies and lectures were con- cause them to initiate the change of the tinued after the distribution of the vol- NACA, the National Advisory Commit- ume. Among the subjects I studied were tee for Aeronautics, from a small gov- the rotational motion of a free body in ernment organization reporting directly space and later, the relative motion of to the president, to the NASA, the two bodies, a subject of importance in National Aeronautics and Space Admin- connection with space rendezvous. istration, a large government agency Many other research organizations, in reporting to Congress and involving this period, were equally involved in an many new research centers and opera- intense educational effort to learn tional centers in addition to those in the everything possible about space flight. original NACA. This change, however, These organizations included the aero- did not immediately affect the research nautical departments of engineering programs at the Langley Research Cen- colleges and government research ter. The change in research emphasis at groups in the Army, Air Force, and Navy. this center came primarily from the interest of the center personnel in new In studying these problems, it was fields of work involving flight in space impressive to find how much famous and the desire to learn as much as pos- mathematicians centuries ago, who had sible about a promising new area of no idea of applying their theories to research. space travel, had learned in studying the motions of planetary bodies. These In the Flight Research Division, I was brilliant men had studied these prob- still assigned as Head of the Stability lems as pure academic exercises, with- and Control Branch. At this time, out modern computing facilities and Henry A. Pearson, Head of the Aircraft without the incentive provided by experi- Loads Branch, initiated a program in mental research with artificial satellites.

4 Monographs in Aerospace History Number 40—Journey Into Space Research Education in New Fields of Research

FIGURE 2.1. List of top- ics covered in Henry I Elementary Orbital Mechanics W. B. Huston and J. P. Mayer A. Pearson’s lecture II Satellite Time and Position With Respect to a T. H. Skopinski series. Rotating Earth Surface III The Motion of a Space Vehicle Within the J. P. Mayer Earth-Moon System: The Restricted Three-Body Problem IV Orbital Transfer A. P. Mayo V Reentry With Two Degrees of Freedom A. P. Mayo VI Six Degree of Freedom Equations of Motion J. J. Donegan and Trajectory Equations of a Rigid Fin Stabilized Missile With Variable Mass VII Inertial Space Navigation D. C. Cheatham VIII Guidance and Control of Space Vehicles C. W. Mathews IX Elements of Rocket Propulsion H. A. Hamer X Characteristics of Modern Rockets and J. G. Thibodaux, Jr. and H. A. Hamer Propellants XI Aerodynamic Heating and Heat Transmission W. B. Huston XII Heat Protection W. S. Aiken, Jr. XIII Properties of High Temperature Materials E. M. Fields XIV The Solar System C. R. Huss Appendix on the Earthʼs Atmosphere W. J. OʼSullivan and J. L. Mitchell XV Communication and Tracking P. A. Gainer and R. L. Schott XVI Some Dynamical Aspects of the Special and D. Adamson General Theories of Relativity XVII Environmental Requirements W. A. McGowan

In many cases, they originated mathe- astronometrics, the study of the dis- matical techniques used in space tances to and relative positions of the research, particularly the techniques stars and other heavenly bodies. The required for the calculation of satellite University of Virginia at Charlottesville, orbits. Virginia is one of the colleges closest to Langley with an astronomy department, I was also impressed by the progress and valuable contacts were established made by astronomers. These scien- there that later aided in the work on the tists, who devoted their lives to abstract Apollo program. studies to understand the nature of the universe, soon realized that they had The subjects presented in these lec- knowledge of value in the space pro- tures included many disciplines that gram. An example of this research, had little relation to the aeronautical which could be classed as an engineer- work previously conducted by these ing study as well as a scientific effort, is branches. For example, a lecture on given in a brief note entitled Exploration hypersonic flow was included because of Space from the University of Virginia, such flow conditions would be involved published in the University of Virginia in the flight of rockets or space vehicles News Letter in January 1959 (ref. 2.6). while entering or leaving the atmo- This note points out that the exploration sphere. Orbital mechanics was con- of space there had been going on for sidered important because vehicles 75 years, since the acquisition of a large operating in space would be subject telescope in 1888. The main object of to the same laws that had been devel- these studies is the field known as oped for planets and other heavenly

Monographs in Aerospace History Number 40—Journey Into Space Research 5 Blast Off Into Space

bodies. The theory of relativity was attraction. This problem has been stud- considered important because it had ied by many famous mathematicians been involved previously in astronomi- over the centuries. Despite the amount cal studies. This theory becomes impor- of effort expended in its solution, this tant when the motion of the bodies problem, in all its generality, has never involved approaches the speed of light. been completely solved. As often hap- Such speeds were not contemplated in pens when brilliant mathematicians the type of space operations required in work on a very difficult problem, how- the early years of the space program. ever, the work led to advances in many Nevertheless, the desire to understand fields of mathematics. As an aside, the laws governing the universe made John P. Mayer later joined the Space relativity a subject of interest in the new Task Group working on the Mercury field of space flight. It was known that a Project and was later in charge of all the slight motion of the perihelion of the computing facilities at the Johnson planet Mercury had been detected Space Center during the Gemini and that was not explained by Newtonian Apollo programs. Practically all the engi- mechanics and was one of the exam- neers in the Aircraft Loads Branch later ples presented by Einstein as a method joined the Space Program, either in of verifying his theory of general relativ- the Space Task Group or at the new ity. Thus, relativity might have an effect Goddard Space Flight Center. In the even on the motion of objects in the list of lecturers given in figure 2.l, solar system. In later space projects these engineers include Wilbur B. involving precise measurements of time Huston, John P. Mayer, Ted H. Skopin- or in the use of spacecraft for navigation ski, Alton P. Mayo, James J. Donegan, purposes, inclusion of relativistic effects and Carl R. Huss. Others on the list, was found necessary to obtain the who worked in my branch, are Donald degree of precision desired. C. Cheatham and Charles W. Mathews. Mathews later moved to NASA Head- Fortunately, an engineer with a knowl- quarters in Washington and became edge of Einstein’s theories, working head of the Gemini project, one of the in the Physical Research Division at most successful space flight projects. Langley, was available. He was David Cheatham worked at the Johnson Adamson, an Englishman who studied Space Flight Center and did important physics at Durham University but was work in the design of the Space Shuttle employed at the Royal Aircraft Estab- control system. lishment (RAE) in England during WWII in research on airplane handling quali- ties. After the war he came to work at Langley. As a result of his experience in Initial Space Research at engineering as well as in physics, his the Flight Research Division lectures on the complex subject of rela- tivity were presented to the engineers One of the main objectives of many with unusual clarity. research organizations both in the Item 3 in the list of topics in figure 2.1 is NACA and in the armed services was a lecture by J. P. Mayer on the subject of to beat the Russians in a race to place the three-body problem. This notable a man in space. The Air Force had problem concerns the motion of three its MISS program (Man in Space Soon- bodies in space, such as, for example, est). At Langley, the Pilotless Aircraft the Sun, Moon, and Earth, under the Research Division (PARD) was in influence of their mutual gravitational charge of Robert R. Gilruth, formerly my

6 Monographs in Aerospace History Number 40—Journey Into Space Research Initial Space Research at the Flight Research Division

boss in the Flight Research Division. vehicle, rather than in the form of heat Members of PARD, who had developed that would produce temperatures high experience in handling rockets by using enough to melt or decompose most them to propel test vehicles used in materials. The same concept was aeronautical research, proposed placing employed in the Mercury project by hav- the astronaut in a capsule that could be ing the capsule enter the atmosphere launched into space by the existing blunt end first, with a suitable heat Atlas Rocket and recovered by para- shield on the blunt end. chute with a splashdown in the ocean. In considering the application of this This imaginative program, largely the concept to an airplane-like configura- concept of Maxime A. Faget, an engi- tion, I visualized that the airplane could neer in PARD, later won the approval of enter the atmosphere at near 90° angle NASA and developed into the Mercury of attack, then pitch down to normal Project. The Air Force Program, mean- gliding attitude after the speed had while, continued with several studies of decreased sufficiently that heating vehicles called the Robo, Brass Bell, would not be a problem. A delta-wing and Hywards. Finally these studies configuration seemed suitable for this were consolidated into the Dynasoar purpose, but controls had to be pro- project, also called the X-20. This pro- vided to control the attitude of the vehi- gram received considerable support and cle during entry and to pitch the airplane reached an advanced state of engineer- down to normal attitudes for landing. A ing development, but was cancelled in sketch of the resulting vehicle is shown 1963. The reasons for the cancellation in figure 2.2. To pitch the airplane down were that no well-defined military mis- after entry into the atmosphere, a set of sion for the vehicle could be found, that tail surfaces was provided that folded the cost was excessive, and that into the shielded region behind the vehi- by 1963 the NASA Gemini program cle during entry, but unfolded for the planned to accomplish many of the pitch-down maneuver and subsequent objectives of the Dynasoar program. glide. In addition, four hinged surfaces During the early stages of the Mercury were provided around the outline of the program studies, there was much con- vehicle, which, by suitable combination cern that the ocean splash-down would of deflections, could provide pitch, yaw, be impractical and that the vehicle and roll control during entry. Such con- should allow the astronaut to land trols are now known as controllable “like a gentleman,” with a conventional strakes and are now being used on the airplane-type landing at an airport. nose of a fighter airplane to assist in control at high angles of attack. The In considering this problem, I tried advantage of these controls for an entry to take advantage of the concept vehicle was that they simply extended developed by H. Julian Allen and A. J. the outline of the vehicle at angles of Eggers, Jr. of the NASA Ames attack near 90° and were not subject to Research Center that a blunt-faced any more heating than the lower surface object would be much more suitable for of the vehicle, on which the heating was entry into the atmosphere than a reduced because of its large area. pointed, rocket-like object (ref. 2.7). This concept was based on the fact that such Soon, considerable interest was gener- an object would dissipate the tremen- ated in further studies of a vehicle of the dous energy of the vehicle entering the proposed type. I made a small model to atmosphere in the form of shock waves illustrate the concept. Many engineers that carried the energy away from the in my branch, with some assistance

Monographs in Aerospace History Number 40—Journey Into Space Research 7 Blast Off Into Space

FIGURE 2.2. Drawing of concept for entry vehi- cle studied by mem- bers of Flight Research Division. (a) Reentry. (b) Landing.

from others in the Aircraft Loads Branch measurements and heating problems of and the Performance Branch, made supersonic aircraft. detailed studies of such aspects as heating, structural loads, and stability Among the phases of flight studied were and control. The vehicle was sized to the orbital phase, the atmospheric entry, weigh about 2000 pounds, the same as the transition from flight in the vacuum the Mercury capsule, so that it could be of space to flight in the atmosphere, and carried on the nose of the Atlas Rocket. the control of the vehicle as the air- Preliminary calculations showed that a speed decreased from hypersonic vehicle capable of carrying a single speed during the initial entry to super- astronaut could be made within this sonic speed and finally to low subsonic weight limitation. speed for landing. Most of these opera- tions were unfamiliar to aeronautical The heating studies were made by engineers who had previously dealt John A. Zalovcik of the Performance with conventional airplanes. Some of Branch. He had previously gained con- the newly acquired knowledge of the siderable familiarity with this branch of dynamics of orbital flight was applied in aerodynamics in his studies of airspeed this work.

8 Monographs in Aerospace History Number 40—Journey Into Space Research Initial Space Research at the Flight Research Division

The return from an orbit around the angle by as much ±10° from the flight Earth required firing a rocket to cause direction produced less than a 0.02° the vehicle to change its orbit from a change in the entry angle. In general, at near-circular path around the Earth to shallow entry angles, the entry angle an orbit that entered the atmosphere. had little effect on the variation of decel- Though the initial thought based on air- eration, which reached a maximum plane experience would be to fire a value of about 8 g for the conditions rocket perpendicular to the flight path, considered. Tilting the vehicle longitudi- studies showed that a much smaller nal axis from the perpendicular position rocket impulse would be required if the by just 10° shortly after the start of rocket were fired along the path in a the buildup of deceleration reduced direction to slow the vehicle down. Then the maximum value to the more desir- the force of gravity would take over to able value of about 5 g, as shown in bring the vehicle down into the atmo- figure 2.4. sphere. This rocket was therefore called The effects of variation in the entry a retro-rocket, a term now familiar in dis- angle of the vehicle on the aerodynamic cussions of space flight operations. A heating were also investigated. Some of study was made of the effect of the these results are shown in figure 2.5. magnitude of the retro-rocket impulse The maximum heating rate remained on the angle at which the vehicle about the same for entry angles entered the atmosphere and the between –1° and –0.25°, but this maxi- distance traveled before entering the mum occurred later in the entry at the atmosphere. shallower angles. With too shallow an entry angle, the At that time, some studies had been vehicle would skip back out of the atmo- made of the effects of different heat sphere, whereas with too steep an shield materials for use on the research entry, the vehicle would experience airplanes such as the X-1; heat resis- excessive deceleration, resulting in tant alloys such as Rene 41, a nickel- intolerable loads on the human pilot. chromium alloy, withstood temperatures Tilting the vehicle from an attitude in up to 1600 °F. This material would allow which the lower surface was perpendic- the use of a steeper entry with less total ular to the flight direction to one in which heat input to the vehicle because the the longitudinal axis of the vehicle high temperature surface would radiate made a smaller angle with the flight much heat. On the other hand, beryllium direction, provided a lift force to slow the has a high heat capacity, so that a rea- vehicle’s entry into the denser region of sonable thickness of the material would the atmosphere and a desirable reduc- absorb the heat of entry. These two tion in the deceleration of the vehicle. In materials typified what were called radi- general, the entry angle needed to be ative and heat sink type heat shields, between –0.5° and –1°. Some of these respectively. A combination of these results are illustrated in figure 2.3. The materials was at that time thought suit- range of –0.5° to –1° appears small, but able to protect the vehicle from aerody- the accuracy of the direction and magni- namic heating. At that time, ablative tude of the retro-rocket burn was well materials, such as Teflon®, had shown within the capability of existing rockets promise in tests in hypersonic wind tun- and control systems. A fortunate effect nels, but the amount of data available of the laws of orbital motion is that the was not sufficient to allow consideration entry angle is very insensitive to the tilt of their use on a vehicle. Ablative mate- of the deorbit impulse. Variations of this rials decompose at high temperatures.

Monographs in Aerospace History Number 40—Journey Into Space Research 9 Blast Off Into Space

10 FIGURE 2.3. Effect of entry angle on varia- 8 tions of deceleration, Deceleration, 6 velocity, and flight- g units path angle, γγ, with alti- 4 tude. Wing loading, 20 2 2 lb/ft ; angle of attack, 90°. 0 × 103 28 24

20

16 Velocity, –0.25 ft/sec 12 –0.5 –1.0 8 –2.0

4 0

–16 –12

–8

–4

0 350 300 250 200 150 100 × 103 Altitude, ft

FIGURE 2.4. Time histo- × 103 400 ries showing effect of Altitude, 300 reduction of angle of ft 200 attack on deceleration 100 and other trajectory 0 variables. Initial flight- × 103 path angle –0.5°, wing 30 loading 20 lb/ft2. Velocity, 20 ft/s 10 0 10 8 Deceleration, 6 g units 4 2 0

–100

–80 90 t > 0 s –60 90 t < 370 s Flight-path {80 t > 370 s angle, deg –40 –20 0 × 103 4 Distance, miles 2 0 0100 200 300400 500 600 700 Time, s

10 Monographs in Aerospace History Number 40—Journey Into Space Research Initial Space Research at the Flight Research Division

FIGURE 2.5. Effect of × 3 γγ 10 entry angle, o, on 20 heat-transfer rates for entries at 90° angle of –2.0 attack. Wing loading, –1.0 2 –0.5 20 lb/ft ; entry altitude, –0.25 350,000 ft. 16

12 Heat-transfer rate, q, Btu/sec-ft2

8

4

0 0 100 200 300 400 500 600 Time, s

This process absorbs heat and the for entry vehicles had been proposed. outer layers turn to gaseous products The results of the Flight Research that carry the heat away from the vehi- Division study, however, had been cle. Later, many additional develop- discussed with personnel involved in ments were made both in ablative and aerodynamic research. As a result, radiative heat shields, as typified by the independently of my efforts, branch heat shields on the Apollo capsule and heads in a number of the wind tunnels on the Shuttle Orbiter, respectively. operating in different speed ranges had models with folding wing tip panels con- The studies associated with the winged structed and ran tests. entry vehicle were published as NASA Technical Memorandum X-226 entitled A large model of the Flight Research A Concept of a Manned Satellite Reen- Division vehicle was built for tests in the try Which Is Completed With a Glide spin tunnel. A picture of the model being Landing by the Staff of Langley tested is shown in figure 2.6. These Flight Research Division, compiled by tests were intended to study the ability Donald C. Cheatham (ref. 2.8). This of the four-hinged surfaces around the memorandum was originally classified border of the vehicle to provide stability confidential but has since been declas- and control during descent at 90° angle sified. Also, a patent was issued in of attack. These tests showed that the my name entitled Variable Geometry control was easily accomplished. Later, Winged Entry Vehicle. At that time, per- an air jet was fitted to the rear of the sonnel at many of the wind tunnels at fuselage and the model was mounted in Langley wished to contribute to the a vertical position in the test section space program, but not many designs of the NASA 30- by 60-Foot Tunnel to

Monographs in Aerospace History Number 40—Journey Into Space Research 11 Blast Off Into Space

FIGURE 2.6. Large model of entry vehicle being tested in Lan- gley Spin Tunnel to study control at or near 90° angle of attack.

study the use of the folding tail surfaces tunnel speed was increased from zero to provide transition from 90° angle of to the speed for horizontal flight. In fact, attack to a glide attitude. This maneuver the configuration showed promise as a was again easily accomplished by grad- tail-sitter vertical takeoff and landing ually deploying the tail surfaces as the (VTOL) airplane.

12 Monographs in Aerospace History Number 40—Journey Into Space Research Initial Space Research at Langley

This configuration, or some variations of As a result, the engineers who had it, for a time acquired the most complete joined the space task group and later set of aerodynamic data of any winged worked at the Johnson Space Center entry configuration, covering the entire were able to apply this experience in the range of Mach numbers within the capa- design of the Shuttle Orbiter. bilities of the Langley wind-tunnel facili- ties. A Langley committee called the Steering Committee for Manned Space Initial Space Research at Flight served to communicate the research results among the various Langley groups. Several of these wind-tunnel studies were published as NASA Tech- Just as the Flight Research Division nical Memorandums (refs. 2.9–2.11). rapidly changed its emphasis from These reports also contain long lists of research on airplanes to research on references containing additional tests space vehicles, all the other divisions at on these and other configurations made Langley attempted to use their expertise during the same time period. By the to investigate many problems of space flight. This research was not confined to time these data were published, the studies of the initial brief orbital mis- Space Task Group had been formed to sions but included studies of unmanned implement the Mercury project. This satellites and of manned orbiting space group was relocated in Houston, Texas stations intended for long durations in to form the nucleus of the Johnson space. Many problems confronting the Space Flight Center. Before the group crew of a space station were immedi- moved to Houston, however, they ately recognized. They included numer- requested a presentation of the results ous biomedical problems, such as the of the Flight Research Division study to effects of zero gravity, protection from compare the relative advantages of the the vacuum of space, protection from two approaches. The presentation had ionizing radiation (already discovered by little effect on the planning for the Mer- Van Allen in his initial orbital experi- cury project, mainly because, with the ments), and provision of food, water, desire to put a man in space as soon as and oxygen over long periods. In con- possible, the Mercury development was nection with the vehicle itself, there obviously simpler and involved less were problems of temperature control, development. In addition, Mercury generation and storage of energy, pro- included the feature of an escape tower tection from meteoroids, and effects of that was considered essential for astro- the space environment on materials and naut safety in case the Atlas rocket mal- structures. It is remarkable that much functioned on the launch pad. basic knowledge of many of these sub- jects was obtained in the first two years The beneficial results of the Flight of the Space program. Research Division study included edu- cational background for many of the A brief discussion of some of the work engineers who later joined the Space done at Langley on these space-related Task Group and early analysis of trajec- problems is now presented. I was not tory and heating problems that provided connected with these projects but results applicable to most types of entry learned of them through participation vehicles. In addition, the later develop- in committee meetings and visits to ment of the Shuttle Orbiter involved a see items of research equipment. configuration somewhat similar to that These comments are based on my rec- studied by the Flight Research Division. ollections and may not be historically

Monographs in Aerospace History Number 40—Journey Into Space Research 13 Blast Off Into Space

accurate, but they reflect the opinions of important subject for scientific research, the research that I formed at that time. but they have little bearing on the prob- lems of manned space flight. In the areas of biomedical research, Other groups at Langley were con- Dan Popma of the Instrument Research cerned with methods of overcoming the Division (ref. 2.12) headed an extensive effects of zero gravity. An obvious con- program. He proposed ingenious ideas cept was the use of a rotating space for methods to purify the atmosphere of station to allow the centripetal accelera- a space station, to provide oxygen, and tion to simulate gravity. Paul Hill and to recycle wastewater and urine. Meth- others in the PARD conceived an inflat- ods of testing these devices in ground- able rotating toroidal space station, like based facilities were suggested. He also a large inner tube. This design would considered the medical effects of zero allow the device to be folded compactly gravity and studied centrifuges, rotating for launching and then to be deployed in space stations, and other ideas, many space. A contract was given to the of which were finally tested in space Goodyear Corporation to build an engi- many years later. This promising work neering model of this concept. The came to an abrupt halt when NASA model was about 15 feet in diameter Headquarters ruled that all biomedical and was set up to study inflation, research should be conducted in a new strength, puncture resistance, and leak- facility at the Ames Research Center, age. When Charles Donlan, at that time manned mainly by medical doctors. I the Deputy Director of Langley, heard of considered it a mistake to place this this work, he immediately ordered it work in the hands of doctors rather than stopped because a large space station engineers. Dan Popma was an engi- was not at that time part of the NASA neer, and his approach was much more space program. This and other experi- logical and could have produced the ences made it clear that the freedom to required information for manned space pursue new ideas that had existed flight much more quickly than the pro- under the NACA was curtailed under gram that actually evolved. The doctors, NASA, and projects had to fit the overall who frequently lacked research experi- space program as established by NASA ence and had no experimental data to Headquarters. go on, feared that unknown effects in space might cause a human being to Several groups studied the problem of become disoriented or might even prove durability of materials and electronic fatal, and they therefore proposed that equipment in the space environment. difficult and inconclusive animal experi- An electron accelerator of the Van de ments should be performed before a Graaf type was acquired for this man was allowed to go into space. Test research. Later, a large laboratory, pilots, on the other hand, had experi- called the Space Radiation Effects Lab enced, in high-altitude flight, many of (SREL) was built in Newport News and the problems that at least approached run in cooperation with the College of those expected in space and did not see William and Mary. A large cyclotron sup- any reason to delay placing a man in plied the required radiation. This work orbit. The eventual program represented continued for many years, but after the a compromise of these views. After a end of the Apollo Program, funds for this few manned space flights had been project were discontinued. Later still, the made, the Ames group became involved cyclotron was removed, but the building in other research, such as theories of was used as part of the Thomas the origin of life. Such theories are an Jefferson Research Laboratory, which

14 Monographs in Aerospace History Number 40—Journey Into Space Research Initial Space Research at Langley

contains a large, continuous beam elec- times more alert to new developments tron accelerator for basic research in than professionals. Nickel-cadmium bat- nuclear physics. My rather surprising teries, or NiCads as they are called, involvement in tiding over the use of the have since been used extensively in facility during the lull in funding between spacecraft power systems. the Apollo era and the development of the electron accelerator is described in Another engineer who turned his atten- a later chapter. tion to space power systems was Albert E. Von Doenhoff, a former airfoil expert Engineers at Langley also foresaw the who was mentioned in reference 1.1 for need for studies of other aspects of the his aircraft landing study. Assisted by space environment on many types of Roland Ohlson and Joseph M. Halissy, materials. Some work could be con- he made an analysis of solar regenera- ducted in vacuum tanks. An example of tive space power systems (ref. 2.13). In this research is the study of friction of this type of system, a solar collector moving parts in the vacuum of space. heats and vaporizes a working fluid, Despite the desire to conduct much which drives an engine similar to a research of this type, very little was steam engine or steam turbine. The done because instead of first putting working fluid is then condensed up a space station and later proceed- for reuse by a radiator that radiates ing with more complex missions, Presi- heat to the blackness of space. The dent Kennedy initiated the Apollo pro- steam engine, of course, may then drive gram, which required a relatively short- an electric generator. I thought that duration mission. As a result, the effects Von Doenhoff’s analysis was excellent, of long exposure to space have been but in that same period, solar photo- studied extensively only in recent years voltaic cells were developed to a usable by using the Long Duration Exposure stage. These cells, together with NiCad Facility (LDEF) satellite launched and batteries to store the energy, have been recovered by the Space Shuttle. used since then on practically all space missions. The weight and complication A number of engineers at Langley of the regenerative power systems have investigated space power systems. discouraged their use. Such systems About the time these studies were being still might be candidates for power on started, I learned about rechargeable very large spacecraft or on planetary nickel-cadmium batteries for powering bases. the transmitters, receivers, and servos of radio-controlled model airplanes. The Roland Ohlson, who formerly worked in great advantage of these batteries is the NACA towing tank testing seaplanes that they can be charged and dis- and flying boat hulls, complained to me charged many hundreds of times with- after his retirement that nothing he had out deterioration. I contacted one of the ever worked on was used anymore. I engineers working on the space sys- suppose that many research engineers tems and found that he had not yet must expect this outcome of their career heard about these batteries. This expe- specialties in this time of rapid develop- rience shows that hobbyists are some- ment of scientific and technical ideas.

Monographs in Aerospace History Number 40—Journey Into Space Research 15

CHAPTER 3 Stability and Control of Space Vehicles

This historical document is not intended A term used in analyzing the rotational to present a theoretical discussion of motion of a body is the moment of iner- the problems encountered in designing tia about some specified axis. The space vehicles. The conditions in space, moment of inertia resists rotational however, are so different from the famil- acceleration of the body, just as its mass iar Earth-based environment that some resists linear acceleration. The moment appreciation of the effect of these condi- of inertia of a small element of mass in tions on the dynamics and control of the body is determined by multiplying space vehicles is required to under- the mass of the element by the square stand the reasons for the research stud- of the perpendicular distance from the ies conducted in the course of the axis. The moment of inertia of the entire space program. In flight in free space, body is determined by summing the contributions of all the elements of far removed from any influence of other mass. heavenly bodies, a vehicle experiences no disturbances. This condition is not To allow calculation of the motion of the attainable on Earth. In the solar system body resulting from torques (also called and in particular in Earth orbit, some moments) applied about arbitrary axes, very small disturbing influences exist. the moments of inertia are first deter- These influences include effects of grav- mined about three mutually perpendicu- ity, forces due to flight through rarified lar axes through the center of gravity of gas, moments from magnetic fields, and the body. In all rigid bodies, these val- effects of solar radiation. Although these ues of inertia serve to define an ellip- effects are small, they are of great soid, called the ellipsoid of inertia, the importance because they provide the formula for which is given in the follow- only means of controlling a spacecraft ing text. The axes defining the maximum without the expenditure of fuel or and minimum values of inertia, together energy. This chapter is therefore with the third axis perpendicular to intended to give a discussion of these these two, are called the principal axes of inertia. These axes are of special problems with as little dependence as importance in determining the rotational possible on mathematical derivations. motion of bodies. Primary emphasis is placed on a physi- cal appreciation of the phenomena The ellipsoid of inertia may be the same involved. for many different bodies that have the

Monographs in Aerospace History Number 40—Journey Into Space Research 17 Stability and Control of Space Vehicles

elements of mass distributed differently. Many problems of rotating bodies occur The angular motion of all these bodies when external moments are applied. in response to a given applied torque The equations for the solution of these would be the same. problems are known as Euler’s dynami- cal equations. These equations are taught in standard courses in mechan- ics. I used these equations in the analy- Rotating Vehicles in Free sis of the effect of steady rolling on Space stability of airplanes, a problem later known by the name “roll coupling” (ref. 1.1). One of the most obvious problems encountered in studying the motion of In 1957, before the start of the space rigid bodies is the motion of an arbitrary program, I had some introduction to the body when it is started with a rotating problem of rotation of rigid bodies with- motion. Students of elementary physics out external moments. I became familiar learn that the translational motion of the with a report by G. W. Braün, an engi- center of gravity of a body depends on neer at the Wright Air Development the external forces applied at this point Center, on an analysis of the spinning of and that the rotational motion about the airplanes (ref. 3.1). Braün assumed that center of gravity is independent of the at high altitudes, the aerodynamic translational motion. The problem of the forces on a spinning airplane would be rotational motion can therefore be stud- small compared to the inertial forces. As ied independently of the forces acting at a result, the equations with no external the center of gravity of a body. moments appeared to be a good start- ing point for the studies of such spins. Although the rotation of a rigid body in With the advent of space flight, bodies the absence of all external moments in space outside the Earth’s atmosphere would appear to be the first problem to experience extremely small external study in this field, the solution of this moments. The rotational motion of these problem is not well-known. In my gradu- bodies in space is therefore a problem ate course at MIT, Introduction to Theo- of practical importance. I also looked up retical Mechanics, I do not recall this further information in a book on classi- problem being mentioned. There are cal mechanics by Goldstein (ref. 3.2), on probably two reasons for this neglect. which the subsequent discussion is First, for bodies rotating on the Earth, based. there are always eternal moments Although the mathematical details of applied during the course of the motion. this problem are too complex to present A rather complex mounting system herein, a novel geometric solution of would be required to reduce these this problem obtained in 1834 by Louis moments to very small values. Actual Poinsot, a French mathematician, is freely rotating vehicles, such as base- presented. To describe this solution, a balls, airplanes, boats, etc. have rela- brief discussion of the method of speci- tively large moments applied by the fying the applicable characteristics of a surrounding air or liquid medium. Sec- rigid body is considered desirable. ond, the solution to this problem of motion with no external moments is very For studying the rotational motion of a complex, and little was to be gained by rigid body, the detailed distribution of the teaching this solution when no practical elements of mass in the body or the examples existed. shape of the body is not required. The

18 Monographs in Aerospace History Number 40—Journey Into Space Research Rotating Vehicles in Free Space

FIGURE 3.1. Illustration of motion of ellipsoid Ellipsoid of inertia of inertia during free rotation of body in space. (Taken from ref. 3.2.) Polhode Invariable plane

Herpolhode

Angular momentum vector

body may be described by the moments detailed distribution of mass elements of inertia about three mutually perpen- may have the same ellipsoid of inertia. dicular axes, with their origin at the cen- ter of gravity, called the principal axes of For any given ellipsoid of inertia, a solu- inertia. These values of moments of tion of Euler’s dynamical equations with the external moments set equal to zero inertia, represented as vectors IX, IY, allows the calculation of the motion fol- and IZ along the three principal axes, can be used to determine the axes of an lowing a disturbance in terms of elliptic ellipsoid, called the ellipsoid of inertia. integrals. A closed-form solution is thus The shape of the ellipsoid is determined available, but the calculations required by the mathematical formula for an ellip- are quite lengthy. Poinsot gave a very soid: ingenious method of visualizing the motion of a freely rotating body in space. During the motion, in which the x2 y2 z2 ++=1 body rotates about the center of gravity, a2 b2 c2 the ellipsoid of inertia rolls on a plane, called the invariable plane. Poinsot where a, b, and c represent the dis- called the curve on the invariable plane tances from the center of the ellipsoid to traced out by the point of contact of the its surface along the X, Y, and Z axes, ellipsoid and the plane the herpolhode, respectively. The ellipsoid of inertia is and the curve on the ellipsoid traced out defined by setting by the point of contact the polhode. Thus, the brief explanation of the motion, as quoted in Goldstein’s book aI=1/ bI=1/ cI=1/ x , y , and z on Classical Mechanics (ref. 3.2) is “The polhode rolls without slipping on the Vectors drawn from the origin to other herpolhode lying in the invariable plane.” points on the ellipsoid may be used to A sketch of this construction is shown in calculate the values of moments of iner- figure 3.1. tia about any other axes through the center of gravity by using similar formu- The assumption that no external las. Different bodies with different moments act on the body implies that

Monographs in Aerospace History Number 40—Journey Into Space Research 19 Stability and Control of Space Vehicles

there are no damping moments. A was not known to any other member of motion once started, therefore, will con- the committee. The design of the vehicle tinue indefinitely. The only steady rota- was stopped at this point for further tions occur when the body is rotating study. As a result, we avoided some about one of its principal axes, as seen embarrassment to Langley that might from the fact that the tip of the axis then have occurred had the design been pro- touches the invariable plane in just one posed to NASA Headquarters. point. In general, the motions started in any other manner would be highly oscil- A simple method of visualizing the ten- latory; that is, the principal axes would dency of a rotating body to rotate about oscillate through large angles with its axis of maximum inertia is to con- respect to a fixed reference system. sider that each element of mass in the Such oscillatory motions would be body experiences a centrifugal force very undesirable for most practical tending to pull it into the plane of rota- applications. tion. The body as a whole will then tend to move to a condition in which its ele- A question also arises as to the stability ments are as close as possible to the of the motion when it is started about plane of rotation. This tendency will one of the principal axes. The moments cause a pancake-shaped body, for of inertia may always be classified as example, to approach a condition in minimum, intermediate, and maximum. which the rotation is about an axis nor- If the body is rotating about the axes of mal to the plane of the pancake. With no minimum or maximum moment of iner- source of damping, however, the body, if tia, and is slightly disturbed, it will started in an orientation displaced from acquire a small oscillation or wobble. If it the plane of rotation, will overshoot this is rotating about the axis of intermediate orientation and continue to oscillate inertia, however, and is slightly dis- back and forth about it. turbed, it will immediately swing through a large angle and continue in a large- To illustrate these points further, I had amplitude oscillation. The rotation about a model constructed, sketched in fig- the axis of intermediate inertia may ure 3.2, in which a flat slab of aluminum therefore be considered statically unsta- was pivoted on a universal joint and ble and is unsuitable for a vehicle could be rotated about a fixed axis by intended to perform a steady rotation. turning a handle. The aluminum slab In the early days of the space program, represented the dynamic characteristics there was considerable discussion of of a rotating space station. If the rotation the design of rotating space stations were started with the plane of the slab intended to provide a centrifugal force in the axis of rotation, a condition of on the bodies of the astronauts to simu- rotation about the intermediate axis of late the gravitational force existing on inertia, the slab performed a large- Earth. Once, in a meeting of one of amplitude oscillation. The oscillation the research coordinating committees damped out fairly quickly because of air chaired by Eugene Draley, an Assistant resistance and bearing friction, and the Director of Langley at that time, a pre- slab ended up with its plane normal to sentation was made for the design of the axis. In space, where the damping a rotating space station that rotated forces are much less, such an oscilla- about its axis of intermediate inertia. I tion would continue much longer. If the pointed out that the vehicle would be rotation is started with the plane of the unstable as a result of the consider- slab perpendicular to the axis of rota- ations described previously. This fact tion, corresponding to rotation about its

20 Monographs in Aerospace History Number 40—Journey Into Space Research Rotating Vehicles in Free Space

FIGURE 3.2. Model used to illustrate stability of rotation of body in space. (a) Rotation about axis of intermediate iner- tia. When rotation is started with the plate in this position, it immediately swings through the position shown in part (b) and continues to oscillate back and forth until the oscillation damps out. (b) Rotation about axis of maximum inertia. When rotation is started with the plate in this position, it remains in this orientation during the rotation.

axis of maximum inertia, the steady change to a rotation about the axis of rotation continues. maximum inertia, because this motion, for a given angular momentum, has the The theory discussed so far assumes a least energy. Detailed consideration of rigid body. All space vehicles, in prac- how damping forces act on various tice, have some flexibility, or some mov- parts of the body might be extremely able parts. When the body distorts, or complicated, but the end result is when parts move, some energy is dissi- always the same. The body assumes a pated by internal damping or friction. steady rotation about the axis of maxi- This energy can come only from the mum inertia. This condition results even rotating body itself. when the body starts rotating about its Newton’s laws state that the angular axis of minimum inertia, a stable condi- momentum of a body will remain con- tion for a rigid body. So far as I know, stant in the absence of external this result has been known to designers moments. If energy is dissipated of satellites launched by the United through internal causes, however, the States, and in some cases intentional nature of the motion must change. If damping devices have been installed to the rotating motion continues for a speed the settling down to a steady long time, the motion will eventually rotation about the axis of maximum

Monographs in Aerospace History Number 40—Journey Into Space Research 21 Stability and Control of Space Vehicles

inertia. Goldstein, in his book, Classical For convenience, the moments are des- Mechanics, however, states: “This fact ignated as rolling, yawing, and pitching was learned the hard way by early moments in a way analogous to the designers of spacecraft.” In other words, usual definitions for aircraft. Thus, if a there must have been some spin- satellite is in an orbit, a rolling moment stabilized satellite that was launched tends to rotate the satellite about a hori- spinning about its axis of minimum iner- zontal axis in the orbital plane, a yawing tia and was intended to continue spin- moment about a vertical axis in the ning in this manner, but which, because orbital plane, and a pitching moment of internal energy dissipation, ended in about an axis normal to the orbital a flat spin about its axis of maximum plane. inertia.

Gravitational and Centrifugal Moments Acting on a Moments Satellite Consider first the effects of gravity and Moments acting on a satellite may come centrifugal force on the pitching from the following sources that are moments of a satellite in an orbit about inherent in the environment or motion of a planet. A dumbbell-shaped satellite is the vehicle. These moments are in addi- used as an example. The dumbbell con- tion to those supplied intentionally by sists of two equal concentrated masses mechanisms such as jets, gyroscopes, connected by a weightless rod. This or inertia wheels. configuration is used because it experi- ences the greatest gravitational and 1. Gravitational fields in space centrifugal moments of any body of the 2. Centrifugal force on parts of the same total weight and size. satellite If such a body is aligned with the direc- 3. Magnetic fields in space tion of flight and rotated to various 4. Radiation pressure angles about an axis normal to the 5. Aerodynamic forces orbital plane (a pitching motion), the centrifugal forces due to the angular In the early days of the Space program, velocity of orbital rotation may be shown I presented a lecture on these moments to be zero, but the gravitational forces to members of the Flight Research Divi- tend to hold the body in a vertical atti- sion as part of the Pearson lecture tude. This effect is usually called the series. Information on these topics was gravity gradient effect; it results because obtained from then current technical the lower mass of the dumbbell is papers. Since then, textbooks and nearer to the center of the planet than reports that discuss these subjects in the upper mass. The gravitational attrac- greater depth have become available tion, which varies inversely as the (ref. 3.3). These texts, however, neces- square of the distance from the center sarily present derivations involving of the planet, is therefore greater on the rather complex mathematics. To avoid lower mass. The moment tending to the need for such derivations, I will con- align the dumbbell vertically is zero fine this presentation to discussion of when the dumbbell is horizontal or verti- the illustrative examples worked out in cal and reaches a maximum at a pitch my lecture. angle of 45°.

22 Monographs in Aerospace History Number 40—Journey Into Space Research Gravitational and Centrifugal Moments

The rolling moment on a dumbbell- To give an idea of the small magnitude shaped body is discussed for the case of these moments, the period of oscilla- in which the body is aligned normal to tion of the dumbbell about its equilib- the orbital plane and is rotated to vari- rium attitude in orbit may be calculated. ous angles about a horizontal axis in the The period is given as a fraction of the orbital plane. The analysis for the gravi- orbital period. tational effects is identical to that for the pitching moments, but in this case, the 1 centrifugal effects due to orbital angular Pitch velocity also tend to align the dumbbell 3 with its long axis vertical. The magni- tude of the rolling moment due to cen- 1 trifugal forces is 1/3 that due to the Roll gravity gradient. As a result, the total 2 rolling moment is 4/3 that of the pitching moment. Yaw 1 . 0 The orbital period of a satellite in a low The yawing moment on a dumbbell- Earth orbit is about 90 minutes; there- shaped body is discussed for the case fore, the period in minutes for these in which the body is aligned with its long three cases is axis horizontal and is rotated to various angles about a vertical axis. In this Pitch 51.96 case, gravity gradient effects are zero, but the centrifugal forces create a Roll 63.6 moment tending to align the body with the flight direction. The magnitude of Yaw 9 0 this effect is the same as for the rolling The periods for any other satellite would moment, that is, 1/3 the magnitude of be longer than those of the dumbbell- the pitching moment. This effect is shaped body considered. caused by the tendency, discussed pre- viously, of all elements of mass of a Another comparison of the magnitude of rotating body to move into the plane of these effects may be made by compar- rotation. ing the periods of the motion with those of a more familiar vehicle such as an air- All pitching, rolling, and yawing plane. For a typical light airplane, the moments on a satellite do not really period of the lateral oscillation in the depend on the fact that the satellite is in cruise condition would be about 3 sec- orbit. A body on the surface of the onds. For a dumbbell-shaped satellite Earth, located on the equator and expe- with the same moment of inertia in yaw riencing the Earth’s rotation, would feel as the light airplane, the period of the the moments from the same sources. lateral oscillation would be 90 minutes These moments are so small, however, or 5400 seconds. For a given moment of inertia, the restoring moment in yaw var- that only in the weightless condition of ies inversely as the square of the space are they noticeable. On Earth, it period. The restoring moment on the would be very difficult to mount the body body in orbit is therefore (3/5400)2 or on a bearing supporting its weight that 3.1 × 10–7 times as great. would be sufficiently close to its center of gravity or sufficiently frictionless to The very small magnitude of the avoid masking these effects. moments acting on a vehicle in space

Monographs in Aerospace History Number 40—Journey Into Space Research 23 Stability and Control of Space Vehicles

requires the development of new con- needed. This problem can be solved cepts for stability and control. Despite analytically only for simple geometric the very small magnitude of moments shapes, such as cylinders or spheres produced by gravity gradient effects, (ref. 3.4). As an example, consider an such moments may be used as the aluminum cylinder that is long com- basis of a stabilization system for a sat- pared to its radius spinning about its ellite. Usually, such moments are used long axis in a low Earth orbit. If the rota- to keep an elongated vehicle in a verti- tion is such as to cut the magnetic lines cal orientation, to keep an antenna or of force at right angles, the rotation is sensor pointed at the Earth. In addition shown to damp to half its initial angular to the restoring moment provided by the velocity in about 3.1 days. To determine gravity gradient, damping devices must the effect of the Earth’s field on an be used to damp out oscillations about actual spinning satellite, however, the equilibrium attitude. Without the pro- experimental measurements are usually vision of damping, oscillations would required. The accuracy of these mea- continue indefinitely. surements may be increased and the time for the test may be greatly reduced by spinning the satellite in an artificial magnetic field many times the strength Magnetic Moments of the Earth’s field. If the actual satellite is too large to test in this manner, a Magnetic moments may arise from the scale model simulating the inertial and interaction of the Earth’s magnetic field electrical characteristics of the full-scale with a permanent magnet or other mag- device may be used. netized material on a satellite, or from eddy-current damping of a rotating sat- ellite made of conducting material. The interaction of the Earth’s field with a Effect of Radiation Pressure magnet on a satellite in orbit is similar to that of the Earth’s field on a compass Frequently, persons unfamiliar with needle. This problem is discussed in space research do not realize that sun- many available textbooks and is there- light shining on a surface exerts a pres- fore not considered further herein. sure. The magnitude of this pressure on a black surface normal to the incident If any conducting body is rotated in a radiation is magnetic field, currents are induced in the body. These currents generate mag- PWc= netic flux that interacts with the original magnetic flux to produce a torque. The where W is the intensity of the incident torque is always in a direction to oppose energy and c is the velocity of light. This the rotation. The energy required to pressure may be approximately doubled maintain the rotation is lost in the form by the use of an aluminized surface to of heat generated by the ohmic resis- reflect the radiation. At the Earth’s tance to the current in the body. radius, W =×13. 106 ergs/s cm2 and The moments due to eddy-current 10 damping are quite important in many c =×310 cm/s. Hence P for a reflect- satellite applications because of the −4 ing surface is 0. 866× 10 dyne/cm2. practice of spinning bodies to provide attitude stabilization. An estimate of the A moment tending to point a satellite time for the original rotation to decay is towards the Sun may be obtained by

24 Monographs in Aerospace History Number 40—Journey Into Space Research Effect of Aerodynamic Forces

equipping the satellite with a fin of erned by aerodynamic effects existing in reflecting material supported on a light a flow condition called Newtonian flow. structure. The moment provided by such The laws for aerodynamic forces in a fin varies as the sine squared of the Newtonian flow are equivalent to those angle between the sunlight and the fin. for radiation pressure. The density of the The fin is therefore quite ineffective at atmosphere falls off continuously with small angles of deviation. This problem increasing altitude. A point of interest is may be overcome by using a pair of fins the altitude at which the impact pres- in the form of a V. The maximum effect sure due to this rarified gas on a satel- is obtained by setting each fin at an lite traveling at orbital speed would be angle of 45°. equivalent to the radiation pressure. A rough calculation indicates that this The magnitude of the restoring moment condition exists at an altitude of about provided by such an arrangement may 420 miles. The satellite with V-shaped be illustrated by the following example. fins discussed previously, flying at this Consider a 20-inch-diameter satellite altitude in the shadow of the Earth equipped with a V-shaped pair of fins 2 would have the same oscillation period each 1 m . Assume the following char- due to aerodynamic forces alone acteristics: (13.5 minutes) as that of the satellite exposed to solar radiation in a com- Moment of inertia in 2 × 106 gm cm2 plete vacuum. At lower altitudes, the yaw period due to aerodynamic forces Distance from center 75 cm would decrease until at an altitude of of gravity to centroid 200 miles; the period would be about of V 48 seconds. At an altitude of 420 miles, assuming a The period of small oscillations about nearly circular orbit, the loss in altitude the equilibrium position is 13.5 minutes. per orbit due to aerodynamic drag on The period is a much smaller fraction of the fin-stabilized satellite would be only the orbital period than that obtained by 0.0058 miles (30.6 feet) while at an alti- gravity gradient or centrifugal effects. tude of 200 miles, the loss would be 1.47 miles. These figures illustrate that The effect of radiation pressure acting aerodynamic forces can have a large on large, light “solar sails” has been effect on the angular motions of a satel- studied as a means of propulsion in lite while they are still small enough to space. The acceleration produced by have a minor effect on the trajectory. this method is small, but because of the lack of aerodynamic resistance in Another application of this effect is space, large changes in velocity may be the ability of aerodynamic forces to obtained by allowing this force to act align an entry vehicle in the correct over a long period of time. direction before aerodynamic heating and deceleration become large. For example, consider a vehicle weighing 4000 pounds and having directional sta- Effect of Aerodynamic bility equivalent to a fin area of 18 feet2 Forces acting at a moment arm of 3 feet from the center of gravity. If the vehicle enters The forces and moments acting on an the atmosphere at a sideslip angle object in the rarified atmosphere at of 90°, the aerodynamic forces at an high altitudes above the Earth are gov- altitude of 350,000 feet will be sufficient

Monographs in Aerospace History Number 40—Journey Into Space Research 25 Stability and Control of Space Vehicles

Part of spacecraft FIGURE 3.3. Sketch of drag device proposed for causing an orbiting Bobbin spacecraft to enter Earth’s atmosphere in case of failure of retro- Vanes rocket. Many small Side view vanes could produce required drag.

Top view

to produce an angular acceleration of thought was that an ordinary parachute about 0.5° per second2. The vehicle might fail to open because of the small would pick up an angular velocity of dynamic pressure. In the device pro- 5° per second in 10 seconds and would posed, a series of curved vanes would pass through zero sideslip in about be attached to thin cables wrapped 20 seconds. Other studies have shown around a bobbin. The bobbin would be that the aerodynamic heating rate does expelled from the vehicle by a spring or not start to build up, at least in a flat a small explosive charge. As it traveled entry (flight path angle less than –3° at back, the vanes would unwind from the 300,000 feet) until about 100 seconds bobbin, forming a train long enough to after this point. The deceleration does add the required amount of drag. Gyro- not build up until a later time. For suc- scopic effect would keep the bobbin cessful alignment of the vehicle, how- itself in a stable attitude, and it would fall ever, artificial damping would probably free at the end of the deployment. This have to be provided; otherwise, it would device would allow placement of the oscillate back and forth through a large train of drag vanes in a straight line amplitude with very little damping. behind the vehicle without relying on the very small aerodynamic drag to stretch At the time the Mercury capsule was out the thin cables and vanes. I made a under consideration, Mr. Robert Gilruth, small model of the device using a bob- then Chief of PARD, expressed concern bin for sewing thread and vanes of that if the retro-rocket on the capsule paper. The model illustrated the princi- failed to fire, the capsule might be left in ple successfully. The idea was never orbit so long that the oxygen and other used, possibly because the attitude con- supplies onboard the vehicle would be trol rockets could serve as a backup to exhausted. I proposed a device that the retro-rocket. could be deployed to add a sufficiently large amount of aerodynamic drag on The methods mentioned previously for the vehicle to cause it to enter the atmo- applying moments to satellites are usu- sphere in a reasonable time. A sketch of ally slow acting and limited to applica- this device is shown in figure 3.3. My tions requiring small moments. For

26 Monographs in Aerospace History Number 40—Journey Into Space Research Effect of Aerodynamic Forces

applications requiring larger moments, until its axis is in alignment with the axis rockets or flywheels may be used. Both about which the moment is applied. At of these techniques have a limited total this point, the gyroscope is said to be impulse. For example, the rocket can saturated and will no longer resist the be used until its fuel is used up, after applied moment. which it can no longer provide a moment. The flywheel can be spun up A combination of a gyroscope and a by a motor, producing a moment on the rocket may be used so that when the satellite in the opposite direction. The gyro is approaching saturation, a motor turns the flywheel faster and moment may be applied to the satellite faster until it reaches its limiting speed with a rocket to precess the gyro so that or the flywheel fails due to excessive it its gimbals rotate back to the original centrifugal stresses. Alternatively, the orientation. In this way, the gyro may flywheel can be run at a constant speed continue to be used until the rocket fuel and mounted in gimbals like a gyro- is exhausted, or perhaps some other scope. A moment applied to an axis per- slower acting source of torque may be pendicular to the axis of rotation will used to desaturate the gyro. The gyro- then cause the gyro wheel to precess, scope method, in some cases, has an while the gimbal to which the moment is advantage that its ability to hold the sat- applied will not move. As a result, an ellite in the desired orientation is practi- opposite moment will be applied to the cally unlimited except for effects of satellite. The gyro wheel will precess structural flexibility.

Monographs in Aerospace History Number 40—Journey Into Space Research 27

CHAPTER 4 Rendezvous

Rendezvous of a spacecraft with a tar- In an effort to overcome the perception get vehicle means maneuvering the that the Russians were ahead of the spacecraft in such a way that it comes in United States in space research, and, close to the target vehicle with small or by implication, in all scientific and tech- zero relative velocity. Docking, a maneu- nical developments, president John F. ver that often follows rendezvous, Kennedy, on May 25, 1961, made his means that the spacecraft is attached to bold commitment that the United States the target vehicle to allow transfer of should place a man on the Moon within equipment or personnel. a decade. This extraordinarily difficult task required new developments in In this chapter, three aspects of my work many fields. As it turned out, rendez- on rendezvous are discussed. From the vous was an important consideration in earliest days of the space program, the two of the three mission plans that were importance of rendezvous in conducting considered. The three mission plans space operations was realized. If the were direct ascent, Earth orbit rendez- space program had been influenced pri- vous, and lunar orbit rendezvous. The marily by scientific and technical think- part played by engineers in my division ing rather than by political considera- and by others at the Langley Research tions, a logical development would have Center in reaching the decision to use been to test some small manned satel- the lunar orbit rendezvous technique is lites and then place a space station in described in this chapter. orbit. Such a space station would have allowed much basic research on prob- The third subject discussed in this chap- lems of space flight with application to ter is the development of the actual later missions to the Moon or planets. guidance and navigation systems used Obviously, rendezvous of supply ves- in the Apollo mission. Although the orig- sels with the space station would have inal Langley research was effective in been necessary to conduct these oper- convincing officials of the space pro- ations. Many researchers in the space gram that the lunar orbit rendezvous program made studies to determine mode was the correct technique to use, how a spacecraft should be controlled in later developments showed that the a rendezvous maneuver. A brief review original Langley concepts were oversim- will be given of the work that I did in this plified, and that much more sophisti- field. cated methods were used in the actual

Monographs in Aerospace History Number 40—Journey Into Space Research 29 Rendezvous

Apollo mission, both in the theoretical the vehicles in different orbits, requiring developments and in the design of hard- the application of more complex varia- ware. tions of force to perform the desired maneuver.

If the vehicles are in sufficient proximity Effect of Orbital Mechanics and are moving with small relative velocity, differences in acceleration In the early stages of the space pro- caused by differences in gravitational or gram, I and many other researchers centrifugal force are small. In this case, made studies to determine how a the maneuver required to perform a ren- spacecraft should be controlled to per- dezvous is only slightly different from form a rendezvous maneuver. Because that required in space far from the influ- of the relatively short time usually ence of heavenly bodies. As the relative allowed for such a maneuver, the velocity of or displacement between the spacecraft is assumed to be maneu- vehicles increases, the required maneu- vered by use of jets or rockets that allow ver becomes different. My initial studies control of rotation about three axes and were concerned with calculation of the control of velocity along three axes. paths required of the rendezvous vehi- cle at larger values of the displacement If two vehicles are in the vacuum of and relative velocity. space and are sufficiently far from other heavenly bodies that the effects of grav- Although the derivation of the equations itational fields of these bodies are negli- for the motion of the vehicle requires the gible, then rendezvous is a relatively use of mathematics, in this presentation simple procedure. In a two-dimensional an attempt is made to give only the analogy, it would be much like one method involved. The equations for the skater on ice maneuvering to catch up orbit of a body around a planet may be with and move along with another set up considering the initial conditions skater. Obviously there are many paths of the body and the acceleration due to that could be followed in such a maneu- the gravitational field of the planet. Add- ver. Other constraints would have to be ing small increments to the variables in imposed to specify a definite maneuver, these equations gives the equations of such as the time for the maneuver, the a second vehicle that can be considered energy (or fuel) expended, or vehicle the rendezvous vehicle. Subtracting the direction and motion as limited by visual first set of equations from the second or other constraints. then gives the equations describing the relative motion of the rendezvous vehi- In maneuvers within the solar system, cle and the orbiting vehicle. Readers the effects of gravitational fields of the familiar with differential calculus will Sun and planets are always of some realize that this procedure incorporates importance. In most practical cases, the derivation of the methods of calcu- both vehicles are in orbit about the lus. The procedure may be simplified same planet. If the two vehicles are at using the methods of calculus by simply the same altitude and speed, but dis- taking the differential of the original placed in different positions in a circular equation. orbit, they continue to move in the same relative position because both The resulting equations are nonlinear vehicles are subject to the same gravita- because the gravitational force varies tional acceleration. Any attempt to per- inversely as the square of the distance form a rendezvous, however, places from the center of the planet. In general,

30 Monographs in Aerospace History Number 40—Journey Into Space Research Simulation Experiments

there is no closed-form solution of these was one of the first simulation studies equations. A closed-form solution is one conducted, and because it had an that can be expressed in terms of important bearing on subsequent devel- known functions, such as trigonometric opments in the space program. functions or other tabulated functions. To obtain a closed-form solution, the The purpose of the simulation was to variables may be expanded in power investigate whether a spacecraft pilot series and terms higher than the first could rendezvous with a target vehicle order may be neglected. If the relative by observing the illuminated vehicle displacements and velocities of the against a star background. Simulation vehicles are sufficiently small, the equipment at that time (about 1959) did higher order terms will be very small not have the advantage of computer- and the solution of the linearized equa- generated displays and digital computer tions will be reasonably accurate. solutions of vehicle dynamics that were available in the nineties. Considerable As explained in the book Journey in ingenuity was required to provide a real- Aeronautical Research (ref. 1.1), the lin- istic simulation in a short time at low earized equations for the motion of an cost. airplane for small disturbances are highly accurate because the aerody- A planetarium can provide a visual namic forces and moments vary linearly scene of the stars and heavenly bodies, with the displacements and velocities but planetarium projectors and spherical in the range used in normal flight. It screens of the type used in museums is natural, then, that I should try the and observatories are quite expensive. same method in the problem of rendez- Max Kurbjun and other engineers in my vous. Many other research groups division provided the necessary spheri- derived these linearized rendezvous cal screen by obtaining a hemispherical equations about the same time that I inflatable radome, about 50 feet in diam- did. A paper by W. H. Clohessy and eter, of the type that was used for pro- R. S.Wiltshire of the Martin Company, tecting radar equipment in Alaska. A containing these equations, was pre- sufficiently complete star background sented in June 1959 (ref. 4.1). As a was provided by using a point light result, these equations were usually source to project beams of light through referred to as the Clohessy-Wiltshire small lenses, providing simulated equations, although these authors stars on the inside of the radome. Origi- acknowledge that an earlier paper con- nally, the simulation was performed taining these equations was brought to under conditions of free space, without their attention after the presentation of the effects of gravitation of a nearby the paper. My work was never published planet. Later, an early type of analog because of the earlier appearance of computer was obtained that allowed these other papers in the literature. including the effects of orbital mechan- ics as represented by the Clohessy- Wiltshire equations. Simulation Experiments Carrying out a rendezvous in space was a maneuver that had not been A complete chapter devoted to simula- attempted at that time, and many people tion of space operations will be pre- expressed doubt that such a maneuver sented subsequently. At this point, would be sufficiently safe to be practical. however, some early work on simulation The main result of the simulation study of rendezvous is introduced because it was to show that the rendezvous could

Monographs in Aerospace History Number 40—Journey Into Space Research 31 Rendezvous

be made quite simply with a minimum of lier had been working on stability and guidance equipment or instrumenta- control of aircraft. tion. These runs were made at relatively The Apollo mission could be controlled close range, so that the visual cues by radio guidance from the ground or by played an important role providing the the astronauts onboard the vehicle by required information to the pilot or astro- using the navigation instruments pro- naut. Furthermore, the rendezvous sim- vided. The main reason for the onboard ulator provided a convenient tool for navigation capability was to give the demonstrating such a maneuver to astronauts a guidance capability when other personnel involved in the space the vehicle was on the back side of the program. Moon and as a backup in the unlikely failure of the radio guidance system. Both the LM and the CM were provided Implementation of with onboard computers of an unusual and advanced (for that time) design that Rendezvous in Apollo provided a high degree of reliability. Program These computers were hard-wired to solve the equations representing the orbital motions of the vehicles during The Lunar Orbit Rendezvous mode was the rendezvous operation as well as in selected as the design basis of the all the other phases of the mission. Apollo Mission. In this mode, the Lunar Module (LM) must take off from the The lunar orbit rendezvous technique Moon and rendezvous and dock with requires that the LM take off from the the Command Module (CM) that is in Moon and rendezvous and dock with orbit around the Moon. Some of the the CM that is in orbit around the Moon. lengthy arguments and conferences At the time, other engineers in the Aero- involved in selecting this mode of opera- space Mechanics Division at Langley tion are discussed later. For the present and I were deriving the rendezvous purpose, a brief comparison is made equations, we were unaware that the between the methods employed in exact solution of these equations had the simulation described previously and been obtained over two centuries ago the actual guidance and navigation by famous mathematicians, including techniques used in the Apollo mission. Carl Frederich Gauss, Joseph-Louis These methods are discussed in detail Lagrange, and Johann Heinrich in the excellent book by Richard H. Lambert. In fact, the equations for deter- Battin (ref. 4.2). mining the orbit of a body to go from one given point in space to another given The Apollo mission managers selected point with a specified transfer time are the Draper Lab in Cambridge, Massa- known as Lambert’s equations. As chusetts to design the Apollo guidance pointed out previously, these equations and control system. This organization cannot be solved in closed form, but the had extensive previous experience in early mathematicians had derived itera- development of inertial navigation sys- tive solutions to solve the equations to tems and in missile guidance. The engi- any desired accuracy. Such iterative neers in this organization obviously solutions, improved by modern develop- had much more experience in the ments, were programmed into the com- design of sophisticated guidance equip- puters used in the Apollo mission. ment than the Langley engineers in Inertial platforms used in conjunction my division, who just a year or so ear- with radar equipment were used to

32 Monographs in Aerospace History Number 40—Journey Into Space Research Implementation of Rendezvous in Apollo Program

measure the relative positions of the future positions of the vehicle and its vehicles. Sextants operated by the deviation from the planned trajectory. astronauts were used to determine the One young engineer in my division, location of the vehicles with respect to Robert Collins, had learned enough in the Earth, Moon, and other reference his college studies to know of Lambert’s objects. Thus the rendezvous navigation equations. He devised an iterative tech- was performed in a highly sophisticated nique of solution, using the Langley manner, taking into account all the grav- computer complex, to solve the rendez- itational influences of nearby heavenly vous problem using Lambert’s equa- bodies and allowing accurate calcula- tions. This solution brought the tion of rendezvous trajectories of any possibility of an exact solution to the length or orientation. attention of Langley engineers but was too late to influence the Apollo guidance The same equations and navigational system. The experience gained in the equipment were used in all phases of early simulation studies proved very the Apollo mission, such as insertion helpful to the Langley engineers who into translunar orbit, midcourse correc- later joined the Space Task Group and tions in all phases of the mission, inser- took part in the actual design of the tion into lunar orbit, and so on. Many of Apollo system. For example, Walter J. these mission phases required taking Russell, who operated a small analog into account gravitational influences of computer in the Langley rendezvous the Sun, Earth, and Moon. In addition, studies, was later placed in charge of all the locations of over 50 navigational analog computer equipment at the stars, as well as the corrections to the Johnson Space Center. John Mayer, position of the Earth and Moon when one of the Langley engineers who par- using the sextant to track the horizons, ticipated in the lecture series conducted rather than the centers of these bodies, by Henry Pearson, was later in charge were all programmed into the Apollo of all digital computing equipment at computers. In using these data, the the Johnson Space Center. John M. astronauts were required to punch into Eggleston, who made studies of optimal the computer on a keyboard the data rendezvous at Langley, participated in obtained in celestial readings. The com- the design of the Apollo control system puter would then automatically solve the and held important administrative posi- equations to obtain the present and tions at the Johnson Space Center.

Monographs in Aerospace History Number 40—Journey Into Space Research 33