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I» *4-4 ^ -4 - -4 ^-4 .4 35V, 7 3 «B.^£ 4< f*DK A n A/o C - -4 tLlOT-0* 7r l ° S >4-4 K it �41 . ! �� Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Gerald Wittenstein Collection Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Identifier An unambiguous reference to the resource within a given context sdsp_skyl_000065 Title A name given to the resource "SKYLAB REENTRY LOG July 8 ⟶ July 11." Description An account of the resource Pages 14 to 159 are unused. Creator An entity primarily responsible for making the resource United States. National Aeronautics and Space Administration Date A point or period of time associated with an event in the lifecycle of the resource 1979-07-11 Temporal Coverage Temporal characteristics of the resource. 1970-1979 Subject The topic of the resource Skylab Program Skylab 1 Reentry Type The nature or genre of the resource Notebooks Notes (Documents) Text Source A related resource from which the described resource is derived Gerald Wittenstein Collection Box 8, Folder 4 University of Alabama in Huntsville Archives and Special Collections, Huntsville, Alabama Language A language of the resource en Rights Information about rights held in and over the resource This material may be protected under U. S. Copyright Law (Title 17, U.S. Code) which governs the making of photocopies or reproductions of copyrighted materials. You may use the digitized material for private study, scholarship, or research. Though the University of Alabama in Huntsville Archives and Special Collections has physical ownership of the material in its collections, in some cases we may not own the copyright to the material. It is the patron's obligation to determine and satisfy copyright restrictions when publishing or otherwise distributing materials found in our collections. Relation A related resource Skylab Document Scanning Project Metadata Skylab 50th Anniversary https://digitalprojects.uah.edu/files/original/201/14401/sdsp_skyl_000066_001.pdf f209bcd1b458b451f84b4da9bef890fe PDF Text Text MSFC'S ROLE IN THE SKYLAB REACTIVATION MISSION INTRODUCTION ide 1 On July 11, 1979, Skylab impacted the Earth surface. The debris dispersion area stretched from the South Eastern Indian Ocean across a sparsely populated section of Western Australia. This presentation discusses in some detail the events leading to the reentry of Skylab and the role the Marshall Space Flight Center played in these events. BACKGROUND On February 9, 1974, Skylab systems were configured for a final power down and Skylab was deactivated. Prediction of solar cycle 20 activity, the solar cycle predicted to begin in 1977, indicated that the final attitude in which Skylab was left, the gravity gradient attitude, would result in a potential storage period of 8 to 10 years. However, in the fall of 1977 it was determined that Skylab had left the gravity gradient attitude and was experiencing an increased orbital decay rate. This was a result of greater than predicted solar activity at the beginning of solar cycle 21. This increased activity increased the drag forces on the vehicle. Skylab was not predicted to reenter the Earth's atmos­ phere in late 1978 or early 1979 unless something was done to reduce the drag forces acting upon it. It was necessary to make a decision to either accept an early uncontrolled reentry of Skylab or to attempt to actively control Skylab in a lower drag attitude thereby extending its orbital lifetime until a Shuttle mission could effect a boost or deorbit maneuver with Skylab. �2 0 S^3e 1A (leave up) In order to verify what options could be accomplished with the onboard Skylab systems, a small team of NASA engineers went to the Bermuda Ground Station to establish communications and interrogate Skylab systems. On March 6, 1978, the Airlock Module (AM) command and telemetry (TM) systems were commanded on from the Bermuda Ground Station. The reception of t"he AM TM carrier at Bermuda was proof that the onboard AM system had responded to the commands. For the next several days, the AM electrical power system was properly configured and the AM batteries charged whenever the simultaneous conditions of ground station coverage at Bermuda and solar energy availability permitted. Subsequently AM power was trans­ ferred to the Apollo Telescope Mount (ATM) systems and the operational status of the ATM systems was determined. On March 11, 1978, power was ^applied to the ATM Attitude and Pointing Control System (APCS) bus which in turn supplied power to the primary Apollo Telescope Mount Digital Computer (ATMDC)/Workshop Computer Interface Unit (WCIU). Power was maintained on the APCS bus for approximately 5 minutes and the receipt of ATMDC telemetry data at Bermuda was confirmed. The receipt of fcbi'fiC data~irid±cated thatrthe primary ATMDC/WCIU hardware and attendant .soft­ ware were operational and cycling. On March 13, 1978, engineers concluded the interrogation test on Skylab. The resulting data indicated no discernible degradation of the Skylab systems during its 4 years of — orbital storage. Aided with this data,(the knowledge that Skylab was in an unstable tumble prompted investigation into schemes which might extend the orbital lifetime of Skylab. # �o 3 <§) Scheme Development and Operational Modes March 1978 to July 1979 The first scheme investigated was to use the onboard Thruster Attitude Control System (TACS) to maintain a quasi-stable tumble of Skylab. However, it was soon determined that this option would not extend Skylab lifetime sufficiently to correspond to the operational readiness of the Space Shuttle for a possible reboost or deorbit mission. The only alternative left was to reactivate and continuously control the Skylab in a minimum drag attitude. Slide 2 In order to accomplish this the End On Velocity Vector (EOVV) minimum drag attitude control scheme was developed by MSEC engineers..and-used-afher-bhe-4ntt-i-at- Skylab-reatrivatton- on .June—1-4-r—4-9-7#: The EOVV mode was a minimum drag attitude with the relatively small surface areas of the front or back ends of Skylab being pointed approximately along the velocity vector. This mode was a modification of the Z Local Vertical (ZLV) mode (vehicle "Z" axis along the local vertical) that was used during the original Skylab Mission. fe-rte^e¥V-™rfer^^«W^^rdinat«-axe8 were r,ffs«^iightiy^om_theJLV^xes-d:o-align-the- vehicle-principle ^ax^S—with—the-ZLV-axes-*—lhajzehicl-e-was—theirToiled—such-that-i-ts soLar—amays—poiinred-toward—the—sun near"orbttTn~n7ToTrWTaa^imum power—eoHectrftra—and—att-ltude—reference—updatir.gr—Desaturation oiL-GMG—momentum—was done-with- gravity gradient torques-and-was _continuously~active ground-the~orbx"t. �T h e r e w e r e two s u b s e t s o f t h e EOVV mode, EOVV A a n d EOVV B. The EOVV B mode c a n b e t h o u g h t o f a s a b a c k w a r d EOVV A mode. The EOW A mode h a d t h e S k y l a b Command M o d u l e d o c k i n g p o r t p o i n t e d a l o n g t h e v e l o c i t y v e c t o r w h i l e t h e EOW B mode h a d t h e aft end of the workshop pointed along the velocity vector. The EOVV B mode w a s d e v e l o p e d t o p r o l o n g t h e l i f e o f CMG # 2 by a l l o w i n g maximum s o l a r i m p i n g e m e n t o n CMG # 2 s p i n b e a r i n g s f o r n e g a t i v e s u n a n g l e s . F o r t h e same r e a s o n , EOW A was u t i l i z e d when t h e v e h i c l e e x p e r i e n c e d p o s i t i v e s o l a r a n g l e s . The EOVV mode w a s u s e d i n t h e f i r s t p a r t o f t h e S k y l a b R e a c t i v a t i o n M i s s i o n t o r e d u c e a s much a s p o s s i b l e t h e S k y l a b d e s c e n t r a t e . I t was h o p e d t h a t t h e o r b i t a l l i f e o f S k y l a b c o u l d b e e x t e n d e d u n t i l a r e b o o s t / d e o r b i t m i s s i o n by t h e S p a c e S h u t t l e c o u l d b e l a u n c h e d . T h e e f f e c t o f t h e EOVV mode o n t h e d e s c e n t r a t e i s shown i n t h e n e x t v i e w g r a p h ( S l i d e 3 ) T h e r e was a n o t i c e a b l e s l o w i n g down o f t h e S k y l a b f a l l when EOVV was e n t e r e d J u n e 1 1 , 1 9 7 8 . I t was e s t i m a t e d t h a t i f S k y l a b h a d r e m a i n e d i n EOVV t h a t r e e n t r y c o u l d h a v e b e e n d e l a y e d u n t i l a t lide 3 lease April of 1980. Referr-in-g--to—slxde—1-A~,—the—transition - f r o m a-.no...control- -to a c o n t x a l i e d - T o o d e - C s o l a r i n e r t i a l ) — o c c u r r e < i ~ J u n e ~ £ . -^Tlris -was -feTJ-owed—two—days—1-a t er"wh en—th-e—t-r an sifion-t o - -the- EOW-A at-titude—oc-euired~. 1/ IA It should be noted that transitions from the o l d S k y l a b o p e r a t i o n a l modes ( S I , ZLV, e t c . ) t o t h e s e new o p e r a t i o n a l ¥ a Or "— 11 0: (^ | rt) m o d e s ^ r e q u i r e d many h o u r s o f e q u a t i o n a n d s c h e m e d e v e l o p m e n t , s o f t ­ ware implementation and verification, the generation of computer �0 uplink loads and their verification, and finally close surveillance of vehicle operation once the operational mode was activated to insure that the vehicle responded as the theory p r e d i c t e d i t w o u l d . N u m e r o u s m e e t i n g s w e r e h e l d w i t h i n MSFC a n d J S C a n d a t NASA H e a d q u a r t e r s t o e n s u r e a l l l e v e l s o f NASA Management approved and agreed with proposed Skylab operations. For example, between January 1978 and the i n i t i a l Skylab SI acquisition on June 9, 1978, four Skylab presentations were made to Mr. Yardley, one to Dr. Frosch, and two to other headquarters personnel. , ±.t) , , -j ,v^ B.e£er°rl«^^ on June 28. ^ A J ^ a transition from SOW A to SI occurred This was necessary because of abnormal momentum s t a t e s w h i c h c o u l d l e a d t o CMC g i m b a l s t o p p r o b l e m s . T h e p r o b l e m w a s eventually traced to the inability of the strapdown updating scheme to compensate f o r the movement of the i n e r t i a l reference, particularly at high sun angles. When t h i s w a s u n d e r s t o o d , a strapdown update bias term was successfully used to compensate for the observed Z axis drift due to orbit plane and sun motion. On J u l y 6 the vehicle was again returned t o the EOW-A a t t i t u d e . On J u l y 9 t o t a l v e h i c l e c o n t r o l w a s l o s t i n c l u d i n g a t t i t u d e reference. The cause of t h i s loss of a t t i t u d e was due to a power f a i l u r e . The vehicle had been placed in a power configuration w i t h t h e AM b a t t e r i e s n o t o n t h e l i n e d u e t o t w o u n e x p l a i n e d b a t t e r y c h a r g e r f a i l u r e s i n t h a t s y s t e m . T h e ATM p o w e r s y s t e m a n d ^ t h e AM s o l a r a r r a y s w e r e n o t s u f f i c i e n t t o c a r r y t h e l o a d i n t h e EOVV m o d e c a u s i n g t h e ATM b a t t e r i e s t o a u t o m a t i c a l l y t r i p o f f line. As was done previously, i t was decided t o go from the the �a m T c e • ...J u n c o n t r o l l e d a t t i t u d e t o S I A (Uuly 1 9 ) a n d t h e n t o t h e EOVV-A attitude (July 25). \ — (X c-c -?1 c J EOW-A t o EOVV-B T r a n s i t i o n On November 4 , 1 9 7 8 , S k y l a b w a s m a n e u v e r e d 1 8 0 d e g r e e s f r o m i t s n o r m a l EOW o r i e n t a t i o n w i t h t h e MDA p o i n t i n g t o w a r d t h e p o s i t i v e v e l o c i t y v e c t o r (EOW-A) t o a new EOW o r i e n t a t i o n w i t h t h e MDA p o i n t i n g t o w a r d t h e n e g a t i v e v e l o c i t y v e c t o r ( E O W - B ) . The p u r p o s e o f t h i s m a n e u v e r was t o i n c r e a s e t h e p r o b a b i l i t y f o r extended Skylab lifetime by providing the most favorable thermal c o n d i t i o n s , i n EOW o p e r a t i o n , t o r e d u c e t h e s t r e s s o n CMG 2 . A n a l y s i s o f S k y l a b d a t a a t MSFC o b t a i n e d d u r i n g EOW o p e r a t i o n u p t o t h i s t i m e showed a r e l a t i o n s h i p b e t w e e n t h e o c c u r r e n c e o f CMG 2 a n o m a l i e s , t h e s u n a n g l e , a n d t h e o p e r a t i n g t e m p e r a t u r e o f CMG 1i d e 4 2 (Slide 4). This data indicated that the stress conditions on CMG 2 c o u l d b e a v o i d e d o r r e d u c e d b y p r o v i d i n g a h i g h e r o p e r a t i n g t e m p e r a t u r e e n v i r o n m e n t f o r CMG 2 . The EOW-A o r i e n t a t i o n p r o v i d e d m o r e s o l a r e x p o s u r e f o r CMG 2 a t p o s i t i v e s u n a n g l e s w h i l e a n EOW-B o r i e n t a t i o n w o u l d p r o v i d e m o r e s o l a r e x p o s u r e lide 2 a t n e g a t i v e s u n a n g l e s a s shown i n S l i d e 2 . The 1 8 0 ^ t r a n s i t i o n m a n e u v e r was s c h e d u l e d f o r e a r l y November 1978 to coincide with the upcoming positive-to-negative change in s u n a n g l e ( i . e . , s u n a n g l e movement f r o m N o r t h t o S o u t h o f t h e orbit plane). A modification to the flight software had to be d e v e l o p e d (SWCR-S4016, b u f f e r 1 5 ) a n d was i m p l e m e n t e d t o a c c o u n t f o r c o m p u t a t i o n a l d i f f e r e n c e s a s s o c i a t e d w i t h t h e EOW-A a n d �7 EOVV-B orientations and to automate maneuver sequencing to support the transition maneuvers. Slide 5 The transition maneuver sequence was developed and simulated, based on available station coverage (Slide 5) to minimize TACS utilization and to provide favorable conditions for initiation of EOVV-B operations. Normal and contingency procedures to support the transition maneuver were developed and executed from 11/2/78 through 11/4/78 with the transition maneuver taking place on 11/4/78 as the sun angle passed through zero. It should be noted that trh^P design and simulation effort enabled the maneuver plan to be executed for no TACS usage, saving this limited resource for future operations. Skylab remained in the EOW-B attitude from November 4, 1978 until January 25, 1979. During this time, all Skylab systems functioned satisfactorily. After Skylab was brought under active control, in a low-drag attitude to minimize its rate of decay, it was decided in mid-1978 to accelerate the development of an orbital retrieval system that might be accommodated on an early flight of the Space Shuttle, increasing chances of rendezvousing with Skylab. A proposed mission sequence with the Skylab boost/deboost 51ide 6 options is shown in Slide 6. �8 The rate of orbital decay, however, continued to increase due to increased solar activity. Skylab's onboard systems also showed signs of deterioration and there were increasing concerns over the Space Shuttle's schedule. For these reasons, the concept of Skylab recovery was terminated in December 1978. At that time, it was decided to reorient and control the vehicle in a solar inertial attitude which was the normal vehicle orientation for original Skylab mission operations. accomplished January 25, 1979. This was The vehicle remained in SI control until June 20 when TEA control was activated. TEA Control As Skylab's altitude decreased, the magnitude of the aerodynamic torques on the vehicle increased. Studies/indicated that vehicle control in the solar inertial orientation would no longer be possible below about 140 n.m. due to these increased aerodynamic torques and the limited control authority available from the Vehicle Control Moment Gyros (CMG's). Aerodynamicists and control engineers at the Marshall Space Flight Center (MSFC), while investigating vehicle orientations which produced minimal disturbance torques on the vehicle, found certain orientations where the summation of these disturbance torques was zero. These attitudes were called Torque Equilibrium Attitudes (TEA s). A TEA attitude control law was developed to utilize these zero torque points. This control law was unique because it was the first spacecraft control scheme which used upper atmospheric �9 aerodynamic torques to desaturate CMG momentum. In the normal SI Skylab mode, CMG momentum was managed by dumping excess momentum using gravity gradient torques. In a TEA attitude, the aerodynamic torques and gravity gradient torques are equal and opposite. By offsetting the vehicle slightly from this attitude, the relative magnitudes of the gravity gradient and aerodynamic torques can be increased or decreased as desired to maintain the CMG momentum at the desired state. Through a concentrated effort at MSFC, the TEA control law was developed, programmed and verified between January and May 1979. There were several TEA attitudes and each resulted in different atmospheric drag on the vehicle. The limiting factors in maintaining control were meeting the electric power requirements and being in a dense enough atmosphere to generate the desired aerodynamic torques. Most of the TEA attitudes were unuseable because the Skylab solar arrays could not collect sufficient solar energy to run the various Skylab systems in the specified attitude. Other attitudes could be used only during a range of certain sun angles and below certain altitudes. Two of the 7 useable attitudes, the T275 and T121G are shown in Slide 7. The T275 attitude has a smaller projection of surface area into the direction of flight and a corresponding lower atmospheric drag than the T121G attitude. By maneuvering between TEA attitudes the drag on the vehicle could be modulated to slow or speed the desired descent rate of Skylab. This provided the capability to shift the reentry time several orbits if necessary. �Based on early reentry predictions of mid-June 1979, initial procedures were developed to begin TEA operations in the T121G attitude on May 26, 1979. At this time the vehicle altitude was predicted to be approximately 150 n.m. and the sun angle profile such that the T121G attitude would supply sufficient solar power from this point to the predicted reentry. However, as the time approached, it became apparent that Skylab was not reentering as fast as predicted and reentry slipped to early July 1979. As a result of this delay in reentry, a maneuver from T121G to T121P or T275 would be required to provide sufficient power over the sun angle range from 20 of May to reentry. Because of this, and the fact that Skylab would be around 157 n.m. on May 26, it was decided to delay TEA operations and stay in SI control until late June 1979. In the June 18-20 time frame, Skylab altitude would be approaching its lower limit for SI control (140 n.m.) and the sun angle would be such that the T121P (Slide 8) attitude would provide sufficient power all the way to reentry. In addition to requiring only one TEA orientation for solar power, this delay provided additional benefits. More time was available for TEA control analysis; development of procedures for power management, rate gyro bias compensation, TEA parameter updating, and contingencies; and ground controller training. Ori—May—2ff,—the alt ifu"de~"af"the vehicle was! approximately 160 TIJJL Skylab-could be-controlled in the SI~ mode only until. �11 The Skylab _.descent rate- and- expected :Ms solar—aetdv±ty~daTHicalTe3TTfhat altitude VouSd be reached_ .th.is_time,-the_-. - »•» ware-avai-ia^ • Thw. velocity vector and perpendicular to th.orbital plane and the v provide a high atmospheric drag on the ve T^un- angle would-allows the T275 atmospheric density wo —— attitude to he useable, ^ ^ ^ It was planned to use the ,f ^ modulation during the fnal 36 hour P lated orbit. predicted reentry occurred during a high y P P 20 the delayed ZLV commands contain 7 3:24 «T, dune 20. t ^^ o£fsets were uplinked. At time to start TEA acquxsxt At At 8-17 GMT, the CMG ,30 CHT, TACS control ^^^second „ gimbal rate limit was mc more TEA Slide 9 ^ to lnltlal control authority during •^ initiated at 12:53 for phases of ^lnertial control. The maneuver: sequ ^ ^ ^ X121P is shown in Slid allM ^ ^ ^ q£^ Santiago station was eoverage. At 13:01 CKT, shortly^ ^ ^ ^^ acquired, the OlO's were c a g ^ ^ ^ new TEA attitude. was complete. At xo- ^^ ^ �12 TEA Control Reacquisition (?— To support the normal maintenance of TEA control, t-he~ slope matrix was programmed to receive updates by ground uplink. This was a 3 by 3 matrix relating the momentum errors to attitude errors about the torque equilibrium attitude. This slope matrix was a function of atmospheric density and the TEA attitude. Since atmospheric density was increasing with the vehicle descent, the slope matrix required periodic adjustment. At 16:13 GMT on June 24, the first slope matrix update since TEA acquisition was uplinked. However, the flight program required that the slope matrix elements be commanded in row order, but it was uplinked from the ground in column order. This meant that the elements in the slope matrix were reversed and the flight program had actually received the transpose of the desired , . .. C-O-vJ Vt matrix. With an incorrect slope matrix, the vehicle cannot Ujo-asJL cL properly manage the CMG momentum and wifd slowly lose attitude control. The first pass through the TEA control calculations with the transposed slope matrix occurred after the telemetry station was lost. There was no indication of a problem until the next station acquisition occurred at 17:44 GMT. The telemetry data from this station showed that the CMG momentum was becoming saturated, TEA control parameters were off-nominal, TACS had been used and that, in general, TEA control authority was being lost. An analysis of the DCS commands issued during the previous pass was made and it was discovered that the transpose of the slope matrix had been transmitted. �13 A contingency TEA control reacquisition procedure had been previously developed for use during the initial TEA control acquisition. Although this contingency procedure was developed assuming that TEA control might be lost due to an offset in the assumed X axis center of pressure, it also applied to the current situation. The commands necessary for the procedure were already resident at the ground stations and were readily available. The reacquisition procedure was executed when the next station coverage occurred over Madrid. The entire process of losing TEA control and reacquiring TEA control used approximately 1100 lb f-sec of TACS fuel. Although this fuel usage had been unplanned, there was still sufficient fuel remaining for any required maneuvers prior to reentry. �14 Skvlab Reentry ............< ™ i ,Hlitv amount of reentry control capability. Procedures to maneuver Skylab from the high-drag T121P attitude to a low-drag (T275) or ZLV attitude were developed to provide a means o prion By maintaining a low-drag shifting the reentry predrctron. y i A Tip pxtended over that m attitude, the orbit lifetime could be exten T121P attitude. This would make it possible to s r h.f o£ high population density predicted reentry from an orbrt of hrgh p one with a lower population density. A <-Viat~ TEA control (T121P) °r T275 Since several factors indicate m ^gQ and TACS only control could no n m procedures were also developed to initiate VI ilts in a predictable average drag tumble. A random tumble results P A-i «-t-i nns By controlling tne ;rr.r:::irr::-r;rrrir ::::rr:jr.-::.......----« . i.n predicted reentry and July 9 at ^ hours prior Beginning on July Headquarters, h six hours thereafter, NORAD supplied NASA Hea each six nour reentry predictions. MSFC and JSC with Skylab tracking a a these centers was constantly maintained K Communications between these staining cxi-Tvrrk loop. Decisions pertami g — * -lecosraunications netw k p _^ ^ to executing procedures M A cA H P A dnuarters under or ^ ^ �15 3 As it turned out, only T121P control was required. NORAD data received between the 48 and 18-hour-to-go time points indicated that Skylab would reenter during the minimum population density orbit, but the predicted impact point was in the highest population density area of the orbit. This was reconfirmed at the 12-hour time-to-go point. The decision was therefore to continue the T121P attitude until altitude of approximately 80 n.m., at which point the vehicle control was finally terminated when Skylab was commanded to tumble approximately 9 hours before reentry. (Reentry took place on July 11, 1979, at 16:37 - 28 GMT.) The vehicle tumble Slide /O decreased the vehicle drag and by selection of the time before entry to initiate the tumble, an extension of the footpring by approximately one quarter of a revolution was realized. �16 FINAL COMMENTS D u r i n g b o t h t h e p r i m a r y a n d r e a c t i v a t i o n m i s s i o n s , many S k y l a b s y s t e m s w e r e r e q u i r e d t o o p e r a t e i n modes n e v e r i n t e n d e d b y i t s designers and to accomplish tasks dictated by unforseen events. T h e S k y l a b r e a c t i v a t i o n m i s s i o n o f f e r e d NASA a u n i q u e o p p o r t u n i t y to evaluate complex power generation, mechanical, computer and environmental control systems after having been in a space environment for over six years. Further, these systems were in orbital storage for over four of the six years in an uncontrolled s p a c e e n v i r o n m e n t b e f o r e b e i n g r e a c t i v a t e d i n March 1 9 7 8 . S y s t e m degradation was found to be minimal. U n i q u e c o n t r o l s c h e m e s w e r e d e v e l o p e d (EOW a n d TEA) w h i c h e n a b l e d Skylab to fly through the gravity gradient/aerodynamic torque transition region. Torque equilibrium points were discovered where gravity gradient and aerodynamic torques balanced. At these points very l i t t l e vehicle control authority was required to maintain control. Moreover, vehicle orientations at these points were such that one could choose attitudes exhibiting high or low v e h i c l e - d r a g c h a r a c t e r i s t i c s . By m o d u l a t i n g b e t w e e n t h e s e orientations, the rate of vehicle descent could be increased or decreased, forcing i t into an impact orbit characterized by a low population density. After control of Skylab was regained in June 1978, a t an a l t i t u d e of 218 nautical miles, the vehicle was e s s e n t i a l l y u n d e r c o n t r o l down t o a p p r o x i m a t e l y 8 0 n a u t i c a l m i l e s b e f o r e i t w a s commanded t o r e e n t r y . ( R e e n t r y t o o k p l a c e o n J u n e 1 1 , 1 9 7 9 , a t 1 6 : 3 7 - 2 8 GMT.) The vehicle tumble decreased �17 the vehicle drag and by selection of the time before entry to initiate the tumble, an extension of the footpring by approximately one quarter of a revolution was realized. XIKKKXEBKIMKMES � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Gerald Wittenstein Collection Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Identifier An unambiguous reference to the resource within a given context sdsp_skyl_000066 Title A name given to the resource "MSFC'S ROLE IN THE SKYLAB REACTIVATION MISSION." Creator An entity primarily responsible for making the resource United States. National Aeronautics and Space Administration Date A point or period of time associated with an event in the lifecycle of the resource 1979-07-11 Temporal Coverage Temporal characteristics of the resource. 1970-1979 Subject The topic of the resource Skylab Program Skylab 1 Reentry George C. Marshall Space Flight Center Type The nature or genre of the resource Reports Text Source A related resource from which the described resource is derived Gerald Wittenstein Collection Box 8, Folder 4 University of Alabama in Huntsville Archives and Special Collections, Huntsville, Alabama Language A language of the resource en Rights Information about rights held in and over the resource This material may be protected under U. S. Copyright Law (Title 17, U.S. Code) which governs the making of photocopies or reproductions of copyrighted materials. You may use the digitized material for private study, scholarship, or research. Though the University of Alabama in Huntsville Archives and Special Collections has physical ownership of the material in its collections, in some cases we may not own the copyright to the material. It is the patron's obligation to determine and satisfy copyright restrictions when publishing or otherwise distributing materials found in our collections. Relation A related resource Skylab Document Scanning Project Metadata Skylab 50th Anniversary https://digitalprojects.uah.edu/files/original/201/14402/sdsp_skyl_000067_001.pdf 54af932513b89bdd15db2aa0321af4ff PDF Text Text MEXK » ME iCO CAfilttHKA.\ SEA I* fOllnh* ItAMCAiU �I IHOUR MRTl ^ULV '11^197©' M0 Q 0 11 j-*rA\ AlflHiA Oi'TH4 R A H PmUPPiRR" 1619> CEA N 2t*UKD � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Gerald Wittenstein Collection Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Identifier An unambiguous reference to the resource within a given context sdsp_skyl_000067 Title A name given to the resource "JULY 11, 1979 1 HOUR REPORT." Description An account of the resource This is a report about the Skylab 1 reentry. Creator An entity primarily responsible for making the resource United States. National Aeronautics and Space Administration Date A point or period of time associated with an event in the lifecycle of the resource 1979-07-11 Temporal Coverage Temporal characteristics of the resource. 1970-1979 Subject The topic of the resource Skylab Program Skylab 1 Reentry Type The nature or genre of the resource Reports Diagrams Still Image Source A related resource from which the described resource is derived Gerald Wittenstein Collection Box 8, Folder 4 University of Alabama in Huntsville Archives and Special Collections, Huntsville, Alabama Language A language of the resource en Rights Information about rights held in and over the resource This material may be protected under U. S. Copyright Law (Title 17, U.S. Code) which governs the making of photocopies or reproductions of copyrighted materials. You may use the digitized material for private study, scholarship, or research. Though the University of Alabama in Huntsville Archives and Special Collections has physical ownership of the material in its collections, in some cases we may not own the copyright to the material. It is the patron's obligation to determine and satisfy copyright restrictions when publishing or otherwise distributing materials found in our collections. Relation A related resource Skylab Document Scanning Project Metadata Skylab 50th Anniversary https://digitalprojects.uah.edu/files/original/201/14403/sdsp_skyl_000068_001.pdf 9ce0453d7e9289409afc7552a73c8644 PDF Text Text j?CMO ll/tS/fcO SATELLITE DRAG STUDY G r a n t Number NSG 8 0 6 9 Final Report F o r t he period 1 April 1979 to 3.1 July 1980 Principal Investigator • J a c k W. Slowey Prepared for National Aeronautics and Space Administration Marshall Space Flight Center, Alabama 35812 October 1980 Smithsonian Institution Astrophysical Observatory Cambridge, Massachusetts 02138 The S m i t h s o n i a n A s t r o p h y s i c a l O b s e r v a t o r y and the Harvard College Observatory a r e members o f t h e Center for Astrophysics The NASA T e c h n i c a l O f f i c e r f o r t h i s g r a n t i s D r . R . E . S m i t h , Code E S 8 1 , Space Science Laboratory, Marshall Space Flight Center, Marshall Space Flight Center, Alabama 35812. �... ' •* m m i m. m wmm IotB.'S 1 ' • m 1 • BpM 5 9 |$ V ' mm • - •'%• m •• m �SATELLITE DRAG STUDY G r a n t Number NSG 8069 F i n a l Report For t h e p e r i o d 1 A p r i l 1979 t o 31 J u l y 1980 Principal Investigator Jack W. Slowey Prepared for National A e r o n a u t i c s and Space A d m i n i s t r a t i o n Marshall Space F l i g h t C e n t e r , Alabama 35812 O c t o b e r 1980 Smithsonian I n s t i t u t i o n Astrophysical Observatory Cambridge, M a s s a c h u s e t t s 02138 The S m i t h s o n i a n A s t r o p h y s i c a l O b s e r v a t o r y and t h e Harvard C o l l e g e Observatory a r e members o f t h e Center for Astrophysics The NASA Technical O f f i c e r f o r t h i s g r a n t i s Dr. R.E. S m i t h , Ctide ES81, Space S c i e n c e L a b o r a t o r y , Marshall Space F l i g h t C e n t e r , Marshall Space Flight Center, Alabama 35812. ��1. Introduction The Smithsonian Astrophysical Observatory (SAO), under a grant from NASA (NSG8058), first began an analysis of the effects of atmospheric drag on the Skylab satellite, 1973-27A, in the fall of. 1977. Under that grant we determined, from orbital data obtained from NORAD, the observed atmospheric drag on Skylab with a resolution of 5 days or better in an interval that was eventually to extend from March 1974 to the end of 1978. At the same time, we compared the observed drag with that predicted by an atmospheric model and made numerous forecasts, based on predicted solar and geomagnetic activity, of the orbital lifetime of the satellite. The current grant work at SAO is essentially a continuation of the original grant work. Under this grant we continued to monitor the drag on Skylab and to make lifetime predictions up to the time of final decay. These activities were described in some detail in an earlier report and will not be covered again here. We also continued to act as consultant to MSFC in matters relating to our various atmospheric models and, in particular, to the implementation at MSFC of our most recent (1977) model. These activities have been conducted on a relatively informal basis, by telephone and letter, and will not be described here. We have also conducted a "post mortem" analysis to determine how the techniques that we used on Skylab might be improved in the future, especially with respect to the question of separating possible variations in area-mass ratio from departures of the atmospheric density from model values. A short summary of this work, together with some suggestions for future work, are given in what follows. 2. Technical Progress We had hoped to utilize a second satellite in a comparative analysis of atmospheric drag during the final portion of the lifetime of Skylab. The object was to see if the drag on a second satellite could be successfully used to separate variations in the observed drag on Skylab that might be due to variations in the area-mass ratio from those that are due to variations in atmospheric density. An atmospheric model alone is not entirely adequate for this purpose since present models, as good as they are in representing the large variations in density that occur, are subject to appreciable systematic errors having characteristic times of up to a month or more. These errors are due mainly to failure of the decimetric solar flux to adequately represent the variations in the solar EUV radiation that actually heats the thermosphere and to similar inadequacy of the planetary geomagnetic index as an indicator of the heating associated with geomagnetic disturbance (and of present models of that very complex phenomenon). Except for short intervals in which �'tci 5 • lv : j. :l -• -rsoa -»w • : fj e«fca .&iill9cre» srf:J io ' -1 P isniglio arte :o �PAGE 2 geomagnetic disturbance may dominate, the densities determined from drag on different satellites are generally quite consistent among themselves. Thus it should be possible to infer density at another location with greater accuracy from the drag on a satellite of known area-mass ratio than is possible from any model based on the usual geophysical parameters. Of course, a model would still be required to provide a means for interpolating between the location of the probe satellite and the desired location. Unfortunately, the problems of data acquisition, program development (mainly the conversion of several large programs to run on a new computer), and processing proved to be too great to carry out the projected analysis within the constraints of this grant. Instead, we made use of densities we had previously obtained from the drag on the Explorer 32 satellite (1966-44A) to make a comparison with the orbital accelerations of Skylab that we had determined under the earlier grant (NSG 8058). The interval covered by this comparison was 265 days beginning in late March, 1974. The orbital acceleration (rates of change of the mean motion) of Skylab that we determined from the available NORAD orbits are plotted for this interval in Figure 1. These were obtained by drawing a smooth curve through the observed values of the argument of latitude (M +w) and numerically differentiating the curve using a 5-day time-step. In the same figure are shown the corresponding accelerations determined by differentiation of the results of numerical integration of the orbit using an atmospheric model and, at the bottom, the ratios of the observed accelerations to those computed from the orbit integration. The model used in the integration was an updated version of Jacchia's 1970 model (Jacchia, 1970) and the assumed area-mass ratio was 0.0369 cm2/g. The drag coefficient was allowed- to vary around the orbit, but the effective value was very close to 2.24 throughout the interval. The relatively large short-term variations in the orbital acceleration seen in Figure 1 are due to variations in thermospheric heating by both solar EUV radiation and the particle precipitation and/or ionospheric currents associated with geomagnetic disturbance. In models, these two heat sources are tied, in the first instance, to the 10.7-cm solar radio flux and, in the second instance, to the Kp planetary geomagnetic index. In Figure 2, we have plotted 5-day means of both the 10.7-cm flux and the Kp geomagnetic index, on scales that are roughly equal in terms or their expected effect on the exospheric temperature of the atmosphere (the scale for Kp is slightly exaggerated in this regard compared to that for *10#7). As can be seen, the two indices are quite independent of each other and may act either in unison or in opposition. ��PAGE 3 In Figure 4, we have plotted the exospheric temperature Tj that results from . the 1970 model when only the short-term variation in the 10.7-cm flux (the so-called "27 day" variation) and the geomagnetic variation are taken into account. These temperatures were computed from Tj = 665.2+1.8 ( P10#7 ~ 85.) +28 Kp+ 0.03 exp (Kp), where F Kpare the 5-day mean values from Figure 2. We 7and give them"here only to better illustrate the expected short-term variations in density in the interval being studied. Note that these temperatures differ somewhat in detail from the computed accelerations in Figure 1. This is because the values in Figure 1, as a result of the differentiation process, actually represent means over a slightly longer interval than 5 days. In comparing the observed acceleration of Skylab from Figure 1 with either the corresponding model values or the temperatures of Figure 3, it will be noticed that several of the expected sharp maxima or minima are missing in the observed values. It is these points on the plot that result in the more prominent "outlyers" in the ratio of observed to computed acceleration. There is little doubt that some of th,ese apparent departures from the model are, in fact, due mostly to errors in the observed values resulting from the relatively crude method used to derive them. And, it follows that the scatter in the values of the ratio is adversely effected generally for the same reason. This difficulty could, of course, be overcome by differentiating with a considerably larger time step, but this would automatically rule out the possibility of resolving shorter-term variations of any kind. In Figure 4 we have plotted 5—day means of atmospheric densities obtained from analysis of the drag on the Explorer 32 satellite. The densities are those at the effective height (approximately 1/2 scale-height above the true height) of perigee. The average effective height in the interval plotted was about 300 km. These densities were obtained by direct analysis of radar observations from selected sensors. The densities were originally determined with a general resolution of 1 day and a resolution of 0.5 day during larger geomagnetic disturbances. The 5-day means plotted in the figure should have a relative precision of close to 1%. These densities confirm the accuracy of the model with respect to 4 of the 5 worst values of the ratio in Figure 1. The exception is the point at MJD 42205, where the minimum predicted by the model and missing in the Skylab accelerations is also missing in the densities determined from Explorer 32. Other differences in the Skylab accelerations are also confirmed as being atmospheric in origin and not due to errors in the observed values. �V �PAGE 4 At t h i s p o i n t we s h o u l d m e n t i o n t h a t our o l d e r a t m o s p h e r i c models a r e n o t c u r r e n t l y o p e r a t i o n a l a t SAO due p r i m a r i l y t o a change i n c o m p u t e r s . I t was o u r i n t e n t i o n i n t h e p r e s e n t analysis t o compute model v a l u e s f o r both S k y l a b and E x p l o r e r 32 using our most r e c e n t a t m o s p h e r i c model ( J a c c h i a , 1 9 7 7 ) , which i s operational, and t o make a c o m p a r i s o n between t h e two s a t e l l i t e s using the r a t i o s t o t h e s e v a l u e s . Much t o our s u r p r i s e , however, the o r b i t a l a c c e l e r a t i o n s of S k y l a b computed w i t h t h e new model did not agree i n d e t a i l w i t h t h e o b s e r v e d v a l u e s a s w e l l a s d i d those from t h e o l d e r model n o r d i d t h e d e n s i t i e s computed f o r Explorer 32 r e p r e s e n t t h e d e t a i l s of t h e o b s e r v e d v a l u e s a s w e l l as i t was e x p e c t e d t h e y would. I t i s n o t y e t known w h e t h e r t h i s a p p a r e n t d i f f i c u l t y w i t h the new model i s i n t r i n s i c o r i s somehow due t o t h e way i t was implemented i n t h e p a r t i c u l a r c i r c u m s t a n c e s . The o n l y o t h e r application of t h e model t h a t we have made i n a d r a g s i t u a t i o n was during t h e f i n a l d e c a y of S k y l a b . I t seemed t o perform q u i t e well in t h a t c a s e . T h a t was h a r d l y a d e f i n i t i v e t e s t , however, and i t may w e l l b e t h a t t h e most i m p o r t a n t r e s u l t of t h e p r e s e n t analysis i s t h a t it revealed a major difficulty in the model-related, a p p a r e n t l y , t o t h e "improved" model of the geomagnetic v a r i a t i o n t h a t i t i n c o r p o r a t e s . When means of t h e computed d e n s i t i e s f o r E x p l o r e r 32 were taken over 10-day i n t e r v a l s , t h e y were q u i t e smooth and d i d reproduce most of t h e s y s t e m a t i c d e p a r t u r e s o b s e r v e d i n t h e ratios for S k y l a b . I n view of t h e d i f f i c u l t i e s w i t h t h e model, we do not f e e l t h a t we a r e j u s t i f i e d i n p r e s e n t i n g t h o s e r e s u l t s as proven f a c t , however. We m u s t , a t l e a s t f o r t h e time b e i n g , consider t h e a n a l y s i s t o have been " i n c o n c l u s i v e " . 3. Conclusions and Recommendations an Our e x p e r i e n c e w i t h S k y l a b d e m o n s t r a t e d t h e need automated p r o c e d u r e f o r t h e h i g h - r e s o l u t i o n d e t e r m i n a t i o n of densities from s a t e l l i t e d r a g . For r e a s o n s of e f f i c i e n c y , should be an a n a l y t i c p r o c e d u r e a n d , l i k e t h e program t h a t previously e x i s t e d a t SAO, s h o u l d be based on d i r e c t a n a l y s i s of the individual o b s e r v a t i o n s of t h e p a r t i c u l a r s a t e l l i t e i n o r d e r to yield t h e g r e a t e s t p o s s i b l e p r e c i s i o n and t i m e r e s o l u t i o n . As a p r a c t i c a l m a t t e r , i t s h o u l d b e f u l l y a u t o m a t i c and f r e e of reliance on hand methods of any k i n d . I t would be e x t r e m e l y valuable i n a v a r i e t y of programs i n o r b i t a l dynamics, such a s the s t u d i e s we made of t h e d r a g on S k y l a b and t h e kind of comparative a n a l y s i s we s u g g e s t i s f e a s i b l e i n t h e c a s e of s a t e l l i t e s w i t h unknown o r v a r y i n g a r e a - m a s s r a t i o s , and a s a research t o o l that could contribute significantly to the improvement of models of t h e t h e r m o s p h e r e and e x o s p h e r e . We recommend t h a t MSFC s e r i o u s l y c o n s i d e r t h e development of such a program. �' �PAGE 5 We would a l s o recommend t h a t c o n s i d e r a t i o n be g i v e n t o t h e possibility of u t i l i z i n g t h i s program i n a p r o j e c t t o monitor a small number of s a t e l l i t e s on a c o n t i n u o u s b a s i s . No d e n s i t i e s from s a t e l l i t e d r a g have been d e t e r m i n e d i n any a p p r e c i a b l e quantity s i n c e 1 9 7 4 . T h e r e a r e , however, some problems i n thermospheric s t r u c t u r e and model development t h a t would b e n e f i t greatly from t h e a v a i l a b i l i t y of s u c h d e n s i t i e s . There i s s t r o n g evidence i n measurements of s o l a r EUV i r r a d i a n c e , f o r example, of major d i f f e r e n c e s between t h e c u r r e n t s o l a r c y c l e and t h e previous one ( H i n t e r e g g e r , 197 9 ) . Our a n a l y s i s of S k y l a b revealed t h a t t h e r e s p o n s e of t h e a t m o s p h e r e r e l a t i v e t o t h e decimetric f l u x was n o t g r e a t l y d i f f e r e n t i n t h e two c y c l e s . The exact nature of what d i f f e r e n c e may e x i s t remains t o be s e e n , however, and i t would seem t h a t d r a g a n a l y s i s o f f e r s t h e o n l y means by which i t can be a c c u r a t e l y d e t e r m i n e d . Densities determined from d r a g h a v e t h e a d v a n t a g e of c o n t i n u i t y ( t h e d r a g record extends back t o 1958) and freedom from t h e problems of cross-calibration between experiments that is lacking in densities determined by mass-spectrometers and other satellite-borne i n s t r u m e n t s . �• �PAGE 6 References Hinteregger, H.E. 1979 Development of s o l a r c y c l e 21 o b s e r v e d i n EUV spectrum and J. Geophys. R e s . , 84, pp atmospheric a b s o r p t i o n s . 1933-1938. Jacchia, L.G. 1970 New s t a t i c models of t h e t h e r m o s p h e r e and e x o s p h e r e w i t h empirical t e m p e r a t u r e p r o f i l e s . Smithsonian Astrophys. Obs. Spec. R p t . No. . 3 1 3 , 87 p p . 1977 Therraospheric t e m p e r a t u r e , d e n s i t y , and c o m p o s i t i o n : models. Smithsonian Astrophys. Obs. Spec. Rpt. 375, 106 p p . new No. ��PAGE 7 These captions pertain to the following figures (1-4). Figure 1. Observed orbital acceleration of Skylab (top), acceleration computed from an atmospheric model (middle), and ratio of observed to computed acceleration (bottom). Figure 2. 5-day means of 10.7 cm solar Kpgeomagnetic index (bottom). Figure 3. Exospheric temperature computed for just variation and the geomagnetic variation. the 27-day Figure 4. 5-day means of observed densities at effective for the Explorer 32 satellite. height flux (top) and ��PAGE 8 42125 . MJD 42225 42325 1974 Figure 1 ��PAGE 9 IT) IU- ��lO CM CM M" in CM to CM m CM CM CM Q "3 in O £ o ^ N ��o> CM E o Figure 4 ���� Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Gerald Wittenstein Collection Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Identifier An unambiguous reference to the resource within a given context sdsp_skyl_000068 Title A name given to the resource "Satellite Drag Study." Creator An entity primarily responsible for making the resource United States. National Aeronautics and Space Administration Date A point or period of time associated with an event in the lifecycle of the resource 1980-11-25 Temporal Coverage Temporal characteristics of the resource. 1980-1990 Subject The topic of the resource Smithsonian Astrophysical Observatory Skylab Program Skylab 1 Reentry Type The nature or genre of the resource Reports Text Source A related resource from which the described resource is derived Gerald Wittenstein Collection Box 4, Folder 9 University of Alabama in Huntsville Archives and Special Collections, Huntsville, Alabama Language A language of the resource en Rights Information about rights held in and over the resource This material may be protected under U. S. Copyright Law (Title 17, U.S. Code) which governs the making of photocopies or reproductions of copyrighted materials. You may use the digitized material for private study, scholarship, or research. Though the University of Alabama in Huntsville Archives and Special Collections has physical ownership of the material in its collections, in some cases we may not own the copyright to the material. It is the patron's obligation to determine and satisfy copyright restrictions when publishing or otherwise distributing materials found in our collections. Relation A related resource Skylab Document Scanning Project Metadata Skylab 50th Anniversary https://digitalprojects.uah.edu/files/original/201/14404/sdsp_skyl_000069_001.pdf 04cb3141c651fc91493a85341d0b004a PDF Text Text 4i C2 \A/v V ^ TABLE OF CONTENTS Page LIST OF TABLES AND FIGURES iii ABSTRACT iv I. INTRODUCTION 1 II. PREDICTED FOOTPRINT 2 III. RECONSTRUCTION OF SKYLAB FOOTPRINT 4 IV. SUMMARY 7 V. CONCLUSIONS 9 REFERENCES 19 ii �LIST OF TABLES AND FIGURES TABLE 1 PAGE Recovered Skylab Debris 10 1 Predicted Skylab Reentry Scenario 12 2 Skylab Predicted Debris Footprint Comparison 13 3 Relationship of Footprint Size to Breakup Altitude and'BC 14 4 Skylab Altitude from NORAD Vectors 15 5 Skylab Debris Envelopes 10 6 Skylab Impact Corridor -17 7 Map of Footprint FIGURE 18 iii �ABSTRACT This report documents the location and extent of the impact corridor of the Skylab vehicle. Included in this discussion are summaries of the predicted breakup sequences and result­ ing footprint, methodology for reconstructing the actual breakup sequence and footprint, and an assessment of the overall impact footprint size. Questions concerning information contained herein should be directed to Marshall Space Flight Center, EL25, Lee Varnado, AC 205, 453-1163. iv �I. INTRODUCTION When the Skylab vehicle reentered the earth's atmosphere on July 11, 1979, a great deal of attention was, quite naturally, focused on the question of where the surviv­ ing elements would impact- This report addresses that question. Included in this report are brief descriptions of the predicted size of the impact area (footprint), the reconstruction of the actual impact area, and some per­ tinent conclusions. 1 �II. PREDICTED FOOTPRINT In studies performed in 1970 and 1973, personnel of the Lockheed Missiles and Space Company (LMSC) predicted that, assuming, the Sky lab vehicle began to break up at 400,000 <rfeet (65 nmi or 120 km), debris from the Solar Arrays would begin to impact approximately 3600 nmi (6667 km) downrange from the breakup point (See References 1 and 2). This would define the "heel" of the impact footprint and all other debris would impact downrange of this point. The maximum distance any debris would travel, the "toe" of the footprint, would be 7400 nmi (13705 km) from the initial breakup point. Debris from the ATM was predicted to impact in this area. Figure 1 shows the predicted breakup sequence and associated altitudes, as forecast by LMSC. This figure is only included to illustrate the breakup sequence expected, and no significance should be attached to the relative positions of the traces after breakup, as it is not to scale. Shortly before Skylab reentered, in preparing for the footprint reconstruction activity, personnel from MSFC (EL25) performed a brief reentry study to assess the adequacy of our preparations. As a check, we compared our impact dispersions with those from the LMSC study (using the LMSC-predicted breakup sequence) and the results, with one exception, were in reasonable agreement. The one ex­ ception was the ATM case, with MSFC predicting impact some 2500 nmi (4630 km) farther downrange than LMSC. This difference could be accounted for by a difference in the Coefficient of Drag (CQ) used in the two studies. Although no detailed LMSC data are now available, one of the engi­ neers involved in the LMSC study recalled using a CD term no smaller than 0.3 for the intact ATM. The MSFC-determined CD for the ATM was 0.1, a difference which could easily cause the downrange shift in the impact location. The results of the two studies are compared in Figure 2. This study also served to provide us with a priori knowledge of the sensitivity of the footprint size to reentry parameters. The size of the footprint is a function of both the breakup altitude and the Ballistic Coefficient (BC) of the resulting pieces. The BC of any element is a function of its mass (M), area (A), and drag characteristics (CD) and is calculated: BC = M CDA For breakup at any specified altitude, elements with greatly different BC's will produce a larger footprint than 2 �elements whose BC's are substantially the same. Also, given a set of BC's, breakup at higher altitudes will create a larger footprint than if these same elements break up at a lower altitude. The sketches in Figure 3 illustrate this relationship; however, this generality must be applied to Skylab with some caution. Due to the nature of the vehicle, the aerodynamic characteristics change markedly each time an element separates from the OWS. For example, when the OWS and ATM Solar Arrays separated, the mass loss was more than offset by the reduced area, with the result that the BC actually increased. This moved the impact point of the re­ maining elements farther downrange. The problem of estimating impact location is complicated by the fact that each time a breakup event occurs, a new BC for the resulting elements must be determined. The general philosophy we used was to determine the trajectory of each major element (Solar Arrays, OWS, ATM, etc.) to a specified breakup altitude, then run only the resulting pieces with the smallest and largest BC's (taken from the LMSC reports) to impact. This defined the expected limits of each major element without requiring undue amounts of computer time or manpower. The same philosophy was used in the footprint reconstruction activity which is discussed in the succeeding paragraphs. 3 �III. RECONSTRUCTION OF SKYLAB FOOTPRINT As has already been pointed out, the length of the impact footprint is determined by the altitude at which an element breaks up and the Ballistic Coefficients of the resulting pieces. Since LMSC had previously estimated, in References 1 and 2, the BC's of the major assemblies (OWS, ATM, AM, IU), the prime concern was to define the altitude(s) at which these assemblies began to break up. It should be emphasized that this activity, while based on available data, is still somewhat of an art and not purely analytical, due to the lack of precise event timing,uncertainty about size and shape of elements after breakup, and the accuracy limits of observations. Essentially, the procedure was to adjust the breakup altitudes so that the predicted BC's resulted in reentry profiles which agreed with the available data. Data used to reconstruct the reentry history came from several sources. They were: o Special perturbation vectors from NORAD o Tracking from the radars at Bermuda and Ascension Islands on the final revolution o Telemetry data while over Ascension and Bermuda Islands o Special altitude observations from NORAD o Locations of recovered debris In addition to the above data, state vectors provided by NORAD, especially those received in the final 24 hours, were assessed to insure continuity of the trajectory. The follow­ ing paragraphs describe in more detail the data used and how it affected the footprint determination. During the 48 hours prior to reentry, a very important source of data was the special state vectors provided by NORAD. These vectors differed from the standard vectors in that they were determined from fewer sets of tracking data and thus were not as strongly influenced by the effects of long-term perturbations, such as solar activity. The im­ portance of these vectors is that they were used to help determine the time at which any maneuver to shift the probable impact point should be initiated. This determina­ tion was made possible by analyzing the family of impact points resulting from these vectors. The initial reconstruc­ tion activity used these predicted impact points before other data was available to help assess the probability of a 4 �specific breakup sequence. These state vectors also proved valuable in helping determine the aerodynamic profile affect­ ing the vehicle as it approached the reentry altitude. The altitude history derived from these vectors is shown in Figure 4. This altitude profile fit our reconstruction very well and increased our confidence that we had a good estimate of the altitude profile to begin the analysis which was based on the T—7 hrs NORAD vector. : Sets of tracking data were provided by the Bermuda and Ascension Island radars on the final revolution. Data from these trackers indicated an altitude of approximately 62 nmi (115 km) and 57 nmi (105 km) respectively, over these sites. Each of these trackers reported contact with a single target during their observation; this would indicate that separation of the ATM from the OWS had not occurred, at least down to 57 nmi (105 km). This is a significant point, since footprint ^ predictions had been predicated on breakup beginning at 65 nmi (120.4 km). Further confidence in a later-than-expected breakup was pro­ vided by analyzing downlinked electronic data received while the vehicle was over Bermuda. This data indicated that the OWS and ATM solar arrays were still intact and functioning at that point. It appears that sometime after the Bermuda pass, and prior to Ascension acquisition, the OWS solar arrays, while still attached, may have folded back against the OWS. Analysis of the Ascension telemetry data supports this conclusion since, during this pass, downlinked electrical data indicated the OWS Array was still intact but no longer yielding expected currents and voltages. Subsequent to the tracking data provided by the Ascension Island site, some special observations were received from NORAD. These observations consisted of the altitude and time at which various elements disintegrated. It is not possible to concretely establish a relationship between the observations and the specific element, but given other data (aerodynamic characteristics, predicted breakup sequence, and location of recovered pieces), it is possible to use this data to support a probable sequence of events. The procedure used was to construct theoretical breakup sequences based on available data and then determine a "most probable" sequence based on how well the altitude profile (vs. time) of each fit these special observations. Figure 5 shows how the ATM, OWS and AM debris envelopes from the most probable breakup sequence compared to these observations. It is clear from this data that nearly all of the observations are contained within the OWS/IU/AM debris envelope. This further supports the breakup sequence reconstruction. 5 �The final set of data available was the actual location of recovered debris. While much of the debris impacted in the Indian Ocean, several pieces were recovered on land. These pieces were from the OWS and AM and were found in an area within the reconstructed reentry corridor and between Esperance and Rawlinna in Southwestern Australia. No debris from the ATM has been recovered, and it is assumed that, because of its higher BC, all this debris probably impacted northeast of Rawlinna (See Figure 6). This is a very sparsely settled area, practically inaccessible, and it is doubtful if any of the ATM debris will ever be found. Table 1 provides a list of the recovered pieces and their location. 6 �mm IV. HI SUMMARY Analysis of all available data leads us to believe that breakup of the Skylab assembly occurredat somewhat lower altitudes- than predicted. While it is impossible to be precise concerning the breakup sequence, the one deter­ mined by this effort, summarized below, does fit well with all the data available to date. The Solar Arrays, instead of breaking off cleanly, probably folded back against the main structure, and remained attached to a much lower altitude than expected before breaking off. This served to reduce the size of the footprint, since the minimum uprange point (the "heel") is determined by the Solar Arrays impact. The actual breakup process probably -fadid not begin until approximately 54 nmi (100 km) altitude, when the ATM and Solar Arrays separated from the OWS assembly. The ATM, which as a separate entity had a very high BC compared to the other elements, traveled the greatest dis­ tance downrange, probably impacting northeast of Rawlinna (dashed area on Figure 6). Failure to recover any ATM debris makes this a somewhat hypothetical conclusion, but the separation point is almost mandated by the better-known reentry histories of other elements, discussed below, and the resultant ATM trajectory is based on known aerodynamic data. The failure to recover any ATM debris could well be due to the probability that all of it impacted northeast of Rawlinna. The special NORAD observations, shown in Figure 5, do not fall within the ATM envelope resulting from this analysis, indicating that those observations were the result of OWS, IU, or AM debris. With the currently available data, it is not possible to determine the point at which the ATM itself began to break up and the length of the resulting footprint (Figure 6 represents a maximum dispersion). The IU and AM probably separated from the OWS around 44 nmi (81.5 km). Locations of AM debris support this conclusion and also indicate that this element probably did not break up until near impact. Additional support is provided by the relationships of the OWS, AM, and IU debris envelopes to the special NORAD tracking observations, as shown in Figure 5. Separation at altitudes different than 44 nmi (81.5 km) do not fit these observations nearly so well. The location of recovered debris indicates the OWS probably began break­ ing up around 42 nmi (77.8 km), which caused much of this debris to impact in the Indian Ocean. 7 �• a table compares the -P^Se^edfcfe^ fe^fncHevllo^ by this analysts wrth th sequence. - Predicted IKES™ SSSSST. Element Solar Arrays (nmi/Hm) 54/100 54/100 ATM IU, AM/MDA 44/81.5 (nmi/km) (nmi/km) 54/100 65/120.4 (1) 58/107. 4 (2) 48/88.9 42/77.8 45/83.3 UJKSU (nmi/knO 65/120.4 45/83.3 48/88.9 45/83.3 OWS (1) (2) • • t data to estimate accurately, insufficien * Afferent BC's recovered xn nea indicates bieahup near impact. 8 proximity �V. CONCLUSIONS The reconstructed Skylab footprint begins with the impact (theoretical) of the Solar Arrays' debris at 46.9S, 94.4E and extends to 26.OS, 131.2E, the maximum distance expected to be traveled by any of the surviving pieces. Figure 7 is included to illustrate the extent of the footprint, the length of which is approximately 2140 nmi (3963 km), 1660 nmi (3074 km) less than predicted. The difference is due to the lower-than-predicted occurrence of all the separation and breakup events. The reluctance of Skylab to break up not only reduced the size of the footprint, but moved the entire footprint farther downrange than expected. The absence of debris to pinpoint a "heel" (Solar Arrays) or "toe" (ATM debris) precludes any concrete determination of the footprint size, but the reentry sequence proposed here fits all available data quite well. Thus, it appears that the impact footprint described is a reasonable one. 9 �TABLE 1 RECOVERED SKYLAB DEBRIS ITEMS PROBABLE SOURCE LOCATION Charred Fragments OWS 33.9S, 121.9E (In Esperance) Burned Material ows 33.9S, 121.9E (In Esperance) Aluminum 356 Casting OWS 33.7S, 122.IE (20 mi NE of Experance) Foam Fiberglass Beam Section OWS 33.9S, 122.0E (9 mi E of Esperance) H20 Tank OWS 33.8S, 122.0E (9 mi NE of Esperance) OWS 33.9S, 122.IE (10 mi E. of Esperance) Aft End H20 Tank 10' Steel Strip OWS (H20 Tank) 33.9S, 122.3E (25 mi E of Esperance) Heat Exchanger OWS (H20 Cooler) 33.9S, 122.IE (12 mi E. of Esperance) Segment of Fiberglass Sphere 33.9S, 122.IE (11 mi E of Esperance) OWS 33.9S, 122.IE (11 mi E of Esperance) Insulation OWS (Bulkhead) Aluminum Gear and Housing 33.7S, 122.5E (40 mi NE of OWS Esperance) (Urine Separator] N2Tank AM 33.2S, 122.6E (60 mi NE of Esperance) Electronics Module AM 33.5S, 122.3E (35 mi NE of Esperance) N2Sphere AM 33.5S, 122.8E (49 mi ENE of Esperance in Neridup area) Pressure Tank IU 33.2S, 122.6E (60 mi NE of Esperance) 10 �TABLE 1 (Continued) RECOVERED SKYLAB DEBRIS ITEMS PROBABLE SOURCE LOCATION Film Vault Door OWS 32.4S, 123.9E (5.5 mi NE of Balladonia) 02Tank AM 31.IS, 125.3E (5 mi S of Rawlinna) 0.,Tank AM 31.IS, 125.2E (15 mi SW of Rawlinna) Steel Fragment 31.IS, 125.4E (5 mi SE of AM Rawlinna) (Part of 02 Tank) Steel Dome AM (02 Tank) 31.2S, 125.2E (15 mi SW of Rawlinna) 11 �I FIGURE 1 PREDICTED SKYLAB RE-ENTRY SCENARIO 12 ' * ��BC2 ( > BC3) BC3{ > BC4) B C 4 - MINIMUM BC FIGURE3 RELATIONSHIP OF FOOTPRINT SIZE TO BREAKUP ALTITUDE AND BC 14 ��1 o ID 1 o 1 o CO 1 o CM 16 ��� Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Gerald Wittenstein Collection Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Identifier An unambiguous reference to the resource within a given context sdsp_skyl_000069 Title A name given to the resource Skylab Impact Corridor Report. Description An account of the resource This is a report about the Skylab debris field in Australia. Creator An entity primarily responsible for making the resource United States. National Aeronautics and Space Administration Temporal Coverage Temporal characteristics of the resource. 1980-1990 Subject The topic of the resource Skylab Program George C. Marshall Space Flight Center Skylab 1 Reentry Type The nature or genre of the resource Reports Text Source A related resource from which the described resource is derived Gerald Wittenstein Collection Box 4, Folder 9 University of Alabama in Huntsville Archives and Special Collections, Huntsville, Alabama Language A language of the resource en Rights Information about rights held in and over the resource This material may be protected under U. S. Copyright Law (Title 17, U.S. Code) which governs the making of photocopies or reproductions of copyrighted materials. You may use the digitized material for private study, scholarship, or research. Though the University of Alabama in Huntsville Archives and Special Collections has physical ownership of the material in its collections, in some cases we may not own the copyright to the material. It is the patron's obligation to determine and satisfy copyright restrictions when publishing or otherwise distributing materials found in our collections. Relation A related resource Skylab Document Scanning Project Metadata Skylab 50th Anniversary