nasa technical memorandum 100556 aeroassist guidance … · nasa technical memorandum 100556...
TRANSCRIPT
NASA Technical Memorandum 100556
AEROASSIST "QUIET TME"
. EXPERMENT GUIDANCE
Scott A. Stnepe and Wiliam T. Suit
fNASA-Tf3-1QOS56) AEEUASSIST PLIGHT N88-21244 EXPERIMENT G U X D R N C E Q U I E T TIHE :NASA) 76 p
CSiL & 2 S U n c l a s
G 3 / l i 3 0140247
February 1988
National Aeronautlcs and Space Admmtratton
Langley Research Center Hampton, Virginia 23665-5225
https://ntrs.nasa.gov/search.jsp?R=19880011860 2020-08-03T11:21:15+00:00Z
ABSTRACT
The s c i e n c e exper iments for t h e Aeroassist F l i g h t Experiment (AFE) w i l l be g r e a t l y enhanced by t a k i n g measurements w i t h no Reaction C o n t r o l System (RCS) contaminat ion j u s t before perigee. Methods of modifying the AFE qui- dance t o accomplish t h i s are discussed. S e v e r a l methods t h a t c o u l d give up t o 30-seconds of " q u i e t time" were i n v e s t i g a t e d and t h e r e s u l t s of t h e s e guidance m o d i f i c a t i o n s shown. A 20-second " q u i e t time" is d e f i n i t e l y possible and a 30-second " q u i e t t i m e " may be possible i f t h e guidance c a n be i n a c t i v e past perigee. Some q u e s t i o n s t h a t w e r e raised dur ing t h e guidance modification tests are listed; t h e most s i g n i f i c a n t b e i n g t h e c r i t e r i o n f o r d e t e r m i n i n g i f the m i s s i o n is t h r e a t e n e d . A l imited follow-on t es t program is o u t l i n e d .
INTRODUCTION
The Aeroassisted F l i g h t Experiment (AFE), scheduled t o f l y i n t h e m i d - 1990'~~ w i l l he used €or t h e v a l i d a t i o n and ver i f icat ion of t h e v a r i o u s t h e o r i e s , t e c h n i q u e s , equipment, and procedures t h a t w i l l be v i t a l i n any f u t u r e aeroassisted orb i ta l t r a n s f e r v e h i c l e s (AOTV's) or i n t e r p l a n e t a r y m i s s i o n s which w i l l u t i l i z e a e r o c a p t u r e techniques . The AFE is a f l i q h t exper iment which w i l l use t h e E a r t h ' s atmosphere t o assist i n t h e d i s s i p a t i o n of t h e energy r e q u i r e d t o decelerate a v e h i c l e from qeosynchronous o r b i t (GEO) to low-earth o rb i t (LEO). After b e i n g deployed from t h e Space S h u t t l e i n LEO, t h e AFE w i l l accelerate ( u s i n g a small s o l i d r o c k e t motor) t o speeds matchinq a r e t u r n t r a j e c t o r y t o E a r t h from GEO. Upon e n t e r i n q t h e E a r t h ' s a tmosphere, w i t h a l o w f l i g h t p a t h a n g l e , t h e v e h i c l e w i l l e f f e c t i v e l y "skim" t h e atmo- s p h e r e d i s s i p a t i n g k i n e t i c energy i n t h e form of d r a g on t h e v e h i c l e . By c o n t r o l l i n g t h e amount of t i m e s p e n t i n t h e atmosphere, t h e enerqy lost can be r e g u l a t e d , t h u s a l l o w i n q t h e desired o r b i t a l apoqee to he a t t a i n e d . A f t e r r e a c h i n g t h e desired apogee (200 nomi.) , t h e AFE w i l l c i r c u l a r i z e i t s orb i t and t h e S h u t t l e w i l l r e t r i e v e it.
The AFE w i l l f l y through a r e q i o n of t h e atmosphere known as t h e t r a n s i - t i o n r e q i o n . This r e g i o n is l o c a t e d hetween t h e l a y e r s of t h e atmosphere which can be modeled by free molecular f low t h e o r y and t h o s e modeled hy continuum f l o w theory . One of t h e o b j e c t i v e s of t h i s exper iment i s t o deter- mine which t h e o r e t i c a l r e p r e s e n t a t i o n of t h a t t r a n s i t i o n r e q i o n is m o s t accu- rate. S i n c e t h e r e are s e v e r a l t r a n s i t i o n t h e o r i e s a t t h e p r e s e n t t i m e , t h e guidance of t h e AFE w i l l he expec ted to s u c c e s s f u l l y complete i t s miss ion no matter which t h e o r y is used. The f o u r t r a n s i t i o n t h e o r i e s used to c a l c u l a t e v a r i o u s a t m o s p h e r i c d a t a i n t h e Proqram to Optimize S imula ted T r a j e c t o r i e s (POST) are the Vikinq T r a n s i t i o n Equat ions , t h e Lockheed Rr idginq Formula, t h e Potter Method, and t h e Shock Reynolds Number Method 1 1 1 . POST a l so i n c l u d e s a v a r i a t i o n of t h e Shock Reynolds Number Method which corrects for t h e d r a q o n l y and assumes a c o n s t a n t l i f t c o e f f i c i e n t .
For t h i s paper t h e AFE's t r a j e c t o r y w a s s i m u l a t e d hy POST. The basic c o n f i g u r a t i o n of t h e AFE i n p u t i n t o POST €or t h e s e tests (see f i q . 1 ) weiqhs 2741 pounds, h a s a n aerodynamic r e f e r e n c e area of 154 s q u a r e feet and a n aero- dynamic r e f e r e n c e l e n g t h of 14 feet. The quidance a l g o r i t h m which h a s been proposed f o r t h e AFE [2] w a s implemented i n t o POST. T h i s qu idance a d j u s t e d t h e bank a n q l e to m a i n t a i n a c o n s t a n t a l t i t u d e ra te d u r i n q e n t r y u n t i l a spe- c i f i e d v e l o c i t y had been reached ( t h e e n t r y p h a s e ) , a t which p o i n t t h e e x i t
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phase of the a e r o b r a k i n g p a s s began. During t h e e x i t phase, t h e guidance u t i l i z e s a n a n a l y t i c predictor/corrector t o a t t a i n t h e d e s i r e d apoqee a l t i t u d e for rendezvous w i t h t h e S h u t t l e .
T h i s paper addresses t w o main i s s u e s concern ing t h i s guidance. F i r s t , a n i n v e s t i g a t i o n of c e r t a i n gains w a s made c o n c e n t r a t i n g on t h e i r v a l u e s and t h e effect t h e s e v a l u e s had on t h e bank a n g l e commanded by t h e guidance. Second, a d a p t a t i o n s to t h e guidance were s t u d i e d t o allow v a r i o u s exper iments on t h e AFE t o be conducted w i t h l i t t l e or no contaminat ion from t h e Reac t ion C o n t r o l System (RCS).
APPROACH
For t h e s i m u l a t i o n of adequate s o l u t i o n , c e r t a i n i n i t i a l c o n d i t i o n s f o r t h e tude; 28.5 degree-angle of
t h e AFE t r a j e c t o r y and t h e d e t e r m i n a t i o n of a n s p e c i f i c cr i ter ia were used and monitored. The AFE t r a j e c t o r y w e r e g i v e n as: 400,000 feet a l t i - i n c l i n a t i o n ; an i n e r t i a l v e l o c i t y of 33,828.6
fee t / second; f l i g h t p a t h a n g l e of -4.5 degrees ; and a n a n g l e of a t t a c k of 17 degrees. A s u c c e s s f u l r u n c o n s i s t e d o f : ( a ) a maximum h e a t rate less t h a n 150 B t u / f t -sec +/-5 B t u / f t -sec; (b) a n i n c l i n a t i o n a n g l e of 28.5 d e g r e e s +/- .1 degrees; (c ) a n apogee a l t i t u d e of 200 n.mi. +/- 20 n.mi.; ( d ) a tendency of t h e l i f t vector toward t h e down p o s i t i o n (0 d e q r e e bank a n g l e ) a t e x i t ; and ( e ) less t h a n 5 hank r e v e r s a l s before e x i t i n q the a t m o - sphere . A r u n w a s c o n s i d e r e d a c c e p t a b l e i f it m e t t h e above cr i ter ia except : ( c ) an apoqee a l t i t u d e of 200 n.mi. +/- 25 n.mi. Otherwise, i f none of t h e above cri teria was m e t , t h e run w a s c o n s i d e r e d u n s u c c e s s f u l . The tendency of t h e l i f t v e c t o r towards t h e down p o s i t i o n reflects t h a t t h e f l i q h t p a t h a n g l e i s small which is d e s i r e d f o r rendezvous w i t h t h e S h u t t l e [21. The smaller the f l i g h t p a t h a n q l e i s , t h e less chanqe i n v e l o c i t y is r e q u i r e d for c i r c u - l a r i z a t i o n t o maneuver i n t o a s h u t t l e rendezvous orb i t . A l i f t vector up (180 d e g r e e bank a n g l e ) is n o t d e s i r e d as t h e perigee a l t i t u d e is n o t raised (as i s t h e case f o r t h e l i f t v e c t o r down) and more chanqe i n v e l o c i t y is needed f o r c i r c u l a r i z a t i o n . The number of r o l l reversals of t h e bank a n q l e reflects t h e amount of RCS f u e l r e q u i r e d t o m a i n t a i n t h e d e s i r e d o rb i ta l p l a n e ( t h e f e w e r t h e r e v e r s a l s , t h e less t h e amount of f u e l u s e d ) .
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Some q a i n s i n t h e AFE quidance were v a r i e d t o f i n d a set which m e t pre- a r r a n g e d e x i t c r i t e r i a and commanded a minimum number of r o l l r e v e r s a l s i n t h e bank angle . The g a i n s examined f o r t h i s purpose were GHDOT d u r i n q t h e e n t r y phase, GHDOT d u r i n g t h e e x i t phase, and K1. GHDOT is t h e q a i n a s s o c i a t e d w i t h a n a t t i t u d e dampinq f a c t o r to n e q a t e r a p i d a l t i t u d e chanqes, ( e q n . ( 5 ) i n [ 2 ] ) . K1 i s a s s o c i a t e d w i t h t h e amount of d e n s i t v v a r i a t i o n from t h e 1962 Standard Atmosphere t a k e n i n t o a c c o u n t [ 2 ] . The v a r i o u s q a i n s w e r e t e s t e d on POST for t h e 1976 Standard Atmosphere w i t h no t r a n s i t i o n e q u a t i o n s implemented as a n i n i t i a l run. Next t h e most promisinq sets o€ q a i n s w e r e t e s t e d on t h e 1962 Standard Atmosphere w i t h no t r a n s i t i o n e q u a t i o n s . F i n a l l y , t h e hest set OS g a i n s f o r b o t h r u n s w a s t e s t e d on t h e t w o a tmospheres w i t h t h e t r a n s i t i o n e q u a t i o n s inc luded .
During e n t r y i n t o t h e atmosphere, t h e AFE w i l l be conduct inq s e n s i t i v e s c i e n t i f i c exper iments [31. I n a c e r t a i n r e q i o n ( r e f e r r e d to as t h e c l e a n or c o a s t r e g i o n ) , t h e exper iments w i l l be e s p e c i a l l y a c t i v e and s e n s i t i v e . T h i s r e g i o n l ies between a p o i n t known a s t h e s c i e n c e " p o i n t " (approximate ly 85 km
.
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from t h e Earth's s u r f a c e ) and perigee ( i n t h i s paper perigee is d e f i n e d as t h e lowest a l t i t u d e a t which t h e f l i g h t p a t h a n g l e i s z e r o ) . S i n c e t h e React ion C o n t r o l System (RCS) releases gases close t o t h e v e h i c l e (and t h e exper iments ) when per forming bank a n g l e changes, a " q u i e t " ( n o maneuvering) period i n t h e guidance is r e q u e s t e d d u r i n g t h e coast r e g i o n t o reduce t h e p o s s i b i l i t y of c o n t a m i n a t i n g the area around t h e experiments . I n order t o i n s u r e t h a t no maneuvering o c c u r s i n t h i s r e g i o n , s l i g h t m o d i f i c a t i o n s were made t o t h e guidance a l g o r i t h m and t h e AFE's t r a j e c t o r y . Three methods were s t u d i e d t o o b t a i n t h e " q u i e t " p e r i o d ( a t least 20, p r e f e r a b l y 30 seconds long) d u r i n q the coast region. These methods w e r e : t h e t w o ( 2 ) d e g r e e out-of-plane hold ; t h e " p l a t e a u " hold ; and t h e "add-out/take-in" method.
The t w o (2) degree out-of-plane h o l d c o n s i s t s of s t a r t i n g t h e AFE t w o ( 2 ) degrees o u t of t h e desired orb i ta l i n c l i n a t i o n p l a n e ( a t 26.5 and n o t 28.5 d e g r e e s ) and h o l d i n g t h e bank a n g l e a t some v a l u e so t h a t t h e aerodynamic forces w i l l b r i n g t h e v e h i c l e back i n t o plane. Then t h e quidance is c u t on a t perigee. For t h i s paper , a hank a n g l e of 97.6 deqrees w a s chosen from a POST o p t i m i z a t i o n r u n which s t a r t e d t w o deqrees o u t of p l a n e , used no quidance or c o n t r o l l i n g mechanism, and m e t a l l e x i t cr i ter ia ( e x c e p t t h e i n c l i n a t i o n a n g l e ) . The t w o d e g r e e out-of-plane h o l d w a s sugges ted as a v i a b l e method because of i t s re la t ive s i m p l i c i t y , t h u s i ts e a s y implementat ion.
The " p l a t e a u " hold h e g i n s t h e trajectory w i t h t h e quidance t u r n e d on ( b e i n g accessed) u n t i l a n a t u r a l " p l a t e a u " i n t h e bank a n g l e p r o f i l e o c c u r s . The "p la teau" ' (see fig. 2 ) is t h e n a t u r a l " l e v e l i n q o f f " or t h e n e a r i n q of a c o n s t a n t v a l u e of t h e bank a n q l e to some q u a n t i t y (approximate ly 100 deqrees) j u s t p r i o r to t h e coast req ion . A t t h i s "p la teau" , t w o d i f f e r e n t methods w e r e used. F i r s t , a t a n i n e r t i a l v e l o c i t y of 32,500 f e e t / s e c o n d t h e bank a n q l e is i n c r e a s e d t o 106.88 deqrees and h e l d throuqh t h e coast req ion . Second, the guidance is c u t o f f through t h e coast r e g i o n when t h e bank a n q l e r e a c h e s 106.88 d e g r e e s . The v e l o c i t y v a l u e chosen i n method ( 1 ) o c c u r s d u r i n q t h e aforementioned " l e v e l i n q o f f " s e c t i o n of t h e bank p r o f i l e . The c h o i c e of 106.88 d e g r e e s for t h e bank a n q l e w a s p u r e l y a r b i t r a r y , b u t i t w a s r e t a i n e d a f t e r some s u c c e s s u s i n g it i n i n i t i a l runs. The " p l a t e a u " hold attempts t o d e l a y the f i r s t ro l l reversal o f t h e bank a n q l e p rof i le u n t i l a f t e r t h e coast r e g i o n , minimizing t h e RCS f i r i n q s d u r i n g t h a t t i m e . This method w a s a l so r e l a t i v e l y e a s y to implement.
The "add-out / take- in" method involved i n c r e a s i n q t h e commanded bank anqle a c e r t a i n amount a t a predetermined v e l o c i t y for a brief period (add-out ) . Then, t h e i n c r e a s e d a n g l e w a s d e c r e a s e d a small amount j u s t p r i o r t o t h e coast r e g i o n ( t a k e - i n ) . F i n a l l y , t h e la tes t bank a n q l e v a l u e w a s h e l d d u r i n q t h e coast r e q i o n . The a n q l e w a s h e l d i n t h e coast r e q i o n i n i t i a l l y f o r 20 sec- onds, t h e n 25 and 30 seconds , i f the i n i t i a l r u n w a s a c c e p t a b l e . The v e l o c i t y used €or t h e add-out t r i q q e r w a s 33,000 f e e t / s e c o n d . T h i s v a l u e for t h e v e l o c i t y w a s chosen s i n c e i t o c c u r s a t approximate ly 25 t o 30 seconds b e f o r e p e r i g e e ( f o r the nominal atmosphere case). This method a l so attempts to d e l a y t h e f i r s t r o l l r e v e r s a l of t h e bank a n q l e by reducinq t h e amount of o u t of p l a n e v e l o c i t y , t h u s , c a u s i n g t h e guidance to " t h i n k " t h e AFE does n o t r e q u i r e a r o l l r e v e r s a l u n t i l a later t i m e . This method w a s n o t as e a s y t o implement as t h e o t h e r t w o , b u t must, on a v e r a q e , have a bank a n q l e f o r t h e " q u i e t t i m e " t h a t e n a b l e s t h e AFE t o r e a c h the d e s i r e d apoqee a l t i t u d e .
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RESULTS AND DISCUSSION
The f i rs t task i n t h i s i n v e s t i q a t i o n w a s t o e s t a b l i s h a set of q a i n s t h a t would give acceptable results for t h e 1962 and 1976 atmospheres and a l l t r a n - s i t i o n e q u a t i o n s . The g a i n s varied w e r e : GHDOT(entry), GHDOT(exit1, and K1. GHDOT(entry1 w a s se t t o .18,.20,or .25. GHDOT(exit1 w a s set t o -15, .20, or .25. K1 w a s se t t o .10,.15, or .20. Var ious combinat ions of t h e s e q a i n s w e r e tested €or t h e t w o a tmospheres and f i v e t r a n s i t i o n s . The g a i n sets w i t h GHDOT(entry)=.2, GHDOT(exit)=.25, and any of t h e t h r e e values for K1 gave t h e smoothes t bank a n g l e t i m e h i s tor ies and t h e apogee a l t i t u d e s closest to 200 n a u t i c a l miles. S i n c e o t h e r i n v e s t i g a t o r s had used K1=.2 [2] t h i s v a l u e w a s used for t h e " q u i e t t i m e " s t u d i e s . The r e s u l t s of u s i n g t h e q a i n se t described above are g i v e n i n Table I and t h e bank a n g l e t i m e h i s t o r i e s are shown as f i g u r e s 2 and 3. As observed i n [ l l t h e atmosphere assumed had a s i g n i f i c a n t e f f e c t on t h e bank a n q l e r e q u i r e d t o keep the vehicle i n plane and meet t h e apogee a l t i t u d e requirement . Also, t h e t r a n s i t i o n s u s i n q t h e P o t t e r number and t h e Shock Reynolds number changed t h e bank a n g l e profiles r e q u i r e d t o meet t h e miss ion o b j e c t i v e s .
Next, t h e r u n s of t h e v a r i o u s methods t o provide a " q u i e t " p e r i o d i n t h e guidance w e r e examined. The s u c c e s s cr i ter ia w e r e reduced to meet inq t h e h e a t i n g , o r b i t a l i n c l i n a t i o n , and apoqee c o n s t r a i n t s l i s t e d above. The t w o ( 2 ) d e g r e e o u t - o f - p l a n e . h o l d r e s u l t s w e r e n o t a c c e p t a b l e . The AFE never e x i t e d the Farth's atmosphere; it lost too much enerqy t r y i n q to correct for t h e out-of-plane veloci t ies d u r i n q t h e i n i t i a l p a r t of t h e e x i t phase. The c o r r e c t i v e maneuvers caused the AFE t o spend too much t i m e i n t h e h i q h e r d e n s i t y reqion of t h e atmosphere r e s u l t i n q i n t h e e x c e s s i v e loss of enerqy. T h i s w a s t h e o n l y a t t e m p t u s i n g t h i s method t h a t w a s made, and o t h e r imple- m e n t a t i o n s of t h i s t e c h n i q u e could poss ib ly q i v e b e t t e r r e s u l t s t h a n t h e ones r e c o r d e d h e r e .
The f i r s t method i n t h e " p l a t e a u " hold gave acceptable r e s u l t s fo r b o t h t h e nominal a tmospheres ( n o t r a n s i t i o n e q u a t i o n s used) and a l l of t h e t r a n s i - t i o n r u n s when h e l d €or 20 seconds (see f i q u r e s 4 and 5 and Table 11). Rank a n g l e is d e s i g n a t e d as 4 i n Tables I1 through V I I I . When t h e " p l a t e a u " hold w a s used f o r 30 seconds t h e s t a r t i n q v e l o c i t y w a s chanqed t o 33,000 ft/sec so t h a t t h e guidance would c u t on b e f o r e p e r i g e e . Even w i t h t h i s chanqa t h e r u n s u s i n g the 1962 atmosphere went pas t p e r i q e e b e f o r e the quidance w a s c u t on (see f i g u r e 6 ) . The p a s s i n g of periqee is i n d i c a t e d when t h e dashed c u r v e ( f l i q h t p a t h a n q l e GAMMA1 on t h e f i q u r e s ) crosses zero . The r e s u l t s of t h e 30-second " q u i e t t i m e " r u n s were a c c e p t a b l e €or t h e 1976 s t a n d a r d atmosphere and a l l t r a n s i t i o n s e x c e p t t h e P o t t e r t r a n s i t i o n . The r e s u l t s w e r e acceptable €or t h e 1962 s t a n d a r d atmosphere and a l l t r a n s i t i o n s (see Table I11 and f i g u r e s 6 and 7 ) . s p h e r e s and t h e Lockheed and Shock Reynolds number t r a n s i t i o n s . t h e " q u i e t t i m e " was h e l d f o r 30 seconds w i t h b o t h nominal atmosphere cases and a l l of t h e t r a n s i t i o n r u n s q i v i n g s u c c e s s f u l r e s u l t s e x c e p t f o r t h e Shock Reynolds Number €or t h e 1976 atmosphere (see Table I V and f i q u r e s 8 and 9 ) . As was t h e case f o r t h e f i r s t " p l a t e a u " method, t h e r u n s u s i n q t h e 1976 atmo- I
s p h e r e c u t t h e guidance back on b e f o r e perigee, b u t t h e r u n s u s i n q t h e 1962 atmosphere d i d n o t ( f i g u r e 8 ( a ) ) . had a c c e p t a b l e apoqee a l t i t u d e s , t h e e f f e c t of keepinq t h e guidance o f f p a s t perigee is n o t c l e a r . c u t t i n g it back on by p e r i g e e seemed a d v i s a b l e ; however, t h e t r u e impact can
The second " p l a t e a u " method w a s run for b o t h nominal atmo- I n t h e s e r u n s
S i n c e t h e r u n s t h a t went past p e r i q e e s t i l l I
Because t h e quidance changes modes a f t e r p e r i q e a
4
o n l y be assessed when cases w i t h errors are examined. The first v a r i a t i o n of the " p l a t e a u " h o l d w a s used t o show the s e n s i t i v i t y of t h e performance of t h e g u i d a n c e to t h e c h o i c e of t h e bank angle. When the bank a n q l e w a s i n c r e a s e d t w o ( 2 ) degrees from t h e 106.88 degrees used, t h e o r i g i n a l l y acceptable 1976 atmosphere nominal r u n w a s no longer a c c e p t a b l e ( t h e apogee a l t i t u d e f e l l from 194.5 n.mi. to 153.7 n.mi.1. However, when t h e bank angle w a s increased 1.5 degrees t h e r e s u l t s w e r e acceptable ( a n apogee a l t i t u d e of 186.0). T h i s shows t h a t a t w o (2) d e g r e e error i n t h e bank a n g l e a t t h i s c r i t i ca l t i m e i n t h e guidance c o u l d c a u s e t h e t r a j e c t o r y to become unacceptab le . There is, however, about a d e g r e e and a h a l f of tolerance. I n a l l cases t h e i n c l i n a t i o n a n g l e v a r i a t i o n from nominal w a s less t h a n .OS degrees and a l t h o u g h no t shown i n tables t h e n o d a l var ia t ion w a s less t h a n .15 degrees. This n o d a l error c o u l d add up t o 30 ft/sec t o t h e AV required for c i r c u l a r i z a t i o n i n a proper orb i t for rendezvous.
A number of bank a n q l e combinat ions w e r e t r i e d d u r i n g t h e o u t - i n tech- n ique s t u d y . A t y p i c a l bank p r o f i l e i s shown as f igure l O ( a ) . The f i n a l bank a n g l e t h a t w a s t o be h e l d f o r 20 t o 30 seconds w a s cr i t ical . If t h e bank a n g l e h e l d v a r i e d more t h a n 5 d e g r e e s from t h e " p l a t e a u " v a l u e t h e n t h e r e s u l t s u s i n g t h e 1962 atmosphere were a d v e r s e l y a f f e c t e d . Rank a n q l e s of 5 degrees or greater from t h e " p l a t e a u " v a l u e i n many cases gave improved r e s u l t s when a 1976 atmosphere w a s used. The cases shown i n t h i s paper t o i l l u s t r a t e the t e c h n i q u e a l l r e t u r n e d t o t h e " p l a t e a u " bank a n g l e . These r u n s were chosen because t h e y went throuqh a normal quidance sequence w h i l e i n t h e atmosphere for b o t h atmospheres and a l l t r a n s i t i o n s . While t h e apoqee a l t i - t u d e w a s n o t acceptable f o r most of t h e 1976 atmosphere r u n s , t h e s e r u n s w e r e n o t t e r m i n a t e d e a r l y because t h e y had v i o l a t e d a c r i t e r i o n of t h e s i m u l a t i o n .
The r e s u l t s of t h e 25 d e q r e e s out - in r u n s w i t h a 20-second " q u i e t t i m e " are shown as Table V and f i g u r e s 10 and 1 1 . The r e s u l t s were acceptable f o r a l l t h e r u n s u s i n g t h e 1962 atmosphere, b u t f o r o n l y one r u n u s i n q t h e 1976 atmosphere. when t h e " q u i e t t i m e " w a s extended t o 30 seconds a l l t h e 1962 atmosphere r u n s w e r e s t i l l a c c e p t a b l e , b u t t h e apoqee a l t i t u d e a t t a i n e d degraded and t h e number of r o l l r e v e r s a l s became larqe. Only one of t h e 1976 atmosphere r u n s had an acceptable apoqee a l t i t u d e , b u t t h e apoqee a l t i t u d e s of a l l r u n s w e r e closer to t h e d e s i r e d a l t i t u d e (Table V I and f i q u r e s 12 and 13) .
The r e s u l t s of the 35 degree o u t - i n r u n s are shown as Tables VI1 and V I I I , and f i g u r e s 14 through 17. The 20-second " q u i e t t i m e " r u n s were a c c e p t - able for b o t h atmospheres and all t r a n s i t i o n s , b u t the number of r o l l rever- sals a g a i n w a s l a r g e f o r t h e 1962 atmosphere runs . When t h e " q u i e t t i m e " w a s ex tended to 30 seconds none of t h e 1976 atmosphere r u n s had acceptable apoqee a l t i t u d e s , b u t t w o of t h e r u n s were close. All of t h e 1962 atmosphere r u n s had a c c e p t a b l e apoqee a l t i t u d e s and t h e run u s i n q t h e Shock Reynolds number had o n l y t w o r o l l r e v e r s a l s .
An examinat ion of t h e r u n s shown i n d i c a t e s t h a t a combinat ion of o u t - i n a n g l e s could be found t h a t would q i v e acceptable r e s u l t s €or a l l cases under i d e a l c o n d i t i o n s . However, i n t h e presence of p o t e n t i a l d i s p e r s i o n s any com- b i n a t i o n of o u t - i n a n g l e s could have t r o u b l e meet ing t h e apoqee requirement . Another possible problem is t h e amount of maneuverinq r e q u i r e d j u s t b e f o r e t h e " q u i e t t i m e " . S i n c e t h e purpose i n c u t t i n g t h e quidance off is to have as l i t t l e RCS contaminat ion as p o s s i b l e d u r i n q t h e t i m e o f maximum s c i e n t i f i c i n t e r e s t , a minimum of maneuverinq b e f o r e t h e " q u i e t time" seems d e s i r a b l e .
5
As before many of t h e r u n s d i d n o t c u t t h e guidance on u n t i l a f t e r p e r i g e e . F i g u r e s 14 th rough 17 show t h a t t h e guidance w a s o f f f o r a l l cases when the 35 degree out - in m o d i f i c a t i o n to the guidance w a s used. When t h e 25 d e g r e e out- in m o d i f i c a t i o n w a s used the guidance w a s c u t back on b e f o r e p e r i g e e for t h e 1976 atmosphere runs , b u t n o t f o r t h e 1962 atmosphere runs. S i n c e a l l t h e 1962 atmosphere runs had a c c e p t a b l e apogee a l t i t udes f o r t h e i d e a l case l e a v i n g t h e guidance off p a s t p e r i g e e cou ld be acceptable, bu t a t p r e s e n t t h i s does n o t seem d e s i r a b l e . As w i t h t h e p l a t e a u runs , t h e i n c l i n a t i o n a n g l e error w a s less than .05 d e g r e e s and t h e nodal error less than .15 degrees .
CONCLUDING REMARKS
A set of g a i n s t h a t gave r easonab le guidance performance f o r t h e 1962 and 1976 s t a n d a r d atmospheres and f ive t r a n s i t i o n models w a s e s t a b l i s h e d . The guidance wi th t h e s e g a i n s w a s used t o i n v e s t i g a t e m o d i f i c a t i o n s t o t h e AFE guidance t o e s t a b l i s h a " q u i e t t i m e " j u s t before p e r i g e e of t h e a e r o p a s s t r a j e c t o r y . The m o d i f i c a t i o n s w e r e i n v e s t i q a t e d f o r i d e a l c o n d i t i o n s and no errors or a tmospher ic d i s p e r s i o n s were in t roduced . S e v e r a l o f t h e methods gave a c c e p t a b l e resu l t s f o r a 20-second " q u i e t t i m e " . When t h i s t i m e w a s extended to 30 seconds , runs u s i n g some of t h e t r a n s i t i o n models w i t h t h e 1976 atmosphere d i d n o t meet t h e apogee a l t i t u d e t a r g e t s . The runs u s i n g t h e 1962 atmosphere d i d n o t c u t t h e guidance back on u n t i l a f t e r p e r i g e e i n a l l cases, b u t a l l cases meet the cr i ter ia for s u c c e s s f u l runs . The g e n e r a l conc lus ion from t h e s t u d i e s was t h a t a 30-second " q u i e t time" was p o s s i b l e , b u t t h e effect of l e a v i n g t h e guidance o f f f o r t h a t lonq must be better assessed t o assure t h e s a f e t y of t h e miss ion .
The s t u d i e s r a i s e d a number of q u e s t i o n s and po in ted toward s e v e r a l i n v e s t i g a t i o n s t h a t are r e q u i r e d to de termine t h e m o d i f i c a t i o n s t h a t can be s a f e l y made to t h e AFE guidance to g i v e a " q u i e t t i m e " of 30 seconds. The q u e s t i o n s t h a t need t o be answered are:
1 . What is t h e e f f e c t o f l e a v i n g t h e guidance o f f p a s t pe r iqee?
2. What q u a n t i t y o r q u a n t i t i e s should be monitored to de termine i f t u r n i n g t h e guidance o f f is endanger ing t h e miss ion?
3. The methods examined assume a f i x e d bank a n g l e f o r 20 o r 30 seconds. I n t h e a c t u a l case t h e r e w i l l be some d r i € t i n t h e bank anqle. How much can t h e bank a n g l e change and have t h e v e h i c l e e x i t t o a n a c c e p t a b l e apoqee?
4. W i l l t h e " q u i e t t i m e " r equi rement for t h e AFE miss ion d i c t a t e a d i f f e r e n t gu idance scheme?
5. Can q a i n s be de te rmined so t h a t t h e quidance w i l l be less s e n s i t i v e to t h e atmosphere encountered?
To better assess t h e impact of a 30-second " q u i e t t i m e " a d d i t i o n a l s t u d i e s must be conducted. A part ia l l i s t is:
1. Conduct s t u d i e s t h a t i n c l u d e system e r r o r s and a tmospher ic d i s p e r s i o n s to see i f l e a v i n g t h e guidance o f f p a s t p e r i g e e is p r a c t i c a l .
6
2. If t h e guidance must be c u t on a t or b e f o r e p e r i g e e , de te rmine t h e best way t o modify t h e e x i s t i n g guidance t o g e t a 30-second " q u i e t t i m e " b e f o r e p e r i g e e u s i n g a minimum of maneuvers.
3. I f m o d i f i c a t i o n of t h e e x i s t i n g guidance becomes too invo lved , c o n s i d e r o t h e r guidance schemes such as an a d a p t i v e system o r a p r e d i c t o r - c o r r e c t o r method f o r t h e e n t r y phase of t h e b r a k i n q maneuver.
4. A f i x e d set of g a i n s is d e s i r e d f o r t h e AFE guidance. The g a i n s used i n t h i s s t u d y are €or t h e "nominal" case, n o t t h e " q u i e t t i m e " case. When a " q u i e t t i m e " method h a s been determined a set of g a i n s t h a t r e s u l t i n f e w e r r o l l r e v e r s a l s must be designed.
REFERENCES
1. Minier , E l i z a b e t h A. and S u i t , W i l l i a m T.: " E f f e c t of T r a n s i t i o n A e r o - dynamics on A e r o a s s i s t F l i g h t Experiment T r a j e c t o r i e s " , NASA TM-100546, 1988.
2. C e r i m e l e , C. J. and G a m b l e , J. D.: " A S i m p l i f i e d Guidance Alqorithm f o r L i f t i n g A e r o a s s i s t O r b i t a l T r a n s f e r Veh ic l e s" , A I A A Paper 85-0348, A I A A 23rd Aerospace Sc iences Meeting, Reno, Nevada, .January 1985.
3. Jones , J i m J.: "The R a t i o n a l e € o r an A e r o a s s i s t F l i g h t Experiment", A I A A Paper 87-1508, A I A A 22nd lhermophysics Conference, Honolulu, H a w a i i , ,June 1987.
7
~
Max h e a t rate
Btu 151.9
f t2-sec
Btu 151.8 f t -sec
__-
Rtu 2
151.8 f t -sec
Rtu 149.9
f t -sec
AV to c i r cu lar i ze
300.4 f t / s e c 1962
Atmosphere -
269.9 it/sec 1962
- 1962 298.5 i t /sec
297.5 i t /sec 1 1962 I
Rtu 149.6
f t -sec
R t u 2
147.1 f t -sec
~
I
315.9 f t / s e c : 1976
i I I
325.9 ft/sec 1 1976 !
Rtu 147.6
i t -sec
- I
318.0 f t / s e c I 1976
I
TABLE I.- RUNS WITH GUIDANCE
Trans i t i on mode 1
None
Viking
Lockheed
P o t t e r
I n c l i n a t i o n angle
Apogee a1 ti tude
28.43 deg 199.65 nm'
~~ ~
196.7 nm 28.41 deg
_ - - - ~
28.42 deq
28.47 deq
~~ ~
197.7 nm
195.2 nm
I
i 150.4 264.4 ft /sec 1 1962
i t -sec 28.46 deq 192.8 nm Shock Revnolds
Number
Vikinq
Rtu 149.6
f t -sec 28.43 Aeg 197.3 nm 313.5 f t / s e c , 1976
!
1976 I i 1 322.1 f t / s e c ' 2
149.5 f t -sec
28.48 deg 196.2 nm
Lockheed I- P o t t e r
28.46 deq 195.7 nm
192.8 nm 28.45 deq
28.48 deq 195.1 nm Shock Reynolds Number
8
Run d e s c r i p t i o n Atmosphere
AV to and c i r c u l a r i z e Reve r sa l s t r a n s i t i o n
~~ -~ ~~ ~
:ut B V = 3i ,500 f t / s e c io ld + = 106.88O
L i f t tendency a t e x i t
:ut @ v = 32,500 f t / s e c iold + = 106.88O
293.8 f t / s e c
295.7 f t / s e c
- TABLE 11.- BANK ANGLE HOLD POR 20-SECOND "QUIET TIME"
1962 8 Vikinq Down
- 1962
8 Lockheed Down
: nc l ina t io r ang le
183.0 f t
184.3 f t
184.4 f t
180.0 f t
28.53
Btu 2 148.7 -
f t -sec
R t U
2 148.8 -
f t -sec
Btu 146.7 - f t2-sec
147.3 - f t l - s e c
Btu
28.52
301.2 f t / s e c
304.5 f t / s e c
304.6 f t / s e c
298.2 f t / s e c
28.53
1976 7 Vikinq Down
1976 7 Lockeed Down
1976 6 P o t t e r UP
1976 6 Shock
Reynolds UP
28.53
~
28.54
28.54
28.53
28.53
28.53
28.54
Z J T Apoqee Max h e a t
f t -sec 1 Btu 195.4 f t 151.7 7
f t -sec 1 Btu 196.2 f t 151.7 -
f t2-sec
Btu 2 190.2 f t 150.4 -
f t -sec
1962 303.2 f t / s e c l 10 I None I Up I
9
TABLE 111.- BANK ANGLE HOLD FQR 30-SECOND "QUIET TIME"
Atmosphere and
Angle a l t i t u d e rate c i r c u l a r i z e Reversals t r ans i t i on
I RIM d e s c r i p t i o n I n c l i n a t i o n Apogee Max h e a t AV t o
1962 NOne 8
u t B v = 33,000 f t / s e c B t U o l d 0 = 106.88' 28.52' 198.5 f t 151.7 7 298 f t / s e c
f t -sec
L i f e Tendency a t e x i t
Down
28.52'
Viking I I l o I Lockheed Down B t U 196.7 f t 151.6 7 295.7 f t / s e c
f t -sec 8
1962
I I I I I I I
I I 1962 I
I I I I I
:ut @ v = 33,000 f t / s e c [old 0 = 106.88'
v
I Down
P o t t e r I 28.520 194.8 f t 150.0 7 295.3 f t / s e c 8 I I I f t BtU -sec I t Shock B t U 10 28.52' 186.4 f t 150.2 7 288 f t / s e c
Reynolds UP f t -sec
1976 B t u 8 None UP 28.540 181.2 f t 148.1 7 290 f t / s e c
f t -sec
1976 UP Vikinq R t U
28.53O 175.2 f t 148.0 7 282.2 f t / s e c 8 f t -sec
1976 B t U Lockheed UP 28.52' 180.3 f t 148.0 7 290.4 f t / s e c 8
f t -sec
1976 P o t t e r Down B t U R 28.52' 173.6 f t 146.1 7 279 f t / s e c
f t -sec
1976 Shock 6 B t U
213.52~ 176.5 f t 146.7 7 291 f t / s e c R e yno Ids UP f t -sec
.
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TltlE (a ) No t rans i t ion .
Figure 2. Bank angle time h i s tory for runs w i t h 1962 atmosphere.
17
B N K A N C
(b) Viking t rans i t ion .
Figure 2. continued.
(c) Lockheed t r a n s i t i o n .
Figure 2 . Continued.
T IHE ( d ) Potter t r a n s i t i o n .
Figure 2. Continued. .
20
.-
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TiHE
(e ) Shock Reynolds number t r a n s i t i o n .
Figure 2. concluded
21
TlHE
( a ) N o t rans i t ion .
Figure 3. Bank angle t i m e h i s t o r y for runs with 1976 atmosphere.
22
B N
b. Viking t rans i t ion .
Figure 3 . Continued.
1
TIHE
23
TIHE (c ) Lockheed t r a n s i t i o n .
Figure 3 . Continued.
24
200
in0
0
- 100
-200 0 100 208 380
TIME ( d ) Potter t r a n s i t i o n .
Figure 3. Continued.
25
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(e) Shock Reynolds t r a n s i t i o n .
Figure 3 . Concluded.
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Report Documentation Page 1. Repon No. 2. Gomnmrnt Acamon No.
NASA TM-100556
4. T i and Subtitb
3. R ~ i p i r c r t ' ~ c a w NO.
5. Repof t 0810
Aeroassist F l i g h t Experiment Guidance "Quiet Time"
I 8. Performing Organization Repoft No. 7. AuthorW
S c o t t A. S t r i e p e and W i l l i a m T. S u i t
February 1988
9. Performing Orgrnuation Name and Address
NASA Langley Research Center Hampton, VA 23665-5225
12. Sponsoring Agency N w and Address
Nat iona l Aeronaut ics and Space Adminis t ra t ion Washington, DC 20546-0001
70: Work Unit No.
506-46-21-01
11. Contract or Grant No.
13. T W Of R W and P ~ i o d COW^
Technica l Memorandum
14. Sponsoring 4gency Coda
19. Socuritv C l a d . (of tha report) 20. s.Curm C M . (of tha pagal 21. No. of pages
U n c l a s s i f i e d Unc las s i f i ed 77
S p a c e c r a f t Guidance Aeroass is t S pacec ra f t Maneuvers AFE F l i g h t P r o j e c t
22. Prue
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