Lunar Surface Mechanical Properties

JOURNALOF GEOrItYSICAL RESEARCIt VOL. 73, NO. 22, NOV•-a•B•.R15, 1968

E. M. CHRISTENSEN,1 S. A. BATTERSON,2 H. E. BENSON,3 R. CHOATE,1 R. E. HUTTON,4 L. D. JAFFE,1 R. H. JONES,5 H. Y. No,6 F. N. SCHMIDT,7 R. F. SCOTT,8 R. L. SPENCER,1 AND G. H. SUTTON9 The surfacematerial at the Surveyor5 site is granular and slightly cohesive.Spacecraft footpadsplowed trenchesin this material as the spacecraftslid during landing.For a com- pressiblesoil model, a static bearing capacity of 2.7 newtons/cm•' gave best agreementwith the observations. Static firing of the vernier engines against the surface moved surface particles; a crater 20 cm in diameterand about 1 cm deep was produced,apparently at engine shutdown.The permeability of the soil to gases,to a depth of about 25 cm, is 1 X 10-s cm.ø, correspondingto soil particles mostly 2 to 60 t• in diameter.

SPACECRAFTLANDING angular positions, one accelerometerreading, Description. The basic configuration and and three vernier engine thrust commands. landing mechanismfor Surveyor 5 were essen- 2. Analog signals monitoring three strain- tially the same as for Surveyors 1 and 3. Dur- gage bridges, one on each landing leg shock ing the landing, the three landing legs rotate absorber,indicating its axial loading. upward against the resistance of the shock 3. Post-landing television camera coverage absorbers. Following the initial impact, the of footpads,crushable blocks, and areas on the shock absorbers re-extend, returning the legs lunar surface in which these spacecraft mem- bers contacted the surface and came to rest. to their pre-touchdown positions. Additional capability for energy dissipationis provided by 4. Post-landingattitude determinationsbased crushablefootpads and by crushablehoneycomb on the position of the planar array antenna, blocks mounted on the underside of the space- horizon sightings,and star sightings. frame, inboard of each leg. An evaluation of the data indicates that, at an The actual landing of Surveyor 5 can be re- altitude of 4.8 _ 0.7 meters, all three vernier constructedquite accurately from various telem- engines were cut off, resulting in a free-fall etry signals together with available landing- period, during which the spacecraft vertical dynamic simulations.Pertinent telemetry data velocity increased to 4.2 __+0.4 m/sec at the are: time leg 1, the first to contact, encounteredthe 1. Digital indicationsof spacecraftaltitude, lunar surface.After leg 1 contacta suddenpitch three orthogonal velocities, three orthogonal motion, away from leg l, occurred with a velocity in excessof 13 deg/sec.Legs 2 and 3 contacted the ground almost simultaneously:leg 2 x Jet Propulsion Laboratory, California Insti- 190 msec,and leg 3 197 msecafter leg 1 impact. tute of Technology, Pasadena, California 9'1103. 2 Dynamic Loads Division, NASA Langley Re- A period of spacecraft slide lasting approxi- search Center, Hampton, Virginia 23361. mately 1.7 secfollowed, during which the space- a Manned Spacecraft Center, Houston, Texas craft rolled approximately +5.9 ø counterclock- 77058. wise, as seen from above. Christensen et al. 4 TRW Systems,Inc., Redondo, California 90278. [1967b] give further details of the landing and • Hughes Aircraft Co., E1 Segundo, California 90045. of the observationsand analysesoutlined below. o University of Colorado, Boulder, Colorado Figure 1 showsthe time histories of the axial 80302. forcesin the landing-gearshock absorbers from 7 Bellcomm, Washington, D.C. 20036. before surface contact until after the space- s California Institute of Technology, Pasadena, California 91109. craft reached its final position. For each leg, 9 Hawaii Institute of Geophysics,University of the high loading caused by the first impact Hawaii, Honolulu, Hawaii 96822. lasted approximately 0.2 to 0.25 sec. This was

7169 7170 CHRISTENSEN ET AL.

8øøø1LEG I

-4000 8000/ LEG2 r•-7280 1 ! ......

-4000 'øøø/'• rV •øø / 7 - _ _ ......

.ooo-80001 I• I sec •Fig. 1. Strain-gage telemetry data showingshock-absorber axial load historiesduring landing of Surveyor 5. Maximum forces shown are ___350newtons. followedby a near-zero force period that lasted oscillation, the strain gages indicated a small approximately 0.6 to 0.8 sec, indicating a re- loading, correspondingto the static loading of bound of the spacecraftcaused by the landing- the shock absorbers and to the 480-newton lunar gear spring forces.Finally, a secondlow-energy weight of the spacecraft. (1 newton = 105 impact was registered, followed by a poorly dynes = 0.225 lb; I newton/cm2 = 1.45 lb/in?) defined,low-amplitude oscillation. Similar oscil- The final position of the spacecraft, follow- lations, observedduring' the Surveyor I and 3 ing the slide, is about 19.5ø from vertical. touchdowns,were related to the combinedelastic Television observationso] spacecraft-soilin- properties of the spacecraft and the lunar sur- teractions. Surveyor 5 landed on the inner face [Christensenet al., 1967a, 1968]. After the slope of a crater, 9 meters wide, about 12

Fig. 2. Wide-angle mosaic of footpad 2 and the trench formed during landing of Surveyor 5. The depressionformed during the first impact of footpad 2 can be seen at the right-hand end of the trench. The top of the footpad is 30 cm in diameter. (Picture taken September 14, 1967,between 04h 00m and 06h 00m GMT; catalog 5-MP-19.) SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7171 meters long, and more than I meter deep.After material consistsprimarily of very small par- initial touchdown,the spacecraftslid downslope, tides. creating clearly visible surface disturbances. The movement of footpad 3 also causedsome At impact, footpad 2, after possiblygrazing a trenching during the landing and subsequent fragment 12 cm in diameter, penetrated the sliding motion. The visibility of this area to soil at the right-hand end of the trench shown the televisioncamera is p.artiallyobscured, but in Figure 2, forming a depressionabout 12 cm it appearsthat footpad 3 moved approximately deep and ejecting material radially for a dis- the same distance as footpad 2. Like footpad 2, tance up to 80 cm. As the spacecraft slid, the it tipped downward during the trenching, and outboard rim of footpad 2 tipped downward lunar material was deposited on top of both and the soil in front of it was pushedand thrown footpads as they plowed the surface (Figures forward and sideward, forming the trench, 2 and 3). No visible soil was depositedon the which is 8 to 10 cm deep at the uphill end, near footpad tops of Surveyors1 or 3 during land- the impact depression,and 3 to 6 cm at the ing. Surveyor 5 footpads2 and 3 are tilted .ap- downhill end. The ejected material extends approximately 16ø relative to the plane of the proximately 30 cm beside the trench and 75 three footpads. cm beyond the resting position of footpad 2, in The crushable blocks contacted the lunar the direction of spacecraft slide. The trench is surface during the landing. Figure 4a is a nar- almost straight and its length indicatesthat the row-angle view of crushableblock 3 in which footpad slid 81 -----2 cm. The rim of the trench a small rock or clod appears to be wedged be- crumbled, partially filling the bottom. The tween the block and its thermal shield. In a smoothappearance of an area wherethe footpad picture taken later, at a low sun angle, this scrapedalong the trench wall showsthat the fragment is no longer visible, but a deposit of

Fig. 3. Narrow-angle picture of •op of footpad 3 showing •he lunar material on the footpad (September 22, 19.67,13h 48m 13s GMT). 7172 CHRISTENSEN ET AL. (a)

(b)

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•ig. •. •arrow-angle view •aEenthrough the auzilia• mirrorashowing crushable blocE with m•erigl probablypicEed up during •he 8r• landingimpact. (•) •ote small rote or clod wedgedbetween •he blocE •nd •he thermal shield (September•0, 1967,ll• (•) •o•e lunar material adheringto the bottom o• the blocE G•). SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7173

22,200 MISSION DATA respectively. Because of limited visibility, it is not possibleto estimate the initial penetrations 17,760 A0 LEGLEGS I 2 AND :5 } ANALYTICALRESULTSof footpads 1 and 3 of Surveyor 5. The analytical simulation indicates that crushableblocks 2 and 3 each penetrated about 8 cm and that 15,$20 crushable block 1 did not touch the surface. Because of computer program limitations at 8880 LEG :5 this early stage of the dynamic study, the soil model used in this analysis is completely com- 444 pressible; the force F developedon the footpad is expressedby IDletrick et al., 1966]

0 0.1 0.2 0.$ 0.4 0.5 0.6 TIME AFTER INITIAL CONTACT, sec F = poA(1 q- cs)q- P'p• A.•• P2 -- Pt Fig. 5. A comparison of Surveyor 5 landing where data with analytically obtained shock-absorber force/time histories. p0 = static bearing pressureof surface. A_= effectivefootpad area. soil particles can be seen adhering to the bot- c - frictional constant. tom edgeof the block (Figure 4b). No clearevi- s = depth of penetration. dence of crushableblock imprints was obtained. & - velocity of penetration. However, the probablelocations at which crush- p• - original density of soil. able block imprints might exist are in areas p• = density of soil compressedby footpad. obscured or shaded by the spacecraft. Figure 6 illustrates the soil model being pene- The appearance of the lunar surface material trated by a footpad. The surface material ini- at the Surveyor 5 landing site is similar to that tially of density pl is compressedat pressurepo in the vicinity of the Surveyor 1 and 3 landing to a density p• under the penetration of a foot- sites [Christensenet al., 1967a, 1968]. The soil pad. Forces resisting penetrations are the static is granular, slightly cohesive,and generallyfine- bearing pressurethat is assumedconstant with grained. Some lighter-appearing fragments are depth, friction that increaseslinearly with pene- seen and presumably are rocks. Darker-appear- tration, and soil inertia. For the above soil ing fragments are presumed to be soil aggre- model the assumed relationships between the gates, some of natural origin and others pro- density of the soil, density of the soil com- duced by the spacecraft landing. The material pressedby a footpad, and static bearing pres- ejected from the surface by the Surveyor 5 sure are shownin Figure 7. As indicated,for a footpads exhibits less brightnesscontrast with surfacewith a bearing strength of 2.7 newtons/ the undisturbedsurface than did the soil ejected cm•, the assumeddensity of the undisturbed by Surveyors1 and 3. material would be 1.1 g/cm8. This value is Dynamic simulations o/ landing. Computer lower than some estimatesderived from pre- simulation studies of landings, using several vious Surveyor landingsand may be changed analytical soil models, are being performed to as the agreementbetween simulation and actual estimate the mechanical properties of a surface material that will display penetrations and shock-absorber axial loads similar to those obtained during the Surveyor 5 landings.The best correlation obtained to date is shown in Figure 5, which compares the shock-absorber force histories for a simulated landing on a lunar surface with a 2.7-newtons/cm2 static bearing strength, with the histories from Surveyor 5. The penetrationsby footpads 1, 2, and 3 ob- Fig. 6. Soft-surface model for the landing dy- tained in this simulation are 6, 12, and 12 cm, namics analysis. 7174 CHRISTENSEN ET AL.

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02 • 4.0 b.I

z 3.0

(-) 2ø0

0 0.5 1.0 1.5 2.0 SOILDENSITY, p, g//cm 3 Fig. 7. Assumed relationship between surface bearing pressureand the soil density at the surface. landing is improved in subsequent solutions. and in the normal force of the footpadsagainst Dynamic analyses using incompressible soil the lunar surface, as indicated by the strain models have not yet been performed for Sur- gagesand trench depths. Perhaps a more mean- veyor 5. ingful value is the average stopping pressure Simulationsof the earlier missions,using the supplied by the soil against the footpads.Esti- samecompressible soil model,showed good cor- mating an average depth of penetration of 5 relation with landing data for static bearing cm, this pressureis 0.9 newtons/cm2. This value pressuresof 3.4 newtons/cm•, rather than the is consistentwith the stalling pressureobserved 2.7 newtons/cm•, which best matches Surveyor duringthe Surveyor3 lunar trenchingoperations 5 data. However,the downslopelanding of Sur- using the soil mechanicssurface sampler [Scott veyor 5 produceda horizontal loading on the and Roberson,1968]. soil with a possible deformational mode that LUNAR SOIL EROSION TEST--OBSERVED EFFECTS would result in greater footpad penetration. These preliminary results suggestthat the To provide data for estimatingthe amount lunar surfacematerial at the Surveyor 5 land- of soil erosionduring a landing of the Apollo ing site is somewhat weaker than the material Lunar Module (LM) and to determine such at the previous landing sites, or that its re- lunar surfaceproperties as permeabilityto gases, sistanceis less becauseof the slopingsurface cohesion,and particle size, the Surveyor 5 on which the landingtook place. liquid-propellant vernier engines were fired Analysis o[ spacecraft. Since the spacecraft against the lunar surface for 0.55 _--+0.05 sec is at rest on a 20ø slope,the minimum effective on September13, 1967, at 05h 38m GMT. En- coefficient of friction (braking) between the gines i and 3 fired at a 120 _+ 22-newton thrust footpads and surface is tan 20ø -- 0.36. An level, and vernier engine2 fired at a 76 _+ 18- analysisof the dynamicsof the spacecraftslide newton thrust level. During the firing, the by Christensenei al. [1967b] indicatesthat the spacecraftfootpads remained in place on the average stopping force correspondsto an ef- lunar surface. fective coef•cient of friction of 0.73. These re- The effectsof the vernier enginefiring were sults are, however,of limited value becausethe observedby comparingtelevision pictures ob- footpads penetrated the surface. There were tained beforeand after the firing. The location obvioussignificant variations in the trenching and relationshipof the Surveyor 5 vernier en- SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7175 gines and television camera to the areas where Area around a-scattering sensor head. The erosioneffects were detectedand to other space- surface area immediately adjacent to the a- craft componentsare shown in Figure 8. The scattering instrument shows most clearly the erosioncaused by vernier engine3 provided the extent and amount of soil disturbance caused primary erosion experimental data. The area by the vernier engine firing. Figures 12a and under vernier engine i was partially visible. 12b are controlled mosaics composed of nar- Lunar surfacebelow vernier engine3. Changes row-angle, pre- and post-firing pictures of the in the lunar surface causedby erosion can be area (Aw in Figure 8). A comparisonindicates seen in Figures 9 and 10 in the areas labeled that the firing caused a number of changes, E3Mand E3. in Figure 8. It is evident that there including movement of the a-scattering instru- has been: ment sensorhead, movements of rock and soil 1. Erosion of soil from area E•M; most soil fragments, and alteration to general surface fragments identifiable in Figure 9 were moved features. by the firing. In places the basic soil surface has been 2. Formation of a shallow crescent-shaped changedby the firing. Clear evidenceof this is crater directly below vernier engine 3. The shown by the track, about 2 mm deep and 58 crater is 20 cm in diameter and 0.8 to 1.3 cm cm long, indicated by the dotted line in the pre- deep; its open end points approximatelytoward firing picture (Figure 12a); fragment h, prob- the sensor head of the a-scattering instrument. ably ejected during the landing, made the track Figure 11 is a contour map of the crater and as it rolled downhill. In Figure 12b, this track profilesacross it. no longer exists,having been filled in or eroded

/ / /-- AREAAv3 DEPLOYED - Y AXIS• / SENSOR HEAD •./.,•--AR EA PVN AREAv3'•'•.•f • VERNIER I • ENGINE5 HELIUM ]

FOOT PAD $ TANK [ /

EROSION NITROGEN • J CRATER TANK

CAMERA AREA E3O CRUSHABLE BLOCK 2

AREA E3M VERNIER ENGINE2

CRUSHABLE BLOCK 5 +)('AXIS / / / L Irn I COMPARTMENT B

LARGE AUXILIARY AREA ElD MIRROR

VERNIER ENGINE I CRUSHABLE BLOCK I

i Fig. 8. Drawing showing the relationship of the spacecraft to lunar surface areas affected by the vernier engine firing and viewed by camera. 7176 CHRISTENSEN ET AL.

(a)

(b)

away by the firing. Another example is the evidencesuggests the latter. For example,in material to the right of the helium tank, ejecta area Z, somesoil aroundfragment g and partly depositedduring the landing, which is not visi- coveringrock a was definitelyswept away, since ble after the firing. Although the eject• could rock a is exposedto • somewhatgreater depth have beencovered or sweptaway by the firing, after the firing (Figure 13). For this area, 40 SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7177

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Fig. 9. •arrow-anglemosaics of the lunarsurface beneath vernier engine 3, asseen through the auxiliarymirrors, before (a) and after (b, c) on-surfacefiring. Part of the spacecraftframe and one crushableblock are visible at the top of the picture. (a) September13, 1967, 01h 15m37s GMT; catalog5-MP-17. (b) September13, 19'.67,07h 33m 40sGMT; catalog 5-MP-18. (c) September22, 19'.67,14h 08m 49s GMT; catalog5-MP-22.

to 60 cm from vernier engine 3, the estimated partly buried.Figure 14 is a plot of fragment depth of deposition,erosion, or soil replace- diameterversus pre-firing distance from vernier ment is I cm or more. Soil movementwas prob- engine 3, for some of the larger fragments, ably aided by the 20ø surface slope. showingwhich moved during the firing and None of the fragments that moved can be which did not. The circlesplotted in this fig- positively identified in both the pre- and the ure indicate fragments that probably were post-firing mosaics.In some cases,this could lying on the surfaceand not partly buriedprior be due to movement of particles into the area to firing. The figure indicatesa size-distance from locationsnot in the pre-firing mos_aic,or, boundarybelow which a fragmentcould have in other cases,particles shownin the pre-firing been moved by the firing. mosaic could have moved out of the area during The s-scatteringsensor head was displaced the firing. It is also possiblethat some of the by the vernierengine 3 firing.In Figure12b, fragments appear, but, becauseof the move- pointsA and B are the pre-firingpositions of ment and their irregular shape, they present corners A' and B • of the sensor head. different distinguishingfeatures to the camera, In Figure 15a (pre-firing),the imageof the or broke apart, and therefore cannot be iden- sensorhead circularplate is clearlyreflected by tified as the same fragment. Fragmentsa the gold-platedfront of faceI). After the firing through f did not move and therefore appear (Figure 15b), no reflectedimage of the plate in both Figures 12a and 12b. In general,these can be seen.The entire surface of D appears to are the larger fragments; many appear to be be nonreflective,with the bottom 3 em appear-

SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7179

(c)

Fig. 1•). Narrow-anglemosaics of the lunar surfacebeneath vernier engine3, before (a) and and after (b, c) on-surfacefiring. Two spacecra.œttanks and the electronicsbox on the bottom bound the pictures. A spacecraœtstructural member divides the visible lunar surœace.(a) Sep- tember 12, 1967, 00h 58m 58s GMT; catalog 5-MP-37. (b) September 21, 1967, 05h 36m 31s GMT; catalog 5-MP-40. (c) September22, 1967, 13h 36m 49s GMT; catalog 5-MP-42. ing darker than the top 10 cm. This changeis firing. The area is 120 to 130 cm from the probably causedby the adherenceof fine lunar engine center line; the largest fragment dis- material. Erosion debris covers the intersection placed was 2.0 cm in diameter. of face D and the plate. Fragments and soil Footpad 2 area. In the footpad 2 area, appear to have landed on and near the plate fragments that can be seen to have moved are after the sensorhead stoppedsliding when the entirely limited to the lower-left quarter of leadingedge of the plate dug into the soil (Fig- Figure 16. Though only a relatively few frag- ures 15b and 12b). ments of 1-cm diameter or larger have been dis- Footpad 3 area. A fragment-by-fragment placed, the fine soil between the larger frag- study of the footpad 3 area was made by com- ments in the area to the left of footpad 2 was paring individual pre- and post-firing pictures disturbed by the firing. Soil was blown off the using a blink technique. None of the numerous magnet bracket and control bar on the left of soil fragmentsoutboard of footpad 3 were found the footpad. to be displacedby the firing. This area is, how- Footpad 2 trench area. Frame-by-frame ever, at least partly shielded from the direct comparisonof the footpad 2 trench, using a blast of vernier engine 3 by footpad 3 and its blink technique,shows no visiblechange caused leg. The soil to the right of the footpad is not by static vernier firing. Distance along the shieldedfrom vernier engine 3, and many of ground from the center line of vernier engine the fragments here were swept away by the 2 to the trench ranges from 90 to 115 cm. I ./ /

o 5 io I i ' ' ' I , , , i I CENTIMETERS ORIENTATION OF CRATER AS CONTOUR INTERVAL = O. Z5 cm SEEN THROUGH MIRROR BY TV CAMERA (b) PLAN VIEW OF CRATER

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DISTANCE, cm

Fig. 11. Plan view and profiles of the crescent-shapedcrater produced by the vernier engine 3 firing (taken from a drawing by the Mapping Science Branch, Lunar and Earth ScienceDivision, NASA Manned SpacecrMt Center). SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7181

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• ....•::• •'•.:. ' bz..""':,. '" "%•'-" •,'..':'" •':•':• k•-:..... &::'•> .• •e:?:•-•' :'•-::':•:-:'.'"" ß<' ß:•: ":•'•":':" '

•- --• ,•.._a:• ß ...?,•..•..... >::::•::.,...•:• ...... ,•-,-...... :.::"y:.., :., .... •:,•-:.,, •'::::,?:•:•":•-:•:'" •"'•.'::•? ":•-:"..:. ,•::• :...':':• :"•:•:'...... '""::"..-'•:.-L:.: •"':•'•••'••'••:::::.•.•, ...... :.::N:::-...... : :.-':'•.- .... ""... •.. '..:'•:•: • .....',:::".:•:":':..: -:::,. •' .... ": : '....:'.:,,-:-:::.... -",• .. .. •...... -.•-.?' .... .•. :.:•.•;? :::..?.a"' ..:•-':::•:----:•:•:..•.•..-,?.:,::.•:: ..-'.•:.-::.. '::.....::,:,'/.... .:..... :•::.'.....

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'•.. :'...::.::-;,•::,:.-.:'..t •.&:-• : ...... :...... •::.,,..:.' ....======...... ::::::::::::::::::::::::::::::::::: .:...... •:,.,-:•. ...,:?: - . ...•.:-- ...-...... ::.::•.:"... ::::::::::::::::::::::::::::::::.::::::::::::::::::::::: :•,:...... ß :..,:. ,• .,.:...... :.:-.,•,•.. .',,.:>•...... • •.:

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Fig. 12a.

Electronic compartment tops. An indication a near-vertical trajectory to reach the top of of erosionby diffusedgas eruption was obtained compartment B from the area under vernier by comparing pre- and post-firing pictures of engine 1. There were no noticeablechanges in the compartment B top (Figure 17). In Fig- spacecraft temperatures, although the thermal ure 17b, clumps of soil, which landed on the characteristics of the electronic compartment compartment top and broke, are visible. Some top would have been significantlymodified even of the small fragments appear to have rolled by a thin layer of soil. downslope or splattered in the plane of the LUNAR SOIL EROSION TEST-- trajectory. The relationship of compartment B SI1VIULATIONSAND ANALYSIS to vernier engine I is shown in Figure 8; the top of the compartment was 1.1 meters above For vernier engine3 static firing, Table 1 the lunar surface. The particles must have had givesthe engineparameters, the positionof the 7182 CHRISTENSEN ET AL.

Fig. 12b.

Figure 12 shows an annotated •nosaic of lunar surface near a-scattering instrument (a) before firing and (b) after firing. Rock and soil fragments that were not moved by the firing are outlined; some are labeled by letter. Fragments that were moved are marked with an x; some are labeled by number. The top of the a-scattering sensor head is 17 cm on a side. Sep- tember 12 and 14, 1967; catalog 5-MP-24 and 5-MP-24). SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7183

'•'"'•'-:"•**%*,".*'.-*•':½•F %... -Ji::......

!•-.• :•":.:......

.... .:• ......

..•:.;:;

ß:*•: ;•:•½: ,:**•;•::• :• ½:,:• ,:;.•;• P:: .:;':?; .;:::•:;i'*:: ;::i:.½;$f•{½':::;:: :8•::•...•½•L•,:•;• ...... , ...... ß-.8

Fig. 13. Lunar surface beside the helium tank (see Figure 12). Soil erosion from around rock a and the trail left by impact of fragment 26 were causedby the firing. (Top) Pre-firing picture (September 12, 1967; catalog 5-MP-45B). (Bottom) Post-firing picture (September 14, 196.7; catalog 5-MP-46B). 7184 CHRISTENSEN ET AL.

_ •. • PROBABLERANGEFORDISTANCE FROMVERNIERENGINE $ • •/ FORFRAGMENTS 25 AND 27 SEENIN POST-FIRINGMOSAIC; "'•••2•,,3 ORIGINPOINTSNOTKNOWN 4.( ß O½

2.8 ••,• •----BOUNDARYOFDIAMETERvs. '•.• • MAXIMUMDISTANCE FROM X '• . • VERNIER ENGINE3 FOR 2.4 x p • Oh•DISPLACED FRAGMENTS 2.0

_ 0 = LOOSE(?)FRAGMENT NOT MOVED BY FIRING x , x=FRAGMENT MOVEDBYFIRING x x x x • • 0.8

x x X Xx X•- -- 0.4

o ! i I I I I I I i 0 20 40 60 80 I00 I ZO 140 160 180 200 DISTANCE OF FRAGMENT ORIGIN POINT FROM VERNIER ENGINE $,cm Fig. 14. Graph of diameter versus distance for fragments moved by the firing. The dashed line represents the probable maximum sizes for fragments that could be moved by the firing. engine relative to the lunar surface below it, by entrainment of soil particles as the gas flows and the dimensions of the erosion crater over the surface. formed. Figure 18 showsthe theoretical surface 3. Diffused gas eruption [Dodge, 1966; pressure, gas radial velocity, and the corre- Scott and Ko, 1968]: movement of the soil sponding dynamic pressure, as derived from causedby the upward flow of gas through the Roberts' [1963, 1964] theory. These data cor- pores of the soil during and after the firing. respond to conditionsunder which the engine These three types will be consideredsepa- is exhaustingonto a fiat plane parallel with the rately. nozzle exit plane. For surfacestilted 0 ø, 10ø, Bearing load cratering. The bearing pres- and 20ø from the nozzle exit plane at a radial sure produced on the lunar surface by engine distance of 76 cm, for example, along the pro- exhaust was less than 0.3 newton/cm2 (Figure jection of the exhaust vector, the theoretical 18); bearing load cratering, therefore, is con- s•rface pressuresare about 14, 76, and 210 sideredunlikely, and no evidenceof suchcrater- dynes/cm•', respectively.The dynamic pressures ing was observed.(Alexander et al. [1966] call vary similarly with tilt. this type of erosion 'explosive cratering.') Soil erosion causedby rocket engine exhaust Viscouserosion. When a soil is subjectedto gas impingment is of three basic types. rocket engine exhaust, the gas that flows ra- 1. Bearing load cratering [Alexander et al., dially along the surface may dislodgesoil par- 1966]: rapid cratering caused by exhaust gas ticles from the surface and entrain them. The pressureon a soil surfaceexceeding the bearing erosioncharacteristics of a bed of particlesunder capacityof the surface. vacuumconditions (10-' torr) were investigated 2. Viscous erosion [Roberts, 1963]: erosion by Land and Clark [1965] for variousparticle SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7185 (a)

.- ..,.,..i•;•.• .;.• •:-'c"'"':½';.TX•.'!•i

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.•- .... :..

(b)

;...::..,:....:

l•ig. 15. Gold-platedface D (see Figure 12) of the a-scatteringsensor head. (Top) Pre- firingpicture; face D is highlyreflective (September 12, 1967, 05h 06m 27s GMT). (Bottom) Post-firingpicture; face D is nonreflective(September 14, 1967,07h 15m09s GMT). 7186 CHRISTENSEN ET AL.

Fig. 16a. sizes and nozzle heights. Their results showed were taken to be 0.3 and 2, respectively[Rob- that, for the nozzle heights where erosion oc- erts, 1964; Hutton, 1966]. The soil internal curred, erosion was more rapid in soils with friction angle was taken to be 35ø [Christen- coarser particles (within limits) than in fine- sen et al., 1967a]. The results shown in Figure grainedsoils and that the maximumcrater depth 19 indicate that a soil composed of 100-•- was not necessarilydirectly below the nozzle. diameter particles with a cohesion of 1430 Often the resulting crater is in the shape of the dynes/cm' would erode at a maximum rate of lower half of a toroid. Observationsshow, and 0.36 cm/sec. The observed rate is 1.5 cm/sec; theory predicts, that the soil particles leave the according to Figure 19, this rate could have surface in a fairly fiat trajectory when the sur- occurredby viscousaction only on a soil com- face erosion is small. As the erosion depth in- posed of particles larger than 500 .g. Since creases,the trajectory angle between the par- most grainspresent are smallerthan 500 g, the ticle and the surface increases. comparison of theoretical and observed crater Roberts' [1963] theory was used to estimate depths suggeststhat viscous erosion was not the theoretical amount of viscous erosion for the major erosion mechanism for the removal a range of soil cohesionfor four particle sizes, of fine-grained material. Comparison of theo- for the engine conditionslisted in Table 1. In retical and observedcrater diametersprovides these calculations,the aerodynamicfriction and additional evidence. For a soil composedof drag coefficientsacting on the soil particles 600-g-diameter particles with a cohesion of SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7187

• - . •.'"::• ?:.

s :'.'7f.:•... -:• .½•,•...,.:• ...... :..::;:•.,... .-.,.• ..: ..:-.... ß.% *-':•:,...'.. 4:;..':. ,::.•' •.•.,.::..:.

:• ,.... ,-.,....

Fig. lfib. Figure 16 showsmosaics of footpad 2 area (a) pre-firing and (b) post-firing. Representative fragments that were moved by the vernier engine firing are marked with an x. Soil on top of the footpad was little disturbed by the firing. (September 12 and 13, 1967; catalog 5-MP-26 and 5-MP-27).

960 dynes/cm2 (selected to approximate the gion. If a crater formed during this period of observedaverage erosion rate), Roberts'theory firing, it would have the shapeof half a toroid. gives a crater diameter of 66 cm, whereasthe On sudden removal of the surface pressure at observed crater diameter was about 20 cm. engine shutdown,some of the gas diffusedinto Thus, viscouserosion was not a major factor the soil during firing will flow to the surface in forming the crater. It probably was the and may produce an eruption. Such a dis- mechanism that moved soil fragments across turbancewould occurin the high-pressureregion the surface from positions outside the crater. directly under the engine. Diffused gas eruption. I)uring a firing, ex- The conditionsused in the Surveyor 5 static haust gasesflow into and through the porous lunar firing test were chosen to emphasize dif- soil, exiting upward at some radial distance fused gas erosion at shutdown as the major and possiblylifting soil from the surface.For erosion mechanism. Because the lunar surface soil displacementto occur during this period, loadings developedby exhaust gasesfrom the the enginemust be firedfor a time sufficientto LM descent engine during landing and by a achieve a significantupward flow of gasesat a Surveyor vernier enginefiring at low thrust on distance from the central higher-pressurere- the moonare similar,it waspossible to simulate, 7188

Fig. 17. Top of compartment B taken before and after the firing. A lump of material was transported to the compartment top and splattered in a direction away from vernier .engine1. (Top) September 12, 1967, 02h 29m 29s GMT. (Bottom) September 22, 1967, 05h 48m 58s GMT. SURVEYOR5--LUNAR SURFACEMECHANICAL PROPERTIES 7189 TABLE1. VernierEngine 3 Static-Firing:Data [from zontal surface of a homogenous, isotropic, Christensenet al., 1967b] porous medium. The diffusionprocess is essen-

Measured Used in tially independent of the direction of gravity, Quantity Value Calculations-'.':and the diffusion-causedsoil erosionon a slope of 20ø , calculated from the diffusion theory, is Firing time, sec 0.55 q- 0.05 0.55 hardly distinguishable from that on a hori- Thrust, newtons 120 q- 22 120 Chamber pressure, zontal surface. The time for diffusion of gas newtons/cm 2 46.9 through the soil to reach a steady state and Chamber gastemperature, øK 2950 correspondinglythe extent of the soil eruption Chambergas viscosity, poise 5.6 X 10-4 Gas specific heat 1.313 depend on the soil porosity and permeability. Gas constant, m¾sec2 øK 367' Diffusion theory indicates that the diameter of a Nozzle exit radius, cm 6.46 diffusion-causederuption crater is almost inde- Nozzle exit Mach number 5.2 Nozzle height, cm 37 q-1 39.4 pendent of the cohesion[Scott and Ko, 1968]. Angle of nozzle exit plane to The calculationsindicate that a diffused-gas surface below it 0ø-10 o Projection of nozzle exhaust eruption would have formed a crater 32 cm axis, on surface below it Toward a-scatteringhead in diameter and 3.5 cm deep in a completely Erosion crater diameter, cm 2O cohesionlessand very permeablesoil (i.e., one Erosioncrater depth, average, cm 0.8 q- 0.2 Erosion crater depth, maximum, cmt 1.3 Erosioncrater crescent, o.31 direction of cusps Toward a-scatteringhead

* Valuesused in calculationsin somecases differ from measuredbecause calculations were made before latest measure- ments. t From Figure 11.

4000

SURFACE PRESSURE in thelunar experiment, thesoil erosion effects to beexperienced during a LM landing.To simulatetheviscous erosion anticipated byLM, a firingtime of about5 secwould have been GAS VELOCITY 3000 requiredfor theSurveyor engines at theirmini- mum thrust level. The simulationof the diffusedgas eruption phenomenon would require muchshorter Surveyor engine firing time, pre- ferablyabout 0.1 sec. It wasapparent, there- 2000

fore,that botherosion effects would not be 0.1 simulatedwith a singlevernier engine firing. Indicationsof the viscouserosion effects were availablefrom the Surveyor 3 second landing PRESSURE event[Christensen et al., 1968];therefore, it IOOO wasdecided to devotethe Surveyor5 lunar soilerosion test to obtainingthe bestsimula- tionof engineshutdown eruption effects possible withinspacecraft constraints. The 0.55-sec firingtime usedwas considered the minimum o IO 20 30 40 50 thatwould ensure predictable performance and RADIAL STATION, cm provideadequate telemetry of engineperform- ance. Fig. 18. Theoretical static pressure, dynamic Theanalysis of diffusedgas eruption used the pressure. and exhaust gas radial velocity at the surfacepressure obtained from Roberts' theory surface of a plane, parallel to the engine nozzle exit plane; engine thrust = 120 newtons, nozzle (Figure18) for a jetfiring normally onto a hori•- height = 39A cm. 7190 CHRISTENSEN ET AL.

of silts having grain sizesbetween 2 and 60 The lunar material also contains particles larger than this range and possibly some smaller. However, the estimated lunar permeability in- dicatesmost of the particlesare in the 2- to range to a depth of around 25 cm. This estimate agrees with conclusions reached from simulations of Surveyor 3 footpad imprints D-- IOOO/• [Christensenet al., 1968]. The observed crater is attributed to soil removal at engine shutdown, not during firing. If the lunar soil has the permeability indicated, then during an 0.5-sec Surveyor 5 firing the z gasesflowing into and through the soil would o n- 2 not have produced diffused-gas eruptions. If the firing time had been increased or the soil permeability were different, so that the eruption did occur before engine shutdown,the resulting crater would have formed at a radius of 19 to 25 cm from the stagnation point. Since the erosioncrater on the moon has only a 10-cm radius, it is concludedthat it was formed by IOO diffused-gaseruption at engine shutdown. The crescentshape of the lunar crater could have been causedby one or several of the fol- I x IO 3 2 x IO 3 lowing factors: SOILCOHESION, dynes/cm 2 1. Flow of gasesout of the nozzlemay not Fig. 19. Theoretical viscous erosion rates as have been symmetric. functions of soil cohesion and particle size' engine thrust -- 120 newtons. nozzle height -- 39.4 cm. 2. Material that erupted upward couldhave settled preferentially on one side because of the surface slope. in which steady-state gas flow conditionswere 3. Lunar soil below the nozzle may have reached by an 0.5-secfiring), for surface load- been nonhomogeneousor the surfacemay have ing conditions correspondingto the Surveyor had an irregular shape, and so it was tilted 5 test. For the same test conditions, but in a relative to the nozzle exit plane. The direction cohesionlessand less permeable soil (requiring of the crater cuspsis in line with the projec- 5 sec to reach steady-state gas flow), however, tion of the exhaustaxis on the surface,suggest- the eruption crater would have been 18 cm in ing that this explanationis likely. diameter and 1.5 cm deep. If the soil porosity is assumedto be between 0.3 and 0.5, the viscosity of the vernier exhaust •ases in the soil is LIMITS OF GRAIN SIZE, rnm 1 '4 10-4 to 3 x 10-' poise [Pao, 1967], and, (MIT CLASSIFICATION SYSTEM) if it assumedthat an 0.5-sec firing is equivalent 2 0 0 06 0.002 to about one-tenth the time required to reach steady-state conditions, the soil permeability required to match calculated and observed SURVEYOR E ESTI MATES crater diameters is between 1 x 10-8 and 7 X I0 -I 10I-2 I 10I-4 I 10I-6 I[ ]I -. ,o-,O 10-8 cm2 [see Scott and Ko, 1968]. The per- meabilities of soils of different uniform grain PERMEABILITY,cm Z sizes as measured on earth are shown in Fig- Fig. 20. Permeability of lunar soil compared ure 20. The permeability range for the lunar with permeability of earth soil with various grain surfacematerial fits into the permeability range sizes. (Earth data from Scott [1963].) SURVEYOR 5--LUNAR SURFACE MECHANICAL PROPERTIES 7191 Movemento[ a-scatteringhead. An analysis Permeabilityof this lunar soil, to a depth of the forcesrequired to move the a-scattering of about 25 cm, is I x 10-• to 7 x 10-8 cm'•. sensor head the observed distance during the This correspondsto the permeability of earth time of firing indicatesthat the effectivecoeffi- silts and indicates most of the lunar particles cient of friction between the aluminum skirt of are in 2- to 60-/• size range. the head and the lunar surface was over 0.59, Capability of lunar material to adhere to a and probably over 0.84; this includes a con- smooth vertical surface is indicated by the tribution from digging of the skirt into the change of reflectivity of the a-scattering sensor surface. The analysis showed that the coc•qi- head as a result of the vernier engine firing. cient of friction was less than 1.38 [Christen- Vernier enginefiring did not causeany degra- sen et al., 1967b]. dation in the functional capability of the spacecraft. CONCLUSIONS AND SUMMARY Acknowledgments. We thank Frank Sperling, During the l•:nding,Surveyor 5 slid about JPL, for his major contributions to the section on 0.8 meter down the inner slope of a 9- x 12- spacecraft landing analysis and dynamic simula- meter crater. During this sliding,at least two of tions; Charles Goldsmith, William Peer, Alex the footpads dug trenchesin the lunar surface Irving, Albert Plescia, and Lloyd Starkes. JPL, material. The initial depth of penetration for for their work on spacecraft shadow predictions and assembly of photographic mosaics; Robert one footpad was about 12 cm. Ejecta was Breshears, John Stocky, and Charles Dodge, for thrown 80 cm or more. their vernier engine performance analyses; and Soil pressuredeveloped in resistingthe foot- Dave Conaway, Margaret Dove, and John Hinchey, pad sliding during the landing was about 0.9 Hughes Aircraft Company, for assisting in the landing dynamic simulations. newton/cm2, which agrees with the stalling pressureduring trenching with the soil me- ][{EFERENCES chanicssurface sampler on Surveyor 3. Alexander, J. D., W. Roberds, and R. F. Scott, Best agreementobtained for a compressible Soil erosion by landing rockets, Final Report to soil model with the observed Surveyor 5 foot- NASA Manned Spacecraft Center, Houston, pad penetrationsand landingleg loadsis for a Contract NAS9-4825, Hayes International Co., soil static-bearing capability of 2.7 newtons/ Birmingham, Alabama, 1966. cm2 and a density of 1.1 g/cm3. Incompressible Christensen, E. M., S. A. Batterson, It. E. Benson, C. E. Chandler, R. H. Jones, R. F. Scott, E. N. soil model analyses have not yet been per- Shipley, F. B. Sperling, and G. It. Sutton, Lunar formed for Surveyor 5. Preliminary analyses surface mechanical properties--Surveyor 1, J. suggestthat soil at the Surveyor 5 landing site Geophys. Res., 72, 801-813, 1•)67a. is slightly weaker than at previous Surveyor Christensen, E. M., S. A. Batterson, It. E. Benson, landing sites, or that its resistanceto bearing R. Choate, R. E. Hutton, L. D. Jaffe, R. H. Jones, H. Y. Ko, F. N. Schmidt, R. F. Scott, load is reducedby the slope. R. L. Spencer, and G. H. Sutton, Lunar surface Surface material is granular, slightly cohesive, mechanical properties, in Surveyor 5 Mission and generally fine-grained,as at the Surveyor Report, Part 2, Science Results, Jet Propulsion I and 3 landing sites. The differencesin reflec- Lab. Tech. Rept. 32-1246, 1967b. (Also in Sur- veyor 5, A Preliminary Report, NASA SP-163, tivity betweendisturbed and undisturbedlunar Washington, D.C.) soil is less than at the Surveyor I and 3 land- Christensen, E. M., S. A. Batterson, It. E. Benson, ing sites. R. Choate, L. D. Jaffe, R. It. Jones, H. Y. Ko, During the 0.5-seevernier enginefiring, soil R. L. Spencer, F. B. Sperling, and G. H Sutton, fragmentswere moved by viscouserosion from Lunar surface mechanical properties, J Geophys. Res., 73, 4081-4094, 1968. areas below and adjacent to at least one vernier Deitrick, R. F., C. D. Conaway, L. W. Gammell, engine. Fragments up to 4.4 cm in diameter and R. H. Jones, Structures and mechanical were moved from near the vernier engine, and environment, in Surveyor I Flight Performance 0.6-cm fragments were distances up to 1.9 Final Report, SSD 68223R, vol. 3, section 5.11.2, meters. Exhaust gas that had diffused into the Hughes Aircraft Co., E1 Segundo, California, 1966. soil erupted at engine shutdown, producing a Dodge, C. It., Space exploration programs and crater 20 cm in diameter and 0.8 to 1.3 cm spacesciences, Space ProgramsSummary 37-40, deep under one engine. vol. 6, pp. 11-15, Jet Propulsion Laboratory, 7192 CHRISTENSEN ET AL.

California Institute of Technology, Pasadena, the lunar surface,paper presentedto meetingon California, July 1966. fluid dynamic aspectsof spaceflight, Fluid Dy- Hutton, R. E., Investigation of soil erosion and namics Panel, Advisory Group for Aeronautical its potential hazard to LM lunar landing, TR W Research and Development, Marseilles, France, Systems,Eng. MechanicsRept. 16-11, Redondo April 20-24, 1964. Beach, California, 1966. Scott, R. F., Principles o• Soil Mechanics,Addison- Land, N. S., and L. V. Clark, Experimental in- Wesley, Reading, Mass., 1963. vestigation of jet impingement on surfaces of Scott, R. F., and H. Y. Ko, Transient rocket- fine particles in a vacuum environment, NASA engine gas flow in soils, AIAA J., 6, 258-264, TND-2633, NASA Langley Research Center, 1968. Hampton, Virginia, 1965. Scott, R. F., and F. I. Roberson, Soil mechanics Pao, R. H. F., Fluid Dynamics. C. E. Merrill, surface sampler: Lunar tests and analysis, J. Columbus, Ohio, 1967. Geophys.Res., 73, 4045-4080, 1968. Roberts, L., The action of a hypersonic jet on a dust layer, paper presented at the IAS 31st an- nual meeting, New York, January 21-23, 1963. (Received March 28, 1968; Roberts, L., Interaction of a rocket exhaust with revised July 15, 1968.)