Lunar Surface Mechanical Properties

JOURNALOF GEOrItYSICAL RESEARCIt VOL. 73, NO. 22, NOV•-a•B•.R15, 1968 Lunar Surface Mechanical Properties 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) ......•, .:-•.'"•:•' •' -,,'•.-:•:.•?,:.;:..:.,.:,..• %1':...".:. ';'/• "¾ :?./,•'•' ....:........•-'•' . •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 ana- lytical 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.

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