Surveyor Soil Mechanics Surface Sampler

NASA. Public domain (NASA / US government work).

Overview

The Soil Mechanics Surface Sampler was a motorized scoop on an extending arm, flown on Surveyor 3 and Surveyor 7 and operated successfully on both [1]. Surveyor 3 landed in the southeast of Oceanus Procellarum on 20 April 1967, a typical mare site, coming to rest tilted 12.5 degrees [4]; Surveyor 7 landed eight and a half months later on 10 January 1968 on the ejecta blanket north of the crater Tycho, in the southern highlands.

It is the first instrument to measure the mechanical properties of another world’s surface by acting on it. Prior measurements were passive: footpad penetration depths, crushable block imprints and the tracks of rolling fragments [1]. Surveyor 1 footpad interaction combined with television particle-size analysis had given a bulk density of 1.5 g/cm3 in June 1966, and Luna 13’s gamma-ray densitometer returned a double-valued calibration consistent with either 0.8 or 2.1 g/cm3, of which the lower was chosen [9].

The sampler measurements confirmed the 1.5 g/cm3 value and identified the Luna 13 choice as the wrong branch of the calibration curve [9]. The Surveyor parameters, a slightly cohesive sand at about 1.5 g/cm3 with 0.7 kN/m2 cohesion and 35 to 37 degrees friction angle, carried forward into Apollo planning as Model B, and reproduce the measured Apollo 16 and 17 Lunar Roving Vehicle energy consumption [5]. A pre-Apollo reevaluation of the same data proposed 1.1 g/cm3 at the surface rising linearly to 1.6 g/cm3 at 5 cm depth. In retrospect the Surveyor Model values sit near the lower bound of actual lunar soil shear strength, and the spread between it and the later Apollo Model is accounted for by differences in in-situ relative density.

The program flew five successful landers between June 1966 and January 1968; only Surveyors 3 and 7 carried the sampler [1].

Specifications

Parameter	Value	Source
Configuration	motorized scoop on a pantograph extending arm, fixed to the lander	[1]
Maximum extension	1.48 m, given on the program pages as up to 5 feet	[1], [6], [7]
Maximum digging depth	about 18 cm (7 inches)	[6]
Mass	9.09 kg (20.05 lb) on Surveyor 3, 9.62 kg (21.20 lb) on Surveyor 7	[4]
Force measurement	none fitted; applied force recovered from motor current
Position measurement	position-indicating potentiometers deleted

Mission profile

Parameter	Surveyor 3	Surveyor 7	Source
Launch	17 April 1967	7 January 1968, 06:30 UT	[6], [7]
Landing	20 April 1967, 00:04:17 UT	10 January 1968, 01:05:36.3 UT
Landing site	Oceanus Procellarum, 3 S, 23.41 W	Tycho ejecta blanket, 40.97 S, 11.44 W
Lander attitude	tilted 12.5 degrees	not stated	[1], [4]
Trenches dug	4	7	[1]
Bearing tests	7	16
Impact tests	13	2
Alpha-scattering instrument repositioned	not carried	to 3 locations	[7]
Last contact	4 May 1967	21 February 1968	[6], [7]

The program pages and the mission reports differ on reach: the Surveyor 3 page gives the sampler as extending up to 5 feet, about 1.52 m [6], against the 1.48 m in the flight documentation [1]. The Surveyor 7 sampler operated over two lunar days, with communications ending on 21 February 1968 [7].

Build and deployment

The sampler is fixed to the lander and reaches surface only within its own workspace.

The flight units were built by Hughes Aircraft Company [4]. Two flight samplers and one spare were authorized in August 1966 for Surveyors III and IV, simplified from the original design by deleting the force-measuring instruments and the position-indicating potentiometers so that the unit could replace the approach television camera on an unmodified spacecraft. Deleting the force transducers is why applied force had to be recovered from motor current. The spare flight unit was added to Surveyor VII in May 1967 [4]. Sampler mass was 20.05 lb on Surveyor III and 21.20 lb on Surveyor VII, against a total scientific payload of about 71 lb on Surveyor VII, the only spacecraft carrying the survey camera, the alpha-scattering instrument and the sampler together [4].

The mechanism is a scoop about 12 cm long and 5 cm wide with a motor-driven door, carried on a pantograph arm [1]. Surveyor 7 records the scoop width as 5.1 cm and a maximum extension of 148 cm, with calibration points at 106 and 148 cm [2]. Three motor-driven degrees of freedom are available: extension and retraction of the pantograph, azimuth rotation, and elevation. The arm can also be released to drop onto the surface under gravity and spring force, which is how the impact tests were performed. Motors could be stepped in 0.1 s or 2.0 s increments [1].

The Surveyor 7 mechanism was identical to the Surveyor 3 unit except for two horseshoe magnets set into the bearing plate of the scoop door, added to test for magnetic material in the soil, at 0.15 lb [1], [4]. The sampler mounting to the spaceframe was also rotated 30 degrees, and the mechanical interface was modified so that the sampler could pick up the deployed alpha-scattering sensor head and move it to a different location, allowing that instrument to analyze material from more than one site.

Power and energy

Lander bus power. The only sampler-specific figure published is the electronics auxiliary heater, 5 W on Surveyor 3 and 7.5 W on Surveyor 7 [1]. Total sampler operating time was 11 hr 22 min on Surveyor 3 and 36 hr 21 min on Surveyor 7 during the first lunar day, of which 8 hr 45 min on Surveyor 7 was spent on the alpha-scattering instrument recovery [2].

Thermal

Thermal design was a heater in the electronics auxiliary [1] with the mechanism otherwise exposed. On Surveyor 7 the motors showed no measurable change in operating characteristics, in particular distance traveled per command, across their temperature range, operating normally up to 180 F, with the retraction motor working at minus 167 F during post-sunset operations [2]. Lunar surface temperature at 45 degrees latitude runs from a mean 350 K at local noon to a mean 89 K before sunrise [8]. Temperature sensors were attached directly to the retraction and elevation motors on Surveyor 7, a channel that had not been available on Surveyor 3.

The subsystem was operated again during the second lunar day on Surveyor 7, demonstrating that both the mechanism and its electronics survived the lunar night [1].

Compute and avionics

There is no onboard computation. The sampler is a set of motors driven directly by spacecraft commands, and its only sensing is motor current, motor temperature and the television view.

Force measurement is derived from motor current because the force-measuring instruments and position-indicating potentiometers of the original design were deleted when the sampler was simplified to fit the Surveyor III and IV spacecraft without modification [4]. On Surveyor 3 the motor-current readout was assigned to a single commutator frame, giving a maximum of eight current samples for a 2 s actuation [1]. Worse, the state of spacecraft telemetry after landing precluded motor current measurement entirely, so forces on Surveyor 3 had to be inferred indirectly: tests were run until the motor was deliberately stalled, and the stall force was recovered from pre-flight calibration against motor temperature. Surveyor 7 fed the current signal to five symmetrically spaced commutator frames, giving up to 40 samples for a 1 s motion at the 4400 bit/s spacecraft rate, a 50 ms sampling interval that resolved the envelope of the current pulse and gave a much more accurate indication of applied force [2].

The two flights therefore differ by a factor of five in current samples per actuation from an identical mechanism, the difference being commutator frames allocated to one telemetry channel [1]. Two telemetry points were added across the later spacecraft, one of them sampler temperature [4].

Autonomy

None. Every motion is commanded from Earth. Because a single motion required a minimum of five commands [2], operations were driven from pre-built command tapes rather than individually composed instructions, with a special-purpose tape, number 907, used to apply standard 0.5 s load sequences for bearing tests. Telemetry was displayed in real time, motor current at 50 ms and other channels at 250 ms.

Between Surveyor 3 and Surveyor 7 the operations technique, not the hardware, was substantially rebuilt: analysis of the Surveyor 3 tests generated new command tapes for Surveyor 7, including tapes that returned a picture between each command during a trenching test so the sequence could be assembled into a motion picture [1].

Communications

Through the lander’s telecommunications subsystem. The relevant number for the sampler is the spacecraft’s highest bit rate of 4400 bit/s, which set the achievable motor-current sampling rate [1]. No sampler-specific link parameters are published.

Payload and instruments

The sampler is itself the instrument, and it measures by four kinds of action:

• Bearing tests. The scoop, open or closed, is pressed into the surface and the load-penetration curve recovered from motor current, giving bearing capacity and shear resistance as a function of depth over roughly 1 to 20 cm [1].
• Trenching. The scoop is dragged to cut a trench, exposing the subsurface for photometry and color comparison and showing how the material deforms and fails.
• Impact tests. The arm is dropped onto the surface from height, giving a dynamic penetration measurement.
• Manipulation. Picking up, weighing, dropping and overturning rocks, and breaking them [2].

Mission	Spacecraft commands	Subsystem functions
Surveyor 3, first lunar day	5,179	1,191
Surveyor 7	12,649	6,956

Counts from [1]. Surveyor 7’s larger totals follow from its 36 hr 21 min of operating time against Surveyor 3’s 11 hr 22 min, and from the command tapes rebuilt between the two missions [2].

Results

The bearing strength profile assembled across the Surveyor program comes from four independent surface interactions.

Depth	Bearing strength	Evidence	Source
Uppermost millimeter	less than 0.1 N/cm2	imprints of small rolling fragments	[1]
1 to 2 mm	0.2 N/cm2	alpha-scattering sensor head imprint	[1]
About 2 cm	1.8 N/cm2	crushable block imprints	[1]
About 5 cm	5.5 N/cm2	Surveyor 1 footpad penetration	[1]

Strength rises by nearly two orders of magnitude in the first five centimeters. The sampler measured a bearing capacity of about 2.1 x 10^5 dyn/cm2 for the 2.54 cm wide closed scoop at roughly 3 cm penetration [1]. In-situ relative density is about 65 percent in the top 15 cm and exceeds 90 percent below 30 cm, against a 65 to 75 percent practical limit for terrestrial field compaction with heavy equipment [9].

From the sampler tests the derived soil parameters were a cohesion of 0.035 to 0.05 N/cm2, an internal friction angle of 35 to 37 degrees, and a bulk density of about 1.5 g/cm3, with porosity of 0.4 to 0.5 from a few millimeters down to about 10 cm [1]. The material behaves as a fine-grained terrestrial soil with a small cohesion and the friction angle of a medium dense soil. Strength increases with depth at the Surveyor 3 site to about 20 cm; that increase was not found at the Surveyor 7 site [1].

The mare and highland sites differ in failure mode rather than in strength. At Surveyor 3 the soil cracked and split, breaking into relatively large chunks, with evidence of a brittle crust 2.5 to 5 cm deep; at Surveyor 7 the material worked plastically, cracked less, and produced smaller clods [1], [2]. The Surveyor 7 report’s own conclusion is that essentially the same density, friction and cohesion values are representative of the Tycho area as of the Surveyor 3 mare site, as a first estimate, which is the basis on which one soil model was applied Moon-wide in Apollo planning [5].

One rock was weighed on Surveyor 7 at about 0.36 N lunar weight, giving a density of 2.4 to 3.1 g/cm3, an estimate the report itself qualifies as accurate to within 7 or 8 percent on mass but only about 30 percent on volume [2]. Buried fragments resisted retraction forces of 1.8 to 2.0 x 10^6 dyn without moving. A rock was broken by an impact blow delivered from 35 to 40 cm [2].

Recovery of the alpha-scattering instrument

Surveyor VII’s sampler interface had been modified before flight to let it move the deployed alpha-scattering sensor head between sites [4]. In flight the instrument failed to deploy to the surface by its own mechanism at all, and the sampler forced the sensor head free and lowered it, then moved it to a second sample position on a rock and a third on ground the sampler had itself disturbed [1], [2]. The recovery cost 8 hr 45 min of sampler operating time. The alpha-scattering sensor head’s own imprint is one of the four data points in the bearing strength profile above, at 0.2 N/cm2 for 1 to 2 mm depth [1].

Modes of operation

The sampler has no autonomous modes. Its operating states are the four test types above, each invoked by a command tape, plus stowed. Operations were confined to the lunar day and to the periods when the lander could be commanded, and were structured around the tape library rather than around vehicle state.

Ground operations

Operated from JPL by the surface sampler team under principal investigator R. F. Scott, with F. I. Roberson [1]. The working method was to command a motion, take a television picture, evaluate, and command the next, with the picture cadence itself under command; on Surveyor 7 pictures were interleaved between every command during trenching to build a motion sequence.

Between the two missions the operations plans were revised and new techniques devised on the basis of the Surveyor 3 experience [1]. The Surveyor 7 plan originally called for the alpha-scattering instrument to be deployed before any sampler operation, so that a chemical analysis could be taken before the surface was disturbed; the deployment failure inverted that order and made the sampler the enabling instrument for the chemistry.

Technologies developed

The Surveyor soil model sized Apollo landing gear, extravehicular activity and the Lunar Roving Vehicle; its Model B parameters reproduce the measured Apollo 16 and 17 rover energy consumption [5]. Pre-Apollo estimates of lunar soil strength that were not derived from the sampler were wide of the mark, and the sampler-derived values were the best available before Apollo 11 [9].

Bearing capacity, shear strength, density, compressibility and failure mode all follow from pressing, dragging, dropping and lifting a scoop under camera observation, with force recovered from motor current [1]. The same four action types recur in every subsequent lander arm.

A manipulator interface designed for instrument relocation [4] recovered a complete deployment failure on Surveyor VII [1], [2], turning a single-site chemical analysis into a three-site one.

Force sensing derived from motor current is bounded by the telemetry allocated to sampling it. The Surveyor 3 sampler was mechanically identical to the Surveyor 7 unit but had eight current samples per 2 s actuation against 40 per 1 s motion, and lost the current channel entirely after landing, so its forces had to be recovered from deliberate motor stalls against pre-flight calibration [1].

The Lunokhod cone-vane penetrometers extended the same in-situ approach: about 1000 tests to 10 cm depth over 47 km of traverse, of which 327 were made by Lunokhod 1 over a 5 km traverse in western Mare Imbrium [9].

References