PI/Engineer: James K. Mitchell, Univ. of Calif./Berkeley
W. David Carrier, III/JSC
R. F. Scott, N. C. Costes/MSFC
Apollo Flight Nos.: 11, 12, 14, 15, 16, 17
Apollo Exp't No. S 200
Discipline: soil mechanics
Weight: 2.3 kg for SRP
Manufacturer: Murdock Engineering (SRP)
Broad objectives: (1) To enhance the scientific understanding of the nature and origin of the materials, and the mechanisms and processes responsible for the present morphology and consistency of the lunar surface. (2) To provide engineering data on the interaction of crewed systems and operations with the lunar surface, thereby aiding in the evaluation of the mission, and in the planning of future lunar surface scientific investigations and related engineering tasks supporting these activities.
Specific scientific objectives: (1) To verify lunar soil models previously formulated from Earth-based observations and lab investigations and from lunar orbiting and unmanned lunar landing missions. (2) To determine the extent of variability in lunar soil properties with depth and lateral position. (3) To aid in the interpretation of geological observations, sampling, and general documentation of features.
Specific engineering objectives: (1) To obtain information relating to the interaction of the LM with the lunar surface during landing and to lunar soil erosion caused by the spacecraft engine exhaust. (2) To provide a basis for altering mission plans because of unexpected conditions. (3) To assess the effect of lunar soil properties on astronaut and surface vehicle mobility. (4) To obtain at least qualitative information needed for the deployment, installation, operation, and maintenance of scientific and engineering stations and equipment to be used in extended lunar exploration. The Soil Mechanics Investigation was included at a late phase of the Apollo 11 mission planning; consequently, no special soil mechanics testing or sampling devices were to be added to the hardware already planned for that mission. The main sources from which data could be extracted included (1) real-time astronaut observations (2) television (3) cameras (4) flight mechanics telemetry (5) various objects of known geometry and dead weight that came in contact with the lunar surface, including the LM, astronauts, EASEP, hand tools, and various poles and shafts inserted into the lunar surface (contingency sampler handle, SWC, flagpole, and core tubes).
Simple observation of tasks such as walking, the interaction of the wheels of the MET, the effect of the LM descent engine on the soil, the depth of the LM foot pads, and other phenomena provided a good qualitative to semi-quantitative estimate of many geotechnical properties of the regolith.
When a firm decision was made to build a rover (around the time of A-12), it was decided that more quantitative data was needed to design it and predict its perform-ance. The ASP and SRP were approved and became part of the timeline. This was at a time when there were to be several "H" missions in which to gather the data. Subsequently, the delay after the A-13 problem allowed the hardware enhancements for the "J" missions to be ready by Apollo 15. It turned out that the first substantial soil mechanics measurements (presumably justified on the basis of input to the LRV design) flew on the same mission as the first rover. They also stayed in the time-lines for A-16 and 17, despite the efforts of some to have them removed.
Operation of experiment:
The A-15 timeline presents these coordinated activities: LMP - Unstow penetrometer from pallet, attach to extension handle; attach cone or plate to penetrometer; index penetrometer drum to next position; move reference plane to the tip (fully extended) position; position penetrometer vertically on surface; press tip into surface with downward force on extension handle (if cone penetration, attempt 1 inch per sec penetration rate); Withdraw penetrometer from surface and move reference plane full up; restow penetrometer on pallet; proceed to next sample; CDR - select area for penetrometer test and place gnomon; take locator photo using prominent feature - cross sun, f/8, 1/250, 15 feet; take "after" stereo pair cross-sun, f/8, 1/250, 7 feet, when penetrometer removed; retrieve gnomon.
Vertical walls on trenches were observed. On A-14, a trench dug by Shepard collapsed at a shallower depth than predicted, evidently because of lessened soil cohesion - as small as 10% of the values calculated for soils at previous landing sites. This was on the rim of a crater, however. Also at this trench site, the crew was to step onto the pile dug out of the trench to observe the uncompacted behavior of regolith. On A-15, Irwin dug a trench with the small scoop attached to the extension handle (see lunar geology - tools) on EVA-2. This went smoothly and without difficulty until a much harder layer was reached. After this, further excavation required chipping out material. The trenched was caused to fail by inducing a load at the top with the SRP, although forces beyond its range were required. Collapse was sudden and complete. The pre-mission timeline allowed 10 minutes for this soil mechanics trench study.
On A-14, both core tubes driven into the soil in the vicinity of the SWC were easily pushed to a depth of 3 to 5 inches, but further penetration of ~2 inches required hammering as vigorously as possible - to the degree that the hammer dented the extension handle attached to the core tube. In general, core tube holes remained intact after removal of the sample.
On A-15, thinner-walled, larger diameter core tubes were used so as to reduce sample disturbance, increase the size of the sample, and facilitate ease of sampling by the crew. Operationally, this new core tube required the astronaut to insert a "rammer-jammer" rod into the core tube after it had been driven into the soil to push a "keeper" down until it came into contact with the soil. This then held the sample in place while it was extracted. On A-17, the lunar drill deep core hole remained open, as predicted, for the insertion of the Lunar Neutron Flux Probe.
Were any special tools required?
On A-14, the geophone/thumper anchor was used as a penetrometer to obtain 3 two-stage penetrations into the lunar surface. This device (figure 4-11 in the A-14 Preliminary Science Report) was a 68 cm long aluminum shaft which was 0.95 cm in diameter and had black and white stripes 2 cm long to provide a depth scale. It had a 30deg cone tip (apex angle) on one end and a connection for the extension handle on the other. When so used, it was referred to as the Apollo Simple Penetrometer, or ASP. After completion of these tests, the device was used to anchor the geophone cable when the cable was placed in position for the ASE.
On A-15 four new data sources were available for the first time. These included 1) new, larger diameter, thin-walled core tubes, 2) a self-recording penetrometer (SRP), 3) the LRV, and 4) the Apollo Lunar Surface Drill (ALSD). The SRP could penetrate to a maximum of 76 cm with a penetration force of up to 111 Newtons. The record of each penetration was scribed on a recording drum contained in the upper housing. The lunar surface reference plane, which folded for storage, rested on the surface during a measurement and served as datum for measurement of penetration depth. Three penetrating cones, each of 30deg apex angle and base areas of 1.29, 3.22, and 6.45 cm2, were available for attachment to the shaft, as well as a 2.54 by 12.7 cm bearing plate.
The middle cone and the bearing plate were used for a series of six measurements at station 8 on A-15. The records were scribed on the data drum, which was returned for analysis. The surface-reference pad tended to ride up on the shaft when the SRP was vibrated, however, and it is therefore difficult to determine precisely the depth of penetration from 4 of the tests. On A-16, eleven SRP tests were performed during the 2nd EVA. The lunar-reference plane (zero-point) was repositioned after each test back to the zero-point, but while moving to the next test station this plate moved up the shaft slightly. Also, placing the SRP onto the surface while holding it by the housing could have led to some penetration (because of inertia) without recording the accompanying force. An improved procedure might eliminate these two sources of error. Also, test 5 at station 10 did not record on the SRP drum, probably because the LMP placed his left hand around the upper housing in such a manner that the indexing lever was depressed, thus locking the recording drum and preventing inscription of the data (this based on viewing the films). Design with greater sensitivity to operation could have prevented this. At station 4 on the 2nd EVA the 1.29 cm2 cone fell off the penetrometer but was recovered. Once set up properly, the test was performed but a steady push was not easy to provide. Once the crewman leaned on it he lost his balance and came up off it. When it would "give", it would go fast enough to allow the spring to back off. Spiked readings on the recording drum resulted.
Were there related experiments on other flights?
All landing sites included some element of soil mechanics investigations. Lunokhod 1 & 2 both carried cone penetrometers.
What was different between training and actual EVA? No comments by crew.
The Preliminary Science Reports of all landed missions include a chapter on soil mechanics investigations based on a variety of observations.
Geotechnical Engineering on the Moon, document from the Planet Surface Systems Office, NASA - JSC, 12/90.
Memo from Leon T. Silver to Members of the SWP Subpanel on Soil Mechanics Experiment (S-200), November 30, 1971, in JSC History Office.
Apollo 14 Final Lunar Surface Procedures, JCS, December 31, 1970.
Apollo 15 Final Lunar Surface Procedures, JCS, July 9, 1971.
Apollo Scientific Experiments Data Handbook, JSC-09166, NASA TM X-58131, August, 1974, in JSC History Office.
Apollo Program Summary Report, JCS-09423, section 3.2.7, Geology and Soil Mechanics Equipment, April, 1975.
Apollo 16 Technical Crew Debriefing, 5 May 1972, in JSC History Office.
"Lunar Sourcebook - A User's Guide to the Moon" G. Heiken, D. Vaniman, and B. French, Eds., Cambridge University Press, Cambridge, 1991.