Experiment Operations During Apollo EVAs

Experiment: Lunar Surface Gravimeter

(a.k.a. Lunar Tide Gravimeter)
Acronym: LSG


The LSG deployed at the Apollo 17 ALSEP site. The sunshield is tipped appropriately for the latitude of the site. The central station, RTG, and LM are visible in the background (AS-17-134-20501).

PI/Engineer: Joseph Weber, Univ. of Maryland
Other Contacts:
John. J. Giganti, J. V. Larsen, J. P. Richard/Univ. of Maryland

Apollo Flight Nos.: 17
Apollo Exp't No.: S 207

Discipline: space physics, lunar geology, lunar gravimetry

Weight: 12.7 kg
Dimensions: 27.7 x 25.4 x 38.4 cm, stowed plus a 7.6 cm (dia.) cable reel

Manufacturer: Univ./Maryland & Bendix

Description/Purpose:
This was designed to make very accurate (1 part in 10^11) measurements of the lunar gravity and of its variation with time. It was essentially a sensitive spring balance, and also functioned as a one-axis seismometer. It was considered part of the ALSEP. Its intent was to measure gravity waves by using the Moon as an antenna and also investigate tidal distortions of the shape of the Moon. Following deployment of the gravimeter, problems occurred in trying to balance the beam. These problems were caused by a mathematical error in the sensor mass weights. Several reconfigurations of the instrument were made during the previous year.

Unloading from the LM: As part of the ALSEP

Transporting by foot or MET: As part of the ALSEP

Loading/unloading tools/exp'ts on LRV: NA

Site selection: As part of the ALSEP, ~8 m from the central station.

Deploying experiment:
As part of ALSEP on EVA 1. After releasing it from the sub-pallet with the UHT, it was carried (using the UHT) to its site. The crew had to raise and tilt a sunshade, set the instrument on a firm surface with approximate orientation, level and align it using a bubble and shadowgraph, perform initial uncaging, and report level and alignment. It was planned to take ~3 minutes. There were no known anomalies in the deployment of the LSG that would account for the problems encountered upon the commanded activation of the experiment.

Check-out of experiment:
It was discovered on EVA 2 that the sensor beam of the LSG could not be nulled (using the micrometer screw adjustment of the instrument), even though the LMP reverified that the instrument was level and the gimbal was free. Later analysis showed that a design (arithmetic) error of the sensor mass weights. They were ~2% lighter than the proper nominal weight for 1/6 g operation of the flight unit. The sensor mechanism allowed up to only 1.5% adjustment from the nominal for possible inaccuracies. The error was made in the conversion calculations from 1 g to 1/6 g for the flight unit by including an erroneous value in the calculations from the uncorrected calculations for the qualification unit.

Operation of experiment:
From JSC via the ALSEP command system. After determining the design error in the instrument, it was reconfigured to obtain long-term seismic and free-mode science data. However, the sensitivity of the system was considerably reduced.

Repairs to experiment:
Attempted on EVA 2 and 3, but unsuccessful due to design error. The LMP rapped the exposed top plate on the gimbal; rocked the experiment in all directions; releveled the instrument, working the base well against the surface; and verified the sunshade tilt. These actions were taken to free a mass assembly or a sensor beam that was suspected of being caught or bound, but no change was apparent. The problem was at least partly overcome by applying pressure on the beam with the mass-changing mechanism beyond the design point by addition of all included masses so that it contacted the beam. Much valuable EVA time (~ 30 minutes) was spent on the attempt.

Recovery/take-down of experiment: NA

Stowing experiment for return: NA

Loading/unloading samples on LRV: NA

Loading of exp't/samples into the LM: NA

Stowing of package once in the LM: NA

Sampling operations - soil, rocks: NA

Trenching: NA

Raking: NA

Drilling: NA

Navigating/recognizing landmarks: NA

Were there any hazards in the experiment?
i.e. hazardous materials (explosive, radioactive, toxic), sharp objects, high voltages, massive, bulky, tripping hazards, temperatures?
No.

Was lighting a problem? NA

Were the results visible to the crew? Instrument level and gimbal release were visible.

Would you recommend any design changes?
Yes. The PI considered his experiment proprietary and was able to by-pass NASA reviews and supply his experiment as a "black box." Perhaps the design error would have been caught in the proper reviews if they focused on the technical aspects of the device, but not if they only looked for hazards.

Were any special tools required?
The UHT was used during deployment. It was also used later to attempt to jar the gimbal loose during a repair attempt.

Was the orientation of the experiment (i.e. horizontal/vertical) important? Difficult?
The unit needed to be leveled.

Was the experiment successful?
No, but the signals received were processed and analyzed for seismic, free mode, and gravity wave information.

Were there related experiments on other flights?
There was also a traverse gravimeter (S 199: Traverse Gravimeter) on A-17 which took 7 measurements on the 1st EVA, 7 on the 2nd, and 9 on the 3rd. Orbital measurements of mascons were made in several orbital missions, both manned and unmanned. There were also seismic experiments (S 031: Passive Seismic Experiment and S 033: Active Seismic Experiment) performed on landed missions.

Where was it stored during flight? Part of ALSEP

Were there any problems photographing the experiment? No

What pre-launch and cruise req'ts were there?
power, thermal, late access, early recovery?
None.

What was different between training and actual EVA? No comments by crew.

What problems were due to the suit rather than the experiment? No comments by crew.

Any experiences inside the LM of interest from the experiment/operations viewpoint? No

References:

A-17 Preliminary Science Report

Apollo 17 Mission Report

Apollo Lunar Surface Experiments Package - Apollo 17 ALSEP (Array E) Familiarization Course - Handout for class of 1 September 1972, in JSC History Office

Apollo Scientific Experiments Data Handbook, JSC-09166, NASA TM X-58131, August, 1974, In JSC History Office.

Apollo Program Summary Report, section 3.2.15 Lunar Surface Gravimeter Experiment, JCS-09423, April, 1975.

Apollo 17 Final Lunar Surface Procedures, Vol. 1: Nominal Plans, MSC, 11/6/72

ALSEP Array E Critical Design Review Presentation Material, NASA/MSC - Bendix Aerospace Systems Division, NAS9-5829, 14 - 18 June 1971, at JSC History Office.

ALSEP Termination Report, NASA Reference Publication 1036, April, 1979.