Heat Flow

Description

The Heat Flow Experiment (HFE), which was part of the Apollo Lunar Surface Experiment Package (ALSEP), was designed to determine the rate of heat loss from the lunar interior by temperature and thermal property measurements at the surface and in the subsurface. The experiment was carried on the Apollo 15, 16, and 17 missions and was essentially identical on all three missions. The experiment apparatus consisted of two probes connected by 8 meter long cables to an electronics box which was in turn connected by a flat ribbon cable to the ALSEP station. The astronauts would drill two holes with the Apollo lunar surface drill (ALSD). The ALSD was equipped with borestem caps and retainers, borestems, bore bits, a bore bit/drill adapter, a treadle, and a bore stem/core stem wrench. The borestem assemblies used in drilling consisted of lengths of hollow fiberglass tubes, 2.5 cm in diameter, which would be connected together as the drilling progressed, and remained in the holes to provide a casing to prevent collapse of the hole walls during insertion of the probes. Nominally the holes were to be drilled to a depth of 3 meters but in practice no holes reached this depth. The probe would be lowered down into the borestem until it came to rest on top of the drill bit at the bottom of the hole. The borestem tube would project out of the surface a distance dependent on the depth of the hole.

Heat Flow Probes

Each heat flow probe was constructed of two rigid cylinders connected by a flexible joint. Each cylinder was 50 cm in length and held four platinum resistance elements which were electrically connected in pairs to form two accurate (+/-0.001 K) differential thermometers. The first pair of elements were located with one element near the top and one near the bottom of the cylinder, 47 cm apart from each other, and were connected in a bridge circuit. These sensors were designated the gradient bridge (DTG). The other two sensors were located 9 cm below the top DTG sensor and 9 cm above the bottom DTG sensor, 29 cm apart from each other connected by a bridge circuit. This pair was designated the ring bridge (DTR).

A thermocouple (the probe thermocouple) was mounted near the top of the upper cylinder, colocated with the top gradient sensor. Attached to the top of the upper cylinder was a long cable which connected the probe to the electronics box. The cable contained 3 more thermocouples spaced 65 cm, 115 cm, and 165 cm from the probe thermocouple. The thermocouples were spaced so that at least some of them were outside the borehole on the lunar surface.

1000 ohm Karma-wire platinum resistor heaters surrounded each of the four gradient bridge sensor housings on each probe. These were used for conductivity experiments and could be energized at either 0.002 W (low conductivity mode) or 0.5 W (high conductivity mode). The heaters would be turned on for approximately 36 hours for the low conductivity experiments and 6 hours in high conductivity mode.

The probes return absolute temperature data, differential temperature data (across the bridges), low- and high-thermal conductivity data, and thermocouple temperature data. Specifically, the experiment measured lunar temperatures of the following types, with corresponding accuracies noted in parentheses -- gradient bridge high-sensitivity temperature difference measurements (0.001 K) low-sensitivity temperature difference measurements (0.01 K), and absolute temperature measurements (0.05 K) over the temperature range 190 - 270 K; ring bridge temperature difference measurements (0.002 K) and absolute temperatures (0.05 K) over the range 190 - 270 K; thermocouple temperatures (0.07 K) over 70 - 400 K; and the reference bridge temperature (0.01 K) over 23 - 363 K.

The electronics box contained two multiplexers and amplifiers, dc/dc converter, and an isothermal block which contained a reference thermocouple and reference bridge. The electronics box was nominally kept at 278 to 328 K using heating elements, power control thermostats, a layered aluminized mylar insulation bag. fiberglass case, radiator plate, and sunshield. The instrument was powered by 29 volt d.c. from the central station.

Sensor Naming Convention

The sensors on probe 1 were individually designated as follows; for the upper cylinder the second number is "1", the upper gradient bridge sensor was DTG11A and the lower sensor was DTG11B; the upper ring bridge sensor was DTR11A and the lower DTR11B. For the lower cylinder the second number was "2", the upper gradient bridge sensor was DTG12A, the lower DTG12B, and the upper ring bridge sensor was DTR12A, the lower DTR12B. The bridge sensor pairs were designated DTG11, DTR11, DTG12, and DTR12. The sensors on probe 2 had the same format except that the first number represented the probe number, the first "1" in each designation was replaced with a "2", so the upper cylinder, upper gradient bridge sensor was DTG21A. Two numbering conventions exist in the literature for the thermocouples. We are using the convention that TC14 designates the thermocouple at the top of probe 1, TC13 is the cable thermocouple closest to probe 1, followed by TC12 and TC11. Probe 2 would have TC24, etc. (Another convention, seen in the Preliminary Science Reports, has the probe thermocouple designated as TC11, followed successively in the cable by TC14, TC13, and TC12.)

Instrument Operation

In normal operating mode a 7.25 minute measurement sequence is used to collect the ambient high- and low-sensitivity differential data from the gradient sensors and the thermocouple outputs. The same measurement sequence would be used when the heaters were commanded on for the low conductivity (0.002 W) mode, with the heaters activated in turn for typically 36 hours. For the high-conductivity (0.5 W) sequence, the ring bridge sensors were used and were read every 54 seconds. This mode nominally would last 8 hours. This mode could also be done without the heaters on, with measurements simply being made by the ring bridge sensors. This mode, known as a ring bridge survey, would be used approximately every 6 hours at first and less frequently later in the experiment.

Apollo 17 Operational History

The Apollo 17 heat flow electronics box was set up 12.3 meters north of the ALSEP central station with the hole for probe 1 drilled 5.7 meters east of the box and the probe 2 hole drilled 5.4 meters west of the box. Both holes were drilled about 250 cm into the lunar regolith. For probe 1 the sensors were at the following depths: DTG12B - 233 cm; DTR12B - 224 cm; DTR12A - 194 cm; DTG12A - 185 cm; DTG11B - 177 cm; DTR11B - 168 cm; DTR11A - 139 cm; DTG11A - 130 cm. Cable thermocouple TC13 was at a depth of 66 cm in the borehole, TC12 was right at the top of the hole, and TC11 was out on the surface. For probe 2 the sensors were at depths of: DTG22B - 234 cm; DTR22B - 225 cm; DTR22A - 195 cm; DTG22A - 186 cm; DTG21B - 178 cm; DTR21B - 169 cm; DTR21A - 140 cm; DTG21A - 131 cm. Cable thermocouple TC23 was at a depth of 67 cm in the borehole, TC22 was right at the top of the hole, and TC21 was out on the surface. Probe 1 was placed in the hole on 12 December 1972 at approximately 02:44 UT. The instrument was turned on at 03:02:00 UT and the first reading from probe 1 came at 03:05:48. Probe 2 was emplaced at 03:08. The first readings from probe 2 came at 03:08:28. On 18 February 1977 an anomaly occurred in probe 2 at the 230 cm level. The instrument was commanded off along with the other ALSEP experiments on 30 September 1977.

Heater schedule

The heaters were turned on and off in the low power (0.002 W) mode in January 1973 as follows, with the heater designation followed in parentheses by the depth of the heater, the date and time the heater was turned on, and the date and time the heater was turned off, in UT. For probe 1: H11 (130 cm, 3 Jan. 05:58 - 4 Jan. 18:00); H12 (177 cm, 14 Jan. 00:03 - 15 Jan. 11:48); H13 (185 cm, 21 Jan. 00:03 - 22 January 12:31); H14 (233 cm, 8 Jan. 06:21 - 9 Jan. 16:02). For probe 2: H21 (131 cm, 5 Jan. 05:18 - 7 Jan. 06:07); H22 (178 cm, 16 Jan. 12:06 - 18 Jan. 00:05); H23 (186 cm, 23 Jan. 00:31 - 24 Jan. 12:30); H24 (234 cm, 10 Jan. 05:59 - 11 Jan. 17:59). On 25 January at 18:00 UT H14 was turned on at high power, 0.5 W, and was shut off at 20:30 UT. The ring bridge DTR12 went off scale high.