This Skylab science demonstration was designed to simulate the low-gravity melting of a material in a containerless fashion. The specific objective of the experiment was to visibly study a melting process not governed by gravity-driven convective flows.
During the Skylab SL-3 mission, a plastic, cylindrical pill container (30 mm diameter by 75 mm long) was filled with drinking water. An astronaut "...shook the water to the point where it did not have any bubbles to speak of in the solid mass." (1, p. 1) The water was frozen using the onboard food freezer. (A wooden cotton swab was included in the container to provide support for the ice.) After freezing, the cylinder of ice was removed from the pill container and the ice was weighed. Reportedly, there were 39.7 +/- 0.2 grams of ice available for melting. The ice, which was mounted so the cylinder could melt freely, was placed beside a ruler, thermometer and timer (all of which was in the field of view of a 16 mm camera). The camera was used to photograph the melting process approximately every 10 minutes (22 photographs total).
The low-gravity experiment was duplicated on Earth using the same experimental conditions (75% O2, 25% N2) and temperature (78 ¡F, 25.5 ¡C). During the ground-based experiment, the melted water was collected in a beaker and the volume measurements (+/- 0.1 cm 3 ) were made every 5 minutes. This allowed an accurate determination of the accumulative heat input to the material.
Initial post-flight examination of the returned photographs indicated:
(1) The total melting time of the ice cylinder was 190 +/- 5 minutes. (The total melting time of the identical Earth-based cylinder was 130 +/- 2 minutes.)
(2) In space, the cylindrical ends melted first while the diameter of the cylinder remained approximately constant.
(3) The water from these melting ends was driven onto the cylindrical surfaces by surface tension.
(4) The overall shape progressed from cylindrical to spherical with an intermediate, nearly ellipsoidal form. The spherical shape became evident at about half the melting time when the water totally surrounded the remaining ice.
Dimensional analysis of the ice was performed using the returned photographs. It was reported by the investigator that the photographs were slightly out of focus. Therefore the determination of the solid/liquid interface was not very accurate. The accuracy of the data was estimated to be about 10%. The volumes of the ice and water, as a function of time, were determined. "It should be mentioned that the change in volume of ice or water with time is a smooth non-linear curve for the ground experiment, whereas it is a linear function of time for the skylab experiment (t > 70 min)." (1, p. 3) These curves were used to calculate the total heat flow into the ice (using the heat of fusion of 79.8 cal/gram and the known mass and volume of the ice). The individual heat inputs to the ice from radiation, conduction, and convection were theoretically calculated for both ground-based and low-gravity experiments (see Reference (1) for details of these calculations). The sum of these values was then compared to (1) the calculated values from the photographs of the flight experiment and (2) the volume measurements from the ground-based experiment.
It was reported that, for the ground-based experiment, the theoretical and experimental values of total heat flow into the ice agree to within 6% over the duration of melting. "Therefore, since both values agree, it can be concluded from these calculations that, for the ground-based melting, the total heat flow is achieved by 55% convection, 38% radiation, 7% conduction." (1, p. 6)
For the Skylab experiment, "An accurate record of the quantitative analysis for the first 10 min of melting...[was]...not available [no photographs were taken during this time]. However, very good data of volumes...[were]...obtained for t > 50 min up to the total melting time." (1, p. 6) Comparison of the theoretical calculation of heat flow to those determined from the experiment (t > 60 min) indicated no contribution from convection. Therefore, it was concluded that in the low-gravity experiment the latent heat of melting was supplied by 81% radiation and 19% conduction.
Two major conclusions were reported:
(1) As illustrated in the Skylab demonstration, surface tension effects are important during low-gravity containerless melting.
(2) On Earth, convection is the dominant heat transfer mechanism for supply of latent heat of melting. However, in space, radiation is dominant heat transfer mechanism.
Information from this experiment provided knowledge of containerless melting and heat transfer in the absence of convective flow.
(2) Otto, G. H. and Lacy, L. L.: Observation of the Liquid Solid Interface in Low Gravity Melting. In Material Sciences in Space with Applications to Space Processing, AIAA Progress Series in Astronautics and Aeronautics, Edited by Leo Steg, Vol. 52, p. 483, 1977. (post-flight)
(3) Chassay, R. P. and Schwaniger, A.: Low-G Measurements by NASA. In Workshop Proceedings of the Measurement and Characterization of the Acceleration Environment on Board the Space Station, August 11-14, 1986, Guntersville, Alabama, p. 9-1. (acceleration measurements)
(4) Naumann, R. J. and Herring, H. W.: Ice Melting Demonstration. In Materials Processing in Space: Early Experiments, NASA SP-443, p. 87. (post-flight)
(5) TV111-Ice Melting. In MSFC Skylab Corollary Experiment Systems, Mission Evaluation, NASA TM X-64820, September 1974, pp. 7-39 - 7-43. (post-flight)
(6) Bannister, T. C.: Skylab III and IV Science Demonstrations Preliminary Report, NASA TM X-64835, March 1974, pp. 4-6. (post-flight)
(7) Otto, G. H. and Lacy, L. L.: The Liquid/Solid Interface in Low Gravity Melting. In Scientific Investigations on the Skylab Satellite, AIAA Progress Series in Astronautics and Aeronautics, Vol. 48, p. 455, 1976. (post-flight, short description)
(8) Naumann, R. J. and Mason, E. D.: Ice Melting. In Summaries of Early Materials Processing in Space Experiments, NASA Technical Memorandum NASA TM-78240, August 1979, p. 38. (post-flight)
(9) Ice Melting. In MSFC Skylab Mission Report- Saturn Workshop, NASA TM X-64814, October 1984, pp. 12-88.
(10) Input received from Principal Investigator G. H. Otto, July 1989 and August 1993.
(11) MSFC Science Demonstrations Performed by Pilot J. Lousma on Skylab 3. Quick Look Report, November 5, 1973, NASA Marshall Space Flight Center, Space Sciences Laboratory.