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| Getaway Specials (Capillary Pumped Loop; Hearts in Space) |
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| Overview |
| Four Getaway Special (GAS) payloads will be aboard the STS-95 mission. G-467 (Capillary Pumped Loop) and G-779 (Hearts in Space) are described below. The other two GAS experiments, G-238 and G-764, are part of the International Extreme Ultraviolet Hitchhiker (IEH)-03 payload. For more information, see Hitchhiker Experiments. G-467: Capillary Pumped Loop (CPL) The objective of the G-467 GAS payload is to demonstrate in space the working principle and performance of a two-phase capillary pumped loop (CPL) with two advanced evaporators, a two-phase vapor quality sensor (VQS) with two condensers in parallel, and a control reservoir. Capillary pumped loops are two-phase heat transfer systems that use wicks with small internal tubing to move the working fluid from the instrument that needs cooling to the spacecraft radiator for heat rejection to space. A two-phase flow loop system allows heat to be transferred but contains no moving parts and requires minimal power. Another objective is to compare data on CPL behavior in low gravity with analytical predictions resulting from computer modeling and performance on Earth. The in-orbit experiment is to demonstrate that the CPL has the capability to operate under different heat loads imposed on two evaporators in parallel; to share heat load between two evaporators; to prime an evaporator by a controlled management of the reservoir fluid content; to start up from low-temperature conditions; and to adjust and maintain a temperature set point while operating under different heat load and sink conditions. G-467 will also provide low-gravity calibration of the VQS, carry out simple control exercises to demonstrate the usefulness of a VQS for system control, and determine the performance limits of the CPL and its evaporators. This experiment is a modified reflight of G-557, which was flown on STS-60 in February 1994. Experiment G-467 differs from G-557 by a new, more accurate controllable bypass valve and tuned vapor bypass line flow resistance; increased number of sensors; condensers in parallel instead of in sequence; new evaporator internal design; increased number of sensors; new flight scenario; update and extension of experiment controlling software; and updated position of reservoir-loop connection. The G-467 GAS payload is sponsored by the European Space Agency, Paris, France. The payload manager for this experiment is Andre Robelet. G-779: Hearts in Space G-779, or Hearts in Space, was developed by researchers at Bellarmine College in Louisville, Ky. The original Hearts in Space experiment (G-572) was initially flown aboard the space shuttle Discovery on STS-85 in August 1997. Because of the failure of a tape recorder electrical connector, which prevented the on-orbit recording of physiological pressure and flow data, the payload is being reflown. The purpose of the payload is to study why astronauts' hearts become smaller while in space. NASA scientists have noted that the size of the heart, known as the ventricular volume index, decreases in astronauts following adaptation to weightlessness, usually by the second day. In addition to becoming smaller in size, the heart also pumps approximately 15 to 20 percent less blood per heartbeat (the stroke volume index) during zero gravity. The body automatically makes adjustments via hormones and nervous control that maintain blood pressure and heart rate so that the astronauts suffer no ill effects from the reduced size of the heart. The underlying physiological reason for these changes has never been explained. The Hearts in Space experiment will demonstrate how these changes in cardiac size happen. The goal of the experiment is to prove that the biological changes to astronauts' hearts are directly attributable to the loss of gravity, since blood is weightless in space. On Earth, several factors cause the heart to fill, including the overall velocity and pressure of the blood entering the heart; how stiff or flaccid the muscles of the heart wall are; and the weight of the volume of blood in the heart (technically referred to as the gravitational acceleration-dependent hydrostatic pressure difference, that is, fluid pressure due to weight that is caused by gravity). An analogy would be filling a water balloon. As the balloon fills, the weight of the water helps stretch the balloon, allowing more water to enter (the pressure and velocity of the water, and the stiffness of the balloon, also would be factors). The scientists' hypothesis is that the heart fills less (hence its size is reduced) because the incoming blood has no weight in space. Researchers have calculated that the weight of the blood contributes about 15 to 20 percent of the total filling of the heart. Therefore, in space the heart should be 15 to 20 percent smaller because the blood is weightless. This is the same reduction seen on the echocardiographs taken during space flight. G-779 will document what the mathematical calculations predict and the echocardiographs show. Thus, the researchers hope to prove that the change in heart size is because of the lack of weight of the blood and not caused by other factors, such as fluid shifting in the body. The science team has built an experimental apparatus consisting of an artificial human heart and mechanical circulatory system that simulates the blood pressure and flow in a normal adult. The payload uses the same clinical artificial heart that has been implanted in patients with heart disease. Since the apparatus will not be influenced by normal biological reactions to zero gravity, such as hormone release, blood vessel constriction, nerve impulses, etc., scientists should get a more accurate picture of the physical forces affecting the heart in outer space. In the astronauts, their bodies compensate for this loss in filling capacity by increasing the heart rate and constricting blood vessels so that blood flow and pressure stay normal, whereas the mechanical model that will fly on the shuttle allows researchers to study the changes in pressure and flow directly without the biological corrective measures. Principal investigators are Thomas E. Bennett, Ph.D., Department of Biology, Bellarmine College, Louisville; George M. Pantalos, Ph.D., Department of Surgery, University of Utah, Salt Lake City; and M. Keith Sharp, Sc.D., Dept. of Civil Engineering, University of Utah, Salt Lake City. |
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