The Gemini environmental control system is capable of providing life support for the biological systems of two astronauts. It provides for ingestion of appropriate gases and fluids, and the dispersal of by-products which are created as well as for cooling of spacecraft equipment, and the cabin interior.The system consists of a water management subsystem, an oxygen supply subsystem, and a cooling subsystem. It provides gaseous oxygen for breathing, for suits and cabin pressurization, and for suit and cabin ventilation. It provides for the removal of small solids, carbon dioxide, odors, and moisture from the suit and cabin atmosphere. For the flight of GT-3, it provides a drinking water supply-. It provides for storage and disposal of water accumulated as a condensate, and for disposal of urine. It includes a dual, recirculating coolant system for regulating the temperature of the suits, the cabin and items of electrical equipment.
PRIMARY OXYGEN SUBSYSTEM
The primary oxygen subsystem stores and dispenses oxygen for breathing and for suit and cabin pressurization. It supplies oxygen during the entire flight, commencing two hours prior to launch and terminating with the equipment section jettison at retrograde.
Oxygen pressure in the crew compartment is limited to 5 5 to 6.0 pounds per square inch above ambient by the cabin pressure relief valve. Primary oxygen supply capacity for a two-day mission is 15.3 pounds, located in a single spherical container in the equipment section, and is the primary source of oxygen during prelaunch, ascent and orbit. Oxygen is stored in this container in cryogenic form. It is heated to a gaseous state by a heat exchanger, passes to a pressure reducing regulator, then on to the cabin pressure regulator which automatically maintains cabin pressure as desired for the mission. Oxygen remaining in the primary supply is jettisoned with the tank when the equipment section is jettisoned prior to retrograde. The two-day absorber cartridge removes both odors and up to eleven pounds of carbon dioxide. The suit heat exchanger transfers heat from suit circuit oxygen to coolant flow. The heat transfer capacity is 1500 BTU per hour. The exchanger also removes moisture from the suit circuit oxygen and transfers it to the water management subsystem.
Pressure is maintained automatically- in the cabin through all phases of the Gemini mission. During the launch phase, cabin pressure relief valve permits outflow of any overpressures which might exist. This valve seals the cabin when the ambient pressure falls to 5.5 pounds per square inch below cabin pressure. This will occur between 20 and 40 seconds after launch.
If the cabin should decompress for any reason, the supply of oxygen provided through the dual cabin pressure regulator automatically turns off when the pressure reaches 4.0 pounds per square inch. When decompression occurs, either as programmed or as a result of a malfunction, the astronaut's pressure suit automatically takes over the pressurization responsibilities previously provided as a cabin environment.
If one or both astronauts chose to work with the face-plate of their suits open, a manually operated valve permits circulating cabin ah- through the suit circuit for carbon dioxide and water vapor removal. The same valve provides for relief of the vacuum created at the compressor inlet if the snorkel valve is momentarily closed by water after a water landing.
The crew compartment will be maintained at a nominal 65 degrees F during orbital flight. It is expected to rise to a high of approximately 120 degrees F during reentry. The maximum acceptable temperature in the crew compartment during launch and reentry of the Gemini Spacecraft is considered to be 200 degrees F.
SECONDARY OXYGEN SUBSYSTEM
Secondary oxygen is contained in two tanks in the pressurized compartment of the reentry module. There is sufficient quantity in each of the secondary oxygen supply tanks to provide oxygen adequate for one orbit at a normal flow rate and reentry at a nominal oxygen high rate of 0.08 pounds per minute to each astronaut.
The secondary oxygen subsystem operates when the pressure in the primary oxygen line falls below an allowable 75 pounds per square inch. When the primary oxygen container is jettisoned. the secondary oxygen subsystem assumes the primary role.
The oxygen line from the suit-demand and cabin pressure regulators is common to both the primary and secondary supplies so that flow is continuous to the suit circuit if a malfunction occurs.
EGRESS OXYGEN SUBSYSTEM
Oxygen for breathing and for suit pressurization in the event the astronaut's initiate ejection at 45,000 feet or below during launch or reentry is provided by the egress oxygen subsystem.
A tank containing approximately 1/3rd a pound of usable oxygen is located in the seat-mounted egress kit for each astronaut. The egress oxygen circuit is lanyard-opened with seat ejection.
WATER MANAGEMENT SUBSYSTEM
The water management subsystem collects and stores water for drinking, dumps waste water overboard, and manages the water used for cooling. Components of the subsystem include a water tank, a uriceptacle, a drinking nozzle, controls and valves, an evaporator, a reservoir, and a water pressure regulator.
The water is stored in a tank located in the equipment section. For GT-3, it contains approximately 16 pounds of water stored at 7.5 psig. A second water tank in the reentry module also contains approximately 16 pounds of water. Another seven pounds of liquid is contained in the launch cooling heat exchanger reservoir. A tank in the equipment section is charged with oxygen at 1,000 psig and this pressure is available upon demand to pressurize the water storage tanks. Oxygen-pressurized diaphragms force the water in the line to the cabin water tank and to the water dispenser.
URINE DISPOSAL
Urine Disposal equipment is Government furnished, installed by McDonnell. It consists of a urine line, bellows assembly, quick disconnect coupling, and a uriceptacle. On GT-3, urine is routed to the water evaporator for disposition upon actuation of the cabin water dump valve.
TEMPERATURE CONTROL SUBSYSTEM
A distinctively new feature of the Gemini Spacecraft is the fluid coolant system, which maintains cabin temperature, astronauts' suit temperature, and equipment temperature within acceptable limits.
Since the Gemini equipment and crew will generate heat at approximately three times the rate of the Mercury Spacecraft, and will do so for almost ten times as long, it became necessary to provide a new method of heat rejection, namely, a space radiator. The entire outer skin of the adapter module serves as the radiating surface, and the hollow stringers, through which the coolant passes, transfer the heat to the skin.
At the heart of this system is the space radiator in the adapter module, a coolant fluid reservoir, a low-level coolant sensing device, two identical positive displacement pumps for each of two redundant loops of coolant lines, and two regenerative heat exchangers.
The redundant systems provide protection against loss of a loop due to failure such as meteorite penetration.
A silicon ester coolant fluid (Monsanto's MCS 198) is pumped through each, then through heat exchangers which heat the primary cryogenic oxygen supply, cool the cabin and suits, cool such equipment as electrical power supplies, and various electronic equipment. The cabin and suit heat exchangers are conventional heat exchangers, and the electronic equipment heat exchangers are coldplates on which the equipment is mounted.
In the cabin each coolant circuit is divided into two parallel paths, one for the cabin and one for the suit. Ahead of each heat exchanger is a manual valve which permits crew adjustment of the flow through the heat exchanger.
In the section involving coldplates (there are 24 of them in each spacecraft), as many of them as possible have been arranged in parallel to minimize pressure loss. However, all of the flow is required through some high power density units, and these coldplates are arranged in series.
Two regenerative heat exchangers in the equipment section are provided for each of the loops. A temperature- sensitive valve modulates to maintain an outlet coolant temperature at between 36 degrees F and 42 degrees F. When the coolant temperature is 36 degrees F or lower, the valve causes all coolant to flow to the regenerative heat exchanger. When the coolant temperature is 42 degrees or above, the full coolant flow is directed to the space radiator.
The adapter structural shell provides an area of 165 square feet to radiate the excess heat into space.
Each of the redundant coolant loops follows alternate stringers in the adapter. so that when operating on one loop, the tubes of the other loop are prevented from freezing by heat from the adjacent warm tubes. The coolant fluid, however, has a freezing point well below -100' F. and has a characteristic of low viscosity at extremely low temperatures.
During preflight operation a ground heat exchanger removes heat from the spacecraft by circulating a coolant fluid from a servicing cart through a secondary loop in the heat exchanger. During launch, and for a short period thereafter, the space radiator is too warm for effective cooling. Then, a water boiler cools the coolant fluid by the simple process of absorbing the heat through vaporization.
In thermal balance tests run in the 30-foot space chamber at McDonnell, it was determined that the emissivity for the GT-3 spacecraft, which does not have fuel cells, was actually higher than desired. The GT-3 adapter was striped with .66 inch aluminum tape with black coating for deliberate and controlled reduction of the total heat emission of the radiating surface. There are 72 strips on the retrograde section, and 88 on the equipment section. A tape strip is aligned vertically between each stringer.
The temperature control subsystem circuits which connect with the reentry module are lost when the adapter section is jettisoned.
Temperature control subsequent to initiation of retrograde is provided by cabin wall insulation and the heat shield.
SUPERCRITICAL STORAGE OF CRYOGENIC FLUIDS
Primary oxygen is stored in the equipment section of the Gemini Spacecraft in a cryogenic state in especially designed cryogenic containers. Because cryogenic storage is at a lower pressure than tanks of equal size holding the same quantity of oxygen, danger of structural failure is reduced; associated components are lighter; and the heat absorption capacity of the fluid is available to help dispose of spacecraft waste heat.
The difference in weight between a vessel for storage of oxygen as a gas and in its cryogenic state is significant. A gas oxygen requires a heavily constructed tank to withstand a pressure of 7,500 pounds per square inch. In the supercritical cryogenic system used in Gemini, oxygen is first stored as a liquid, then in a confined tank volume, it is heated to a point where it becomes a homogeneous compressed fluid. This process takes place in the cryogenic storage container at a pressure of only 750 pounds per square inch. A cryogenic storage subsystem weighs less than four-tenths of a pound for each pound of oxygen stored. A high-pressure vessel of comparable capacity would weigh five times as much.
Gemini's supercritical storage of fluids insures a steady delivery of oxygen under all gravity conditions just as high-pressured gas does.
Copyright 1997, 1998, 1999 by John
Duncan |