Robert W. Mellette
The survival of the crew of the Space Shuttle depends upon the successful establishment of a closed ecological system that maintains a habitable environment. Shuttle astronauts are fortunate to enjoy a relatively high standard of living as a result of the knowledge gained from the earlier flights of the Mercury, Gemini, Apollo and Skylab spacecraft. The Space Shuttle Environmental Control and Life Support System provides a comfortable, shirt-sleeve environment for the crew as well as a conditioned thermal environment for the electrical components on-board.
As the name implies, the Life Support System ensures the survivability of the crew in the near-vacuum of space. If for any reason the cabin should depressurize suddenly, the results would be devastating. The astronauts blood would literally boil and turn to a gas. In order to avoid such a calamity, the cabin atmosphere is constantly monitored electronically. If this instrumentation detects the slightest drop in air pressure, from 6.66 kg. (14.7 lbs.) to 6.62 kg. (14.61 lbs.), an electronic “‘smart box”’ is alerted. This “‘smart box”’ checks all three oxygen sensors in the spacecraft. If two out of three sensors agree about the oxygen level, the majority opinion rules. If the sensors indicate a loss of cabin pressure additional oxygen and nitrogen is immediately introduced and an alarm is sounded.
Other serious hazards to life also exist. All spacecraft must be shielded against radiation. This shielding consists of special metal alloys that are resistant to cosmic radiation and atomic nuclei which constantly bombard the Shuttle. On earth we are also constantly bombarded by this radiation, however the amount we receive is basically harmless due to the shielding effect of the earth’s atmosphere. Additionally, heat shields in the form of specially manufactured tiles protect the crew not only from the dangers of radiation, but also from the tremendous heat generated as a result of friction with air molecules on entry into the atmosphere during landing.
There are many experiments and demonstrations that can help to demonstrate some of the principles and concepts involved in the section of the curriculum on Life Support Systems. These experiments range from a simple demonstration of forcing a hard boiled egg into a milk bottle, to show the effects of air pressure in a dramatic fashion, to constructing a cloud chamber to visibly demonstrate gamma rays. Students can also be instructed to establish balanced aquariums as models of a closed life support system.
A basic life support system includes not only a breathable atmosphere, but other basic requirements of life such as food, water, shelter and the management of waste products. Each of these necessities of life will now be discussed in detail.
A. AIR
Air pressure inside the cabin of the Shuttle is maintained at 1,033 grams per square centimeter (14.71 lbs.), the same as that on earth at sea level. As a safety measure the system is redundant. The system consists of two separate oxygen systems, two nitrogen and one emergency oxygen system. Cabin atmosphere closely approximates that of earth. The Shuttle’s atmosphere consists of a mixture of 20 percent oxygen and 80 percent nitrogen. A study of the composition of earth’s atmosphere would be appropriate at this point.
Astronauts breathe this mixed gas atmosphere during the mission, however there are three times when they do not. All astronauts breathe pure 100 percent oxygen at lift-off, landing, and while on extra-vehicular activities. The astronauts wear helmets or a space suit which delivers the pure oxygen to help keep them more alert during these critical phases of the space flight.
In addition to providing a breathable atmosphere, the Orbiter,s Environmental Control System circulates the cabin air through filters of granular lithium hydroxide and activated charcoal that remove excess carbon dioxide and odors. A fine mesh screen catches debris, such as lint, hair and crumbs. A build up of the carbon dioxide level would cause discomfort for the crew at low levels, and in large amounts would displace the oxygen required for human respiration.
The humidity of the cabin atmosphere is also regulated at a relative humidity of between 35 and 55 percent. If the humidity level is too high the astronauts would be uncomfortable and their efficiency would be reduced. Humidity control is important when you consider that an astronaut doing strenuous physical work can give off up to two pounds of water per hour in the form of perspiration. Temperature can be regulated between 16 and 32 degrees Celcius (61 and 90 degrees Fahrenheit).
An investigation into the Shuttle’s atmosphere suggests many corresponding terrestrial topics to explore. A natural avenue to investigate is the composition and layers of earth’s atmosphere. Mathematical computations of the force of earth’s atmosphere on the surface of a one gallon metal can be calculated. A dramatic demonstration of the weight of the air pressing on a one gallon metal can follows. To remove part of the air (the oxygen) from the can, light a slip of paper on fire and drop into the can, or a small quantity of water can be placed in the can and boiled to produce steam. The top of the can is then quickly screwed on and the can placed into ice water. The can is crushed by air pressure in a most spectacular fashion.
After completing a series of investigations into the physical science of the earth’s atmosphere, the biology of human respiration can be studied. Circle graphs can be constructed to show the relative amounts of oxygen and carbon dioxide in inhaled and exhaled air. Directions for constructing a simple apparatus for testing for the presence of carbon dioxide using limewater is included in the sample lesson plans unit. Limewater, a clear colorless liquid, changes to a milky white color in the presence of carbon dioxide. In this experiment students exhale through the limewater solution. The resulting change in the solution indicates a positive test for carbon dioxide. It is instructive to produce oxygen by combining peroxide and chlorox at this point and to demonstrate how oxygen is detected using a glowing splint. A tangential study of weather could be introduced along with this section on the earth’s atmosphere.
B.
WATER
Students may be surprised to learn that no water is brought onboard the Shuttle. All water is supplied as a byproduct from the three fuel cells that generate electrical power for the spacecraft. These fuel cells produce electrical energy through the chemical reaction of hydrogen and oxygen. As part of this chemical reaction drinkable water is produced at the rate of 3.2 kilograms (7 lbs.) per hour. This supply is sufficient to supply not only water used for drinking, but also for the water necessary for re-hydration of foods and beverages, and for personal hygiene. Water is not recycled. If the water supply generated exceeds demand, it is routed to storage tanks. Excess water is automatically dumped overboard. As crew size and distances traveled increase, it is unlikely that such a precious resource as water will not be recycled. A study of earth’s natural water or hydrologic cycle is a good model to investigate in this discussion of valuable resources that must be conserved. The chemical symbol H20 becomes meaningful to the middle school student if an electrolysis apparatus is available to break water into its component parts. Students can compare the volumes of the gases collected, (twice as much gas on one side as the other) and then test each gas using a glowing splint test.
C.
FOOD
Weight and volume are the primary design criteria for any object launched into space from earth. The Space Shuttle has the capability of delivering a total payload weight of 29,500 kilograms (65,000 lbs..). This relatively large payload capability is due in part to deliberate weight reduction design of all hardware and equipment. One area where weight reduction may not at first appear significant is in the food supply for the astronauts. All savings on weight at lift-off translate directly into fuel economy which results in increased payload capability. The actual total weight allowed for food is limited to 3.4 pounds per astronaut per day. One pound of this figure represents the weight of packaging. The total weight of the food supply for a typical seven day mission may not seem to be of much consequence, however, the significance becomes clearer when we compute the weight and volume required for future missions that will last up to thirty days or more.
Having discussed the above information with your students, challenge them to “‘brainstorm”’ ways in which weight reduction of the food supply could be accomplished. Try to elicit the response that weight reduction could be achieved by removing some or all of the water from some of the food items prior to launch, and then the food could be rehydrated in space. At this point it is helpful to have on hand freeze dried instant coffee as a familiar example of the type of process used to prepare food for space flight. Freeze dried ice cream can be purchased which provides children with a more exciting and tasty sample of astronaut food. Health food stores are also a source of common low moisture foods such as dried peaches, pears, apricots and beef jerky. After discussing the process of preparing foods for space flight and tasting both foods and re-hydratable beverages such as Tang and instant tea, explain some of the other techniques used in food preparation such as thermostabilizing, irradiating and removing some, but not all, the moisture from food items.
In the earlier Mercury missions astronauts had to squeeze their food out of toothpaste-like tubes. Unable to see or smell the food, there was little interest in eating. Today, astronauts aboard the Shuttle enjoy a menu of over seventy food items and twenty beverages. This is a wider selection than most restaurants offer:
Astronauts may eat from a standard menu or they may substitute food items to accommodate their own taste. All personal preference items are checked by a NASA dietician to insure that the foods selected supply the astronaut with the Recommended Dietary Allowances (RDA) of vitamins and minerals. The daily menu is designed to provide about 3,000 kilocalories per day for each astronaut.
This discussion of food, a favorite topic of middle school children, presents an opportune time to introduce lessons on digestion, the four basic food groups, vitamins and minerals kid caloric intake. Once these topics are explored, challenge your students to create a personal preference menu for a typical seven day mission aboard the Shuttle. This is an opportunity for students to see the practical application of mathematical skills in computing total caloric intake, RDA requirements, food grouping and, if possible, actual weight. Once students gain practice in this activity, they can assemble actual meals to take on a field trip or schoolyard adventure.
