Apollo Expeditions to the Moon



At the heart of each spacecraft were its subsystems. "Subsystem" is space-age jargon for a mechanical or electronic device that performs a specific function such as providing oxygen, electric power, and even bathroom facilities. CSM and LM subsystems performed similar functions, but differed in their design because each had to be adapted to the peculiarities of the spacecraft and its environment.

Begin with the environmental control system - the life-support system for man and his machine. It was a marvel of efficiency and reliability, with weight and volume at a premium. A scuba diver uses a tank of air in 60 minutes; in Apollo an equivalent amount of oxygen lasted 15 hours. Oxygen was not simply inhaled once and then discarded: the exhaled gas was scrubbed to eliminate its CO2 recycled, and reused. At the same time, its temperature was maintained at a comfortable level, moisture was removed, and odors were eliminated. That's not all: the same life-support system also maintained the cabin at the right pressure, provided hot and cold water, and a circulating coolant to keep all the electronic gear at the proper temperature. (In the weightless environment of space, there are no convective currents, and equipment must be cooled by means of circulating fluids.) Because astronauts' lives depended on this systern, most of the functions were provided with redundancy - and yet the entire unit was not much bigger than a window air conditioner.

A picture of the cross-section of the command module and the service module Above: Looking like a huge toy top the conical command module was crammed with some of the most complex equipment ever sent into space. The three astronaut couches were surrounded by instrument panels, navigation gear, radios, life-support systems, and small engines to keep it stable during reentry. The entire cone, 11 feet long and 13 feet in diameter, was protected by a charring heat shield. The 6.5-ton CM was all that was finally left of the 3000-ton Saturn V stack that lifted oft on the journey to the Moon.

Below: Packed with plumbing and tanks, the service module was the CM's constant companion until just before reentry. So all components not needed during the last few minutes of flight, and therefore requiring no protection against reentry heat, were transported in this module. It carried oxygen for most of the trip; fuel cells to generate electricity (along with the oxygen and hydrogen to run them); small engines to control pitch, roll, and yaw; and a large engine to propel the spacecraft into -and out of- lunar orbit.

A picture of the cross-section of the descent stage
The lunar module was also a two-part spacecraft. Its lower or descent stage had the landing gear and engines and fuel needed for the landing. When the LM blasted off the Moon, the descent stage served as the launching pad for its companion ascent stage, which was also home for the two explorers on the surface. In function if not in looks the LM was like the CM, full of gear to communicate, navigate, and rendezvous. But it also had its own propulsion system, an engine to lift it off the Moon and send it on a course toward the command module orbiting above.

A picture of the cross-section of the descent stage

How do you generate enough electric power to run a ship in space? In the CSM, the answer was fuel cells; in the LM, storage batteries. Apollo fuel cells used oxygen and hydrogen -stored as liquids at extremely cold temperatures- that when combined chemically yielded electric power and, as a byproduct, water for drinking. (In early flights the water contained entrapped bubbles of hydrogen, which caused the astronauts no real harm but engendered major gastronomical discomfort. This led to loud complaints, and the problem was finally solved by installing special diaphragms in the system.) The fuel-cell power system was efficient, clean, and absolutely pollution-free. Storing oxygen and hydrogen required new advances in leakproof insulated containers. lf an Apollo hydrogen tank were filled with ice and placed in a room at 70 F, it would take 8.5 years for the ice to melt. lf an automobile tire leaked at the same rate as these tanks, it would take 30 million years to go flat.

"Houston, this is Tranquility." These words soon would be heard from another world, coming from an astronaut walking on the Moon, relayed to the LM, then to a tracking station in Australia or Spain or California, and on to Mission Control in Houston with only two seconds' delay. Communications from the Moon were clearer and certainly more reliable than they were from my home in Nassau Bay (a stone's throw from the Manned Spacecraft Center) to downtown Houston. At the same time, a tiny instrument would register a reading in the astronauts' life-support system, and a few seconds later an engineer in Mission Control would see a variation in oxygen pressure, or a doctor a change in heart rate; and around the world people would watch on their home television sets. Bebind all of this would be the Apollo communications system-designed to be the astronauts' life line back to Earth, to be compact and lightweight, and yet to function with absolute reliability; an array of receivers, transmitters, power supplies and antennas, all tuned to perfection, that allowed the men and equipment on the ground to extend the capabilities of the astronauts and their ships. (Later on, when the computer on Apollo 11's LM was overloaded during the critical final seconds of the landing, it was this communications system that enabled a highly skilled flight controller named Steve Bales to tell Neil Armstrong that it was safe to disregard the overload alarms and to go ahead with the lunar landing.)

A photo of environmental control system of the lunar module Like a plumber's dream, the LM's environmental control system nestled in a corner of the ascent stage. Those hoses provided pure oxygen to two astronauts at a pressure one-third that of normal atmosphere, and at a comfortable temperature. The unit recirculated the gas, scrubbed out CO2 and moisture exhaled, and replenished oxygen as it was used up.

A photo of two NASA engineers in an anechoic test chamber
Sound is deadened and not an echo can be heard in this anechoic test chamber. Used to simulate reflection-free space, its floor, walls, and ceiling are completely covered with foam pyramids that absorb stray radiation, so that an antenna's patterns can be accurately measured. Here two NASA engineers inspect a test setup of an astronaut's backpack. Any interference between the astronaut and his small antenna could be detected and fixed before a real astronaut set foot on the Moon.

If you had to single out one subsystem as being most important, most complex, and yet most demanding in performance and precision, it would be Guidance and Navigation. Its function: to guide Apollo across 250,000 miles of empty space; achieve a precise orbit around the Moon; land on its surface within a few yards of a predesignated spot; guide LM from the surface to a rendezvous in lunar orbit; guide the CM to hit the Earth's atmosphere within a 27-mile "corridor" where the air was thick enough to capture the spacecraft, and yet thin enough so as not to burn it up; and finally land it close to a recovery ship in the middle of the Pacific Occan. Designed by the Massachusetts Institute of Technology under Stark Draper's leadership, G&N consisted of a miniature computer with an incredible amount of information in its memory; an array of gyroscopes and accelerometers called the inertial-measurement unit; and a space sextant to enable the navigator to take star sightings. Together they determined precisely the spacecraft location between Earth and Moon, and how best to burn the engines to correct the ship's course or to land at tbe right spot on the Moon with a minimum expenditure of fuel. Precision was of utmost importance; there was no margin for error, and there were no reserves for a missed approach to the Moon. In Apollo 11, Eagle landed at Tranquility Base, after burning its descent engine for 12 minutes, with only 20 seconds of landing fuel remaining.

A photo of astronaut, Cunningham looking through a spacecraft window
Like new Magellans, astronauts learned to navigate in space. Here Walt Cunningham makes his observations through a spacecraft window. The tools of a space navigator included a sextant to sight on the stars, a gyroscopically stabilized platform to hold a constant reference in space, and a computer to link the data and make the most complex and precise calculations.

But the guidance system only told us where the spacecraft was and how to correct its course. It provided the brain, while the propulsion system provided the brawn in the form of rocket engines, propellant tanks, valves, and plumbing. There were 50 engines on the spacecraft, smaller but much more numerous than those on the combined three stages of the Saturn that provided the launch toward the Moon. Most of them - 16 on the LM, 16 on the SM, and 12 on the CM - furnished only 100 pounds of thrust apiece; they oriented the craft in any desired direction just as an aircraft's elevators, ailerons, and rudder control pitch, roll, and yaw.

Three of the engines were much larger. On the service module a 20,500-pound-thrust engine injected Apollo into lunar orbit, and later brought it back home; on the LM there was a 10,500-pound-thrust engine for descent, and a 3500 pounder for ascent. All three had to work: a failure would have stranded astronauts on the Moon or in lunar orbit. They were designed with reliabillty as the number one consideration. They used hypergolic propellants that burned spontaneously on contact and required no spark plugs; the propellants were pressure-fed into the thrust chamber by bottled helium, eliminating complex pumps; and the rocket nozzles were coated with an ablative material for heat protection, avoiding the need for intricate cooling systems.

Three other engines could provide instant thrust at launch to get the spacecraft away from the Saturn if it should inadvertently tumble or explode. The largest of these produced 160,000 pounds of thrust, considerably more than the Redstone booster which propelled Alan Shepard on America's first manned spaceflight. (Since we never had an abort at launch, these three were never used.)

A photo of 4 small rocket engines Because there is no air to deflect, a spacecraft lacks rudders or ailerons. Instead, it has small rocket engines to pitch it up or down, to yaw it left or right, or to roll it about one axis. Sixteen of them were mounted on the service module, in "quads" of four. Here one quad is tested to make sure that hot rocket exhaust will not burn a hole in the spacecraft's thin skin.

A photo of the service propulsion engine,LM descent engine,and the LM ascent engine
Similar in shape but not size were the three big engines aboard Apollo spacecraft. Two of them had no backup, so they were designed to be the most reliable engines ever built. lf the service-propulsion engine failed in lunar orbit, three astronauts would be unable to return home; if the ascent engine failed on the Moon, it would leave two explorers stranded. (A descent-engine failure would not be as critical, because the ascent engine might be used to save the crew members.)

There were other subsystems, each with its own intricacies of design, and, more often than not, with its share of problems. There were displays and controls, backup guidance systems, a lunar landing gear on the LM and an Earth landing system (parachutes) on the CM, and a docking system designed with the precision of a Swiss watch, yet strong enough to stop a freight car. There were also those things that fell between the subsystems: wires, tubes, plumbing, valves, switches, relays, circuit breakers, and explosive charges that started, stopped, ejected. separated, or otherwise activated various sequences.

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