Landing a space vehicle on a celestial body devoid of atmosphere, such as the moon, presents special technical problems of braking the vehicle’s descent and accurately controlling the braking action. The conditions of entry are different from those presented by a major planet enveloped in a relatively dense atmosphere. For a moon landing the entire kinetic energy of the vehicle must be braked by a counteracting thrust developed by rockets.
On arrival in the vicinity of the moon, the spacecraft is first slowed down by firing its retro-rocket motors, so that it goes into a circular parking orbit round the moon. This applies more particularly to a manned spacecraft such as the Apollo which the Americans have used for landing men on the moon. In the case of the Apollo XI project the actual descent onto the moon’s surface was made by two astronauts in a special mooncraft, the “lunar (excursion) module” (abbreviated as “LEM” or “LM”), which was detached from the orbiting spacecraft, in which one astronaut remained awaiting the return of the mooncraft.
The entire procedure of releasing the mooncraft, landing it at a predetermined site on the moon and then linking it once again to the spacecraft is one which requires precise control of the direction and velocity of both vehicles. When the spacecraft is accurately in orbit and in the correct position on its orbit to ensure a landing in the desired area, the mooncraft briefly fires its rocket motor so that it moves away from the spacecraft and goes into an elliptical orbit whose point nearest the moon is located some 10 to 20 miles before and six or seven miles above the planned landing area. The periodic time of the mooncraft in this elliptical orbit must be the same as that of the spacecraft in its circular parking orbit. This particular requirement is for the astronauts’ safety; should the landing rockets fail to fire, the mooncraft will simply continue in orbit and automatically encounter the spacecraft; the latter can then be maneuvered into a docking position with the mooncraft, so that the two astronauts in it can return to the spacecraft that will take them back to earth.
When the mooncraft is in the correct position in its orbit, the actual landing maneuver can commence. The landing rocket motor of the mooncraft must be able to develop a thrust that can be suitably varied, because at the start of the landing operation the craft still carries its full load of rocket fuel, and its speed has to be slowed down from about 5000 mph to zero. In doing this, fuel corresponding to about two-thirds of the mooncraft’s initial total weight (with full tanks) is consumed. The power and direction of the thrust developed by the motor are so controlled that the craft lands at a predetermined point and at a predetermined speed. If the orbit in which the mooncraft is moving around the moon deviates a little from the specified orbit, corrections can be made by means of small steering rocket jets. In this way the horizontal and the vertical speed in relation to the landing area are reduced. When the horizontal speed has diminished to zero, the mooncraft will slowly sink towards the surface, the actual speed being kept under control by means of retroactive rocket motor thrust. By this time the astronauts have taken over manual control of the mooncraft. Scanning the lunar surface from an altitude of several hundred feet, they select a zone free from boulders, deep cracks or other hazards and then bring their craft gently down. The final operation calls for very accurate control of the thrust so that it almost exactly balances the mooncraft’s weight. When the feet of the craft touch the surface, the motors are shut off.
On completion of their exploration of the lunar surface, the astronauts return to their mooncraft. The lower half of the craft serves as a launching pad for the upper half, which is provided with a second, smaller rocket motor just under the crew cabin. This motor propels the ascent stage of the mooncraft back to the spacecraft, which will rendezvous and dock with it. The lunar astronauts then transfer to the spacecraft and jettison the mooncraft; the return flight to earth then begins.
The sequence of operations for the Apollo XI moon landing project was as follows:
1. Saturn rocket is launched, carrying the Apollo spacecraft with the mooncraft
enclosed within it.
2. First stage of the rocket is jettisoned, second stage is fired.
3. Second rocket stage is jettisoned, third stage goes into orbit around the earth (1 ½ revolutions).
4. Third stage is fired, thereby increasing the speed from 17,500 mph to the so-called “escape velocity” of almost 25,000 mph.
5. Third stage burns out. Apollo spacecraft is released. Mooncraft (LEM) and command module are now docked together nose to nose by a complex maneuver. They then reconnect with the third stage and continue the flight. Third stage is then finally jettisoned, and Apollo spacecraft starts up its own rocket motors. Apollo comprises the command module (i.e., the crew capsule), the service module, and the mooncraft.
6. Braking rockets are fired; spacecraft goes into orbit around the moon at an altitude of about 70 miles.
7. Two astronauts enter moon craft, which is now detached from the spacecraft.
8. Mooncraft goes into its own orbit bringing it over the landing area.
9. Main braking rocket motor of moon craft is fired. Telescopic legs of mooncraft are extended and it lands on the lunar surface.
10. Ascent stage of mooncraft launched for return flight to orbiting spacecraft.
11. Rendezvous with spacecraft. Lunar astronauts transfer themselves from mooncraft to spacecraft.
12. Mooncraft is jettisoned.
13. Spacecraft starts return flight to earth.
14. Command module is detached from service module.
15. Command module is maneuvered so that the heat shield is facing forward on entering the earth’s atmosphere.
16. Final parachute descent to earth.