Challenger Learning Center
Lunar Orbital Rendezvous
the Apollo Mission Trajectory
The launch profile used in the Challenger Return to the Moon mission is based on the actual Apollo flight plan. Here’s how they really did it.
Choosing a Flight Mode
Three methods of reaching the Moon, known as flight modes, were hotly debated in the early days of Apollo mission planning. One, known as Direct Ascent Mode, would have employed a single enormous rocket, called "Nova", to transport Apollo straight to the Moon with no rendezvous at all. The Earth Orbital Rendezvous technique called for two Saturn V boosters to launch and rendezvous in Earth orbit. In this mode, one rocket would carry a single Apollo vehicle and its crew, and the other, more fuel. The fuel would be transferred to Apollo in Earth orbit, and the spacecraft would then continue on to the Moon.
The mode finally agreed upon required only one three-stage Saturn V booster, and split the Apollo vehicle into two independent vehicles – a combined Command and Service Module (CSM), and a Lunar Module (LM). The principle architect of the Lunar Orbital Rendezvous mode was NASA engineer John C. Houbolt. Houbolt, seen below explaining his ideas with the aid of an early CAD/CAM system, was all but ignored in the beginning. The benefits he described based on his calculations were just too good to be true. But in time it became clear to all that LOR did have many real advantages. The Saturn V technology was nearly ready, whereas the Nova technology was still a long way off. Also, one booster was arguably twice as reliable at launch time as two boosters, and by consuming only one Saturn V booster per mission and incorporating more existing technology, Lunar Orbital Rendezvous would be more economical all around. But most importantly, it made good engineering sense. By separating the spacecraft into two specialized vehicles, the designers could take full advantage of the Moon’s low gravity; the lunar lander could be made quite small and lightweight, reducing bulk, fuel, and thrust requirements, all with one stroke.
How Apollo got to the Moon
Hold-down arms restrain the Saturn V booster for a full seven seconds after ignition. The 7.5 million pounds of thrust generated by Saturn’s first stage engines place the vehicle at an altitude of 41miles, with a velocity of 6,200 miles per hour, about 2 ½ minutes after liftoff. The first stage is jettisoned at this point, and the second stage is ignited. Stage 2 boosts Apollo to its orbital altitude of around 115 miles, at the required orbital velocity of 15,400 miles per hour. Following a burn duration of about six and a half minutes, stage two is jettisoned.
Saturn’s third stage is ignited twice. Its first job is to convert the vehicle’s launch trajectory into a parking orbit, a low-altitude, circular orbit which allows the crew to check out all systems one last time before committing to a lunar mission. This requires an engine burn of about two minutes. The next event is translunar injection, at which time the crew reignites the single J-2 engine to accelerate the vehicle to escape velocity - about 25,000 miles per hour. The six-minute burn takes Apollo out of Earth orbit, and places them on a curved path toward the point in space just ahead of where the Moon is calculated to be at their estimated time of arrival.
Shortly after establishing themselves on this translunar trajectory, the crew performs a maneuver called transposition and docking. The Command/Service Module (CSM) separates from the Saturn third stage, executes a 180° about face, and docks head-to-head with the Lunar Module (LM). The wedded CSM/LM spacecraft sheds the spent third stage by means of pyrotechnic bolts and springs, and then heads for the Moon.
Along the way, Apollo maintains a rotisserie-like motion known as "barbecue mode". By rotating slowly around its long axis, the thermal stresses created by the extreme heat of direct and unfiltered solar radiation on one side of the vehicle, versus the severe cold of interplanetary shade on the other, are kept in control. If not for this simple but necessary action, Apollo would soon split itself apart.
One or more midcourse corrections might be required should the navigator’s triangulation measurements show Apollo to be veering off course. A sextant is used to determine the angles between the vehicle, a star, and a common reference point on Earth or Moon. Such readings are taken with reference to three different stars, giving the crew the data they need to pinpoint their position in space with accuracy.
About 215, 000 miles into the voyage, Apollo slows to a speed of around 2, 000 mph due to the decreasing but persistent effects of Earth’s gravity. As Lunar gravity begins to supercede Earth’s gravity, the vehicle begins to accelerate once again. To achieve lunar orbit insertion, Apollo must retrofire (engine facing in the direction of motion) its service module engine to slow the spacecraft to orbital velocity.
Once established in a 60 nautical mile lunar parking orbit, the LM crew powers up their landing vehicle. Every system is checked out carefully in an effort to eliminate surprises during the final descent. The Apollo vehicles gently disunite at CSM/LM separation, leaving the Command Module pilot alone to begin a course of station keeping and scientific observation. Meanwhile, the LM crew executes two carefully planned descent engine burns to first drop the lander’s altitude to about 50, 000 feet, and then to begin the climactic 12-minute powered descent phase to the Moon’s rocky surface. The astronauts are not seated in the LM during these maneuvers. Engineers reasoned that sitting down was immaterial to comfort in a microgravity environment, and so designed a system of belts and cables to support them in a standing position.
The descent engine reaches full throttle about thirty seconds after ignition, with the onboard computer in control as the vehicle begins its graceful arc toward the surface. The Commander eventually takes over manual control to guide his vehicle and its crew around the rugged terrain. He must drop the LEM onto a level landing site with no sideward motion whatsoever, lest its four spidery landing legs collapse from the lateral stress. By the time the crew is safely on the surface of the Moon, there is barely more than vapors left in the fuel tank.
Some say that our conquest of the Moon was a success of engineering, rather than of science. Certainly it was both. As for the classroom, here is one more rich story that brings history, math and science, social studies, teamwork, problem-solving and decision making, and other important lessons, together - and to life.
SATURN V BOOSTER|
|WEIGHT ||6,100,000 lbs|
|PROPELLANT/PAYLOAD RATIO ||50/1|
|TIME FROM IGNITION TO ORBIT ||11 minutes|
|STAGES OF THE SATURN V BOOSTER |
| ||FIRST STAGE ||SECOND STAGE ||THIRD STAGE ||APOLLO
|DIAMETER|| 33'|| 33'|| 21' 8"|| 21' 8"|
|LENGTH||138'|| 81' 6"|| 58' 7"|| 81' 10¼" |
|WEIGHT ||4,792,000 lbs (fueled)|| 1,037,000 lbs (fueled)|| 262,000 lbs (fueled)|| 109,608 lbs |
|ENGINES ||five F-1 rocket engines ||five J-2 rocket engines ||one J-2 rocket engine|| - |
|PROPELLANT ||kerosene & liq. oxygen ||liq. hydrogen & liq. oxygen ||liq. hydrogen & liq. oxygen ||- |
|THRUST ||7.5 million lbs ||1 million lbs ||200,000 lbs ||- |
|1st BURNTIME ||2 min, 50 sec ||6 min, 35 sec ||2 min, 45 sec ||- |
|VELOCITY at 1st burnout|| 6,200 mph ||15,400 mph||17,500 mph ||- |
|ALTITUDE at 1st burnout ||36 miles ||114 - 118 miles ||115 miles ||- |
|2nd BURNTIME ||- ||- ||5 min, 12 sec ||- |
|VELOCITY at 2nd burnout ||- ||- ||24,300 mph ||- |
|ALTITUDE at 2nd burnout|| - ||- ||- ||- |
For more detailed information on the Apollo program, visit NASA's Apollo Mission website web site.
All photos and graphics courtesy NASA.
Bruce Mattson is the Assistant Director and Science and Technology Specialist at the McAuliffe/Challenger Center
© 2012 Christa Corrigan McAuliffe Center for Education and Teaching Excellence
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