Spaceflight Now: Apollo 13 Retrocast

The launch and mission trajectory
Retro-posted: April 10, 1970

The information presented here is based on an on-time April 11 launch and is subject to change before or during the mission to meet changing conditions.

The Saturn V rocket for Apollo 13 is rolled toward the launch pad. Photo: NASA/KSC
A Saturn V launch vehicle will lift the Apollo 13 spacecraft from Launch Complex 39A, NASA-Kennedy Space Center, Fla. The azimuth may vary from 72 to 96 degrees, depending on the time of launch. The azimuth changes with launch time to permit a fuel-optimum injection from Earth parking orbit to a free-return circumlunar trajectory.

April 11 launch plans call for liftoff at 2:13 p.m. EST on an azimuth of 72 degrees. The vehicle will reach an altitude of 36 nautical miles before first stage cutoff 51 nm downrange. During the 2 minutes 44 seconds of powered flight, the first stage will increase vehicle velocity to 7,775 feet per second.* First stage thrust will reach a maximum of 8,995,108 pounds before center engine cutoff. After engine shutdown and separation from the second stage, the booster will fall into the Atlantic Ocean about 364 nm downrange from the launch site (3O degrees North latitude and 74 degrees west longitude) about 9 minutes 4 seconds after liftoff.

The second stage (S-II) will carry the space vehicle to an altitude of 102 nm and a distance of 892 nm downrange. At engine shutdown, the vehicle will be moving at a velocity of 21,508 fps. The four outer J-2 engines will burn 6 minutes 32 seconds during the powered phase, but the center engine will be cut off 4 minutes 47 seconds after S-II ignition. At outboard engine cutoff, the S-II will separate and, following a ballistic trajectory, plunge into the Atlantic about 2,450 nm downrange from the Kennedy Space Center (31 degrees North latitude and 33.4 degrees West longitude) some 20 minutes 41 seconds after liftoff.

The single engine of the Saturn V third stage (S-IVB) will ignite about 3 seconds after the S-II stage separates. The engine will fire for 143 seconds to insert the space vehicle into a circular Earth parking orbit of 103 nm beginning about 1,468 nm downrange. Velocity at Earth orbital insertion will be 24,243 fps at 11 minutes 55 second ground elapsed time (GET). Inclination will be 33 degrees to the equator.

*NOTE: Multiply nautical miles by 1.1508 to obtain statute miles, multiply feet per second by 0.6818 to obtain statute miles per hour.

The crew will have a backup to launch vehicle guidance during powered-flight. If the Saturn instrument unit inertial platform fails, the crew can switch guidance to the command module systems for first-stage powered flight automatic control. Second and third stage backup guidance is through manual takeover in which spacecraft commander hand controller inputs are fed through the command module computer to the Saturn instrument unit.

Lightning Precautions
During the Apollo 12 mission the space vehicle was subjected to two distinct electrical discharge events. However, no serious damage occurred and the mission proceeded to a successful conclusion. Intensive investigation led to the conclusion that no hardware changes were necessary to protect the space vehicle from-similar events. For Apollo 13 the mission rules have been revised to reduce the probability that the space vehicle will be launched into cloud formations that contain conditions conducive to initiate similar electrical discharges although flights into all clouds is not precluded.

May Launch Opportunities
The three opportunities established for May -- in case the launch is postponed from April 11 -- provide, in effect, the flexibility of a choice of two launch attempts. The optimum May launch window occurs on May 10. The three day window permits a choice of attempting a launch 24 hours earlier than the optimum window and if necessary a further choice of a 24 hour or 48 hour recycle. It also permits a choice of making the first launch attempt on the optimum day with a 24-hour recycle capability. The May 9 window (T-24 hrs) requires an additional 24 hours in lunar orbit before initiating powered descent to arrive at the landing site at the same time and hence have the same Sun angle for landing as on May 10. Should the May 9 window launch attempt be scrubbed, a decision will be made at that time, based on the reason for the scrub, status of spacecraft cryogenics and weather predictions, whether to recycle for May 10 (T-O hrs) or May 11 (I424 hrs). If launched on May 11, the flight plan will be similar for the May 10 mission but the Sun elevation angle at lunar landing will be 18.5 degrees instead of 7.8 degrees.

Flight Data File
Mission: Apollo 13
Flight crew:
James A. Lovell, Jr.
John L. Swigert, Jr.
Fred W. Haise
Launch vehicle:
Saturn V AS-508
1913 GMT, April 11, 1970
Lunar landing site:
Fra Mauro

Pre-launch briefing
The rocket - A description of the Saturn V launch vehicle.

Jim Lovell - Meet the mission commander.

Jack Swigert - Meet the command module pilot.

Fred Haise - Meet the lunar module pilot.

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