Apollo 6

Apollo 6, launched in April 1968, was the Apollo program's second and last unmanned test flight of its Saturn V launch vehicle.

Objectives

This was the final qualification flight of the Saturn V before its first manned flight (Apollo 8). It was also the first mission to use High Bay 3 in the Vertical Assembly Building (VAB), Mobile Launcher 2 and Firing Room 2. Another objective was testing the Command Module re-entry system under extreme conditions simulating a worst-case return from the Moon. This objective was not met due to J-2 engine failures.

The pieces come together

The S-IC first stage arrived by barge on 13 March, 1967 and was erected in the VAB four days later, with the S-IVB third stage and Instrument Unit computer arriving the same day. The S-II second stage was two months behind them and so was substituted with a dumbbell shaped spacer so testing could proceed. This had the same height and mass as the S-II along with all the electrical connections. The S-II arrived 24 May. It was stacked and mated into the rocket on 7 July.

Testing was slow as they were still checking out the launch vehicle for Apollo 4, a limitation of the system where there wasn't two of everyone and everything. The VAB could handle up to four Saturn Vs but could only check out one at a time.

The Command and Service Module arrived 29 September and was stacked 10 December. It was a hybrid, featuring the Command Module Number 20 and Service Module Number 14 after SM-020 was destroyed in a tank explosion and Command Module Number 14 was dismantled as part of the investigation into the Apollo 1 fire. After two months of testing and repairs the rocket was moved to the pad on 6 February, 1968.

Flight

Launch

Unlike the near perfect flight of Apollo 4, Apollo 6 experienced problems right from the start. Two minutes into the flight, the rocket experienced severe Pogo oscillations for about 30 seconds. George Mueller explained the cause to a congressional hearing:

Pogo arises fundamentally because you have thrust fluctuations in the engines. Those are normal characteristics of engines. All engines have what you might call noise in their output because the combustion is not quite uniform, so you have this fluctuation in thrust of the first stage as a normal characteristic of all engine burning.

Now, in turn, the engine is fed through a pipe that takes the fuel out of the tanks and feeds it into the engine. That pipe's length is something like an organ pipe so it has a certain resonant frequency of its own and it really turns out that it will oscillate just like an organ pipe does.

The structure of the vehicle is much like a tuning fork, so if you strike it right, it will oscillate up and down longitudinally. In a gross sense it is the interaction between the various frequencies that causes the vehicle to oscillate.

In part due to the pogo, the spacecraft adaptor that attached the CSM and mockup of the Lunar Module to the rocket started to have some structural problems. Airborne cameras recorded several pieces falling off it at T+133.

After the first stage was jettisoned at the end of its task, the S-II second stage began to experience its own problems. Engine Number Two (it had five) had performance problems from 206 to 319 seconds after liftoff and then at 412 seconds shut down all together. Then two seconds later Engine Number Three shut down as well. The onboard computer was able to compensate and the stage burned for 58 more seconds than normal. Even so the S-IVB third stage also had to burn for 29 seconds longer than usual.

The S-IC first stage impacted the Atlantic Ocean east of Florida (), while the S-II second stage impacted south of the Azores ().

Orbit

Due to the less than nominal launch, the CSM and S-IVB were now in a 178 by 367 km orbit instead of the planned 160 km circular orbit. But after two orbits of checking out the spacecraft and rocket stage, the S-IVB failed to restart to simulate a Trans-Lunar Injection Burn, that would send the astronauts to the moon.

So it was decided to use the Service Module engine to raise the spacecraft into a high orbit in order to complete some of the mission objectives. It burned for 442 seconds, longer than it would ever have to on a real Apollo mission and raised the apogee of the orbit to 22,200 km. There was now however not enough fuel to speed up the reentry and the spacecraft only entered the atmosphere at a speed of 10,000 m/s instead of the planned 11,270 m/s. This meant it landed 80 km from the planned touch down point.

Ten hours after launch it was lifted on board the USS Okinawa.

SIV-B reentered April 25, 1968.

Causes and fixes of problems

The cause of the pogo during the first stage of the flight was well known. However, it had been thought that the rocket had been 'detuned'. To further detune the rocket, it was decided to fill the cavities with helium gas.

The failure of the two engines in the second stage was traced to the rupturing of a fuel line that fed the engine igniters. The ignitor was essentially a miniature rocket motor mounted in the wall of the J-2 engine's pressure chamber. It was fed by small-diameter flexible lines carrying liquid hydrogen and liquid oxygen. During the S-II second stage burn, the hydrogen line feeding the engine number three ignitor broke due to vibration. As a result, the igniter fed pure liquid oxygen into the pressure chamber. Normally the J-2 engine burns a hydrogen-rich mixture to keep temperature down. The liquid oxygen flow caused a much higher temperature locally and eventually the pressure chamber failed. The sudden drop in pressure was detected and caused a shutdown command to be issued. Unfortunately, the shutdown command signal for engine three was cross-wired to engine two. Engine two shut down and in turn its pressure sensor sent a shutdown signal back to engine three.

The problem in the ignitor fuel lines was not detected during ground testing because a stainless steel mesh covering the fuel line became saturated with liquid air due to the extreme cold of the liquid hydrogen flowing through it. The liquid air damped a vibration mode that became evident when tests were conducted in a vacuum after the Apollo 6 flight. This was also a simple fix, involving replacing the flexible bellows section where the break occurred with a loop of stainless steel pipe. The S-IVB used the same J-2 engine design as the S-II and so it was decided that an ignitor line problem had also stopped the third stage from reigniting in Earth orbit.

The spacecraft adapter problem was caused by its honeycomb structure. As the rocket accelerated through the atmosphere, the cells expanded due to trapped air and water. This would cause the adaptor surface to break free. To stop this occurring again, small holes were drilled in the surface to allow for expansion.

Cameras

Often during documentaries, footage is needed of a Saturn V launch. One of the most used pieces shows the interstage between the first and second stages falling away. Often this is attributed to the Apollo 11 mission, when in fact it was filmed on the flights of Apollo 4 and Apollo 6.

The cameras filmed at high speeds causing the slow motion look of the sequence when seen in a documentary. The camera capsules were jettisoned soon after the first stage separation and though at about 200,000 feet in altitude, were still below orbital velocity. They then reentered the atmosphere and parachuted to the ocean where they floated waiting for recovery. One of the two S-II cameras on Apollo 6 was not recovered.

Public impact

Ultimately, the American public was relatively uninterested in the Apollo 6 mission, due the fact that on the same day as the launch, Martin Luther King, Jr. was shot dead in Memphis, Tennessee, and five days prior President Johnson had announced he would not seek reelection.

Capsule location

The Apollo 6 Command Module is on display at the Fernbank Science Center, in suburban Atlanta, Georgia, a few miles northeast of the gravesite of Martin Luther King, Jr.

External links

http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1968-025A
Moonport: A History of Apollo Launch Facilities and Operations
Chariots for Apollo: A History of Manned Lunar Spacecraft
Fernbank Science Center

Apollo program | 1968 in the United States

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