Editing Apollo 6 (section)

==Flight==

===Launch===
[[File:S68-27366.jpg|thumb|250px|right|This view of the Apollo 6 launch was taken from a chase plane.]]
Apollo 6 launched from Launch Complex 39A at Kennedy Space Center on April 4, 1968, at 7:00 am (1200 UT). For the first two minutes, the Saturn V launch vehicle behaved normally. Then, as the Saturn V's S-IC first stage burned, [[pogo oscillation]]s shook the vehicle. The thrust variations caused the Saturn V to experience a [[g-force]] of ±{{convert|0.6|g-force}}, though it had only been designed for a maximum of {{convert|0.25|g-force}}. The vehicle suffered no damage, other than the loss of one of the panels of the Spacecraft-Lunar Module Adapter (SLA).{{sfn|Brooks 1979|p=248}}

NASA Associate Administrator for Manned Space Flight [[George Mueller (NASA)|George Mueller]] explained the cause to a congressional hearing:

{{blockquote|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 resonance 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.<ref name = "moonport 6a">{{cite book |last1=Benson |first1=Charles D. |last2=Faherty |first2=William Barnaby |title=Moonport: A History of Apollo Launch Facilities and Operations |url=https://www.hq.nasa.gov/office/pao/History/SP-4204/contents.html |access-date=September 27, 2021 |year=1978 |publisher=NASA |id=NASA SP-4204 |chapter=Two engines out but still running |chapter-url=https://www.hq.nasa.gov/office/pao/History/SP-4204/ch20-3.html |archive-date=January 23, 2008 |archive-url=https://web.archive.org/web/20080123133438/https://www.hq.nasa.gov/office/pao/History/SP-4204/contents.html |url-status=dead }} Ch. 20-3.</ref> }}

After the first stage was jettisoned, the S-II second stage began to experience problems with its [[Rocketdyne J-2|J-2 engines]]. Engine number two had performance problems from 225 seconds after liftoff, abruptly worsening at T+319 seconds. At T+412 seconds the Instrument Unit shut it down altogether, and two seconds later, engine number three also shut down.<ref name="lver" /> The fault was in engine two, but due to cross-connection of wires, the command from the Instrument Unit also shut down engine three, which had been running normally.{{sfn|Orloff & Harland 2006|p=153}} The Instrument Unit was able to compensate, and the remaining three engines burned for 58 seconds longer than planned. The S-IVB third stage also had to burn for 29 seconds longer than usual. The S-IVB also experienced a slight performance loss.<ref name="lver" />

===Orbit===
Due to the less-than-nominal launch, the CSM and S-IVB were inserted into a {{convert|93.49|nmi|km|adj=on|order=flip|sp=us}} by {{convert|194.44|nmi|km|adj=on|order=flip|sp=us}} parking orbit, instead of the planned {{convert|100|nmi|km|adj=on|order=flip|sp=us}} circular parking orbit.<ref name="lver" /> This deviation from the flight plan did not preclude continuing the mission.{{sfn|Orloff & Harland 2006|p=154}} During the first orbit, the S-IVB maneuvered, changing its attitude towards the horizon to qualify techniques that future astronauts could use in landmark tracking. Then, after the standard two orbits to assess the vehicle's readiness for [[trans-lunar injection]] (TLI), the S-IVB was ordered to restart, but failed to do so.{{sfn|Orloff & Harland 2006|pp=354–356}}

Deciding on a pre-planned alternate mission,{{sfn|Orloff & Harland 2006|p=356}} the [[Flight controller#flight director|flight director]], [[Clifford E. Charlesworth]] and his team in [[Christopher C. Kraft Jr. Mission Control Center|Mission Control]] chose to use the SM's [[Service Propulsion System]] (SPS) engine to raise the spacecraft into an orbit with a high [[apogee]] (point of furthest distance from Earth), with a low [[perigee]] that would result in re-entry,<ref name = "legacy" /> as had been done in Apollo 4. This plan would complete some of the mission objectives. The SPS engine burned for 442 seconds to get to the planned {{convert|11,989|nmi|km|adj=on|sp=us|order=flip}} apogee. There was now, however, not enough propellant to speed up the [[Atmospheric entry|atmospheric reentry]] with a second SPS engine burn, and the spacecraft only entered the atmosphere at a speed of {{convert|33,000|ft/s|m/s|order=flip|sp=us}} instead of the planned {{convert|37,000|ft/s|m/s|order=flip|sp=us}} that would simulate a lunar return.{{sfn|Brooks 1979|p=249}} While at high altitudes, the CM was able to return data on the extent to which future astronauts would be protected from the [[Van Allen Belts]] by the skin of the spacecraft.{{sfn|Orloff & Harland 2006|p=356}}

Ten hours after launch, the CM landed {{convert|43|nmi|km|order=flip|sp=us}} from the planned touchdown point in the North [[Pacific Ocean]] north of [[Hawaii]], and was lifted on board {{USS|Okinawa|LPH-3|6}}.{{sfn|Brooks 1979|p=249}} The SM was jettisoned just before reaching the atmosphere and burned up.{{sfn|Orloff & Harland 2006|p=157}} The S-IVB's orbit gradually decayed and it reentered the atmosphere on April 26, 1968.{{sfn|Orloff & Harland 2006|p=156}}