Apollo 6

• 1. Mission Objectives And Payload

  ›

  Apollo 6 launched on the 4th of April 1968. The mission aimed to test the Saturn V rocket's third stage, known as the S-IVB. This stage was designed to propel the spacecraft toward lunar distances. Engineers placed a Lunar Test Article inside the vehicle. This object, designated LTA-2R, simulated a real lunar module without landing gear. It contained no flight systems but held vibration sensors and structural instruments. Water-glycol mixtures filled its fuel tanks while freon occupied oxidizer tanks. The goal was to measure how the entire rocket vibrated under near-full loads. A command and service module, CSM-020, rode atop this payload. This specific module featured a new crew hatch tested under return conditions. That hatch replaced one condemned after the Apollo 1 fire of the 27th of January 1967. The mission plan called for a direct return abort using the service module engine. Flight controllers intended to simulate lunar return conditions with an entry angle of negative 6.5 degrees.

• 2. Preparation And Launch Delays

  ›

  The first stage arrived by barge on the 13th of March 1967. It took four days to erect in the Vehicle Assembly Building. The second stage did not arrive until May 24, causing significant delays. Workers substituted a dumbbell-shaped spacer to keep testing moving forward. High Bay 3 of the VAB proved inadequate for air conditioning needs. Portable units were brought in to cool equipment and workers. Personnel remained busy with Apollo 4 through April 1968. The spacecraft stack finally rolled out to Launch Complex 39A on the 6th of February 1968. Heavy rain fell during the all-day rollout process. Communications failed for two hours, delaying arrival at the pad until dark. High winds prevented movement of the mobile service structure for another two days. A flight readiness test concluded on the 8th of March 1968. Launch was initially set for March 28 but postponed multiple times due to guidance system issues. Final countdown began on April 3 with liftoff scheduled for the next day. All subsequent problems were fixed during built-in holds without further delay.

• 3. Pogo Oscillation And Engine Failures

  ›

  For the first two minutes after launch, the Saturn V behaved normally. Then pogo oscillations shook the vehicle as the S-IC first stage burned. Thrust variations caused g-forces reaching plus or minus 7.5 Gs. The rocket had been designed for a maximum of 6.6 Gs. No structural damage occurred except for one panel loss from the Spacecraft-Lunar Module Adapter. At T-plus-319 seconds, engine number two in the second stage abruptly worsened its performance. The Instrument Unit shut it down completely at T-plus-412 seconds. Two seconds later, engine number three also shut down despite running normally. Cross-wiring between engines caused this cascading failure. Three remaining engines burned for 58 seconds longer than planned to compensate. The third stage had to burn 29 seconds longer than usual. This resulted in an elliptical parking orbit instead of the planned circular path. The damaged third-stage engine failed to restart for trans-lunar injection. Propellant lines leading to spark igniters could fail in vacuum conditions. Metal bellows vibrated rapidly and failed at peak flow during spaceflight. Frost formed on LOX lines during ground testing but vanished in the vacuum.

• 4. Abort Maneuvers And Return Trajectory

  ›

  Flight director Clifford E. Charlesworth chose an alternate mission profile after engine failures. Controllers used the Service Propulsion System engine to raise the spacecraft into high apogee. The SPS engine burned for 442 seconds to reach a target altitude. There was no propellant left for a second atmospheric reentry burn. The spacecraft entered the atmosphere at 36,000 feet per second instead of the planned 37,000 feet per second. Ten hours after launch, the command module landed 100 miles from the planned touchdown point. Splashdown occurred north of Hawaii in the North Pacific Ocean. Recovery ships lifted the capsule aboard USS Hornet. The service module jettisoned just before reaching the atmosphere and burned up. The Saturn V's third stage gradually decayed and reentered on the 26th of April 1968. During high altitudes, the command module returned data on Van Allen Belt protection. Engineers tested techniques for landmark tracking by changing attitude toward the horizon. This deviation did not preclude continuing the mission despite the less-than-nominal launch. The vehicle reached orbit even with two engines lost during ascent.

• 5. Post-Mission Engineering Fixes

  ›

  NASA engineers worked to eliminate pogo oscillations for future flights. About 1,000 government and industry engineers joined the effort. Cavities in valves leading to F-1 and J-2 engines were filled with helium gas as shock absorbers. Propellant lines had metal bellows that vibrated rapidly in vacuum conditions. These bellows failed at peak flow causing burn-throughs. Engineers replaced them with rigid bends and strengthened the lines. The Spacecraft-Lunar Module Adapter suffered from honeycomb structure issues. Trapped air and water caused cells to expand and break free. Small holes drilled into the surface allowed trapped gases to dissipate. A thin layer of cork was placed on the adapter to absorb moisture. Deke Slayton opposed an automatic abort system for excessive pogo. Work began on a pogo abort sensor but was abandoned by August 1968. The Senate Committee on Aeronautical and Space Sciences reported quick analysis and diagnosis. Corrective actions were taken within weeks of the flight. Engineers concluded a third uncrewed test flight was unnecessary after detailed Saturn V performance review.

• 6. Decision To Cancel Third Uncrewed Flight

  ›

  Apollo Program Director Samuel C. Phillips called it less than a perfect mission. He noted the launch vehicle reaching orbit despite engine loss was a major accomplishment. George Mueller later stated Apollo 6 would have to be defined as a failure. Despite these mixed assessments, data proved sufficient for crewed missions. The next Saturn V to fly would carry a crew on Apollo 8. Apollo 7 remained the first crewed mission using a Saturn IB rocket instead. NASA satisfied the Senate Committee on Aeronautical and Space Sciences with their response. Late April reports showed agencies had quickly analyzed abnormalities. Engineers at Marshall Space Flight Center in Alabama made final conclusions. They determined no further uncrewed testing was required before human flights. The command module CM-020 transferred to the Smithsonian Institution afterward. It now sits on display at the Fernbank Science Center in Atlanta, Georgia. Public impact remained low due to Martin Luther King Jr.'s assassination on the same day. President Lyndon B. Johnson announced he would not seek reelection four days prior. Little press coverage existed because national attention focused elsewhere during the launch window.