A little after 9:04 a.m. on September 13, 1961, MA-4 was launched on its one-orbit mission. During the first 20 seconds from liftoff, fairly severe booster vibrations were detected by the flight dynamics officer in the Control Center. The "thick-skin" Atlas passed its max-q test. At the 52-second point, a spacecraft inverter that was converting electrical power from direct to alternating current failed, but the standby inverter switched on automatically. Guidance data soon disclosed that the trajectory was .75-degree high; later, at engine cutoff, it was .14-degree low. Although booster engine cutoff occurred 2.5 seconds early, booster velocity was about 100 feet per second too high. Then the sustainer engine cut off 10 seconds early, so the desired velocity was essentially achieved.  Despite these dispersions, which were within design limits, perigee and apogee of the orbit were only slightly more than a mile and 12 miles, respectively, below plan. The Goddard computers instantly indicated a go for the mission. The powered phase, plus posigrade rocket increment, provided a peak velocity of 25,705 feet per second; g loads during the powered phase reached a peak of 7.6.12
Despite a slight disturbance in the roll, pitch, and yaw of the booster, separation occurred properly, and after a 5-second steadying or damping period the capsule began its turnaround maneuver. Soon, however, large attitude excursions were observed, and the spacecraft took 50 seconds to reverse its ends to heatshield forward, as opposed to a normal 20 seconds, using 9.5 pounds of hydrogen peroxide attitude control fuel against the 2.2 pounds supposedly required. Even with the abnormal turnaround, the spacecraft attitude gyros and scanners soon transmitted nominal readings, and there seemed no doubt that the mission would proceed to its orbital conclusion. The cause of these undue excursions later was found to be an open electrical connection in the pitch- rate gyro.13
A high oxygen usage rate like that on Grissom's suborbital mission cropped up early and continued throughout the flight. At the 27,000-foot point the system sealed off at 5.5 pounds per square inch; then an abrupt drop was indicated in the primary oxygen supply and a concurrent rise in cabin and suit pressure values to 6 pounds per square inch. "Primary oxygen going down fast," Paul E. Purser jotted in his notes as he listened to the communications circuit. "Zanzibar reported 30 percent of primary oxygen left," he later added. Toward the end of the mission, with the primary supply depleted, the system switched over to secondary. Usage from this source was so slight, however, that Walter Williams commenting on the high usage problem in a press conference following the mission, said that the secondary supply was virtually untouched. Throughout the flight the crewman simulator continued to use oxygen to produce moisture and carbon dioxide, and to monitor the operations while recording heat and suit pressure changes.14
Despite the abnormalities with the oxygen supply, once the automated Mercury spacecraft was on its orbital course, the computers indicated that the mission could go for more than seven orbits. In general, the control systems operated well, although on three occasions the spacecraft dropped out of its 34-degree, Earth-reference mode, once just before the ignition of the retrorockets and twice just before the .05-g light telemetry signal. These attitude variations came from the failure of a one-pound yaw-positive thruster and a one-pound roll-negative thruster.
Communications between the capsule and the tracking stations were good, especially on high frequencies, which on the earlier suborbital flights had been virtually unsuccessful. In some cases radar tracking was not good, largely because a few of the operators lacked experience. Telemetry reception was excellent, with some 137 observations received by the various tracking stations during the flight.15
 One hour, 28 minutes, and 59 seconds after MA-4's liftoff, the first retrorocket fired in the vicinity of Hawaii. Monitors at the Guaymas station in Mexico indicated that retrofire, triggered by the spacecraft clock, had gone off as planned. Within the range of the Cape Canaveral control center, telemetry data disclosed that MA-4 was in the proper reentry attitude. Over the Atlantic the drogue parachute opened at 41,750 feet, and the main chute deployed at 10,050 feet. At 10:55 a.m. the capsule splashed down 176 miles east of Bermuda. After an hour and 22 minutes, the destroyer Decatur, which had been about 34 miles from the impact point, pulled alongside the spacecraft and hoisted it aboard. From there the capsule and its robot "astronaut" rode to Bermuda, whence they were airlifted to the Cape for an exhaustive examination.16
The cause of the oxygen supply malfunction was immediately attacked by the STG and McDonnell engineers. Onboard film, they found, disclosed that the oxygen supply emergency light had blinked on, which would have signaled an astronaut to take corrective action. The inspectors also learned that vibration had dislodged the rate handle from its detent, allowing a valve to crack open. But the flow rate had not been sufficient to trip the microswitch that would have given the Mercury Control Center a telemetry indication of an emergency rate actuation while the mission was in progress. Normally a force of from three to eight pounds was needed to break the handle free from the detent, whereas in this case the inspectors moved the handle with very little force. A new emergency rate handle with a positive latching mechanism was to be devised for later missions.17
Other postflight analyses by the engineers found the MA-4 spacecraft and its systems in good condition. There was no afterbody shingle buckling or warping, and the structural materials were only mildly discolored. The horizon scanner window was partially coated with a film of oxidized material caused by aerodynamic heating. Some internal debris, including solder balls and washers, had apparently escaped preflight tumbling and vacuum cleaning. Six buckled skin panels between the base ring and the lower pressure bulkhead indicated that the capsule landed with the heatshield edge striking the water first. Still the inspectors concluded that the structural damage was not enough to have endangered an astronaut. The center section of the heatshield was partially delaminated and the center plug was loose, conditions apparently caused by water impact and cooling. Two cracks were found on the shield in the vicinity of the water-impact point. The depth of the char on the ablation shield was very shallow.18
NASA officials showed their pleasure at the success of MAŚ4 at the press conference held at the Cape immediately after the flight. Gilruth pointed out that this had been the hardest test flight in the whole NASA program. He added that the Atlas had demonstrated that it was capable of boosting a man into orbit, as he, Maxime A. Faget, Purser, and others from NACA-Langley days had long believed. Without hesitation Gilruth concluded that a man would have survived the flight.
 At that point a reporter asked whether a man would fly the next Mercury orbital mission. Walter Williams answered that a three- orbit circuit, either unmanned or carrying a chimpanzee, was still necessary. Then why was the upcoming Mercury-Scout mission necessary, asked a newsman. Again Williams affirmed his confidence in the wisdom of the agreed-upon schedule of flights.19
12 Memo, Carl R. Huss to Flight Dir., "Postlaunch Report on Test 1254," Sept. 15, 1961; Purser, penciled notes on MAŚ4 countdown and flight, Sept. 13, 1961; memo, Walter J. Kapryan, Capsule Systems Monitor, to Flight Dir., "Report on Test 1254," Sept. 29, 1961; memo, Tecwyn Roberts to Flight Dir., "Report on Test 1254," Sept. 25, 1961. Countdown procedures for MA-4 resembled those of the Mercury-Redstone missions. They were conducted in a 500-minute split-count with a 12- to 14-hour hold at T minus 300 for peroxide and pyrotechnics servicing. When the MA-4 count began Sept. 12 the operations crew feared that hurricane "Debbie" might adversely affect the recovery area, but the count proceeded to T minus 300. At 4 p.m. a weather review found conditions improving, so hydrogen peroxide servicing was begun. The count resumed at 2 a.m., Sept. 13. Weather reviews and a peroxide check, plus the problems described in the text, accounted for holds totaling 2 hours and 4 minutes during countdown.
13 "Postlaunch Report for MAŚ4"; Kapryan memo.
14 Memo, unsigned, to Flight Dir., "Verbal Debriefing at End of Flight-Test 1254," Sept. 13, 1961; Purser notes; Schler memo. The crewman simulator was a gray box, 24 by 12 by 8 inches, which took oxygen out of the environmental control system, emitted carbon dioxide, simulated minor suit leakage of oxygen, and initiated dumping. NASA News Release 61-206, "News Conference, Mercury-Atlas No. 4," Sept. 13, 1961.
15 Purser notes; "Project Mercury Status Report No. 12 for Period Ending October 31, 1961," undated; Roberts memo; memo, Network Control Group to Flight Dir., "Network Control Group (NCG) Report on Test 1254," undated; memo, Alan B. Shepard to Flight Dir., "Report on Test 1254," undated; "Postlaunch Report for MA-4." During the mission several Mercury astronauts deployed to some of the remote tracking stations; Carpenter to Muchea, Australia; Cooper to Point Arguello, Calif.; Schirra to Guaymas, Mexico; Slayton to Bermuda. Glenn, Grissom, and Shepard were in the control center at Cape Canaveral, with Shepard serving as Capsule Communicator. (Message, STG to NASA Hq., Sept. 9, 1961.) This was the first time that the automatic stabilization and control system, the reaction control system, and the horizon scanner subsystem could be fully evaluated for orbital missions. The mission proved that the attitude control system was adequate for reentry.
16 Kapryan memo; Purser notes; memo, George M. Low to NASA Administrator, "Preliminary Results of MA-4 Flight," Sept. 15, 1961. R/A John L. Chew, commanding Destroyer Flotilla Four, said at the Cape press conference following the flight that the seas were running only about a foot high in the recovery area - which meant that hurricane Debbie was ineffective in those waters. During the spacecraft's descent, a C-54 aircraft sighted its reentry contrails, shortly thereafter noted deployment of the main parachute, and finally observed water impact. Pickup by the destroyer Decatur was effected at 12:15 p.m. The main chute and the antenna fairing were retrieved about 1,000 yards from the spacecraft. All spacecraft recovery aids performed well with the exception of the radar chaff. "Postlaunch Report for MA-4."
17 "Status Report No. 12."
18 Kapryan memo; Purser notes; Low memo.
19 "News Conference, Mercury-Atlas No. 4." The principals at the news conference included Gilruth, Williams, R/A Chew, Col. R. S. Maloney, Col. Paul R. Wignall, Astronaut Virgil I. Grissom, and John A. Powers.