Most of their training period was spent at the Cape so that Grissom and Glenn could follow the technical progress of spacecraft and launch vehicle by participating in minute checkout operations. In Hangar S the astronauts exercised themselves and all their capsule systems in the simulated high-altitude chamber tests. Their physicians recorded metabolic data and refined physiological reactions. Communication checks, manual control system checks, sequence system verifications, and many simulated missions in the procedures trainer kept them busy. Twice Grissom and Glenn went back to Langley for sessions in the ALFA trainer. In all, each simulated about 100 Mercury-Redstone flights before the upcoming MR-4 launch, scheduled for July.41
Spacecraft No. 11, designated since October for the second manned Mercury flight, had come off the production line at McDonnell in May 1960. As the first operational capsule with a centerline window, No. 11 more nearly approximated the orbital version of the Mercury capsule than Shepard's Freedom 7, or spacecraft No. 7.42
Among other innovations in No. 11 for MR-4 was an explosive side hatch, whose evolution, encouraged by the astronaut corps, had begun early in the Mercury program. The original egress procedure had been to climb out through the antenna compartment,a difficult maneuver that required the removal of a small pressure bulkhead. Since all the astronauts had found it hard to snake out the top of the frustum and cylinder, the STG and McDonnell designers had concluded that removal of an injured astronaut would be even more precarious. Moreover, valuable time would be lost in such a rescue  operation; to open the hatch from the outside, someone had to remove several shingles and 70 bolts.
McDonnell engineers set to work on the problem and came up with two egress hatch models - one with a latch, which was used on Ham's MR-2 and Shepard's MR-3 missions, the other with an explosive hatch cover. The simple latch mechanism weighed 69 pounds, too much of a weight addition for incorporation in the orbital version of the spacecraft. The explosive hatch, on the other hand, utilized the 70 bolts of the original design; a .06-inch hole was bored into each of the quarter-inch titanium bolts to provide a weak point. When a mild detonating fuse, placed in a groove around each bolt, was energized, the bolts were sheared simultaneously and the hatch sprang open.
There were two ways to activate the explosive egress hatch during recovery. About six to eight inches from the astronaut's right arm, as he lay in his couch, was a knobbed plunger. The pilot would remove a pin and press the plunger with a fist-force of five or six pounds, detonating the small explosive charge and blasting the hatch 25 feet away in a second. If the pin was in place, a fist-force of 40 pounds was required. A rescuer outside the capsule could blow open the hatch simply by removing a small panel from the fuselage side and pulling a lanyard. This complete explosive hatch weighed only 23 pounds.43
The welcome new trapezoidal window assembly on spacecraft No. 11 replaced the two 10-inch side ports through which Shepard strained to see. The pilot now could look upward slightly and see directly outside. Visually the field covered 30 degrees in the horizontal plane and 33 degrees in the vertical. The Corning Glass Works of Corning, New York, designed and developed the multilayered panes. The outer pane was made of Vycor glass, .35-inch thick, and could withstand temperatures on the order of 1500 to 1800 degrees F. Three panels were bonded to make the inner pane, one a .17-inch- thick sheet of Vycor, the two others made of tempered glass. This fenestration was as strong as any part of the capsule pressure vessel.44
The manual controls for the second manned flight incorporated the new rate stabilization control system. With it the astronaut could control the rate of spacecraft attitude movements by small turns of his hand controller rather than by jockeying the device to attain the desired position. This rate damping or rate augmentation system, like power steering on an automobile, gave finer and easier handling qualities and another redundant means of driving the pitch, yaw, and roll thrusters.
By the time of the MR-4 flight, Lewis Research Center and Space Task Group engineers had analyzed the thrust rating of the posigrade rockets and had made a valuable discovery. Fired into the booster-spacecraft adapter, the posigrade rockets developed 78 percent greater thrust than when fired openly. Accordingly the capsule separation rockets when ignited inside the adapter,producing what the NASA testers called a "popgun effect," afforded an initial  separation velocity of about 28.1 feet per second. This determination provided the engineers with the confidence that spacecraft-booster separation would occur with little likelihood of recontact.
STG's calculations indicated that the Redstone booster and the Mercury spacecraft should be about 4,000 feet apart on their suborbital trajectory at retrofire. The unbraked booster would hit the water some 566.2 seconds after launch, while the longer and steeper trajectory of the spacecraft would keep it aloft 911.1 seconds. The booster would land about 16 1/2 miles beyond the spacecraft.45 Because of the relatively short distance between the two impact points, STG was concerned enough to assign John P. Mayer and Ted H. Skopinski to study the problem, especially as related to possible recontact of the spacecraft and the booster after separation. As a result of the studies, Skopinski's recommendations for minor changes in the sequencing of retrofire were accepted as solutions to prevent recontact.
Other hardware changes involved attaching a redesigned fairing for the capsule adapter clamp-ring, rearranging the capsule instrument panel, and adding more foam padding to the head area of the contour couch. The fairing and some more insulation should overcome the vibration and consequent blurred vision Shepard had complained about, while the rearrangement of the instruments sought to improve the eye-scan pattern, which Shepard had found poor. These changes cost several more weeks' time. On July 15, 1961, Gilruth affirmed that Grissom would be the prime pilot for Mercury-Redstone No. 4 and that Glenn would be his stand-in. Grissom in turn announced that he had chosen the name Liberty Bell 7 as the most appropriate call-sign for his bell-shaped capsule, because the name was to Americans almost synonymous with "freedom" and symbolic numerically of the continuous teamwork it represented.46
Modifications made on Grissom's pressure suit reflected the experiences of Shepard's flight. Nylon-sealed ball-bearing rings were fitted at the glove connections to allow full rotation of the wrists while the suit was pressurized. A new personal parachute harness was designed to keep the chute out of the way. On the chest of Grissom's suit was a convex mirror, called a "hero's medal" by the astronaut corps, that served simply to allow the pilot-observer camera to photograph instrument readings. Another welcome addition to the suit was a urine reservoir, fabricated the day before the flight. Although during his flight Grissom would find the contraption somewhat binding, it did work. Lastly, Grissom's helmet was equipped with new microphones that promised to filter out more noise and make transmission quality even better.47
Materials successfully used in other phases of the space program also became a part of the second manned flight. In the continuing quest for weight reduction, a lightweight, radar-reflective life raft was developed jointly by the Langley Research Center and the Space Task Group. Weighing three pounds and four ounces (45 percent lighter than the original version), this raft was constructed  of Mylar and nylon, the same materials used in Echo I, the passive communication satellite balloon that began circling the globe in August 1960. The survival pack, with the raft inside, was secured on a shelf in the spacecraft conveniently near the astronaut's left arm.48
Grissom's flight plan was revised rapidly and altered substantially as a result of MR-3. Shepard had really been overloaded with activities during his five minutes of weightlessness. Now Grissom was given a chance to look through his new trapezoidal window to learn more about man's visual abilities in space. If he could recognize landmarks for flight reference, the pilot tasks for the Mercury orbital flights might be considerably simplified. Shepard had assumed manual control of only one axis of movement - yaw, pitch, or roll - at a time, whereas Grissom had instructions to assume complete manual control as soon as he could, to make three maneuvers in about one minute instead of Shepard's 12 minutes, and then to spend as much time as possible making exterior observations.
Mercury-Redstone booster No. 8 had arrived at Cape Canaveral on June 8. Kurt H. Debus' contingent of Wernher von Braun's team and G. Merritt Preston's capsule checkout team had proceeded with the mating of the launch vehicle and capsule and the checkout requirements. On July 13, the flight safety review was held and the spacecraft was pronounced ready for flight. Two days later Walter Williams heard the reports during the mission review; the Redstone and Liberty Bell 7 were pronounced ready to go. The recovery ships, anticipating the launch date on Tuesday, July 18, moved into their assigned positions.
Essentially a repeat of MR-3, Grissom's flight was to reach an apogee of 116 miles, over a range of 299 miles, with the astronaut feeling a maximum acceleration load of 6.33 g and deceleration of 10.96 g. Only the launching azimuth, changed by three degrees to stay within range bounds, varied from Shepard's flight into space.49
On July 16 the news media received a weather bulletin predicting that the cloud cover in the launch area for the next 48 hours would be below average, but that the impact area would be slightly cloudier than usual. The mission was postponed early Tuesday, the 18th, in hope of better weather. Fortunately the frosty liquid oxygen had not been loaded so the launch delay was only 24, rather than 48, hours.
Early Wednesday, July 19, Grissom, asleep in his quarters on the balcony of Hangar S, was awakened by his physician, William Douglas, who told him that Walter Williams' operations team was pushing for a 7 a.m. launch to beat the weather. The launch day routine began again. By 5 a.m. Grissom was up in the gantry. He slid into his niche; the count resumed and continued unbroken until 10 minutes and 30 seconds before launch, when a hold was called to wait for a rift in the cloud cover. When no break appeared, the mission was scrubbed again. This time the liquid oxygen had been tanked, so a dreary 48-hour delay would be necessary.50
The weather conditions on July 21 were still not ideal. The view from an  altitude of a hundred miles would show that all the northern portion of Florida was completely obscured by high cirrus and lower patches of cumulus clouds. Southern Florida and Cuba would be splotched by scattered cumulus. The operations team nevertheless decided that since the view was not essential to the success of the mission, the launch should come off as scheduled.51
Because Grissom had shaved and showered before going to bed rather than before his low-residue breakfast, and because Slayton, the blockhouse communicator, briefed the astronaut on the status of the capsule and booster during the van ride to the pad rather than just before gantry ascent, the routine was a bit less hurried. George E. Ruff, an Air Force psychiatrist, had time to interrogate Grissom about his feelings before he lay in his contour couch for MR-4's liftoff.52
Grissom was unruffled, calm, and poised as he entered Liberty Bell 7 again. The count resumed and proceeded smoothly until 45 minutes before launch time, when a gantry technician discovered that one of the 70 hatch bolts was misaligned. A 30-minute hold was called, during which the McDonnell and STG supervisory engineers decided that the remaining 69 bolts were sufficient to hold and blow the hatch, so the misaligned bolt was not replaced. The countdown was resumed, but two more holds for minor reasons cost another hour's wait.53
Alone in his capsule awaiting liftoff, Grissom experienced a wide range of impressions. As the gantry, or service structure, moved back from the launch vehicle, he had the illusion that he was falling. His pulse rate ranged from 64 to 162 beats per minute, depending upon his feelings. His heart beat rose during the oxygen purge, fell while the hatch bolt repair decision was being made, rose again when the go decision was made, and finally doubled at launch. His liftoff was at 7:20a.m.54
41 Virgil I. Grissom, interview, Houston, April 12, 1965; "Postlaunch Memorandum Report for Mercury-Redstone No. 4 (MR-4)," Aug. 6, 1961. During his debriefing Grissom complained about having to travel so much for training missions. He suggested that an ALFA trainer be installed at Cape Canaveral.
42 Excerpts from messages compiled by Purser; Morton J. Stoller, "Some Results of NASA Space Flight Programs in 1960-61," paper, Third International Symposium on Rockets and Astronautics, Tokyo, 1961.
43 Results of the Second U.S. Manned Suborbital Space Flight, July 21, 1961 (Washington, 1961), 4; "Postlaunch Memorandum Report for MR-4"; "Astronaut Recovery Handbook (Capsules No. 11 and 15)," McDonnell Aircraft Corp., St. Louis, June 1, 1961.
44 "MR-4 Press Kit," June 29, 1961; Results of the Second U.S. Manned Suborbital Flight, 3, 4. The window measured 19 inches high, 11 inches across the base, and 7 1/2 inches across the top. NASA News Release 61-152, "MR-4 Design Changes," July 16, 1961. The contract change proposal providing for the observation window was submitted in October, 1959. Memo, Purser to Langley Research Center, "Contract NAS 5-59; Contract Change Proposal No. 73, Astronaut Observation Window Installation," Oct. 1, 1959.
45 "Postlaunch Memorandum Report for MR-4"; memo, Future Projects Br., Aeroballistics Div., Marshall Space Flight Center, "Project Mercury-Redstone: Additional Trajectory Data for MR-4," June 3, 1961.
46 Newport News Daily Press, July 16, 1961. Someone had done Grissom the favor of painting a likeness of the crack in the original Liberty Bell on spacecraft No. 11. Other astronaut assignments for the MR-4 mission put Shepard and Schirra in the Mercury Control Center, the former as "Cap Com," the latter as observer; Slayton and Carpenter in the blockhouse; and Cooper flying the chase plane.
47 "Postlaunch Memorandum Report for MR-4"; memo, Sjoberg to Assoc. Dir., "MR-4 Postflight Debriefing of Virgil I. Grissom," undated. Grissom became chilled while waiting in the spacecraft for launch on Wednesday, July 19. His suit inlet temperature was about 61 degrees F. On the day of the flight, the suit inlet temperature (55 degrees F) was more comfortable because the astronaut's underclothing remained essentially dry. Reception of medical data from Grissom's flight was better than that from Shepard's.
48 "MR-4 Design Changes," 61-152.
49 Grimwood, Mercury Chronology, 214; "Postlaunch Memorandum Report for MR-4"; "MR-4 Press Kit"; memo, Low to NASA Administrator, "Mercury-Redstone-4 Launching," July 17, 1961. The recovery forces were deployed in the same manner as for Ham and Shepard. Under direction of R/A J. L. Chew, stationed in Mercury Control Center as an advisor to Williams, the main forces consisted of an aircraft carrier, three destroyers, and two destroyer escorts. Five P2V aircraft, supplemented by Air Rescue Service planes, provided contingency recovery support. Carrier and shore-based helicopters were assigned to pick up the spacecraft, while just off Cape Canaveral a rescue salvage vessel stood by for action in the event of a mission abort. And, once again, land vehicles were deployed around the launch site for duty in case of a catastrophe.
50 Memo for news media representatives, July 16, 1961; Virgil I. Grissom, "The Trouble with Liberty Bell," in John Dille, ed., We Seven, by the Astronauts Themselves (New York, 1962), 216-219.
51 "Postlaunch Memorandum Report for MR—4."
52 Dille, ed., We Seven, 217-218.
53 The count was resumed, but after another 15 minutes a 9-minute hold was called for turning off pad-area searchlights - which in the past had caused telemetry interference.
Next came a 41-minute hold because cloudy skies had reduced light conditions to below par for camera coverage. During this hold, the main inverter began to overheat, reaching 190 degrees F, and so Grissom switched to the standby unit to allow the main component to cool. When the count resumed at 15 minutes before launch, he switched back to the main inverter. Significantly, during the 80 extra minutes from astronaut insertion to liftoff, not one of the holds was chargeable to the booster. Sjoberg undated memo; "Postlaunch Memorandum Report for MR-4."
In an interview with Grissom on April 12, 1965, the pilot stated that the misaligned bolt had nothing to do with the premature explosion of the hatch. In fact, if a number of bolts were misaligned it would be unlikely that the hatch would blow off at all. Grissom now has the misaligned bolt as a souvenir.
54 The following detailed account of Grissom's flight is based, like that for Shepard, on the evidence of the motion picture camera, the tape transcript of communications, the confidential postflight report, the debriefing records, telemetry transcripts, and personal interviews.