Mercury-Redstone 4
Mercury-Redstone 4 was the second United States human spaceflight, on July 21, 1961. The suborbital Project Mercury flight was launched with a Mercury-Redstone Launch Vehicle, MRLV-8. The spacecraft, Mercury capsule #11, was nicknamed Liberty Bell 7. It was piloted by astronaut Virgil "Gus" Grissom.
The spaceflight lasted 15 minutes 30 seconds, reached an altitude of more than, and flew downrange, landing in the Atlantic Ocean. The flight went as expected until just after splashdown, when the hatch cover, designed to release explosively in the event of an emergency, accidentally blew. Grissom was at risk of drowning, but was recovered safely via a U.S. Navy helicopter. The spacecraft sank into the Atlantic and was not recovered until 1999.
Mission parameters
- Mass: 1 286 kg
- Maximum altitude: 190.39 km
- Range: 486.15 km
- Launch vehicle: Redstone rocket
Spacecraft
Explosive hatch
Liberty Bell 7 also had a new explosive hatch release. This would allow an astronaut to exit the spacecraft quickly in the event of an emergency. Emergency personnel could also trigger the explosive hatch from outside the spacecraft by pulling on an external lanyard. Both the pop-off hatch and the lanyard are standard features of ejection seats used in military aircraft, but in the Mercury design, the pilot still had to exit the craft himself, or be removed by emergency personnel. The original exit procedure was to climb out through the antenna compartment, after removing a small pressure bulkhead. This was a difficult and slow procedure. Removal of an injured or unconscious astronaut through the top hatch would be nearly impossible. The original side hatch was bolted shut with 70 bolts and covered with several spacecraft shingles, making it a slow process to open the original hatch.McDonnell Aircraft engineers devised two different quick release hatches for the Mercury spacecraft. The first had a latch, and was used on Ham's MR-2 and Shepard's MR-3 missions. The second design was an explosive release hatch. The quick release latching hatch weighed 69 lb, too much of a weight addition to use on the orbital version of the spacecraft. The explosive hatch design used the 70 bolts of the original design, but each quarter-inch titanium bolt had a hole bored into it to provide a weak point. A mild detonating fuse was installed in a channel between the inner and outer seal around the periphery of the hatch. When the MDF was ignited, the resulting gas pressure between the inner and outer seal would cause the bolts to fail in tension.
There were two ways to fire the explosive hatch during recovery. On the inside of the hatch was a knobbed plunger. The pilot could remove a pin and press the plunger with a force of 5 or 6 lbf. This would detonate the explosive charge, which would shear off the 70 bolts and propel the hatch away in one second. If the pin was left in place, a force of 40 lbf was required to detonate the bolts. An outside rescuer could blow open the hatch by removing a small panel near the hatch and pulling a lanyard. The explosive hatch weighed.
Window
The new trapezoidal window on Liberty Bell 7 replaced the two side portholes that were on Freedom 7. The Corning Glass Works of Corning, New York designed and developed the multilayered panes that comprised the new window. The outer pane was thick Vycor glass. It could withstand temperatures of. The inner pane was made of three inner glass panels bonded to form a single inner pane. One panel was a thick sheet of Vycor, while the others were tempered glass. This new window assembly was as strong as any part of the spacecraft pressure vessel.Controls
The manual controls of Liberty Bell 7 incorporated a new rate stabilization control system. This allowed fine control of spacecraft attitude movements by small turns of the hand controller. Previously a lot of jockeying of the device was needed to maintain the desired attitude. This rate damping, or rate augmentation system, gave finer and easier handling qualities and a redundant means of firing the pitch, yaw, and roll thrusters.Before the Mercury-Redstone 4 mission, Lewis Research Center and Space Task Group engineers had determined that firing the posigrade rockets into the booster-spacecraft adapter, rather than in the open, developed 78 percent greater thrust. This achieved a greater spacecraft-booster separation through a kind of "pop-gun" effect. By using this technique, the spacecraft separated at velocity of about rather than using the old procedure. The Mercury-Redstone 4/Liberty Bell 7 mission would take advantage of this new procedure.
Additional hardware changes to Liberty Bell 7 were a redesigned fairing for the spacecraft-Redstone adapter clamp-ring and additional foam padding added to the head area of the contour couch. The fairing changes and additional foam were used to reduce vibrations the pilot experienced during the boost phase of flight. The spacecraft instrument panel was rearranged to provide a better eye scan pattern.
Naming
Captain Grissom dubbed his capsule "Liberty Bell 7", the seven in honor of the seven original astronauts selected for Mercury, a tradition inadvertently started by prior Mercury pilot, Alan Shepard, who incorporated the seven in "Freedom 7" as his spacecraft was factory model no. 7. The other astronauts liked the symbolism, and so each appended 7 to their spacecraft names as well. Grissom chose "Liberty Bell" due to the capsule's resemblance to a bell, and because it evoked the iconic Liberty Bell. Grissom went as far as having the Liberty Bell on the spacecraft complete with the crack that characterizes the real bell. This detail motivated a degree of teasing of Grissom after the spacecraft sank in the ocean at the end of its mission.Mission description
In January 1961, NASA's Director of the Space Task Group, Robert Gilruth, told Gus Grissom that he would be the primary pilot for Mercury-Redstone 4. John Glenn was the backup pilot for the mission.Redstone launch vehicle MRLV-8 arrived at Cape Canaveral on June 8, 1961. A mission review on July 15, 1961, pronounced Redstone MRLV-8 and Mercury spacecraft #11 ready to go for the Mercury-Redstone
4 mission.
The Mercury 4 mission was planned as a repeat of MR-3. It was to reach an apogee of. The planned range was. Grissom would experience a maximum acceleration of 6.33 g and deceleration of 10.96 g.
The launch of Liberty Bell 7 was first planned for July 16. The cloud cover was too thick and the launch was postponed until July 18. On July 18, it was again postponed due to weather. Both times, the pilot had not yet boarded the spacecraft. On July 19, 1961, Grissom was on board when the flight was delayed again due to weather. At that point, it had just 10 minutes 30 seconds to go before launch.
On the morning of July 21, 1961, Gus Grissom entered the Liberty Bell 7 at 8:58 UTC and the 70 hatch bolts were put in place. At 45 minutes prior to the scheduled launch, a pad technician discovered that one of the hatch bolts was misaligned. During a 30-minute hold that was called, McDonnell and NASA Space Task Group engineers decided that the 69 remaining bolts should be sufficient to hold the hatch in place and blow it at the appropriate time. The misaligned bolt was not replaced.
Liberty Bell 7 was launched at 12:20:36 UTC, July 21, 1961.
Launch
Grissom later admitted at the postflight debriefing that he was "a bit scared" at liftoff, but he added that he soon gained confidence along with the acceleration increase. Hearing the engine roar at the pedestal, he thought that his elapsed-time clock had started late. Like Shepard, he was amazed at the smooth quality of the liftoff, but then he noticed gradually more severe vibrations. These were never violent enough to impair his vision.Grissom's cabin pressure sealed off at the proper altitude, about, and he felt elated that the environmental control system was in good working order. The cabin and suit temperatures, respectively about were quite comfortable. Watching his instruments for the pitch rate of the Redstone, Grissom saw it follow directions as programmed, tilting over at about one deg/s.
During a 3 g acceleration on the up-leg of his flight, Grissom noticed a sudden change in the color of the horizon from light blue to jet black. His attention was distracted by the noise of the tower-jettison rocket firing on schedule. Grissom felt the separation, and he watched the tower through the window as it drifted off, trailing smoke, to his right. At two minutes and 22 seconds after launch, the Redstone's Rocketdyne engine cut off after building a speed of. Grissom had a strong sensation of tumbling during the transition from high to zero acceleration, and, while he had become familiar with this sensation in centrifuge training, for a moment he lost his bearings.
The Redstone coasted for 10 seconds after its engine cut off; then a sharp report signaled that the posigrade rockets were popping the spacecraft loose from the booster. Although Grissom peered out his window throughout his ship's turnaround maneuver, he never caught sight of his booster.