Mercury 4

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Mercury 4
Mission Insignia
Mercury 4 insignia
Mission Statistics
Mission Name:Mercury MR-4
Call Sign:Liberty Bell 7
Number of
Crew Members:
Launch:July 21, 1961
12:20:36 UTC
Cape Canaveral
Complex 5
Landing:July 21, 1961
12:36:13 UTC
Template:Coor dms
Duration:15 min 37 s
Number of
Apogee:118.30 mi
190.39 km
302.07 mi
486.15 km
5,168 mph
8,317 km/h
Peak acceleration:11.1 g (10.9 m/s²)
Mass:1,286 kg
Crew Picture
Mercury 4 crew portrait (Grissom)
Mercury 4 crew portrait (Grissom)
Gus Grissom

Mercury 4 was a Mercury program manned space mission launched on July 21, 1961 using a Redstone rocket. Its capsule was named "Liberty Bell 7" and performed a suborbital flight piloted by astronaut Virgil I. "Gus" Grissom. It reached an altitude of over 118.26 miles (190 km) and traveled about 300 miles (480 km). The Redstone was MRLV-8 and the spacecraft was Mercury spacecraft # 11, the first one with a centerline window instead of two portholes.



Backup Crew

Mission Parameters

  • Mass: 1286 kg
  • Maximum Altitude: 190.39 km
  • Range: 486.15 km
  • Launch Vehicle: Redstone rocket

Liberty Bell 7

Mercury spacecraft # 11, was designated to fly the second manned suborbital flight in October, 1961. It came off McDonnell's St. Louis production line in May 1960. Spacecraft # 11 was the first Mercury operational spacecraft with a centerline window. It was closer to the final orbital version than was Alan Shepard's Freedom 7.

Explosive hatch

Spacecraft # 11 also had a new explosive side hatch. 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. 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. It was a slow process to open the original hatch.

McDonnell engineers came up with two different quick release hatches for the Mercury spacecraft. One with a latch, used on Ham's MR-2 and Shepard's MR-3 missions. The other design was an explosive release hatch. The quick release latching hatch weighed 69 pounds (31 kg), 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 (6.35 mm) titanium bolt had a 0.06 inch (1.5 mm) hole bored into it to provide a weak point. A mild detonating fuse (MDF) is installed in a channel between the inner and outer seal around the periphery of the hatch. When the MDF is ignited, the resulting gas pressure between the inner and outer seal causes the bolts to fail in tension.

MR-4 Explosive Hatch Diagram. (NASA)
MR-4 Explosive Hatch Diagram. (NASA)

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 five or six pounds force (25 N). This would detonate the explosive charge which would shear off the 70 bolts and propel the hatch 25 feet (8 m) away in 1 second. If the pin was left in place, a force of 40 pounds force (180 N) was required to detonate the hatch. An outside rescuer could blow open the hatch by removing a small panel near the hatch and pull a lanyard. The explosive hatch weighed only 23 pounds (10 kg).


The new rectangular window on spacecraft # 11 replaced the two 10 inch (254 mm) side portholes that were on Freedom 7. The Corning Glass Works of Corning, New York, designed and developed the multilayered panes that made up the new window. The outer pane was Vycor glass was 0.35 inch (8.9 mm) thick. It could withstand temperatures of 1500 to 1800 F (820 to 980 C). The inner pane was made up of three inner glass panels bonded to make the inner pane. One panel was a 0.17 inch (4.3 mm) thick sheet of Vycor, the two others made of tempered glass. This new window assembly was as strong as any part of the spacecraft pressure vessel.


The manual controls for Mercury 4 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 attain 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 4 flight, 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 28.1 feet per second (9 m/s) rather than 15 feet per second (5 m/s) using the old procedure. Mercury 4 flight would take advantage of this new procedure.

Additional hardware changes to Mercury 4 were a redesigned fairing for the spacecraft-Redstone adapter clamp-ring and additional foam padding was added to the head area of the contour couch. The faring 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.

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 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 4 mission.

Also, on July 15, 1961 Gus Grissom announced he would name Mercury 4, Liberty Bell 7. Grissom said the name was an appropriate call-sign for the bell-shaped capsule. He also said the name was synonymous with "freedom".

The Mercury 4 mission was planned as a repeat of MR-3. It was to reach an apogee of 116 miles. The planned range was 299 miles (881 km). Grissom would experience a maximum acceleration of 6.33 g (62 m/s²) and deceleration of 10.96 g (107 m/s²).

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 aboard the Liberty Bell 7 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.

The Liberty Bell 7 was launched at 12:20:36 UTC, July 21, 1961.


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, never violent enough to impair his vision.

Grissom's cabin pressure sealed off at the proper altitude, about 27,000 feet (8.2 km), and he felt elated that the environmental control system was in good working order. The suit and cabin temperature, about 57.5 and 97 F (14 and 36 C), respectively, were quite comfortable. Watching his instruments for the pitch rate of the Redstone, Grissom saw it follow directions as programmed, tilting over about one degree per second.

Under a 3 g (29 m/s²) 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. The pilot felt the separation and 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 velocity of 6561 feet per second (2 km/s). 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 capsule loose from the booster. Although Grissom peered out his window throughout his ship's turnaround maneuver, he never caught sight of his launch vehicle.


With turnaround accomplished, the Air Force jet pilot for the first time became a space pilot, assuming manual-proportional control. A constant urge to look out the window made concentrating on his control tasks difficult. He told Shepard back in Mercury Control that the panorama of Earth's horizon, presenting an 800 mile (1,300 km) arc at peak altitude, was fascinating. His instruments rated a poor second to the spectacle below.

Turning reluctantly to his dials and control stick, Grissom made a pitch movement change but was past his desired mark. He jockeyed the handcontroller stick for position, trying to damp out all oscillations, then made a yaw movement and went too far in that direction. By the time the proper attitude was attained, the short time allocated for these maneuvers had been used, so he omitted the roll movement altogether. The manual controls impressed Grissom as very sluggish when compared to the Mercury procedures trainer. Then he switched to the new rate command control system and found perfect response, although fuel consumption was high.

After the pitch and yaw maneuvers, Grissom made a roll-over movement so he could see the ground from his window. Some land beneath the clouds (later determined to be western Florida around the Apalachicola area) appeared in the hazy distance, but the pilot was unable to identify it. Suddenly Cape Canaveral came into view so clearly that Grissom found it hard to believe that his slant-range was over 150 miles (240 km).

He saw Merritt Island, the Banana River, the Indian River, and what appeared to be a large airport runway. South of Cape Canaveral, he saw what he believed to be West Palm Beach.


With Liberty Bell 7 at an altitude of 118.26 miles (190 km), it was now time to position the spacecraft in its reentry attitude. Grissom had initiated the retrorocket sequence and the capsule was arcing downward. His pulse reached 171 beats per minute. Retrofire gave him the distinct and peculiar feeling that he had reversed his backward flight through space and was actually moving face forward. As he plummeted downward, he saw what appeared to be two of the spent retrorockets pass across the periscope view after the retrorocket package had been jettisoned.

Pitching the spacecraft over into a reentry attitude of 14 degrees from Earth-vertical, the pilot tried to see the stars out his observation window. Instead the glare of sunlight filled his capsule, making it difficult to read the panel dials, particularly those with blue lights. Grissom felt that he would not have noticed the 0.05 g (0.5 m/s²) light if he had not known it was about to flash on.

Reentry presented no problem. Grissom could not feel the oscillations following the acceleration buildup; he could only read them on the rate indicators. Meanwhile he continued to report to the Mercury Control Center on his electric current reading, fuel quantity, acceleration, and other instrument indications. Condensation and smoke trailed off the heatshield at about 65,000 feet (20 km) as Liberty Bell 7 plunged back into the atmosphere.

The drogue parachute deployed on schedule at 21,000 feet (6.4 km). Grissom said he saw the deployment and felt some resulting pulsating motion, but not enough to worry him. Main parachute deployment occurred at 12,300 feet (3.7 km), which was about 1,000 feet (300 m) higher than the design nominal altitude. Watching the main chute unfurl, Grissom spotted a six inch (150 mm) L-shaped tear and another two inch (50 mm) puncture in the canopy. Although he worried about them, the holes grew no bigger and his rate of descent soon slowed to about 28 feet per second (9 m/s). Dumping his peroxide control fuel, the pilot began transmitting his panel readings.


A "clunk" confirmed that the landing bag had dropped in preparation for impact. Grissom then removed his oxygen hose and opened his visor but deliberately left the suit ventilation hose attached. Impact was milder than he had expected, although the capsule heeled over in the water until Grissom was lying on his left side. He thought he was facing downward. The capsule gradually righted itself, and, as the window cleared the water, Grissom jettisoned the reserve parachute and activated the rescue aids switch. Liberty Bell 7 still appeared watertight, although it was rolling badly with the swells.

Preparing for recovery, he disconnected his helmet and checked himself for debarkation. The neck dam did not unroll easily; Grissom tinkered with his suit collar to ensure his buoyancy if he had to get out of the spacecraft quickly. When the recovery helicopters, which had taken to the air at launch time and visually followed the contrails and parachute descent, were still about two miles (3 km) [373] from the impact point, which was only three miles (5 km) beyond the bullseye, Lieutenant James L. Lewis, pilot of the primary recovery helicopter, radioed Grissom to ask if he was ready for pickup. He replied that he wanted them to wait five minutes while he recorded his cockpit panel data. Using a grease pencil with the pressure suit gloves was awkward, and several times the suit ventilation caused the neck dam to balloon, but the pilot simply placed his finger between neck and dam to allow the air to escape.

After logging the panel data, Grissom asked the helicopters to begin the approach for pickup. He removed the pin from the hatch-cover detonator and lay back in the dry couch. "I was lying there, minding my own business," he said afterward, "when I heard a dull thud." The hatch cover blew away, and salt water swished into the capsule as it bobbed in the ocean. The third man to return from space was faced with the first serious emergency; Liberty Bell 7 was shipping water and sinking fast.

Grissom had difficulty recollecting his actions at this point, but he was certain that he had not touched the hatch-activation plunger. He doffed his helmet, grasped the instrument panel with his right hand, and scurried out the sloshing hatchway. Floating in the sea, he was thankful that he had unbuckled himself earlier from most of his harness, including the chest restraints. Otherwise he might not have been able to abandon ship.

Lieutenant John Reinhard, copilot of the nearest recovery helicopter, reported afterward that the choppers were making their final approach for pickup. He was preparing to cut the capsule's antenna whip (according to a new procedure) with a squib-actuated cutter at the end of a pole, when he saw the hatch cover fly off, strike the water at a distance of about five feet (1.5 m) from the hatch, and then go skipping over the waves. Next he saw Grissom's head appear, and the astronaut began climbing through the hatch. Once out, the pressure-suited spaceman swam away.

Instead of turning his attention to Grissom, Lewis completed his approach to the sinking spacecraft, as both he and Reinhard were intent on capsule recovery. This action was a conditioned reflex based on past training experience. While training off the Virginia beaches the helicopter pilots had noted that the astronauts seemed at home in and to enjoy the water. So Reinhard quickly clipped the high-frequency antenna as soon as the helicopter reached Liberty Bell 7. Throwing aside the antenna cutting device, Reinhard picked up the shepherd's hook recovery pole and carefully threaded the crook through the recovery loop on top of the capsule. By this time Lewis had lowered the helicopter to assist Reinhard in his task to a point that the chopper's three wheels were in the water. Liberty Bell 7 sank out of sight, but the pickup pole twanged as the attached cable went taut, indicating to the helicopter pilots that they had made their catch.

Reinhard immediately prepared to pass the floating astronaut the personnel hoist. But at that moment Lewis called a warning that a detector light had flashed on the instrument panel, indicating that metal chips were in the oil sump [375] because of engine strain. Considering the implication of impending engine failure, Lewis told Reinhard to retract the personnel hoist while he called the second chopper to retrieve the pilot.

Meanwhile Grissom, having made certain that he was not snared by any lines, noticed that the primary helicopter was having trouble raising the submerged spacecraft. He swam back to the capsule to see if he could assist but found the cable properly attached. When he looked up for the personnel line, he saw the helicopter start to move away.

Suddenly Grissom realized that he was not riding as high in the water as he had been. All the time he had been in the water he kept feeling air escape through the neck dam. The more air he lost, the less buoyancy he had. Moreover, he had forgotten to secure his suit inlet valve. Swimming was becoming difficult, and now with the second helicopter moving in he found the rotor wash between the two aircraft was making swimming more difficult. Bobbing under the waves, Grissom was scared, angry, and looking for a swimmer from one of the helicopters to help him tread water. Then he caught sight of a familiar face, that of George Cox, aboard the second helicopter. Cox was the copilot who had retrieved both the chimpanzee Ham and Astronaut Shepard. With his head barely above water, Grissom found the sight of Cox heartening.

Cox tossed the "horse-collar" lifeline straight to Grissom, who immediately wrapped himself into the sling backwards. Lack of orthodoxy mattered little to Grissom now, for he was on his way to the safety of the helicopter, even though swells dunked him twice more before he got aboard. His first thought was to get a life preserver on. Grissom had been either swimming or floating for a period of only four or five minutes, "although it seemed like an eternity to me," as he said afterward.

As the first helicopter moved away from Grissom, it struggled valiantly to raise the spacecraft high enough to drain the water from the impact bag. Once the capsule was almost clear of the water, but like an anchor it prevented the helicopter from moving forward. The flooded Liberty Bell 7 weighed over 5,000 pounds (2,300 kg), a thousand pounds (450 kg) beyond the helicopter's lifting capacity. The pilot, watching his insistent red warning light, decided not to chance losing two craft in one day. He finally cast loose, allowing the spacecraft to sink swiftly. Martin Byrnes, aboard the carrier, suggested that a marker be placed at the point so that the capsule might be recovered later. Rear Admiral J. E. Clark advised Byrnes that in that area the depth was about 2,800 fathoms (5.1 km).

Substantial controversy ensued as Grissom reported that the hatch had blown prematurely without his authorization. Engineering teams concluded this was unlikely. Mrs. Grissom was not invited to the White House as per the forming tradition with Shepherd's wife upon his successful mission completion. Subsequent independent technical review of the incident has raised doubts as to the veracity of the incident report conclusions that Grissom blew the hatch and was responsible for the loss of the spacecraft. There is strong evidence that the Astronaut Office didn't accept Grissom's guilt in the fact that he was chosen to command the first Gemini flight.

Several years later, during an interview on April 12, 1965, Grissom said he thought the hatch may have been triggered because the external release lanyard came loose. On Liberty Bell 7, the external release lanyard was only held in place by a single screw. It was better secured on later flights. Ironically, the inability to open a hatch swiftly contributed to the death of Grissom, as well as Ed White and Roger B. Chaffee, in the Apollo 1 launch-pad fire.

Recovery of Liberty Bell 7

One day shy of the 38th anniversary (July 20, 1999), of Mercury 4's suboribital flight, a team led by Curt Newport and financed by the Discovery Channel, lifted the Liberty Bell 7 capsule off the floor of the Atlantic ocean and onto the deck of a recovery ship. The capsule was found after a 14-year effort by Newport and his team at a depth of nearly 15,000 feet (4.5 km), 300 miles South East of Cape Canaveral and was in surprisingly good condition. Some of the interior aluminium panels showed deterioration but some fabric pieces, including Grissom's personal parachute were perfectly intact. The recovery failed to answer some of the questions surrounding the prematurely blown hatch. The recovery team ran out of time and was not able to continue the search for the hatch itself. A camera that was running during the flight was located but was found broken open and the film inside was damaged beyond repair. After Liberty Bell 7 was secured in the deck of the recovery ship, experts removed and disposed of an explosive device that was supposed to detonate in the event of the capsules sinking but which failed to explode. After the capsule was made safe, it was placed in a container filled with seawater to prevent further corrosion. The Kansas Cosmosphere and Space Center disassembled and cleaned the capsule and keeps it on display in their museum.

Liberty Bell 7 was recovered exactly 30 years to the day after man first landed on the moon.

See also

External link

  • Field Guide to American Spacecraft ( Pictures of Liberty Bell 7 on display after recovery and restoration.


Mercury Redstone Sub-Orbital Flight Events

T+ Time Event Description
T+00:00:00 Liftoff Mercury-Redstone lifts off, onboard clock starts.
T+00:00:16 Pitch Program Redstone pitches over 2 deg/s from 90 deg to 45 deg.
T+00:00:40 End Pitch Program Redstone reaches 45 deg pitch.
T+00:01:24 Max Q Maximum dynamic pressure ~575 lbf/ft² (28 kPa).
T+00:02:20 BECO Redstone engine shutdown - Booster Engine Cutoff. Velocity 5,200 mph (2.3 km/s)
T+00:02:22 Tower Jettison Escape Tower Jettison, no longer needed.
T+00:02:24 Capsule Separation Posigrade rockets fire for 1 s giving 15 ft/s (4.6 m/s) separation.
T+00:02:35 Turnaround Maneuver Capsule (ASCS) system rotates capsule 180 degrees, to heat shield forward attitude. Nose is pitched down 34 degrees to retro fire position.
T+00:05:00 Apogee Apogee of about 115 miles (185 km) reached at 150 miles (240 km) downrange from launch site.
T+00:05:15 Retrofire Three retro rockets fire for 10 seconds each. They are started at 5 second intervals, firing overlaps. Delta V of 550 ft/s (168 m/s) is taken off forward velocity.
T+00:05:45 Retract Periscope Periscope is automatically retracted in preparation for reentry.
T+00:06:15 Retro Pack Jettison One minute after retrofire retro pack is jettisoned, leaving heatshield clear.
T+00:06:20 Retro Attitude Maneuver (ASCS) orients capsule in 34 degress nose down pitch, 0 degrees roll, 0 degrees yaw.
T+00:07:15 .05 G Maneuver (ASCS) detects beginning of reentry and rolls capsule at 10 deg/s to stabilize capsule during reentry.
T+00:09:38 Drogue Parachute Deploy Drogue parachute deployed at 22,000 ft (6.7 km) slowing descent to 365 ft/s (111 m/s) and stabilizing capsule.
T+00:09:45 Snorkel Deploy Fresh air snorkel deploys at 20,000 ft (6 km). (ECS) switches to emergency oxygen rate to cool cabin.
T+00:10:15 Main Parachute Deploy Main parachute deploys at 10,000 ft (3 km). Descent rate slows to 30 ft/s (9 m/s)
T+00:10:20 Landing Bag Deploy Landing Bag Deploys, dropping heat shield down 4 ft (1.2 m).
T+00:10:20 Fuel Dump Remaining hydrogen peroxide fuel automatically dumped.
T+00:15:30 Splashdown Capsule lands in water about 300 mi (500 km) downrange from launch site.
T+00:15:30 Rescue Aids Deploy Rescue aid package deployed. The package includes green dye marker, recovery radio beacon and whip antenna.

Previous Mission:
Mercury 3
Mercury Next Mission:
Mercury-Atlas 4 (unmanned)
Mercury 6 (manned)
de:Mercury-Redstone 4

nl:Mercury MR-4 pt:Liberty Bell 7

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