Mercury 4 Lapel Pin
Liberty Bell 7 MR-4 (19)
Liberty Bell 7
Pad LC-5 ()
Crew: Virgil I "Gus" Grissom
Backup Crew: John H. Glenn, Jr.
Milestones: 3/7/61 - Spacecraft delivered to Hanger S CCAFS
Payload: Spacecraft # 11, Launch Vehicle S/N MR-8
Mercury-Redstone 4 was the fourth mission in the Mercury-Redstone series of flight tests and the second U.S. manned suborbital spaceflight. It was the next step in the progressive research, development and training program leading to the study of man's capabilities in a space environment during manned orbital flight.
The main objective was to corroborate the man-in-space concept. The main configuration differences between the MR-3 spacecraft was the addition of a large viewing window and an explosively actuated side hatch.
The addition of the large viewing window was a result of a change requested by Mercury astronauts. This window allowed the astronauts to have a greater viewing area than the original side port windows. The field of view was 30 degrees in the horizontal plane and 33 degrees in the vertical. The window is composed of an outer panel of 0.35-inch thick Vycor glass and a 3-layer inner panel.
The explosively actuated side hatch was used for the first time on the MR-4 flight. The mechanically operated side hatch on the MR-3 spacecraft was in the same location and of the same size but was considerably heavier (69 pounds rather than 23 pounds). The explosively actuated hatch utilizes an explosive charge to fracture the attaching bolts and thus separate the hatch from the spacecraft. Seventy 1/4-inch titanium bolts secure the hatch to the doorsill. A 0.06-inch diameter hole is drilled in each bolt to provide a weak point. A mild detonating fuse (MDF) is installed in a channel between an 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. The MDF is ignited by a manually operated igniter that requires an actuation force of around 5 pounds, after the removal of a
safety pin. The igniter can be operated externally by an attached lanyard, in which case a force of at least 40 pounds is required in order to shear the safety pin.