The Four-Inch Flight: Mercury-Redstone 1 Launch Failure
Updated: Jul 18, 2023
Mercury-Redstone 1 was the first flight of the Mercury-Redstone Launch Vehicle and the
first attempt to launch a Mercury spacecraft with that booster. It was an uncrewed suborbital flight because the booster was not powerful enough to put a spacecraft in orbit. It was launched from Launch Complex 5 at Cape Canaveral Air Force Station in Florida on November 21, 1960. The rocket was designated Redstone MR-1 and the capsule was Mercury spacecraft #2. It is commonly referred to in the space community as the "four-inch flight."
The Mercury spacecraft's launch escape system firing after engine shutdown (NASA)
The purpose of the flight was to qualify the Mercury spacecraft and Redstone launch vehicle for suborbital missions. This included certifying the spacecraft's automated flight control and recovery systems, as well as ground launch, tracking, and recovery operations. The flight would also test the spacecraft's automatic inflight abort sensing system, which would be unable to trigger an abort itself during this mission. This was important to test because if the signal was faulty it could jeopardize a manned launch.
On November 21 at 9:00 a.m. EST, the engines of the Redstone ignited but shut down immediately after lift-off from the pad. It only rose about four inches (ten cm) before settling back onto its pedestal. There it simply sat. To everyone's surprise and confusion, it did not explode as expected and in similar fashion to the failure of Vanguard TV-3, the attempt to launch the first American satellite three years prior. Instead, the escape tower jettisoned itself and left the capsule attached to the booster. Typically it would pull the capsule with it to safety during a launch if an anomaly arose. Three seconds later the capsule deployed its drogue chute followed by its main chute and reserves. Basically, all that had launched during the entire mission was the escape rocket. A fully fueled and powered-up rocket sitting on the pad with nothing holding it in place is extremely dangerous, especially with a blockhouse full of people only a few hundred feet away. It was feared the parachutes, hanging to the side of the booster, would fill with air and tip the rocket over. Fortunately weather conditions for the day were favorable and the chances of that occurring were slim. Technicians and flight controllers decided to wait until the rocket's flight batteries died and the liquid oxygen boiled off. Flight Director Chris Kraft rejected someone's idea using a rifle to shoot holes in the booster's propellant tanks in an effort to depressurize them. [1]
The Four-Inch Flight, from ignition to chute deployment (Reddit)
Shutdown of the Redstone was caused by two of its electrical cables, the control cable and the power cable, separating in the wrong order. The control cable provided various control signals while the power cable provided electrical power and grounding. Both were plugged into the bottom edge of one of the rocket's tailfins and would separate at liftoff, first the control and then the power. However, the control cable was longer in length than expected because the one used for this flight in particular was designed for military PGM-11 Redstone missile launches, not the Mercury-Redstone Launch Vehicle. Therefore, the power cable separated first instead of second. This caused both the engine to shut down and the escape tower to fire. Normally, an electrical relay triggers normal engine cutoff at the end of powered flight after the rocket has left the atmosphere. Since the rocket now lacked grounding, and therefore power, the relay tripped and caused the booster to shut off its engine and signal to the capsule things were nominal. Usually during this point in flight, after engine cut-off, the spacecraft would jettison its launch escape system, now useless, and fire the explosive bolts holding it to the booster. In this case, the spacecraft fired the escape tower like it was supposed to but separation from the Redstone did not occur because its sensors detected acceleration: The explosive bolts would not fire if the spacecraft was still accelerating and sitting on the ground, its sensors detected a constant "acceleration" of 1-g, which was simply its own weight. Additionally, the reserve parachutes were deployed for the opposite reason: Because the parachute system did not detect any load on the main parachutes, since they were not supporting the capsule's weight, it thought they did not fire and sent out the reserves.
Redstone MR-1was refurbished after it was slightly damaged coming back down onto the launch pad and was held in reserve. Mercury-Redstone 1A was a new test flight using the Redstone MR-3 rocket and Mercury spacecraft #2, which was undamaged. It launched on December 19. To prevent an incident like MR-1 from occurring again, a grounding strap was added to electrically connect the rocket to the launch pad. It would separate only after all other electrical groundings were severed. Redstone MR-1 is now on display at the Marshall Space Flight Center in Huntsville, Alabama. Spacecraft #2 is displayed at the NASA Ames Exploration Center at Moeffet Federal Airfield in California. After MR-1A, there would be three more flights of the Mercury-Redstone rocket, two of which were crewed: MR-2 launched Ham the chimpanzee in January 1961 while MR-3 made Alan Shepard the first American to fly in space that May. Gus Grissom closed out the manned suborbital flights with MR-4 in July.
The MR-1 incident was depicted in the 2020 National Geographic miniseries The Right Stuff.
Author's note: This is one of my favorite missions of the Space Race because of its peculiarity. Let me know your thoughts in the comments below and remember to like and share this post. Thanks for reading!
Bibliography
[1] Kranz, Gene. Failure Is Not an Option: Mission Control from Mercury to Apollo 13 and Beyond. Simon & Schuster Paperbacks, 2000.
Swenson, Loyd S, et al. “This New Ocean - Ch9-7.” NASA, NASA, 1966, history.nasa.gov/SP-4201/ch9-7.htm.