The Devil In The Details: The Story of MR-1, or the “Four-Inch Flight”
How half an inch of wire sent America’s first spacecraft crashing back to the pad, and the dangers of overlooking the most minute issues.
It’s a tale as old as time — the best of plans being undone by a single forgotten detail. Whether it’s the detective unmasking the culprit using a piece of overlooked evidence, or flunking a math test because you forgot a single negative sign somewhere, these types of stories are ubiquitous.
However, there are times when this trope goes from a silly little quirk of human nature to the instigator of disaster, which is why companies and agencies across the globe have teams of highly trained and skilled engineers to review their plans and ensure that no details have been missed. Despite this, even the best engineers are only human, and tiny things can often escape the most stringent scrutiny — a fact that NASA learned while the agency was still in its infancy.
“Details make perfection, and perfection is not a detail.”
~Leonardo da Vinci
On October 4, 1957, the Soviet Union achieved a massive milestone in human history. For the first time, a man-made object — Sputnik — left Earth’s atmosphere to orbit the planet. This immediately forced their geopolitical rivals, the United States, to attempt to catch up, starting what we now know as the Space Race.
By late 1958, it became clear that the next major milestone to achieve would be the first human spaceflight, and NASA, the newly-formed American space agency, was eager to reach that cherished goal. By 1960, the design of America’s first human-rated spacecraft, the Mercury capsule, was largely completed, as well as the construction of the booster that would carry it into space, named the Redstone. The first test launch of an unmanned Mercury spacecraft atop a Redstone rocket was scheduled for November 21 of that year. The flight, named Mercury-Redstone 1, or MR-1, was slated to test the automated flight control system, as well as ground-based tracking and recovery procedures.
As the clock ticked towards the scheduled liftoff time, controllers at Mercury Control — the predecessor to modern-day Mission Control — conducted their final checks to ensure that every onboard system was functional. At 9:00 AM, to the jubilation of everyone present, MR-1 blasted off of Launch Complex 5 with a brilliant burst of flame. However, the joy was short lived. A split-second after leaving the pad, the Redstone’s engine shut down, sending the entire stack crashing back to the pad, where it precariously balanced on its engine bell. At the same time, the escape tower — a rocket motor intended to pull the Mercury capsule to safety in the event of a booster failure — was jettisoned and flew off the top of the rocket. Three seconds later, with a noise reminiscent of a champagne cork popping, the drogue chute deployed, followed almost instantaneously by both the main and reserve parachutes, which flopped unceremoniously to the side of the booster.
While onlookers gawked at the now-stationary rocket on the pad, Mercury Control was simultaneously perplexed and petrified. All ground connections to the Redstone had, by design, severed during the rocket’s brief flight, leaving them with no way to control or shut down the booster, which was full of highly volatile fuel. Additionally, there was the huge risk of the deployed parachutes catching a gust of wind and tipping the stack over, which would cause the entire thing to explode. To add the cherry on top of the danger sundae, MR-1’s self-destruct package, intended to blow the rocket up should it malfunction in flight, had been armed at the moment of liftoff, and without the connections in place, there was no way to disarm the system. Clearly, Mercury Control needed to find a way to safely de-activate the booster — and fast.
Chris Kraft, the Flight Director at Mercury Control, opened the floor to suggestions from the various engineers present. Several ideas were floated, including using a cherry-picker basket crane to send up workers with garden shears to cut the parachute cords, which would alleviate the risk of the rocket tipping over, then draining the tanks using a series of pumps, as well as the quintessentially American idea of using a high-powered rifle to shoot holes in the fuel tanks to drain them. All of these plans were, understandably, rejected due to the risk they posed to the personnel involved.
Ultimately, it was concluded that the safest course of action was, oddly enough, to do nothing. The winds were forecast to remain calm, which meant that it was unlikely for the parachutes to tip the stack, and the Redstone’s batteries would die by the following morning, which would disarm the self-destruct package and cause the liquid oxygen in the fuel tanks to boil off, thus allowing the ground crews to safely approach the rocket. Glad to finally receive a reasonable solution to the problem, Kraft agreed, and the rocket was left alone until the battery died, at which point it was de-fueled and recovered.
News of the almost cartoonish failure of MR-1 spread quickly, and the media was quick to compare it to the historic firsts of the Soviet space program. Journalists sardonically dubbed the mission the “Four-Inch Flight,” in reference to the approximate distance traveled by the booster.
Meanwhile, NASA was urgently tasked with investigating the cause of the failure — after all, if an astronaut had been on board, the outcome could have been catastrophic.
As the investigation unfolded, engineers traced back the issues with MR-1, and came to a startling conclusion — the entire incident had been caused by half an inch of wire.
Like many early-generation rockets, the Redstone booster was essentially a modified missile — in this case, NASA altered the PGM-11 Redstone short-range ballistic missile (SRBM) used by the US Army at the time to launch the Mercury capsule. During their launch sequence, both the PGM-11 and the NASA-modified Redstone were connected to ground control by two wires — a control cable, which was used to transmit signals from controllers to the rocket while it was still on the ground, and a power cable, which provided the booster with electrical power on the pad, in addition to grounding the rocket to prevent stray currents or sparks from damaging it. Both of these cables were connected to the rocket through a plug point near the fins at the booster’s base, and were designed to disconnect as the rocket ascended. However, it was imperative that the cables detached in a specific order — first the control cable, then the electrical cable.
Due to the many modifications to the booster, the Redstone rocket required a shorter control cable than the PGM-11 to ensure that the cables separated in the right order. However, on MR-1, a mix-up had occurred; the wire for the PGM-11 was used instead of the one for the Redstone. While the difference in length between the two cables was only about half an inch, it meant that the cables separated in the incorrect order — electrical before control — as MR-1 ascended.
When the electrical cable separated, it caused the rocket to become un-grounded, which, in turn, sent a stray current through the engine-cutoff relay that was supposed to shut the engine down at the end of the rocket’s ascent. This resulted, somewhat predictably, in the engine shutting down less than a second after liftoff. Sensing the engine shutdown, the automatic flight control system then sent a command to jettison the escape tower and blow the explosive bolts holding the capsule to the booster. The escape tower promptly flew off of the top of the rocket; the capsule, however, sensed that the rocket was subject to a constant 1-g acceleration rather than the 0-g it was expecting to detect at the moment of engine shutdown, and thus did not fire the bolts to detach itself from the rocket.
At this point, since the escape tower had been jettisoned, the parachute deployment system was armed. The parachutes were supposed to deploy as the capsule descended through 10,000 feet, and the control system used a barometer to measure atmospheric pressure to determine when to deploy them. Since the rocket had by now settled onto the pad and was well below 10,000 feet, the system deployed the drogue chute, followed by the main parachute. As the capsule was sitting still, the parachutes didn’t catch any incoming air and fell to the side of the booster, which caused a sensor measuring tension in the parachute lines to determine that the parachutes hadn’t deployed at all, causing the reserve parachute to deploy as well.
Ultimately, NASA modified the Redstone to prevent the engine-cutoff relay from being energized until approximately 130 seconds into the flight, thus ensuring that a similar situation could not occur even if the cables were somehow mixed up a second time. Additionally, a second grounding strap was added to the rocket as a form of redundancy. Unfortunately, the Mercury program’s goal of putting an American into space before the Soviets would not be met; less than six months after the “Four-Inch Flight,” the Soviet cosmonaut Yuri Gagarin would become the first human in space.
Despite this, the Redstone went on to have a near-perfect launch record, with the remaining five flights in the program going off without a hitch. Additionally, the troubleshooting process of how to disarm MR-1 gave rise to one of the most common adages in spaceflight, and engineering more generally — “if you don’t know what you are doing, don’t do anything.”
However, the key lesson that the “Four-Inch Flight” holds is that every last detail matters, no matter how small. Tiny oversights may seem like no big deal, but they can often mean the difference between failure and success, and — in some particularly high-stakes cases — life and death. It therefore falls on us to defeat the devil in the details, and pay close attention to every last attribute in our projects, as we never know what the consequences of neglect may be.
