Switched

STS-68
Imagine, for a moment, that you are Terrence W. Wilcutt and it's August 18, 1994. Your blood is cold as ice as you suit up for your first-ever spaceflight, STS-68. The many years you've spent as a United States Marine Corps test pilot have steeled your nerves and steadied your hand, and you don't bat an eye as you're strapped into the pilot's seat of the most high-performance vehicle ever built. Imagine your nervous excitement as the launch time counts down. Five seconds: shaking begins as the main shuttle engines start. Four, three -- you glare at the sky, squint, and swallow hard -- two, one, blast... wait... nothing. At T-3 seconds, the shuttle's computer detected a problem with the engines and automatically shut down the launch in what's called a Redundant Set Launch Sequencer (RSLS) abort. No space for you today.



STS-93
STS-93, the 26th launch of the space shuttle Columbia, was aborted only seven seconds before it went up. Just before the shuttle's main engines fired, excess hydrogen was detected in the aft compartment of the orbiter, and the cutoff command was given. Sound familiar? That's because the exact same problem stalled Endeavor's launch this week, nearly ten years later.



STS-51
STS-51F holds the dubious distinction of being on our list twice. Its first attempt at launch on July 12, 1985 was halted three seconds before liftoff due to a malfunction of a coolant valve on one of the shuttle's main engines. During launch countdown, at T-minus-31 seconds, the shuttle's onboard computer takes over, making the final decision about "go" or "no go." In this case, that faulty coolant valve prompted a trusty auto-RSLS abort. Because it's automatic, an RSLS abort tends to take everyone by surprise, which isn't exactly good for the nerves of observers; just listen to the reaction of the video's narrator, and you'll hear what we mean.


STS-51F
Seventeen days later, Challenger was back on the launch pad for another go at STS-51F. This time, the shuttle lifted off without a hitch, but five minutes and 45 seconds into the launch an engine temperature sensor malfunctioned and automatically shut off the Number 1 main engine. Fortunately, the launch can continue with one of the main engines out. Unfortunately, the same malfunction happened at the exact same time in the Number 2 engine. A quick-thinking technician recognized that the problem was with the sensor, not the engine itself, and disabled the auto-shutdown, likely saving the crew's lives. Challenger initiated an immediate Abort To Orbit (ATO), which automatically flies the spacecraft to an orbit lower than that specified by the mission. Not a bad result, but a very near miss.

In addition to ATO, there are Abort Once Around (AOA) and Return To Launch Site (RTLS) mid-air abort protocols. Both are considerably more difficult, and the RTLS abort is one of the most dangerous maneuvers ever dreamt up by NASA flight engineers. It's worth reading about, here.

"The Twang"
When searching for these videos, we came across some amazing footage of a shuttle launch phenomenon affectionately referred to by astronauts as "the twang." When the main engines fire at T-5 seconds, the entire shuttle stack bends thanks to the instant spike of thrust. As soon as the shuttle snaps back upright, the solid rocket boosters ignite and the shuttle blasts off. It's too cool not to include.