According to NASA website, about 5 seconds after liftoff, flight controllers noted a voltage drop on one of the shuttle's electrical buses. Because of this voltage drop, one of two redundant main engine controllers on two of the three engines shut down. The redundant controllers on those two engines -- center and right main engines -- functioned normally, allowing them to fully support Columbia's climb to orbit.
The orbit attained, however, was 7 miles short of that originally projected due to premature main engine cutoff an instant before the scheduled cutoff. This problem was eventually traced to a hydrogen leak in the No. 3 main engine nozzle. The leak was caused when a liquid oxygen post pin came out of the main injector during main engine ignition, striking the hot wall of the nozzle and rupturing three liquid hydrogen coolant tubes.
The orbiter eventually attained its proper altitude and successfully deployed the Chandra X-ray Observatory into its desired orbit.
"Eileen Collins was the Commander on that flight. As it happened, I was on the Engine Console for that launch. I don't really know what the 'outside world' thinks happened on that flight, but the context leads me to believe that people think it was a lot worse than it actually was. To correct any misunderstandings, I'll try to answer what actually happened in some detail.
"Very shortly after liftoff, there was some sort of a 'transient' on AC Bus 1 (probably the bus wiring intermittently shorted to the vehicle structure thru some of the wiring insulation (kapton) that doesn't 'age' very well. There are sensors in the Orbiter's electrical system called 'AC Bus Sensors' that take the bus 'off-line' in the event of something like this, and they functioned properly. So, AC Bus 1 went 'off-line'. This leaves 2 AC Buses (AC 2 and AC 3) still operating.
"The Shuttle Main Engines (SSMEs) operate with a device called an 'Engine Controller'. It's a small computer that supervises and controls the engine operation, and monitors what's going on. Each engine controller actually has 2 independent 'Digital Control Units' (called 'DCU A' and 'DCU B') that are operating. Each DCU is powered by a separate AC Bus. For example, Engine #1 (the center engine) controller DCU A and B are powered by AC Bus 1 and 2, respectively. Engine #2 DCUs are powered by AC 2 and AC 3. Engine #3 DCUs are powered by AC 3 and AC 1 respectively.
All engines continued to operate normally
"When AC Bus 1 shut down, DCU A on Engine #1 went with it, as did DCU B on Engine #3. Engine #1 switched operation from DCU A to DCU B automatically. Engine #3 was not impacted, as its DCU B was not in control at the time. All engines continued to operate normally. This entire set of events happened at about the time the vehicle cleared the umbilical tower, and all of the above 'excitement' was over faster than you can blink your eyes. It just took us (myself, at least) a few seconds to absorb it!
"Unbeknownst to us at liftoff, some 'pins' (they look like a golf tee and are about the same size) used to de-activate main injector elements were 'spit out' during the engine start. I don't remember how many of these pins were involved. They damaged the thin-wall tubes that make up the engine's nozzle extension, and caused the tubes to start leaking hydrogen."
Mohr continues;
"The sensor in the engine that senses fuel flow (H2 flow) is upstream of these leaks, and so was not able to 'know' that hydrogen was leaking from the system. The major effect of these fuel leaks was to 'starve' the main combustion chamber for fuel, and cause the main combustion chamber pressure (Pc) to drop somewhat. The engine's control system responds to a Pc dropoff by increasing oxidizer mass flow. Since the system was not 'aware' of the fuel leak, this action caused the engine to run at a ratio of oxidizer-to-fuel (called the 'mixture ratio') that was somewhat higher than planned. The discrepancy was not large, but it caused oxidizer to be depleted sooner than it should.
Engine cutoff command
"The way we knew that something 'non-normal' was happening was that the turbine that drives the oxidizer pump was working a bit harder than it should (it's gas temperature was up a little bit) and operating pressure in the fuel system was a bit low. This continued for essentially the entire ascent to orbit, and was being closely monitored. This condition is a classic 'textbook' type of condition for us, so the effect it would have was no mystery. We trained VERY thoroughly for stuff more complicated than this, so this particular case was not a big deal (as long as you're on the ground....).
"As it happened, at just about the time that was anticipated for the guidance system to shut down the engines, the liquid oxygen supply was depleted, and this was sensed by the Orbiter's data processing system, which issued an immediate engine cutoff command. This happened so close to the intended engine cutoff time that it took a few seconds of 'staring' at the data to get thru to ourselves that we had experienced a 'LOX Depletion Cutoff'.
"Since the engines cut off sooner than intended, the vehicle's velocity at cutoff (MECO) was a little lower than planned (it was thirty-something feet per second low, if I remember correctly). This magnitude of 'underspeed' has to be factored into what happens next, but was of no significant consequence (other than some hurt feelings!).
Space Shuttle Columbia had never been close to being lost after STS-93 Liftoff
"I reviewed documents I have from this flight. In doing so, I found that Wikipedia has a page for STS-93. I come from the 'pre-Internet' generation, so going to the Internet for information is usually not what occurs to me first. Still, I suppose I shouldn't be surprised.
"In any event, the Wikipedia page on STS-93 contains the following verbage:
"....the vehicle safely achieved its intended orbit and completed the mission as planned. This incident brought on a maintenance practice change that required damaged oxidizer posts to be removed and replaced as opposed to being intentionally plugged, as was the practice beforehand.'
Verbage flat out 'wrong'
"I would like to state here, and 'for the record', (drum roll please....) that this verbage is flat out 'wrong' -- 'erroneous' -- 'untrue!'
"Reason -- Oxidizer posts are welded into the SSME's Main Injector. Removal of them (let alone replacement!) is a physical impossibility. To do so, the Main Injector would have to be removed from the engine's Powerhead, which requires grinding out welds. Then removal of a particular LOX post would entail grinding out the weld that secures it into the injector's 'interpropellant plate'. I might add that even accessing that weld (in anything but the outermost row of LOX posts -- the so-called 'Row 13' posts -- would itself be impossible without removing LOX posts around it. LOX posts in the Main Injector are VERY tightly packed into a circularly symmetric pattern of 13 concentric rows.
"The maintenance procedure at the time was to drive the gold-plated plug into the upstream end of the LOX post, and fly 1 flight with the 'plugged' post. After return of the Orbiter, a 'closeout plug' was driven into the LOX post and welded into the post at the post's downstream end (i.e., the end of the post that you can see by looking at the injector thru the nozzle's throat)."
Mohr concludes;
"Someone's going to ask 'why didn't you weld the closeout plug into the post to start with??'
"Answer -- I haven't any idea. I didn't write the maintenance procedure."