Understanding STS-93: the key is Mixture Ratio

Some time back I started to tell the story of the most interesting shuttle launch:  STS-93.  I think it is time to return to that topic.  To understand what happened, some background is necessary.

If this is too engineering-geeky for you, well, what are you doing thinking about rockets and space travel?  Consider this part of your education.

Consider the following graph – I certainly spent many hours studying it and its relatives.  I would tell you frankly, I am sure I never completely understood it.  So don’t feel bad if you don’t either.  But it gives a summary of some very complex interactions.

FPRFlight Performance Reserve (FPR) is the mass of fuel (Hydrogen) and Oxygen left in the External Tank when it is jettisoned just short of orbital velocity.  Minimizing FPR is a good thing – every ounce thrown away is an ounce that could have been payload – food, water, experiment, satellite – something useful.  FPR thrown away is . . . .wasted.

At the same time, keeping too little FPR, or making the mistake of not keeping any reserve at all, means that one likely comes up short.  Short of energy, short of velocity, not in orbit, but on a ballistic trajectory that re enters the earth’s atmosphere very soon.  Too soon.  STS-93 was nearly that case.

If you look at the burning of Hydrogen and Oxygen – the second highest energy release possible on the Periodic Table – you would find that the stoichiometric ratio for complete combustion and maximum energy release is 16: 2 hydrogen atoms (atomic mass = 1) attached to one oxygen atom (atomic weight 16) for a complete combustion MR of 8.0.  But the space shuttle’s main engines have a mixture ratio – reminiscent of Avogadro’s number without the exponent – of 6.02.  If you look at the chart above, you will find at that mixture ratio, the unusable masses are just about at minimum.  But anything that drives combustion in the engines away from that optimum point will lead to increased unusable mass.

The reason for such a wretchedly low mixture ratio is that the closer the MR is driven toward stoichiometric, the hotter the fire.  The turbine blades in the turbines that power the pumps feeding the engines can’t take a much hotter fire than results from 6.02.  Blades would melt, casings too, bad things indeed would happen.  Temperature sensors in the turbines should trip the engine to shut itself down before that happened. On STS-51 F in July of 1985, both voting temperature measurements failed and started reading higher than the turbine temps actually were:  51F became the only case of an SSME shutdown in flight and it was caused by faulty temperature readings.  But back to our story:   6.02 is ‘just right’. At least to two decimal points.  We spent 30 years arguing about what the next decimal point should be.

Important Safety Note:  if propellant depletion occurs, it must occur first on the oxygen side.  If the hydrogen runs out first, the last sputters at the turbine will be much closer to stoichiometric, and, well, bad things wil happen.  Did occasionally happen early in ground testings.  Big mess in the bottom of the flame trench at Stennis.  Not what anybody wanted in flight.

So STS-78 was  a real wake up call.  On that flight, the low level sensors in the ET flashed ‘dry’ just a fraction of a second before the engines shut down.  No problem ensued, there was enough fuel in the line to shut down safely, but it scared the bejabbers out of everybody.  At least everybody that understood what that meant.

Bet you never heard about that close call.

There is supposed to be a ‘fuel bias’ (extra hydrogen) of almost 1000 lbs.  But on STS-78, due to another instrumentation failure and some funny mixture ratio business, the engine burned right through that extra thousand pounds of hydrogen and all the other ‘dispersion’ allowances that were loaded in the tank.

It all started with plugged LOX posts.  LOX post plugs played a part in STS-93, too.

All space geeks need to stay tuned.  It really is rocket science.



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O Columbia

When I first heard that the Houston Grand Opera proposed to produce an opera about the Columbia disaster, I was appalled.

If HGO wanted a spaceflight opera, it would better be the moon landing of Apollo 11; or the successful story of Hubble repair; or the construction of the ISS. But not Columbia.
I envisioned a maudlin, inaccurate, titillating soap opera treatment of a still too painful event. Something that would be disrespectful of the dead, their families, and all those who worked so hard and yet failed a decade ago. Something that Hollywood might produce.
I resolved to try to shut this down.

Then I received an invitation to talk with the HGO leadership, the writers and creative talent who would develop and produce the opera. I drove downtown with the mission burning in my heart to stop this, direct them away, keep this project stillborn.

When I met the HGO team, they were respectful, considerate, and affirming. They reached out to more than a few members of the NASA community to gain input. We talked about everything possible aspect and I expressed my concerns in my best program manager brusque style. They listened. I came away grudgingly admitting that it might be barely acceptable. Not accurate, but possibly respectful.

Great tragedy, as I am reminded, can focus attention on great human values. Art, music, dance can lead to an emotional understanding which can transcend and reach people who otherwise would be unaware. The most powerful works of the stage – from Shakespeare to Faulkner – use tragedy to highlight values and meaning.

After the HGO team spend several weeks at work, we were invited back to hear the reading of the libretto; the words. A crowd of familiar NASA faces joined at the stage door of the Wortham center in downtown Houston to hear what had been created.

First we heard from the composer designated to work on the project. A soprano brought some of his previous work into glorious voice.

Then we were ushered into a room for the reading of the libretto. Very informal, no sets, no costumes, just a stark rehearsal room where we sat on folding chair while several performers on stools faced us over music stands holding the pages.

The various voices read, did not sing, the text, all the while in character.

It was not merely good, it was great.

It put space exploration in a historical context. It used the point of view of a young person aspiring to be an astronaut. There was nothing neither maudlin nor titillating. At one point there were tears among the audience. It was powerful.

After the reading we were asked for comments; the astronauts spoke first, then we all had the chance; praise for the approach, minor corrections to the script, overall an emotional release.

The music is in composition, much later the sets and costumes will be constructed, rehearsals will begin. The curtain should go up in October 2015. Just like all space projects, the schedule will take some time.

I think it will be great.
You should mark your calendar and go.

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Manifest Destiny or Pipe Dream?

“I started out as a child” – William H. Cosby, Jr., Ed.D.

I have been ruined by the timing of my childhood. Grew up with the space race; 3 years old when Sputnik launched, 7 when Gagarin and Shepard flew and JFK promised the moon, allowed to stay up late to watch if Ranger or Surveyor were going to be successful, watched Gemini through grade school, Apollo through High School, and somewhere in between read a host of science fiction stories.
SF definitely ruined my life; captivated as I was at an impressionable age by Asimov, Clark, Heinlein, and so very many others. Watched “Destination Moon” a thousand times along with every other B grade SF movie ever made. Then, of course, came Star Trek. After that show went off prime time network programming, one of the local TV stations in my area showed reruns every night. So I watched Star Trek reruns every night – every night! – all through high school. There was a time I could quote the dialog from any episode verbatim. My social life suffered accordingly.

Read about Goddard in his tree reading War of the Worlds and deciding to find a way to go to space.  “It is difficult to say what is impossible, for the dream of yesterday is the hope of today and the reality of tomorrow.”
Long after he was dead, JFK’s speeches were the proof text for the future. Space – its exploration and exploitation – was the future. I read and re-read the works of Gerard K. O’Neill and believed. I had no doubt. It was our Manifest Destiny to explore space.
One of my middle school friends proclaimed in about 1967 that we would never reach the moon, it was impossible. Somehow he and I lost touch over the years but I expect he has become one of those fringe types who believe Apollo 11 was filmed on some sound stage in Arizona. I reveled in the fact that he was wrong.
Both fact and fiction pointed to an inevitable future.
Somehow it hasn’t quite worked out as expected.
2001 came and went without lunar bases, without any excavation of black obelisks in Tycho, without manned spaceships heading out to Jupiter or Saturn. I suspect that Zefram Cockrane wasn’t born last year, and I fear that warp drive might just not be possible in the universe we inhabit.
What happened to those flying cars which we should have by now? At least we should have the hover board. But no.
In a year when one’s birthday ends in a zero, brooding comes easily. Thinking about how there is more runway behind than ahead. Wondering if a career’s worth of work and worry were well invested or wasted. Wondering when – or if – the promise will come true.
It becomes easy to fall into cynicism, to write the whole thing off as a stunt. A one-time geopolitical ploy rooted in a particular time and policy made for reasons which no longer exist. Leaving us with the rump vestige of a dream which lives on merely to siphon off public money into some sort of jobs program. Something which exists merely to exist.
Hardnosed taxpayers want to know what they are going to get for their money. Spending billions to plant a flag on some distant point in the universe merely for prestige does not constitute a business case which shows a return on investment. They insist that the ISS better come up with something more important than videos of astronauts in zero G chasing water globules around with straws.
Space travel could just be a pipe dream. Something like believing in fairies or unicorns. Something for children which adults dismiss.
Certainly makes it hard to listen to idealistic pronouncements of ‘horizon goals’ and the inevitability of our future in space.
Ok then, shut ‘er down, put the pieces in a museum, and tell kids to study finance and marketing because that’s where the big money lies. Gordon Gekko was right after all: get filthy rich by any means necessary, win every contest by any means available. That’s what counts. That’s all that counts.
But you know . . .
Pessimism never improved the human condition. Cynics never accomplished anything positive. Real and lasting satisfaction only comes when heart and strength are given to something bigger than yourself.
Roddenberry showed us the future could be better than the present. Heinlein really got it right; it’s not about money. It’s about freedom, achievement, and the joy that comes from accomplishing something hard and worthwhile. There has to be meaning in life or it is not worth living.
Those who go forth into the world – into the universe – have at least the chance to succeed. Those who stay home in fear or greed inexorably fall into stagnation, into dissolution, decay, and finally destruction. Grow or die, that is the law of the universe.
So what if the timeline runs a little longer than we expected? Does that mean we stay home, let the cynics run the world, and wait for Malthus to say ‘I told you so’?
Today we are chasing our tails because the space experts debate destinations: Moon, Mars, or asteroid? O’Neill and I say do them all. The ‘horizon goal’ isn’t Mars, it is the entire solar system. When we have built colonies on every habitable niche, then maybe we will find a way to go to the stars.
I didn’t say bankrupt the treasury. Don’t squander other people’s money; figure out how to do it anyway.
Nobody said that it would be easy. But what is our choice?


It’s been a long road, getting from there to here.
It’s been a long time, but my time is finally near.
And I will see my dream come alive at last. I will touch the sky.
And they’re not gonna hold me down no more, no they’re not gonna change my mind.

‘Where My heart Will Take Me’ written by  Diane Warren

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Risk Tolerance

I am a big fan of the author Bill Bryson. I have enjoyed all his books and recently have been rereading “One Summer: America, 1927”. His sparkling account joyously brings that time to life. Looking back at 1927 from April to October, Bryson chronicles an amazing time: Babe Ruth and Lou Gehring, Coolidge and Harding, the first ‘talking’ pictures bringing revolution to Hollywood, and of course, Lucky Lindy. Yes, I recommend you buy Bryson’s book and read it.

Especially study the part about the Orteg prize and the competition to be the first to fly nonstop from New York to Paris. As I read about aviation in 1927, I am powerfully aware of the many folks — people who would influence space policy today — who continue to draw comparisons between aviation in the ‘golden age’ and space exploration.

Remembering the actual events of those years should give those advocates pause to consider the risks that were considered acceptable in 1927. An unbelievable number of those early aviators were ill-prepared, took risks that were outside the limits of good judgment, or let ego and the pursuit of fame blind them to the realities of what they were about to do. Stupidity seemed to be more typical than not.

Let me excerpt some passages from Bryson’s book which gives an accounting of aviation in those electric months of 1927 (with sincere apologies to the author as I do violent excision of his words):


“America had three teams in the running . . . Columbia, America, American Legion . . . The leader of the America team was thirty-seven-year-old naval commander Richard Evelyn Byrd . . . On April 6, 1927, just before six in the evening, [Anthony] Fokker . . . copilot Floyd Bennett, navigator George Noville, and Byrd himself eagerly crowded into the cockpit . . . As America came in to land . . . it came down nose-first. The problem was that the weight was up front and there was no way for any of the four men to move to the back to redistribute the load . . . a piece of the propeller ripped through the cockpit and pierced Bennett’s chest . . . Byrd . . . failed to notice that his left arm had snapped like a twig . . . For the time being, the Byrd team was out of the running.

Clarence Chamberlin . . . a short flight above Long Island . . . the landing gear fell apart during the takeoff . . . the wing hit the ground and the damage to the plane was sufficient to set back the Columbia’s plans considerably.
Noel Davis and Stanton Wooster . . . were smart, able aviators, and their plane, a Keystone Pathfinder . . . was 1,150 lbs heavier than it was supposed to . . . On April 26 . . . they would take off with a full load of seventeen thousand pounds . . . the plane struggled to get airborne . . . not enough to clear a line of trees . . . stalled and fell to earth with a sickening crash. Davis and Wooster died instantly . . .

. . . Paris, where at dawn on May 8 . . . Captain Charles Nungesser and Captain Francois Coli . . . war heroes . . men at ease with danger . . . called their plane L’Oiseau Blanc (The White Bird) and painted it white so that it would be easier to find if it came down at sea. . . they could carry no more than about forty hours’ worth of fuel, which left them almost no margin for error. . . after loading supplies, their plane weighed almost eleven thousand pounds. It had never taken off with that much weight before . . . the plane slowly gathered speed . . . lifted briefly, but then came down again and bouncily proceeded another three hundred yards before finally, agonizingly, and barely getting airborne. The chief engineer, who had run along beside the plane much of the way, fell to his knees and wept. . . . One hour and twenty-seven minutes later . . . Nungesser and Coli reached the chalky sea cliffs of Normandy at Etretat. A squadron of four escort planes tipped their wings in salute and peeled away and L’Oiseau Blanc flew off alone in the direction of the British Isles and the cold Atlantic beyond. . . . Nugesser and Coli . . . missing and feared lost. . . The one thing that wasn’t found was any trace of the White Bird or its occupants.

At the same time . . . another ambitious French flight . . . got under way when three aviators, Pierre de Saint-Roman, Herve Mouneyres, and Louis Petit, took off from Senegal . . . and headed for Brazil . . . no wreckage was ever found . . . In nine months, eleven people had died in the quest to fly the Atlantic . . . nothing was going right for anyone . . .

. . . Lindbergh . . .

With the Atlantic conquered, attention turned to the Pacific – specifically the 2,400 miles . . . between California and Hawaii. . . the Dole Pacific race with $35,000 in prize money . . . from the municipal airport in Oakland, California . . . to Oahu. . . . Three competitors died in crashes before they even reached Oakland. Another plane crashed in the sea as it approached the Oakland airfield . . . another plane was not allowed to depart after it became evident that the pilot had no idea how much fuel he needed to reach Hawaii and didn’t have a fuel tank nearly big enough . . . By the day of the race, the number of planes taking part had been reduced to eight, and four of those scratched before takeoff or turned back soon after. Of the four planes that set off, two made it to Hawaii and two more were lost en route. . . . When word got back that six people were missing, a pilot named William Erwin took off from Oakland to look for them, but he disappeared, too. . . ten people died in the Dole race . . .

. . . people were suddenly announcing daring and risky flights all over the place. Paul Redfern . . . proposing to fly 4,600 miles – farther and anyone had evern flown before – over ocean and jungle, into a realm far beyond the range of reliable maps and weather reports . . . it would take him at least sixty hours to reach Rio. But before he even cleared the Caribbean he was lost . . . it was the last anyone ever saw of him . . . in 1938, at the request of Redfern’s wife . . . Redfern was declared officially dead by a court in Detroit.

In Britain an unlikely sixty-two-year old woman, Princess Anne of Lowenstein-Wertheim-Freudenberg . . . a dashing young captain named Leslie Hamilton expressed a desire to cross the Atlantic from east to west, she funded the flight on the understanding that she would accompany the fliers. With Lieutenant Colonel Frederick Minchin as copilot, they took off from an airfield near Salisbury in Wiltshire. The princess wore a stylish hat and an ocelot coat . . . They were sighted over Ireland and again from a ship about halfway across the Atlantic, but they never reached American and no trace of them was ever found.

At about the same time, a plane called Old Glory . . . took off from Old Orchard Beach in Maine . . . bound for Rome . . . piloted by Lloyd Bertaud . . . copilot was James DeWitt Hill, and along for the ride as passenger was Philip A. Payne, editor of Hearst’s Daily Mirror. Just three and a half hours after takeoff they issued an urgent, unexplained SOS. They were never seen again. A few hours later, two Canadian airmen, Captain Terrence Tully and Lieutenant James Medcalf took off from Newfoundland, bound for London in a plan called the Sir John Carling. They were never heard from again either. “


So the question remains: how good an analogy is early aviation to the current state of space exploration?
And how much risk are we willing to take to go to space these days? A prudent amount? What does that mean? Is your standard the same as mine? And what will the lawyers say when the heirs file their lawsuits? And who will be the next Lindbergh?

2014 is not 1927: no Babe Ruth in sight. Hopefully nobody will repeat those days . . . . Sadly, nobody will repeat those days . . . .

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Finding Columbia

IMG_0877[1] IMG_1087[1]A couple of days ago, I had the rare opportunity to visit the Columbia Debris Repository on the 16th floor of the VAB at Kennedy Space Center.  It is a solemn experience to walk among the remaining parts of the space shuttle which bear witness to the tremendous forces that broke the vehicle apart and ended the lives of seven brave astronauts. 

This experience is not available to the general public.  But you can find places to see parts of Columbia.  The data recorder, found miraculously intact in a field in east Texas, is on display in public areas at the Johnson Space Center in Houston, and the frame for the side hatch window is in the entrance display case of the Kennedy Space Center headquarters building in Florida.  There is another part reportedly on display at the Marshall Space Flight Center in Huntsville, Alabama, although I have never been able to find it on my many visits there.

These artifacts have great engineering value showing how future spacecraft should be built to withstand the extreme conditions of space flight. 

But there is a backstory that is more important. There is a story that must be told of how poor decisions were made, mistakes, and hubris.  This more important story is not found in the artifacts, although they emphasize the consequences. It is for us who lived through those days to share that story.

It is not enough to remember the sacrifice of our brave friends, although we should do that.  It is not enough to study the wreckage for clues of how to build better aerospace systems although we should do that.  It is mandatory that we remember how those sacrifices and that wreckage came to be.  And to prevent it from happening in the future.

Recently, some have said that loss of human life in spaceflight is to be accepted as a part of the cost of exploring the universe.  That may be, but those of us in the business must always believe that going forward we have to all we can to prevent accidents.

Because forward into the universe we must go.

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Keeping Eileen on the Ground: Part II – or – How I Got Launch Fever

The STS-93 launch attempts proved the adage we knew well in the Flight Director office: When the weather is good, the vehicle will break; when the vehicle works perfectly the weather will be bad. On the first launch attempt – scrubbed due to hydrogen sensed in the aft compartment – the weather was good. The Space Flight Meteorology Group post mission report says all sites were GO – launch, RTLS, TAL, AOA, etc. All GO. Hard to believe that there were no issues anywhere. And of course, something broke.

Two days later, we had another great weather forecast, no issues. The SMG post mission report for this day says “Thunderstorms occur at KSC about 3.2% of the time for a 0400 to 0500 UTC launch window.” Guess Murphy doesn’t believe in statistics.

Deploying the Chandra X-ray telescope on its Inertial Upper Stage required a complicated launch window. The plan was to deploy the payload early in the flight. But you can’t just kick it out just anywhere. The IUS had a complicated navigation and guidance system and the telescope needed to get its solar arrays going pretty quickly – a long period of time in the earth’s shadow wouldn’t be good. And there probably were other constraints that I’m not remembering. Given the orbital constraints on the deployment window, a launch window could be calculated. But that is not the whole story; we should plan a backup deploy opportunity just in case we ran into trouble. And an alternate deploy window in case we wound up in a lower than expected orbit. And another backup deploy window in case we had to wait until the second day of the flight to deploy. Etc., Etc., Etc. Each of these deploy windows worked back into a constraint on the launch window. Covering for all the backups and contingencies meant that the launch window would be 47 minutes long.

As the countdown progressed, rain showers which built into thunderstorms popped up just off shore – not over the pad where they would be real trouble, but offshore to the east. And the prevailing winds were pushing the storms to the east, away from the launch pad.

After Apollo 12, we knew that thunderstorms meant lightening and lightening was bad for our space ship. No launching if the edge of a thunderstorm was within 20 miles of the launch pad. And one of the storms was right there. On the edge.

Sitting behind John Shannon, on the MOD console, Bryan Austin and I were not happy. The Launch Director was not happy. The entire team, having scrubbed once, was not happy.

The Ops Manager, who chairs the MMT down in Florida in the firing room bubble, called me on the loop and asked about the constraints on the launch window. Could we extend it? Yes, by golly, we could, I decided. Bryan and I got on the loop with the IUS team out in Sunnyvale and with the Telescope POC. If we gave up the third backup deploy opportunity, that would add ten minutes to the launch window. Everybody agreed that it was unlikely that we would need that backup deploy opportunity. We all wanted to go. We were all frustrated that the launch had scrubbed earlier. This flight had been delayed several times in the planning cycle before we got to the pad; it was time for Chandra to fly!

The SMG forecaster put his electronic cursor on the satellite image of the edge of the storm, projected the storm movement based on wind speed, and opined that we would be good to go at the end of the window. So we waited.
Time passes very slowly.

Then the storm started building up, and expanded, and all of a sudden, and now that ten minutes would not be enough.

We were back on the loops again. Giving up another backup deploy opportunity would gain another 7 minutes or so. Were we all OK with that? Sure, let’s light this candle. GO.

I could tell that the Ascent Flight Director was not really comfortable with this broken field running. When John queried his FDO about the launch window close time, the FDO said he didn’t know when the window would close based on all the back room deals that the MOD was making.

That should have been a red flag. Instead, it made me mad. Hadn’t FDO been listening?

Just about that time, the thunderstorm on the edge of the launch area exploded. The trailing edge of the storm rapidly grew farther and farther to west. Against the wind. Over the launch pad. Time was up, no matter how you calculated it. We were done.

STS 93 weather

The crew rode the elevator back down. Launch was reset for 24 hours later.

We had an interesting debrief out by the coffee pot in the hall. That is the place all the really significant conversations in Mission Control happened. Not on the loop. In the hall at the coffee pot.

It became suddenly clear what a risk I had been running by negotiating real time changes to the launch window. We could have launched into a miscalculated window that left us no options but to return with the telescope in the payload bay. Of if we needed one of those backup deploy windows we would have been out of luck. What had I been thinking?

So when folks point to the damage that has been done on other launches when officials got launch fever and let go of good judgment, I think about the second scrub for STS-93. And how sometimes an errant thunderstorm can save you.

Next time: what happened when we actually launched.

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STS-93: Keeping Eileen on the Ground, Part 1

The summer of 1990 is remembered in shuttle circles as the hydrogen leak summer. We tried to launch Columbia and found that there was a leak in some of the plumbing carrying that volatile gas. The launch team scrambled into a series of hydrogen loading tests with various sensors to find the source. During the trouble shooting on OV-102, we tried to launch Discover and found leaks in OV-103 too. All summer we sat on the ground running test after test. There is a really good summary on the NASA lessons learned page: http://appel.nasa.gov/2008/01/01/the-summer-of-hydrogen/ This was the set up for our story about STS-93, the most interesting shuttle launch of them all.

Hydrogen is a powerful fuel for rockets, but it is hard to work with. The molecules are very small and can slip through the tiniest of openings; and exposed to any oxygen (in the air), well, everybody has seen the Hindenburg movie. Not good.

The aft compartment is the engine room of the space shuttle. It is surprisingly large and open on the inside – the volume is bigger than your bedroom but oddly shaped. And it is stuffed full of pipes of various sizes running every which way; large ones and small ones, some carrying gases and some carrying cryogenically cold fluids, hydrogen and oxygen lines, ammonia and Freon lines, hydrazine in two varieties and nitrogen tetroxide – a powerful oxidizer so deadly that it was said if you whiffed its fishy vapor in the air you were already dead. During main propellant loading and right up to engine start, dry nitrogen gas is pumped into the compartment to keep the possibility of fire as low as possible.

And to detect if we had a problem? We went through an evolution in sensor technology, but in 1999, inside the shuttle aft compartment was one (1) inlet to a hose to detect a hazardous gas leak. The detection equipment was actually located well away from the shuttle itself, buried deep in the mobile launch platform. A variety of a mass spectrometer searched for various gases and forwarded concentration information to a console operator in the firing room, three miles away. A sample of gas “collected” in the top of the aft compartment wended its way down through the tubing and plumbing maybe a hundred feet before hitting the analyzer. And the analyzer needed frequent calibration to ensure that it was operating correctly. This system could tell us, within a reasonable expectation, that we had a leak and what gas it was. But the detection system was practically useless to tell us where the source of the leak could be found. This system was slow, unwieldy, and the best we had at the time.

The launch commit criteria was carefully written to tell the operators exactly what concentrations at what times were allowable and which were not – for example, sometimes when External Tank loading was started the seals weren’t thermally stabilized and leaked a little until they chilled down. Later on, that same concentration of hydrogen free in the aft compartment wouldn’t be acceptable.

And at certain times, parts of the shuttle system were “committed” to launch – the firing room personnel were not longer required to monitor nor call a hold. But there was the big exception statement in the front of the LCC that required “senior console operators” to use their “best judgment” to call a hold to stop a hazardous situation, no matter how late in the countdown.

So it was a real surprise that during the first countdown attempt for STS-93, the senior haz gas operator called a cutoff – the technical term for a launch scrub that late – at T-8 seconds for high hydrogen gas concentration. There is a really good youtube video here: http://www.youtube.com/watch?v=T7vGqQUhciE – which has an erroneous title and writeup, but the video is good.

At T-10 seconds, the ground computers send the very last command the onboard system needs to fly: Go for Main Engine Start. With no other electronic word from the firing room computers, the shuttle launches itself. Right after that T-10 seconds command, the big sparklers at the base of the MLP towers fire off to ignite any small patches of free hydrogen that might have escaped from the main engines. At T-6.6 seconds, the orbiter computers command the main engines to start, staggered by about 120 milliseconds – center first, then left, and finally right. Shutting down the count at T-8 seconds is dicey. If the mains start there is a lot of free hydrogen left around and plenty of heat to ignite it. The ROFI sparklers were firing – those take 48 hours to replace – and if even the first engine cracks open one valve in the start sequence, it is a three week turnaround to remove and replace any SSME that started. So it was a surprise to get the call, especially after haz gas was “committed” for launch. It was one of those senior console operator things.

It is not technically a pad abort, no fire or smoke occurred – other than the sparklers – but the adrenaline is very high all around the program when it gets that close.

Safing was complete, the crew unstrapped and rode the elevator down to wait for the next attempt; 48 hours later at the earliest.

The KSC management was ready to pin a medal on the guy that called the hold. But the first story we got in Houston was that the sensor had just gone through an automatic calibration and the high concentration reading was a ghost.

I never heard the end of that story. Did Ozzie get a medal? Did we unnecessarily scrub a launch? Somebody needs to fill me in. All I remember is it set me up for a bad case of launch fever two days later.

That story next time.

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