STS-93: Dodging Golden Bullets


Calling it Rocket Science is, of course, a misnomer. Science provided the background but today it is definitely Rocket Engineering. Scientists and Engineers mix together like, well, cats and dogs. Friendly détente some days, not so much other days.

But there is one part of building liquid rocket engines that can still be called Rocket Art – making the injectors work. All combustion engines mix fuel and oxygen to make a fire; but for really complete combustion – really good gas mileage – that mixing can be very tricky indeed.

In a large, high pressure, high thrust liquid rocket engine like the SSME, a great deal of art is involved in designing the injector plate at the top of the combustion chamber. In a poor design, spray patterns from the injector can create hot spots on the wall of the chamber, defeating the cooling mechanisms and melting out the side – like letting loose a welder’s cutting torch. In other cases poor mixing can lead to combustion instability. Think of an overloaded out of balance washing machine, but much more powerful. The mighty F-1 engines of Apollo were plagued by combustion instability which was never really quite solved. That made each moon launch more of a gamble at the very start than you probably realized.

In the world of technical and economic secrets, injector design is protected by ITAR and economic espionage laws. We won’t go to that level of course.

In the SSME, the very hot partially burned Hydrogen gas must be mixed with the still super cold liquid Oxygen in just the right way to protect the engine through the start transient, main stage with its varying throttle settings, and the shutdown transient. Did I tell you that the propellant flow is over a half a ton a second? And it burns at over 3,000 degrees F? And passes through the throat of the nozzle – about the size of a dinner plate – at the speed of sound? And in all this, the engine is 99.9% of the maximum theoretical efficiency for this type of heat engine? And that’s not all, its reusable, too. Only one in the world.

LOX posts and pin

Looking at the picture, the liquid oxygen is introduced through a forest of stainless steel tubes called LOX posts. Cooled by the LOX inside, heated by the hot Hydrogen outside, the tubes are both robust and at the same time frighteningly fragile. Hugh forces work on the LOX posts, especially during start up. Vibration forces are high throughout. And if one of those posts breaks off at the root, well, very bad things can happen. In the cool NASA parlance, a LOX post failure is CIL Crit 1. Loss of vehicle, loss of crew ‘promptly’ upon just one failure.

To eliminate the potential of a LOX post failure, inspections of the hundreds of LOX posts is performed with ultrasound. If a post shows signs of ‘fatigue’, the remedy is to plug it at the base. They use a gold pin, about the size and shape of a bullet. In the history of the program, over 200 LOX posts were pinned in this way, and only a couple ever worked loose. STS-93 was one of those.

Back to the case of STS-78 discussed in the last edition of this blog; the ‘overboard mixture ratio’ for the vehicle was changed because of the number of LOX posts that were pinned on the engines of that flight. Instead of being the expected 6.03-ish, it turned out to be more like 6.002-ish. That difference of .028 meant more fuel was used, less oxygen, and, in combination with other factors, resulted in fuel depletion just at the guidance commanded MECO. A bigger difference would have cut the engines off early – safe except for the trajectory implications. We got lucky.

On STS-93, a different set of circumstances was in play and the results were worse. Manageable, but that is due to being luckier than we deserved.

As my old boss used to say: “It’s better to be lucky than smart.” I hate that but it’s true.

Stay tuned.

About waynehale

Wayne Hale is retired from NASA after 32 years. In his career he was the Space Shuttle Program Manager or Deputy for 5 years, a Space Shuttle Flight Director for 40 missions, and is currently a consultant and full time grandpa. He is available for speaking engagements through Special Aerospace Services.
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12 Responses to STS-93: Dodging Golden Bullets

  1. spacebrat1 says:

    your writing is both plainly understandable and delightful, even when discussing such intense subjects.

  2. walt pinkston says:

    You’ve got me hanging by my fingernails!

  3. Beth says:

    Definitely staying tuned!

  4. Bob Rollins says:

    Wow, this takes me back to United Aircraft in 1957 and a combustion chamber with a single lox post in a gaseous hydrogen wall-injected environment. Post went screwy and exhaust flame quickly turned green as the copper throat caught fire. Colorful, but not good.

  5. Chris Ramsay says:

    When I was an Instructor over in the SCA (Simulation Control Area), our scripted cases came close to “Killing the Crew” sometimes. Sometimes when we dodged a “simulated” bullet, he would say “If you can’t be good, be lucky”…

  6. Vince says:

    …[“It’s better to be lucky than smart.”]…or, as we’d say in fighter squadrons in the AF: I’ll take luck over skill day of the week…and all of the other semantic variations in the other posts. We seem to all comprehend the limits, of mind and science. Thanks Wayne.

  7. Cliff says:

    Good read – thanks. Staying tuned…

  8. James Knapper says:

    I wish you would write a book based on your experiences.

  9. Miles Archer says:

    Can you please spell out the acronyms the first time you use them? I have a technical background, but struggle with acronyms like MECO (Main Engine Cut Off?)

    Combustion instability is a very hard problem to solve at the scale of these engines.

  10. M. Mraz says:

    Some of the best reading anywhere is the late Bob Biggs’ very approachable history of SSME development … utterly fascinating. Some of it is on the web, here’s one site: Even better is to ask your public library to interlibrary-loan a copy of “Space Shuttle Main Engine: The First Twenty Years and Beyond” (AAS History Series, Volume 29) ISBN 978-0877035473. Another excellent book is “Rocketdyne: Powering Humans into Space” ISBN 978-1563477546 which is notable because it contains both technical history and history of the personnel involved.

  11. jerr says:

    I look forward to the rest of the story. Thanx Wayne!

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