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.
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.