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.
Flight 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.