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.



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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18 Responses to Understanding STS-93: the key is Mixture Ratio

  1. Goody, Rocket Science!

    • STS-93 marked the 95th launch of the Space Shuttle, the 26th launch of Columbia, and the 21st night launch of a Space Shuttle. Eileen Collins became the first female shuttle Commander on this flight. Its primary payload was the Chandra X-ray Observatory. It would also be the last mission of Columbia until March 2002. During the interim, Columbia would be out of service for upgrading, and would not fly again until STS-109. The launch was originally scheduled for 20 July but the launch was aborted at T-7 seconds. The successful launch of the flight occurred three days later.

  2. Steve Pemberton says:

    I remember the problems with the ECO sensors that would sometimes fail wet during tanking causing a scrub, since a similar failure during launch could cause the computers to think that there was still hydrogen in the tank when there wasn’t and allow the engines to keep burning until the hydrogen was depleted.

    In the STS-115 post-scrub press conference I vividly remember you explaining that this would result in a “bad day”.

    So I take it that the ECO sensors worked correctly during the STS-78 launch and that it’s the fact that they even came into play that was so alarming?

  3. Andrew_W says:

    No doubt a stupid question, but: Don’t the SSME’s always shut down before orbital insertion, so couldn’t a longer OMS burn compensate?

  4. Chris Ramsay says:

    Excellent Blog Wayne! Like I said on an earlier installment, I remember the STS-93 Ascent very well. I was the DPS-SPT Operator in the back room. I won’t spoil it for your readers!

  5. Susan Bates says:

    I wish you had been my math teacher as well as my Sunday School teacher.:)

  6. CGN38DAVE says:

    Hooray! Been waiting for the continuation of this story for almost a year…

  7. Beth says:

    You’ve got my attention; next chapter, please!

  8. Vince says:

    What next? OBTW, loved the chart. More of them?

  9. spacebrat1 says:

    thanks for sharing your amazing tales…

  10. DH2VA says:

    Isn’t the 6:1 mixture ratio also optimum from an energetic point of view? While 8:1 is stoichiometric and converts fuel and oxidizer into plain water, a 6:1 MR produces water and hydrogen in the exhaust effectively lowering its molecular mass and therefore its exhaust speed (and Isp). the fuel/oxidizer is still burning stoichiometric, but the generated fire (energy) accelerates the excess hydrogen in addition.

    • waynehale says:

      There are many factors in practical rocket engine design to consider, and the optimum point can be very complex to find. While it is true that lower molecular weight is a factor in improving ISP, it is a secondary effect. Mixture ratio in the SSME is constrained more by turbine temperature than by lower molecular weight in the exhaust.

      • patb2009 says:

        I get running fuel rich helps you keep the peak temps down, but it sounds like you had a dual constraint of not just Pre-burner and Turbopump temperature limits but also
        a limit in the main combustion chamber. Only a fraction of the propellants run through the hot sections, the vast bulk will run through the cold sections as a liquid phase, but a small chunk is burnt in the pre-burner run through the hot section and then into the combustion chamber.

        Now i’ve always focused on the idea that Hydrogen engines like to run a little fuel rich in large part because that increases Isp. If you look at the stuff coming out the nozzle if it burns Stochiometric it’s all Steam so that’s H2O, if you have a bit of free hydrogen in there,
        that’s a very light species and gives you the maximum Exhaust Velocity, which increases Isp.

        if you look at the RL-10, which is an expander engine and doesn’t have any of the rotating bits running very warm at all, you find that the mix ratio there is running 5.8 or so

        To me, that was always driven by the maximization of Isp, but if you say it’s really driven by Main Combustion Chamber Temp limits that would be interesting….

        I’ll have to look at the Creep study we performed on engine cooling tubes and look to see what peak temps and trades we ran.

  11. Kenneth L Glaze says:

    A basic chemistry question: if H2/O2 is the second highest energy possible in the Periodic Table, what is the first? One of the hydrocarbons?

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