Tending the Fire in the Sky


Words are only painted fire; a look is the fire itself. –Mark Twain

F_N =( \dot{m}_{air} + \dot{m}_f) V_{j} – \dot{m}_{air} V

That’s the violence hanging in the air, waiting for you to torch it off. Starts simple, starts at a cold standstill. Tons of metal locked inert, waiting. Fill ‘er up.

Then:

\dot{m}_{air}     is the rate of flow of air through the engine
\dot{m}_f     is the rate of flow of fuel entering the engine
V_j\;     is the speed of the jet (the exhaust plume) and is assumed to be less than sonic velocity
V\;     is the true airspeed of the aircraft
(\dot{m}_{air} + \dot{m}_f) V_j     represents the nozzle gross thrust
\dot{m}_{air} V     represents the ram drag of the intake

Say what? All I know is the magic incantation of “Starting Engines Checklist,” the ragged rush of high pressure air channeled by a flick of my wrist into the right engine starter. The brute force of hot air at 45 PSI drives the rotor blades like Niagra Falls spins the turbines that light half of the east coast.

Fuel lever up, wing spar and engine shut-off valves snap open and dual high-pressure pumps ram jet fuel through lines metered by a bank of computers in the lower deck below your feet: spray nozzles, burner cans and a whomping thud as the pressure builds and the dragon breathes a ring of blue fire, a scorching gale at 700 degrees and a hundred miles an hour that would knock a dumptruck sideways. Seen it myself.

Now we’re cooking, smoothly whirling a blowtorch driven series of rotors, compressors and turbines idling at 30,000 rpm and 400 degrees centigrade. You’re saddled up, strapped on–never felt better than to have a fistful of thrust to move you and the metal at mach speed, whenever you say so.

And there are those who live with the aggregation of interlocking numbers, the formulas and structures of chemical reactions that gather in your right hand and though everyone riding the cliche in back thinks you’re that guy–you sure ain’t.

It’s never been about the fifty-headed abacus of numerical relationships that while you have to acknowledge put the beast together, forged of alloys and bonded of thousand degree welds and strung with heartstrings of titanium and vessels coursing with combustibles of unspeakable explosive energy, channeled just feet from where you sit in a controlled explosion that will continue for hours–you aren’t even thinking about ground stuff, things that don’t move–because when it’s all in play, we move like lightning.

That’s the real stuff–don’t give a damn about the paperwork or the tons of pulp and blather to make everyone riding the fire not notice that they are.

But they are.

And every flinch of an engine indication, the jet’s EKG synthesized on a bank of CRTs before you, and every nuance of the fuel burn and the hand-in-hand air nautical miles per pound of fuel, every bit of that is the pulse you feel and notice with the slightest shift, tending the fires.

Everything in the sky once you’re there is paid in the currency of fuel. Every air mile is a consumable and there’s only so much on board. Don’t know so much distance and altitude as I do minutes of fuel.  Don’t really care.

It’s that glass blue flame, the thousands of degrees and the 450 miles per hour cooling and feeding the twin blazes that gulp the air then blast it out the other end with fifteen times as much force. It’s out tons of steel and fuel and bone and flesh arched overhead and flung across the sky, dragging the twin white vapor wakes that testify to the tremendous engineering wonder holding us up like it was easy. And it won’t stop till I say so.

Some say the world will end in fire, some say in ice. –Robert Frost

I have my own idea.

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13 Responses to “Tending the Fire in the Sky”

  1. Whenever I fly, I always think about the fire. It’s awesome and frightening at the same time. But I never forget it’s there.

  2. Lego Spaceman Says:

    As a propulsion systems engineer, this brings a tear to my eye.

    If you like the giddy-up in those CFM-56 engines, you are going to love the GE90s that power the 777.

    • Tell me more about your job–might make a good podcast interview some day.

      • Lego Spaceman Says:

        You don’t want me. I am a boring systems guy (fuel, bleed air, fire detection / suppression, electric, hydraulic, etc.). I put all of these on an already functioning engine.

        You want a guy from GE, PW, or RR. They are the brains behind the turning and burning.

  3. Joel Genung Says:

    And the thing that amazes me is all that magnificent power derives itself from one moving part: the central shaft. Sure, there are other components inside that engine doing their separate thing but the beauty is in the simplicity. What a great post, Chris!

  4. Chris,
    You express perfectly the wonder of science and engineering which is the modern airliner.
    I watch the huge machines gracefully making their way through the sky and then, with a bump, remember the violence which makes it possible.
    I then marvel at the skill of those who made this violence controllable and placed control in the hands of the man up front in such a way that he does not have to be the god of fire to manage it.

  5. Charlie Jones Says:

    Bravo! A true poet (of the science nerd variety)!

  6. Such a great read. Thanks again, Captain. 🙂

  7. I’mm interested in being a B738 pilot for Copa Airlines in the future. I like a lot what you write and would like to recomend you write a book as your perspective of aviation and flying is amazing and for sure people would like to have them all compiled in their hands.

    Btw, check out http://www.airliners.net…For sure you will like it. 🙂

  8. Love the off-the-wall way you chat about the mechanics of lighting a controlled explosion off! Glad to have found the blog. Come visit me over at mine from time to time 😉

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