Hopefully if you've been reading along for the past 16 months that I've been writing Astronomical Returns, or maybe even stumbled upon my primer on rocket fuel, you're familiar with RP-1 (a highly refined form of kerosene) and liquid oxygen as being one of the most common rocket propellant combos since the dawn of the Space Age, fueling everything from the Saturn V of the Apollo Program to the Falcon 9 of today. But just as no car can run without its spark plugs, no discussion of rocket fuel is complete without understanding the ignition source that lights the engine! 

Rocket engines all have an extremely complex system of turbopumps to get the fuel into the combustion chamber, but once it's in there, the ignition system has a single, crucial job: ignite the fuel IMMEDIATELY and evenly. The reason it has to happen ASAP is because if excess fuel pools in the combustion chamber, and then ignites all at once, it can over-pressurize and cause the engine to explode (known euphemistically as a "hard start"). Think of it like a gas barbecue grill - you'd never open the propane tank, then let the gas collect for several minutes before turning on the ignition... unless the goal is to blow your eyebrows off 

Operational rockets today usually use one of two ignition methods: electric pyrotechnics or pyrophoric ignition fluid. Starting with electric pyrotechnics, these systems in rockets are conceptually no different than automobile spark plugs: a high-voltage electrical circuit makes a spark jump across a gap that's exposed to the rocket fuel, igniting it. But since rocket fuel is so energetic and under such high pressure, you can't just light the fuel at a single point, since that localized energy spike can produce uneven shock waves or cause unlit fuel to pool dangerously at the other end. Everything needs to light at once. To solve this, both the J-2 engine that powered the Saturn V upper stage and the Space Shuttle Main Engine (two famous hydrogen-powered engines) had a mechanism that swirled the hydrogen and oxygen at the moment of ignition before blasting it into the combustion chamber

In contrast to hydrogen, RP-1 is surprisingly hard to ignite despite being a petroleum-derived product, and electric pyrotechnics proved unreliable, so pyrophoric ignition fluids are typically used for RP-1 burning engines. The term pyrophoric describes any substance that spontaneously combusts with air, no ignition source needed. For engine ignition, rocket scientists found that a ~15/85 mixture of triethylaluminum-triethylborane (TEA-TEB) was so pyrophoric, it could spontaneously ignite cryogenic oxygen so that by the time it reached the combustion chamber, the already-ignited oxygen would burn nicely with the RP-1 

While also reliable and flight-proven, TEA-TEB has a few drawbacks. It's highly toxic, and the pyrophoric quality that makes it useful in the first place unfortunately requires that it be carefully stored in pure nitrogen when not in use. Plus, since TEA-TEB is yet another consumable the rocket needs to carry, the number of engine restarts you can perform is limited by your TEA-TEB supply (particularly crucial for Falcon 9 booster reentry burns)

Well there you have it - the two families of rocket engine ignition systems! Keep in mind, the above only pertains to standard bipropellants like RP-1 or liquid hydrogen burning with liquid oxygen. Hypergolic propellants like UDMH/N2O4 don't need an ignition source because the fuel and oxidizer spontaneously ignite on contact, simplifying engine design (the downside is hypergolic propellants offer slightly lower performance and happen to be extremely toxic)

There is one other fun fact I found worth sharing. The decades-old Russian Soyuz that still flies to this day uses an incredibly rudimentary yet unbelievably reliable ignition source: enormous T-shaped wooden matchsticks inserted into each combustion chamber with a pyro charge. You can read up more on it with this great Popular Mechanics article!

And broadly speaking, to learn more about the rocket ignition systems described above, I recommend this page here

Hailing back to my high school science days (Mrs. Singh, Mr. Bunch, and Mrs. Brown, this one's for you!!), here's a brief overview of the concepts in thermodynamics that define rocket engine ignition and any other combustion reaction

As a basic definition, a chemical reaction occurs when compounds break their existing chemical bonds and rearrange them into new compounds. Even for exergonic reactions (thermodynamically favorable, meaning the the Gibbs free energy** of the products is less than that of the reactants), there is a required "activation energy" that must be externally supplied in order to break the initial bonds and start the reaction. Assuming this energy barrier is met and the reaction begins, the energy released by the reaction can continue to sustain it for as long as there are available reactants

This is best visualized by the chart below: 

How does this appear in practice? Consider the example of a match: it's thermodynamically favorable for phosphorus to combust with the oxygen in the air, but matches don't spontaneously catch fire. I have to supply the activation energy of striking the match, producing friction, to break the initial chemical bonds. Once I do and the fire starts, it will continue to burn until the match is spent. Rocket engines are no different - all the ignition systems discussed above are just glorified ways of "striking the match" to provide the activation energy necessary to ignite the propellants

And in the case of hypergolic propellants (meaning they instantly combust on contact), this simply means that the activation energy is so negligible, the motion of the molecules at room temperature is all the energy needed to begin the reaction! Pretty neat huh? All these years later, high school is still serving me well