As noted in the previous post, SpaceX CEO Elon Musk has tweeted that the problem that halted Thursday's launch attempt was with the system that lights the Falcon 9's nine Merlin 1D first-stage engines using a chemical mixture called TEA-TEB.

I asked Ben Brockert (aka @wikkit on Twitter), CEO of Denver-based Able Space Co., to expand on that — and boy, did he ever come through. For the tl;dr crowd (that's how the cool Internet kids say "too long; didn't read"), the money quote: "[I]t was like they expected 180 proof TEA-TEB but only got 100 proof. The fire in the engines was monitored and was less than the launch computer expected, so it shut the engines down rather than proceeding to full thrust."

Here's the full response:

Hypergolic ignition is one of the neat concepts in rocketry that don't have a great parallel in day-to-day life. It's an rare thing that would be scary if it was something that happened often. Hypergolic simply means that, when exposed to each other, two chemicals will burst into flame without a spark or other ignition source.

The ignition process on a rocket engine is critical and must be of high reliability. On a vehicle with multiple engines, if 8 out of 9 lit but one was just dumping un-lit propellant out the end, the fire from the others would ignite that propellant. The fire would then travel up into the engine where it would create a massive pressure spike, definitely destroying the engine and possibly destroying engines nearby. In propulsion testing the euphemism for this is a "hard start" leading to "rapid unplanned disassembly".

SpaceX is solving the problem of absolute ignition reliability by using hypergolic ignition. They use a mix of two chemicals, triethylaluminum and triethylborane, aka TEA-TEB. Each is basically a metal atom (aluminum or boron) holding on to three hydrocarbon molecules (tri-ethyl), ready to break at a moment's notice.

These two chemicals will spontaneously and near instantaneously burst into flame upon contact with oxygen. It can be oxygen in air or liquid oxygen in a rocket engine. The boron in the TEB is what causes the green flame when the engines start. To start the engine, LOX is flowed through the rocket injector into the chamber from the vehicle's tank, TEA-TEB is injected into the chamber to create ignition, then RP-1 (fancy kerosene) is flowed in from the vehicle tank to start burning. The flows are increased, thrust is made, and the rocket launches.

To save weight on the first stage apparently they are flowing the TEA-TEB from a tank on the ground. Elon's tweet was "Abort was caused by oxygen in ground side TEA-TEB. Upper stage on separate internal circuit, so doesn't face same risk."

So there was some oxygen that inadvertently got in the TEA-TEB tank. Could have just been atmospheric oxygen that got in when the tank was refilled, or could have been a bit of oxygen that flowed back in from a previous engine test. That would then react with the TEA-TEB in the tank and reduce its concentration, so that the tank then contained a mix of TEA-TEB and combustion products.

In other words, it was like they expected 180 proof TEA-TEB but only got 100 proof. The fire in the engines was monitored and was less than the launch computer expected, so it shut the engines down rather than proceeding to full thrust.

The other common approach for igniting restartable liquid engines is the "spark torch igniter". A spark plug ignites normal propellants in a little rocket engine, which fires into the big rocket engine. This is what most of the smaller New Space rocket companies use because working with hypergols is unpleasant. Hypergolic chemicals are expensive, usually poisonous, some are carcinogenic, and by nature any spills or contamination can result in unexpected fires.

In theory, hypergols are more reliable. In practice, that seems to not necessarily be the case.

Edited 6:44 p.m. to add P.S.: