Okay. So now let's talk SRBs (solid rocket boosters) because they are really fascinating.
Why do we use them? Basically they are small compact little tubes of insane power. One of the Shuttle's SRBs kicks out 80% more power than the F-1 engine (5 of those took the Saturn V up on the Apollo missions). They are bitchingly efficient. Only problem? You cannot turn those things off. They burn until they are done. I'll get to that in a minute. So once they were lit the Space Shuttle was going up no matter what. That's why they lit the main engines first (to make sure they were good). And then once the Shuttle came back from the twang they lit the SRBs.
So how do the SRBs work exactly? So it's obviously a big metal tube. My original thought was that burnt like a candle. And it could. But that's a bad way to burn it. The real way it burns is from the inside out. So imagine filling that tube with a bunch of donut-shaped propellant (known as grains). That's basically how the SRB looks on the inside. It's filled with propellant and it has a hollow core. It actually also has some gaps between the grains but that's not important to understand how it physically burns. When the SRB is lit, it is lit along the entire length of the SRB igniting this inner hollow core. The SRB then burns from that center cylindrical hole to the outside. The benefit of this is that you get a nice even burn and thrust through the 2 minutes. And while I can't verify this I'm pretty sure you can 'tune' the SRB to what you need by altering the dimension of the tube. A wider tube will burn longer. A longer tube will provide more thrust. Make sense? Totally fascinating. Here's a video to show how it works.
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