Jun 11, 2013


Since my little visit to a solid fuel rocket test a while back I've been reading up on them. Turns out my guesses on how they work is completely wrong. Before we get to that, video.

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.

So what burns inside the SRBs? Now this is really interesting. I had no idea what they were burning. What they are burning for all practical purposes is metal. The propellant is made up of a few things. First is ammonium perchlorate. This is just the oxidizer. Or in other words it provides oxygen in sufficient quantity that the burn isn't left wanting for more. Mixed in with that is aluminum. This is the fuel really. Aluminum burns? It does. It's very reactive. Tough to start burning but once it burns it burns well. The reason we don't think of it as reactive is that when it does react (e.g., aluminum combines with oxygen) you get aluminum oxide or rust. But wait a second, aluminum doesn't rust. In fact it does. Whenever you are looking at aluminum you are looking at aluminum rust. Unlike iron oxide (the more familiar rust) it's a very stable covering and doesn't crack and fall off or change color. The third ingredient ironically is rust (iron oxide). It acts as as a catalyst for the reaction and not much is needed. And finally a rubber is incorporated which makes the homogeneous mixture hold together.

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