As you can probably tell, I tend to get ahead of myself. This blog has talked about all sorts of crazy different ways of getting into space, except for the two most common methods, which are solid rocket engines and liquid rocket engines. The difference may seem to be subtle, but they are actually quite big. I will talk about solid rocket engines first.
Most people have probably seen a solid rocket engine (maybe even held one in their hands), since they are what are used in model rockets. In fact, they have been used for about a thousand years. The first fire arrows were arrows with a little solid rocket engine tied on to them. Basically, people used to take gun powder, shove it in a tube, tie it to an arrow, and light it. The arrow would travel at much higher speeds than if an archer fired it, but they were also very hard to aim. Before World War II, the largest improvement in the technology came when William Hale, in about 1845, packed the gun powder into a cylinder and had the exhaust come out in such a way that the cylinder would spin, which provided stabilization to the rocket. It also eliminated the need for a big stick.
Did you know that we launched rockets against Mexico City? How crazy is that?
When the Civil War started, we started to use modern artillery, like guns, which were much more accurate and could be used by a vast number of the solders. Rockets basically went out of favor during this time, although there were a few people working on liquid propellant rockets (like Goddard in the early 20th century), just to try to get the ideas right. I will talk about this when I talk about liquid rockets.
Anyways, during World War II, the Germans figured out how to really use rockets. They used liquid propellant engines in order to launch rockets against the allies. Now, the liquid propellant technology was a huge breakthrough, but they also came up with a bunch of other breakthroughs, namely a guidance system to actually steer the rockets on their 200 mile journey as well as a telemetry system to see if they actually were headed in the correct direction. This sounds like a simple thing to do, but this was before computers were invented, let alone GPS. But, that is once again, for another post.
The lesson here is that the Germans developed the underlying technologies that allowed the rocket to go from being a spinning cylinder that would sometimes work, to a supersonic vehicle that could hit a target accurately from 200 miles away.
Once World War II was over, the US and USSR militaries wanted to used rockets for a couple of uses, but one of the most important was for launching atomic bombs at other countries. This was because an airplane could be shot down when trying to deliver an atomic bomb. Also, it could take an airplane hours to get to a target. With a rocket, almost any place on Earth could be hit in 45 minutes or less.
What rocket scientists learned is that using liquid propellants on Intercontinental Ballistic Missiles is not really smart. This is because liquid propellants can’t actually be stored in the rocket – they have to be stored in coolant tanks that keep the liquids very cold. For example, liquid oxygen needs to be stored at about -300° F (-183° C), which is a wee bit cold. So, when someone wants to fire a liquid propellant rocket, they have to fuel it first, which can take a long time.
On the other hand, those old rockets from the 1840s were packed with gun powder and were stable for a many years, and could be fired immediately when needed. A perfect solution for global thermonuclear war! Therefore, scientists developed solid propellant engines. These are pretty simple. You take a solid fuel and a solid oxidizer and mix them together with something that will bind them. You put it in a tube and shape it, and ta-da, you have a solid rocket engine. There is obviously more to it than that, but that is the basic premise. There are a lot of resources out there that will tell you the chemical makeup of solid rocket fuels.
When you pack the fuel (called grain) into the cylinder, where are a wide variety of shapes that it can be. The simplest is packing the outer walls and leave a hole in the center (see wonderful drawing below). The point here is that only the part of the grain that is exposed will burn, so when you ignite this, then a very small portion of the grain starts to burn. Then, as the grain burns away, more surface area is exposed, so more grain can burn. If you are looking up the rocket from the bottom, you will see a little circle of clear area at first, then as the grain burns, the circle gets larger and larger, with more and more grain burning. Since more grain is burning, the thrust actually increases as a function of time, until the outer wall is reached, and the thrust stops, since the fuel is completely depleted. This is called a progressive burn. Model rockets have this grain pattern.
If you pack the grain in exactly the opposite way, with a big circle in the middle, and a thin shell of clear area at the outer shell, then as the grain burns, the circle of grain gets smaller and smaller, which creates a regressive burn.
Interestingly, you can do some strange shapes that will allow the burn to be neutral, where the exposed surface area of the grain remains mostly constant as a function of time. (If you think about it, the area always has to roughly be the area of outer shell, since this is the final area.) This is what the space shuttle’s solid rocket booster engines used – an 11-point star configuration for a roughly constant burn for the entire 104 seconds that they thrusted. More on the space shuttle’s solid booster in another blog post!
One huge disadvantage of solid rocket boosters is that once you fire them, you can’t stop them – they burn until all of the grain is used up. With a liquid propellant rocket engine, you can throttle the engine and even turn it off and back on. There is none of that control in a solid rocket engine. Engineers design the shape of the grain to give them the type of burn that they want, and when it is lit, it goes. This can be somewhat mitigated by making the top of the rocket so that it can pop off, exposing the grain at both the top and bottom, allowing the rocket to thrust both down and UP at the same time. This creates a balance in the (upward and downward) forces, allowing the rocket to go nowhere. That is pretty insane, and, I would imagine, quite dangerous, but it is something.
Another disadvantage is that, because the rocket exhausts all of the fuel in one go, the rocket basically can’t do any final adjustments. Therefore, the uncertainty on where it will end up can be rather large. For nuclear weapons, being off by a few (10s of) miles is not a big deal, since there are a lot of warheads on each one and they all go sort of cluster-bomb at the end anyways. If you want to get something to orbit, being off by a few (10s of) miles can be somewhat bad.
Ok, let’s summarize. Some advantages of solid rocket engines:
- Solid rocket engines are dead simple and are therefore easy to design and cheap.
- You can store a fully fueled solid rocket engine for a long time and fire it whenever.
- Solid rocket engines can be strapped on to liquid propellant rockets to give them an initial boost. Hence, solid rockets are sometimes called boosters.
- Did I mention that they are so simple that you can buy them at Walmart?
- Once you ignite the solid rocket engine, it will fire until used up.
- Because they can’t be relit, they are not super accurate.
- They are almost always not as powerful as liquid propellant engines. I didn’t talk about this, but it is true!
Solid rocket engines end up being used as strap-on boosters and intercontinental ballistic missiles, and are not used very often to put stuff into orbit on their own. That is, unless you don’t have a lot of money and need to get to space for cheap. You can pick up a bottom of the line Pegasus-XL for about $28,000,000 from Orbital-ATK (and build a paper one for free!). And yes, they accept bitcoin.
- My pretty, pretty drawings.