Rockets, on the face of it, sound like fairly simple devices.

You have a large cylinder with fuel, one end mounts the engine to burn that fuel and make it go, and at the other end, you put something you want to go somewhere else really quickly.

It’s the engine, though, that’s not as simple as you might think. Because what it does is control one or more potentially unstable fuels, and explodes them in large quantities in a controlled way — controlled enough to get the rocket going, but not to make the rocket disappear into a fiery ball of fire. Petrol cars are also fuelled by explosions, but those are contained inside the engine block, and fairly small in comparison.

But why even do it this way? Aircrafts, after all, don’t need barely controlled explosions to fly, right?

The first one has to do with the fact that rockets usually travel to places where there’s little or no air left. Airplanes can use lift from their wings to fly, and need engines only to provide enough thrust to keep that air rushing over its wings.

However, that doesn’t really preclude rockets from using wings to generate additional lift. What does, however, is the “tyranny” of the rocket equation. The equation itself, of course, has no ambitions to subjugate earth, but what it describes is the answer to a fairly simple question: Given a payload (and an empty rocket) with a certain weight, how much fuel do you need to get to low earth orbit, for example?

And it’s here where it starts to get tricky: Because if you want to launch a satellite into orbit, you need to add a certain amount of fuel to get it there. But adding that fuel adds more weight to your rocket, so you need to add even more fuel to account for that additional weight. Except it’s even heavier now, so you need to add even more fuel, and so on.

For this reason, the rockets themselves are kept as simple and light as possible, so as to allow as much fuel as possible to go towards fighting gravity, and getting the payload where it needs to go. Therefore, stuff like wings, which only work in the lower parts of the atmosphere anyway, are left off.

The other factor in a rocket engine is the fuel it burns. A lot of research has gone into different kinds of rocket fuel. One of the most popular today is actually fairly simple: highly refined Kerosene (RP-1), and liquid oxygen. RP-1 has certain properties that make it very efficient to use inside the atmosphere, it is easy to store, and more importantly, it’s comparatively cheap and easy to produce. Liquid oxygen is also plentiful, and easily available.

When in the vacuum of space, another fuel mixture is preferred: That of liquid hydrogen and liquid oxygen. It is more efficient in the vacuum of space, and brought the Apollo astronauts from low earth orbit to the moon and back, for example.

Both of those fuel combinations have one drawback, though: The engine needs to ignite them, even in the vacuum of space (if it’s an engine capable of restarts), before being able to generate thrust. This is unsuitable for other kinds of engines, like those used for attitude control. Those should ideally be small and simple. Chemistry has a solution for that, though: Hypergolic fuels.

“Hypergolic” means that two fuels spontaneously ignite when they come into contact. A popular combination today is Monomethylhydrazine (MMH) and Nitrogen tetroxide (NTO), which is used in the Draco thrusters of the SpaceX Dragon spacecraft, for example.

Hypergolic fuels is also where we get into some positively hard to handle, toxic, and horrifying fuel mixtures. For example, Hydrazine and nitric acid earned the nickname “Devil’s Venom” by Russian rocket scientists for its role in the Nedelin catastrophe.

And then there’s Chlorine trifluoride. Its effects are best described by John D. Clark in his book “Ignition!”:

It is, of course, extremely toxic, but that’s the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water—with which it reacts explosively. It can be kept in some of the ordinary structural metals—steel, copper, aluminum, etc.—because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminum keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.

And:

[Rocket engineers] nevertheless knew enough to be scared to death, and proceeded with a degree of caution appropriate to dental work on a king cobra. And they never had any reason to regret that caution. The stuff consistently lived up to its reputation.

(John D. Clark, “Ignition! An Informal History of Liquid Rocket Propellants”, Rutgers University Press, ISBN 0-8135-0725-1, p. 73-74)

Unsurprisingly, Chlorine trifluoride is not used in modern rockets due to its properties.

So. Rockets. On the face of it, fairly simple devices, but in reality, they are quite complicated, and it’s a small miracle that they don’t explode more often, in my opinion.

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