How Rocket Engines Work, Part 2: Plumbing

This time, we’re covering the plumbing of liquid fueled rocket engines. We learned last time that liquid engines are by far the most complicated engines to produce and run – but why is this?

Liquid engines, like all rocket engines, need a fuel and oxidizer. While these can just be packed together and burned in a solid rocket, liquid fueled engines require pipes to bring the fuel to the engine nozzle (the hole on the bottom).

Image result for space shuttle external fuel tank

Take the space shuttle for instance. It uses 3 liquid fueled engines at the bottom of the orbiter (the part that looks like a plane). The fuel for the engines is stored in the external tank (the orange thing) and is pumped into the engines via some pipes. Even in normal rockets, getting the fuel to the engine nozzle is very hard and requires a lot of thinking.

Generally, the fuel and oxidizer meet in a combustion chamber, where they are burned. The gas that comes out expands in the nozzle and pushes the rocket forwards.

The main problem is that rocket engines need to burn a lot of fuel very quickly. They can’t rely on gravity to push fuel down into the engine or use a weak pump like the one that pushed gas into the engine of your car.

Rocket scientists have come up with many ways of getting the fuel and oxidizer together:

Pressure-fed Engine:

This is by far the most simple type of rocket engine. A pressurized gas is heated so that it expands, then is used to force fuel and oxidizer into the engine. It is very safe, as the engine can be turned on and off by opening or closing the fuel and oxidizer valves.

The OMS (Orbital Maneuvering System) on the Space Shuttle consists of pressure-fed engines, and uses a mixture of monomethyl hydrazine as fuel and dinitrogen tetroxide as oxidizer. These compounds instantly ignite when mixed together (this is called hypergolic fuel) and remove the need for an igniter. All you need to do is pressurize the system, and the fuel will do the rest of the work.

Weak rocket engines are also pressure-fed, such as the AJ-10, which was used on the Delta II rocket’s second stage.

However, pressure feeding doesn’t work on larger engines that need to be more powerful. More powerful engines rely on more propellant going into the combustion chamber in a shorter time. This can be achieved by adding more pressure, but it is clear that the pressure cannot be increased beyond a certain point, as the pipes will break.

In summary:
Advantages: Safe, simple
Disadvantages: Weak, not scalable to larger engines.

Expander Cycle:

This next method is also fairly simple and reliable.
The Expander Cycle consists of a turbine, which sucks and pumps the fuel and oxidizer into the engine. We see turbines and pumps in a lot of modern liquid engines.

Fuel is taken from the fuel tank and is pumped around the engine nozzle, which gets very hot when the engine is burning. This makes the fuel expand and take up more space, which forces it into a turbine and causes it to pump fuel and oxidizer into the engine. After going through the turbine, the fuel goes into the combustion chamber. This process is self sustaining, meaning that it can keep going on.

The combustion creates heat, which pumps in more fuel and oxidizer, which makes more heat.

An interesting benefit of this design is that the engine nozzle stays very cool, due to the fuel absorbing the heat of the engine. Also, starting the engine and shutting it down is almost as simple as with a pressure-fed engine – all you need to do is open and close some valves and ignite the fuel at the right time – no need for complicated ignition systems. Due to the fact that the expander cycle engine is simple, it is also very safe to use and reliable. That’s why the Delta IV uses the RL10 expander cycle engine on its upper stage.

RL-10 with cutaway.JPG
The RL10 engine. If you saw the last post on rockets, this image was one of the real-life examples.

The expander cycle is used typically on upper stage rocket engines – meaning that they are only used when almost in space or already in space. Expander cycle engines produce much less thrust than first stage engines, because they rely on the expansion of fuel to work.

The usual fuel of choice is liquid hydrogen, which is stored at around -250 C or -420 F, so that it expands a lot when heated.

A mathematical problem arises when you try to make an expander cycle engine bigger. The surface area of the nozzle increases 2 dimensionally, while the amount of fuel needed to be heated increases 3 dimensionally (volume), and eventually a point is reached where the engine cannot heat the fuel enough to flow through the engine.

Advantages: Safe, simple, efficient
Disadvantages: Cannot be scaled up to larger engines.

This next section is where we get into really powerful engine designs. All of the following engines use turbopumps to drive fuel into the engine, rather that pressure or expansion due to heat.

Gas Generator Cycle:

The next type of engine that we’re covering covers some of the most famous engines known to date.  The Rocketdyne F-1 engine was used on the Saturn V rocket, used to send people to the moon.

This may seem quite complicated at first, but the principle of operation isn’t too hard to grasp. The pumps used to move fuel and oxidizer into the combustion chamber have to be turned somehow. In the expander cycle engine, it was the pressure from the expanding propellant that turned the turbine. Here, it is a mini-rocket engine that allows the turbine to spin.

Some of the fuel and oxidizer goes into another combustion chamber, the preburner, which produces hot gases and turns a turbine, pumping more fuel and oxidizer into the engine.

This allows for a very powerful engine, since you can get fuel into the engine more quickly. However, note that all of the propellant going into the preburner is wasted, as it is dumped out the side of the engine.

Gas generator engines are a lot more complicated to use compared with pressure-fed or expander cycle engines, but are a lot simpler than other engines in its power class. Gas generator engines are lighter than staged combustion engines because the spent gas can just be thrown away.

For instance, in a staged combustion engine (which I will cover below), you have to push the gas into the combustion chamber, which means you need more pressure and need to work the engine harder.

However, despite being somewhat inefficient, gas generator engines are very popular and are used on the majority of first stages.

SpaceX Testing Merlin 1D Engine In Texas.jpg
SpaceX’s Merlin 1D uses a gas generator cycle. Note the main flame coming out of the centre, and some extra gas coming from the side. That is the preburner exhaust. Even though that part of the fuel is wasted, the Merlin 1D is the most powerful engine in the world for its size.

Advantages: Powerful, lightweight when compared to other engines in its class
Disadvantages: Inefficient

Staged Combustion

Finally, we arrive at one of the most interesting engine designs: staged combustion. Remember how we discussed the fact that gas generator engines throw away the preburner gas and lose efficiency? The goal of staged combustion is to raise the efficiency of the engine. This combustion process has made some of the most powerful and efficient engines in the world.

Staged combustion is just like a gas generator engine, except the gas that comes out of the preburner is injected into the combustion chamber. This allows 100% of the propellant to be used in some form for moving the rocket forwards, and therefore increases efficiency.

This might sound easy – you just put one extra pipe on the engine, but keep in mind that the combustion chamber is a very high pressure. If the preburner doesn’t have enough force to drive the gas into the combustion chamber, it will leak back out!

Making a staged combustion engine work is a tricky balancing act. The Space Shuttle Main Engine exploded several times during tests before it finally worked.

The main problem is that fuel has to be split up while oxidizer flows directly to the engine. This puts a lot of stress on the engine.

Note: The example shown above is “fuel rich” combustion, meaning that the gas coming from the preburner has a lot more fuel than oxidizer. Oxidizer rich engines have also been made, but they are harder to make because the metal inside the engines can get destroyed from the oxidizer.

Full flow staged combustion aims to solve the problem by making both propellants flow at similar rates. However, it is even more complex and more expensive.

An example of staged combustion engines are those used on the Space Shuttle.A rocket engine firing. A blue flame is projecting from a bell-shaped nozzle with several pipes wrapped around it. The top of the nozzle is attached to a complex collection of plumbing, with the whole assembly covered in steam and hanging from a ceiling-mounted attachment point. Various pieces of transient hardware are visible in the background.

Staged combustion engines perform very well, but they are complex, cost a lot, and are heavy.

Advantages: Efficient, powerful
Disadvantages: Expensive, heavy, complex, prone to failure

In conclusion, liquid-fueled engines can take on a variety of forms. Each is useful in its own applications, and each has its own advantages and disadvantages.

Next time, I will be looking into other forms of propulsion for rockets. I will post some other stuff in the meantime. If you want me to elaborate on anything of cover something I haven’t already, leave a comment below or on this page.
Otherwise, thanks for reading!

Stay tuned and stay sciency,

Mingde

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