With jet engines, it’s relatively easy to light it up and throttle. I know you can’t shut them down mid-flight and then turn them back on while still in the air, but you can easily throttle them. Now with rocket engines, you can only light most engines 1-3 times, and their throttle ability is minimal. Why is this so?
Thanks in advance.
In: Engineering
It varies a lot between engine types. As a general rule of thumb, the more complicated the rocket engine, the harder it is to relight. The only exception to this rule are solid rocket engines that can’t really be shut off in any practical way once they’ve started going.
Firstly, it’s really important to note that a rocket engine needs the same pressure going into the combustion chamber as there is leaving the combustion chamber through the nozzle, or the combusting gases will just flow back into the tanks, which you can probably guess is not optimal. This is where 95% of the complexity of rocket engines comes from: How do I get the fuel up to the right pressure in an efficient way?
There are rocket engines that are actually extremely simple to start, as simple as turning a valve or two. These are pressure-fed monopropellant engines, typically used on reaction control systems for fine maneuvers in space, and they’re so easy to start because they’re nothing more than a pressurized tank or two, a valve, and a nozzle. Turn the valve and the pressurized gas flows out, generating thrust. They’re called pressure-fed because the fuel is already at the same pressure inside the tank as there will be going into the nozzle, so you dont need any pumps.
Pressure-fed engines can get more efficient if you pick fuels that can decompose, like hydrogen peroxide, and in turn generate heat. In terms of complexity its still practically the same thing, just now with a combustion chamber between the valve and the nozzle. If you want to get even more efficient you can use bipropellant engines that take advantage of a property of hypergolic fuels. Essentially, a hypergolic mixture is a mix of two gases that instantly combust when they meet, meaning you dont need any igniters. Open the valve and the engine burns.
It starts getting difficult when you leave the realm of pressure fed engines. The problem is that pressure fed engines are limited by how much pressure you can have in the tanks, which isn’t a lot when you consider the tanks need to be as light as possible. The alternative is to use a pump to get the fuel up to pressure. This has been done in endless different ways throughout the history of spaceflight, and is overwhelmingly the most expensive part of an engine to develop every single time. It’s also the eason why most rocket engines are so hard to start.
On the low end of startup difficulty, you have something like Rocket Lab’s Rutheford engine, which uses electric pumps. That’s pretty straight forward, you just have to spin up the pumps to get the fuel flowing, then find a way to ignite it once its flowing. Easy. The problem is that batteries are heavy, and weight is the biggest enemy of spaceflight.
The other option is to combust a little bit of the fuel you already have onboard to drive pumps. This is called a gas turbine, the most common solution rockets tend to use, with endless variations. The reason this is so complex and difficult to start is because you’re relying on the pump, driven by the turbine, to supply the fuel that drives the turbine. Its a feedback loop. You need something external to start the pump and get the whole thing going. This is the reason why most gas turbine rocket engines arent designed to relight at all, because they use ground-side hardware to get the pumps running. They don’t need to relight, anyway, since they burn once and are discarded once they’ve done their job.
Jet engines aren’t actually much easier to start. They suffer many of the same problems, including that feedback loop of needing the engine to be running to get the fuel flowing. It’s just that with a jet engine, even if it shuts off mid-flight you still have the airflow that can force it to spin up and allow you to relight it. Also, weight on planes isn’t nearly as much of an issue as on rockets, so you can afford to have something called an Auxiliary Power Unit (APU), essentially a smaller jet engine, to spin up the main ones and help them start under any condition.
In terms of throttleability, the problem is, again, pressure, just in a different way. Rocket nozzles have a very specific shape and size because they’re designed to take the extremely high pressure gas from the combustion chamber and expand it, transforming the heat and pressure into velocity. The ideal rocket engine expands the gas until its pressure is exactly the same as the environment, because thats how it can extract the maximum amount of energy from the hot exhaust gas.
For this reason, rocket engines are designed so that the pressure at the very tip of the nozzle is roughly the same as the atmospheric pressure outside when at 100% throttle, where the engine will most often be operating. If they were to throttle down, which is to say, reduce the fuel flow to the engine, now there’s less gas, which means less pressure, which means the pressure at the tip of the nozzles actually drops *below* atmoshperic pressure. This becomes a problem when the pressure difference is so high that the outside air starts to force itself into the nozzle, in a phenomenon called flow separation, which can destroy the nozzle.
If you want more detail, there’s a youtube channel called Everyday Astronaut that explains [rocket engine cycles](https://youtu.be/Owji-ukVt9M) and [engine relights](https://youtu.be/bAUVCn_jw5I) really well, with accompanying graphics that we can’t match through text.
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