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
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Solid rocket engines: it’s like lighting a firecracker. There is no off switch.
Liquid rocket engines: they just require a lot of fuel and energy to start up. It’s not that practical. They can control the throttle to an extent. However, they are generally designed for very specific high-output applications (like launching a satellite into orbit) so they don’t generally need to be designed to work in a variety of conditions.
Even from your example with regular jet engines, it actually is not that easy to start them up like a car. It takes a huge amount of power (and time) to get the compression high enough to start producing its own power.
Startup is expensive in terms of power because the initial reaction needs to be started. Things like turbopumps that are their own gas turbines need to start before that can happen.
And rocket engines are hot and violent things… the mechanical wear is Huge.
Liquid fuel engines can be throttled some. The engines on the space shuttle could be throttled from 67% to 109% of rated power. The problem with lower power levels is that at some point the gas coming out of the throat of the combustion chamber can’t fill the nozzle fully and tends to stick to one wall. This makes the thrust very asymmetric and is a very bad thing. There are deeply throttleable liquid fuel engines that have been developed, but they aren’t very high thrust.
>i know you cant shut them down mid flight and turn them back on while still in the air
Inflight restarts after flameouts are absolutely a thing you can do and even train for, provided you didnt shut them down with the fire suppression system. In fact, an inflight restart has the plane’s own airspeed working to assist this, as the airflow into the engine causes it to passively windmill, which can and does provide enough spin in the engine for a re-light to have enough compression to self sustain again. On the ground, you would need bleed air from the APU or another already-running engine to get the thing to start turning.
You typically *dont* do an inflight restart because of the reasons that caused you to shut it down in the first place
It basically comes down to complexity. The scale goes something like:
steam engine < Otto engine < turbine engine < rocket engine
In terms of complexity of combustion and thermodynamics.
A rocket engine is generally running right on the edge of what’s physically possible. If you disturb it too much, bad things tend to happen.
Moreover a lot of (first stage) rocket engines are designed for single use, and are most efficient on their highest power setting. There’s just no reason to add restart or deep throttle capabilities. The recent development of reusable first stages has somewhat changed that, but even on reusable rockets (Falcon 9) only 3 out of the 9 engines have the ability to restart, because for the others that’s just not needed.
Rockets involve very explosive fuels. So the trick is to ignite them without exploding them. Many of the engines now in use the cyrogenic reactants to cool the bell nozzle, and they use the heat of the bell nozzle to turn the cyrogenic liquid into gases – so without the heat of the burning fuels, you have to have other systems to prepare the fuel for ignition.
Solid fuel rockets like the space shuttle booster don’t have any way to turn them off. Once you ignite them, you are going….someplace, hopefully space.
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.
There are rocket engines you can start up hundreds of times. They are used to control the orientation of spacecraft or do small course correction maneuvers.
For the big rocket engines that launch things to space there is often no reason to ignite them more than once. You start them up, you run them until their stage runs out of propellant, you shut them down. They might throttle down a bit on the way, but running them at very low thrust would be inefficient.
For the first stage you can install some of the startup hardware in the launch pad instead of the rocket. That way it doesn’t add to the rocket mass.
Big rocket engines are incredibly optimized for their specific task – produce as much thrust with as little mass as possible. Let’s compare this to aircraft engines: The [Rolls-Royce Trent 900](https://en.wikipedia.org/wiki/Rolls-Royce_Trent_900) (powering e.g. the Airbus A380) produces ~350 kN of thrust with a mass of 6.2 tonnes. The [Raptor](https://en.wikipedia.org/wiki/SpaceX_Raptor) (the engine of Starship) produces 2600 kN of thrust with a mass of just 1.6 tonnes. That’s 7 times the thrust with 1/4 the mass. Rocket engines are always just a second away from destroying themselves. Everything needs to work as planned – that is easier if you can keep them running continously.
I’d guess the most famous engine with throttling ability would be the Apollo LEM descent stage. It was restarted 4 times on Apollo 9 (earth orbit test), though the design criteria was two restarts. Wouldn’t be a first choice these days due to hypergolic propellants I’d think, but it was mid-1960’s tech. There’s development these days on a throttling RL-10 (a 1960’s vintage that’s been upgraded over time), since NASA wants a much more powerful engine for future lunar use.
Rocket engines and jet engines are vastly different things though – jet engines use spinning vanes to increase air and fuel pressure, rockets require pressurization or pumps, or even a “mini rocket engine” that functions as a massive fuel and oxidizer pump.
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