Science fact of the day- Liquid rocket engines!

Because solid rockets are not quite effiecent enough and lack control, other types of engines are very needed. Liquid engines make up the other type of chemical rocket, although these two come in two main types.

Pressure Fed engines are the simpler type of engine. these engines will have their fuel tank, and then behind that, there will be a large tank filled with pressurized gas of some kind. whenever they need to burn the engine, they open valves and the pressure pushes the fuel down into the engine so that it can burn inside the combustion chamber. There are a few major advantages to pressure fed engines: for one, theyre incredibly reliable. with other engine types like we'll see later, there's a lot more systems that all have to work better for the engine to work, and if just one fails, your engine could be done, or even worse, your whole rocket could explode. with a pressure fed engine, theyre far more likely to just work when you turn them on, every single time. Which is another advantage- they can be used over and over and over again as long as you have fuel. they're great for missions that will need to do multiple smaller burns, such as those exploring other planets. Cassini, for example, fired its two engines a combined 200 ish times or so in order to line in up to have the best orbit for studying different things about Saturn as well as lining up orbital encounters. Only a pressure fed engine can have that sort of reliability. however, they have a major downside: theyre not super effecient- in fact, theyre often similar to solid rocket motors, in that 2 ish km/s range. still, for the reliability they provide theyre still often the best option.

I should also mention cold gas thrusters- theyre basically this taken to the extreme. they cut out the middle man by just straight up pushing out pressurized gas instead of pushing fuel into a combustion chamber. they're incredibly simple, they're incredibly reliable, and they're like, way less effecient. they do still have a good use, and that's attitude control. if you look at a picture of the space shuttle, you see all these little holes in the front and some itty bitty thrusters on the back- it's a large number of cold gas thrusters that can be used to rotate the spacecraft in literally any way they need. because cold gas thrusters are really really simply and reliable, this makes them the best suited to this tast, which requires precision in firing the engines and reliability to opperate them literally hundreds of times, maybe even within a single mission. these kinds of systems are called Reaction Control Systems (at least it was on the space shuttle) and that's basically the only niche these guys fill but theyre still pretty neat

finally, we have pump fed rockets. these are the real big boys, your F-1s and your RS-25s and your Merlins and your Raptors. The defining aspect here is a turbopump- basically its a pump built to spin at insanely fast speeds (on the RS-25, it was THIRTY THOUSAND rpm), and with high torque too, because they need to to move this much fuel, and again using the RS-25s as an example, it was something like 800 kilos per second for each individual engine, although the F-1 engine burned like 2500 kilos per second per engine, which is like an entire car's weight of fuel in a single second flowing through those pumps. so this raises the question? how the fuck do you power a pump that powerful? well, we have a bunch of rocket fuel lying around, so why not use just a little bit of that? only a very small amount of fuel is actually burned in the turbopump, so its a fairly small price to play for being able to achieve the absurdly high thrusts of rockets. Now, the engines i've talked about here are probably doing more than the average rocket engine, most probably have rpms in the 1-10 thousand range, and handle far less propellant per second, but i like going to the extremes to show the kinda shit these engines do.

You can probably see why these engines are so complicated. However, the benefits are easily worth it. They're the only way you can come even close to the power of solid rocket boosters, and they have the added bonuses of being more efficiency, while also being controllable. while many of them are made to only fire one time, they can usually be gimbled (pointed a little bit to steer the rocket) while executing their burn, and some of them can be operated multiple times in one flight, or at least be refurbished between flights (such as the RS-25). so how do they do on efficiency compared to the other engines? well it actually depends massively on their fuel- which i'll talk about tomorrow so byeeeeee