Nonetheless, I dutifully started reading through the more approachable sections, and I came across an interesting topic I haven't discussed before: the selection of the optimal propellant feed system. Most of my rocket science posts on Astronomical Returns have focused on the propellant itself, but having the perfect fuel and oxidizer is pointless if you can't get it out of the tanks and into the combustion chamber! Propellant feed systems tend to fall into one of two categories: those that use pressurized gas, and those that use turbopumps. There's no simple rule for choosing between the two, as mission requirements vary drastically and materials science is always advancing, but in general pressurized gas has been used for simpler, lower impulse propulsion systems while turbopumps have been used on larger vehicle applications

Starting with pressurized gas: a separate, smaller tank storing a gas at high pressure (about 3,000 - 5,000 psi) is connected to the main fuel and oxidizer tanks inside the rocket, with a start valve and a regulator to ensure the gas always flows out at the proper rate. Technically any gas could be used, but modern applications all use helium due to its low molecular weight, low boiling point, and non-reactivity. As propellant is consumed and the pressure in the tanks starts to drop, the helium is pumped in to pressurize the vacuum and force the propellants down out of the tanks and into the combustion chambers for the engines to burn. At its simplest, the cold, pressurized helium can be pumped directly into the tanks, but better performance can be achieved by first running the helium around the combustion chamber, where fuel is already being burned, as a heat exchanger to heat up and expand the helium as much as possible, maximizing the volume and reducing the mass of helium needed to be carried 

Notable past and present pressure-fed propulsion systems include the Apollo Command Module main engine, Lunar Module ascent/descent engines, the Space Shuttle reaction control systems and orbital maneuvering systems, the AJ10 used in the Delta II second stage, and the SpaceX SuperDraco engines

While pressure-fed systems are quite simple and reliable, the helium is another consumable that adds weight to the rocket, especially since the helium tank needs to be very thick to contain the high pressure. Given practical limits on tank pressure, larger propulsion systems (i.e. virtually all 1st stage engines) must use turbopumps to generate the pressure necessary to pump propellant into the combustion chamber. The advantage here is that only low inlet pressures are required since it's the turbopump's job to raise the pressure of the propellants, avoiding the need for heavy, thick-skinned tanks. How turbopumps do that is actually quite interesting (and new to me; prior to researching this post, it was one of those things that just "magically" happened) - an impeller (the spinning wheel) is made to rotate, imparting centrifugal force to the fluid that translates into increased velocity and pressure via conservation of angular momentum. This motion generates suction that draws in more propellant from the tanks, while high pressure propellant can now be discharged into the combustion chamber 

It's worth noting, turbopump assemblies still need some pressurized helium, both to spin up the turbopump at ignition sequence start as well as to fill the ullage (empty space) in the propellant tanks to prevent them from collapsing as they're depleted (think what happens to an empty plastic bottle if you try to suck the air out). Additionally, the intricate plumbing adds to the engineering complexity of the rocket: the power source of the turbopump is a separate discussion in and of itself! (Long story short: some of the fuel and oxidizer is burned separately from the combustion chamber, and the hot gases from that reaction power the turbine. See here). But when big engines are consuming vast quantities of propellant, turbopumps are the way to go!