Propellant Feed Systems - Pump or Pressurize?

"Success in space demands perfection... unless [the engines] operate flawlessly first,
none of the other systems will get a chance to perform
- Wernher von Braun


As someone who works at SpaceX as a financial analyst, not an engineer, I don't exactly need to know advanced rocket science to do my job. But it's still super helpful for me to understand the major components of a rocket, especially from an accounting and cost perspective. Over the past year I've been trying to expand my knowledge beyond just the top level assemblies (engines, first stage, second stage, payload) and go one level deeper (for example: thrust chamber assembly, gas generator, composite overwrapped pressure vessel, etc). To that end, my SpaceX engineer roommate sent me a PDF of the book Design of Liquid-Propellant Rocket Engines, which is apparently like the Bible for aerospace engineers. In my opinion, he's either vastly overestimated the extent of my self-taught math and engineering knowledge, or he's intentionally trying to send me down a rabbit hole of rocket science torture! 

I don't think anything I learned for my finance degree is going to help me out with this one :(

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

The Saturn V burned 40,000lbs of fuel per second. Obviously, gravity alone isn't going to get all that into the combustion chamber!

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

Two excellent diagrams showing the flow of helium pressurizing the propellant tanks as they're being depleted

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 

The propellant is drawn down from the tanks via suction, spun around in the impeller, and discharged to 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!

Turbopump cutaway of the RL10, used on the Delta IV and SLS upper stages


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