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LSET 4: Feed System Design
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By the end of this LSET, you should have a rough understanding of how to size and choose the components needed for a simple pressure-fed engine feed system. In other words, you should be able to (roughly) understand the design of the Helios P&ID (piping & instrumentation diagram) below, and what each component in the system does.
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Pressure-fed Engine Scheme
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Shown above is an example graph pulled from a regulator’s company datasheet
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Shut-off Valves
“Shut-off” valves really just refer to all valves in the system that serve as gates to the flow. We add valves to our system to have control over different steps of operation. This includes propellant tank pressurization, engine firing, system venting, propellant filling, and anything in between. The below table provides a brief overview of some types of shut-off valves that are often used in a simple pressure-fed engine feed system. Note: although I literally just found these pictures off google, choosing valves for your system is a CRUCIAL (and surprisingly fun) area of design for pressure-fed engines. In industry, things get even more interesting, as it is common to design custom valves in-house.
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One of the main things to consider when choosing valve components is the resulting pressure drop through the valve. This is important to consider because if we have many valves in our system that cause large pressure drops, the resulting pressure at the engine inlet will be much lower than desired. A COTS (commercial off-the-shelf) valve will usually have an accompanying flow coefficient from its datasheet. If you know the basic characteristics of your feed system, you can use the flow coefficient equation (discussed earlier) to calculate the pressure drop across each valve and ensure your valve pressure drops are not detrimental.
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Pressure Relief Components
Relief components are extremely important safety mechanisms used in feed systems. Their function is to release pressure from the system if it gets too high. Without pressure relief components, a scenario where the system becomes significantly overpressured could cause catastrophic damage.
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Let's first go over the layout of the Helios feed system. Refer to the key below for all diagrams.
aA. First, we separate our pressurant tank, propellant tank, and engine with the necessary valves. The manual valve opens the pressurant, and it is regulated down to system pressure on both sides.
bB. Actuating ball valves are on either side of the propellant tanks. Check valves are on either side as well to prevent backflow to the pressurant tank and propellant tanks respectively.
cC. Now let’s incorporate the relief valves and fill valves. The LOX side has a relief valve and burst disk for extra precaution.
dD. The fill valves allow us to pour the Ethanol and LOX into the propellant tanks (when unpressurized).
eE. Next, we incorporate all of our solenoid valves. Our main control valves are pneumatically actuated ball valves and have solenoid valves attached that are pressurized with low psi air.f
F. We also added solenoid vent valves next to the relief components, which we can actuate from a distance to depressurize the system after an operation, or in a failure event. It’s not shown in this diagram, but we actually have 2 solenoid vent valves on each side for redundancy.
gG. At this point, the feed system is complete, and we just need to add all of the pressure sensors. Pressure gauges are used to view the live pressure transients over video, and the pressure transducers are used to log the pressure profile.
Closing
There’s still a lot more to talk about for pressure-fed feed system design, and we’ll go over that soon! And this is still just the tip of the iceberg for feed systems in general. If you want to learn about more complex feed systems, look up Gas Generator and Staged Combustion engine cycles. These are the systems used commonly for large industry-level engines, and they introduce another awesome topic: ~turbomachinery~
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