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LSET
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4: Feed System Design
For any questions: Original Author: Alexander Hodge '22, ahodge@mit.edu, or Slack DM
Brief Intro
“The propellant feed system of a liquid rocket engine determines how the propellants are delivered from the tanks to the thrust chamber. These systems are generally classified as either pressure fed or pump fed. The pressure-fed system is simple and relies on the tank pressure to feed the propellants into the thrust chamber. This type of system is typically used for in-space propulsion applications and auxiliary propulsion applications requiring low system pressures and small quantities of propellants. In contrast, the pump-fed system is used for high pressure, high performance applications.” ->NASA Encyclopedia of Aerospace Engineering
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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.
1. Pressure-fed Engine Scheme
Let’s start by looking at a simple pressure-fed system.
The image to the right is a diagram of a simple pressure-fed engine. We will ignore the heat exchanger, and assume the pressurized gas feeds directly into the fuel and oxidizer tanks.
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2. Tanks
From the above diagram, 3 tanks will be necessary to effectively feed propellant to the engine, with 1 pressurant tank for both the fuel and oxidizer. For these problems, let’s assume that we have already been supplied with a proper pressurant tank, and only need to design the propellant tanks. Each propellant tank now needs to be properly sized & chosen to ensure that:
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There’s a lot of constraints and conditions that go into the design of propellant tanks, but we're only going to cover the basics for now. We can justify this because our team is currently testing from a stationary stand, so many sizing and loading constraints that an actual rocket would impose are not relevant here.
Tank Volume Sizing
Read Huzel & Huang, Section 8.2, “Shape and Size of Propellant Tank” subsection for more info.
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Find the required usable oxidizer volume
- Find the design volume of the oxidizer tank
- Find the required usable fuel volume
- Find the design volume of the fuel tank
Tank Thickness Sizing
Read Huzel & Huang, 8.2, “Safety Factors” section, and 8.3, “Cylindrical Section” for more info
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BF = burst factor, t = wall thickness, r = tank radius, Fu = material ultimate stress, pt = MEOP
3. Regulators
Regulators are used in a feed system to maintain a desired pressure level. In most design schemes, the regulator takes in the high pressure gas from the pressurant tank at its inlet, and regulates it down to the desired system pressure at its outlet, which continues towards the propellant tanks. In a perfect world, the regulator will continue to supply a constant output pressure during the duration of operation. In practice, the regulator outlet pressure will change, and that change depends on a few factors, described below: a. Flow coefficient(Cv), Cv=QSGP, Q=volumetric flow rateSG=fluid specific gravity,P=pressure drop across regulator
b. Changing inlet pressure
c. Changing flow rate
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B. Shown above is a graph pulled from a regulator’s company data sheet. Using this graph, the flow rate/delivery pressure from question A, and an inlet pressure of 4500 psig, determine if the regulator represented in this graph would satisfy our requirements. You will need to “guess” an additional curve that fits with the trends of the existing curves. (hint: 8.02 SCFH = 1 gal/min)
4. 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 on 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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System Constraints - Q=100,000 gal/min - fuel: pure ethanol @25C FIND THIS HERE | Valve 1  Cv = .02 | Valve 2 Cv = 0.15 | Valve 3
Cv = 0.008 |
5. 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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mdot_failure=1.5 kg/s | p_sys=800psi | fluid = pure ethanol @25C FIND THIS HERE | Cd = .25 | A_orifice= 6.95e-5 m2 |
6. Check Valves
In pressurized fluid systems, it is possible to have something called “back-flow”, in which the flow, as you would imagine, travels in the opposite direction that you desire. In a pressure-fed system, one scenario could be the cryogenic oxidizer traveling backwards out of the propellant tank, and into the pressurant gas lines. This could be detrimental, as the pressurant side components are likely not rated to cryo temperatures, and would be susceptible to damage. To prevent any backflow scenarios, we strategically place check valves in the feed system, creating one-way gates along the flow.
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Check valves are subject to the same pressure drop and flow requirements as the shut-off valves in the system. Flow coefficients for check valves can be found in supplier data sheets, and pressure drop calculations are done in the same way as discussed earlier.
7. Overall Design
Understanding the proper placement of these components is the next step in learning feed system design. There is an aspect of intuition and experience with this, and these are developed from a full understanding of each component’s use.
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Referencing the P&ID diagram above, label the valve components on the images shown. Specifically, label all of the control valves, relief valves, regulators, tanks, etc. I’d recommend copying the image to powerpoint for easier editing, or you can just explain it verbally.
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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