*Note: This page is not meant as a primer on the design or sizing of liquid engines. There are many fantastic resources created by other groups, here are some:
Liquid Rocket Engine Sizing - USC Viterbi
Charlie Garcia's Playlist
Sizing a Rocket Engine - Liquid Propulsion Lab
USC Liquid Propulsion Laboratory
Overview
Liquid engine sizing is the process that relates the chemical processes of combustion to physical parameters. It is separate, but related, to engine design which will consider materials, heat transfer, and many other physical constraints.
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Engine sizing involves a large number of variables both quantitative and qualitative. However, nearly all of these can be constrained by the selection of a propellant combination and two of the following three variables: chamber pressure (
), thrust (
), and mass flow rate (
). Selecting two of these and a propellant combination will allow you to proceed to NASA CEA and define engine performance and size.
Oftentimes, these parameters are driven by systems-level requirements such as certain vehicle performance/flight profile requirements, propellant selection dictated by a challenge/competition/school rules, or any number of program requirements. See the following pages for more details on: Selecting Propellant Combination , Selecting a Design Thrust, and Selecting a Preliminary Chamber Pressure.
Again, we chose to size with respect to chamber pressure (
) and thrust ().
It would be theoretically possible to select a , , and and to then drive propellant requirements but this technique is not feasible or productive. Propellant combination selection is driven by overarching program or organization requirements (ex. our school does not allow LOX effectively requiring us to use N2O). NASA Combustion Equilibrium Analysis
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Variable Name | Symbol | Value & Units | Description | Methodology |
|---|
Chamber Pressure | | | Pressure of the chamber, measured at the injector. | One of two initial values, driven by metallurgy and COTS valves/fittings. |
Thrust | | | Thrust of the engine! | Second of the two initial values, driven by competition requirements and simulations. |
Mass Flow Rate | | | Mass flow rate through the engine. Mass is conserved so is the same at the injector, throat, and nozzle exit. | The Matlab engine sizing program interates through various values until the correct thrust is achieved. |
Exhaust Pressure | | | Pressure of the exhaust. In the ideal case , if engine is “underexpanded” and vice-versa. | We design around the ideal case ( ). Some groups/teams may size for to avoid backflow. |
Mass Ratio (also called Mass Fraction or Oxidizer Oxidizer/Fuel Ratio (OF)) | | | Eazy math inline |
|---|
| body | 4.5~\textrm{(unitless)} |
|---|
|
| Mixture mass ratio between propellants as Ox/Fuel. | NASA CEA allows you to enter multiple MRs. Multiple were entered until the combustion temps and were ideal. |
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Specific Weight (also called Gamma) | | | | |
| | | | |
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