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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 (

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bodyP_{c}
), thrust (
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bodyF_{t}
), and mass flow rate (
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body\dot{m}
). 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.

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Variable Name

Symbol

Value & Units

Description

Methodology

Chamber Pressure

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bodyP_{c}

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body2500~\textrm{kPa}

Pressure of the chamber, measured at the injector.

One of two initial values, driven by metallurgy and COTS valves/fittings.

Thrust

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bodyF_{t}

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body2500~\textrm{N}

Thrust of the engine!

Second of the two initial values, driven by competition requirements and simulations.

Mass Flow Rate

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body\dot{m}

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body1.1~\textrm{kg/s}

Mass flow rate through the engine. Mass is conserved so

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body\dot{m}
is the same at the injector, throat, and nozzle exit.

The Matlab engine sizing program interates through various

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body\dot{m}
values until the correct thrust is achieved.

Exhaust Pressure

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bodyP_{e}

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body100~\textrm{kPa}

Pressure of the exhaust. In the ideal case

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bodyP_{e}=P_{atm}
, if
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bodyP_{e} > P_{atm}
engine is “underexpanded” and vice-versa.

We design around the ideal case (

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bodyP_{e}=P_{atm}
). Some groups/teams may size for
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bodyP_{e} > P_{atm}
to avoid backflow.

Mass Ratio (also called Mass Fraction or Oxidizer Oxidizer/Fuel Ratio (OF))

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body\mathit{MROF}

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body4.5~\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

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body\textrm{ISP}
were ideal.

?

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body{M}

Specific Weight (also called Gamma)

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body{\gamma}

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