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.

Engine sizing can seem somewhat complex, much of this is due to the berth of prerequisite knowledge that is needed to fully grasp the formulas, concepts, and processes. The reality is that the engine sizing approach used by many collegiate teams benefits from, but does not require, advanced knowledge of fluid mechanics, gas dynamics, or propulsion. Again, engine sizing may seem complex and while it can be the basics can be grasped without an extensive technical background.

The propellant combination is one of the most significant factors in engine sizing. Nitrous Oxide, N2O is our oxidizer, and Isopropyl Alcohol, IPA is our fuel. IPA was selected as fuel due to its low cost and accessibility.
Three main variables constrain engine sizing, , thrust, and . We used a Simulink simulation to decide on an of and metallurgy and COTS valve/fitting pressure ratings to arrive at a of or .
Using NASA CEA. We input,, and for our N2O oxidizer and IPA fuel. CEA allows you to simulate various mass fractions. We chose 4.0 as it gives the best ISP performance, this burns hot but not as hot as a more OX-rich mix. CEA outputs the , (equivalent to ),, , and .
The following parameters are fed into a Matlab script: , , , , and (L* is somewhat arbitrary and selected) as well as chemical properties. m_dot is adjusted! We chose an L* of 1 m, and m_dot 1.1 kg/s. That yields V_c. We pick 0.15 m L which gives a D_c of 7.73cm and exit diameter of 6.3 cm and a throat diameter of 2.99 cm.

Variable Table

Variable Name	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 Fuel Ratio (OF))	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.
?
Specific Weight (also called Gamma)

Engine Sizing

Variable Table