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Rocket Thrust Simulator

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This is the beta version 1.4b of the Rocket Thrust Simulator program, and you are invited to participate in the beta testing. If you find errors in the program or would like to suggest improvements, please send an e-mail to

With this software you can investigate how a rocket nozzle produces thrust by changing the values of different factors that affect thrust. By changing the shape of the nozzle and the flow conditions upstream and downstream, you can control both the amount of gas that passes through the nozzle and the exit velocity.

There are several different versions of RocketThrust which require different levels of experience with the package, knowledge of thermodynamics, and computer technology. This web page contains the on-line Version 1.4 of the program. An upgraded Version 1.5 is also available. Version 1.4b includes an on-line user's manual which describes the various options available in the program and includes hyperlinks to pages in the Beginner's Guide to Rockets describing the math and science of rockets. More experienced users can select a version of the program which does not include these instructions and loads faster on your computer. You can download these versions of the program to your computer by clicking on this yellow button:

Button to Download a Copy of the Program

NOTE: If you experience difficulties when using the sliders to change variables, simply click away from the slider and then back to it. If the arrows on the end of the sliders disappear, click in the areas where the left and right arrow Images should appear, and they should reappear.

If you see only a grey box at the top of this page, be sure that Java is enabled in your browser. If Java is enabled, and you are using the Windows XP operating system, you need to get a newer version of Java. Go to this link:, try the "Download It Now" button, and then select "Yes" when the download box from Sun pops up.


The program screen is divided into four main parts:

  1. On the left of the screen is a graphics window in which you can display a drawing of the nozzle you are designing. You can control the appearance of the graphics by using your mouse and the slider located in the graphics window. Details are given in Graphics.
  2. On the top right of the screen are choice buttons to select English and metric units for input and output and to select the particular input panel. Computed Thrust and Flow through the nozzle are also displayed here. The red "Reset" button is used to return the program to its default conditions.
  3. On the middle right of the screen are the interactive inputs to the program. Inputs to the program can be made using sliders or input boxes. To change the value of an input variable, simply move the slider. Or click on the input box, select and replace the old value, and hit Enter to send the new value to the program. Details of the Input Variables are given below.
  4. At the bottom right of the screen is additional output from the program displayed in output boxes. By convention, input boxes have a white background and black numerals, output boxes have a black background and yellow numerals. Details of the Output Variables are given below.


On the left is a schematic drawing of the nozzle you are designing. Flow is from top to bottom for the rocket nozzle. The combustion chamber (or plenum) conditions are noted by the "Plenum-0," and the throat is at "Throat-th." The "Exit-ex" and "Free Stream-fs" conditions are also noted.

You can move the schematic in the graphics window by clicking on the figure, holding the left mouse button down and drag the schematic to a new location. You can change the size of the schematic by using the Zoom slider at the left of the graphics window. Click on the bar and move it along the line. If you lose the schematic, click on the word "Find" to restore the schematic to its default location.

You can change the length of the nozzle in the schematic by using the "Length" slider on the "Geometry" input panel. In real nozzles, the length to throat area ratio is important for keeping the flow attached. In this simulator, viscous effects are ignored, and the length is used only for "nice" graphics--it does not affect the calculation of thrust.


The input variables are located at the middle right on three panels; geometry, flow, and propellant. You select the type of input panel by using the choice button above the panel.

  • If you select the Geometry input, you must specify the throat area Ath. For a rocket or convergent-divergent nozzle, you must also specify the exit area ratio Aex/Ath. The plenum area ratio Ao/Ath and the Length of the nozzle are given for pleasing graphics, but are not used in the calculation of performance.
  • If you select the Flow input, you can change the chamber total pressure Pto, total temperature Tto, and free stream static pressure Pfs. The pressure and temperature are used in the calculation of the mass flow through the nozzle. For rocket calculations, if you change the propellants, the plenum chamber temperature is re-set to the average combustion temperature of the propellants. You may then change the chamber temperature to see its effect on thrust by using the sliders and input box on the Flow input panel.
  • If you select the Propellant input, you can change the gas which passes through the nozzle. The names of several propellants are given on the choice button next to the "Mol. Wt." label. Selecting a propellant re-sets the value of the molecular weight, the ratio of specific heats gamma and the combustion temperature . The change in molecular weight changes the gas constant used in the calculation of the mass flow through the nozzle. You can select to use a typical value for the molecular weight of the products of combustion, or you can input your own value by using the choice button. The value of the ratio of specific heats depends on the temperature of the flow, and you can use a typical curve for the variation of gamma, or input your own value by using the choice button next to the "Gamma" label. Finally, the combustion of the propellants generates a typical combustion temperature. You can use the typical value, or input your own value on the Propellant input panel by using the choice button.


Output variables are located at the top and bottom on the right. At the top of the output group are the weight flow and the computed thrust of the rocket nozzle. At the bottom are the computed values of specific impulse Isp and exit velocity Uex. The exit velocity depends on the exit area Aex and the exit Mach number Mex, which are also displayed. The exit Mach number is determined by the throat Mach number Mth and the area ratio, which is input. The thrust depends also on the pressure at the exit Pex and on the overall nozzle pressure ratio NPR. The analysis used to compute these variables is based on isentropic flow through the nozzle.

Related Sites:
Rocket Index
Rocket Home


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Jun 12 2014

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