Motor Guide Help

The Motor Guide helps you to pick an appropriate motor for your rocket from the list of all motors. The guide includes a quick-and-dirty flight simulator and will pick motors from the list that will probably work for your rocket.

Enter Key Info

The best way to use the guide is to save your rocket first. That way, you can run the guide multiple times and have access to more options such as clusters and MMT adapters.

Note that weight and length values may be entered in various units. Use the drop-down box to the right of the entry field to pick the units each measurement uses.

Body Diameter
the overall outside diameter of the rocket.
Enter the O.D. of the largest body tube in your rocket. Note that you must select the appropriate length units.
Dry Weight
the weight of your rocket as it would fly (everything except the motor).
Note that you must select the appropriate weight (mass) units.
MMT Diameter
the inside diameter of the motor mount tube.
This determines the size of motors that will fit into your rocket. (This value is matched approximately since motor diameters aren't consistent between manufacturers.) Note that you must select the appropriate length units.
MMT Length
the available length of the motor mount tube.
This limits the length of motors that will fit into your rocket. Note that you must select the appropriate length units.
CD
coefficient of drag.
This helps to determine the resistance of your rocket as it passes through the air. See Explanation of CD for more info.
Guide Length
usable length of the rod or rail.
This determines how much length the rocket has to reach a velocity where its fins can keep it flying straight.

Search Criteria

In addition to which motors will physically fit your rocket, you may also narrow down the motors that will be simulated to ones that are available to you.

Impulse Class
limit the search to a particular class.
If you only want results for motors of a single impulse (letter) class, select it from the list.
Type
limit the search to a particular motor type.
If you only want results for motors of a certain type, select it from the list.
Manufacturer
limit the search to a particular manufacturer.
If you only want results for motors from a single manufacturer, select it from the list.

For a saved rocket, there are more options and you can choose multiple values for the options above.

Understanding the Results

The first paragraph packs a lot of information into a compact form. Generally, the details here don't matter, but if you're getting too few results, you might make sure that you entered the data properly and have sensible filters.

  • N motors didn't fit
    tells you how many of all the motors in the database wouldn't fit into your rocket. Note that this means the motor mount tube (MMT) inner diameter must be approximately the same as the motor outer diameter and that the MMT length must be at least as long as the motor.
    Tip: If all motors are failing, check the MMT dimensions.
  • N motors eliminated by filters
    tells you how many motors, that would otherwise fit, were eliminated by filters you specified when setting up the guide.
    Tip: If all motors are failing, check that you don't have overly restrictive filters.
  • N motors failed safety checks
    tells you how many motors aren't safe with your rocket. This is most often caused by motors having too low an initial thrust for the weight of your rocket.
    Tip: If all motors are failing, check the dry weight.

If one or more thrust curve data files are present for a motor that fits your rocket, a quick simulation is run with each data file. The information in the Motors That Work table shows an average of some of the key flight statistics.

  • Weight is the mass at lift-off. You enter the dry weight of your rocket (that is, everything except the motor) and the total weight at lift-off (including the motor being tested) is shown.
  • Launch is the speed attained by the rocket before it leaves the launch guide. This is used to determine whether the motor has enough initial thrust for the rocket.
  • Velocity is the maximum speed attained by the rocket in flight. This gives you an idea of how harsh a ride this motor will give your rocket.
  • Accel is the maximum acceleration attained by the rocket in flight. This gives you an idea of how hard the motor is pushing the rocket. Note that this is expressed as a multiple of gravity ("Gs" are 9.8m/s² or 32ft/s²).
  • Altitude is the maximum height above ground (apogee) achieved by the rocket.
  • Time is the ascent time (from lift-off to reaching apogee).
  • Delay is the optimal delay length (the time from burn-out to apogee).

Finally, there may be some warnings listed for the motors. Keys for the warnings are listed in the first column of the table and the warning text appears at the bottom as a footnote. Here are all possible warnings:

  • F MMT dimensions not known, so fit not known.
    This means that valid motor mount dimensions were not specified for your rocket and the fit could not be checked.
  • 5 Rocket weight and/or average thrust unknown; 5:1 ratio not checked.
    This means that a valid dry weight was not specified for your rocket or the motor data does not include the average thrust.
  • G Rocket launch guide length unknown; minimum velocity not checked.
    This means that a valid launch guide length was not specified for your rocket.
  • W Motor total weight unknown, safety checks not accurate.
    This means that the motor data does not include the total weight and thus the lift-off weight cannot be determined accurately.
  • D No simulation data found for this motor.
    The motor appears to work, but no one has yet submitted a data file for it and no simulations could be run.

Note that this really is a quick and dirty simulation. For more accurate simulations, please use a real rocket flight simulator; see the Simulators page for many options.

Explanation of CD

CD stands for the "coefficient of drag," which is a fudge factor that corresponds to the amount of drag a rocket will have. This term takes into account things like the shape of the rocket, the shape of the fins and the smoothness of the finish. (This value has no units and is a constant by which the frontal area drag is multipled.)

For most models, the CD will be in the range 0.5 - 0.9. An ultra-optimized, perfectly finished model with airfoil fins might have a CD as low as 0.3 and a rocket with a complex shape or no paint could easily have a CD above 1.0.

The motor guide defaults the CD to 0.6, which is a reasonable starting value. If you have prior flight results, you can enter a more accurate value to improve the estimates. (Values below 0.1 or above 5.0 are not allowed.) The combo box allows quick selection of plausible values:

  • perfect: 0.3, for a competition rocket with airfoil fins and polished finish
  • good: 0.45, for a simple rocket with good finish
  • average: 0.6, for most sport rockets with average finish
  • high: 1.0, for complex rockets with rough finish

This gives you an idea of the roughness of this basic simulation method (using a static CD) and leads naturally into the next section.

Accuracy Disclaimer

The simulations are done using the basic algorithms incorporated in the original RASP simulator. In creating this software, we relied primarily on the Rocket Math 3: Simulations articles by Norm Dziedzic, published in NIRA newsletter and later in Extreme Rockety. Thanks Norm!

Disclaimer: This guide is only meant to give you a starting point for choosing motors. For more accurate simulations, please use a real rocket flight simulator; see the Simulators page for many options.