User Manual

The User Manual for the Chute Tamer™ recovery deployment control contains lots of pictures and how-to information. The following sections are included in the user manual:

Easy Calc Delay Estimator Worksheet

The easiest way to estimate a delay time is to add a couple of seconds to the total engine burn time (engine thrust plus delay time). This can be done without a worksheet. To get a better delay time estimate use the Easy Calc worksheet.

The Easy Calc worksheet is a fast way to conservatively estimate the delay time for delayed parachute deployment. This method assumes a rocket tumble velocity (after the engine's ejection charge and before the main parachute is released) of 100 feet per second. This is faster than a "standard" mid power rocket built from "standard" materials is likely to tumble, but information about the rocket geometry and weight is not required to use the Easy Calc worksheet.

The Easy Calc worksheet can be used with a small drogue parachute, or no drogue parachute. This worksheet provides space for three different rocket/engine combinations. To use the Easy Calc worksheet, you will need to know:

• The engine thrust burn time in seconds - see engine manufacturer specifications; typically between one and three seconds.
• The engine delay burn time in seconds - the length of the engine's delay.
• The expected altitude of this rocket/engine combination - see the rocket kit manufacturer's estimate for each recommended engine; use a computer simulation tool such as Rocksim.

Standard Calc Delay Estimator Worksheet

As you get to know your rocket and desire to use a bigger engine to gain more altitude, a more accurate delay time is desirable. To calculate a better estimate, use the Standard Calc worksheet. The added calculations on this worksheet provide a better estimate of your rocket's tumble velocity (after the engine's ejection charge and before the main parachute is released).

The rate at which the rocket will tumble depends on the surface area of the tumbling rocket and/or the drogue parachute, if one is used. First calculate the effective surface area of the rocket and then compare it to the effective surface area of the drogue parachute. (If the drogue is a flat circle, then use the formula for the surface area of a circle = pi*r^2. If the drogue is hemi-spherical, then use one-half the formula for the surface area of a sphere = ½ * 4*pi*r^2. Where "r" is the radius or ½ the diameter of the drogue parachute and pi is approximately 3.14 and "^2" means squared or raised to the second power.)

If the drogue parachute's effective surface area is not larger than that of the rocket, then the drogue is not big enough to control the tumbling airframe. The effective surface area of the rocket is an estimate since the fins rotate into and out of the rising airstream (see Standard Calc worksheet). Use the larger of the two effective surface areas in the tumble velocity calculation.

The Standard Calc worksheet can be used with a drogue parachute, or no drogue parachute. This worksheet provides space for three different rocket/engine combinations. To use the Standard Calc worksheet, you will need to know:

• The engine thrust burn time in seconds - see engine manufacturer specifications; typically between one and three seconds.
• The engine delay burn time in seconds - the length of the engine's delay.
• The expected altitude of this rocket/engine combination - see the rocket kit manufacturer's estimate for each recommended engine; use a computer simulation tool such as Rocksim.
• The outside diameter of the rocket's airframe - measure it. If the rocket has multiple airframe dimensions calculate a weighted average using the length of each section.
• The length of the overall airframe, including the nose cone - measure it.
• The weight of the empty engine casing that you will be using - weight it or check the manufacturer's specifications.
• The weight of the rocket including recovery "laundry" - weight it or look at the suggested built-weight provided by the kit's manufacturer.
• The altitude at which the main parachute should be released for deployment - remember to leave room for the parachute to open after it is released (1,000 feet is a good starting point).

Tumble Velocity Table

When using the Standard Calc worksheet, there is a rather interesting formula involving the square root of a large number. If your calculator does not do square roots, or you just promised yourself that you would not do another square root after high school, then the Tumble Velocity Estimator table is for you! The Tumble Velocity Estimator table is copied onto the back of the orange Standard Calc worksheets provide with the Chute Tamer™ control.

This calculation can be bypassed by looking up the estimated tumble velocity in this table. Simply look for the row (effective surface area) and column (weight of rocket, engine casing, and Chute Tamer™ control) that match your rocket. The shaded cells going from the upper left corner to the lower right corner represent combinations that will produce a tumble velocity of 50 feet per second. Below this diagonal line the tumbling velocities are slower. Above this line, the tumbling velocities are faster.

If your estimated tumbling velocity is higher than 50 feet per second, then it is strongly recommended that you use a larger drogue parachute with a larger effective surface area. The maximum recommended safe tumbling velocity is 50 feet per second.

Included at the bottom of the Tumble Velocity Estimator table are the formulas used to derive the table along with the constants that were used. Using these formulas (or improved formulas) with different assumptions (such as the coefficient of drag or atmospheric density) you can create your own table that is better suited to your flight conditions.

Chute Tamer™ and Rocksim

The Rocksim simulation software can be used to simulate rocket flights including the Chute Tamer™ control. This PDF document provides a tutorial including several Rocksim screen shots and simulation plots showing exactly how to include the Chute Tamer™ control in your Rocksim design. Although not required, Rocksim can be a fun and educational way to estimate the total delay time from launch to main parachute release. Central to this tutorial is how to simulate a tumbling rocket in Rocksim.

Also included in this PDF file is a real plot from an early Chute Tamer™ test flight. In this test a PerfectFlite logging altimeter was used to capture actual altitude data over time. The resulting plot clearly shows the time at which the Chute Tamer™ control releases the parachute slowing the descent of the rocket.