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The Challenges of a Successful Rocket Flight

Another Separation

Simulations are often skipped by the inexperienced in the rush to get the rocket into the air. DO NOT SKIP THIS STEP! It is important to understand how the rocket will perform during various conditions in order to successfully recover the rocket. There are many questions that must be answered before taking flight. How high will the rocket fly on xxx engine? How long should the delay for recovery system deployment be? To vent or not to vent? How much ejection charge to use? These questions can only be answered by various simulations.

There is a very well written article titled Rocket Simulations by Randy Culp that describes the various calculus equations required to simulate rocket flight characteristics. Please read this article now BUT DO NOT BE INTIMATED BY THE MATH.

Randy provides various JAVA calculators and Excel spreadsheets to assist in the simulations. It is important to understand the forces that conspire to ruin your rocket during flight. Bookmark Randy’s website as you will refer to it frequently. Once Randy’s article is read, continue below.

There are also software packages that conduct rocket simulations and do the entire math for you. The most popular is Rocksim from Apogee Rockets.The price tag might scare people away but it is well worth the slightly over 100 bucks for the ability to plug in values and instantly receive the simulated results. Of course any simulations are only accurate as the data fed into it. Even Rocksim requires that all components be entered accurately, right down to the type of finish. However, due to Rocksim’s popularity, many rocket kits and engine data are already available in Rocksim format and are free to download.

Failed Ejection

Ejection charge never ignited thus no deployment occurred. Not good!

Always compare downloaded Rocksim data with your rocket as everyone’s construction techniques are different. At the very least, prep your rocket as if it were to fly, without the engine, and accurately weight it. Compare this weight with Rocksim and account for any differences

How high will the rocket fly on xxx engine? This is important because it will provide the basis for the entire flight. Of course weather conditions will influence rocket performance. Wind can rob valuable altitude from the rocket as well as humidity. Simulate each engine that the rocket might use and the weather conditions that the rocket might fly in. Record the results. The rocket altitude is important to make sure that the rocket stays within the altitude waiver. It is also important so that an educated answer can be provided to the common spectator question, “How high did you think it went?”.

Just as important as the rocket altitude is the elapsed time from engine ignition to apogee. The elapsed time from engine ignition to apogee will be used to set the delay. The delay element is usually a solid material made of various compositions that burns at a known rate under the specific impulse of the engine. The delay element is situated between the rocket propellant and the ejection charge. The delay is ignited at the same time as the rocket’s propellant is ignited. The delay element eventually burns through, thus igniting the ejection charge, thus forcing out the recovery mechanism. The goal is to eject the recovery mechanism just as the rocket arcs over at apogee. Rocksim will provide an optimal delay as part of its simulation. Most rocket engine vendors will provide known delay times with their engines. Some will provide adjustable delays. An early or late ejection can lead to rocket damage such as separation or zippering.

The delay time is still important even when rocket electronics are used. The proper time must be programmed into the electronic timer if timers are utilized. Altimeters will automatically detect apogee. However, knowing the delay time is still useful because the delay can be set to function as a backup if the electronics fail.

To vent or not to vent? As a rocket gains altitude, the air pressure becomes thinner. If the air pressure becomes too thin, the difference in air pressure outside of the rocket and the air pressure inside the rocket will cause premature separation as the air pressure tries to equalize. This can lead to damage of the rocket. To avoid this, a hole is drilled in the recovery section of the rocket to allow the air pressure to equalize or vent. The size of the hole depends on the available volume of the recovery section of the rocket. Tim Van Milligan of Apogee Rockets has a very nice news letter (issue 68) titled Pressure Relief Holes that explains everything.

And finally, the ejection charge must be tested. How much ejection charge is required to successfully deploy the recovery mechanism? Most ejection charges utilize black powder but some rocketeers will use pyrodex or compressed air. The charge should separate the rocket and push the recovery mechanism clear from the recovery compartment without blowing up the rocket or causing the rocket to come apart. For this reason, most high power rockets utilized long shock cords to dissipate the extra ejection energy. The rule of thumb is that the shock cord should be 5 times the length of the rocket. There is a nice Black Power Sizing article from Rocketry Online. The black powder charge can be and should be tested on the ground before flight. Some of the ejection charge might vent out of the Pressure Relief Holes. Also, add a tad more to the black powder charge as the black powder will lose some of its punch as altitude increases.

Armed with the simulation results and measurements, a detailed launch checklist can now be developed. Every rocket should have its own checklist before heading to the launch field. Remember, a true rocketeer needs to remember their offsprings and spouse’s birthday as well as anniversaries, everything else can be put on the checklist to be referenced as needed. The rocket is now ready to be prepared for flight.


Houston, we have separation, Not good!

Last Updated: August 7, 2009 6:09 PM
By Peter E. Abresch Jr.
Simulating Your Rocket

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