(AKA: Model Rocket Flight Profile)
So, you’ve gotten a kit, motors, and built your rocket. Now what?
Phase 1 – Ignition and Liftoff
A model rocket always uses an electrical ignition system. That means that you need to have a device that starts the motor burning. This device is called a “Launch Controller.” Basically, it consists of a battery, a switch, a safety key, and some wire. There are many different controllers available either at your local hobby store or online. If you have some basic electrical skills, you could even choose to design and build your own. The launch controller, along with a “Launch Pad” are the items you need to launch your new rocket.
Igniters
The actual device that starts the motor burning is the “igniter” (AKA “starter”). It looks a little like a match head, with wires coming off the tip. These wire leads are then connected to the launch controller. When the electrical current passes through the igniter, it heats it up and causes it to flame up. This flame is what actually starts the propellant burning in the rocket motor. Igniters always come with rocket motors that you buy, but can also be purchased separately.
So, Why electrical ignition?
This is a common question. The answer has to do with safety. Many new people think a fuse would be simpler. However, that would sacrifice safety. Once a fuse is lit, there is no stopping it. If the rocket should tip over, or an airplane suddenly appear in the sky, you could not halt the launch.
What about the Launch Pad?
Sometimes, new modelers do not understand the requirement for a launch pad that includes a launch rod. After all, SpaceX and NASA don’t have a big launch rod for their rocket launches.
The purpose of the launch rod is to guide the rocket until it reaches sufficient speed where the fins take over and keep the rocket moving in a straight path. This speed is approximately 30 miles per hour. Without a rod, the model can easily tip over at lift-off, and come screaming right at you or others in your group. Therefore, for safety, we have a launch rod that keeps the rocket pointed up. The rest of the launch pad is there to hold the rod, and to keep it from tipping over in a breeze.
By the way, the reason the large rockets do not have launch rods is because they have steerable rocket engines. In other words, the direction the rocket engine pushes controls the path of the rocket. Model Rockets have a fixed nozzle that only points in one direction, so we need a rod to keep the rocket moving in that “upward” direction while it builds up enough speed to become stable on its own.
Lift Off
Once the motor ignites, it begins to generate thrust. It is this thrust force that pushes the rocket into the air. While the motor is making thrust, you’ll normally see a flame coming out the back of the motor. Sometime it is hard to see because the rocket moves so fast. At the same time, the rocket motor is making a loud roar and usually a lot of smoke.
Phase 2 – Engine Burnout
The propellant inside the motor burns quickly. In most motors, the propellant is consumed in less than three seconds, at which point “burnout” occurs. This means the motor is no longer producing a thrust force. By the time the motor burns out, the rocket has already reached its top speed. It cannot go any faster from this point on. Most people are surprised that burnout occurs at a very low altitude. While the rocket may eventually reach hundreds or even thousands of feet in altitude, the burnout location on most rockets is about 50-80 feet after leaving the pad.
Phase 3 – Coasting
When the motor burns out, the rocket may be traveling hundreds of miles per hour. We don’t want the parachute to come out of the model while it is going this fast. Otherwise, it will be ripped to shreds. We want the model to coast upward and bleed off some speed. The period of time that starts at engine burnout, and ends when the parachute is ejected out of the rocket is called the “Coast Phase.” Even though the rocket motor isn’t making thrust, there is something happening inside of it. The special composition called the delay element (or delay grain) is burning at a slow rate. It is obvious that something is going on, because there is still smoke coming out of the rocket motor. Maybe this is what causes confusion among new modelers that they think the motor keeps burning all the way until it reaches apogee (the highest point in the flight). The smoke serves a purpose though. It allows us to track the rocket — in other words, to follow its progress into the air. Sometimes the rocket moves so fast, that it is hard to follow with our eyes. So the smoke gives us a visual indication where the model is.
Something worth mentioning, is that the smoke produced by the delay grain is not as heavy or as thick as the smoke produced by the motor while it is producing thrust. The delay smoke is whiter, and wispy.
Phase 4 – Apogee and Ejection
When the delay composition is done burning, it ignites the “ejection charge” that is also built into the motor. This ejection charge burns quickly, and is directed inside, toward the top of the rocket. Its goal is to push off the nose cone, and eject the parachute out of the rocket. Typically, we desire the ejection to occur right at apogee (the highest point in the trajectory of the rocket). It is at this point the rocket has slowed down to its minimum velocity. So when the chute comes out, it isn’t hit by a huge gust of air.
The modeler controls when the ejection charge pushes out the parachute by proper motor selection. If you use a motor that has too long of a coast phase delay, the rocket will arc over, and will
eject the chute while the rocket has built up some speed when it is coming back down to the ground. Like-wise, too short of a delay will mean the rocket hasn’t coasted to its highest point.
Phase 5 – Recovery
After the parachute has ejected, it fully inflates, and the rocket has begun its recovery phase. Nothing much happens during the recovery phase. The model just drifts slowly to the ground under the
canopy of the parachute. But it is at the mercy of any wind that is blowing. The stronger the wind, the further the model will drift away from the launch pad. Because of this, modelers have searched for ways to keep the model from drifting out of sight. The most common thing they do is to switch from a parachute to a streamer. A streamer does the same thing as a parachute, but it falls faster, so it doesn’t drift as far. Some other things a modeler can also do to prevent the rocket from drifting too far include cutting a hole in the canopy of the chute, to make it fall faster. Similarly, you can tie the suspension lines together to reef the chute (to prevent it from opening fully). Again, this makes the rocket fall faster, so it doesn’t drift as far during the recovery phase.
Final Note
With the exception of the recovery phase, the flight of the rocket is controlled by the rocket motor. Because of this, proper motor selection is a very important part of the launch process. Fortunately for most new and inexperienced modelers, they usually fly rockets that are built from a kit. The manufacturer of the kit will always have a list of motors they recommend for the flight. As long as you use a motor on that list, your flight should be a success!