Issue 98, September 1998
Smart Rockets - Data Acquisition in Model Rocketry

INTRODUCTION TO MODEL ROCKETRY

Model rockets are lightweight, low-cost, solid-fuel rockets designed to permit safe experimentation with the principles of rocketry. Factory-made, single-use, solid-fuel engines are a key feature in model-rocket safety.

A model-rocket engine consists of, from bottom to top, a nozzle, propellant, delay charge, ejection charge, and an end cap. This design governs the overall flight characteristics of the model.

The acceleration phase (i.e., when the propellant burns) lasts from a fraction of a second to up to 2 s for typical rocket engines (see Figure 1). It is followed by a longer (several seconds) tracking phase when the rocket coasts up to maximum altitude. The model is not powered during this phase, as the delay charge that generates smoke for visibility has negligible thrust.

At the end of the tracking phase, the ejection charge ignites at the peak altitude, when the velocity of the rocket is near zero. The ejection charge blows forward, bursting the end cap, and pressurizing the interior of the model, which pushes the nose cone/payload section out of the rocket body.

Both pieces, connected by an elastic shock cord, then drift safely back to earth via a parachute. During this recovery phase, the rocket is at the mercy of the wind which can blow it onto rooftops, into trees or power lines, or, in our case, dump the model in the middle of the Charles River! If all goes well, however, the rocket is recovered, the engine is replaced, and the model can be flown again.

Model-rocket engines are designated by a letter and two numbers. The letter indicates the total impulse of the engine, which is just the integral of the thrust versus time curve of the engine (the dashed line in Figure 1).

We used A-, B-, and C-sized engines with total impulses of 2.5, 5, and 10 N, respectively. The first number of the engine designation is the average thrust in newtons, and the second number is the period of the tracking phase in seconds.

Our rocket—the Estes Nova Payloader—is a typical single-stage model rocket. It’s about 21? long, 1? in diameter, and weighs 1.8 oz. (50 g) without a payload or engine.

The rocket consists of a paper tube, balsa wood fins, and a 4¾?-long plastic payload section topped by a nose cone. The wood and paper parts are assembled with wood glue, and the plastic parts with model cement. The rocket body and the nose cone/payload section are connected by an elastic rubber shock cord, which attaches to the 12? diameter parachute.

Prior to launch, an engine is inserted into the rear of the rocket and held in place with a spring clip. A piece of recovery wadding (fireproof tissue) is placed in the rocket body just forward of the engine to protect the plastic parachute and shock cord from hot gasses during ejection.

Next, the parachute and shock cord are coiled up and inserted into the rocket. The nose cone/payload section is inserted into the forward end of the body, and the model is placed on the launch pad, ready for flight.

Our launch platform—an Estes Porta-Pad II—consists of a plastic tripod, a steel blast deflector, and a meter-long launch rod. The rocket has a small tube attached to its side, the launching lug, that holds the model to the launch rod during the initial phase of powered flight.

Model-rocket engines are fired electrically using an igniter that typically consists of a piece of nichrome wire coated with a flammable material. A current passed through the wire ignites this material, which then ignites the propellant.

The igniter connects to a long pair of wires that lead to the launch control box at a safe distance (~20?) from the rocket on the pad. The launch control box has a switch (the launch button) that connects the igniter to the 6-V battery (four alkaline AA cells). We discuss the details of the launch control box later.