ROCKET MOTORS

In physics, Power = Force times Time. In rocketry, force is called Thrust and Power is called Impulse, so Impulse = Thrust times Time, where Thrust is in Newtons and Time is in seconds. Impulse is therefore in Newton-Seconds.

In the example above C is the total Impulse available, 5 is the average impulse applied to the rocket while the propellant is burning, and 3 is the delay between the end of the burn time and the beginning of the ejection charge that pushes the parachute and nose cone out of the rocket’s body. Burn time is equal to the total impulse of the motor divided by its average thrust level. The higher the average impulse of a motor is, the faster it burns through its propellant. A C5 and a C10 motor have the same total impulse power (10 Newton-Seconds) but the burn time of a C5 motor is 2.0 seconds (10 ÷ 5 = 2), whereas the burn time of a C10 motor is 1.0 second (10 ÷10 = 1.0). Motors are made with the same total impulse but different average impulses because light rockets require low average thrust so they aren’t ripped apart on the launch pad, and heavy rockets require high average thrusts so they get up to speed quickly enough for their fins to begin stabilizing the flight path.

The perfect delay allows the rocket to coast to its peak altitude just as the parachute is deployed. Light rockets use long delays because they coast longer. Heavy rockets use short delays because they reach apogee sooner. If the delay is too long, a heavy rocket will start down before the parachute is deployed.

MANUAL ALTITUDE MEASUREMENTS

You can approximate the altitude your rocket reaches using a sighting tool like the one sold at Estes. You’ve found a spot 300 feet from where your rocket will be launched. As the rocket rises, sight the tool on it. If you’ve chosen the right delay time, the ejection charge will push the parachute and nose cone out of the rocket just as the rocket reaches apogee and you’ll see a puff of smoke. Note the angle on your sighting tool, then calculate the altitude using the formula above and the table of tangents below. You know that B is 300 feet. If the sighting tool indicates 45 degrees, the table tells you the Tangent of 45⁰ is 1.00. So the altitude your rocket reached is 300 feet (300 = 1.00 x 300.) If your sighting angle is 60⁰ your rocket reached an altitude of 519 feet (1.731 x 300 = 519). If your sighting angle is 75⁰ your rocket reached 1,119 feet (3.7321 x 300 = 1119).

POWER RANGE

Motors are manufactured in specific categories according to a minimum and maximum total impulse for that range. The power doubles with each range.

HIGH POWER CERTIFICATION

Stores will sell you one high power motor for your NAR certification launch. It must have 124 or more grams of propellant and more than 80 Newtons of average thrust.

Level 1 » your rocket must be inspected and that your launch is observed by a certifying rocketeer at an NAR sanctioned event. The rocket must be built by you, have the center of pressure visually identified, have a parachute recovery system, and be in flyable condition after recovery.

Level 2 » same requirements as Level 1 but you must also pass a written test.

Level 3 » same requirements as Level 1 and 2 but your rocket must be inspected by two members of the Technical Advisory Panel and it’s recovery procedure must include two separate electronics with one being the deployment system.

BLACK POWDER MOTORS

Black powder motors are made by hydraulically compressing charcoal, potassium nitrate, and sulfur into a paper casing. Those ingredients are less expensive than the fuel and oxidizers used in composite motors, but they create less energy per kilogram of fuel than the ingredients used in composite motors. Black powder doesn't require the intense heat of a flame to ignite, just the heat generated by the current flowing through an igniter. So you can stage a second rocket from the first because you only need the heat of the ejection charge to ignite the second motor. But this makes black powder more volatile than a composite motor, so shipping large E and F motors requires HAZMAT shipping.

COMPOSITE MOTORS

Composite motors are made by mixing the fuel and oxidizer together then solidifying the mixture with a chemical reaction similar to a two-part epoxy or resin. The result is a rubbery, resilient, low-risk mixture with twice the energy density of a black powder motor and far less risk of fracture if dropped. They require a direct flame to ignite the propellant. When electricity passes through the Starter it heats up and bursts into flame to starts the propellant burning.

Unlike black powder motors, composite motors can be made with chemicals to create different colors of flame, like fireworks on the fourth of July. Staging a rocket with a composite motor requires electronics to start the second motor because you need a flame, not just the heat of the ejection charge, to ignite the second composite motor.

Clustering (more than one rocket engine) is possible, however, if all your starters ignite simultaneously and all of the motors begin their delay simultaneously so the rocket doesn’t veer off of a vertical profile.

ESCAPE VELOCITY

In theory, a rocket could be launched at or above escape velocity into an orbit around Earth or on to Mars, the Moon or the Milky Way because escape velocity is the speed at which the gravitational pull of the Earth decreases faster than it can slow the rocket down. Escape velocity is calculated using the values for G, M and R, which works out to be 11.2 km/sec or 7 mi/sec… that’s Mach 33!

In practice, rockets are launched into an orbit around Earth or on to Mars, the Moon or the Milky Way far below escape velocity because the engine keeps burning with enough thrust to overcome the decreasing force of gravity. They take off at a few miles per hour then accelerate to thousands of miles per hour.

STABILITY

Model rockets are also launched far below escape velocity but far above the speed of a full sized rocket because the path of a full-sized rocket is controlled by swiveling the nozzles of its engines and positioning its fins, whereas the path of a model rocket is controlled by the speed of the air flowing over its fixed fins.

Stability for a full sized rocket means keeping it on a programmed trajectory to orbit the Earth, reach the Moon or some location in space. Stability for a model rocket means keeping it on a vertical path from launch to burn out. And speed isn’t the only criteria for a stable launch. Stability also depends on the distance between the COG and COP because increasing the distance increases the leverage the airflow has on the pivot point of your rocket, which is the center of gravity. The rocket is stable if the COG is ahead of the COP because the airflow decreases the angle of attack. The rocket is unstable if the COG is behind the COP because the airflow increases the angle of attack.

The speed of the air flowing over a rocket’s fins should be a minimum of 15 meters per second off the rail, and the minimum thrust-to-weight ratio to achieve that speed is 5 to 1. Those minimum values will make it more likely that your rocket and its engine will work well together… BLAST OFF… WOW!

The Rock Sim Launch Simulator can help you evaluate how well a rocket and an engine work together because it uses the engine’s actual thrust curve. But it’s expensive and complicated to use. A simpler method is to calculate the velocity that results from the thrust-to-weight ratio of a rocket plus its engine. Velocity is where the rubber meets the road… uh, where a rocket meets the sky. The calculations below are for Aerotech’s Initiator and three of the engines Aerotech recommends for it. The F67 is the best choice for the Initiator because...

>> the velocity with the F44 is below 15 meters per second
>> the G74’s initial thrust isn’t as steep as the F67
>> the G74 requires Hazmat shipping… $$$