Reloadable Rocket Motor Knowledge
Reloadable Rocket Motors
(NH4ClO4) The oxidizer used in most composite rocket motors. Other components are Aluminum powder (fuel) and polybutediene rubber (the binder holding it all together). This is the propellant mixture that the Shuttle SRB’s use. See Binder
Ammonium Perchlorate Composite propellant.
A type of reusable rocket motor in which the fuel and oxidizer are kept separate and in different material states (such as a compressed liquid and a solid) until combustion occurs. A typical hybrid motor currently uses nitrous oxide gas as an oxidizer, and cardboard or plastic as fuel. See Micro Hybrid
(N2O)Gaseous oxidizer used in current hybrid rocket motors. See Hybrid and Micro Hybrid
- The slugs of propellant (called grainssource: rocketreviews.com).
In rocketry, any pellet or cast section of propellant or other combustible material. The black powder propellant packed and shaped into a single-use model rocket motor is referred to generically as a grain. Also, the direction of the fibers in a piece of wood, such as balsa. See Wood Grain
- A motor liner (to protect the aluminum casing).
- The delay grain and its components (this is a very slow burning cylinder of propellant that acts like a fuse going to the ejection charge … which is installed as part of the motor even if an ejection charge isn’t being used).
- O-ringssource: rocketreviews.com
A rubber gasket used to contain gasses in solid rocket motors
- A nozzlesource: rocketreviews.com (one company has a graphite nozzle that is reuseable – others are not).
The part of the rocket motor that smoothly expels gas from the combustion chamber; typically made from phenolic (in SU or AT reloads), graphite (Kosdon-style reloads), or clay (Estes motors). Components of the nozzle include the entrance cone, throat, and expansion cone
- End closures that attach by threading or snap rings.
- And detailed set of instructions that must be followed to the letter. Even experienced rocketeers set these instructions in front of them and use them to carefully double-check their assembly.
Fig. 1 These are APCP propellant slugs (called grains). They are cast in dense cardboard or phenolic liners. The propellant grains are installed in “sections” so the ends will burn as well. This keeps the burn surface area and thrust relatively consistent.
Fig. 2 A protective liner has been slid over the stacked propellant grains. This is to protect the aluminum motor case from the heat during the burn time. The liner is fitted with the nozzle and its O-rings that seal the gasses so they can’t escape around the nozzle. A sealing cap is fitted at the head end on this motor.
Fig. 3 The propellant, liner, and nozzle are installed and the “end closures” are ready to be installed.
Fig. 4 The completed motor ready to install into the rocket
Fig. 5 Once the motor is installed into the rocket and the rocket is on the pad, the electric igniter fires to begin the launch. The igniter is covered with a more easily ignited sort of propellant mixed with magnesium or titanium to start the motor burning good. It only takes a fraction of a second in smaller motors but may take a second or two on big motors.
Fig. 6 The propellant grains ignite and the motor begins to pressurize. Thrust builds very quickly at this point.
Fig. 7 Here’s a perfect illustration of the burning process in a motor. See the burning grains of propellant? The grains burn both at the core area and the ends. This way the burning surface area doesn’t keep increasing.
Fig. 8 The motor is still burning fiercely but you can see the APCP fuel grains are getting small.
Fig. 9 The propellant grains are nearly gone and the motor exhausted. This does not mean that the rocket has stopped. In fact, the rocket is traveling at it highest velocity at motor burn-out and continues to climb (coast) for a considerable length of time.
It is usually easy to find a more experience rocketeer that is willing to assist the uninitiated with the assembly of most motors. Always follow the manufacturer’s assembly instructions.
PHYSICAL SIZE (DIAMETER)Rocket motors come in standard diameters. In the section below lists them by diameter and the powers usually associated with that diameter; however, know that there is overlap from one diameter to the next. This allows for versatility in rocket design. Each diameter also comes in different lengths to make the different powers possible.
- 24mm — This small diameter is typically used for E and F power motors.
- 29mm — Used for H through I power motors. G motors are also normally 29mm.
- 38mm — The most common high power motor size. Used for H through J power motors.
- 54mm — Motors for 54mm casings usually begin at J and go to L power.
- 75mm or 76mm — Also called 3-inch motors. They are usually K through M in power.
- 98mm — Also called 4-inch. These big honkers are for L through N power motors.
- 152mm — This is six inches. These are the mega-motor cases for O power.