Reloadable Rocket Motor Knowledge

Reloadable Rocket Motors

Propellant for high power rocket motors is usually a solid fuel mixture of ammonium perchlorate

(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

called APCP

Ammonium Perchlorate Composite propellant.

. There are also Hybrid

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

motors that use a cored paper or plastic element and Nitrous oxide

(N2O)Gaseous oxidizer used in current hybrid rocket motors. See Hybrid and Micro Hybrid

that when in combination are ignited in a combustion chamber, ie, motor, produces thrust. However, I’m going to speak of APCP motors first. APCP is a mixture of ammonium perchlorate and a few other elements. In its raw cast form as a cylinder of solid propellant, it can be ignited and it will burn, but not fast enough to create any kind of thrust. As APCP is pressure sensitive, this propellant burns much faster once you contain it in a combustion chamber (motor case) and you carefully calculate the nozzle exit throat to produce just the right amount of pressure to create a fast but controlled burn, resulting in that all important thrust. As the pressure builds inside the combustion chamber, the hot gases exit the nozzle throat, expanding into the nozzle exit cone and continues to accelerate (depending on the design of the nozzle), resulting in an even more efficient use of the stored kinetic energy of the propellant.

There are single use motors and reloadable motors. The single use motors are common up to G power. After that, most motors are reloadable. High power motors begin at H power so most High Power rockets use reloadable motors. What’s a reloadable motor? Reloadable motors have separate hardware and propellant that you assemble yourself. The motor case and the two ends are reusable. You purchase the motor hardware and reload it over and over. Rocketeers messed up when it came to the description of the “motor casing hardware.” They use the term “motor” interchangeably and it confuses people. Just so you know. Anyway, you buy the propellant and the disposable internal parts as a motor, but actually it’s a kit.

Nowadays there are lots of motor manufacturers and each sells hardware that works only for their motors. One company has a system in which the propellant and inner section comes pre-assembled and all you do is screw it into the case. However, the average motor kit usually contains:
  • The slugs of propellant (called grains

    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-rings

    A rubber gasket used to contain gasses in solid rocket motors

  • A nozzle

    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

    (one company has a graphite nozzle that is reuseable – others are not).
  • 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.
Here is how a reloadable solid fuel motor works:

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’s always easy to find a rocketeer to help you assemble your first couple of motors.


Motors also come in standard diameters. I’ll list them and the powers usually found in that size but know that they overlap somewhat. This allows for versatility in rocket design. Each diameter also comes in different lengths to make the different powers possible.
  • 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.