APCP Ammonium Perchlorate Composite Rocket Propellant

APCP is the most common high performance solid rocket propellant used by experienced amateurs for high performance rocket motors. It is the same stuff that NASA uses in the space shuttle boosters. It consists of Ammonium Perchlorate, a fuel-binder, and usually aluminum powder, magnalium powder or magnesium powder. The dual purpose fuel/binder is usually HTPB or PBAN, however other binders such as silicone¹ have been used. For high performance, a rough AP/HTPB/Al mass ratio is 70/15/15, and for low smoke compositions, the mix is closer to 80/18/2.² Other fuels used with AP include rubber, polyurethane and nitropolymer.³ A small percentage iron(III) oxide (around 7%) is added as a catalyst in large high power motors like the Nasa shuttle boosters.  Most APCP motors are designed as core-burners, especially when large in size.  Common bore shapes include circular, C-slot, moon and finocyl.  The geometry of the motor and core is called the propellant grain.  Each type of bore shape will significantly affect the thrust curve of the motor.  Nakka-rocketry has very nice page discussing different propellant grains.  Particle size also has an affect on burn rate and the thrust curve.  Generally, smaller particle size of the propellant constituents will yield a faster burn.  Red iron oxide powder is often added as a catalyst to uniformly increase the burn rate.  The ideal amount of added oxide usually lies within 0.5% to 3%⁴ depending on the binders and partical sizes.  Many amateurs move to APCP propellant after experimenting with the black powder propellant because it is much more energy dense.

High Performance:

Ammonium Perchlorate............................70%

HTPB....................................................15%

Aluminum Powder...................................15%

Low Smoke:

Ammonium Perchlorate............................80%

HTPB....................................................18%

Aluminum Powder...................................2%

Advantages:

• High performance, fast burn rate
• About twice the energy density of black powder
• Vast amount of information and data available
• Easier than PB or R-candy to make very large motors reliable
• No longer considered an explosive by the BATFE! (as of 3-16-2009)
• Easier to light than ANCP

Disadvantages:

• More expensive compared to R-candy propellant or ANCP
• Harder to get than R-candy propellant
• Less environmentally friendly than ANCP or R-candy (KNO₃/sugar)

Cautions:

Mixes with fast burn rates can rupture the motor in an explosive manner if the casing strength is insufficient or the pressure and rate are incorrectly calculated.  Ammonium perchlorate is sensitized by organic matter, and detonation is possible, releasing large amounts of gas.

Interesting Facts:

An amateur rocket named GoFast built by the Civilian Space eXploration Team (CSXT) achieved an amateur record breaking altitude of 72 miles (120 km).  The APCP motor provided over 1800 lbs of thrust for seven seconds, accelerating the rocket up to 36 miles at MACH 5 and allowing it to coast un-powered another 36 miles into space.

Some amateurs and motor suppliers like RCS, Aerotech alter APCP  propellant to achieve colored flame.  The addition of different metal salts (oxidizers) can be used to change the flame color.  Strontium nitrate can be used to get a red flame, while barium nitrate is used to get green flames.

1.  US Patent 4332631

2.  Wikipedia 9/26/2009

3.  NASA (http://www.hq.nasa.gov/pao/History/conghand/propelnt.htm)

4.  US Patent 7011722