AP and HMX are the two oxidizers used most often in modern solid propellants, either composite propellants or composite modified double base propellants. Although the two oxidizers have very similar combustion characteristics as monopropellants, they lead to significantly different characteristics when combined with binders to form propellants. Also, different characteristics result depending on the particular binder system used. This paper discusses various flame structures and mechanisms that

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... apparently lead to these similarities and differences, with emphasis on the qualitative effects of flame structure on combustion mechanisms. For AP composite propellants, the primary flame is more energetic than the monopropellant flame, leading to an increase in burn rate over the monopropellant rate. This also leads to a very strong particle size dependence. In contrast the HMX primary flame is less energetic than the HMX monopropellant flame and ultimately leads to a propellant rate significantly less than the monopropellant rate in composite propellants. In HMX composite propellants the primary flame apparently robs energy from the monopropellant flame leading to a reduced rate with little particle size dependence. In double base propellants HMX has little effect on the burning rate.