Characterization of combustion behavior of newly formulated NF2-based solid propellants

In an effort to increase the performance of solid propellant rockets, new and novel energetic propellant ingredients were synthesized and several families of propellant formulations were tested. These new propellants were characterized experimentally in order to determine the burning-rate characteristics, temperature sensitivity, the Arrhenius form of burning-rate law, and the stability behavior in terms of the Novozhilov parameters. Propellant samples were tested in an optical strand burner for a broad range of pressures (0.35 MPa < P < 20.7 MPa) and initial temperatures (- 25 degreesC < T < 50degreesC). Propellant burning-surface temperatures were determined from the subsurface temperature measurements made using fine-wire thermocouples. Some of the new propellants tested contained difluoramino (NF2-based) energetic groups. The theoretical performances of NF2-based propellants are substantially higher than those of the traditional propellants. This paper also compares the combustion behavior of NO2-based propellants [such as DNP, 1,4-dinitropiperazine (C4H8N4O4), and TNP, 1,4,4-trinitropiperadine (C5H8N4O6)] to that of NF2-based propellants [CL22, 4,4-bis-difluoramino-N-nitropiperadine (C5H8N4O2F4)]. Results of strand-burning experiments showed that all three CL22 propellants (CL22(B) with boron additives, CL22(Al) with aluminum additives, and CL22 without any energetic particles as additives) consistently had higher burning rates than those of DNP and TNP propellant formulations tested. The DNP propellant was found to be unstable near 10.3 MPa. The particle size of DNP has a significant effect on the burning-rate pressure exponent, n. The n value of the TNP propellant was found to be unacceptably high for rocket applications. CL22 propellants show satisfactory stability characteristics for limited ranges of pressure and initial temperatures. For Ti greater than or equal to 50 degreesC, a drastic increase of burning rate of CL22 propellants was observed. This phenomenon is believed to be associated with the relatively low melting temperature behavior of CL22 oxidizer crystals, and recommendations for future improvements are given in the paper.