A. A. Paletsky

Molecular-Beam Mass-Spectrometry to Ammonium Dinitramide Combustion Chemistry Studies

The methods for the study of flame structure and kinetics of the thermal decomposition of solid propellant by probing mass spectrometry are described. The developed methods were applied to the study of ammonium dinitramide (ADN) combustion chemistry. The study has shown that along with ADN decomposition, sublimation takes place to give gaseous ADN followed by dissociation to yielding ammonia and dinitraminic acid (HD). Gaseous ADN has been observed in ADN decomposition products. The structure of ADN combustion zones at 1-6 atm was studied using a molecular-beam mass-spectrometry as well as a microthermocouple technique. Three combustion zones have been observed. Gaseous ADN has been discovered in the first cool flame zone at 3 atm. Gaseous ADN dissociation on NH3 and HD followed by HD decomposition in the near-surface zone are key reactions resulting in a temperature rise of about 150 K. The second high-temperature zone is found within 6-8 mm from the ADN burning surface at 6 atm. The main reaction in this zone is ammonia oxidation by nitric acid and the combustion temperature is 1400 K. The third zone was observed at 40 atm, the measured final temperature was —2000 K. The obtained data form the basis for the development of a chemical mechanism of reactions in both the ADN flame and combustion model.

Modeling the chemical reactions of ammonium dinitramide (ADN) in a flame

Ammonium dinitramide (ADN) is a new energetic material that can be used as an oxidizer in solid rocket propellants. In the last few years, a number of papers devoted to the study of ADN combustion mechanism have been published. Molecular beam mass-spectrometry and thermocouple measurements are here used to study ADN flame structure at pressures of 0.3 and 0.6 MPa. Measurements include species concentrations and temperature profiles. A general mechanism for describing the chemical reactions in an ADN flame (172 reactions and 31 species) is developed based on these experimental studies and literature data. The scheme includes a sub-mechanism (98 reactions and 22 species) for propellants combustion suggested by Yetter and Dryer. The latter is further supplemented by a set of 74 reactions, including 63 steps suggested by Lin and Park and the ADN dissociation reactions suggested by the authors of this paper. The correlation between the experimental and calculation results is satisfactory.

Study of Solid Propellant Flame Structure by Mass-Spectrometric Sampling

Abstract A method of mass-spectrometric sampling (MSS) of solid propellant (SP) flames for SP flame structure investigation is described. Two types of instruments have been developed: 1) microprobe sampling and 2) molecular-beam mass-spectrometry (MBMS)using time-of-flight mass spectrometry (TOFMS). Addressing an important question concerning estimated errors of probe measurements, detailed investigations on substantiating the sampling technique for flames with narrow combustion zones have been carried out. Examples of the mass-spectrometric probing technique applications to study the flame structure of the following solid propellants and their ingredients are presented: RDX, HMX, sandwich type systems based on HMX and double-base propellants. Applications of MSS to study of unstable combustion of SP are described.

Flame structure of ADN/HTPB composite propellants

Abstract Visual flame characteristics, burning rates, and final flame temperatures for propellants based on ammonium dinitramide (ADN) and hydroxyl-terminated polybutadiene (HTPB) at different HTPB concentrations (3–20%) have been studied at pressures of 0.05 to 0.6 MPa. The flame structure was studied by probing molecular-beam mass-spectrometry, thin thermocouples, and video recording. The burning-zone width at 0.1 MPa was ∼1.5 mm. Thermocouples revealed temperature fluctuations of about ±400 K at 0.1 MPa in the flame zone within 1.5 to 4 mm from the burning surface. Along with temperature fluctuations, fluctuations also occurred in the intensities of the mass peaks of mass spectra of samples withdrawn from the flame. These are a consequence of the nonhomogeneous and nonstationary combustion of the propellant. Video recordings revealed the existence of several brightly luminous flame jets (torches) of about 1 mm in diameter at the burning surface, disappearing from one site and re-appearing at another. Combustion products were found remote from the burning surface and in its immediate vicinity by using molecular-beam mass spectrometry. The reactions in the condensed phase (mainly the ADN decomposition reaction) control ADN/HTPB propellant combustion. The oxidation of HTPB decomposition products in the gas phase increases the heat release there and accelerates reactions in the first and second zones of the ADN flame.

Mass spectrometric study of combustion of GAP- and ADN-based propellants

Abstract The flame structure of composite propellants and sandwiches based on ammonium dinitramide (ADN) and glycidyl azide polymer at 0.015 to 0.3 MPa was studied by molecular beam mass spectrometry. A zone near the surface, ∼1.5 mm wide, was detected, where reactions occur. The gas composition near the surface of burning ADN laminae at 0.1 MPa was close to that near the surface of burning pure ADN at 0.3 MPa. Among the species responsible for reactions in the flame near the surface, the most probable are HNO3, dinitraminic acid, and the vapor of ADN. The luminous zone of the flame extends more than 10 mm from the surface. The composition of the final combustion products has been determined by freezing at the temperature of liquid nitrogen and indicates incomplete combustion. The temperature profiles measured with thin thermocouples confirm the measured widths of the near-surface and luminous zones. The final temperature at the pressure of 0.3 MPa is as high as 2600 K.

Study of Combustion Characteristics of Ammonium Dinitramide/Polycaprolactone Propellants

This paper is devoted to the investigation of main characteristics and mechanism of combustion of the composite solid-rocket pseudopropellant based on ammonium dinitramide and polycaprolactone. Experimental data on the dependence of the burning rate on pressure in the pressure range of 4 ‐8 MPa for ammonium dinitramide/polycaprolactone propellant with different additives and with polycaprolactone of different molecular weight are presented in the paper. The dependence of propellant burning rate on particle size of oxidizer and initial temperature has been also investigated. Composition of the combustion products of the propellant at pressures of 4 MPa using two different systems of sampling has been determined. The temperature proe le in the combustion wave of some propellants has been obtained with aid of thin e at thermocouples. Temperature of the e nal combustion products of the propellant without additive and for some propellants with additive has been determined by a thermocouple method. Also the ine uence of a CuO catalyst on temperature proe le has been investigated. Flamestructureofammonium dinitramide/polycaprolactonepropellant at 0.1 MPa has been studied. Data obtained elucidating combustion mechanism and place of action of catalyst are discussed.

Probe Method for Sampling Solid-Propellant Combustion Products at Temperatures and Pressures Typical of a Rocket Combustion Chamber

The paper describes a new probe method for determining the quantitative composition of solid-propellant combustion products at temperatures of 2500–3200 K and pressures of 4-8 MPa under conditions typical of rocket motor conditions. A two-step probe is described, which allows a sample to be frozen without passing through the main shocks inside the sampler. The gas dynamics and the kinetics of chemical reactions were simulated to asses the correctness of sampling. It is shown that during sampling from a flame, the relative change in concentrations for most of the stable components does not exceed 3%, and for H2 and O2, it does not exceed 12%. The method permits additional operations with a sample, in particular, separation of CO and N2 with subsequent analysis on a time-of-flight mass spectrometer. The CO and CO2 concentrations in the combustion products of the model composite solid propellant — ammonium dinitramide (ADN) with polycaprolactone (pCLN( — were determined at a pressure of 4 MPa.

Combustion Chemistry of Energetic Materials Studied by Probing Mass Spectrometry

The methods of probing mass spectrometry (PMS) for diagnostic of flames and for the study of kinetics and mechanism of the thermal decomposition products of energetic materials (EM) are described. Several types of instruments based on microprobe and molecular beam mass spectrometric sampling have been developed. Time of flight mass spectrometer has been used. Apparatuses for high (10 atm) and low (

Multistage Mechanism of Thermal Decomposition of Hydrogen Azide

A kinetic mechanism for combustion of hydrogen azide (HN3) comprising 61 reactions and 14 flame species (H2, H, N, NH, NH2, NNH, NH3, HN3, N3, N2H2, N2H3, N2H4, N2, and Ar) was developed and tested. The CHEMKIN software was used to calculate the flame speed at a pressure of 50 torr in mixtures of HN3 with various diluents (N2 and Ar), as well as the self-ignition parameters of HN3 (temperature and pressure) at a fixed ignition delay. The modeling results of the flame structure of HN3/N2 mixtures show that at a 25–100% concentration of HN3 in the mixture, the maximum temperature in the flame front is 25–940 K higher than the adiabatic temperature of the combustible mixture. Analysis of the mechanism shows that burning velocity of a HN3/N2 mixture at a pressure of 50 torr is described by the Zel’dovich-Frank-Kamenetskii theory under the assumption that the burn rate controlling reaction is HN3 + M = N2 + NH + M (M = HN3) provided that its rate constant is determined at a superadiabatic flame temperature. The developed mechanism can be used to describe the combustion and thermal decomposition of systems containing HN3.

A numerical study of the superadiabatic flame temperature phenomenon in HN3 flames

The phenomenon of superadiabatic flame temperature (SAFT) was discovered and investigated in a low-pressure HN3/N2 flame using numerical modelling. A previously developed mechanism of chemical reactions in the HN3/N2 flame at the pressure 50 Torr and the initial temperature T0 = 296 K was revised. Rate constants of several important reactions involving HN3 (HN3 (+N2) = N2 + NH (+N2), R1; HN3 (+HN3) = N2 + NH (+HN3), R2; HN3 + H = N2 + NH2, R4; HN3 + N = N2 + NNH, R5; and HN3 + NH2 = NH3 + N3, R7) were calculated using quantum chemistry and reaction rate theories. Modified Arrhenius expressions for these reactions are provided for the 300–3500 K temperature range. Modelling of the flame structure and flame propagation velocity of the HN3/N2 flame at p = 50 Torr and T0 = 296 K was performed using the revised mechanism. The results demonstrate the presence of the SAFT phenomenon in the HN3/N2 flame. Analysis of the flame structure and the kinetic mechanism indicates that the cause of SAFT is in the kinetic mechanism: exothermic reactions of radicals with hydrogen atoms occur in the post flame zone, which results in the formation of super equilibrium H2 concentrations. The flame propagation velocity is largely determined by the second-order HN3 decomposition reaction and not by the reaction of HN3 with H, as was previously assumed. Calculation of the flame propagation velocity according to the Zeldovich-Frank-Kamenetsky theory with the decomposition reaction as a limiting stage yielded a value that agrees with that obtained in numerical modelling using the complete reaction mechanism.