Volker Weiser

Modelling Nitromethane Combustion

Summary In combustion research, nitromethane is an important nitro-compounded liquid propellant offering high performance. Additionally it can be used as a model substance for understanding mechanisms of combustion of solid propellants. The objectives of investigations were the burning behaviour and the flame spectrum in the UV=VIS spectral range because of the appearance of strong radical emissions also present in flames of other energetic materials. The evaluation is based on a detailed chemical kinetics and a linear combustion model including a reduced mechanism.

Burning Behaviour of ADN Formulations

The application of ADN for an effective oxidizer of propellants and explosives requires a detailed knowledge of the burning behaviour. The physical and chemical mechanisms of the combustion depend on pressure. Especially profiles of temperature and species in the flame are important to design propellant formulation of high performance and low signature of the rocket plume. In the presented study, pure ADN and ADN/paraffin mixtures were investigated as strands in an optical bomb at pressures of 0.5 MPa to 10 MPa. The application of non-intrusive combustion diagnostics for the investigation of fast burning energetic materials allowed the measurement of burning rates and profiles of temperature and gas components at various distances above the burning propellant surface. The burning rate was determined by using a video system and a special frame analysis. The acquisition and analysis of emission spectra in the UV/VIS allowed the investigation of rotational temperatures, the determination of particle temperatures and the identification of transient flame radicals. The vibrational temperatures of final combustion products resulted from band spectra emitted in the near and mid infrared spectral range. Burning rates of 5 mm/s to 70 mm/s were recorded showing a mesa/plateau-effect in the pressure range of 4 MPa to 7 MPa. The UV/VIS spectra indicated an emission from OH, NH and CN radicals. The strong emission of OH bands of the ADN/paraffin mixture allowed the investigation of rotational temperatures with a mean value of 2700 K which is closely below the adiabatic flame temperature of 2950 K. Additionally, one-dimensional intensity profiles of the flame radicals were measured. As combustion end products H2O, CO, CO2 and NO were found. NO could only be detected at a distance up to 2 mm above the propellant surface. The measured CO/CO2 fraction was higher as 10/1. Water could only be detected far above the propellant surface.

Ballistic Characterization of AlH 3 -Based Propellants for Solid and Hybrid Rocket Propulsion

Experimental analyses of laboratory-type composite solid rocket propellants and hybrid solid fuels based on �-AlH3 (alane) are presented. Chemical and physical properties of this peculiar energetic ingredient are discussed, ballistic properties experimentally evaluated under a variety of configurations, and flame structures compared with the corresponding aluminized propellants. The obtained results overall disclose a competitive performance of alane-based propellants with respect to the aluminized formulations and encourage its use in both solid and hybrid rocket propulsion for space exploration.

Investigation of the Burning Behavior of Cryogenic Solid Propellants

Cryogenic Solid Propellant (CSP)-technology is a new approach to develop more powerful rocket motors. CSPs include the advantages of classical solid propellants to save weight as well as those of a high energy content and safety of modern liquid propellants. The charges consist of liquid and/or gaseous fuels and oxidizers, both frozen. Two main versions of CSP-technology can be realised: 1. Mono-CSPs show the burning behavior of solid propellants. Experiments with mono-CSPs have been carried out under inert pressure conditions in a window bomb. Mono-CSPs have a stable burning behavior with a constant regression rate which follows the Vieilles law under varying pressure conditions. 2. The advantage of high safety is obtained by assembling oxidizer and fuel in sandwich configurations. The grain geometry governs the burning behavior. Such systems can be externally controlled, e.g. by the heat from a gas generator or they can work self-sustained. A Rod-in-Matrix burner shows self-sustained combustion in an inert pressure atmosphere with overall burning rates in a similar range as solid rocket propellants which obey also a Vieille-like pressure law. Disc stack burners have also been investigated, the combustion of which is strongly dependent on the disc thickness. For a short time Machs nodes have been observed in the exhaust plume of a disc stack burner. Currently, the temperature ranges are limited to the boiling temperature of liquid nitrogen. Therefore, liquid oxidizers like H2O2 have been used. However, for the first time a propellant strand of polymer rods embedded in solid oxygen was prepared and burnt. The experiments with CSPs end in the combustion of a small rocket motor showing no serious technical obstacles. Simplified models based on the heat flow equation can simulate the burning characteristics of the frozen energetic materials including phase transitions.

2,4,6‐Trinitrotoluene: A Surprisingly Insensitive Energetic Fuel and Binder in Melt‐Cast Decoy Flare Compositions

Aircraft are facing a steadily increasing threat by infraredguided ground-to-air and air-to-air missiles working in the l = 1–5 mm range. To fight these threats, aerial platforms eject pyrotechnic flares, which create an intense infrared signature to distract the seeker of the incoming missile and causes it to lose track of the target. Common firstand secondgeneration missile seekers track the hottest spot in the field of view (FOV) in the 1.9–2.6 mm range (a-band). Hence they are most advantageously countered by pyrotechnic flares that yield hot flames (T= 2000–2500 K) and a graybody type signature. Typical payloads of this type comprise magnesium/ fluorocarbon compositions. However, true aerial targets do not exhibit a graybody-type signature but radiate selectively in the range of the combustion products H2O (1.87, 2.7 mm) and CO2 (2.7, 4.3 mm). Thus so-called two-color seekers have been devised for advanced missiles that are able to distinguish between hot spot flares and true targets. These seekers evaluate the intensity ratio qb/a in two spectral ranges, a-band and b-band (3.5–4.8 mm). Thus hot graybodies yield qb/a< 1, whereas aircraft yield values between qb/a 5–20. To fight these threats, pyrotechnic flares which generate predominantly CO2 and little H2O have been described in the literature; Table 1 shows the composition of typical formulations. These formulations often comprise potassium perchlorate (KClO4) as oxidizer and oxygenated aromatic compounds, such as potassium benzoate (C7H5O2K; 1) or pyromellitic dianhydride (1,2,4,5-benzenetetracarboxylic anhydride, C10H2O6; 2), as fuel. [4] Mixtures of potassium benzoate and perchlorates are extremely sensitive to friction and impact and have caused disastrous explosions while mixing and handling. Pyromellitic dianhydride and potassium benzoate have a large negative enthalpy of formation DfH8 ( 954 and 529 kJmol , respectively), and thus yield insufficient spectral efficiency El (J g 1 s r ). In moist air, compound 2 is prone to undergo hydrolysis to give the tetracarboxylic acid, C10H6O8, which does not burn well with KClO4. The color ratio qb/a increases with increasing oxygen balance L (wt %) of a fuel; however, the spectral efficiency El decreases with the oxygen balance of the fuel. Thus it was hitherto impossible to increase both El and qb/a. Other payload types used in spectral flares resemble double base propellants and are based on nitroglycerine, (NG, C3H5N3O9), diethylene glycol dinitrate, (DEGN, C4H8N2O7), and nitrocellulose (NC, C6H7N3O11)n). [7] Payloads of this type are very sensitive and can detonate when struck by a bullet or nearby shock. In view of the deficiencies of current payloads, there is an urgent need for spectrally matched compositions that are safer to produce and less vulnerable to accidental stimuli and that prove to be at least equally or even more powerful than prior formulations. 2,4,6-Trinitrotoluene (TNT) was probably the most widely used explosive in military stores in the 20th century. Today it still plays an important role as energetic binder in melt-cast explosives based on nitroguanidine (NQ) and guanylurea dinitramide (FOX-12). TNT has been proposed as a component for detonating obscurant charges. However, owing to its character as a high explosive, it has never been considered as an energetic fuel and binder in slow burning pyrotechnic compositions. Herein, we report the findings on the use of TNT as both energetic fuel and melt-cast binder in pyrotechnic formulations. In the present study, mixtures of TNT and KClO4 were considered for use in spectrally matched decoy flare compositions. Table 2 shows the compositions investigated. The stoichiometry was varied between 35–50 wt% TNT to investigate the effect of oxygen balance of the composition, and Lx [Eq. (1)] on burn rate, color ratio, and spectral Table 1: Common spectral flare formulations A–C.

Experimental Investigation on Cryogenic Solid Propellants in a Combustion Chamber

The burning behavior of modular cryogenic solid propellants was investigated in two different test chambers. In a first stage, one optically accessible chamber allowed the observation of the diffusion controlled combustion of linear propellant grains using photographs, IR-cameras, pyrometers and spectrographs. The second phase led to the assembly of a rocket test chamber in which, for first time, a cylindrically symmetric 1kg propellant charge of cryogenic disc stacks of H2O2 as oxidizer could be investigated.

Combustion Behavior of Aluminum Particles in ADN/GAP Composite Propellants

Propellants containing ADN/GAP are regarded as a promising green alternative to AP/HTPB solid rocket propellants because of avoiding chloric acid emission. The addition of aluminum powder is a classical approach to increase the theoretical specific impulse of composite propellants. The optimum aluminum content is in the range of 16–18 % Al. But propellant formulations with ADN, a chlorine-free oxidizer, and GAP, an energetic binder, generate no chloric acid but more nitrogen on combustion achieving similar performance. Significantly different thermal and chemical conditions occur to the aluminum particles close to the burning surface. This study investigates the combustion behavior of aluminum particles in an ADN/GAP matrix in comparison to AP/HTPB at various pressures up to 15 MPa. The agglomeration of Al particles at the surface and burning behavior of aluminized AP/HTPB propellants has already been investigated and is extended to the ADN/GAP propellants. The temperature measurements close to the propellant surface indicate higher values near the Al boiling point that accelerates the melting of Al particles and influences the agglomeration process. At higher pressure the temperatures are in the magnitude of Al2O3 evaporation and decomposition close to 3000 K.