HNF Composite Propellants

When HNF or ADN particles are mixed with a GAP copolymer containing aluminum particles, HNF-GAP and ADN-GAP composite propellants are formed, respectively. A higher theoretical specific impulse is obtained as compared to those of aluminized AP-HTPB composite propellants.However, the ballistic properties of ADN, HNIW, and HNF composite propellants, such as pressure exponent, temperature sensitivity, combustion instability, and mechanical properties, still need to be improved if they are to be used as rocket propellants. 

Ammonium dinitramide (ADN) is a crystalline oxidizer with the formula NH4N(N02)2, that is, it is composed ofionicaUy bonded ammonium cations, NH4, and dinitramide anions, N(N02)2- Though ADN is crystalline and has a high oxygen content, similar to AP and KP, it has no halogen or metal atoms within its structure. ADN is used as an oxidizer in smokeless composite propellants, similar to AN and HNF. It melts at about 364 K, accompanied by the latent heat of fusion. 

Hydrazinium nitroformate (HNF) contains a relatively high concentration of oxidizer fragments, as shown in Table 2.6. When GAP is used as a binder of HNF particles, HNF-GAP composite propellants are made. The maximum of 285 s and the maximum Tf of 3280 K are obtained at (HNF) = 0.90 with an optimum expansion from 10 MPa to 0.1 MPa, as shown in Figs. 4.20 and 4.21, respectively. Since a... 

Fig.4.21 Adiabatic flame temperatures of HNF-GAP and HNF-HTPB composite propellants.

The ballistic properties of ADN, HNF, and HNIW as monopropellants and as oxidizers in composite propellants have been extensively studied.P2-351 Since ADN, HNF, and HNIW particles produce excess oxygen among their combustion products, these particles are used as oxidizer crystals in composite propellants. The pressure exponents of crystalline ADN and HNIW particles are both approximately about the same value as those for HMX and RDX when they are burned as pressed pellets. However, the pressure exponent of HNF is 0.85-0.95,135] higher than those of the other energetic crystalline oxidizers. 

When these oxidizer particles are mixed with a binder such as HTPB, a nitropoly-mer, or GAP, the burning rate decreases with increasing mass fraction of ADN or HNF particles.[3+l Though the temperature sensitivity of an ADN composite propellant is significantly high in the low-pressure region, 0.005 at 1 MPa, it decreases... 

Composite propellants consist of an oxidizer (AP/AN/ADN), a metallic fuel such as Al, Mg etc and a binder, usually a polymer which also serves as a fuel. Vacuum stability tests (VSTs) suggest that composite propellants are intrinsically more stable than SB, DB and propellants. However, use of more exotic ingredients such as oxidizers (ADN and hydrazinium nitroformate, HNF), binders [poly([NiMMO)] and poly([GlyN)] are likely to introduce severe compatibility-related problems [30, 31]. Some recent research in this direction indicates that stability of such propellants is largely determined by the chemical and mechanical properties of propellants. However, early evidence of deterioration generally comes from a change in their mechanical properties rather than from chemical investigations [32]. 

In search of a high performance and eco-friendly oxidizer, extensive research has been going on in this area for more than two decades and as a result, two oxidizers ammonium dinitramide (ADN) and hydrazinium nitroformate (HNF) have emerged as strong contenders of AP in composite propellants and their important aspects are as follows. 

The m.p. of HNF lies in the range of 115-124 °C depending on its purity and it is suitable for processing of propellant formulations. The m.p. of an oxidizer is important because curing of composite propellants is usually done at elevated temperature. Further, there is no measurable moisture uptake for HNF. 

Ammonium dinitramide (ADN) and hydrazinium nitroformate (HNF) are potential high performance and eco-friendly replacements for AP in composite propellants and efforts are being made all over the globe in this direction. Similarly, there is a need to study these high performance oxidizers and their salts for pyrotechnic applications. Some groups of researchers have already initiated research in this direction and several alkali dinitramide salts have been synthesized and characterized for their elemental content, solubility, thermal behavior and crystal structures. 

The research into energetic molecules which produce a large amount of gas per unit mass, led to molecular structures which have a high hydrogen to carbon ratio. Examples of these structures are hydrazinium nitroformate (HNF) and ammonium dinitramide (ADN). The majority of the development of HNF has been carried out in The Netherlands whereas the development of ADN has taken place in Russia, USA and Sweden. ADN is a dense non chlorine containing powerful oxidiser and is an interesting candidate for replacing ammonium perchlorate as an oxidiser for composite propellants. ADN is less sensitive to impact than RDX and HMX, but more sensitive to friction and electrostatic spark. 

The ballistic properties of ADN, CL-20, and HNF as monopropellants and composite propellants used as oxidizers have been studied extensively133-39. Since ADN, CL-20, and HNF produce excess oxygen in their combustion products, these are used as oxidizer crystals of composite propellants. Though the pressure exponent of ADN and CL-20 is approximately 0.7 3X, about the same value as that of HMX and RDX when burned as pressed pellets, the pressure exponent of HNF is about 0.85-0.95[39]. 

Presently, research is on-going into trying to find alternatives to AP/A1 (see also Ch. 1.2.4). The problems with the AP/A1 mixtures which contain HTPB as a binder, are two-fold. On the one hand AP is toxic and should be substituted for this reason alone (see Ch. 1.2.4). On the other hand, such formulations are also problematic in slow cook-off tests (SCO test, see Ch. 6.2). It appears to be the case that here the AP slowly decomposes during the formation of acidic side-products. These acidic side-products then react with the HTPB binder, which can result in the formation of cracks und cavities in the composite, which consequently negatively affects the performance and sensitivity. Possible alternatives for AP are ADN, HNF and TAGNF. However, they cause other problems, such as, for example, the low thermal stability (ADN melts at 93 °C and already decomposes at 135 °C) and the binder compatibility is not always guaranteed either. Further research work is absolutely necessary in order to find better oxidizers for solid propellants. In this context, the following requirements must be fulfilled ... 

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