The present invention is directed to solid propellants for rocket motors, gas generators and comparable devices, based on a high energetic oxidizer, combined with a binder material.

Solid propellant combinations are prepared by blending solid oxidizers such as ammonium perchlorate or hydrazinium nitroformate with a liquid precursor for the matrix material. By curing of the binder a solid propellant is obtained, consisting of a polymer matrix and oxidiser in the form of solid inclusions.

For ammonium perchlorate quite often liquid hydroxyl terminated polybutadienes are used as precursor for the matrix material. However, for hydrazinium nitroformate these precursors were not used, as they were deemed unsuitable for combination with hydrazinium nitroformate (US-A 3,658,608 and US-A 3,708,359). It was expected that the hydrazinium nitroformate combination with the polybutadiene would be unstable, due to reaction of the hydrazinium nitroformate with the double C=C bond.

The present invention is based on the surprising discovery that it is possible to combine hydrazinium nitroformate with hydroxyl terminated unsaturated hydrocarbon compounds and accordingly the invention is directed to a stable solid propellant for rocket motors, gas generators and comparable devices, comprising a cured composition of hydrazinium nitroformate, an unsaturated hydroxyl terminated hydrocarbon compound and a curing agent.

A chemically stable solid propellant, with sufficient shelf life for practical use can be obtained, provided that hydrazinium nitroformate of high purity is used, which can, among others, be realized by improvements in the production process like the use of pure starting materials, containing substantially less impurities (e.g. chromium, iron, nickel, copper, and oxides of the metals, ammonia, aniline, solvent and the like).

A chemically stable material shows absence of spontaneous ignition during storage at room temperature (20oC) of at least 3 months, although it is preferred to have an absence of spontaneous ignition for at least 6 months, more preferred one year.

A further improvement in the stability of the solid propellant can be obtained by using hydrazinium nitroformate which contains substantially no hydrazine or nitroform in unreacted form. This can for example be obtained by changes in the production process, as discussed in WO-A 9410104 and a strict control of the addition rate of hydrazine and nitroform during the production of hydrazinium nitroformate, resulting in a purity of the recrystallised hydrazinium nitroformate between 98.8 and 100.3, based on H₃O⁺ and a pH-value of a 10 wt.% aqueous solution of hydrazinium nitroformate of at least 4. It is preferred to use hydrazine and nitroform in substantially equimolar ratio', more in particular a molar ratio of hydrazine to nitroform of from 0.99:1 to 1:0.99.

Further, the water content of the different propellant ingredients, especially the water content of the binder components influences the stability and accordingly a water content of less than 0.01 wt.% in the binder is preferred. In addition to the aforementioned aspects, stabilisers may be added to further improve the shelf-life.

Further important variables in the production of the solid propellant are the selection of the curing temperature of the matrix material, the choice of the curing agent and the curing catalysts and inhibitors.

The solid propellant combinations according to the invention have various advantages. They possess an increased performance, expressed as an increased specific impulse for rocket applications and as an increased ramjet specific impulse for gasgenerator applications. The ramjet specific impulse is defined as: I_sp,r = (I+ϕ)I_sp - ϕ U₀/g.

In which ϕ is the weight mixture ratio of air and gas generator propellant, I_sp is the specific impulse with ambient air as one of the propellant ingredients and U₀ is the velocity of the incoming air.

As the energy content of the system is high, it may become possible to use less oxidiser, thereby increasing the overall performance.

Further, it is to be noted that the material is chlorine free, which is an advantage from both corrosion and environmental considerations.

Depending on the actual use various compositions of the solid propellant according to the invention are possible. According to a first embodiment a solid propellant can comprise 80 to 90 wt.% of hydrazinium nitroformate, in combination with 10 to 20 wt.% of binder (hydroxyl terminated unsaturated hydrocarbon). In case a fuel additive, such as aluminium is added, 10 to 20% of the hydrazinium nitroformate in the above composition can be replaced by the additive. These formulations are especially suited as rocket propellants with improved performance.

For the purpose of a gas generator propellant for ramjets or ducted rockets, the following combinations are preferred. 20 to 50 wt.% of hydrazinium nitroformate, combined with 50 to 80 wt.% of hydroxyl terminated unsatured hydrocarbon and a curing agent. As in the above composition it is also possible to use an amount of fuel additive for increased performance, such as Al, B, C and B₄C, whereby this fuel additive may be present in 10 to 70 wt.%, in combination with 10 to 70 wt.% of the hydrocarbon, keeping the amount of hydrazinium nitroformate identical.

As indicated above, the solid propellant is prepared from a cured composition of hydrazinium nitroformate and a hydroxyl terminated unsatured hydrocarbon. The hydrazinium nitroformate preferably has the composition described above, whereby the amount of impurities is kept at a minimum.

The binder or polymeric matrix material is prepared from a hydroxyl terminated unsaturated hydrocarbon. In view of the production process of the solid propellant this hydrocarbon preferably has a low molecular weight, making it castable, even when containing substantial amounts of solids. A suitable molecular weight for the hydrocarbon ranges from 2000 to 3500 g/mol. After blending the solid hydrazinium nitroformate with the liquid hydrocarbon and a curing agent it can be poured in a container and cured.

Curing is preferably carried out by crosslinking the hydroxyl terminated hydrocarbon, preferably hydroxyl terminated polybutadiene, with a polyisocyanate. Suitable polyisocyanates are isophorone-di-isocyanate, hexamethylene diisocyanate, MDI, TDI, and other polyisocyanates known for use in solid propellant formulations, as well as combinations and oligomers thereof. In view of stability requirements it is preferred to use MDI, as this provides the best stability (longest shelf-life). The amounts of hydrocarbon and polyisocyanate are preferably selected in dependence of the structural requirements so that the ratio of hydroxyl groups in the hydrocarbon and the isocyanate groups is between 0.7 and 1.2. Curing conditions are selected such that an optimal product is obtained by modifying temperature, curing time, catalyst type and catalyst content. Examples of suitable conditions are curing times between 3 and 14 days, temperatures between 30 and 70°C and use of small amounts of cure catalysts, such as DBTD (< 0.05 wt.%)

In case further fuel additives are included in the propellant these are added prior to curing.

Generally speaking, also minor proportions, especially up to no more than 2.5 wt.% of substances such as phthalates, stearates, metal salts, such as those of copper, lead, aluminium and magnesium, said salts being preferably chlorine free, such as nitrates, sulfates, phosphates and the like, carbon black, iron containing species, commonly used stabiliser compounds as applied for gun propellants (e.g. diphenylamine, 2-nitrodiphenylamine, p-nitromethylaniline, p-nitroethylaniline and centralites) are added to the propellant combinations according to the invention. These additives are known to the skilled person and serve to increase stability, storage characteristics and combustion characteristics.

The invention is now further elucidated on the basis of the following examples.

Cured samples of HNF/HTPB formulations with different polyisocyanates and additives have been prepared. Typical examples are shown in table 1, showing the stability of the compositions as a function of time and temperature.

For all cured samples (unless stated differently): NCO/OH = 0.900; curing time is 5-7 days at 40 °C, after which samples are either stored for an additional week at 40 °C, or at 60 °C for 1-2 days; solid load 50 wt%; additives 2 wt% (and 48 wt% HNF), unless stated differently.

Example 2. HNF/HTPB as a high performance propellant composition.

In table 2 the specific impulse of HNF/HTPB and HNF/AL/HTPB combinations are presented. Similar AP based compositions are presented for reasons of comparison. From table 2, it becomes apparent that HNF/AL/HTPB compositions possess higher specific impulses compared to AP/AL/HTPB compositions of similar solid load, whereas the HNF/HTPB composition has the additional advantage of low smoke properties due to the abundance of Al in the composition (at cost of some performance loss). Specific impulse(s) Solid load w% AP/HTPB HNF/HTPB AP/AL/HTPB

(19% AL) HNF/AL/HTPB

(19% AL) 80 276.6 290.8 314.2 327.3 82 283.1 296.9 318.6 330.8 84 289.9 303.4 324.8 334.3 86 296.9 310.2 329.1 338.2 88 303.6 317.2 331.7 344.4 90 309.0 324.1 332.9 348.8

Table 2. Comparison of the theoretical performance of new HNF/HTPB propellants compared to conventional AP/HTPB propellants (NASA CET 89 calculations, vacuum specific impulse, chamber pressure 10 MPa, expansion ratio 100, equilibrium flow conditions).

HNF/HTPB as a high performance fuel for a ducted rocket gas generator for ramjet applications. In Table 3 the ramjet specific impulses of a 30% and a 40% solids HNF/HTPB are listed in comparison to 40% solids AP/HTPB fuel and a GAP fuel. The latter two represent typical state-of-the-art fuels for ducted rocket gas generator propellants. In ducted rockets, fuel rich reaction products of a propellant are injected into a combustion chamber where it reacts with oxygen from the incoming air.

From Table 3 it becomes apparent that HNF/HTPB compositions possess higher ramjet specific impulses compared to other compositions which are momentary under consideration for ramjet fuel applications. In addition to high performances, HNF/HTPB has the additional advantages that it has a low signature (HCl free exhaust), potentially a high pressure exponent, increasing the gas generator throtteability and possibly lower oxidator loadings compared to AP-based gas generators, resulting in overall performance gains. Ramjet specific impulse (s) Oxygen/ fuel ratio GAP AP/HTPB

(40% solids) HNF/HTPB

(40% solids) HNF/HTPB

(30% solids) 2.5 369.1 298.6 304.3 289.6 10 743.0 901.9 936.0 1010.0 15 785.6 981.5 1023.4 1121.1 20 799.3 1022.1 1070.1 1182.3 30 783.1 1044.8 1100.7 1234.7 40 737.3 1025.7 1087.2 1236.4

Table 3. Ramjet specific impulse for three different ducted rocket gas generator propellants (NASA CET 89 calculations, chamber pressure 1 MPa, exit pressure 0.1 MPa, exit pressure 0.1 MPa, sea level at 2.5 M, equilibrium flow conditions).