DINITRAMIDE BASED LIQUID MONO-PROPELLANTS The present invention relates to liquid propellants for the purpose of generating hot gases, or for the generating of energy-rich gases on decomposition, which gases can be combusted in a secondary reaction. These gases are suitable for driving a turbine, vane or piston motor, inflating air bags or for rocket propulsion, or other vessel or vehicle propulsion. More particularly the present invention relates to such propellants especially suited for space applications.

Background of the invention A high performing, low risk and low cost mono-propellant is the most attractive concept for rocket propulsion. A mono-propellant will require a minimum of hard ware components to build up a propulsion system and thus will lead to a minimum of complexity and cost.

The dominating mono-propellant used today for spacecraft propulsion is hydrazine. The major advantages of hydrazine systems are long flight heritage and well-established technology. The major drawbacks of hydrazine systems are the hazards involved.

Hydrazine is highly toxic and carcinogenic and hence, rigorous routines are required for manufacturing, handling and operation of hydrazine systems.

Due to hazards, and therefore the total cost, an alternative propellant is highly attractive.

Thus, hydrazine will sooner or later be replaced due to cost reduction, safer handling and new requirements on personal safety and environmental requirements. However, this requires that the alternative propellant reach maturity and has been flight qualifie.

Thus, new, less toxic, less hazardous, more energetic and higher density mono-propellants are desired.

Ammonium dinitramide (ADN) is a new solid oxidizer, mainly intended for high performance composite rocket propellants. ADN and other similar compounds are the subject of several patents for application as solid composite rocket propellants and as explosives, both for pyrotechnic applications in general and for other uses, such as in inflators for air-bags. The composite explosives of this type typically comprise ADN (or some other compound) as an oxidizer, an energetic binder (e. g. energetically substituted polymers), a reactive metal and other typical propellant ingredients such as curatives and stabilizers. One of the disadvantages of ADN, as a solid oxidizer, is its high hygroscopicity.

Summary of the Invention Thus, the existing liquid mono-propellants are subject to a number of disadvantages, such as health hazards for personnel handling the propellants, environmental hazards in general due to the toxic nature thereof. A further disadvantage of these liquid mono-propellants is the costs associated with the additional safety arrangements required for handling and usage of these mono-propellants. Therefore it is an object of the present invention to provide a novel liquid propellant that is low-hazardous both from a handling point of view and from an environmental one, and preferably does not develop smoke. In summary the propellant should exhibit the following properties: -low toxicity -low flamability -higher theoretical specific impulse (as compared to hydrazine) -higher density (as compared to hydrazine) -easily ignitable, by means of a controlled ignition mechanism -storable at a temperature between-10 and +70 °C, preferably +10 and +50 °C -low sensitivity.

The above stated object is achieved according to the present invention with a liquid propellant as defined in claim 1, comprising a solution of an oxidizer of the general formula X-D (I)

wherein X is a cation; and D is the anion dinitramide (-N (N02) 2), and a fuel. The cation can be selected from the group consisting of metals, organic ions and inorganic ions.

Examples of suitable cations are OHNH3+, NH4+, CH3NH3+, (CH3) 2NH2+, C2H5NH4+,(C2H5)2NH2+,C2H5)3NH+,(C2H5)4N+,(CH3)3NH+,(CH3)4N+, (C2H5)(CH3)2N+,(C3H7)4N+,(C4H9)4N+,(C2H5)(CH3)NH2+,(C2H5)(CH 3)NH2+, N2H5+, (H2NOH) 2+. CH3N2H +, (CH,) 2 3) 3 2H2+, (CH,) 4 (CH3)5N2+ The preferred cations are N2H5+, (H2NOH) 2+, OHNH3+, and NH4+.

Metal ions can be used, but will generally lead to the generation of smoke, which is often undesirable. Examples of groups of metals, which can be used, are the alkali metals, and the alkaline earth metals, especially the former, specific examples being lithium, sodium, and potassium ions.

The propellant comprises a fuel which can be selected from the group consisting of mono-, di-, tri-and poly-hydric alcohols, aldehydes, ketones, amino acids, carboxylic acids, primary, secondary and tertiary amines, and mixtures thereof, or any other compound which can be combusted with the dinitramide oxidizer, and in which said oxidizer is soluble, and/or which is soluble in a suitable solvent, such as water and/or hydrogen peroxide, wherein the dinitramide salt is soluble, thereby forming a liquid monopropellant exhibiting the above-mentioned desirable characteristics.

Thus, when ADN is used as the oxidizer in the propellants of the present invention, the high hygroscopicity of ADN is a major advantage, especially when said propellants contain water.

Examples of compounds usable as the fuel are polyhydric alcohols such as ethylene glycol, glycerol, erythritol, diethylene glycol, triethylene glycol, tetramethylene glycol, ethylene glycol monoethyl ether, propylene glycol, dipropylene glycol, dimethoxytetraethylene glycol, diethylene glycol monomethyl ether, the acetate of ethylene glycol monoethyl ether

and the acetate of diethylene glycol monoethyl ether; ketones, such as for example, acetone, methyl butyl ketone and N-methyl pyrrolidone (NMP); monohydric alcohols such as methanol, propanol, butanol, phenol and benzyl alcohol; ethers, such as dimethyl and diethyl ether, and dioxane; also, the nitriles such as acetonitrile; the sulfoxides such as dimethylsulfoxides; formamides such as N, N-dimethylformamide, N-methylfomamide; sulfones such as tetrahydrothiophene-1,1-dioxide; the amines such as ethylamine, diethylamine, ethanolamine, hydroxylamine; substituted hydroxylamines such as methyl and ethyl hydroxylamine; and any mixtures thereof. Polar fuels are preferred for their ability to dissolve the dinitramide salt. Thus, by using polar fuels, the use of any added water in order to increase the solubility of the dinitramide salt, can be minimised, or even avoided, as water will lower the impulse, as will be explained below.

To further increase the impulse, metallic fuels, such as Al, Mg, B, Zr, Ti, graphite, boron carbide, or carbon powder, or any combination thereof, can be suspended in the liquid propellant. A preferred metallic fuel is Al. However, as already mentioned above, the inclusion of a metal will lead to the generation of smoke on combustion.

The invention will now be described by way of non-limiting examples and the detailed description of preferred embodiments thereof, with reference to the attached drawing figures, in which: Solvent mixture refers to fuel + water (i. e. solvent for the oxidizer, in this case ADN); Fig. 1 shows a graph over the theoretical specific impulse for glycerol as compared to hydrazine, given a saturated solution of ADN at 0 °C, as a function of percentage by weight of fuel in solvent mixture; Fig. 2 depicts a Differential Scanning Calorimetry (DSC) chart showing the progress of the exothermal reactions of different propellants of the invention as the temperature is gradually increased.

Fig. 3 shows a graph over the theoretical specific impulse for different ADN based propellants saturated at 20 °C, having different fuels, as a function of the percentage by weight of the fuel in the solvent mixture, as compared to hydrazine.

Fig. 4. describes the solubility of ADN in different water/fuel-solvent mixtures.

Detailed description of preferred embodiments The present invention is directed to a family of liquid propellants having high specific impulse. The preferred propellants include a dinitramide salt, water and a mono-, di-, tri-or polyhydric alcohol as a fuel. The propellants according to the invention have several avantages over e. g. hydrazine, as already indicated above, the main ones being low toxicity per se, and essentially non-toxic combustion products.

Preferred examples of the fuel are alcohols, amino acids, and ketones, a suitable example of an amino acid being glycine. Also, ammonia (i. e. ammonia in water) can be used. By way of example a preferred ketone is acetone. More preferably, alcohols usable in the present invention are linear or branched lower alcohols comprising from 1 to 6 carbon atoms.

Specific examples of the latter are any of the isomers of methanol, ethanol, ethanediol, propanol, isopropanol, propanediol, propanetriol, butanol, butanediol, e. g. 1,4. butanediol, butanetriol, pentanol, pentanediol, pentanetriol, pentaerythritol, hexanol, hexanediol, hexanetriol, trimehylolpropane. Most preferably the fuels are non-volatile such as for example glycerol and glycine, the former of which is being preferred due to its good ignitability as seen in figure 2.

Examples of oxidizers usable according to the invention are hydroxyl ammonium hydroxyl amine dinitramide, hydrazine dinitramide, hydroxyl ammonium dinitramide (HADN), and ADN, of which hydrazine dinitramide and ammonium dinitramide are preferred. The most preferred oxidiser is ADN. Typical fuels are represented by methanol, ethanol, acetone, glycine, and glycerol, the latter being a most preferred fuel.

The specific impulse for a given propellant is a qualitative measure of the impulse generated by one unit of mass of the specific propellant under certain standard engine conditions. Specific impulse is inter alia related to the pressure and temperature inside the engine, the composition and thermodynamical properties of the combustion products, the ambient pressure, and the expansion ratio.

In order to determine the specific impulse for various propellants, calculations have been performed using the CET93 thermo-chemical program (Gordon, S., McBride, B. J., "Computer Program for Calculation of Complex Chemical Equilibrium Compositions, Rocket Performance,...". NASA SP-273, March 1976). This program uses the heat of formation, chemical composition, chamber pressure and expansion ratio as input data, and the obtainable output is the combustion temperature, specific impulse (Isp), characteristic velocity (C*) and reaction products.

Calculations were performed with the above program on ADN/water/fuel solutions for different fuels, which will be in more detail in the Examples. Also, in order to obtain results for said solutions under a temperature within the conventional operating interval of hydrazine, so as to have results comparable to those for hydrazine, the calculations were based on solutions saturated at 0 °C.

Calculations for glycerol and hydrazine, respectively were performed using the following data: Reactant Sum Formula Heat of formation (kJ/mol) ADN N4H404-146 Glycerol C3H803-668. 6 Water H201-285. 83 Hydrazine N2H4 50. 63 The calculations were based on a chamber pressure of 1.5 MPa assuming frozen flow, and the nozzle area ratio was set to 50. with the assumption of expansion to vacuum.

In the thermo-chemical calculations the heat of solution was not taken in to consideration.

The saturated mix compositions are according to measured data.

Thermo-chemical calculations were made for points at 10% intervals.

As can be clearly seen from Fig. 1, the theoretical specific impulse for a propellant according to the invention containing glycerol as the fuel is markedly higher than for hydrazine, for a certain concentration range, i. e. 20-50 % by weight.

Pure ADN decomposes at temperatures above 95°C but can be decomposed by acids at lower temperatures. Therefor it is assumed that a solid acid catalyst can decompose ADN or any ions thereof. An example of a solid acid catalyst is the silica-alumina catalyst. The silica to alumina ratio can tune the pH of this catalyst.

A typical liquid propellant formulation (saturated solution at 0°C) within the scope of the present invention has the following ingredients: Ingredient Weight-% ADN61 Water26 Glycerol13 It is to be understood that although this is a presently preferred formulation, the percentages given above can be varied within certain intervals, which can easily be established by the person skilled in the art by means of merely routine experimentation, as long as a liquid propellant is obtained. Thus, for a propellant according the invention containing water, and glycerol as the fuel, a suitable composition is from 15 to 55 % by weight of the fuel in solvent mixture (solvent mixture = water + fuel), and with reference to Fig. 1, a preferred composition is from 10 to 50 % by weight of fuel in solvent mixture, and more preferably,

25 to 45 % by weight of fuel in solvent mixture and most preferably about 61 % of ADN, about 26 % of water, and about 13 % by weight of glycerol.

As will be obvious to the person skilled in the art, the preferred composition of a specific propellant of the invention will, inter alia, be dependent upon the temperature selected at which the solution will be saturated. Said temperature should be selected so that the propellant will be storable and usable at a selected minimum temperature without the precipitation of any component thereof.

Water can be added in order to increase the solubility of the oxidiser, such as ADN, in a liquid fuel. Solid fuels might also be used if they dissolve in ADN/water solutions. For example, a dinitramide salt having an excess of oxygen could be used as the oxidiser, together with a fuel, consisting of a dinitramide salt having an oxygen deficit, dissolved in water.

To lower the flame temperature and/or the sensitivity of the specific propellant, the amount of water can be increased. However, increasing the amount of water will lower the specific impulse of the propellant. In order to reduce the lowering of the impulse due to the addition of water, some, or all, of the water can be substituted with hydrogen peroxide, having a comparable polarity to that of water. It is believed that the hydrogen peroxide will act as an additional oxidiser, and, will thus allow for a corresponding additional amount of fuel to be added to the propellant. As will be realised by the person skilled in the art, the amount of hydrogen peroxide used, if any, will be governed by the stability of the propellant obtained therewith.

Since the major function of the water in the liquid propellant according to the present invention is considered to be the function of a solvent for the oxidizer and the fuel, it is also conceivable to reduce or even omit the added water from the propellant if a fuel or a mixture of fuels is used in which the oxidizer can be dissolved, i. e. a fuel being a solvent for the oxidizer. This might also lead to an increase in the specific impulse for the specific propellant.

In order to study the behaviour of different combinations and compositions of ADN, water and fuel, solubility and density measurements have been made. Solubility at 0°C was measured with UV spectroscopy for higher boiling fuels, and density of saturated solutions was measured at room temperature. For volatile fuels the solubility at 0°C of ADN in water and different fuels were measured in a TGA (thermogravimetric analyzer), where possible, at different water/fuel ratios.

EXAMPLES The following is an explanation of the concept of"fuel in solvent mixture", used in the Examples.

A solution describes a liquid or solid phase containing more than one substance. where, for convenience, one of the substances is called the solvent, and may itself be a mixture, and the other substances, are called solutes.

In this case the solvent mixture comprises water (Sw>50%) and an organic fuel. The weight fraction of fuel in the solvent mixture is expressed as, <BR> <BR> <BR> <BR> <BR> <BR> S _ mF<BR> F- mFmW+ where m is the amount of the respective substance, and the indexes F and W are for fuel and water, respectively. The solute is the oxidiser salt ammonium dinitramide, ADN.

The solubility of ADN in the solvent is a function of SF and the temperature. We have studied the solubility of ADN in the solvent at 0 °C since it is close to the freezing point of hydrazine.

Super saturated solutions of ADN in the solvent were prepared at 0 °C and the solubility was then measured by UV-spectroscopy at 284 nm. The results are shown in Figure 4.

The curves in Figure 4 defines the upper limit for respective fuel, were the solution can exist. Above that line ADN will precipitate. To maximise the specific impulse (Ns/kg) for this type of propellants the amount of water shall be as low as possible. This means that the composition that gives the maximum impulse will be found somewhere on that line.

By fitting a second order polynomial to the data in the figure the weight fraction of ADN, W2, can be calculated as,,<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> wA=aO+alS,. +a, S,, ~.

The weight fraction of fuel and water in the solution are calculated as, (1-wA)SFwF= 1-wA-wF.wW= Now the specific impulse can be calculated by using, for example, CET 93. The <BR> <BR> composition (WA, WF, WW) that yields the maximum impulse is presented in the Tables 1 and 2.

In the Examples, the theoretical specific impulse (Isp) was calculated for a number of ADN/water/fuel solutions using the CET-93 program (vide supra), and the results of each example are presented in the following tables 1 and 2.

The results should be compared with hydrazine, for which, at the same conditions, Isp = 2,200 Ns/kg, and Ivsp of about 2,200 Ns/dm3.

In the following Tables, the temperature given is the theoretical temperature generated on combustion of the specific propellant.

Table 1. Composition at maximum theoretical vacuum specific impulse. Propellants saturated at 0°C. Pc=1.5 MPa, E=50. 12345Exampleno. Fuel Acetone Ammonia Ethanol Methanol NMP Fuel in solvent 25.0 40.0 29.0 32.0 32 mixture(%) ADN (%) 67.18 77.27 61.0 64.30 63.0 Fuel (%) 8. 20 19. 3 11.42 11.7 Water (%) 24.62 13.64 27.7 24. 28 25. 3 Density (g/cm3) 1.349 1.372 1.309 1 324 Isp (Ns/kg) 2541 2515 2422 2518 2455 Ivsp (Ns/dm') 3428 3449 3153 3333 * Temperature (K) 2157 2109 1872 2077 2046 * Density not measured.

Table 2. Composition at maximum theoretical vacuum specific impulse. Propellants saturated at 0°C. Pc=1.5 MPa, E=50. 6789ExampleNo. Fuel glycolEthylene 1,4-Butane- Trimethylol- diol. BDO propane. TMP Fuel in solvent mixture 33 33 24 25 ADN (%) 61.00 62. 00 63.62 64. 08 Fuel (%) 12. 87 12. 54 8. 73 8. 98 Water (%) 26.13 25. 46 27. 65 26. 94 Density (g/cm') 1.420 1. 391 1. 390 1.402 Isp (Ns/kg) 2425 2457 2460 2470 Ivsp(Ns/dm) 3444 3418 3418 3463 Temperature (K) 1972 2009 2005 2029

The propellant of Example 5, as measured with the DSC as shown in Fig. 2, ignites at 120 °C. In practical experiments, ignition has been observed when the propellant is dropped on a hot plate heated to a temperature of 200 °C.

From the above table it can also be seen that the propellants of the invention exhibit a high density, as compared to a hydrazine containing one, leading to an attractively high volume specific impulse.

It is to be understood that the specific impulse, and especially the volume specific impulse for any of the above-mentioned ADN/water/fuel solutions, in contrast to hydrazine, will be increased if solutions saturated at a higher temperature than 0 °C are used, since the solubility of the oxidizer and fuel generally increases with the temperature. Thus, the above-mentioned values based on solutions saturated at a temperature of 0 °C are to be regarded only as exemplary, and indicative of the excellent impulse characteristics of the liquid propellants of the present invention.

Thus, as can be clearly seen from Figure 3, the maximum specific impulse (Isp) values for different propellants, comprising solutions saturated at 22 °C, are higher than the ones presented in Table 1 for the corresponding solutions saturated at 0 °C.

The solubility of HADN in water or water + fuel is expected to be markedly higher than that of ADN, and thus, when used in the propellants of the present invention, HADN will lead to even higher Isp values, and, more importantly, to even higher Ivsp values.

An excess of fuel in relation to oxidiser may be useful for generation of energetically rich gases, which in turn. can be combusted in a secondary reaction.

An at present preferred composition is ADN/water/glycerol, mainly because it ignites at approximately 200°C on a hot plate, and it does not emit toxic or flammable vapours prior to ignition, unlike fuels such as ethanol, methanol and acetone, and is thus not volatile.

Also, small amounts of added substances, such as stabilizers or any other conventionally used substances in the art can be included in the propellants of the invention without departing from the scope of the invention. For example, since ADN is not stabile in acidic environment, small amounts of a suitable base might be added in order to stabilize the dinitramide. When a metal is used in the propellant of the invention for increasing the impulse, an agent for inhibiting sedimentation of the metal could be included, or an agent stabilising the suspension thereof.

However, it is conceivable that other combinations of oxidizer/water/fuel within the broad definition of the invention may have better performance, and it is to be regarded as being within the abilities of the man skilled in the art to find such combinations without undue experimentation.