NO139916B - SMOKE-FREE, STABLE-BURNING FUEL - Google Patents

Description

The present invention relates to stable burning, smokeless propellants, more particularly, high-energy ammonium perchlorate propellants based on a polybutadiene binder.

An absence of visible exhaust from a rocket engine is highly desirable, especially for military purposes. It is possible to achieve such an absence by eliminating from the propellant any material which, upon combustion, will form a solid particulate material (primary smoke). Double-based (nitrocellulose-nitroglycerin) propellants have been the usual propellants that have been used for this purpose. High-energy composite propellants based on ammonium perchlorate in an organic binder have used various compounds that form solid particles, aluminum in particular, to eliminate combustion instability and obtain a maximum specific impulse. By eliminating aluminum from the system, you eliminate the primary smoke, but instead get a problem with an unstable combustion when the propellants are combined with a high content of oxidizing agent, thereby obtaining a high specific impulse.

Investigations have shown that smokeless ammonium perchlorate propellants (AP) using a hydroxy-terminated polybutadiene (HTPB) will produce a smokeless exhaust (primary smoke) and burn stably in an engine if the burning speed is approx. 1.0 cm/sec. or lower at a pressure of ' JO kg/cm 2 . At burning rates above the aforementioned level, combustion instability has limited the use of such compositions.

Smoke is defined as solid propellant exhaust and includes all visible effects with the exception of flame or light effects. However, smoke can strictly be divided into two general categories: It can be primary, where solid particles in the exhaust affect the passage of light in the exhaust itself, regardless of the surroundings, or secondary (induced), where some of the gas components in the exhaust, such as HC1, HF, NOg or condensable water vapor affect the surrounding air, so that visible aerosols of solid particles or of liquid particles are produced. Sources of primary smoke from the propellant include unburnt carbon or metal oxides.

A choice of any propellant involves a choice between propellant factors, safety factors, storage factors and economic factors. The driving factors include specific impulse, density and thermal expansion characteristics, mechanical properties, burning rate,

combustion stability, the sensitivity of the chamber pressure to grain temperature as well as the erosiveness of the propellant. Safety factors include sensitivity to impact, friction, dropping, fire and sparks. Furthermore, heat stability or self-ignition temperature, risk during processing and manufacture, toxicity and possible toxicity of exhaust products can be specified as safety factors. Storage factors include polymer degradation, sensitivity to moisture, migration of plasticizers, and catastrophic phenomena associated with grain cracking and degradation of propellant bonds. If smokelessness was previously developed in the propellant, this usually resulted in one or more of the above-mentioned factors not being met to the desired extent.

Combustion instability is a complex phenomenon that includes a combination of the engine's internal configuration and dimensions, as well as the propellant. An engine shows instability when the propellant's combustion reactions to pressure and speed variations interact with the acoustic properties in the chamber in such a way that at In or more frequencies the acoustic energy is added to the energy developed by the propellant itself, and where this total energy exceeds the which is led away by friction damping or led out of the chamber by convection. Because said phenomena include an interaction between the engine configuration and the properties of the propellant and because these interactions are not fully understood, it is not always possible to design propellants and combustion chambers that will guarantee stable combustion.

Combustion instability is currently the primary problem when using smokeless propellants. For many years now, large quantities of aluminum have been used in solid propellants because this almost completely inhibits combustion instability. If the aluminum is removed by thereby making the propellant smokeless, then this means that the propellant itself shows completely unacceptable tendencies towards unstable combustion.

It is therefore a purpose of the present invention

to provide a smokeless propellant in which combustion instability is essentially suppressed.

Furthermore, there is an object of the present invention

to provide a propellant which is essentially free of primary smoke in the exhaust and which at the same time has a high specific impulse and burning rate without showing combustion instability.

Furthermore, it is an object to provide an ammonium perchlorate-containing propellant without aluminum and which maintains combustion stability at burning velocities greater than 1 cm/sec. by

o

a pressure of approx. 70 kg/cm.

It has now been discovered that by adding small amounts of additives selected from the group consisting of refractory metal carbides or oxides, will provide a stable burning, smokeless propellant for certain chamber-propellant interactive resonance frequencies and at burning rates above approx. 1 cm/sec. When you add small amounts of carbon in the form of hollow, thin-walled balls, whole or broken, or in the form of flakes, you also expand the resonant frequency range for stable combustion. Combustion stability in an even wider environment

prevail with respect to resonance frequencies, is achieved when adding small amounts of carbon powder together with metal carbides or oxide and carbon balls or flakes.

These and several other purposes and advantages of the present invention will be apparent from the following description,

where reference is made, among other things, to the attached drawings, where

figure 1 is a diagram showing the firing curve for a double reaction engine where the propellant contains additives according to the present invention, and

figure 2 is a diagram showing the combustion curve for

a propellant without such additives.

According to the present invention, it is provided

a stable burning, smokeless solid propellant containing a

cured intimate blend of:

a major amount of a solid inorganic oxidizing salt, a minor amount of a combustible synthetic hydrocarbon elastomer formed by chain extension and cross-linking of functionally terminated liquid butadiene polymers, and 0.2-5 wt-# of the propellant of a combustion stabilizing additive, characterized in that the additive essentially consists of a combination of: (1) an oxide or carbide with a melting point of at least 2000°C selected from thorium, tungsten, silicon, molybdenum, aluminium, hafnium, zirconium and vanadium oxide and - carbide, and (2) particulate carbon in the form of small plates or hollow spheres with a thickness, or wall thickness, of between 1 and 10 microns and a length, or diameter, of between 25 and 400 microns.

The propellant usually contains a high amount of combustible solids, typically over 65% by weight, and an amount of the elastomeric binder usually below 15% by weight. A smaller amount of burning rate accelerators is also used, usually below 3% by weight. The oxidizing agent is preferably ammonium perchlorate, as other possibilities are HMX and RDX.

The hydrocarbon elastomer can be carboxy-terminated polybutadiene hardened with amines or epoxides, polybutadiene-acrylonitrile-acrylic terpolymers hardened with epoxides, as well as hydroxy-terminated polybutadienes hardened with diisocyanates. It is preferred to use hydroxy-terminated polybutadienes for economic reasons, reactivity, they are easily available and have good mechanical properties. The butadiene can be produced by so-called lithium-initiated polymerization (Li-HTPB) or a free-radical-initiated polymerization

(FR-HTPB).

The propellant may further contain a smaller amount, i.e. usually below 10%, of various additives, such as curing agents, stabilizers or thixotropic regulators or reactive polymeric modifying compounds, such as one or more diols or polyols. The isocyanate is usually present in an amount at least equivalent to that required to react with the hydroxy prepolymer and hydroxyl-substituted modifying compounds. The equivalent weight of the liquid prepolymer is the least

7,.iS.

1,000, usually not more than 5,000. The functionality of the polymer

is advantageously from 1.7 to 3.0, preferably from 1.9 to 2.3, so that elastomeric polymers with a molecular weight of at least 3,000 are formed by cross-linking and chain extension. As high molecular weight prepolymers may require heat to reduce viscosity, the molecular weight is preferably from 1,000 to 4,000.

The pdlyisocyanate for curing the prepolymer can be chosen from those with the general formula R(NC0)m where R is a di- or polyvalent organic radical with from 2 to 30 carbon atoms, and where m is 2, 3 or 4* R can be alkylene, arylene , aralkylene or cycloalkylene. It is preferred that the organic radical is essentially a hydrocarbon in character, although the presence of unreactive groups containing elements other than carbon and hydrogen may be permitted, which is also the case with reactive groups which are not capable of reacting with isocyanate- groups that are possibly capable of forming urea or carbamate bonds, which will affect the desired reaction.

Examples of suitable compounds of this type include benzene-1,3-diisocyanate, hexane-1,6-diisocyanate, toluene-2,4-diisocyanate (TDI), toluene-2,3-diisocyanate, diphenyl-methane-4,4 '-diisocyanate, naphthylene-1,5-diisocyanate, diphenyl-3,3'-dimethyl-4,4'-diisocyanate, diphenyl-3,3'-dimethoxy-4,4'-diisocyanate, butane-1,4- diisocyanate, cyclohex-4-ene-1,2-diisocyanate, benzene-1,3,4-triisocyanate, naphthylene-1, 3 > 5 >7-tetraisocyanate, metaphenylene diisocyanate (MDI), isocyanate-terminated prepolymers, poly- aryl polyisocyanates and the like.

The polyols are preferably, but not limited to, diols or triols and can be either saturated or unsaturated aliphatic, aromatic or certain polyester or polyether products. Examples include glycerol, ethylene glycol, propylene glycol, neopentyl glycol, pentaerythritol, trimethylolethane, glycerol triricineolate or alkylene oxide adducts of aniline such as Isonol which is N,N-bis-(2-hydroxy-propyl)-aniline and many other well-known polyols which can be added to the binder composition to control the degree of cross-linking.

The particular compound as well as the amount of this compound used in depends on the functionality and nature of the hydroxyl-terminated prepolymer and the polyisocyanate used in the binder composition.

As the functionality on Li-HTPB is usually somewhat below 2, the polyol is preferably a triol, so that a cross-link between polymer chains is obtained by a reaction with isocyanates.

As examples of polyols, one can mention glycerol triricinoleate (GTRO) and Isonol (a propylene oxide adduct of aniline), N,N-bis-(2-hydroxy-propyl)-aniline. The polyisocyanate is present in a sufficient amount to satisfy the stoichiometry, i.e. the functionality of HTPB and other polyols present. The polyisocyanate can be di-, tri- or higher functional and can be aliphatic in nature, such as hexane diisocyanate, but is preferably an aromatic polyisocyanate such as TDI. An analytical curing agent can be used. These agents can be metal salts, such as metal acetylacetonates, preferably thorium acetylacetonate (ThAA) or iron acetylacetonate (FeAA).

The combustion stability-promoting additives can

are used alone, but are preferably used in combinations and used in concentrations of 0.2-5 wt-# of the propellant as single ingredients or combined.

The use of carbon in the refractory metal compounds, such as zirconium carbide, will have a minimal effect on the freedom from smoke. Carbon will of course burn completely to CO and C02, while zirconium carbide in an amount of 0.5$ will produce 0.7 g of solid Zr02 per 100 g of pre-burnt propellant. Smoke measurements during the firing of propellants produced with an additive and one without, showed that the light transmission through the exhaust jet was the same for both propellants, which shows that said ZrC had no measurable effect on the amount of primary smoke produced.

It is assumed that both the carbon and ZrC function via a special damping mechanism. Moreover, the carbon and ZrC present represent two different classes of compounds. One acts as a particulate damper near the burning surface. Carbon is completely consumed during combustion and cannot act to provide particulate damping the entire time the gas is present in the engine. The other agent, namely ZrC is particulate and is present in the gas phase either as ZrC or ZrOg, most likely as ZrC.

As carbon can be used "Thumax" (0.3v0, and when the product is used in the form of flakes or small plates, the preferred size is from 10 to 150 u x 1 to 8 p thick. Whole spheres give improved efficiency compared to broken spheres , eliminating an instability that occurs around 2200 Hz. An elimination of frequencies above 5-000 Hz can be provided by further adding carbon powder.

Carbon spheres that have proven effective in the present invention are the following carbon spheres.

Theoretically, the combustion rate of a propellant depends only on the chamber pressure. In reality, it also depends on the speed of the gas flow that passes over the burning surface. The higher the gas velocity across a point on a grain, the higher the combustion rate will be at this point. Some propellants are more sensitive to erosive burning than others. However, erosive burning is more prevalent in propellants with low burn rates than in those with high burn rates.

Unstable combustion is a phenomenon that is common in all propellant systems, although not for all specific propellants within a single system. Furthermore, it is the case that additives which in one system can regulate the combustion instability, can have no effect or a harmful effect on another propellant or other binder system. Apparently, unstable combustion is more common with high-energy propellants than with low-energy propellants. Tests indicate that unstable combustion is due to the occurrence of transverse or longitudinal acoustic oscillations in the combustion gas during combustion. These fluctuations result in areas of high and low velocity around and along the grains, and this has a marked effect on the local burning rate. In high-velocity areas, the rate of combustion rises very rapidly, producing a further increase in pressure. In low-velocity areas or at junctions, the burning rate will be very low. In such cases, it can be seen that an uneven combustion of the grains can produce a premature breakdown, even if the average chamber pressure does not exceed the maximum chamber pressure. Extremely erratic operation and chamber breakdown are usually associated with worsened and uncontrollable resonance or unstable combustion, although in some rockets this can only be detected with the help of high-frequency equipment. It seems that erosive and unstable combustion are closely related phenomena.

Combustion instability in smokeless propellants in accordance with the present invention was studied in nT,, burner which is a standard device for experimental measurement of combustion instability. Said combustor uses opposing cylindrical grains and is typically used at pressures from 35 to " JO kg/cm. The chamber length was varied to obtain fundamental acoustic frequencies near 3,000 and 4,000 Hz. These tests were performed to determine the following parameters :

otg = growth constant for acoustic pressure

AP = amplitude for acoustic pressure fluctuations

Rk = response function, burning rate changes

in relation to pressure changes

Cylindrical 12-part grains of a hydroxy-terminated polybutadiene binder system containing a stoichiometric amount of TDI and an appropriate amount of ammonium perchlorate and various additives were prepared. The preparation was worked up into cylindrical grains which were suitable for the aforementioned "T'* burner test, and the results of the test are given in the following table.

The high burn rate propellant without additive, i.e. Example No. 2, is more unstable at 3,000 Hz, i.e. higher a AP and 11^. The propellants that contained spent carbon balls (Example 5) or zirconium carbide (Example 8) or these additives in combination (Example 7) eliminated the instability at 3,000 Hz, while obtaining some advantage at 2,000 Hz, especially with regard to reduced response function (R^). Propellant No. 6 incorporating standard amorphous, rubber-grade carbon powder, designation P-33> shows some reduction in instability at 3-000 Hz, but is not as effective as carbon in the form of spent hollow spheres (Example 5 )-

This reduction in combustion instability shown in the "T,," burners has been verified in engine combustions in a dual reaction engine where the grains were composed of 88% ammonium perchlorate (AP) in an HTPB binder with 0.5% zirconium carbide (ZrC) .Although some instability due to the DC shift could be observed, this shift was only 10% of that seen with the propellant without additive.Furthermore, the onset of the shift was delayed until the end of the combustion phase.

Another engine was fired using the combination of 0.5% ZrC and 0.5% partially broken down carbon balls, i.e. composition no. 8. The results were even better with this engine. The DC offset was completely eliminated, and a residual pressure coupled with a maximum amplitude of only 0.07 kg/cm at an operating pressure of 84 kg/cm 2 was obtained. This minor instability is perfectly acceptable within the normal operating limits of fixed rocket engines.. The firing curve for this double reaction engine is shown in figure 1, where composition no. 8 was used in the firing phase itself. Figure 2 shows a propellant of the same composition, but without additives, and shows large pressure variations, which is a result of combustion instability.

Additional tests were carried out with the <n>T<M-> torch, to investigate the effect of 1% ZrC (no carbon) on the stability of the same propellants used to evaluate the 0.5% mixture with

carbon, also to investigate the effect of using a lower percentage content of ammonium perchlorate, whereby a propellant with a lower combustion rate was produced. The results are shown in the following table.

Data from the above table shows that 1% ZrC (Example No. 9) corresponds to the propellant which contained the mixture of additives (Example No. rJ). A use of some carbon is considered significantly better, as this does not create any particulate smoke. Firings of the propellants of Examples Nos. 10 and 11 were both stable until the ratio S^/S^ was 1 or less, after which the firing became unstable. Stø indicates the area of burning propellant, while Sqq is the cross-sectional area of the combustion chamber. Slow-burning propellants containing a smaller amount of ammonium perchlorate, as shown in Example No. 12, show improved stability with this composition, being stable at frequencies of 2200 and 2600 Hz. Further <n>T<M->burner data is shown in the following table.

Control propellants 17 and 18 show that without additives all AP propellants were unstable at 2000 and 3000 Hz, although they were stabilized at a frequency of 4000 Hz. It is obvious that higher burn rates, i.e. propellant no. 18, increase the instability at 3000 Hz.

The effect of different forms of carbon can be seen in propellants 14, 29, 13 and 15- Neither the P-33 (Example 14) nor the carbon fibers (Example 15) provided an improvement in stability at 87% AP. Carbon balls A-100 (Example 13) provided improved stability at 87% AP and 3000 Hz. At 88% AP, the carbon balls gave improved stability at both 3000 and 4000 Hz.

Zirconium carbide (Example No. 16) provided significantly improved stability at 3000 and 4000 Hz. Further testing of carbon balls,

1% and ZrC, 0.5%, as single additives in a "T" burner and also in engines (Examples 19, 20 and 21) showed that these compositions were unstable in the <w>T" burner at 2500 Hz. Motor firings of the propellant from Example 21 also showed that these compositions were unstable when the propellant band had burned out to a diameter corresponding to a frequency of 4000 to 5000 Hz.

In combination, the ZrC and the carbon balls (Examples 25, 19> 26, 23 and 24) produced stable combustion in the <n>T" burner at 25OO Hz and was also stable in 22.5 cm O.D. grains, when the propellant was fired in engines with a burnout frequency of 27OO Hz. The effect of the combination provides an improvement in stability that is better than that shown by the single ingredients when used alone.

A further evaluation of the effect of carbon and ZrC was tried in examples 27, 28 and 29, and showed that both carbon powders as well as spheres gave improved stability at 2600 Hz, although all combinations were unstable at 2200 Hz. The amorphous black carbon powders were stable in the "T" burner, even at an S^/S^ ratio as low as 1, i.e., the ratio of the propellant's burning surface to the cross-sectional area of the combustion chamber, while the propellant with carbon balls with ZrC was unstable in this area. Although this instability was detectable in the »»Tn burners, no instability could be observed in engine firings, which is due to the fact that at an S^/S^ ratio of 1, the grains are fired as flat pieces, and you get no contribution from sidewall combustion, which is typical when firing grains with said configuration. The results show that the stability above 2500 Hz is essentially due to the combination of carbon and ZrC and is not dependent on the shape

on the carbon.

The data given in Table 5" show a comparison between the <w>T" burner results at 63 and 175 kg/cm and tests carried out in a normal engine at 63 and 105 kg/cm at 21 C. The binder was in each example a softened HTPB.

The correlation between motor diameter and motor resonance frequency is shown in the following table:

The data given in table 5 show a good correlation between

the results obtained with a <M>T<M->burner and the engines. This is particularly clear for example in No. 471 where the "C burner shows stability at 2600 Hz, while the engine showed stability at 2690 Hz, and where the "TM<-> burner showed instability at lower frequencies below 2200 Hz.

Example No. 36 {89% AP, speed =1.03 cm/sec. by

70 kg/cm 2 ) and which contained no additive, was unstable both in the wT" burner at 2500 Hz and in the engine even at a higher frequency of approx. 4000 Hz.

Example No. 37 (88% AP, 0.5% Fe^, velocity = 1.47 cm/sec. at 70 kg/cm^) was again unstable at higher frequency in the engine, indicating an effect of higher burn rate on the instability.

Example no. 38 (87% SP, speed = 1.22 cm/sec. at

70 kg/cm 2 ) containing 100 carbon balls and 5 ^ ZrC was stable in the <«»>T" burner and in the engine at firings down to 48OO Hz, illustrating the effect of the additive combination.

Example no. 39 (87% AP, speed = 1.35 cm/sec. at

70 kg/cm ) which contained 200 y. carbon balls and 5 ^ ZrC, were stable in the "T<n->burner, showing that the 200 ji carbon balls were as effective as the 100 ^u balls.

Example no. 4-0 (88% AP, speed = 1.40 cm/sec. at

70 kg/cm ) contained only 0.5% ZrC and was unstable in the "T" burner and at ^5000 Hz in the motor, showing that the carbon balls are necessary for stability.

Example no. 41 (87% AP, speed = 1.32 cm/sec. at

70 kg/cm ) containing 1% 200 ^1 carbon spheres was unstable in the "TM<->burner, showing that ZrC is required in combination.

Example no. 42 (88% AP, speed = 1.03 cm/sec. at

70 kg/cm ) containing 1% 200 ^1 carbon balls was unstable in the "TM burner and in the motor at ^4,000 Hz, again showing that the combination was necessary to achieve stability.

Remaining propellants illustrate the effect of the combination of carbon balls and ZrC. The results from both the <M>T<M> burner and the engine show the effectiveness of the additive combination in that stability is achieved within a wide range with respect to combustion rates, i.e. from 1.0 to 1.5 cm/sec. at 70 kg/cm o at frequencies as low as 2500 Hz and a content of oxidizing agent from 87 to 88%. At burning speeds above 1.5 cm/sec. at 70 kg/cm 2 it was

an instability at 2500 Hz in the "T<w->burner. The stability of the engine was maintained at temperatures from -39.6° to 57i2°C as shown by the engine fired with the propellant of Example 47*

It is thus obvious that solid additives, such as refractory metal carbides alone, irregular thin carbon particles, such as broken carbon balls or the combination of refractory metal compounds with various forms of carbon, are capable of providing stable-burning, high-energy, smokeless propellants without significant loss of specific impulse, even though aluminum is completely eliminated from the fuel.

The following table shows a series of tests carried out with a <M>T<n> burner for fuels containing other refractory compounds, such as 0.5% by weight of hafnium oxide, niobium carbide or tantalum carbide in combination with 0.5% by weight 200 micron carbon balls and 87% ammonium perchlorate (AP).

Example 5^

A propellant was prepared with the following ingredients: 19 kg of the propellant was fired in a normal engine. The engine developed a force of from 180 to 360 kg, and no frequencies above 5,000 Hz were found.

Propellants which do not contain additives according to the present invention will not burn stably if the ammonium perchlorate content is not below 80% by weight. This will lower both the propellant's impulse and density. The propellant according to the present invention, containing stabilizing, smokeless additives, makes it possible to process propellants containing over 85% ammonium perchlorate, whereby a solid propellant with high density can be produced which burns stably with a high specific impulse and without visible smoke.

Claims (5)

1. Stable-burning, smokeless solid propellant containing a cured intimate mixture of: a major amount of a solid inorganic oxidizing salt, a minor amount of a combustible synthetic hydrocarbon elastomer formed by chain extension and cross-linking of functionally-terminated liquid butadiene polymers, and 0.2 -5% by weight of the propellant of a combustion stabilizing additive, characterized by. that the additive essentially consists of a combination of: (1) an oxide or carbide with a melting point of at least 2000°C selected from thorium, tungsten, silicon, molybdenum, aluminium, hafnium, zirconium and vanadium oxide and carbide , and (2) particulate carbon in the form of small plates or hollow spheres with a thickness, respectively wall thickness, of between 1 and 10 microns and a length, respectively diameter, of between 25 and HOO microns. 2. Propellant according to claim 1, characterized in that the hydrocarbon elastomer used as binder is present in an amount of no more than 15% by weight, and in that the additive is present in an amount of 0.1-4% by weight. 3- Propellant according to claim 2, characterized in that the binder is a chain-extended and hardened, liquid polybutadiene polymer with an equivalent weight of between 1000 and 5000 and a functionality of between 1.7 and 3.0. 4. Propellant according to claim 35, characterized in that the oxidizing salt is ammonium perchlorate in an amount of between 85 and 90% by weight. 5. Propellant according to claim 1, characterized in that the additive comprises 0.2-1% by weight of a metal carbide with a particle size in the range of 2-10 microns.