Cohen, Joseph (Sacramento, CA)
Zimmerman, Gilbert A. (Sacramento, CA)

05/360867

12/09/1975

06/07/1973

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Aerojet-General Corporation (El Monte, CA)

60/219

149/110, 149/21, 149/2, 149/76, 149/19.900, 149/19.100, 149/20

C06B23/04; C06B33/00; C06B45/02; C06B45/10; C06B23/00; C06B45/00; C06D5/06

149/19.9,20,44,76,21,2,19.1 60/219

Padgett, Benjamin R.

Miller E. A.

Ansell, Edward Jacobs Marvin O. E.

What is claimed is

1. A stable burning, solid propellant composition absent visible smoke on burning comprising a cured, intimate mixture of:

2. 2 to 5% by weight of the composition of a stability enhancing additive consisting essentially of the combination of:

3. an oxide or carbide having a melting point of at least about 2000°C selected from the group consisting of thorium, tungsten, silicon, molybdenum, aluminum, hafnium, zirconium and vanadium oxides and carbides; and

4. particulate carbon.

5. A composition according to claim 1 in which the binder is an elastomeric hydrocarbon polymer present in an amount of no more than 15% by weight and the additive is present in an amount from 0.1 to 4% by weight.

6. A composition according to claim 2 in which the binder is a chain extended and cured liquid polybutadiene polymer having an equivalent weight between 1,000 and 5,000 and a functionality between 1.7 and 3.0.

7. A composition according to claim 3 in which the oxidizing salt is ammonium perchlorate present in an amount of between 85% and 90% by weight.

8. A composition according to claim 4 in which the additive comprises 0.2 to 1% by weight high melting carbide having a particle size ranging between 2 to 10 microns.

9. A composition according to claim 5 in which the carbide is zirconium carbide.

10. A composition according to claim 5 in which the carbide is hafnium carbide.

11. A composition according to claim 4 in which the additive comprises a mixture of high melting carbide and particulate carbon.

12. A composition according to claim 8 in which the particulate carbon is selected from hollow, thin-walled carbon spheres and carbon flakes.

13. A composition according to claim 9 in which said additive further includes carbon powder.

14. A composition according to claim 9 in which the particulate carbon is in the form of hollow carbon spheres having a diameter between 100 and 200 microns and a wall thickness from 2 to 8 microns.

15. A method for producing thrust in the absence of visible smoke comprising the steps of:

16. A composition according to claim 2 in which the butadiene polymer is selected from a carboxy-terminated polybutadiene cured with an amine or an epoxide, a butadiene-acrylonitrile-acrylic terpolymer cured with an epoxide and hydroxy-terminated polybutadiene cured with a diisocyanate.

17. A composition according to claim 11 in which the hollow carbon spheres are unbroken.

BACKGROUND OF THE INVENTION

1. Field of the Invention

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

2. Description of the Prior Art

The absence of a visible exhaust from a solid rocket motor is a highly desirable attribute, particularly for military applications. Such performance is possible by eliminating from the propellant formulation any material which will form a solid particulate on combustion (primary smoke). Double-base (nitrocellulose-nitroglycerin) compositions have been the principal propellants used for smokeless applications. Although more desirable because of higher performance, the composite propellants based on ammonium perchlorate in an organic binder have used materials which form solid particulates, principally aluminum, to eliminate combustion instability and maximize specific impulse. Eliminating aluminum from the composite system eliminates primary smoke but brings in the problem of combustion instability when the propellants are formulated with high oxidizer content for high specific impulse.

Recent work has shown that smokeless ammonium perchlorate (AP) propellants using a hydroxy-terminated polybutadiene (HTPB) binder will yield a smokeless exhaust (primary smoke) and burn stably in a motor if the burning rate is about 0.40 in/sec or lower at 1,000 psia. At burning rates above this level, combustion instability has limited the usefulness of such compositions.

Smoke is defined in terms of solid-propellant exhaust as including all visible signature effects with the exception of flash or luminosity effects. Smoke is more strictly considered to be of two general categories: either primary, wherein solid particles in the propellant exhaust affect its light transmissivity independently of the environment, or secondary (induced), wherein some of the gaseous components in the exhaust such as HCl, HF, NO ₂ or condensible water vapor interact with the ambient air to produce visible aerosols of liquid or solid particles. Sources of primary smoke from the propellant include unburned carbon and metal oxides.

The selection of any propellant involves the determination of performance factors, safety factors, life factors and cost factors. Performance factors to be considered include specific impulse, density and thermal expansion characteristics, mechanical properties, burning rate, combustion stability, sensitivity of chamber pressure to grain temperature and propellant erosivity. Safety factors include sensitivity to impact, friction, dropping, fire and spark. Also to be considered under safety are thermal stability or auto ignition temperature, processing hazards, toxicity and exhaust product toxicity. Life factors include polymer degradation, moisture sensitivity, plasticizer migration and catastrophic phenomena connected with grain cracking and bond failure. Previously, smokelessness resulted in definite penalties in one or more of these factors or determinants of the factors.

Combustion instability is a complex phenomena involving the combination of the inner motor configuration and dimensions as well as propellant. A motor exhibits instability when the burning response of the propellant to pressure and velocity fluctuations interacts with the acoustics of the chamber cavity such that, at one or more frequencies, the acoustic energy added to the system by the propellant exceeds that which is dissipated by frictional damping or carried from the chamber convectively. Because the phenomena does involve the interrelation of motor configuration and propellant properties and because these interactions are not completely understood, it is not always possible to specify propellant or chamber design procedures which will guarantee stable burning.

Presently the primary problem in the use of smokeless propellants is combustion instability. For many years the use of high percentages of aluminum in solid propellants almost completely inhibited combustion instability. The removal of aluminum to make the primary exhaust smokeless causes the propellant to exhibit unacceptable tendencies toward unstable combustion.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a smokeless propellant in which combustion instability is substantially suppressed.

A further object of the invention is to provide a propellant substantially free of primary smoke in the exhaust and exhibiting a high specific impulse and burning rate without exhibiting any combustion instability.

Yet another object is the provision of an ammonium perchlorate loaded propellant that is absent aluminum and which maintains combustion stability at a burning rate greater than 0.40 in/sec at a pressure of about 1,000 psia.

In accordance with the invention it has been discovered that the addition of small amounts of additives selected from refractory metal carbides or oxides will provide a stable burning smokeless propellant for some chamber-propellant interactive resonant frequencies and at a burning rate above 0.40 in/sec. When a small amount of carbon in the form of hollow, thin walled spheres, whole or broken, or flakes, is also added the regime of resonance frequencies for stable combustion is broadened. Combustion stability for a still broader band of resonant frequencies is obtained when a small amount of carbon powder is included with the metal carbide or oxide and the carbon spheres or flakes.

These and many other objects and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the firing curve for a dual thrust motor in which the booster propellant grain of the bipropellant configuration contains additives according to the invention; and

FIG. 2 is a graph showing the firing curve for a propellant grain without additives in the booster grain.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The propellant composition usually contains a high proportion of combustible solids, typically in excess of 65% by weight, a small proportion of binder, usually below 15% by weight, and a small amount below 3% by weight of burning rate accelerator. The combustible solids usually comprise an oxidizer such as ammonium perchlorate, HMX or RDX and 0.2-5% by weight of the combustion stabilizing solid added in accordance with the invention.

Preferred binders are elastomeric hydrocarbon polymers formed by the chain extension and cross-linking reactions of functionally terminated liquid polybutadiene polymers. Such polymers may include carboxy-terminated polybutadiene cured with amines or epoxides, polybutadiene acrylonitrile-acrylic terpolymers cured with epoxides and hydroxy-terminated polybutadiene cured with diisocyanates. Hydroxy-terminated polybutadienes are preferred due to cost, reactivity, availability considerations and mechanical properties. The butadiene may be derived from the lithium initiated polymerization (Li-HTPB) or free radical initiated polymerization (FR-HTPB).

The composition may also contain a minor amount below 10% of various additives such as cure promoters, stabilizers and thixotropic control agents, or reactive polymeric modifiers such as one or more diols or polyols. The isocyanate is generally present in at least an equivalent amount sufficient to react with the hydroxy prepolymer and hydroxyl substituted modifiers.

The equivalent weight of the liquid prepolymer is at least 1,000 and not usually more than 5,000. The functionality of the polymer is advantageously from about 1.7 to about 3.0, preferably from about 1.9 to 2.3 to form by cross-linking and chain extending elastomeric polymers of molecular weight of at least 30,000. Since higher molecular weight prepolymers may require heat to reduce viscosity, the molecular weight is preferably from 1,000 to 4,000.

The polyisocyanate for curing the prepolymer can be selected from those of the general formula (R(NCO) _m in which R is a di- or polyvalent organic radical containing from 2-30 carbon atoms and m is 2, 3 or 4. R can be alkylene, arylene, aralkylene or cycloalkylene. It is preferred that the organic radical be essentially hydrocarbon in character although the presence of unreactive groups containing elements other than carbon and hydrogen is permissible as is the presence of reactive groups which are not capable of reacting with isocyanate groups capable of forming urea or carbamate linkages such as to interfere with 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, polyaryl polyisocyanates and the like.

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. Exemplary compounds include glycerol, ethylene glycol, propylene glycol, neopentylglycol, pentaerythritol, trimethylolethane, glycerol triricineolate, or alkylene oxide adducts of aniline such as Isonol which is N,N-bis-(2-hydroxypropyl) aniline and many other polyols well known in the art which can be incorporated into the binder composition to control the degree of cross-linking. The particular compound and amount utilized is dependent on the functionality and nature of the hydroxyl terminated prepolymer and polyisocyanate employed in the binder composition.

Since the functionality of Li-HTPB is generally slightly less than 2, the polyol is preferably a triol so as to provide cross-linking between polymeric chains upon reaction with isocyanates. As exemplary polyols, mention may be made of glycerol triricinoleate (GTRO) and Isonol (a propylene oxide adduct of aniline), N,N-bis-(2-hydroxypropyl)-aniline. The polyisocyanate is present in an amount necessary to satisfy stoichiometry, that is, the functionality of the HTPB and any other polyol present in the composition. The polyisocyanate may be a di-, tri- or higher functional material and may be aliphatic in nature such as hexane-diisocyanate but is preferably an aromatic polyisocyanate such as TDI. A catalytic cure promoting agent can be utilized. These agents may be metal salts such as metal acetylacetonates, preferably thorium acetylacetonate (ThAA) or iron acetylacetonate (FeAA).

The combustion stability promoting additives in accordance with the invention may be used alone but are preferably used in combination at concentrations as low as 0.2% as single ingredients or combined. While no upper limit is theoretically non-functional, however, with respect to degradation of performance and optimum exhaust smoke characteristics, the concentration of the solid additives should not exceed about 3% by weight of the propellant composition. The refractory metal carbide or oxide should have a melting point of at least about 2,000°C.

Suitable high melting materials are the carbides and oxides of metals including thorium, tungsten, silicon, molybdenum, aluminum, hafnium, vanadium. The refractory compound should be provided in the form of fine particles ranging between 2 to 10 microns. The use of carbon in refractory metal compounds such as zirconium carbide will have a minimal affect on smokeless performance. Carbon will, of course, burn completely to CO and CO ₂ while zirconium carbide at a level of 0.5% will produce about 0.7g of solid ZrO ₂ per 100g of propellant burned. Smoke measurements made on firings of a propellant formulated with additive and one formulated without showed that the light transmission through the exhaust plume to be the same for both propellants, demonstrating that the ZrC had no measurable effect on the amount of primary smoke produced.

It is believed that both the carbon and ZrC function through a particulate damping mechanism. Further, the carbon and ZrC represent two different classes of material. One functions as a particulate damper close to the burning surface. Carbon is completely consumed in the combustion process and cannot act to provide particulate damping the entire time the gas is present in the motor. The other, ZrC, is a particulate which is present in the gas phase either as ZrC or ZrO ₂ , most probably as ZrC.

The carbon additive when utilized in combination with the refractory oxide or carbide can have diverse physical form and size. However, when the carbon is utilized alone as a combustion stabilizer, it should preferably have a thickness between 1 and about 10 microns and a length of between about 25 to 400 microns. A preferred form of carbon is small particles such as platelets or spheres or carbon powder such as Thumax (0.3μ). When in the form of flakes or platelets, the preferred sizes are 10 to 150 μ × 1 to 8 μ thick.

Unbroken spheres provide improved effectiveness with respect to broken spheres, eliminating an instability appearing at 2200 Hz. Elimination of frequencies above 5,000 Hz is provided by the further addition of carbon powder to the composition.

Carbon spheres that have been found effective in the invention are the following grades of carbon spheres described below produced by Kureha Chemical Industry Company, Ltd., Tokyo.

Table 1 ______________________________________ Property A-100 A-200 ______________________________________ App. mean diameter, microns 110 200 Diameter range, microns 75-150 150-250 Wall thickness, microns 2-3 3-8 Bulk Density, g/cc 0.10-0.25 0.07-0.20 Particle density, g/cc 0.15-0.40 0.15-0.35 ______________________________________

Theoretically, the burning rate of a propellant is dependent only on the chamber pressure. Actually it is also dependent on the velocity of gas flow over the burning surface. The higher the gas velocity across the point on a grain, the higher the burning rate at that point. Some propellants are more susceptible to erosive burning than others. In general, erosive burning is more prevalent in lower burning rate propellants than in those with high burning rates.

Unstable burning is a phenomena common to all propellant systems yet not to all propellants within a system. Furthermore, additives which in one system may control combustion instability may have no affect or an adverse affect in another propellant or binder system. It appears that unstable burning is more common with higher energy propellants than with lower energy propellants. Tests have indicated that unstable burning is a result of the production of transverse or longitudinal acoustical oscillations of the combustion gases during burning. These oscillations result in areas of high and low velocity around or along the grain which have a marked effect on the local burning rate. At a high velocity area caused by oscillation of the gas, the burning rate rises rapidly, causing a further increase in pressure. At a low velocity or nodal point, the burning rate is very low. It may be seen that the non-uniform burning of the grain can cause premature break-up even if the average chamber pressure does not exceed the maximum chamber design pressure. Extremely uncontrolled performance and chamber failures are commonly associated with aggravated, uncontrolled resonance or unstable burning, although in some rockets it can be detected only by high frequency instrumentation. It appears that erosive and unstable burning are related phenomena.

Combustion instability of the candidate smokeless propellants in accordance with the invention was studied in a "T" burner which is a standard device for experimental measuring of combustion instability. The T burner device uses opposing cylindrical grains and is usually operated at pressures of 500 and 1,000 psi. The chamber length was varied to provide fundamental acoustic frequencies near 3,000 and 4,000 Hz. The tests were utilized to determine the following parameters:

α _g = growth constant for acoustic pressure

ΔP = amplitude of acoustic pressure oscillations

R _b = response function, ratio of burning rate change to pressure change

Cylindrical grains were formulated with 12 parts of an hydroxy-terminated polybutadiene binder system containing a stoichiometric amount of TDI and an appropriate amount of ammonium perchlorate and different additives. The composition was formed into cylindrical grains suitable for the T burner test and the results of the test are provided in the following table.

Table 2 ____________________________________________________________ ______________ Frequencies 2000 Hz 3000 Hz 4000 Hz Example AP Additive r in./sec. No. Wt% Type Wt% at 1000 psia α _g ΔP R _b α _g ΔP R _b α _g ΔP R _b ____________________________________________________________ ______________ 1 88 None 0 .42 40 115 1.33 12 18 <.15 -- Stable -- 2 88 None 0 .58 70 75 1.73 63 105 1.02 -- Stable <.16 3 88 Aluminum 0.5 .55 -- -- -- 47 60 .55 -- Stable <.13 4 88 Al ₂ O ₃ 1.0 .64 60 60 1.19 34 60 .66 30 7 .39 5 88 Broken Carbon 1.0 .64 52 65 1.06 -- <1 .145 -- Stable <.14 Spheres 6 87 P-33 Carbon 1.0 .51 54 160 1.52 28.5 50 .7 -- Stable <.2 7 87 (Broken Carbon 0.5) .49 36 60 1.15 -- ≉0 .21 -- Stable <.18 (Spheres ) (ZrC 0.5) 8 88 ZrC .5 .57 53 70 1.35 -- ≉0 .24 -- Stable <.14 ____________________________________________________________ ______________

The higher burning rate propellant without additives, Example No. 2, is more unstable at 3,000 Hz, i.e., higher α _g ΔP and R _b . The propellants containing broken carbon spheres (Example 5) or zirconium carbide (Example 8) or these additives in combination (Example 7) eliminate the instability at 3,000 Hz and above with some benefit obtained at 2,000 Hz, particularly in the reduced response function (R _b ). Formulation No. 6 including a standard amorphous, rubber grade of carbon black, P-33, shows some reduction of the instability at 3,000 Hz but is not as effective as the carbon in the form of broken, hollow spheres (Example 5).

This decrease in combustion instability shown in T burners has been verified in motor firings of a dual thrust configuration where a booster grain composed of 88% ammonium perchlorate (AP) in an HTPB binder with 0.5% of the zirconium carbide (ZrC) was used. Although some instability was observed as shown by the DC shift, this shift was only 10% of that shown by the propellant without additive. Further, the onset of the shift was delayed until the end of the boost phase.

A second motor was fired using the combination of 0.5% ZrC and 0.5% partially broken carbon spheres, formulation No. 8 above, in the booster propellant. The results were even better with the second motor. The DC shift was eliminated entirely, leaving a residual pressure coupled maximum amplitude of only 10 psi at the operating pressure of 1,200 psi. This minor instability is well within acceptable operation limits for solid rocket motors. The firing curve for this dual thrust motor is shown in FIG. 1, where formulation No. 8 was used for the boost phase of the operation. FIG. 2 typifies the performance of the composition without additives showing the large pressure spikes resulting from combustion instability.

Further T burner date were obtained on the effect of 1% ZrC (no carbon) on stability of the same propellant used for evaluating the 0.5% mixture with carbon and also on the effect of using a lower percentage of ammonium perchlorate as a lower burning rate propellant. The results are shown in the following table.

Table 3 ____________________________________________________________ ______________ Example AP Additive r, in./sec. Frequency, No. Wt.% Type Wt.% at 1,000 psi Hz Performance ____________________________________________________________ ______________ 9 87 ZrC 1.0 0.59 2600 Stable 2200 Stable 10 87 Broken C Spheres/ 0.5 0.54 2600 Stable ZrC 0.5 0.54 2200 Stable 11 87 Unbroken C Spheres/ 0.5 0.55 2600 Stable ZrC 0.5 0.55 2200 Stable 12 86 ZrC 0.5 0.32 2600 Stable 2200 Stable ____________________________________________________________ ______________

The data given in the above table shows that 1% ZrC (Example No. 9) is equivalent in performance to the propellant containing the mixture of additives (Example No. 7). The use of some carbon is considered significantly superior since it does not create any particulate smoke. The firings of the propellants of Examples No. 10 and 11 were both stable until the ratio of S _b /S _Co was 1 or less and then the firing became unstable. S _b indicates the area of the propellant burning and S _Co is the cross-sectional area of the chamber. The lower burning rate propellant containing a lower amount of ammonium perchlorate as shown in Example No. 12 shows that with this composition stability is improved, being stable at 2200 and 2600 Hz. Further T burner data is shown in the following table.

TABLE 4 ____________________________________________________________ ______________ B.R. a 500 psia 900 psia Ex. AP 1000 2000 Hz 3000 Hz 4000 Hz 2500 Hz No. Wt% Additive Wt% psia α _g ΔP R _b α _g ΔP R _b α _g ΔP R _b α _g ΔP R _b ____________________________________________________________ ______________ 13 87 C Spheres 1 0.56 -- -- -- 32 7 <.13 38 5-10 .475 14 87 P-33 1 0.51 54 150 1.52 28.5 50 .7 Stable <.2 15 87 Carbon Fibers 1 0.49 47 130 1.38 41 65 .8 Stable <.18 16 88 ZrC 0.5 0.56 53 70 1.35 -- 0 .24 Stable <.14 17 88 0.42 40 115 1.33 12 18 <.15 Stable 18 88 0.58 70 75 1.73 63 105 1.02 Stable <.16 19 87 C Spheres 1 0.53 200 450 1 20 88 C Spheres 1 0.41 200 450 1 21 88 ZrC 0.5 0.56 200 500 1.1 22 88 C Spheres/ZrC 0.5/0.5 0.41 Stable 23 88 C Spheres/ZrC 0.5/0.5 0.41 Stable 24 87 C Spheres/ZrC 0.5/0.5 0.58 Stable 25 87 C Spheres/ZrC 0.5/0.5 0.49 Stable 26 87 C Spheres/ZrC 0.5/0.5 0.54 Stable 27 87 Thermay/ZrC 0.5/0.5 0.61 Stable ³ 28 87 P-33/ZrC 0.5/0.5 0.59 Stable ³ 29 87 C Spheres 1 0.67 52 65 1.06 -- 4 .145 Stable .13 30 87 C Spheres/ZrC ¹ 0.5/0.5 0.54 Stable ³ 31 87 C Spheres/ZrC ² 0.5/0.5 0.55 Stable ³ 32 87 Thermay/ZrC 0.5/0.5 0.61 140 >600 ⁴ 33 87 P-33/Zrc 0.5/0.5 0.59 153 >600 ⁴ 34 87 C Spheres/ZrC 0.5/0.5 0.54 171 >500 ⁴ 35 87 C Spheres/ZrC 0.5/0.5 0.55 163 >500 ⁴ ____________________________________________________________ ______________ ¹ 100% broken spheres ² 100% unbroken spheres ³ 2600 Hz ⁴ 2200 Hz

The control propellant batches 17 and 18 show that without additives all AP propellants are unstable at 2000 and 3000 Hz although they stabilized at a frequency of 4000 Hz. It is evident that at higher burning rate, i.e. batch No. 18, instability is increased at 3000 Hz.

The effect of various forms of carbon is seen in batches 14, 29, 13 and 15. Neither P-33 (Example 14) nor carbon fibers (Example 15) gave an improvement in stability at 87% AP. Carbon spheres A-100 (Example 13) gave improved stability at 87% AP and 3000 Hz. At 88% AP, carbon spheres gave improved stability at both 3000 or 4000 Hz.

Zirconium carbide (Example No. 16) gave significantly improved stability at 3000 and 4000 Hz. Additional testing of carbon spheres, 1%, and ZrC, 0.5%, as single additives in the T burner and also in motors (Examples 19, 20 and 21) showed these compositions to be unstable in the T burner at 2500 Hz. Motor firings of Example 21 also showed these compositions were unstable when the propellant web burned out to a diameter corresponding to a frequency of 4000 at 5000 Hz.

In combination the ZrC and carbon spheres (Examples 25, 19, 26, 23 and 24) gave stable combustion in the T burner at 2500 Hz and were also stable in 9-inch O.D. grains, when fired in motors having a frequency at burnout of 2700 Hz. The effect of the combination produces an improvement in stability over that shown by the single ingredients when used alone.

A further evaluation of the effect of carbon and ZrC was tested in Example Nos. 27, 28 and 29 which showed that both carbon powders as well as spheres provided improved stability at 2600 Hz although all combinations were unstable at 2200 Hz. The amorphous blacks were stable in the T burner even at S _b /S _Co ratios as low as 1, the ratio of the area of propellant burning surface to the area of the cross-sectional chamber, whereas the propellant with carbon spheres with ZrC was unstable at this area ratio. Although this instability was evident in T burners, no instability was seen in motor firings evidently because at an S _b /S _Co ratio of 1 the grains are burning as flat slabs and have no contribution from side wall burning as is the case in a typical ID burning grain configuration. The results show that stability above 2500 Hz is primarily due to the combination, carbon plus ZrC, and is not dependent on the form of carbon.

The experiments summarized in Table 5 show the comparison of T burner results at 900 and 2500 psia with full scale motor tests conducted at 900-1500 psia and 70°F. The binder in each example was a plasticized HTPB.

Table 5 ____________________________________________________________ ______________ Full Scale vs T Burner Test Data on Smokeless Composite Propellants Burning* T Burner Tests Example AP Additive Rate 2500 Hz & 900 psia Full Scale Motor Tests No. Wt.% Type Wt.% Size, μ in/lb α _g ΔP R _b Type Gr. O.D. Results ____________________________________________________________ ______________ 36 89 None -- -- .41 ≉200 900 [1] 7.75 Unstable at 6" Dia. 37 88 Fe ₂ O ₃ .5 <1 .59 160 550 1.1 [1] 5.00 Unstable at 4" Dia. 38 87 C/ZrC .5/.5 200/5 .49 -- Stable <.07 [1] 5.00 Stable 39 87 C/ZrC .5/.5 200/5 .54 -- Stable -- Not Tested 40 88 ZrC .5 5 .56 ≉200 ≉500 >1.1 [1] 5.00 Unstable at 4.8" Dia. 41 87 C Spheres 1 200 .53 ≉200 ≉450 >1 Not Tested 42 88 C Spheres 1 200 .41 ≉200 ≉450 >1 [2] 9.90 Unstable at 6" Dia. 43 88 C/ZrC .5/.5 200/5 .41 -- Stable -- [2] 9.00 Stable at 5" Dia.** 44 88 C/Zrc .5/.5 200/5 .41 Stable (2600 [2] 9.00 Stable a 9" Hz) 150 300 (1200 Hz) 45 87 C/ZrC .5/.5 200/5 .67 ≉230 300 Not Tested 46 87 C/ZrC .5/.5 200/5 .64 ≉187 900 Not Tested 47 87 C/ZrC .5/.5 200/5 .58 Stable (2600 [2] 9.00 Stable Hz) ≉206 500 (2200 Hz) ____________________________________________________________ ______________ *Solid Strand at 1000 psia **Lost Nozzle insert [1] 100 lbs. of propellant Grain Design A [2] 100 lbs. of propellant Grain Design B

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

Table 6 ______________________________________ Motor Diameter, Motor Resonant D, inch Frequency, cps ______________________________________ 4.0 6,000 5.0 4,800 6.0 4,000 7.0 3,430 8.0 3,000 9.0 2,670 10.00 2,400 ______________________________________

The data in Table 5 shows a good correlation between the results obtained with T burner and the motors. This is particularly evident for Example No. 47 where the T burner showed stability at 2600 Hz as did the motor at 2690 Hz, while the T burner showed instability at the lower frequency of 2200 Hz.

Example No. 36 (89% AP, r = 0.41 in/sec at 1000 psia) containing no additive was unstable both in the T burner at 2500 Hz and in the motor at even a higher frequency of .about.4000 Hz.

Example No. 37 (88% AP, 0.5% Fe ₂ O ₃ , r = 0.59 in/sec at 1000 psia) again was unstable at a higher frequency in the motor indicating the effect of higher burning rate on the instability.

Example No. 38 (87% SP, r = 0.49 in/sec at 1000 psia) contained 100μ carbon spheres and 5μ ZrC and was found to be stable both in the T burner and in the motor firing down to 4800 HZ, illustrating the effect of the combination of additives.

Example No. 39 (87% AP, r = 0.54 in/sec at 1000 psia) contained 200μ carbon spheres and 5μ ZrC and was found to be stable in the T burner illustrating that 200μ carbon spheres are as effective as the 100μ spheres.

Example No. 40 (88% AP, r = 0.56 in/sec at 1000 psia) contained only 0.5% ZrC and was found to be unstable in the T burner and at .about.5000 Hz in the motor illustrating the need for the carbon spheres for stability.

Example No. 41 (87% AP, r = 0.53 in/sec at 1000 psia) contained 1% of 200μ carbon spheres and was found to be unstable in the T burner illustrating the need for the ZrC in combination.

Example No. 42 (88% AP, r = 0.41 in/sec at 1000 psia) contained 1% of 200μ carbon spheres and was found to be unstable in the T burner and in the motor at .about.4,000 Hz illustrating again the need for the combination to achieve stability.

The remaining batches illustrate the effect of the combination of carbon spheres and ZrC. Both the T burner and motor results show the effectiveness of the combination of additives in achieving stability over the range of burning rates from .about.0.40 to .about.0.60 in/sec at 1000 psia at frequencies as low as 2500 Hz and an oxidizer level from 87 to 88%. At the burning rates above 0.60 in/sec at 1,000 psia instability at 2500 Hz was evident in the T burner. Stability in motors was maintained over the temperature range of -40 to +135°F as shown by the motor fired from Example No. 47.

Thus it is apparent that solid additives such as a refractory metal carbide alone, irregular thin carbon particles such as broken carbon spheres or the combination of refractory metal compound with diverse forms of carbon are capable of providing stable burning, high energy, smokeless propellants without significant loss of specific impulse even though aluminum has been eliminated from the fuel.

A further series of T burner data for propellants containing other refractory compounds such as 0.5 weight percent of hafnium oxide, niobium carbide or tantalum carbide in combination with 0.5 weight percent of 200 micron diameter carbon spheres and 87% ammonium perchlorate (AP) is presented in the following table.

Table 7 ____________________________________________________________ ______________ Motor Resulting Example Burn Rate Pressure, Resonance Resonance, No. Additive in/sec psig design f, Hz f, Hz α _g , sec ΔP, psig ____________________________________________________________ ______________ 48 HfO ₂ .57 1084 2600 0 Stable 0 49 HfO ₂ .58 1072 2200 2300 +160 765 50 NbC .59 1088 2600 0 Stable 0 51 NbC .53 1066 2200 0 Stable 0 52 TaC .53 1044 2600 0 Stable 0 53 TaC .54 1100 2200 2250 +147 670 ____________________________________________________________ ______________

Example 54

A propellant was compounded as follows:

Ingredient Wt.% ______________________________________ AP 87 Carbon Spheres (unbroken) 0.5 Carbon Powder 0.5 Zirconium Carbide 0.5 Binder of Example 1 11.5 ______________________________________

Forty-two pounds of the propellant was fired in a full scale motor. The motor developed 4,000 to 8,000 pounds of thrust and was found to be absent frequencies above 5,000 Hz.

Propellant compositions absent the additives of the invention do not burn stably unless the ammonium perchlorate level is below 80% by weight. This lowers both the impulse and density of the propellant. The propellant composition of the invention containing the stabilizing smokeless additives permits formulation with over 85% ammonium perchlorate to form a high density solid propellant which burns stably with high specific impulse and without visible smoke.

It is to be realized that only specific embodiments of the invention have been described and that numerous substitutions, alterations and modifications are all permissible without departing from the scope of the invention as defined in the following claims.