Minimum Smoke Propellants with High Burning Rates … Meeting Proceedings/RTO... · Their disadvantage however is the ... from formulations with mixed TMETN/GAPA plasticizers can

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Minimum Smoke Propellants with High Burning Rates and Thermodynamic Performance

Klaus Menke; Siegfried Eisele, Manfred Bohn, Peter Gerber Fraunhofer-Institut für Chemische Technologie

– ICT – Joseph-von-Fraunhofer-Straße 7

76327 Pfinztal GERMANY

ABSTRACT

A new system of fast burning rocket propellants will be introduced which is based on the ingredients AP/CL20/GAP with only 20 % AP and energetic plasticizers like TMETN, BTTN and GAP-A.

The propellant system offers a significantly reduced exhaust signature obeying to an AB AGARD signature classification together with high specific impulses up to 2450 Ns/kg ≤ ISPEQ ≤ 2500 Ns/kg (250 – 255 s) and 4300 Ns/dm³ ≤ ISPEQ * ρ ≤ 4400 Ns/dm³ for an expansion ratio of 70:1 (1016 psi). It is endowed with burning rates up to 45 mm/s at 10 MPa and favourable pressure exponents like 0,5 ≥ n ≥ 0,3 in the pressure range from 4 – 25 MPa decreasing to high pressures. Corresponding AP/HMX/GAP formulations reach only 24 – 28 mm/s at 10 MPa which are accompanied by pressure exponents of n = 0,5 – 0,8 increasing to high pressures.

Additionally AP/CL20/GAP formulations are easy processible and offer good chemical stability, convenient glass transition points and satisfying mechanical properties. Their mechanical sensitivity is high, but in the same range as for fast burning composite propellants based on AP/HTPB. Although according to Gap tests their safety classification belongs to class 1.1, their sensitivity to detonation is lower than for pressed explosives or than for high energetic Doublebase or Composite Doublebase propellants.

Although AP/CL20/GAP propellants have only been characterised to a basic level up to now, they appear to open a new class of rocket propellants which offer a unique property profile – connecting high specific impulses and burnrates together with a significantly reduced signature and smoke production. They recommend themselves for further development and investigation and may be applied with great benefits in rocket motors for actively guided highly accelerating tactical missiles or for high performance end burning grains for missiles which require improved camouflage and a hardly detectable trajectory.

1.0 INTRODUCTION

Fast burning high performance composite propellants based on AP/Al/HTPB and on AP/HTPB have been known for a long time[1]. Special formulations exist using high amounts of ultrafine Ammoniumperchlorate and ferrocene derivatives for burn rate catalysts such as Catocene® or Butacene® [2]. The propellants have been well explored, developed and applied for numerous applications. In the fast burning mode they have been used for rocket motors of highly accelerating hypervelocity missile systems like HVM. These have been proposed for the defense against incoming missiles, ICBMs after launching, helicopters with high agility and tanks[3].

RTO-M

Paper presented at the RTO AVT Specialists’ Meeting on “Advances in Rocket Performance Life and Disposal”,held in Aalborg, Denmark, 23-26 September 2002, and published in RTO-MP-091.

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The conventional composite propellants fulfil the goals of high specific impulse, high burn rates with low pressure exponents, high mechanical strength and good viscoelastic properties in the full temperature range for application. They offer good thermal stability and service life together with a detonation sensitivity belonging to class 1.3. Their disadvantage however is the production of large amounts of hydrogen chloride in the exhaust. Together with aluminiumoxide and substantiate air humidity they create large amounts of smoke and an easy detectable trajectory.

Formulations based on CL20/GAP have been taken into consideration as a new upcoming minimum smoke propellant system[4]. It is endowed with a high composite like specific impulse and exhibits considerable higher burn rates compared to other minimum smoke Doublebase- or nitramine propellants. Maximum burnrates however didn’t pass the border of 25 mm/s at 10 MPa.

Formulations based on AP/HMX/GAP were presented some years before as promising candidates for reduced smoke and possible substitute for AP/HTPB propellants[5]. Due to the limited burnrates and high pressure exponents they still have disadvantages and couldn’t reach the performance and application profile of fast burning composite propellants based on AP/Al/HTPB and AP/HTPB.

To overcome these disadvantages our goal was the development of a new propellant system which should combine minimum or reduced smoke behaviour with a high thermodynamic performance and burn rates being comparable to the best fast burning composite propellants based on AP/HTPB and AP/Al/HTPB. Due to the promising features of CL20/GAP formulations[4] and our experiences with the AP/HMX/GAP system we looked for formulations based on AP/CL20/GAP with energetic plasticizers and limited amounts of 20 % AP.

A propellant with only 20 % AP will fall into the AGARD classification B for secondary signature[6] thus producing an almost invisible trail under most environmental conditions. Together with an improved launcher camouflage active guidance by laser beam riding or trajectory control by radar appear to be possible in that case.

For HVM-application, however, the propellant should fulfil a lot of requirements which are not so easy to achieve together:

→ high specific impulse ISP ≥ 250 s (1000 psi, 68,9 : 1 expansion ratio)

→ high burning rates r10MPa ≥ 30 mm/s

→ low pressure exponents n ≤ 0,5

→ high mechanical strength together with viscoelastic properties in full application range from –54 °C to +72 °C

→ good thermal stability and service life

→ low sensitivity to detonation and impact

2.0 PROPELLANT SYSTEM

To achieve high thermodynamic performance together with high burning rates and significantly reduced signature the propellant system of AP/CL20/GAP was designed.

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The propellant compositions are:

20 % AP

42 %; 47 % CL20

35 %; 30 % GAP binder polymer together with 60 and 70 % p. o. b. energetic plasticizers like:

TMETN TMETN / GAPA TMETN / BTTN / GAPA

3 % Fe2O3 and ZrC for burn rate modification

Like CL20, TMETN/BTTN are most convenient to achieve a good internal energy and oxygen balance together with good chemical and thermal stability[4]. GAP and GAPA contribute not only to an increased thermodynamic performance but significantly to increased burning rates and improved combustion behaviour. Last but not least about 3 % burning rate modifiers like 1,5 % iron-III-oxide and 1,5 % zirconium carbide are applied for further adjustment of burning rates and combustion behaviour.

In this contribution the properties of propellant formulations based von AP/CL20/ GAP are compared with a previously reported formulation based on AP/HMX/GAP. Because it was not clear how AP/HMX/GAP propellants with a mixed TMETN/BTTN/GAP-A plasticizer would behave these were formulated and investigated for their burning behaviour.

3.0 THERMODYNAMIC PERFORMANCE

Thermodynamic performance calculations were applied with the ICT thermodynamic code and data base developed by Volk and Bathelt[7,8]. The calculated parameters for an expansion ratio of 70 : 1 from three AP/CL20/GAP formulations and one AP/HMX/GAP propellant are compiled in table 1 together with the calculated reaction products at nozzle. Values of the specific impulse ISPEQ and the volumetric one: ISPEQ ⋅ ρ are plotted in map like diagrams which are presented in figures 1 and 2. Spec. impulse values from formulations with mixed TMETN/GAPA plasticizers can be pointed out by interpolation. The lines present the ISP dependence towards the amount of plasticizer in the binder and towards the mass percentage of CL20 respectively the total amount of solids in the propellant.

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Table 1: Calculated Thermodynamic Values of three AP/CL20/GAP and one AP/HMX/GAP Formulation (Expansion Ratio 70:1)

Nr 182 Nr 181 Nr 189 Nr. 193

AP 20,0 % 20,0 % 20,0 % 20

CL20 47,0 % 47,0 % 47,0 % –

HMX – – – 47,0 %

GAP/N 100 + Plasticizer 30,0 % 30,0 % 30,0 % 30,0 %

Plasticizer TMETN GAP-A / TMETN

GAP-A / BTTN

/TMETN

GAP-A / BTTN

/TMETN

Fe2O3 / ZrC 3,0 % 3,0 % 3,0 % 3,0 %

Expansion Ratio 70:1

Spec. Impulse ISP,EQ (Ns/kg) 2508 2456 2472 2402

Spec. Impulse ISP,EQ (s) 255,7 250,4 252,0 244,9

Vol. spec. Imp. ISP,EQ ρ (Ns/dm3) 4506 4353 4382 4197

Comb. Temperature (K) 3201 3027 3066 2791

Char. Velocity c* (m/s) 1549 1540 1544 1520

Density ρtheor (g/cm3) 1,797 1,771 1,777 1,747

Adiabatic Coeff. 1,234 1,247 1,245 1,25

Mean Mol.Weight (g/mol) 25,38 24,11 24,32 23,33

Oxygen Balance (%) -21,6 -28,6 -27,4 -32,5

AGARD signature classification AB AB AB AB

REACTION PRODUCTS NOZZLE Mol% Ma% Mol% Ma% Mol% Ma% Mol% Ma%

CO2 13,88 23,41 9,33 16,67 9,87 17,47 7,60 14,42

H2O 20,13 13,89 14,96 10,94 15,50 11,23 15,87 12,32

N2 26,03 27,94 27,17 30,91 27,49 30,98 25,45 30,73

CO 22,98 24,68 27,20 30,95 26,51 29,87 26,14 31,56

H2 12,19 0,94 16,89 1,38 16,12 1,30 21,13 1,84

HCl 3,91 5,47 3,64 5,38 3,68 5,34 2,98 4,69

OTHERS 0,61 3,57 0,81 3,77 0,83 3,81 0,77 3,71

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50%

55%

60%

65%

70%

75%

80%(G

AP

82,2

% N

100

17, 8

%: R

=1,2

)PL

AS T

ICIZ

E R O

F B

IND

ER

65% 66% 67% 68% 69% 70% 71% 72% 73% 74% 75%

42% 43% 44% 45% 46% 47% 48% 49% 50% 51% 52%

4780

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GAP-A

SOLIDS

CL 20

AP 20%(1,5% Fe 2O3 ; 1,5% ZrC)TOTAL BINDER:97% - AP - CL 20

SPEC.IMPULSE IN Ns/kg (70:1)

20

24

24

2 0

2

2510

2520

2530

2 2

2 3

25 0

0

20

TMETN

Figure 1: Thermodynamic Optimisation of AP/CL20/GAP Formulations: Change of the Specific Impulse ISPEQ upon Solid Loading and Plasticizer Ratio.

Figure 2: Thermodynamic Optimisation of AP/CL20/GAP Formulations: Change of ISPEQ*ρ upon Solid Loading and Plasticizer Ratio.

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50%

55%

60%

65%

70%

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(GA

P 8

2 ,2%

N10

0 17

,8%

: R

=1,2

)P

LA

ST

I CIZ

ER

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DE

R

65% 66% 67% 68% 69% 70% 71% 72% 73% 74% 75%

42% 43% 44% 45% 46% 47% 48% 49% 50% 51% 52%

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TMETNGAP-A

SOLIDS

CL 20

AP 20%(1,5% Fe 2O3 ; 1,5% ZrC)TOTAL BINDER:97% - AP - CL 20

VOL. SPEC.IMPULSE IN Ns/dm 3 (70:1)


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All formulations pointed out in the diagrams include 20 % AP, 3 % burning rate modifier (Fe2O3 + ZrC) and a binder polymer based on 82,2 % GAP and 17,8 % N100. Formulations with pure TMETN exhibit a strong, formulations with pure GAPA a moderate increase of ISP with the increase of CL20 concentration. An increase of ISP is observed too with increasing amounts of plasticizer in the binder portion. With 70 % solids loading and major amounts of nitrate ester ISP values of 2500 Ns/kg and 4500 Ns/dm3 are quite realistic to achieve. Table 1 and table 2 give an impression about the thermodynamic values of formulations which were made in the lab scale and which are discussed further.

Table 2: Propellant Formulations and Specific Impulses

Propellant Nr.: 178 185 181 182 183 189 192 193 103

Solids loading ma% 65 65 70 70 70 70 70 70 70

AP 2um ma% 20 20 20 20 20 20 20 20 20

CL20 30um ma% 42 42 47 47 -- 47 -- -- --

CL20 5µm ma% -- -- -- -- 47 -- -- -- --

HMX 5-10µm ma% -- -- -- -- -- -- 47 47 45,5

GAP/N100 ma% 10,5 14 12 12 12 12 12 12 12

TMETN ma% 12,25 10,5 9 18 9 2,25 9 2,25 18

BTTN ma% -- -- -- -- -- 6,75 -- 6,75 --

GAP-A ma% 12,25 10,5 9 -- 9 9 9 9 --

Fe2O3 ma% 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 3,0

ZrC ma% 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5

Thermodynamical Performance 70:1

Spec. Impuls Ns/kg 2427 2405 2456 2508 2456 2472 2391 2402 2422

Spec. Impuls s 247,4 245,1 250,4 255,7 250,4 252,0 243,7 244,9 246,9

theor. density g/cm³ 1,737 1,730 1,771 1,797 1,771 1,777 1,742 1,747 1,782

Vol. Spec. Imp. Ns/dm³ 4215 4161 4349 4506 4349 4391 4165 4197 4316

The specific impulses of AP/HMX/GAP formulations are about 2 – 3 % lower than those of corresponding AP/CL20/GAP propellants (189 – 193, 183 – 192, 182 – 103). A similar decrease is observed for replacing 50 % of the TMETN plasticizer by GAP-A (182 – 181). This disadvantage however will be more than compensated by the benefits of GAP-A concerning burning behaviour and chemical stability.

4.0 PROCESSIBILITY, GLASS TRANSITION TEMPERATURES AND MECHANICAL PROPERTIES

The casting viscosities, mechanical properties and glass transition temperatures of several AP/CL20/GAP formulations with 65 % and 70 % solids loading are presented in the tables 3 and 4. Viscosity measurements were performed with a Brookfield spindle viscosimeter at 50 °C casting temperature. Mechanical properties have been determined using a Zwick uniaxial tensile tester with 50 mm/min drawing speed at different temperatures. Glass transition temperatures were determined by dynamic

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mechanical analysis (DMA) and thermomechanical analysis (TMA). For DMA measurements, a typical sample configuration was a rectangular bar of 12 mm x 5 mm x 50 mm. Measurements were carried out at a fixed frequency of 1 Hz (6,28 rad/s). The glass transition temperatures were measured by DMA with a rapid cooling down to –60 °C followed by a ramp up heating rate of 1 °C/min. TMA measurements were performed with 5 °C/min cooling rate starting from room temperature. Table 3 shows the results of the glass transition temperature of the different samples and measurement options. The glass transition temperature TG is defined by NATO STANAG 4540 as the maximum of loss modulus G". Because this peak cannot be detected in all samples the maximum value of tan δ has been determined too and has been pointed out in table 3.

Table 3: Processibilty, Glass Transition Temperatures and Mechanical Properties

Propellant Nr.: 185 178 181 182 189 103

Solids loading ma% 65 65 70 70 70 70

CL20 / HMX ma% 42 42 47 47 47 45,5

GAP/N100 ma% 14 10,5 12 12 12 12

Plast o.f Binder ma% 60 70 60 60 60 60

Plasticizer TMETN GAP-A

TMETN GAP-A

TMETN GAP-A

TMETN TMETN BTTN GAP-A

TMETN

ViscosityEOM (30°C) Pas 52 48 152 148 144 184

Mech. Properties (20°C)

Tensile Str. N/mm² 0,45 0,28 0,59 0,58 0,54 0,65

Elongation % 30 35,4 19,2 19,8 24,9 39,5

E-modulus N/mm² 3,40 1,26 5,78 5,72 4,46 6,43

DMA ramp up 1°/min

Tg maxG" °C -55,8 -50,1 -52,0 -53,7

Tg max tan δ °C -46,3 -44,1 -44,7 -46,9

TMA 5°/min, [1] 10°/min

Tg °C -52 -58 -57 [1]

TEC 10-5/K 8,75 8,50

All CL20 formulations even with 70 % solids are easy and due to the higher density of CL20 better processible than the corresponding AP/HMX/GAP formulations. Larger differences occur as it should be expected between 65 and 70 % solids loading. A small increase in casting viscosity is observed if TMETN is replaced by GAP-A or a decrease if it is replaced by BTTN.

Mechanical properties have not been optimised. If the binder formulation is focussed on one GAP diol and N100 specification and if curing time, temperature and curing catalyst are kept constant, they mainly depend on the mass percentage of solids, on the polymer plasticizer ratio and on the NCO/OH equivalent ratio.

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Table 4: Mechanical Properties of AP/CL20/GAP Propellants in the Temperature Range from –40° To +50°C

Batch Nr / Curing time 191 / 2d 189 / 1d

AP 20,00 % 20,00 %

CL20 42,00 % 47,00 %

GAP / N 100 (R=1,05) + PLAST. 35,00 % 30,00 %

PLASTICIZER TMETN /GAP-A TMETN /GAP-A /BTTN

Plasticizer parts of binder 60% 60%

ADDITIVES 3,00 % 3,00 %

Mech. Properties (50 mm/min) -40°C +20°C +50°C -40°C +20°C +50°C

σmax (N/mm2) 4,40 0,54 0,39 4,35 0,60 0,40

ε at σmax (%) 17,0 25,4 22,2 17,1 21,7 23,3

σ at εmax (N/mm2) 3,14 0,46 0,33 2,60 0,54 0,37

εmax (%) 18,9 28,2 22,8 28,1 24,9 25,7

E modulus (N/mm2) 80 2,7 2,3 116 4,5 3,5

There is no change of modulus and elongation if TMETN is replaced by GAP-A (182 – 181), however, a slight softening in E module and increase in elongation occurs if TMETN is partly replaced by BTTN (182 – 189). With 65 % solids (185, 178 and 191) smaller values of tensile strength and elastic modulus are observed. Remarkable softening occurs with higher amounts of plasticizer (178) and a decrease of the NCO/OH ratio. The best fit to HVM requirements, which demand high modulus and tensile strength, is made with 70 % solid loading and 60 % energetic plasticizer being part of the binder. The mechanical properties of propellant No. 189 at high and low temperatures in table 4 and the glass transition points in table 3 indicate that there is no freezing at low and no significant softening at high application temperatures.

The glass transition behaviour is illustrated in figure 3 by the temperature dependence of G', G" and tan δ of propellant 181 determined from –60°C to +50°C by DMA. The comparatively broad transition indicates the formation of a 3 dimensional crosslinked elastomeric network without crystallisation.

5.0 CHEMICAL STABILITY

The results of short term tests and subsequent storage at 90 °C for determination of chemical stability are pointed out in figure 4 together with table 4. Dutch tests were performed by heating a propellant sample of 2 g in a loosely fitted stopper tube at 105 °C. Weight loss was determined from 8 – 72 h heating. The test fails if more than 2 % weight loss are obtained. Vacuum stability tests were performed by heating 2,5 g propellant sample for 40 h at 100 °C stored under a pressure of less than 1 mm Hg. The test fails if there is more than 2 ml/g gas production during the heating period. Autoignition points were determined by heating a sample of 200 mg in an open test tube with a heating rate of 5°/min.

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-60.0 -50.0 -40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 40.0 50.010 5

106

107

108

109

1010

10-3

10-2

10-1

100

101

Tem p [°C ]

G' (

)

[P

a]

G"

()

[Pa]

HFK 181, temperature ramp up

TG, DM A = -44,1 °C

TG, m ax G '' = -50,1 °C

tan_delta ()

[ ]

Figure 3: G', G" and tanδ of Propellant Nr. 181 determined by DMA Ramp up with 1 °/min.

Figure 4: Weight Loss of AP/CL20/GAP Propellant Samples during Storage at 90°C.

Dutch tests and vacuum stability tests show that all formulations are sufficiently stable even without nitrate ester stabilisers. By replacing TMETN by GAPA the stability is improved (182 – 181), with BTTN however there is a higher weight loss for propellant 189. Slightly decreased vacuum stability is observed too for formulation 178 in connection to a high plasticizer content. All results indicate that chemical

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0.0

1.0

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WE

IGH

T L

OS

S A

T +

90°C

IN

%

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70STORAGE TIME AT +90°C IN DAYS

181182189

182 181 189

TMETN TMETN TMETN

GAP-A GAP-A

BTTN


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stability of the formulations are better than those of conventional Doublebase- or Nitroglycerine containing EMCDB propellants. Fig. 5 compiles the mass loss curves of the BTTN containing propellant 189 at 70°, 80° and 90°C. The measured values clearly demonstrate a better stability and service life than conventional NGl containing DB or CDB propellants. For a detailed discussion see also the paper of M. Bohn[9].

Figure 5: Mass Loss Data of the AP/CL20/GAP Propellant 189 with BTTN/TMETN/GAP-A Plasticizer Mixture at 70°, 80° and 90°C.

6.0 BURNING BEHAVIOUR

Burning rates of the propellants were determined at 20 °C from 2 to 25 MPa by Crawford measurements. Coated strands with 5 mm square section and 150 mm length with two 50 mm measuring distances were used. The results are exhibited in table 2 and in the figures 6 – 9 showing eight logarithmic plots of burning rates and pressure exponents versus pressure. In figure 6 the difference of propellant 182 with pure TMETN is pointed out in comparison to propellant 181 with a mixture of TMETN/GAPA. Burning rates and mainly the pressure exponents are strongly influenced by incorporating GAP-A. If only nitrate esters are applied the pressure exponent tends to increase from n = 0,5 to 0,8, with GAPA the pressure exponent remains constant between 0,4 < n < 0,5. Figure 7 exhibits the burning behaviour of formulations 185 and 178 each with 65 % solid loading, but with 60 % (185) and 70 % (178) plasticizer percentage in the binder fraction. The difference in burn rates i.e. 36 mm/s for 185 and 47 mm/s for 178 is amazing and not fully understood. Due to its high burn rates formulation 178 might be attractive, but clearly shows disadvantages regarding chemical stability and mechanical properties which are too soft and in most cases not applicable for HVM or any other applications of fast burning rocket propellants.

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181

Nr. 181: AP 20% GAP-A / TMETNR10 = 38,0 mm/sISP = 2450 Ns/kg

Nr. 182: AP 20%TMETNR10 = 34,8 mm/s ISP = 2508 Ns/kg

181

182

182

Figure 6: Burnrates and Pressure Exponents determined by Crawford Measurements for Formulation 182 with TMETN and 181 with TMETN/GAP-A.

Figure 7: Burnrates and Pressure Exponents determined by Crawford Measurements for Formulations with 65 % Solids, 60 % (185) and 70 % (178) Plasticizer TMETN/GAP-A.

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Nr. 178Nr. 185

Nr. 185: 20% AP TMETN / GAP-AR10 = 35,9 mm/sISP = 2405 Ns/kg

Nr. 178: 20% AP TMETN / GAP-AR10 = 47,1 mm/sISP = 2427 Ns/kg

185

178


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193

Nr. 193: HMX / 20% APGAP-A / BTTN / TMETNR10 = 25,8 mm/s

Nr. 189: CL20 / 20% AP GAP-A / BTTN / TMETNR10 = 45,3 mm/s

193

189

189

Figure 8: Burnrates and Pressure Exponents for the AP/CL20/GAP Formulation 189 with a BTTN/TMETN/GAP-A Plasticizer Mixture, together with the corresponding AP/HMX/GAP Propellant 193.

Figure 9: Burnrates and Pressure Exponents for the AP/CL20/GAP Formulation 183 with a TMETN/GAP-A Plasticizer Mixture, together with the corresponding AP/HMX/GAP Propellant 192.

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192183

Nr. 192: HMX / 20% AP GAP-A / TMETNR10 = 24,4 mm/s

192

183

Nr. 183: CL20 / 20% AP GAP-A / TMETNR10 = 41,9 mm/s


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In the figures 8 and 9 the burning behaviour of AP/HMX/GAP formulations is compared to those of analogue AP/CL20/GAP propellants. The data are pointed out in table 5. Corresponding formulations with 70 % solids, TMETN/GAP-A and TMETN/BTTN/GAP-A plasticizer mixtures have been formulated and investigated by Crawford measurements. In both cases the CL20 propellants possess strong advantages regarding burning rates – HMX propellants: 24 – 26 mm/s, CL20 propellants 41 – 46 mm/s at 10 MPa – and pressure exponents – HMX propellants 0,6 – 0,8 increasing to high pressure levels, CL20 propellants 0,6→0,3 decreasing to high pressures (183 – 192, 189 – 193). Similar behaviour was observed in previous formulations (103) consisting of AP/HMX/GAP with TMETN or BTTN/TMETN as a plasticizer[5]. The reason for the better properties of AP/CL20/GAP propellants may be assigned not only to the higher energy and reactivity of the cage like CL20, but also to its potential complexing ability with azido groups. This property relation was pointed out by molecular modelling[10]. It might be responsible for the unusual beneficial effects of GAP-A in CL20 propellants concerning burning behaviour and stability.

Table 5: Basic Tests for Chemical Stability

Propellant Nr.: limit 178 185 181 182 189 103

Solids loading ma% 65 65 70 70 70 70

CL20 ma% 42 42 47 47 47 --

HMX ma% -- -- -- -- -- 45,5

Plasticizer ma% 24,5 21 18 18 18 18

Plasticizer TMETN GAP-A

TMETN GAP-A

TMETN GAP-A

TMETN TMETN BTTN GAP-A

TMETN

Autoignition Temp. 5°/min °C

≥170 172 176 175 172 177 177

Dutch Test 8-72h/105°C mass loss %

≤ 2,0 0,78 0,80 0,68 0,90 0,75 0,63

Vacuum Stability 40h/100°C ml/g

≤ 2,0 (1,2)

1,55 1,49 1,32 1,74 1,49 1,44

1 % ML 70°C d 240 252 >300 269 100 --

1 % ML 80°C d 73 73 86 65 45 --

1 % ML 90°C d 18 19 24 17 14 --

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Table 6: Burning Properties of AP/CL20/GAP and AP/HMX/GAP Propellants

Propellant Nr.: 178 185 181 182 183 189 192 193 103

Solids loading ma% 65 65 70 70 70 70 70 70 70

CL20 30um ma% 42 42 47 47 -- 47 -- -- --

CL20 5µm ma% -- -- -- -- 47 -- -- -- --

HMX 5-10µm ma% -- -- -- -- -- -- 47 47 45,5

Plast. of Binder % 70 60 60 60 60 60 60 60 60

TMETN ma% 12,25 10,5 9 18 9 2,25 9 2,25 18

BTTN ma% -- -- -- -- -- 6,75 -- 6,75 --

GAP-A ma% 12,25 10,5 9 -- 9 9 9 9 --

Fe2O3 / ZrC ma% 3,0 3,0 3,0 3,0 3,0 3,0 3,0 3,0 4,5

Burn rate 10 MPa mm/s 47,1 35,9 38,0 34,8 41,9 45,3 24,4 25,8 28,0

n (4-20 MPa) 0,5 ≤0,45 ≤0,45 0,5 →0,8

0,6 →0,35

0,6 →0,3

0,6 →0,8

0,5 →0,8

0,5 →0,7

remarks ++ ++ ++ -- +++ +++ --- --- +/-

7.0 SENSITIVITY AND VULNERABILITY

Impact and friction sensitivity were determined with the BAM hammer and friction apparatus manufactured by Julius Peters Co. Gap tests were only performed according to WIWEB test with 21 mm Gap ∅, using a PMMA block as Gap material. Gap distances of 50 mm ∅ and initiation pressures were estimated from former tests.

Due to the high price and limited availability of CL 20 only a few data on mechanical and detonation sensitivity have been determined. The results of 3 different AP/CL20/GAP formulations are listed in table 5 in comparison to a pressed booster charge consisting of 94,5 % RDX, 4,5 % wax and 1 % graphite (HWC) and to the AP/HMX/GAP propellant 103.

Because AP is present in a very fine particle size the mechanical sensitivity of the formulations is always high but comparable to conventional fast burning composite propellants. Although CL20 may contribute in a similar way, the high values for impact sensitivity should be caused by the presence of ultrafine AP (UFAP) and the combination of AP/nitrate ester, as it is known from the fast burning AP/PU/TMETN propellant 130 and fast burning AP/HTPB propellants[5].

Independently of solid loading and plasticizer amounts all AP/CL20/GAP formulations are sensitive to detonation and clearly belong to 1.1 classification. Compared to a pressed booster charge of HWC and to a conventional Doublebase or Nitroglycerine containing EMCDB propellant the detonation sensitivity is much lower than expected. It looks to be acceptable.

Further data of sensitivity and vulnerability haven’t been collected. Not considered are the hazards which may be connected to the processing of CL20 propellants in larger quantities.

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Table 7: Mechanical and Detonation Sensitivity of AP/CL20/GAP Formulations

103

TOTAL SOLIDS HWC 65 % 65 % 70 % 70 %

AP 20 % 15 % 20 % 20 %

CL20 42 % 47 % 47 % --

HMX -- -- -- 45,5

ADDITIVES 3,0 % 3,0 % 3,0 % 4,5 %

Total Binder 35 % 35 30 % 30

PLASTICIZER TMETN / GAP-A

TMETN / GAP-A

TMETN / GAP-A

TMETN

RDX WAX GRAPHITE

94,5 % 4,5 % 1,0 %

Friction Sensitivity (N) 216 32 32 36 40

Impact Sensitivity (Nm) 7,5 5,0 5,0 4,0 3

GAP-Distance (∅21mm) (mm PMMA)

18+/19- 11+/12- 12+/13-

11+/12-

--

GAP - Distance (estim.; ∅ 50 mm)

≈ 53 ≈ 44 ≈ 45 ≈ 44 ≥44

Initiation Pressure ≈ 21 kbar ≈ 34 kbar ≈ 31 kbar ≈ 34 kbar ≤34 kbar

8.0 SUMMARY AND CONCLUSION

For tactical missile application propellants based on AP/CL20/GAP offer a promising and beneficial property profile. They are endowed with a high specific impulse, the maximum going up to 2450 Ns/kg ≤ ISPEQ ≤ 2500 Ns/kg (250 – 255sec) and 4300 Ns/dm3 < ISPEQ ⋅ ρ < 4400 Ns/dm3. This is quite more than for any other rocket propellant which belongs to the class with minimum or reduced smoke behaviour.

With only 3,7 mol % (5,3 mass %) the HCl content in the exhaust is significantly lower than for conventional composite propellants based on AP/Al/HTPB and for reduced smoke propellants based on AP/HTPB (18,8 mol %, 25,3 ma %). It conforms clearly to the B classification of secondary signature according to NATO AGARD standardisation.

With only 20 % AP burning rates up to r = 45 mm/s at 10 MPa are achieved which are pretty high even for metallized or reduced smoke composite propellants[1,5]. Conventional composite propellants incorporate more than 70 % AP and about 18 % Al, fast burning smoke reduced formulations up to 86 % AP.

CL20 in combination with GAP, nitrate ester and GAP-A appear to be one of the best burn rate promoting ingredient combinations for this type of propellant. High molecular energy and kinetic reactivity are connected to good oxygen balance. With only 20 % AP a maximum of system performance can be achieved.

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Unknown effects may contribute to the enhanced burn rates of formulations with lower solids and high plasticizer ratio. In the same way it is amazing that most of the formulations exhibit good chemical stability and service life without any stabilisers for liquid nitrate esters. These phenomena should have been looked closer and in a more detailed investigation.

Contrary to AP/CL20/GAP propellants corresponding AP/HMX/GAP formulations enable maximum burnrates of 24 < r < 30 mm/s at 10 MPa and are equipped with lower Isp values. Their major disadvantage however are high pressure exponents (n = 0,5 – 0,8) which increase to higher pressures and prevent their application in highly perforated grain geometries. The burning behaviour of AP/CL20/GAP propellants turns out to be very favourable with burn rates up to 45 mm/s at 10 MPa and pressure exponents approaching n = 0,3 towards higher pressure levels, but only when GAP-A together with nitrate esters has become a major part of the plasticizer mixture incorporated in the propellants. This beneficial behaviour may be caused not only by the higher energy and chemical reactivity of the cage like CL20 molecule, but also by its potential of forming chemical complexes with the terminated azido groups of GAP-A. This property relation has been pointed out elsewhere by molecular modelling[10]. It may be responsible too for the unusual high chemical stability of the AP/CL20/GAP formulations without any nitrate ester stabilisers.

The mechanical properties of the presented propellants have not been optimised. They appear sufficient for high performance sustainers with an end burning grain and laser transparent minimum smoke trajectory. Especially for the perforated grain geometry of a HVM propellant a little bit more toughness including an increase in tensile strength and modulus will be convenient. It should be achievable by optimising the GAP binder specification, properties and curing conditions.

AP/CL20/GAP formulations are sensitive to detonation and fall below the 1.1 classification. Their initiation pressure, however, is higher than for a conventional pressed explosive formulation based on RDX/Wax/Graphite. It is higher too, than for a conventional high energetic Doublebase or composite Doublebase propellant. The hazards in manufacturing CL20 are not completely known, but may be overcome if rounded particles or desensitised material will be used.

It is shown that AP/CL20/GAP propellants combine some unique properties which are not covered by the existing rocket propellants: significantly reduced signature, high performance and high burning rates together with low pressure exponents. Endowed with these properties the propellants may become promising candidates for highly accelerating boosters or high performance end burners to be used in actively guided missiles.

9.0 REFERENCES

[1] A. Davenas; “Solid Rocket Propulsion Technology”; Pergamon Press, Oxford 1993.

[2] a) Bernard Finck et. al.; “Non Migrating Ferrocene Derivatives for High Burning Rate Composite Propellants”; ADPA – Conf. Proceedings, Virginia Beach 1989.

b) Hubert Jungbluth, Klaus Menke; “Ferrocenderivate: Effiziente Abbrandmoderatoren für Composittreibstoffe u. Gasgeneratoren”; 27th Intern. Conf. of ICT, 1996, S. 147-1 ff.

[3] Soldat und Technik 1/1998.

[4] Yves Longevialle, M. Golfier, H. Graindorge; “New Generation of Propellants for High Performance Solid Rocket Motors”; AVT Conf. Proc. about “Small Rocket Motors and Gas Generators”, pp. 25-1 ff, Corfu 1999.

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[5] Klaus Menke, Siegfried Eisele; “Rocket Propellants with Reduced Smoke and high Burning Rates”; Prop. Explos. Pyrot. 22, 112 – 119 (1997).

[6] P.A. Kessel in “Rocket Motor Plume Technology”; AGARD LS-188 1993 pp. 3-1 to 3-9.

[7] H. Bathelt, F. Volk; “The ICT-Thermodynamic Code (ICT-Code)”; Proceedings 27th Intern. Annual Conference of ICT, June 25-28, 1996.

[8] H. Bathelt, F. Volk, M. Weindel; “The ICT Thermochemical Data Base”; Proceed. 30th Intern. Annual Conference of ICT, pages 56-1 to 56-12, June 29 – July 2, 1999, Karlsruhe, Germany.

[9] M. Bohn, S. Eisele; “Ageing and Service Time Period Assessment of Novel Solid Rocket Propellant Formulations containing CL20, AP and Energetic Plasticizers”; 32th Intern. Ann. ICT Conf. 2001 (Proc.) pp. 152-1 ff.

[10] V. Thome, P.B. Kempa, M.A. Bohn; “Erkennen von Wechselwirkungen der Nitramine β-HMX und ε-CL20 mit Formulierungskomponenten durch Computersimulation”; Proceed. 31st Intern. Annual Conference of ICT, pages 63-1 to 63-20, June 27-30, 2000.

10.0 ABBREVIATIONS AND SYMBOLS

AGARD Advisory Group for Aerospace Research and Development AP Ammoniumperchlorate BTTN Butantrioltrinitrate CL20 Hexanitrohexaazaisowurtzitane DMA Dynamic Mechanical Analysis EMCDB Elastomer Modified Composite Double Base G*, G', G" Dynamic mechanical moduli ( 22 )''()'(* GGG += ) GAP Glycidylazidopolymer GAPA Glycidylazidopolymerazide HMX Cyclotetramethylentetranitramine – Octogen HTPB Hydroxyterminated Polybutadiene HVM Hypervelocity Missile HWC Hexogene, Wachs, Carbon (Graphite) ISPEQ Specific Impulse (Equilibrium Flow) ISPEQ ⋅ρ Volumetric Specific Impulse (Equilibrium Flow) ICBM Intercontinental Ballistic Missile ML Mass Loss (from chemical stability tests) PMMA Polymethylmethacrylate RDX Cyclotrimethylenetrinitramine Hexogen Tc Combustion Temperature TG Glass transition temperature tan δ Mechanical loss factor (tanδ = G"/G') TMA Thermomechanical analysis TEC Thermal Expansion Coefficient TMETN Trimethylolethanetrinitrate

11.0 ACKNOWLEDGEMENT

The authors want to express their thanks to the German Ministry of Defense for financial support of this work.

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SYMPOSIA DISCUSSION – PAPER NO: 13

Discusser’s Name: Luigi DeLuca

Question: Do you have an explanation for the appreciable changes of the pressure exponent n in your formulations?

Author’s Name: Klaus Menke

Author’s Response: A major difference of decreasing and increasing exponents toward higher pressures is seen by a comparison of AP/CL20/GAP formulations (decreasing n) to AP/HMX/GAP formulations (increasing n). It is also observed for AP/CL20/GAP formulations with TMETN/GAP-A (decreasing n) and with TMETN alone (increasing n). The reason for this behavior is not known. The higher burn rates of CL20 formulations are certainly due to the higher heat evolution corresponding to the exothermic reaction of CL20 in comparison to that of HMX. Replacement of TMETN by Gap-A reduces the heat evolution and combustion temperature of the propellant but increases the amount of evolved gases. Benefit is contributed by the favorable combustion kinetics of the terminated azido functions. This, however, will not explain the difference in the burning behavior and changes in the pressure exponents of AP/CL20/GAP to AP/HMX/GP formulations. Molecular models show that two nitramine groups of CL20 are in a favorable position of complexing to the azido groups of GAP and GAP-A. This position of molecular interactions is not possible for the nitramine groups of HMX. The intermolecular complexes should be more intensive with the end standing N3 functions of GAP-A and should be favored towards higher pressures. This circumstance might influence the reactions at the burning surface and might contribute significantly to the observed favorable changes of pressure exponents. As it is observed, the favorable decrease of n towards higher pressures is always accompanied with significant amounts of GAP-A and CL20 and is not seen for HMX formulations or even for AP/CL20/GAP formulations without GAP-A. A special ratio of CL20 and GAP-A will be required too. Further and detailed combustion experiments, of course, must be performed to establish this thesis.

Discusser’s Name: Tom Johannenssen

Question: I have performed two VTS test at 100 degrees C using AP and TMETN. Both times the samples started to burn violently after about 16-hours. Have you performed a similar test using AP and TMETN? Can you explain why you got low values from the VTS test using a propellant containing these two ingredients?


Author’s Response: Our propellants with AP/TMETN and GAP or inert PU binders are pretty stable and will withstand 40-hours at 100 degrees C in the VST test easily. Problems with TMETN may occur when acidic residues are not washed out completely. Then of course an autocatalytic exothermal decomposition may occur which will lead to further heating until initiation occurs. The autocatalytic decomposition of TMETN may be favored in the presence of AP but it is not observed in any formulations of AP with PU binders and clean and neutralized TMETN as a plasticizer.

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Discusser’s Name: Hans Besser

Question: What will the cost of the “AB” smoke class formulations presented here be in comparison to conventional HTPB/AP propellants?


Author’s Response: Depending on the availability of raw materials, especially CL20 and Gap, the costs will be certainly higher than the traditional systems of AP/HTPB. In future, of course, the costs may be significantly reduced if larger production facilities are built. Despite that circumstance, the ratio of the expected benefits to the higher costs is great enough to pursue this system.

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