NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

H.L.J. Keizers, G.C Reeling Brouwer, J. Weijl, F.P. Weterings TNO Prins Maurits Laboratory

Lange Kleiweg 137, P.O. Box 45 2280 AA Rijswijk

THE NETHERLANDS Fax +31 15 284 39 58

Phone +31 15 284 33 78

E-mail keizers@pml.tno.nl

SUMMARY

The lifetime of composite rocket motors is primarily a function of the mechanical properties of the propellant, the ageing characteristics of the propellant, the grain design and the environmental conditions endured during operational use. For double base systems, lifetime is generally determined by its stability (stabilizer consumption). The propellant properties may degrade because of a number of different processes, i.e. chemical, physical and mechanical. In this paper an overview will be given of techniques currently employed in the Netherlands to predict the safelife of rocket motors, focusing on chemically and physically induced ageing mechanisms. By combining the results of mechanical and chemical analysis, the ageing behaviour of the propellant and dominant ageing mechanisms can be established. Knowledge about both ageing behaviour and the ageing mechanisms are essential to predict the lifetime of rocket motors.

In contrary to the state-of-the-art composite propellants, new energetic formulations are likely to posses some of the inherent stability related lifetime issues as are well known for double base systems. An impression will be given how, by the use of well proven techniques for composite as well as double base propellants, new propellant formulations may be screened in order to evaluate the use of these new formulation in new, higher performing and long life systems.

1.0 INTRODUCTION

The Netherlands armed forces use a variety of missile systems with solid propellant rocket motors (e.g. AMRAAM, Hellfire, Hydra, Maverick, MLRS, Patriot, Sidewinder, Standard Missile, Stinger). Key issues for surveillance of these missile systems are safety, reliability, timely notice of replacement needs and costs. To support the Netherlands armed forces in maintaining its missile inventory, TNO is working on surveillance studies for specific weapon systems as well as more experimental studies to improve lifetime prediction techniques for solid propellants.

Lifetime of modern composite solid rocket motors is primarily a function of the mechanical properties of the propellant, the ageing characteristics of the propellant, the grain design and the environmental conditions endured during operational use (Figure 1).

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.

P-091 25 - 1

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

Mechanicalproperties

Graindesign

Ageingcharacteristics

Operationalconditions

Propellantdegradation

Lifetime

Figure 1: Factors Affecting the Service Lifetime of Rocket Motors.

For CTPB and HTPB based systems, lifetime is generally limited by a degradation of the mechanical properties of the propellant composition. The reduction of mechanical properties may lead to crack formation at the inner bore (loss of strain capacity), debonding at the propellant- liner-casing interface or slump (excessive softening of the propellant, generally induced by hydrolysis type of reactions). In case of double base systems lifetime is in general limited by the amount of stabilizer consumed. Furthermore, since these motors are usually based on a loose charge propellant grain concept, mechanical properties are not as critical as in the case of case bonded composite motors. Ageing of a solid propellant can be caused by a number of processes:

•

•

•

chemical (i.e. oxidation, chain scissioning, stabilizer consumption)

physical (i.e. migration of liquid components of the propellant or adjacent materials)

mechanical (i.e. thermally induced stresses, shock loads, vibrations).

In this paper an overview will be given of a number of techniques employed in the Netherlands to predict the safelife of solid propellant rocket motors, focusing on chemically and physically induced ageing mechanisms. Emphasis is given in the understanding of the dominant ageing process. Based on the dominant ageing mechanisms, safelife of systems can be evaluated with the help of numerical tools and real life operational conditions that are being monitored. These techniques are not only valid for conventional systems, but its significance will grow with the use of new and more energetic propellants.

2.0 ASSESSMENT CHEMICAL AND PHYSICAL AGEING OF COMPOSITE SOLID ROCKET MOTORS

Evaluation of rocket motor safelife after a certain period of in service use consists of two main parts. First, the current status of a rocket motor is assessed. Second, a lifetime projection is made based on accelerated ageing. As rocket motor lifetime is primarily a function of the mechanical properties of the propellant, mechanical testing of the propellant is carried out to assess the current and the projected state of the rocket motor. Additionally, chemical analysis of the propellant composition is carried out to gain insight in the ageing processes that take place. Oftenly, gradients in properties are obtained near the inner bore or at interfaces between different materials (e.g. booster-sustainer interfaces, propellant-insulation interfaces). In these regions, the results of the chemical tests can be used to evaluate the mechanical properties, since mechanical tests are hardly (not) possible in these small area’s with large gradients.

25 - 2 RTO-MP-091

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

2.1 Mechanical Evaluation A range of mechanical characterisation techniques are in use to determine the mechanical behaviour of a propellant before and after (accelerated) ageing. The mechanical properties of a composite propellant are dependent of the temperature and the deformation rate. Based on the assumption that the propellant behaves thermorheologic [1, 2] a range of experiments at different temperatures and deformation rates can be combined into one material model. Main propellant characterisation is carried out with relaxation experiments, tensile tests (range 5 to 500 mm/min @ -60 °C to +74 °C) and bondtests (propellant-casing interface). The mechanical characterisation tests give direct information about possible age-out mechanisms. If data is obtained after different periods of (accelerated) ageing, safelife predictions can be made based up on the obtained data.

When necessary, the “standard” characterisation tests can be complemented with more detailed tests, like gas dilatometer experiments (to study bonding agent efficiency /dewetting behaviour of the propellant), pressurized and pre-strained low temperature tensile testing (simulation ignition pressurisation load at low temperature), cyclic testing (damage) or bi-axial tests. Examples of a gas dilatometer experiment and the pressurized tensile test are given in respectively Figures 2 and 3. When reaching the age-out limits of a propellant, detailed knowledge of the properties reduces the safety factors required while assessing the safety margins of a motor, which may significantly improve the shelf life of a system. For example, composite propellants are strongly dependent up on the pressure [2, 3], see Figure 3. Generally the stress capacity of propellants increase with an increase in external pressure, whereas the Youngs’ modulus remains constant. Effects on the strain capacity are dependent up on the actual propellant and the test conditions used. After a certain pressure level, no significant increase is observed anymore and the stress capacity reaches a sort of “maximum” level (see Figure 4). Figure 4 shows furthermore that the initial modulus remains constant. It is possible to test the propellant with and without pre-strain in order to simulate a cooldown load, followed by ignition pressurization.

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.2

0.4

0.6

0.8

1.0

1.2

dV/V

o *1

0 [-]

& P

.R.

Stress

stre

ss [M

Pa]

strain [-]

0.0

0.1

0.2

0.3

0.4

0.5

dV/V P.R.

Figure 2: Result Gasdilatometer Experiment.

RTO-MP-091 25 - 3

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

0 10 20 30 40 50 60.0

0.5

1.0

1.5

0

stre

ss [M

Pa]

strain [%]

ambient 2 MPa

Figure 3: Propellant Properties with and without Pressurization Loading.

0 1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

max

imum

stre

ss

pressure [MPa]

max stress

0

2

4

6

8

10

12

14

16

modulusin

itial

mod

ulus

Figure 4: Maximum Stress and Initial Modulus versus External Pressure for an AP/HTPB Propellant (Test Conditions: 50 mm/min @ 20 °C).

25 - 4 RTO-MP-091

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

2.2 Chemical Evaluation of Ageing Chemical analysis techniques, like sol/gel, GC, GPC, HPLC, NMR, etc. can be used to assess and quantify the dominant propellant ageing processes). Especially sol/gel analysis is very well applicable to assess the ageing behaviour of composite propellants. For composite propellants, the mobile propellant compounds (especially the free binder polymer chains and plasticizer) play an important role in the mechanical properties of the propellant. Extraction of the propellant in an organic solvent (Soxlet extraction with dichloromethane) will give the soluble part of the binder that consists of the plasticizer and molecular part of the binder that is not connected to the polymeric network. The sol content is directly linked to the mechanical properties of a propellant.

At TNO, small propellant samples of 0.05 – 0.2 grams are used for an accurate determination of the sol content. This makes the sol/gel determination a powerful tool in determining ageing gradients at free or bonded surfaces of a propellant grain. Due to ageing processes the amount and the composition of the sol generally change. Examples of the sol-content as a function of accelerated ageing time at 60 °C for two typical air-to-air missiles with respectively an AP/HTPB and an AP/CTPB based propellant are given in Figure 5. The reduction in sol-content of the HTPB propellant correlated very well with a reduction in strain capacity and an increase in strength, caused by oxidative ageing, whereas the CTPB propellant is less prone to oxidation. By further chemical analysis of the Sol (antioxidant content, plasticizer content, etc.), the dominant ageing process can be determined.

Change in Sol content due to ageingartificial ageing @ 60 °C

5,5

6,0

6,5

7,0

7,5

8,0

0 50 100 150 200Ageing time [days]

Sol [

%] AP/CTPB

AP/HTPB

Figure 5: Sol Content as a Function of Ageing Time at 60 °C for Two Different Composite Propellants.

A change in sol content of the propellant will lead to a change in mechanical properties of the propellant, as there is a direct relation between the sol content, cross-link density of the binder and the mechanical properties [3]. This relation between Sol content, cross-link density and mechanical properties is illustrated in Figures 6 and 7. Figures 6 and 7 give experimental results of propellants with different crosslink densities. These propellants were prepared using different isocyanate levels (NCO-OH ratio). A distinct relationship is obtained between the initial modulus, maximum strain, maximum stress and the sol content for this AP/HTPB propellant. By increasing the NCO-OH ratio of a propellant, the crosslink density will increase. A similar effect occurs due to oxidative ageing, whereas hydrolysis will cause the crosslink density to decrease.

RTO-MP-091 25 - 5

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

NCO-OH ratio, Sol content and maximum stress

7%

8%

9%

10%

0,8 0,82 0,84 0,86 0,88 0,9

NCO-OH ratio [-]

Sol c

onte

nt [%

]

0,4

0,5

0,6

0,7

0,8

0,9

Stre

ss [M

Pa]

SOL

max stress

Figure 6: Sol Content and Maximum Stress as a Function of Crosslink Density for an Experimental AP/HTPB Propellant.

NCO-OH ratio, initial modulus and maximum strain

0

10

20

30

40

50

60

0,8 0,82 0,84 0,86 0,88 0,9

NCO -O H ratio [-]

Max

stra

in [%

]

0,4

1,4

2,4

3,4

4,4

5,4M

odul

us [M

Pa]

max strain

Modulus

Figure 7: Initial Modulus and Maximum Strain as a Function of Crosslink Density for an Experimental AP/HTPB Propellant.

For double base propellants, life is generally limited by its thermal stability (risk for run-away reaction and/or gas cracking). The thermal stability can be assessed by an assessment of the remaining stabilizer content or by means of isothermal storage tests (IST). Although double base systems generally age-out due to long-term thermal instability effects, also a reduction in molecular weight of the NC and NG molecules is known to occur, leading to a reduction in mechanical properties. The reduction in Mc can very well be assessed by means of GPC.

25 - 6 RTO-MP-091

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

3.0 ACCELERATED AGEING AND LIFETIME PROJECTION

Accelerated ageing plays an important role for lifetime projection of solid rocket motors. Chemical and physical ageing processes can be accelerated by increasing the temperature. This process oftenly can be described by the Arrhenius equation [4]. Accelerated ageing can be performed on motor level, propellant blocks or on propellant sample level. At TNO, ageing is mainly carried out on sample level because of the flexibility of the method and the related costs involved. When performing accelerated ageing, the samples should be aged comparable to the (natural) ageing environment of propellant in a rocket motor grain. The relatively large free surface over volume ratio of a propellant sample with respect a rocket motor propellant grain may cause differences in the projected lifetime. This effect is apparent when comparing the effect of 2 different approaches for accelerated ageing of an experimental AP/HTPB propellant:

•

•

Method 1: ageing of JANNAF samples in sealed bags (containing atmospheric air)

Method 2: ageing of propellant block according to draft STANAG 4581 method [5].

In the draft STANAG 4581 [5] ageing is performed with a propellant block (140 x 90 x 40 mm) that is stored in a sealed box at a temperature of 60 °C during resp. 13 and 26 weeks. After ageing, the test samples are prepared as indicated in Figure 8. This method of ageing is expected to predict the bulk properties of an aged propellant grain. In order to be able to establish possible gradients in the STANAG block, the sampling plan as defined in [5] was complemented with measurements to asses possible gradients in the propellant properties (see Figure 8). From the middle of the block, 3 JANNAF samples are prepared. From one side an additional JANNAF sample is made, and from the other side a rectangular propellant sample is produced that can be used for chemical analysis (profile measurements).

Profile measurement

L = 140 mm

B = 90 mm

H = 40 mm

Figure 8: STANAG 4581 (Draft) Block with 4 JANNAF Samples and Chemical Analysis Sample. The section used for ageing profile determination is indicated.

Some of the results of the comparative study are given in Figures 9 and 10. In Figure 9, the normalised maximum strain is plotted as a function of the normalised sol content for the propellant aged at 60 °C according to method 1 (JANNAF sample ageing). In Figure 10, normalised sol content and normalised shore A are given as a function of the distance (depth) below the surface for propellant aged for 13 weeks at 60 °C according to Method 2. Sol content is determined at the location indicated in Figure 8. Sol profile

RTO-MP-091 25 - 7

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

measurements show that after 13 weeks ageing the inner part of the STANAG block hardly changed (normalised sol content still equals one), but towards the edge the normalised sol decreases to 0.87.

Normalised max strainAP/HTPB aged @ 60 °C

0,00

0,50

1,00

0,50 0,60 0,70 0,80 0,90 1,00 1,10

Normalised Sol [-]

Nor

mal

ised

strai

n [-

]

Figure 9: Normalised Strain vs Normalised Sol for Propellant Aged at 60 °C according to Method 1.

Normalised SOL & Shore A profile aged 13 weeks @ 60°C

0,8

0,9

1,0

1,1

1,2

0 10 20 30 40Depth (mm)

Sol ,

Sho

re A

[-] Shore A

SOL

Figure 10: Normalised Shore A and Sol vs Distance of Free Edge (See Figure 8) for Propellant Aged at 60 °C according to Method 2.

Combining the results of Figures 9 and 10 shows that the strain capacity at the free edge of the STANAG block after 13 weeks ageing at 60 °C will be approximately 60 % of the maximum strain at the middle of the block. Lifetime prediction based on samples cut from the middle of the STANAG block would overestimate the strain capacity at the free surfaces of a propellant bore by 67 %, thereby overestimating the actual safelife and potentially leading to dangerous situations. Although the gradient in a naturally aged rocket motor is expected to be less prominent [6], Method 1 may be used as a more conservative approach for determining propellant properties at free surfaces.

25 - 8 RTO-MP-091

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

4.0 FIELD CONDITION MONITORING

When the dominant ageing mechanisms and reaction kinetics involved are known, the results of field condition monitoring can be used for safelife estimations of ammunition systems. At the moment environmental conditions (temperature and humidity) are measured during out of area missions, but also in storage depots in the Netherlands. Using these measurements with the results of the chemical and mechanical tests can not only give insight in the expected safelife of missiles carried during missions. Furthermore, it can be used to quantify differences between missiles stored in Dutch depots and missiles carried during a mission. This provided insight in possible inhomogenities in the quality of the missile population and can be used to prioritise usage of these missiles.

An example of the use of environmental data and experimental propellant data is shown in Figure 11. During a mission in Northern Africa, the temperature was measured in the storage facility and the ammunition transport containers. Measurements started up on departure, so the effect of transport to Africa was monitored as well. The ammunition arrived in Africa approximately 9.5 weeks after start of the measurements. The rockets were transported back to The Netherlands approximately 16 weeks later (at 25 weeks). In Africa, the missiles and rockets were stored in an earth covered bunker, reducing the temperature fluctuations during the stay. After arriving in and before departing from Africa some high temperature peaks occurred. This is caused by the fact that the ammunition containers were placed on the quay exposed to direct sunlight.

As an example, Figure 11 shows the effect of the mission on the lifetime of a double base rocket motor. For double base propellants, the amount of stabiliser consumed is an important parameter for lifetime evaluation. By means of Heat Flow Calorimetry (HFC), an Arrhenius based relation between storage temperature and stabiliser consumption for the double base propellant of the rocket was established. This relation in combination with the measured environmental conditions was used to predict stabiliser consumption due to the out of area mission. The result is shown in Figure 11 (stabiliser content at the start of the measurements has been set to 100 %). Especially the periods at high temperature show to significantly affect the stabiliser content.

-20

-10

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

time [w eeks]

tem

pera

ture

[° C

]

94

95

96

97

98

99

100

Stab

iliser

con

tent

[%]

Bunker Temperature stabiliser content (%)

Figure 11: Storage Temperatures and Predicted Stabiliser Depletion of Double Base Rocket

Systems during an Out-of-Area Mission in Northern Africa.

RTO-MP-091 25 - 9

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

Because this mission was relatively short and storage conditions can be considered as relatively mild (earth covered bunker) the stabilizer depletion remained limited. However, based on the experimental determined relation between storage temperature and stabiliser consumption it can be concluded that the consumption of stabiliser because of this mission is about 4.5 times higher then in case of depot storage in the Netherlands.

4.1 Ageing of New Propellant Formulations Worldwide, developments are ongoing to develop propellants based on new, and generally more energetic ingredients or propellants having specific IM or minimum smoke properties. Typical examples include the use of new oxidisers like HNF and ADN, new energetic binder systems like GAP, BAMO, PGLYN, PNIMMO, HTPE and energetic plasticizers like BTTN, TEGDN, TMETN, etc. or exotic burn rate modifiers and additives.

Whereas these propellants generally behave very well in terms of their intended need, they may posses reduced shelf life compared to their conventional equivalents. Due to their energetic nature, often they combine the conventional composite propellant failure modes (oxidative ageing, hydrolysis) with the double base stability issues. In addition, humidity effects and hygroscopicity of some of its ingredients might aggravate the ageing behaviour.

At TNO, “new” propellant ingredients are assessed in three subsequent phases:

1)

2) 3)

Screening tests to obtain a preliminary screening of compatibility (mass loss at 40 and 60 °C);

Short term stability tests of mixtures using VST (2 – 4 days at 60 °C);

Long term stability tests by storage of propellant samples for different periods at different temperatures.

A wide variety of propellant additives have been tested in this manner. Table 1 gives an overview of some typical results using the new oxidiser HNF, showing that by a proper choice of ingredients significantly improved stability values can be reached.

Table 1: Summary of Typical Mass Loss and VST Tests of a Number of Different HNF Based Compositions

Composition Time @ 60 °C[days]

mass loss@ 60 °C[wt %]

VSTa [ml/g]

HNF/HTPB b 2 1.76 – + IPDI 1 4.21 15.4 +Desm VL+stab.

2 0.26 0.73

HNF/HTPB (80 %SL)

2 0.45 1.1

HNF/Al/HTPB (80 %SL)

– – 0.5

HNF/GAP – – 4.4 a Vacuum stability test, VST (48 hrs @ 60°C). b Uncured sample, 50 % solid load.

25 - 10 RTO-MP-091

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

The processes leading to thermal degradation of energetic binder systems and new oxidisers is generally characterised by high activation energies. For example for HNF, an activation energy of approximately 150 KJ/mole is obtained. Under the assumption that the decomposition of HNF is a first order reaction,

the gas production, G, in the VST would follow from G trT= . The time at different temperatures to produce a set amount of gas may be used to calculate the activation energy, Ea, from the degradation

process with the equation . The time has been determined at 60, 80 and 90 ( )TRE

TaeAr ⋅−⋅= / oC to produce

3 ml gas per gram of HNF, showing an activation energy of approximately 150 KJ/mole [7]. This data can used to estimate the time to generate 3 ml/g gas during normal storage conditions, i.e. at 20 oC. This is respectively 712 and 610 years for two different grades of HNF (respectively HNF-P and HNF-C), confirming the thermal stability of the material.

For these high energetic materials, a clear link between quality and thermal stability is shown. Figure 12 shows the effect of purity on the stability of the HNF produced (expressed in gas production in a VST test, 90 oC). The purity of the HNF produced directly affects the purity of the propellant.

Figure 12: VST of Improved HNF Grades from APP vs. US Patent Values at 90 °C.

Another technique to determine the stability of propellant mixtures is the isothermal storage tests (IST). Test samples are stored at a constant temperature and endo-/exothermic reactions are being measured by measuring the energy flow as a function of time. By comparing the results at different temperatures, either at constant energy production or at the onset of characteristic decomposition modes (e.g. double base propellants go into autocatalyses) the activation energy of the decomposition process can be determined. Based on isothermal storage tests (70, 60, 50 and 40 °C), a preliminary evaluation of the activation energy of the degrading process has been made for a HNF/HTPB propellant. The activation energy came close to the observed values for HNF (approx. 135 – 150 KJ/mole).

Similar high activation energies are also shown for the more energetic binder systems. Reference [8] for example gives typical numbers for GAP based propellants using GAP-A, TMETN, and BTTN as plasticizers, giving activation energies of typically 120 – 130 KJ/mole in the 70 – 80 oC temperature range.

RTO-MP-091 25 - 11

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

Screening of possible propellant ingredients and early identification of possible life issues is a necessity when looking at the introduction of the new ingredients. Small scale tests can be used to identify possible ageing mechanisms (VST, IST, TG/DTA, DSC, ….). Together with the ability to use of numerical modelling techniques, allows for the assessment of the life of such systems.

5.0 CONCLUSIONS

In order to be able to assess the current state and the projected lifetime of a propellant, knowledge of both the mechanical and the chemical changes is essential to understand and quantify the ageing processes (dominant ageing mechanisms).

When predicting rocket motor lifetime based on accelerated ageing, special care must be taken with respect to sample geometry and ageing conditions. A combination of different ageing techniques may be used to get a complete overview of the ageing behaviour of the propellant. By combining the results of chemical and mechanical analysis, possible drawbacks of a chosen ageing method can be accounted for.

Using the method described in the draft STANAG 4581 to asses the life of rocket motors, may overestimate the actual safety margins of a rocket motor, potentially yielding an unsafe situation. Complementing the method currently described with sol-measurement to account for possible surface effects may overcome this effect.

Sol measurements, using samples of 0.05 – 0.2 grams of propellant, allow for an accurate assessment of the mechanical properties (and safety margins) in interface layers.

Monitoring of environmental conditions is essential to assess the current state of ammunition that was deployed during missions to “hot” areas. Missiles specific knowledge of these conditions allow for prioritisation of usage of these missiles when they return to their storage depots.

When evaluating the ageing behaviour of new energetic propellant compositions, a combination of composite propellant ageing aspects (reduction in mechanical properties) together with thermal instability problems is often shown. A combination of DB and composite propellant techniques is required to fully assess the life issues at hand. When combining the ageing parameters of these new compositions with operational (temperature) monitoring data, reliable life predictions of such new energetic systems can be made. Such life predictions will become necessary to fully exploit the possibilities of these new propellant types.

6.0 ACKNOWLEDGEMENT

The presented work is supported by the Dutch Ministry of Defense.

7.0 SYMBOLS

A pre-exponential coefficient Ea Activation Energy G Gas production Mc Average molar weight R gas constant T Temperature

25 - 12 RTO-MP-091

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

NATO UNCLASSIFIED

ADN Ammounum Dinitramide AP Ammonium Perchlorate APP Aerospace propulsion products BAMO 3,3-Bis-azidomethyl-oxetane BTTN Butanetrioltrinitrate CTPB Carboxyl Terminated PolyButadiene DB Double Base DSC Differential Scanning Calorymetry GAP Glycidyl azide polymer GC Gas Chromatography GPC Gel Permeation Chromatografy HPLC High Pressure Liquid Chromatography HNF Hydrazinium nitroformate HTPB Hydroxyl Terminated PolyButadiene HTPE Hydroxy-terminated polyether IM Insensitive Munition IPDI IsoPheroneDiIsocyanate IST Isothermal Storage Test NC NitroCellulose NCO-OH Isocyanate-hydrocyl equivalence ratio NG NitroGlycerine NMR Nuclear Magnetic Resonance PGLYN Polyglycidylnitrate PNIMMO Poly-3-nitratomethyl-3-methyloxetane TEGDN Triethyleneglycoldinitrate TMETN Trimethylolethane trinitrate TNO Netherlands organization for applied scientific research TGA/DTA Thermo Gravimetric Analysis/ Differential Thermal Analysis VST Vacuum Stability Test

8.0 REFERENCES

[1] AGARD Advisory Report 350: Structural Assessment of Solid Propellant Grains, Report of the Propulsion and Energetic Panel Working Group 25, Dec. 1997.

[2] Fitzgerald, J.E., Hufferd, W.L., Handbook for the Engineering Structural Analysis of Solid Propellants, CPIA Publ 214, May 1971.

[3] Ferry, J.D., Viscoelastic Properties of Polymers, Second Edition, 1970.

RTO-MP-091 25 - 13

NATO UNCLASSIFIED

Evaluation of Rocket Motor Safelife Based on Condition Monitoring and Ageing Modelling

25 - 14 RTO-MP-091

NATO UNCLASSIFIED

NATO UNCLASSIFIED

[4] Christiansen, A.G., Layton, L.H., Carpenter, R.L., HTPB Propellant Ageing, AIAA 80-1273, 1980.

[5] STANAG 4581, Explosive Materials: Assessment of Ageing Characteristics of Composite Propellants Containing an Inert Binder, Fifth Draft, May 2001.

[6] Cunliffe, A.V., Davis, A., Tod, D., Ageing and Life Prediction of Composite Propellant Motors, 87th AGARD PEP Symposium, 1996.

[7] Keizers, H.L.J., Heijden, van der, A.E.D.M., Vliet, van, L., Welland-Veltmans, W.H.M., Ciucci, A., Developments on HNF-based High Performance and Green Solid Propellants, ESTEC, NL, ESA Green Propellant Symposium, June 2001.

[8] Bohn, M.A., Ageing and Service Time Period Assessment of Novel Solid Rocket Propellant Formulations containing CL-20, AP and Energetic Plasticizers, 28th Int. Pyrotechnics Seminar, Adelaide, Australia, Nov. 2001.