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American Institute of Aeronautics and Astronautics

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Development of ADN-based Minimum Smoke Propellants

Niklas Wingborg*, Sten Andreasson, John de Flon, Mats Johnsson, Mattias Liljedahl, Carl Oscarsson, Åke Pettersson and Marita Wanhatalo

Swedish Defence Research Agency, FOI. SE-147 25 Tumba, Sweden

There is a need for more environmentally benign minimum smoke propellants to replace

current lead containing double-base propellants. One of the most promising candidates to

replace double-base is composite propellants based on ammonium dinitramide (ADN). This

paper presents development of ADN-based minimum smoke propellants and results from

test firing of a rocket motor containing 3 kg ADN propellant.

Nomenclature

ADN = ammonium dinitramide a = burning rate constant CDB = cast double-base

CMDB = composite modified cast double-base de = external grain diameter di = internal grain diameter EDB = extruded double-base

GAP = glycidyl azide polymer Isp = specific impulse l = grain length n = pressure exponent p = pressure (MPa) r = burning rate (mm/s) w = total burning distance, web; =(de-di)/2 XLDB = crosslinked double-base ∆Hf = heat of formation λ = normalized grain length; = l/de η = normalized grain thickness; = w/de ρ = density

I. Introduction

urrent minimum smoke double-base propellants used in tactical missiles contains lead compounds, such as lead resorcylate and lead maleate, to obtain the desired ballistic properties. Exposure to lead is associated with a

number of acute and chronic health effects, even at very low levels. The US Department of Defense (DoD) organized workshop on advanced strategy for environmentally sustainable energetics identified lead compounds used in propellants as one of the key environmental safety and occupational health issues 1. Hence the DoD’s Strategic Environmental Research and Development Program (SERDP) have stated a need to develop new environmentally benign, insensitive, castable, high-performance, minimum smoke rocket propellant formulations 2. The formulations must meet all of the performance requirements associated with current minimum smoke, double-base propellants, but must not contain lead compounds, ammonium perchlorate or RDX. There has been considerable efforts to develop such a propellant and many different energetic materials have been studied 3-8. One of the most promising energetic material for this application is ammonium dinitramide (ADN). This paper presents some of the properties of ADN and the ongoing work in Sweden to develop a minimum smoke composite propellant based on ADN and glycidyl azide polymer (GAP).

* Senior Scientist, Department of Energetic Materials, [email protected], Senior Member AIAA.

C

46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit25 - 28 July 2010, Nashville, TN

AIAA 2010-6586

Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.


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II. Ammonium dinitramide, ADN

ADN was first synthesized 1971 at the Zelinsky Institute of Organic Chemistry in Moscow. It is claimed that ADN-based solid propellants are in operational use in Russian Topol intercontinental ballistic missiles 9 and that ADN previously was produced in ton-size quantities in the former USSR 10. The USSR’s dinitramide technology was strictly classified and unknown to the rest of the world until 1988 when it was “re-invented” at SRI 11 in the USA. In the beginning of the 1990s, the Swedish Defence Research Agency, FOI, started research on ADN in order to develop minimum smoke propellants for tactical missile applications.

A. Basic properties of ADN

ADN (CAS nr. 140456-78-6) is a solid white salt of the ammonia cation (NH4+) and the dinitramide anion

(N(NO2)2-), Figure 1. It has a high oxygen balance, +25.79%, melts at 93°C and starts to decompose at

approximately 150°C at a heating rate of 10 K per minute, as seen in Figure 2.

-N

NO2

NO2

NH4+

Figure 1. Structure of ADN.

Table 1. Properties of ADN at 25.0 °°°°C.

Molecular weight 124.07 g/mol Density 1.81 g/cm3 13 Oxygen balance +25.79% 12 Melting point 93.2°C Heat of melting 142 J/g Heat of formation -148 kJ/mol 13 Heat of combustion 424 kJ/mol 12 Heat of solution +35.7 kJ/mol 14 Solubility in 100 g H2O 357 g 12 Molar volume in H2O 74.08 g/mol 12 Critical relative humidity 55.2% 12

Similarly to ammonium nitrate, ADN is hygroscopic and readily soluble in water and other polar solvents but

scarcely soluble in non-polar solvents. The critical relative humidity for ADN is 55.2 % at 25.0 °C 12. Thus the relative humidity must be below this level during storage and handling. Some of the properties of ADN are summarized in Table 1.

Temperature (oC)

50 100 150 200 250 300

He

at

flo

w (

w/g

)

-15

-10

-5

0

5

10

ADN 20089012

Figure 2. DCS thermogram of ADN. Heating rate

10 K/min.

N

NO2

NO2

HNO3 / H2SO4

H2NSO3T < -25 oC

Figure 3. Synthesis of dinitramide from sulfamic

acid.


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B. Production of ADN

ADN can be synthesized by different methods and an overview of these has been described by Venkatachalam, et al. 15. One viable method is based on direct nitration of salts of sulfamic acid by ordinary mixed acids (sulfuric and nitric), followed by neutralization and separation of the ADN formed, Figure 3.

This method was developed and patented by FOI in the mid 1990s 16. The method was then scaled up and the technology was transferred to EURENCO Bofors in Sweden, which has been producing dinitramides in a pilot plant scale since 1996, on license from FOI. All chemicals involved in the method are standard industrial chemicals and thus no strategic materials are needed. Sulfamic acid is, for instance, used to produce sweeteners and as an ingredient in cleaning agents. The purity of the material produced is above 99% pure, it has a high thermal stability and thus no stabilizer is required.

Through the years, EURENCO Bofors has delivered samples to more than 50 research establishments and companies worldwide. By providing samples to a large community, a broader range of potential applications for military and civilian purposes has emerged. Dinitramides are currently used in gas generators for automotive air-bags, which is their first full scale application. For this application guanylurea dinitramide, GUDN (FOX-12) 17, is used and EURENCO Bofors has produced GUDN in industrial scale since 2000.

III. Formulation and Testing

A. Specific impulse

The theoretical specific impulse for ADN-based composite propellants were calculated using the NASA CEA 600 computer program 18, 19. An infinite area combustor and shifted equilibrium during expansion was assumed. The binder considered in combination with ADN was poly glycidyl azide (GAP). GAP was chosen because it:

• is compatible with ADN • improves performance • provides good ballistic properties in combination with ADN.

The thermochemical data for ADN used in the calculations are shown in Table 1, and the data for GAP was

taken from the ICT thermochemical database (C3H5N3O, ρ= 1.29 g/cm3, ∆Hf =+114 kJ/mol) 20. The results from the calculations are shown in Table 2 and Figure 4.

In Figure 4 the calculated values are compared with the specific impulse of conventional minimum smoke propellants, such as extruded and cast double-base (EDB/CDB), composite modified cast double-base (CMDB) and cross-linked double-base propellants (XLDB), found in the literature 21. Table 2. Calculated specific impulse and density for ADN/GAP.

Amount ADN (%) Isp (s)a ρ (g/cm3) ρ·Isp (gs/cm3) 60 238 1.56 371 65 245 1.59 390 70 251 1.61 404 75 256 1.64 420 80 260 1.67 434

a) Expansion: 7 to 0.1 MPa, adapted nozzle, 15° exit cone half angle. Already at a solid loading slightly above 50% ADN, the specific impulse for ADN/GAP matches that of

EDB/CDB, and at 60% ADN the specific impulse for ADN/GAP matches that of XLDB. The maximum specific impulse of 260 s is obtained with 80% ADN. This show that ADN/GAP-based propellants have the potential to increase the specific impulse by 10 -15% compared with conventional minimum smoke double-base propellants.


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Solid loading in ADN/GAP propellant (%)

50 60 70 80 90 100

Sp

ecific

im

pu

lse

(s)

210

220

230

240

250

260

270

ADN/GAP

EDB/CDB

CMDB

XLDB

Expansion: 7 to 0.1 MPa, adapted nozzle, 15° exit cone half angle.

Figure 4. Specific impulse for ADN/GAP compared

with conventional minimum smoke propellants. Figure 5. Spray prilled ADN.

B. ADN prilling

In order to obtain high specific impulse a high solid loading is required. However, high solid loading increases the viscosity of the uncured propellant slurry. Thus, the maximum solid loading and impulse is limited by processing constraints. To obtain a castable propellant formulation with reasonable viscosity and high solid loading, particles with minimum spatial extension are required. For this reason the particles should have a low aspect ratio or more preferably spherical shape. The particle shape of ADN received from EURENCO is needle shaped and thus not suitable for formulation. Controlling the shape and size of ADN crystals is one of the most critical problems that have hampered the development of ADN-based propellants until now.

At FOI a method to produce spherical ADN particles, prills, have been developed 22. The prills are produced by spraying molten ADN through a nozzle. In the nozzle the molten ADN is atomized to form droplets which then solidify to the desired prills seen in Figure 5.

The particle size can be controlled to some extent by varying spray nozzle size and pressure. Typical particle size distributions for the fine and coarse prills produced are shown in Table 3, Figure 6 and Figure 7.

Table 3. Typical particle size distributions of prilled ADN.

Grade d10 (µm) d50 (µm) d90 (µm) Fine 30 60 120 Coarse 30 200 400

Particle Size Distribution

0,01 0,1 1 10 100 1000 3000

Particle Size (µm)

0

2

4

6

8

10

12

Volu

me (

%)

Particle Size Distribution

0,01 0,1 1 10 100 1000 3000

Particle Size (µm)

0

2

4

6

8

10

Volu

me (

%)

Figure 6. Particle size distribution of fine ADN prills. Figure 7. Particle size distribution of coarse ADN prills.

Currently the prills are produced using up to 250 g ADN per batch. However, with modifications the method can

be run continuously making the technology suitable for industrial production. Fumed silica (Cab-O-Sil) is added to the prilled material as an anti-caking agent. Without any anti-caking agent, the prilled ADN cakes after a short time of storage, even at dry conditions.

The prills were characterized with respect to particle density, tap density, melting point and purity. The results are shown in Table 4. Properties of as-received ADN are shown for comparison.


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Table 4. Typical properties of as-received ADN and coarse prills 23

.

As received Prilled Amount Cab-O-Sil (%) -- 0.5 Particle density (g/cm3) 1.81 1.79 Tap density (g/cm3) 0.86 1.11 Volumetric loading (%) 47.5 62.0 Melting point (°C) 93.2 92.5 Nitrate content (%) 0.03 0.08

From the results in Table 4 it can be seen that the particle density decreases by 1%, but the tap density of the

prilled material increases by 30%, which is important to obtain high solid loading. The decrease in particle density might be due to inclusions formed during the spraying. The somewhat higher nitrate content in the prilled material is due to degradation of ADN during the prilling and is probably responsible for the change in melting point.

C. Formulation

An ADN propellant with a GAP-based binder have been formulated containing 70% bimodal prilled ADN. The GAP used was obtained from EURENCO France (Lot: 76S04 24). The relatively low solid loading was chosen to ensure low viscosity and hence good quality of the casted propellant. Based on our experience, it is now clear that the solid loading can be increased.

Batches up to 3.75 kg have been mixed using an IKA HKV 5 high performance kneader and samples for characterization and motor testing have successfully been cast and cured.

The thermal stability of the propellants was measured using a heat flow calorimeter. According to STANAG 4582 25, the heat flow should not exceed 63.1 µW/g at 75°C during 19 days. Figure 8 show that the propellant has an excellent thermal stability with a heat flow well below the acceptance limit.

Time (days)

0 5 10 15 20

He

at F

low

(µ

W/g

)

0

10

20

30

40

50

60

70

ADN/GAP, FP09053

Acceptance limit at 75°C according to STANAG 4582

Figure 8. Thermal stability at 75°C of ADN/GAP-

propellant containing 70% ADN.

Pressure (MPa)

2 3 4 5 6 7 8 9 201 10

Bu

rnin

g r

ate

(m

m/s

)

15

20

25

30

40

10

FP10002

FP10011

FP10014

FP10015

Ti=20oC

Figure 9. Burning rate for ADN/GAP 70/30. Four

different batches.

D. Burning rate measurements

The burning rate was determined using a strand burner. The results were evaluated using Eq. 1,

r=apn (1)

where p is the combustion pressure, in MPa, and r is the measured burning rate in mm/s. The pressure exponent,

n, and the burning rate constant, a, where determined by linear regression analysis of the data in a log-log diagram. The results from testing of four propellant batches are shown in Figure 9 and Table 5.


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Table 5. Evaluated constants in Eq. 1 and burning rate at 7 MPa.

a n R2 a r7 (mm/s)

9.2 0.49 0.993 24 a) R2 is the linear correlations coefficient.

The data correlates well with Eq. 1, as shown with a linear correlations coefficient, R, close to unity. It has a high burning rate, 24 mm/s at 7 MPa, and a pressure exponent below 0.5, in the pressure interval examined.

E. Motor testing

Rocket motor firings were performed to verify the calculated specific impulse and to demonstrate the ADN minimum smoke capability. To accurately determine the specific impulse from ballistic test motor firings the pressure should preferably be close to constant during the burning time interval. The steady-state pressure follows the burning area of the grain. Hence grain geometry should be chosen for minimum variation in burning area.

Usually for ballistic test motors cylindrical center perforated grain geometries, burning internally and from both ends, are preferred. For a given grain thickness (web) there is an optimum length, while for a given length the variation in burning area decreases the thinner the web. Typically the web is chosen to obtain a certain burning time.

For a given web the optimum grain length is determined according to Eq. (2),

η

ηηλ

−

+−=

2552

54 2

(2)

where λ and η are the normalized grain length and thickness, with respect to external diameter, respectively. Three 3 kg case bonded grains were casted in steel cartridges as shown in Figure 10. A liner based on HTPB was used. Some minor machining was needed after casting. The propellant was easy to machine yielding a smooth surface. In Figure 11 the heavy walled ballistic test motor used is shown.

Figure 10. Case bonded 3 kg ADN/GAP (70/30)

grain cartridges for motor testing.

Figure 11. Ballistic test motor on test stand.

Figure 12 shows the motor during firing conforming its minimum smoke characteristics. The red curve in

Figure 13 shows the recorded pressure as a function of time, and the black is the calculated pressure using the strand burner data in Figure 9. Some characteristics of the grain and the test motor, as well as some evaluated parameters from the motor firing, are shown in Table 6.

The results from the test show that the burning rate was 14% higher in the rocket motor compared to the strand burner data. Else, the shape of the pressure curve reasonably agreed with the calculations. The measured specific impulse was a few percents lower than predicted, as is usually the case, confirming the high performance potential of ADN/GAP.


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Figure 12. Test firing of ADN/GAP rocket motor. Figure 13. Combustion pressure vs time. Red curve

experimental, black curve calculated.

Table 6. Grain and nozzle characteristics and evaluated parameters.

Propellant ADN/GAP 70/30 batch no. FP10015 Grain internal diameter 73.0 mm Grain external diameter 123.2 mm Propellant weigh t 3025 g Mean pressure 5.55 MPa Burning rate 24 mm/s (calc. 21 mm/s) Nozzle throat diameter 31 mm Nozzle area expansion ratio 5.0 (under expanded) Specific impulse 233 s

F. Sensitivity and mechanical properties

The propellant used in this work did not contain any plasticizer or bonding agent and its mechanical properties needs to be improved to obtain the desired elasticity.

The sensitivity is one of the most important issues to consider when developing new propellants. ADN/GAP-propellants with high solid loading is expected to be a hazard class 1.1 material. However, at low solid loading the sensitivity is expected to be reduced. Preliminary results show that ADN/GAP, containing 68% ADN, is less shock sensitive than double-base. The sensitivity can also be reduced by partially replacing ADN with a low sensitive energetic filler.

IV. Conclusion

Minimum smoke composite propellants based on ADN and GAP have high specific impulse. Already at a approximately 50% ADN, the specific impulse matches that of EDB/CDB, and at 60% the specific impulse matches that of XLDB. At 80% the maximum specific impulse, 260 s, is obtained which is 10 -15% higher compared with conventional double-base propellants.

Spherical ADN particles, prills, can be produced enabling high solid loading. An ADN propellant with a GAP-based binder has been formulated containing 70% bimodal prilled ADN. The propellant had low viscosity and higher solid loading can be used.

Batches up to 3.75 kg propellant have been mixed and casted. The propellant has excellent thermal stability, high burning rate and a pressure exponent below 0.5.

3 kg case bonded grains were casted and test fired. The results from the test firing confirmed the minimum smoke characteristics and the high specific impulse.


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Acknowledgments

The authors thank the Swedish Armed Forces, the Swedish Defence Materiel Administration and the Defence Science and Technology Agency Singapore for funding this work.

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