HNF: Oxidiser for High Performance, Low-Signature Propellants

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1 A.E.D.M. van der Heijden, H.L.J. Keizers TNO Prins Maurits Laboratory P.O. Box AA, Rijswijk THE NETHERLANDS W.H.M. Welland-Veltmans Aerospace Propulsion Products Westelijke Randweg RT Klundert THE NETHERLANDS ABSTRACT The use of hydrazinium nitroformate (HNF) as a main oxidiser ingredient in composite solid propellants has two distinct advantages compared to conventional propellants: first a significant increase in performance of the propellant can be achieved and secondly the propellant s exhaust gases are chlorinefree, leading to a low-signature propellant. At TNO Prins Maurits Laboratory, HNF solid composite propellants have been subject of investigation for several years. Together with APP, manufacturer of HNF, international programmes both for the space and the military market have been and are being conducted. The research is focused on the use of inert (HTPB) as well as energetic binders (GAP, PNM). In this contribution some of the ballistic, mechanical, hazard and thermal properties of HNF/HTPB based composite solid propellants will be summarised, including theoretical results on the combustion products of this class of propellants in comparison with conventional, minimum-smoke AP/HTPB propellants. Keywords: HNF, propellant, performance, signature. INTRODUCTION At TNO Prins Maurits Laboratory (TNO-PML) the development of HNF and HNF based propellants started in the early nineties. In 1992 a pilot plant for the production of the oxidiser HNF was built at Aerospace Propulsion Products (APP). Raw HNF is produced by the addition of hydrazine to nitroform, leading to a strongly exothermic precipitation reaction forming HNF. The raw HNF (HNF-P) can be recrystallized for reasons of control of the mean size and/or its purity by means of cooling (HNF-C), evaporative (HNF-E) and solvent/non-solvent or drowning-out (HNF-S) crystallization. Although patent literature [1,2] indicated that HNF propellants based on binders with unsaturated C=C bonds in the backbone of the polymer did not lead to thermally stable propellants, recent results on HNF/HTPB based propellants have shown that the experimental characterisation of the thermal stability, mechanical, hazardous and ballistic aspects appear promising, although in particular the thermal stability and pressure exponent (~ 1) need further optimisation. Figure 1 shows HNF crystals as produced by APP in The Netherlands. Optimisation of the production process over the years, has led to a significant improvement of the aspect ratio of the HNF particles. Also the mean particle size can be controlled, by using different crystallisation techniques, like drowningout or cooling crystallisation [3-5] or by means of mechanical grinding of the HNF crystals. A recently published review paper on HNF provides an extensive overview of the development and properties of the oxidizer HNF [6]. Paper presented at the RTO AVT Specialists Meeting on Advances in Rocket Performance Life and Disposal, held in Aalborg, Denmark, September 2002, and published in RTO-MP-091. RTO-MP

2 Figure 1: HNF Crystals as Produced by APP. The mean size of the HNF particles produced by APP is in the range of µm. The aspect ratio of the particles is between 1-4. EXPERIMENTAL Apart from an improved theoretical and experimental performance of HNF based composite propellants, an additional advantage is that HNF is chlorine free. This implies that it is more environmentally friendly and that it has an improved signature compared to the conventional AP/HTPB propellants. An advantage of the replacement of AP by HNF in an HTPB based binder system is that the relatively low cost HTPB raw materials and existing HTPB processing knowledge and infrastructure can be maintained. Contrary to patent literature [1,2], recent investigations have shown that the combination of HNF and HTPB containing unsaturated C=C bonds in the polymer backbone is well possible. This mainly depends on the use of a suitable curing system and the fact that the current product quality of HNF produced by APP has improved significantly compared to the quality of the HNF on the basis of which the older patents [1,2] have been issued. HNF/HTPB batches have been produced on 350 gram scale for characterisation purposes, like ballistic, mechanical, thermal and hazard properties. Mixing and casting is carried out in an IKA mechanical mixer with horizontal mixing blades. Curing of the cast items is performed at 40 C, during 5-7 days. Table 1 summarises a selection of the batches that will be discussed in this paper. Additional information can also be found in a review paper [7] RTO-MP-091

3 Batch code HNF [wt%] Al [wt%] Table 1: Formulations of Selected HNF/HTPB Propellants AP [wt%] Stabilizers [wt%] BRM [wt%] Binder c [wt%] Remarks HHU Bimodal mix of HNF-C15 and HNF-S16 (70/30 wt%); non-aluminised HHU HNF-C18; non-aluminised HHU HNF-E8; non-aluminised HHU a HNF-C24 HHU b HNF-C24 HHU b HNF-C24 HHU HNF-C27, ground; non-aluminised HHU HNF-C27, bimodal unground/ground (70/30 wt%); non-aluminised CK b AP-based reference propellant a b c Mean size of Al: 20 µm. Mean size of Al: 6 µm. Binder composition: HTPB/isocyanate/plasticiser. The highest solid load which has been achieved up to now with HNF/HTPB propellants is 80 wt%. These formulations used as-received, monomodal HNF batches. A further increase of the solid load is foreseen by lowering the aspect ratio of the HNF crystals or by using bimodal particle size distributions. CHARACTERISATION OF HNF/HTPB PROPELLANTS The characterisation tests comprised chimney burner tests (ballistic properties), Shore A hardness and compression tests (mechanical properties), VST and TG/DTA (thermal stability) and BAM impact and friction sensitivity (hazard properties). A selection of the results obtained with different HNF/HTPB propellants will be reported in this paper. Ballistic Properties The ballistic properties (Vieille s law: r = a p n ) were determined using a chimney burner test set-up, in which strands of propellant are burnt at a constant pressure p. The dimensions of the strands is mm 3. The regression rate r of the strand is measured as a function of the pressure. By plotting r vs. p, the values of the fitting parameters a and n (pressure exponent) can be determined, see table 2. Table 2: Ballistic Properties of Selected (non-)aluminised HNF/HTPB Propellants Formulation a n r 7 [mm/s] HHU HHU HHU HHU HHU HHU HHU RTO-MP

4 Uncatalysed HNF/HTPB propellants show a pressure exponent of approximately , which is too high for safe operation of solid rocket motors. When using a burn rate modifier, the pressure exponent can be reduced to 0.85 (HHU-7). Also by using very fine, ground HNF particles (5-10 µm), a lower pressure exponent was found (~ 0.8) [8]. The results show that the burn rate properties of HNF/HTPB propellants can be tailored. Additional efforts are ongoing to further control the pressure exponent. Considering the high burn rates achieved, HNF based propellants are particularly interesting for high burn rate applications. Mechanical Properties The mechanical properties of several of the HNF/HTPB propellant samples have been determined by means of compression and Shore A hardness tests. The results are presented in Table 3. Table 3: Mechanical Properties and Shore A Values of a Selection of Propellants (Compressive Tests) Batch code ε σ max [%] E 0 [MPa] σ max [MPa] Shore 0 s / 15 s HHU / 52 HHU / 62 HHU a / HHU / 31 HHU / 50 a σ at 60% strain. The differences in the mechanical properties of the HNF/HTPB propellant series are due to differences in solid load, combined with possible catalysing effects of the burn rate modifiers on the curing reaction between the isocyanate and the hydroxyl groups of the pre-polymer. The latter was in particular observed for HHU-7, which also had a significantly shorter pot-life during processing compared to the non-catalysed batch HHU-6. The Shore A hardness values are similar to conventional state-of-the-art AP/HTPB propellants. Thermal Stability and Ageing The thermal stability of HNF/HTPB propellants has been assessed by means of TG/DTA and VST measurements. The results are summarised in table 4. The conditions during testing with the TG/DTA are: Equipment : Seiko TG/DTA 320 Temperature range : C or 350 C Heating rate Atmosphere Flow rate Sample Reference Sample size : 2.5 or 10 C/min : Nitrogen : ~ 50 ml/min : Open aluminium cup : Empty open aluminium cup : ~ 5 10 mg 10-4 RTO-MP-091

5 Table 4: Thermal Stability of a Selection of HNF/HTPB Propellants Batch Code TG/DTA (scanning) VST [ml/g] HHU-2 No data 1.10 (48 hrs@60 C) 2.42 (100 hrs@60 C) DTA TG HHU-3 T onset dec. 124 C T onset dec. 125 C T peak dec. 131 C T extrapol onset 129 C 5.36 (100 hrs@60 C) Enthalpy dec µvs/mg Total mass loss 82 wt% DTA TG HHU-4 T onset dec. 104 C T onset dec. 126 C T peak dec. 124 C T extrapol onset 4.70 (48 hrs@60 C) Enthalpy dec. Total mass loss 68 wt% DTA TG HHU-6 T onset dec. 124 C T onset dec. 121 C T peak dec. 124 C T extrapol onset 124 C 0.52 (48 hrs@60 C) Enthalpy dec µvs/mg Total mass loss 70 wt% HHU-7 No data 1.20 (48 hrs@60 C) HHU-8 No data 20.5 (70 hrs@60 C) HHU-9 No data 10.6 (95 hrs@60 C) The vacuum stability was determined on ~ 1 g propellant samples in duplicate or triplicate during 48 hrs at 60 C. For some specimens, a period of 100 hrs at 60 C has been applied in order to determine the stability for prolonged periods of time. The thermal stability data indicate that the use of specific burn rate modifiers may lead to an increase of the VST value as compared to a non-catalysed propellant, see e.g. HHU-2 (non-catalysed) and the catalysed propellants HHU-4 and HHU-7. A decreased thermal stability of the catalysed propellants is also evident from the TG/DTA data, in particular for HHU-4 showing an onset temperature for decomposition of ~ 104 C which is approximately C below the onset temperature found for a non-catalysed propellant (HHU-6). The use of ground HNF in formulations HHU-8 and 9, leads to an increase of the VST value compared to the reference using standard HNF (HHU-3). Probably this increase is caused by the increase in specific surface area of the ground HNF crystals, which increases the interaction between HNF and other propellant ingredients, leading to more (chemical) reactions via which additional gases may evolve. The compatibility of HNF with other propellant ingredients and the thermal stability of formulations containing HNF have been significantly improved over the years. This appears to be primarily related to an increased purity of the HNF produced nowadays. Furthermore, stabilizers have been identified which effectively increase the shelf life of HNF based propellants. With the HNF/HTPB propellants mentioned, shelf lives of over 1 year have been obtained. Although the current stabilities are still insufficient for use in tactical missile applications, future efforts are likely to yield more thermally stable formulations. RTO-MP

6 Hazard Properties Table 5 summarises the BAM impact and friction sensitiveness of the HNF/HTPB propellants. For comparison the hazard properties of pure HNF is given as well. The value of the friction sensitivity of the HNF/HTPB propellants is generally slightly higher than that of pure HNF, whereas the impact sensitivity of an HNF-based propellant is generally similar to that of HNF. Table 5: Friction and Impact Sensitiveness of HNF(/Al)/HTPB Propellants (For comparison also typical data of pure HNF have been included) Batch code Friction [N] Impact [Nm] HHU HHU HHU HHU HHU HNF-C The hazard properties of the propellant containing the ground HNF (HHU-8) are an exception, since in this particular case the propellant is rather insensitive, both with regard to friction and impact stimuli. This formulation only contains 50 wt% of HNF, so the low sensitiveness could be a dilution effect. However, previous results on similarly diluted samples did not indicate such a significant reduction in sensitiveness [9]. This strongly suggests that the HNF mean particle size plays a role as well. Internal defects in crystals are often regarded as initiation sites for a reaction like impact, friction or decomposition [10]. By grinding, the number of internal defects per particle is reduced because of the increase of specific surface area combined with the fact that the probability of a crystal fracturing at an internal defect, is high. Less crystal defects in the ground material will therefore reduce the number of initiation sites for a positive reaction in a friction and impact test, leading to significantly improved hazard properties. PERFORMANCE, SIGNATURE AND COMBUSTION PRODUCTS Performance In Table 6 the theoretical performance, based on NASA CET89 calculations (equilibrium flow conditions) and the smoke classification, according to STANAG 6016, of several new propellant formulations based on HNF, is compared to a conventional AP/HTPB propellant. All the formulations are based on a 86 wt% solid load. Both the specific impulse (I sp ) and density specific impulse (ρ I sp ) have been calculated. The primary design driver for solid rocket motors is the specific impulse of the propellant (impulse per unit weight of a propellant). However, the volume required to fit this amount of propellant is directly linked to the density of the propellant. This can be described by using the parameter density specific impulse, ρ I sp. The relative importance of these two properties (I sp, ρ I sp ) is system dependent and a more detailed system specific analysis is required before either of the two options (inert vs. energetic binder) can be chosen. In addition other development aspects (stability, usability of existing infrastructure, availability of raw materials, costs etc.) must be considered in such an assessment. The propellants based on HNF show to have significant potential improvements compared to the state-of-the-art AP based propellants RTO-MP-091

7 Table 6: Theoretical Performance and Smoke Classification (according to STANAG 6016) of different Propellant Formulations, as compared to a conventional AP/HTPB Propellant (conditions: 6 MPa, A e /A t = 60, equilibrium calculations) all formulations refer to a solid load of 86 wt% Composition I sp [s] ρ I sp [g s /m 3 ] Smoke AP / HTPB AC HNF / HTPB AA HNF / PNM AA HNF / GAP AA The highest vacuum specific impulse at a solid load of 86 wt% is found for HNF based propellants in combination with the energetic binders GAP and PNM. Although the density of HNF is lower than that of AP, the higher vacuum specific impulse compensates for this lower density when calculating the density specific impulse. Ispec (s) Solid Load (w%) HNF/HTPB HNF/GAP HNF/PNM Figure 2: Graphical Representation of the Theoretical Performance (Vacuum Specific Impulse) as a Function of the Solid Load of HNF in Combination with an Inert HTPB Binder and the Energetic Binders GAP and PNM. Signature and Combustion Products All the propellants using HNF show an AA smoke classification according to the STANAG 6016, whereas the AP/HTPB has an AC classification. The higher classification of the new propellant families is mainly due to the absence of chlorine in the ingredients. The combustion products of the different propellant formulations are listed in table 7. The most important observation, obviously, is the absence of HCl in the HNF containing formulations. The relatively high CO content for the AP/HTPB and HNF/HTPB compositions is related to the relatively high (negative) oxygen balance of ~ -14 and ~ -32 wt%, respectively, pointing at an incomplete combustion of the carbon atoms. The oxygen balance is ~ -5 and ~ -6 wt% for HNF/PNM and HNF/GAP, respectively. Furthermore, the nitrogen content for the HNF containing compositions, and especially those with the energetic binders, is significantly higher compared to the AP/HTPB composition; this is related to the fact that both HNF and the energetic binders contain much more N-atoms than AP. RTO-MP

8 Table 7: Combustion Products (in mole%) of several Propellant Formulations 298 K), as calculated with the ICT Thermodynamic Code [11] all formulations refer to 86 wt% solids and 14 wt% binder Combustion products AP/HTPB HNF/HTPB HNF/PNM HNF/GAP CO 2 (g) H 2 O (l) N 2 (g) CO (g) H 2 (g) NH 3 (g) CH 4 (g) HCl (g) 18.5 CONCLUSIONS In this paper some of the characteristic properties of HNF based solid composite propellants, in particular using HTPB as the binder, have been presented and compared to those of conventional AP/HTPB propellants. The results show that HNF based propellant formulations show a high potential with regard to the performance gains which can be realised compared to AP/HTPB, especially when using energetic binders like GAP and PNM. Because of the absence of chlorine, also the smoke classification of HNF based propellants is improved from AC for AP/HTPB to AA for HNF propellants. The main issues still to be addressed for HNF based propellants are: Reduction of the pressure exponent n, Improvement of the thermal stability, e.g. by further purification of the HNF batches and/or the use of stabilizers, and Increase of the solid load by reducing the aspect ratio of the HNF crystals and/or by using polymodal powders. Currently, several research programmes are being carried out addressing these issues. ACKNOWLEDGEMENTS Part of the work has been carried out under a European Space Agency s Contract Ref /98/ NL/PA(SC), NIVR (Dutch Space Agency) and NL-MoD contracts as well as private TNO company funding. ABBREVIATIONS AP APP BRM GAP Ammonium perchlorate Aerospace Propulsion Products Burn rate modifier Glycidyl azide prolymer 10-8 RTO-MP-091

9 HNF HNF-C HNF-E HNF-P HNF-S HTPB PML PNM STANAG TG/DTA TNO VST Hydrazinium nitroformate HNF produced by cooling crystallization HNF produced by evaporative crystallization Raw HNF HNF produced by solvent/non-solvent crystallization Hydroxy-terminated polybutadiene Prins Maurits Laboratory Poly 3-nitratomethyl-3-methyloxetane (also: PolyNIMMO) Standard NATO agreement Thermogravimetric/differential thermal analysis Netherlands Organization for Applied Scientific Research Vacuum stability test REFERENCES [1] G.M. Low and V.E. Haury, Hydrazinium Nitroformate Propellant with Saturated Polymeric Hydrocarbon Binder, US Patent 3,708,359, January 2, [2] G.M. Low and V.E. Haury, Hydrazinium Nitroformate Propellant Stabilized with Nitroguanidine, US Patent 3,658,608, April 25, [3] W.H.M. Veltmans, A.E.D.M. van der Heijden, M.I. Rodgers and R.M. Geertman, Improvement of Hydrazinium Nitroformate Product Characteristics, Proceedings of 30th Annual International Conference of ICT, June 29 July 2, 1999, Karlsruhe, FRG. [4] W.H.M. Veltmans and A.E.D.M. van der Heijden, Sonocrystallisation of Hydrazinium Nitroformate to Improve Product Characteristics, Proceedings of 14th International Symposium on Industrial Crystallisation (IChemE), September 1999, Cambridge, UK. [5] W.H.M. Veltmans, A.E.D.M. van der Heijden, J.M. Bellerby and M.I. Rodgers, The Effect of Different Crystallisation Techniques on Morphology and Stability of HNF, Proceedings of 31st Annual International Conference of ICT, June 27 30, 2000, Karlsruhe, FRG. [6] H.F.R. Schöyer, W.H.M. Welland-Veltmans, J. Louwers, P.A.O.G. Korting, A.E.D.M. van der Heijden, H.L.J. Keizers and R.P. van den Berg, Overview of the Development of Hydrazinium Nitroformate, J. Propulsion and Power 18 (2002) 131. [7] H.F.R. Schöyer, W.H.M. Welland-Veltmans, J. Louwers, P.A.O.G. Korting, A.E.D.M. van der Heijden, H.L.J. Keizers and R.P. van den Berg, Overview of the Development of Hydrazinium Nitroformate-Based Propellants, J. Propulsion and Power 18 (2002) 138. [8] W.H.M. Welland-Veltmans, A.E.D.M. van der Heijden, H.L.J. Keizers, L.D. van Vliet, W. Colpa, F. Lillo and G. Marcelli, Improvement of Ballistic Properties of HNF/Al/HTPB Based Propellants, presented at the Symposium on Propulsion for Space Transportation of the XXI st century, Versailles, May [9] Unpublished Results. [10] A.I Atwood, P.O. Curran, K.J. Kraaeutle, T.P. Parr, D.M. Hanson-Parr, Decomposition Studies of Production Grade Ammonium Perchlorate and an Ammonium Perchlorate Based Solid Rocket Propellant, Presented at the 30 th International Annual Conference of ICT, June 1999, Karlsruhe, Germany. [11] ICT Thermodynamic Code, Version 1.00, Fraunhofer ICT, Pfinztal/Berghausen, Germany. RTO-MP

10 SYMPOSIA DISCUSSION PAPER NO: 10 Discusser s Name: Alain Davenas Question: 1) What would be the hazard classification of a rocket motor using a 76% HNF level of solid propellant? 2) Other authors in your organization advocate the use of HNF for space boosters where low burning rate are required. In your paper you say, HNF based propellants are particularly interesting for high burn rate applications. How do you reconcile these two types of statements? Author s Name: A.E.D.G. van der Heijden Author s Response: 1) It is expected that the hazard classification of a 76% HNF based propellant would be 1.1 rather than 1.3. However, other preliminary hazard tests (Slow Cook-off and bullet impact) performed at Fiat Avio indicated that HNF propellants (65 to 70 % by weight) show similar responses as conventional Ammonium Perchlorate propellants. 2) These two characteristics are relevant for two different applications. For a space booster, indeed a lower burn rate is required; whereas, for example, for a hyper velocity application, a high burning rate is required. Special additives and/or the use of co-oxidizers will be required to obtain the required properties for a specific application. Discusser s Name: Hans Besser Question: What are the ballistic and mechanical properties of formulations with HNF/GAP and HNF/PNM in comparison to the HNF/HTPB formulations? Author s Name: A.E.D.G. van der Heijden Author s Response: HNF/GAP: Studies conducted at TNO in 1993 to 1995 show that pressed HNF/AP/GAP propellants have a pressure exponent of 0.7 to 0.8 (uncatalyzed) and about 0.6 (catalyzed). Mechanical properties of these pressed compositions are not available. HNF/PNM: recent work has shown that the ballistic properties are more the same as those of HNF/HTPB, in particular with regard to the pressure exponent being close to 1 (uncatalyzed). At present the mechanical properties of HNF/HTPB are superior to those of HNF/PNM, but further optimization of the PNM binder is foreseen. Discusser s Name: Ron Derr Question: You have drawn conclusions for various propellant ingredient changes using six different lots of HNF. What is the lot-to-lot variation for HNF manufacturing? Author s Name: A.E.D.G. van der Heijden Author s Response: Due to the limited production capacity of HNF at this time, different lots have been used for this work. For most formulations, the cooling crystallization (C-) grade of HNF has been used. Experimental results RTO-MP-091

11 (also results not included in this paper) have shown only small differences between propellant characteristics when using different C- grades of HNF, although slight batch-to-batch variations exist with regards to, for example, mean size and particle size distribution. Only when using, for example, small HNF particles, or changes in other formulation ingredients (for example burn rate modifiers, aluminum, etc.), significant changes in the propellant properties have been found. The conclusions are based on these latter variations and are not related to batch-to batch variations of the HNF product. Discusser s Name: Joe Flanagan Question: What are the solids loadings for propellants used in bullet impact tests? Author s Name: A.E.D.G. van der Heijden Author s Response: The solid loading was 65 to 70 %. Note: these tests were performed not by TNO but by Fiat Avio. Discusser s Name: Luigi DeLuca Question: 1) What approach will you take to reduce the burning rate exponent? 2) Did you investigate how the lower pressure deflagration limit is affected by changing the burn rate exponent? Author s Name: A.E.D.G. van der Heijden Author s Response: 1) We intend to further explore the use of burning rate modifiers, possible in combination with very fine HNF particles. 2) No. RTO-MP

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