- Email: [email protected]
Ballistic anomalies in solid rocket motors due to migration effects
Acta Astronautica Vol. 13, No. 10, pp. 599~05, 1986 Printed in Great Britain. All rights reserved
0094-5767/86 $3.00+ 0.00 © 1986 Pergamon Journals Ltd
BALLISTIC ANOMALIES IN SOLID ROCKET MOTORS D U E TO MIGRATION EFFECTSt M. PR6BSTER~and R. H. SCHMUCKER§ Bayern-Chemie, Gesellschaft fiir flugchemische Antriebe mbH, Postfach l l31, D-8261 Aschau a. Inn, F.R.G. (Received I1 February 1986)
Abstract--Double base and composite propellants are generally used for rocket motors, whereby double base propellants basically consist of nitrocellulose plasticized with an explosive plasticizer, mostly nitroglycerine, and in some cases with an additional inert plasticizer and ballistic additives. Composite propellants consist of an oxidizer like ammonium perchlorate and of aluminum, binder and plasticizer and often contain liquid or solid burning rate catalysts. A common feature of both propellants is that they contain smaller or larger amounts of chemically unbonded liquid species which tend to migrate. If these propellants loose part of the plasticizer by migration into the insulation layer, not only will there be a change in mechanical propellant properties but also the bond between propellant and insulation may degrade. However, depending on the severity of these effects, the change in the ballistic properties of the propellant grain caused by plasticizer migration may be of even more importance. In the past, most emphasis was placed on the behaviour of end-burning configurations. However, more recent theoretical and experimental studies revealed that not only for end-burning grain configurations but also for internal burning configurations there is a common effect which is responsible for ballistic anomalies: migration of liquid species from the propellant into the insulation. By using a plasticizer equilibrated insulation in an internal burning configuration the liquid species migration and thus the previously observed ballistic anomalies are avoided. Using this approach for end-burning configurations provides similar positive results. The various factors affecting plasticizer migration are studied and discussed, and several methods to prevent liquid species migration are described as well as methods to obtain plasticizer resistant insulations.
In the past, most emphasis was placed on the behaviour of end-burning configurations; in part, the above described effects were found to be responsible for ballistic anomalies in these cases[3-6]. The effects of these processes on the ballistics of internal burning grain configurations were studied with much less emphasis[7-9], pointing out that similar trends are to be expected. The influence of this migration on the ballistic properties of double base and composite propellants and how it can be prevented will be the topic discussed herein.
1. INTRODUCTION The solid propellants generally used for rocket motors can be divided into several main groups, with double base and composite propellants being the best known types[l, 2]. Basically, double base propellants consist of nitrocellulose plasticized with an explosive plasticizer, mostly nitroglycerine, and in some cases with an additional inert plasticizer and ballistic additives. Composite propellants consist of an oxidizer like a m m o n i u m perchlorate and of aluminium, binder and plasticizer and often contain liquid or solid burning rate catalysts. A c o m m o n feature of both propellants is that they contain smaller or larger amounts of chemically u n b o n d e d liquid species which tend to migrate. If these propellants loose part of the plasticizer by migration into the insulation layer, not only will there be a change in mechanical propellant properties but also the bond between propellant and insulation may degrade. However, depending on the severity of these effects, the change in the ballistic properties of the propellant grain caused by plasticizer migration may be of even more importance.
2. MIGRATION OF PLASTICIZERS Plasticizer migration is defined as the balancing process in the rates of concentration between two adjacent material layers which contain one or more mobile species. As shown in Fig. 1 two cases can be identified, namely:
#Paper IAF 85-176 presented at the 36th Congress of the International Astronautical Federation, Stockholm, Sweden, 7-12 October 1985. :~Dr. rer. nat., Dipl.-Chem., Technical Staff Member, Propellant Development Department, 8261 Aschau, F.R.G. §Prof., Dr.-Ing., Head, Engine Development Department, 8012 Ottobrunn, F.R.G. 599
(a) the most c o m m o n case, where the propellant contains a plasticizer, a liquid burning rate catalyst or even just a liquid curing agent migrating into the insulation which on its own is short of or free of these mobile liquid species, (b) the case where ingredients of the insulation material, like plasticizer, catalysts, decomposition products, unreacted monomers, etc., migrate into a propellant which is short of these mobile liquid species.
600
M. PR(~)BSTERand R. H. SCHMUCKER
x-
×
x x x
Propellant
Insulation
Insulation
PropeHont
Fig. 1. Migration effects at the propellant/insulation interface. (The additional problems caused by layers of liner shall not be treated here.) In both cases, plasticizer migration will change the • chemical properties of insulation and propellant; • mechanical properties of insulation and propellant; • bonding of insulation/propellant; • ballistic properties of the propellant. The degree and rate of propellant components migrating into the insulation--only this will be treated here--depend on a number of parameters, some of which cannot be influenced in a given polymer[lO]: • concentration; • temperature; • solubility parameters of polymer and plasticizer. The diffusibility of the mobile species is determined by the: • size of molecules; • shape (cross section) of molecules; • polarity of molecules.
subsequently stored at an elevated temperature. As a function of time the weight increase gives an indication of the migration tendency of the various pairs of propellant/insulation materials. For the tests described below discs with a diameter of 50 mm and a thickness of 6 mm were used a n d - - a s shown in Fig. 2--loaded with 5 kp. A sol/gel analysis was carried out beforehand to ensure that the insulation test specimen contained no species which could migrate (e.g. from incomplete curing reactions) and could, by a sort of countermigration lead to an under-estimated migration of the propellant plasticizer[13, 14]. 3. BALLISTIC ANOMALIES DUE TO LIQUID SPECIES MIGRATION What happens if liquid species migrate from a propellant grain into the insulation? Depending on the type of liquid ingredients four principles have to be identified: • migration of an energetic plasticizer (Ngl, DEGN, etc.); • migration of an inert plasticizer (DOA, DOS, IDP, etc.);
How easy a molecule can migrate into a polymer and plasticize it depends on the: • crystallinity; • cross-link density; • phase transitions of 1. and 2. order; • polarity; • H-bonding capability.
50
i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:!:i:i:i:!:!:!'i:i:i:!:i:i:i:i:i:i:!: /
All these facts should be considered when a new type of insulation is being developed with the aim to have a highest possible resistance to plasticizer migration[ 11]. Before discussing the various effects of migration and the means of how to prevent it, some remarks on the measuring of migration effects are necessary. Apart from the quantitative analysis of migrated species by chromatographical or other methods[12-14], the method of analyzing the relative degree of migration in accordance with the German Industrial Standard (Deutsche Industrie-Norm) DIN 53 405 is well established, especially for series analysis[15, 16]. This method uses disc-size test specimen of propellant and insulation material which are brought into contact, loaded with a weight and
N !
/
"
/
/
/
/
/
/
~ /
~'"
/
/
/ / / / / f / / / / / / / / / / / / i
K~
~'~'s'.
Aiuminiurn sheets
!#11
/
/ / / a - / 1 / / / / / / / / / / / ~
/
i!iiTili:i:i:i:i:i:ili~i~i!!~i;!;i!!~!!i:i:!:!:i:!:i?i!iiiiiiiiiiiii k\\\\\\\\\\\\\\\\\\
\ \ \ ~\\\\N
Propealnt
~iiii!iiii!~!:!i!i!iiiiilili::i:.l
ios~o,oo I
I
Fig. 2. Apparatus for migration measurement.
Ballistic anomalies in solid rocket motors
Fig. 3. Coning in an end burning grain configuration. • migration of a liquid catalyst (Catocene, etc.) which, in a wider sense could also be treated as a plasticizer; • migration of a curing agent (diisocyanate, etc.) before the propellant is fully cured. Apart from the decrease in total performance due to the reduced amount of energetic plasticizer in the propellant which should be rather small, a change in the burning rate will occur in the propellant layers adjacent to the insulation. Depending on the mechanism--burning rate increase or decrease--the ballistics will change in a corresponding way. An increasing burning rate, e.g. due to relative higher burning rate catalyst concentration or higher solids loading will cause, in case of an end burning configuration, a progressive burning surface. Radiographic inspections[17] show this tendency at various time intervals as presented in Fig. 3. Various explanations for the conical burning in such configurations can be found in the literature. These are: • heat transfer from the wall to the propellant due to chamber heating; • heating of a propellant layer adjacent to the insulation due to insulation decomposition products; • inhomogeneous propellant caused by the casting process (demixing); • change in chemical composition of the propellant due to migration effects. As the burning rate is normally much faster than the heat transport rate in the chamber wall due to the hot combustion products, a significant heating of the outer propellant layers will not occur. This is especially true for high burning rate propellants. Only for extremely low burning rate propellants may this effect be dominant. The effect of insulation decomposition products or
601
solid particles on burning rate enhancement is well known for gas generators, where it normally does not result in a coning effect but in surface cratering. As a result of improper propellant distribution caused by the casting tools, demixing of the propellant during the casting process and a subsequent inhomogeneous composition is often observed for large propellant grains. The main effect seems to be the "trump" phenomenon, a burning rate enhancement especially noticed in internal burning configurations during mid-web combustion[6, 19]. Therefore, liquid species migration remains the most probable cause for ballistic anomalies as there is the well-known coning in end burning propellant grains. This may also be one of the reasons for the differences in burning rate measurements with small scale motors and Crawford strands. The latter is known to be significantly affected by insulation material type, thickness, configuration, not neglecting additional effects like heat transfer to chamber, insulation, etc. Similar to the described coning effect, for internal burning configurations an unexpected pressure rise can sometimes be observed just prior to the end of the quasi-steady burning period. Using an internal burning grain configuration as an example with an almost neutral pressure history, the influence of plasticizer migration on the pressure curve can easily be observed. There is a distinct pressure peak at the end of the burning time caused by the faster burning propellant layers close to the insulation. In Fig. 4 the pressure trace of a composite motor with 350mm diameter, compared with the normal internal ballistics prediction is presented. When analyzing this pressure peak, in addition to the above mentioned burning rate enhancement effects, debonding of the propellant grain from the insulation must be also considered[8, 9, 20]. However, the reproducibility of the pressure rise indicates that debonding may not be the reason for the observed effects. By plasticizer migration from the propellant into the insulation the solids loading fraction is increased,
~-- Pressure peek
=
Calculation
\\ 'rail
.c
L
Surnin~ time
~>
Fig. 4. Pressure peak in an internal burning grain configuration.
602
M. PROBSTERand R. H. SCHMUCKER -°c
, , , 0 t =£ t
t = 3At
t=6A!
046[
t=9At
~ 1.0 0.8 • ~
o.~i
K
c_
_/c ....
pl,rn
pl, max
1
°
!
•i::I J/
I 0.08
Burning rote increase
oo iy 0 . 0 0 Ir 000
I 002
I 004
I 006
I 008
InsuFotion P r o p e l l o n t
Fig. 5. Qualitative plasticizer concentration in the propellant and insulation at various time intervals.
As~/sweb Fig. 7. Normalized pressure peaks as function of plasticizer affected zone thickness for two burning rates.
thus resulting in a higher burning rate due to a higher ammonium perchlorate concentration. The qualitative plasticizer distribution at several time intervals is shown in Fig. 5, whereby the zones with reduced plasticizer contents progress from the outer layers towards the inner layers. If the insulation is saturated with plasticizer, further migration can only take place within the propellant material untii the concentration is balanced or equilibrated. Normalizing this above mentioned relative pressure peak with the average pressure at the time of this anomaly shows a dependence on storage time of the cast propellant grain, which is presented in Fig. 6. The circles and triangles mark slightly modified
grain geometries. The envelope of the measured data reveals a maximum in the region on 100 to 150 days storage time. For longer storage times the pressure peak diminishes, an indication that there is a balancing or counter migration mechanism[13, 14, 16, 21]. The burning rate increase is inversely proportional to the plasticizer concentration and, therefore, should be already detectable after a short storage time. However, the maximum pressure peak during motor firing strongly depends on the thickness of the layer with reduced plasticizer concentration. This is shown in Fig. 7, where the pressure rise is presented as a function of web thickness for two different burning 0.18 rate rise factors, taking also into account chamber filling effects. This zone-related burning rate and the progressing o ~ o of the burning surface with non-equidistant surfaces 0.12 8 o "-w can be simulated with a suitable grain geometry o program. Fig. 8 shows results of such an internal ballistics calculation together with an experimental <] o.o6 pressure curve, whereby theory and test data correspond quite well. To check the relation of burning rate and plas0.00 I [ I I 50 100 150 200 ticizer migration, a propellant grain was manufactured with a plasticizer equilibrated insulation Storage time (days) Fig. 6. Normalized pressure peak as function of storage layer and tested. The result is the dotted line which corresponds well to the predicted pressure curve. time. 1.0-
S. N
0.5
E
o Z
0.0 00
I 0.2
I 04 Normalized
I 0.6
~ 0.8
I 1.0
time
Fig. 8. Comparison of experimental and predicted pressure histories. - . . . . Propellant/insulation with equilibrated plasticizer; - Insulation without plasticizer, propellant with plasticizer: O Post-test prediction including plasticizer migration effects.
Ballistic anomalies in solid rocket motors This leads to the methods for preventing plasticizer migration.
603
glycerine, the insulation itself becomes flammable due to the absorbed nitroglycerine and its shelf life is affected by the decomposition products of the nitroglycerine. Because of all these facts, a basic study was 4. I N H I B I T I N G PLASTICIZER MIGRATION started to investigate--by combination of theory and In order to prevent plasticizer migration from the test--the fundamental relations between migration propellant into the insulation, the following methods and may be applied, depending on the type of propellant • type of polymer; and insulation: • amount of solids; • cross-link density. • propellant grain without plasticizer[22]; According to the well-known effect "equal dis• insulation material equilibrated to the plassolves equal", the migration of the polar nitroticizer[8]; • barrier coating between propellant and insu- glycerine (dipole moment 3.82 Debye)[10] into nonpolar insulation materials should be very small, as lation[23-26]; • insulation material resistant to mi- only very little plasticizing can occur here. However, such polymers as PE, PTFE, etc. produce severe gration[27, 28]; problems with respect to propellant/insulant bonding • metallized polyesterfilm[29]. and ablative properties. Therefore, and also for other These methods are partly solutions of specific reasons, castable insulation materials like unproblems and cannot be generalized e.g. the use of saturated polyesters, polyurethanes and silicones a composite propellant which does not contain a which are cured after contact with the propellant are plasticizer at all. mainly used. As data is available on polyesters[27] Composite propellants. For propellants which have and on silicones[4, 30] and since these materials do only one type of species able to migrate (mostly not provide the variation capabilities of polyplasticizers), the simplest w a y to reduce migration is urethanes, the study was directed towards the latter. to use an insulation material equilibrated to the As could be shown previously, the type of filler plasticizer. In this case the same type of plasticizer as (organic, inorganic) in PU materials used had only in the propellant will be added to the insulation small influence on the migration tendency[28]. material until the plasticizer concentrations in the Figure 9 shows that the absorbed amount of binder of the insulation and the propellant binder are plasticizer strongly depends on the filler concenmatched. When balancing the plasticizer concen- tration in the insulation material, as, with an intration, the solids contained in the propellant and creasing percentage of filler, the amount of binder insulation are considered to be not affected by the which can be plasticized is reduced. As the castability plasticizer; only soaking of the binder is taken into of the insulation becomes correspondingly poorer account. In order to avoid a concentration gradient and the mechanical properties change (increased between propellant and insulation, with respect to the Shore A hardness etc.), plasticizer migration can only high percentage (often up to 30%) of plasticizer in the be reduced to a limited extent by increasing the filler propellant binder, large quantities of the liquid in- concentration. Therefore, filler materials which allow gredients have to be added to the insulation material good mixing (e.g. proper grain size distribution) which is normally less loaded with solids. With should be used. the high plasticizer concentration in the insulation Increasing the cross-link density in the resin is an material problems can occur with bonding of easy way to reduce plasticizer absorption in polyinsulation/propellant and the mechanical properties urethanes. The desired heat resistance and ablative at high temperatures. properties of an insulation material may be achieved If the propellant contains several mobile species, for polyurethanes by proper choice of the ingredients. e.g. catocene and a plasticizer, it is recommended to The combination of apply a barrier coating which forms a tight network • diol, diisocyanate and low molecular triol, onto the insulation before casting and not to use the method of an equilibrated insulation. Etched poly~ 15 Filler fraction E 35% ester films and metal foils have also been applied, however, there have been problems with the coating ~10 50 % process, the bonding qualities and the low temperature coefficient of expansion. Double base propellants. Due to the high percentage and the high reactivity of the liquid species and its decomposition products, the problem of migration is even more significant for double base ~ 0 I L I I 10 20 30 40 propellants. Apart from reducing the energy content T i m e (days) in the propellant and changing the ballistic properties of the motor by reducing the amount of nitroFig. 9. Filler loading effect on plasticizer migration. A A 13,1~"
604
M. PROBSTERand R. H. SCHMUCKER 40
(M E o -.. 3 0
High c r o s s - link density/filler
2o
High cross-link density
o
1 15
o
I 30 Time
I 45
I 60
(days)
Fig. I0. Cross-link density effect on plasticizer migration. • •
diol and triisocyanate, triol and diisocyanate,
results in properly cross-linked systems. Of these, the combination of diol and triisocyanate was chosen for this study. The length of the diol and thus the cross-link density of the polymer was varied. The migration rates of two pure binder systems manufactured like thus are shown in Fig. 10. Adding 20% of filler, it is surprising to see that the migration rate increases, despite the reduced percentage of binder which can be plasticized. However, this may be due to perturbations in the polymer network caused by the filler. A further increase of filler concentration--which is necessary for heat resistance and ablative properties' requirements--produces a migration curve which lies between the two lower curves in Fig. 10. After selection of a suitable combination of diol and triisocyanate the cross-link density can be further increased by changing the N C O / O H ratio. As can be seen from Fig. 11 the plasticizer absorption can be reduced by this simple method too. Based upon the above mentioned parameters like filler concentration, choice of binder and suitable cross-link density, an insulation material was developed which absorbs only small amounts of plasticizer, together with good mechanical properties and excellent bonding qualities. 5. CONCLUSIONS
In internal and end burning grain configurations, deviations from the predicted ballistic behaviour can NCO/OH ratio
10
1.1:1
.E ._~ o~ 0 0
I 10
[ 20 Storage
I 30 time
I 40
(days)
Fig. 11. NCO/OH ratio effect on plasticizer migration for double base propellants.
be observed sometimes. In the lbrmer case, these are pressure peaks at the end of quasi-steady combustion and in the latter case, a steady increase of the chamber pressure is measured. Theoretical and experimental studies revealed that in both cases often a c o m m o n effect is responsible for this behaviour: migration of liquid species from the propellant into the insulation. By using a plasticizer balanced insulation in an internal burning configuration the liquid species migration is avoided. This results in a complete disappearence of the previously observed ballistic anomalies. Using this approach for end burning configurations also provided similar results. Furthermore, several methods to prevent liquid species migration into the insulation were shown as well as methods to obtain plasticizer resistant insulations. REFERENCES
1. H. Schubert, Raketenfesttreibstoffe. Chemiker Ztg. 97, S. 486 (1973). 2. H. Brachert, Raketentreibstoffe. Ullmanns Enzyklop. d. Techn. Chemie 20, 91 112 (1981). 3. S. J. Bennet and R. L. Carpenter, Migration at interfaces. JANNAF Propulsion Mtg., l, 53-65 (1983). 4. B. F. Gonthier and J. M. Tauzia, Burning rate enhancement phenomena in end-burning solid propellant grains. AIAA-85-1435 (1985). 5. H. W. Jolley, J. F. Hooper, P. R. Hilton and W. A. Bradfield, Studies on coning in end-burning rocket motors. AIAA-85-1179 (1985). 6. T. E. Kallmeyer and L. H. Sayer, Differences between actual and predicted pressure-time histories of solid rocket motors. AIAA-82-1092 (1982). 7. Anon., Solid propellant selection and characterization. NASA SP 8064 (1971). 8. J. V. Simon, Untersuchungen zur Druckfiberh6hung bei Raketentriebwerken. BC, unpublished results (1982). 9. H. P. Sauerwein, Beitrfige zur innenballistischen Leistungsanalyse von Feststoffraketentriebwerken. Diss. Techn. Universitfit Mfinchen (1983). 10. N. W. Gregornik, Barrier films for Mk 12 Mod 2 cartridge plug. AD A 043253 (1974). 11. Anon., Solid rocket motor internal insulation. NASA SP 8093 (1976). 12. H. Meier, D. B6sche and G. Zeitler, Isotopentechnische Untersuchungen fiber die Lebensdauer von Treibsatzisolationen. In Ber. Fraunhoferges. Treib- und Explosivstoffe (ICT). Jahrestagung, p. 179 (1979). 13. G. Wunsch, Untersuchungen fiber die Lebensdauer von Treibsatzisolationen. Techn. Mitt. ICT 9 (1974). 14. G. Wunsch, Untersuchungen fiber die Lebensdauer von Treibsatzisolation PBD-105. Techn. Mitt. ICT 3 (1979). 15. H. Berding, Untersuchungen fiber die Beeinflussung von doppelbasigen Treibstoffen durch lsoliermaterialien, In Ber. Fraunhoferges. Treib- und Explosivstoffe (ICT). Jahrestagung. p. 359 (1971). 16. G. Wunsch, Untersuchungen fiber die Lebensdauer von Treibsatzisolationen. Techn. Mitt. ICT 9 (1972). 17. H. G. Langer, R6ntgenaufnahmen zur Untersuchung des Abbrandverhaltens mit Gaserzeuger. MBB, unpublished results (1980). 18. A. M. Messner, Transient coning in end-burning solid propellant grains. AIAA-80-1138 (1980). 19. K. Klager, Personal communication (1985). 20. S. Heister and E. Landsbaum, Analysis of ballistic anomalies in solid rocket motors. AIAA-85-1303 (1985). 21. L. A. Dee. L. J. Emmanuel, M. E. Fiske and L.
Ballistic anomalies in solid rocket motors
22. 23. 24. 25.
Ninomiya, Solid propellant ingredient migration studies. AD B 066 263 (1982). P. G. Butts, R. N. Hammond and J. Shdo, IUS propellant development and qualification. JANNAF Propulsion Mtg., 1, 237-250 (1983). R. T. Davis, J. D. Byrd, I. G. Shepard and K. E. Bevel, Migration control in solid propellant rocket motors. AIAA-Joint Propulsion Conference (1983). L. v. Nieciecki, Verfahren zur Herstellung yon mit einer Isolierschicht versehenen Feststofftreibs/itzen. DE-PS 1 200 184 (1965). E. Mesmer, Treibsatz mit einer Isolierung und Verfahren zu seiner Herstellung. DE-OS 2 444 930. DE-OS 2447 060 (1976).
605
26. D. V. Clifford, W. G. Williams, S. Gordon and K. G. Reed, GB-PS 1 448 068 (1976). 27. M. Caire-Maurissier and J. Tranchant. La Migration de la Nitroglycerine dans les Inhibiteurs de Poudres a Double-base. Propellant and Explos. 2, 101-104 (1977). 28. M. Pr6bster, R. Strecker and G. v. Taeuffenbach, Rauchlose Isolierung ffir DB-Treibstoffe. BC unpublished results (1983). 29. R. P. Deschner and D. J. Korpics, US-PS 3 381 065 (1968). 30. B. F. Gonthier and J. M. Tauzia, Minimum smoke rocket motors with silicone inhibitors. AIAA-84-1418 (1984).
0094-5767/86 $3.00+ 0.00 © 1986 Pergamon Journals Ltd
BALLISTIC ANOMALIES IN SOLID ROCKET MOTORS D U E TO MIGRATION EFFECTSt M. PR6BSTER~and R. H. SCHMUCKER§ Bayern-Chemie, Gesellschaft fiir flugchemische Antriebe mbH, Postfach l l31, D-8261 Aschau a. Inn, F.R.G. (Received I1 February 1986)
Abstract--Double base and composite propellants are generally used for rocket motors, whereby double base propellants basically consist of nitrocellulose plasticized with an explosive plasticizer, mostly nitroglycerine, and in some cases with an additional inert plasticizer and ballistic additives. Composite propellants consist of an oxidizer like ammonium perchlorate and of aluminum, binder and plasticizer and often contain liquid or solid burning rate catalysts. A common feature of both propellants is that they contain smaller or larger amounts of chemically unbonded liquid species which tend to migrate. If these propellants loose part of the plasticizer by migration into the insulation layer, not only will there be a change in mechanical propellant properties but also the bond between propellant and insulation may degrade. However, depending on the severity of these effects, the change in the ballistic properties of the propellant grain caused by plasticizer migration may be of even more importance. In the past, most emphasis was placed on the behaviour of end-burning configurations. However, more recent theoretical and experimental studies revealed that not only for end-burning grain configurations but also for internal burning configurations there is a common effect which is responsible for ballistic anomalies: migration of liquid species from the propellant into the insulation. By using a plasticizer equilibrated insulation in an internal burning configuration the liquid species migration and thus the previously observed ballistic anomalies are avoided. Using this approach for end-burning configurations provides similar positive results. The various factors affecting plasticizer migration are studied and discussed, and several methods to prevent liquid species migration are described as well as methods to obtain plasticizer resistant insulations.
In the past, most emphasis was placed on the behaviour of end-burning configurations; in part, the above described effects were found to be responsible for ballistic anomalies in these cases[3-6]. The effects of these processes on the ballistics of internal burning grain configurations were studied with much less emphasis[7-9], pointing out that similar trends are to be expected. The influence of this migration on the ballistic properties of double base and composite propellants and how it can be prevented will be the topic discussed herein.
1. INTRODUCTION The solid propellants generally used for rocket motors can be divided into several main groups, with double base and composite propellants being the best known types[l, 2]. Basically, double base propellants consist of nitrocellulose plasticized with an explosive plasticizer, mostly nitroglycerine, and in some cases with an additional inert plasticizer and ballistic additives. Composite propellants consist of an oxidizer like a m m o n i u m perchlorate and of aluminium, binder and plasticizer and often contain liquid or solid burning rate catalysts. A c o m m o n feature of both propellants is that they contain smaller or larger amounts of chemically u n b o n d e d liquid species which tend to migrate. If these propellants loose part of the plasticizer by migration into the insulation layer, not only will there be a change in mechanical propellant properties but also the bond between propellant and insulation may degrade. However, depending on the severity of these effects, the change in the ballistic properties of the propellant grain caused by plasticizer migration may be of even more importance.
2. MIGRATION OF PLASTICIZERS Plasticizer migration is defined as the balancing process in the rates of concentration between two adjacent material layers which contain one or more mobile species. As shown in Fig. 1 two cases can be identified, namely:
#Paper IAF 85-176 presented at the 36th Congress of the International Astronautical Federation, Stockholm, Sweden, 7-12 October 1985. :~Dr. rer. nat., Dipl.-Chem., Technical Staff Member, Propellant Development Department, 8261 Aschau, F.R.G. §Prof., Dr.-Ing., Head, Engine Development Department, 8012 Ottobrunn, F.R.G. 599
(a) the most c o m m o n case, where the propellant contains a plasticizer, a liquid burning rate catalyst or even just a liquid curing agent migrating into the insulation which on its own is short of or free of these mobile liquid species, (b) the case where ingredients of the insulation material, like plasticizer, catalysts, decomposition products, unreacted monomers, etc., migrate into a propellant which is short of these mobile liquid species.
600
M. PR(~)BSTERand R. H. SCHMUCKER
x-
×
x x x
Propellant
Insulation
Insulation
PropeHont
Fig. 1. Migration effects at the propellant/insulation interface. (The additional problems caused by layers of liner shall not be treated here.) In both cases, plasticizer migration will change the • chemical properties of insulation and propellant; • mechanical properties of insulation and propellant; • bonding of insulation/propellant; • ballistic properties of the propellant. The degree and rate of propellant components migrating into the insulation--only this will be treated here--depend on a number of parameters, some of which cannot be influenced in a given polymer[lO]: • concentration; • temperature; • solubility parameters of polymer and plasticizer. The diffusibility of the mobile species is determined by the: • size of molecules; • shape (cross section) of molecules; • polarity of molecules.
subsequently stored at an elevated temperature. As a function of time the weight increase gives an indication of the migration tendency of the various pairs of propellant/insulation materials. For the tests described below discs with a diameter of 50 mm and a thickness of 6 mm were used a n d - - a s shown in Fig. 2--loaded with 5 kp. A sol/gel analysis was carried out beforehand to ensure that the insulation test specimen contained no species which could migrate (e.g. from incomplete curing reactions) and could, by a sort of countermigration lead to an under-estimated migration of the propellant plasticizer[13, 14]. 3. BALLISTIC ANOMALIES DUE TO LIQUID SPECIES MIGRATION What happens if liquid species migrate from a propellant grain into the insulation? Depending on the type of liquid ingredients four principles have to be identified: • migration of an energetic plasticizer (Ngl, DEGN, etc.); • migration of an inert plasticizer (DOA, DOS, IDP, etc.);
How easy a molecule can migrate into a polymer and plasticize it depends on the: • crystallinity; • cross-link density; • phase transitions of 1. and 2. order; • polarity; • H-bonding capability.
50
i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:!:i:i:i:!:!:!'i:i:i:!:i:i:i:i:i:i:!: /
All these facts should be considered when a new type of insulation is being developed with the aim to have a highest possible resistance to plasticizer migration[ 11]. Before discussing the various effects of migration and the means of how to prevent it, some remarks on the measuring of migration effects are necessary. Apart from the quantitative analysis of migrated species by chromatographical or other methods[12-14], the method of analyzing the relative degree of migration in accordance with the German Industrial Standard (Deutsche Industrie-Norm) DIN 53 405 is well established, especially for series analysis[15, 16]. This method uses disc-size test specimen of propellant and insulation material which are brought into contact, loaded with a weight and
N !
/
"
/
/
/
/
/
/
~ /
~'"
/
/
/ / / / / f / / / / / / / / / / / / i
K~
~'~'s'.
Aiuminiurn sheets
!#11
/
/ / / a - / 1 / / / / / / / / / / / ~
/
i!iiTili:i:i:i:i:i:ili~i~i!!~i;!;i!!~!!i:i:!:!:i:!:i?i!iiiiiiiiiiiii k\\\\\\\\\\\\\\\\\\
\ \ \ ~\\\\N
Propealnt
~iiii!iiii!~!:!i!i!iiiiilili::i:.l
ios~o,oo I
I
Fig. 2. Apparatus for migration measurement.
Ballistic anomalies in solid rocket motors
Fig. 3. Coning in an end burning grain configuration. • migration of a liquid catalyst (Catocene, etc.) which, in a wider sense could also be treated as a plasticizer; • migration of a curing agent (diisocyanate, etc.) before the propellant is fully cured. Apart from the decrease in total performance due to the reduced amount of energetic plasticizer in the propellant which should be rather small, a change in the burning rate will occur in the propellant layers adjacent to the insulation. Depending on the mechanism--burning rate increase or decrease--the ballistics will change in a corresponding way. An increasing burning rate, e.g. due to relative higher burning rate catalyst concentration or higher solids loading will cause, in case of an end burning configuration, a progressive burning surface. Radiographic inspections[17] show this tendency at various time intervals as presented in Fig. 3. Various explanations for the conical burning in such configurations can be found in the literature. These are: • heat transfer from the wall to the propellant due to chamber heating; • heating of a propellant layer adjacent to the insulation due to insulation decomposition products; • inhomogeneous propellant caused by the casting process (demixing); • change in chemical composition of the propellant due to migration effects. As the burning rate is normally much faster than the heat transport rate in the chamber wall due to the hot combustion products, a significant heating of the outer propellant layers will not occur. This is especially true for high burning rate propellants. Only for extremely low burning rate propellants may this effect be dominant. The effect of insulation decomposition products or
601
solid particles on burning rate enhancement is well known for gas generators, where it normally does not result in a coning effect but in surface cratering. As a result of improper propellant distribution caused by the casting tools, demixing of the propellant during the casting process and a subsequent inhomogeneous composition is often observed for large propellant grains. The main effect seems to be the "trump" phenomenon, a burning rate enhancement especially noticed in internal burning configurations during mid-web combustion[6, 19]. Therefore, liquid species migration remains the most probable cause for ballistic anomalies as there is the well-known coning in end burning propellant grains. This may also be one of the reasons for the differences in burning rate measurements with small scale motors and Crawford strands. The latter is known to be significantly affected by insulation material type, thickness, configuration, not neglecting additional effects like heat transfer to chamber, insulation, etc. Similar to the described coning effect, for internal burning configurations an unexpected pressure rise can sometimes be observed just prior to the end of the quasi-steady burning period. Using an internal burning grain configuration as an example with an almost neutral pressure history, the influence of plasticizer migration on the pressure curve can easily be observed. There is a distinct pressure peak at the end of the burning time caused by the faster burning propellant layers close to the insulation. In Fig. 4 the pressure trace of a composite motor with 350mm diameter, compared with the normal internal ballistics prediction is presented. When analyzing this pressure peak, in addition to the above mentioned burning rate enhancement effects, debonding of the propellant grain from the insulation must be also considered[8, 9, 20]. However, the reproducibility of the pressure rise indicates that debonding may not be the reason for the observed effects. By plasticizer migration from the propellant into the insulation the solids loading fraction is increased,
~-- Pressure peek
=
Calculation
\\ 'rail
.c
L
Surnin~ time
~>
Fig. 4. Pressure peak in an internal burning grain configuration.
602
M. PROBSTERand R. H. SCHMUCKER -°c
, , , 0 t =£ t
t = 3At
t=6A!
046[
t=9At
~ 1.0 0.8 • ~
o.~i
K
c_
_/c ....
pl,rn
pl, max
1
°
!
•i::I J/
I 0.08
Burning rote increase
oo iy 0 . 0 0 Ir 000
I 002
I 004
I 006
I 008
InsuFotion P r o p e l l o n t
Fig. 5. Qualitative plasticizer concentration in the propellant and insulation at various time intervals.
As~/sweb Fig. 7. Normalized pressure peaks as function of plasticizer affected zone thickness for two burning rates.
thus resulting in a higher burning rate due to a higher ammonium perchlorate concentration. The qualitative plasticizer distribution at several time intervals is shown in Fig. 5, whereby the zones with reduced plasticizer contents progress from the outer layers towards the inner layers. If the insulation is saturated with plasticizer, further migration can only take place within the propellant material untii the concentration is balanced or equilibrated. Normalizing this above mentioned relative pressure peak with the average pressure at the time of this anomaly shows a dependence on storage time of the cast propellant grain, which is presented in Fig. 6. The circles and triangles mark slightly modified
grain geometries. The envelope of the measured data reveals a maximum in the region on 100 to 150 days storage time. For longer storage times the pressure peak diminishes, an indication that there is a balancing or counter migration mechanism[13, 14, 16, 21]. The burning rate increase is inversely proportional to the plasticizer concentration and, therefore, should be already detectable after a short storage time. However, the maximum pressure peak during motor firing strongly depends on the thickness of the layer with reduced plasticizer concentration. This is shown in Fig. 7, where the pressure rise is presented as a function of web thickness for two different burning 0.18 rate rise factors, taking also into account chamber filling effects. This zone-related burning rate and the progressing o ~ o of the burning surface with non-equidistant surfaces 0.12 8 o "-w can be simulated with a suitable grain geometry o program. Fig. 8 shows results of such an internal ballistics calculation together with an experimental <] o.o6 pressure curve, whereby theory and test data correspond quite well. To check the relation of burning rate and plas0.00 I [ I I 50 100 150 200 ticizer migration, a propellant grain was manufactured with a plasticizer equilibrated insulation Storage time (days) Fig. 6. Normalized pressure peak as function of storage layer and tested. The result is the dotted line which corresponds well to the predicted pressure curve. time. 1.0-
S. N
0.5
E
o Z
0.0 00
I 0.2
I 04 Normalized
I 0.6
~ 0.8
I 1.0
time
Fig. 8. Comparison of experimental and predicted pressure histories. - . . . . Propellant/insulation with equilibrated plasticizer; - Insulation without plasticizer, propellant with plasticizer: O Post-test prediction including plasticizer migration effects.
Ballistic anomalies in solid rocket motors This leads to the methods for preventing plasticizer migration.
603
glycerine, the insulation itself becomes flammable due to the absorbed nitroglycerine and its shelf life is affected by the decomposition products of the nitroglycerine. Because of all these facts, a basic study was 4. I N H I B I T I N G PLASTICIZER MIGRATION started to investigate--by combination of theory and In order to prevent plasticizer migration from the test--the fundamental relations between migration propellant into the insulation, the following methods and may be applied, depending on the type of propellant • type of polymer; and insulation: • amount of solids; • cross-link density. • propellant grain without plasticizer[22]; According to the well-known effect "equal dis• insulation material equilibrated to the plassolves equal", the migration of the polar nitroticizer[8]; • barrier coating between propellant and insu- glycerine (dipole moment 3.82 Debye)[10] into nonpolar insulation materials should be very small, as lation[23-26]; • insulation material resistant to mi- only very little plasticizing can occur here. However, such polymers as PE, PTFE, etc. produce severe gration[27, 28]; problems with respect to propellant/insulant bonding • metallized polyesterfilm[29]. and ablative properties. Therefore, and also for other These methods are partly solutions of specific reasons, castable insulation materials like unproblems and cannot be generalized e.g. the use of saturated polyesters, polyurethanes and silicones a composite propellant which does not contain a which are cured after contact with the propellant are plasticizer at all. mainly used. As data is available on polyesters[27] Composite propellants. For propellants which have and on silicones[4, 30] and since these materials do only one type of species able to migrate (mostly not provide the variation capabilities of polyplasticizers), the simplest w a y to reduce migration is urethanes, the study was directed towards the latter. to use an insulation material equilibrated to the As could be shown previously, the type of filler plasticizer. In this case the same type of plasticizer as (organic, inorganic) in PU materials used had only in the propellant will be added to the insulation small influence on the migration tendency[28]. material until the plasticizer concentrations in the Figure 9 shows that the absorbed amount of binder of the insulation and the propellant binder are plasticizer strongly depends on the filler concenmatched. When balancing the plasticizer concen- tration in the insulation material, as, with an intration, the solids contained in the propellant and creasing percentage of filler, the amount of binder insulation are considered to be not affected by the which can be plasticized is reduced. As the castability plasticizer; only soaking of the binder is taken into of the insulation becomes correspondingly poorer account. In order to avoid a concentration gradient and the mechanical properties change (increased between propellant and insulation, with respect to the Shore A hardness etc.), plasticizer migration can only high percentage (often up to 30%) of plasticizer in the be reduced to a limited extent by increasing the filler propellant binder, large quantities of the liquid in- concentration. Therefore, filler materials which allow gredients have to be added to the insulation material good mixing (e.g. proper grain size distribution) which is normally less loaded with solids. With should be used. the high plasticizer concentration in the insulation Increasing the cross-link density in the resin is an material problems can occur with bonding of easy way to reduce plasticizer absorption in polyinsulation/propellant and the mechanical properties urethanes. The desired heat resistance and ablative at high temperatures. properties of an insulation material may be achieved If the propellant contains several mobile species, for polyurethanes by proper choice of the ingredients. e.g. catocene and a plasticizer, it is recommended to The combination of apply a barrier coating which forms a tight network • diol, diisocyanate and low molecular triol, onto the insulation before casting and not to use the method of an equilibrated insulation. Etched poly~ 15 Filler fraction E 35% ester films and metal foils have also been applied, however, there have been problems with the coating ~10 50 % process, the bonding qualities and the low temperature coefficient of expansion. Double base propellants. Due to the high percentage and the high reactivity of the liquid species and its decomposition products, the problem of migration is even more significant for double base ~ 0 I L I I 10 20 30 40 propellants. Apart from reducing the energy content T i m e (days) in the propellant and changing the ballistic properties of the motor by reducing the amount of nitroFig. 9. Filler loading effect on plasticizer migration. A A 13,1~"
604
M. PROBSTERand R. H. SCHMUCKER 40
(M E o -.. 3 0
High c r o s s - link density/filler
2o
High cross-link density
o
1 15
o
I 30 Time
I 45
I 60
(days)
Fig. I0. Cross-link density effect on plasticizer migration. • •
diol and triisocyanate, triol and diisocyanate,
results in properly cross-linked systems. Of these, the combination of diol and triisocyanate was chosen for this study. The length of the diol and thus the cross-link density of the polymer was varied. The migration rates of two pure binder systems manufactured like thus are shown in Fig. 10. Adding 20% of filler, it is surprising to see that the migration rate increases, despite the reduced percentage of binder which can be plasticized. However, this may be due to perturbations in the polymer network caused by the filler. A further increase of filler concentration--which is necessary for heat resistance and ablative properties' requirements--produces a migration curve which lies between the two lower curves in Fig. 10. After selection of a suitable combination of diol and triisocyanate the cross-link density can be further increased by changing the N C O / O H ratio. As can be seen from Fig. 11 the plasticizer absorption can be reduced by this simple method too. Based upon the above mentioned parameters like filler concentration, choice of binder and suitable cross-link density, an insulation material was developed which absorbs only small amounts of plasticizer, together with good mechanical properties and excellent bonding qualities. 5. CONCLUSIONS
In internal and end burning grain configurations, deviations from the predicted ballistic behaviour can NCO/OH ratio
10
1.1:1
.E ._~ o~ 0 0
I 10
[ 20 Storage
I 30 time
I 40
(days)
Fig. 11. NCO/OH ratio effect on plasticizer migration for double base propellants.
be observed sometimes. In the lbrmer case, these are pressure peaks at the end of quasi-steady combustion and in the latter case, a steady increase of the chamber pressure is measured. Theoretical and experimental studies revealed that in both cases often a c o m m o n effect is responsible for this behaviour: migration of liquid species from the propellant into the insulation. By using a plasticizer balanced insulation in an internal burning configuration the liquid species migration is avoided. This results in a complete disappearence of the previously observed ballistic anomalies. Using this approach for end burning configurations also provided similar results. Furthermore, several methods to prevent liquid species migration into the insulation were shown as well as methods to obtain plasticizer resistant insulations. REFERENCES
1. H. Schubert, Raketenfesttreibstoffe. Chemiker Ztg. 97, S. 486 (1973). 2. H. Brachert, Raketentreibstoffe. Ullmanns Enzyklop. d. Techn. Chemie 20, 91 112 (1981). 3. S. J. Bennet and R. L. Carpenter, Migration at interfaces. JANNAF Propulsion Mtg., l, 53-65 (1983). 4. B. F. Gonthier and J. M. Tauzia, Burning rate enhancement phenomena in end-burning solid propellant grains. AIAA-85-1435 (1985). 5. H. W. Jolley, J. F. Hooper, P. R. Hilton and W. A. Bradfield, Studies on coning in end-burning rocket motors. AIAA-85-1179 (1985). 6. T. E. Kallmeyer and L. H. Sayer, Differences between actual and predicted pressure-time histories of solid rocket motors. AIAA-82-1092 (1982). 7. Anon., Solid propellant selection and characterization. NASA SP 8064 (1971). 8. J. V. Simon, Untersuchungen zur Druckfiberh6hung bei Raketentriebwerken. BC, unpublished results (1982). 9. H. P. Sauerwein, Beitrfige zur innenballistischen Leistungsanalyse von Feststoffraketentriebwerken. Diss. Techn. Universitfit Mfinchen (1983). 10. N. W. Gregornik, Barrier films for Mk 12 Mod 2 cartridge plug. AD A 043253 (1974). 11. Anon., Solid rocket motor internal insulation. NASA SP 8093 (1976). 12. H. Meier, D. B6sche and G. Zeitler, Isotopentechnische Untersuchungen fiber die Lebensdauer von Treibsatzisolationen. In Ber. Fraunhoferges. Treib- und Explosivstoffe (ICT). Jahrestagung, p. 179 (1979). 13. G. Wunsch, Untersuchungen fiber die Lebensdauer von Treibsatzisolationen. Techn. Mitt. ICT 9 (1974). 14. G. Wunsch, Untersuchungen fiber die Lebensdauer von Treibsatzisolation PBD-105. Techn. Mitt. ICT 3 (1979). 15. H. Berding, Untersuchungen fiber die Beeinflussung von doppelbasigen Treibstoffen durch lsoliermaterialien, In Ber. Fraunhoferges. Treib- und Explosivstoffe (ICT). Jahrestagung. p. 359 (1971). 16. G. Wunsch, Untersuchungen fiber die Lebensdauer von Treibsatzisolationen. Techn. Mitt. ICT 9 (1972). 17. H. G. Langer, R6ntgenaufnahmen zur Untersuchung des Abbrandverhaltens mit Gaserzeuger. MBB, unpublished results (1980). 18. A. M. Messner, Transient coning in end-burning solid propellant grains. AIAA-80-1138 (1980). 19. K. Klager, Personal communication (1985). 20. S. Heister and E. Landsbaum, Analysis of ballistic anomalies in solid rocket motors. AIAA-85-1303 (1985). 21. L. A. Dee. L. J. Emmanuel, M. E. Fiske and L.
Ballistic anomalies in solid rocket motors
22. 23. 24. 25.
Ninomiya, Solid propellant ingredient migration studies. AD B 066 263 (1982). P. G. Butts, R. N. Hammond and J. Shdo, IUS propellant development and qualification. JANNAF Propulsion Mtg., 1, 237-250 (1983). R. T. Davis, J. D. Byrd, I. G. Shepard and K. E. Bevel, Migration control in solid propellant rocket motors. AIAA-Joint Propulsion Conference (1983). L. v. Nieciecki, Verfahren zur Herstellung yon mit einer Isolierschicht versehenen Feststofftreibs/itzen. DE-PS 1 200 184 (1965). E. Mesmer, Treibsatz mit einer Isolierung und Verfahren zu seiner Herstellung. DE-OS 2 444 930. DE-OS 2447 060 (1976).
605
26. D. V. Clifford, W. G. Williams, S. Gordon and K. G. Reed, GB-PS 1 448 068 (1976). 27. M. Caire-Maurissier and J. Tranchant. La Migration de la Nitroglycerine dans les Inhibiteurs de Poudres a Double-base. Propellant and Explos. 2, 101-104 (1977). 28. M. Pr6bster, R. Strecker and G. v. Taeuffenbach, Rauchlose Isolierung ffir DB-Treibstoffe. BC unpublished results (1983). 29. R. P. Deschner and D. J. Korpics, US-PS 3 381 065 (1968). 30. B. F. Gonthier and J. M. Tauzia, Minimum smoke rocket motors with silicone inhibitors. AIAA-84-1418 (1984).