Effects of Additives on the Ignition of AP-Based Propellants at subatmospheric pressures

Propellants, Explosives, Pyrotechnics 10, 129-138 (1985) 129

Effects of Additives on the Ignition of AP-Based Propellants at Subatmospheric Pressures

T. Saito, T. Yamaya, and A. Iwama

Institute of Space and Astronautical Science, Komaba, Meguro-ku, Tokyo 153 (Japan)

T. Kuwahara

Nissan Motor Co., Ltd., Matoba, Kawagoe, Saitama 350 (Japan)

Die Wirkung von Zusatzen auf die Anziindung von AP-Treibstoffen bei subatmospharischen Drucken

Die Ammoniumperchlorat-Komposittreibstoffe, von denen jeder noch Kupferchromit als Abbrandbeschleuniger und RUB als Trii- bungsmittel enthalt, wurden angeziindet bei subatmospharischen Drucken unter Argongas mittels eines C0,-Lasers; die Wirkung der Zusatze auf das Anziindverhalten wurde untersucht. Es wurde gefun- den, dafi Kupferchromit die Anziindzeit besonders unterhalb 100 Torr verringert, und dafi es gleichzeitig die Anziindfahigkeit steigert, d. h. die selbstunterhaltende Anziindbarkeit.

RUB, der als Triibungsmittel mit abnehmender Reflexion und zunehmender Strahlenabsorption an der Treibstoffoberflache wirkt, kann nicht als aktiver Katalysator bezeichnet werden fur die Anziin- dung bei subatmospharischen Drucken.

Die Ergebnisse der Differentialthermoanalyse fur die genannten Proben zeigten, dafi das Maximum der exothermen Temperaturpeaks verschoben wird nach niederen Bereichen bei Zunahme der Rufikon- zentration, die exotherme Peakstruktur wurde scharfer. Jedoch macht die Zugabe von Kupferchromit die exothermen Peaks schwacher. Die Ergebnisse der Differentialthermoanalyse bestatigen die aus den Anziindversuchen gewonnenen Ergebnisse.

L’intluence d’additifs sur l’allumage des propergols ii base de perchlorate d’amrnonium aux pressions infkrieures a la pression atmosphe- rique

Des propergols composites B base de perchlorate d’ammonium, contenant en outre du chromite de cuivre pour agir sur la vitesse de combustion et du noir de carbone comme agent opacifiant, ont Btt soumis B des essais d‘allumage par l’action du faisceau d’un laser B CO,, B des pressions inftrieures B la pression atmosphtrique; ces essais visaient B examiner l’influence des additifs sur la reaction B l’allumage. On a constatt que la prtsence de chromite de cuivre rtdui- sait le retard ti l’allumage, en particulier aux pressions inftrieures B 100 mm Hg et qu’elle augmentait en m&me temps I’inflammabilite, c’est-B-dire l’auto-entretien de la rtaction d allumagne.

Le noir de carbone qui agit comme agent opacifiant en abaissant le pouvoir de rtflexion et en favorisant I’absorption du rayonnement B la surface du propergol, ne peut pas &re considtrt comme un catalyseur qui active la rkaction d’allumage a w pressions inftrieures B la pression atmosphtrique.

Les rtsultats de l’analyse thermique diffkrentielle, B laquelle on a soumis les tchanitillons en question, ont montrt que, lorsque la concentration en noir de carbone augmente, le maximum des pics de temptrature exothermique se trouve dtcalt vers le bas et que l’allure des pics exothermiques est plus prononcte. Toutefois, I’addition de chromite de cuivre atttnue les pics exothermiques. Les rtsultats de l’analyse thermique difftrentielle confirment ceux obtenus par les essais d’allumagne.

Summary

The ammonium perchlorate (AP)-oxidized composite propellants, each of which contains separately copper chromite (CC) as a burning rate adjuster and carbon black (CB) as an opacifier, have been ignited at subatmospheric pressures of argon gas by means of a carbon dioxide laser, and the effects of the additives on the ignition behavior have been studied. It has been found that copper chromite shortens the ignition time especially below 100 torr and that at the same time it enhances the ignitability, i.e., self-sustaining ignition.

Carbon black, being an opacifier decreasing reflectivity and increasing radiative absorption at propellant surface, can not be recognized to be an active catalyst in ignition at subatmospheric pressures.

The data of differential thermal analysis (DTA) for above specimens have indicated that the maximum exothermic peak temperature is shifted toward a lower one with the increase in CC concentration, the exothermic peak structure becoming sharper. However, CB addition to the basic propellant makes exothermic peaks less distinct. The results of DTA support those obtained from the ignition experiments above.

1. Introduction

Ignition of solid propellants has been investigated exten- sively up to date and a detailed review of the literature on solid

0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1985

propellant ignition published by 1965 was conducted b Price et al.(’), and succeedingly Kulkarni, Kumar and Kuo( have described in detail many reports conducted after then from the view point of both sides of experiment and theory. However, since a complete understanding of ignition event for solid propellants has not been achieved yet, much information about it will be required for the purpose of the development of ignition theory. We have been investigating the ignition phenomena for composite solid propellants at subatmospheric pres- s u r e ~ ( ~ , 4, with the following reasons: (1) Though an up-stage rocket motor, a kick motor and an apogee motor fired at high altitudes or in space, are usually ignited under confined environment kept atmosphere, it may occur for them to have to be operated nearly in vacuum because of an unpredictable acci- dent. Therefore, the ignition characteristics of solid rocket propellants under conditions simulating those at high altitudes or in space must be more fully studied in order to ensure effective ignition in an empty space environment. (2) As a realistic ignition of rocket propellants is a high-speed transient phenomenon taking place during several to scores of milliseconds, it is necessary to extend the ignition process to the order of several seconds in order to examine the detailed behavior which is apt to be immersed in experimental errors.

The present work studied the ignition effects of adding copper chromite (CC) or carbon black (CB) to basic propellant

Y

0721-3115/85/0510-0129$02.50/0

130 T. Saito, T. Yamaya, A. Iwama, and T. Kuwahara

being composed of ammonium perchorate (AP) as oxidizer and carboxyl-terminated polybutadiene (CTPB) as fuel-bind- er. Copper chromite has been found to be an active catalyst for “high-temperature reaction” in the thermal decomp~sition(~, of AP and, based on the principle, the effects of CC on burning rate or ignition time of AP only or AP-based propellants have been in~estigated(~-”). As for carbon black, catalytic effects have been cited in some referencest7> 12) among above ones. However, the effects of CC and CB on ignition at subatmospheric pressures have been publicly little e~plained(’~3 14).

Only Shannon(14) observed that CC decreased ignition time and carbon also had a positive effect on the ignition with polybutadiene-acrylic acid-acrylonitrile (PBAN) propellant, using an arc-image furnace as a radiation source under experimental conditions of below atmosphere of nitrogen gas and the range of 10-100 [cal/cm2 . s] for incident heat flux. Therefore, the objective of the present paper is to contribute to an understanding of ignition mechanism through additive effects on the ignition process at subatmospheric pressures.

2. Experimental

As shown in Table 1, such mixture as containing 80 parts of AP and 20 parts of CTPB was selected as a basic propellant and besides propellants with 0.5, 1.0, 2.0 and 4.0 parts of CC and CB, respectively, added to 100 parts of basic propellant were prepared, totalling to nine kinds of samples. Each strand- like sample, whose surface area exposed to radiative heating occupied 3 mm x 3 mm and length about 15 mm, was cut from a block of propellant. A sample stage with the test speci- men was placed in a subatmospheric vessel having a volume of about 20 litres. The beam of C02 laser with 10.6 pm in wave length was reflected twice by two mirrors, one vibrating hori- zontally at 50 Hz, the other vertically at 100 Hz, and further- more only the central part of the reflected beam was passed through a slit of 9 mm in diameter in order to generate fluxes with uniform intensity spacially and temporally. And then the beam entered into the vessel through a convex germanium window and irradiated a sample surface, the whole surface being covered with it. Consequently, while the original laser possessed output power of 100 W, the heat flux at sample

Power Meter - I

Propellants, Explosives, Pyrotechnics 10, 129-138 (1985)

Table 1. Propellants Used in Experiments. (unit : parts)

AP CTPB cc CB

AP-80 AP- 80(CC-0.5) AP-80(CC- 1) AP-80(CC-2) AP-80(CC-4) AP-80(CB-0.5) AP-80(CB- 1) AP-80(CB-2) AP-80(CB-4)

80 80 80 80 80 80 80 80 80

20 20 20 20 20 20 20 20 20

AP: CTPB: Carboxyl-Terminated Polybutadiene CC: Copper Chromite CB: Carbon Black

Ammonium Perchlorate (Particle Diameter: 100 - 150 pm)

surface decayed below 10 cal/cm2s. Ignition event was detected with a photo-transistor (0s-14; Toshiba Co., Japan) showing maximum sensitivity over a range of infrared wave length and sensing incipient flame in gas phase or radiaton emitted from surface of solid samples. Exposure time was controlled using two shutters operated automatically or manually. Ignition time is defined as duration from the initiation of irradiation on a sample surface till the time when initial flame in gas phase or runaway emission from solid surface was detected. The incident heat flux at surface was calculated on the base of an output measured immediately before each run by a power meter, also being reconfirmed just after it.

After the vessel was evacuated once, it was filled with argon gas at various prescribed pressures and then an experiment was started. Ignition time was measured at each fixed pressure for some incident heat fluxes at surface as a parameter.

On the other hand, differential thermal analysis (DTA) instrument was employed for each sample at various pressures in order to examine the relationship between ignition time and thermal properties for nine kinds of propellant specimens. The sample weighing 5.0 mg was in the form of small slice. The maximum heating rate, 20 Wmin, of the instrument was adopted to gain the heating speed as high as possible. The schematic of ignition experimental apparatus is shown in Fig. 1.

p+----j Pen- Recorder

TP+t Chamher

4 - t Vacuum Gas Figure 1. Experimental apparatus.

Propellants, Explosives, Pyrotechnics 10, 129-138 (1985) Effects of Additives on the Ignition of AP-Based Propellants 131

3. Results and Discussion

3.1. Characteristic maps for ignition

3.1.1. Basic propellant (AP-80)

Figure 2 indicates ignition time (ti) plotted against ambient pressure (p) for three heat flux levels at surface (qs) as a parameter. One point in all the figures shows average value over several values. Generally, ti decreased with q, and p, respectively, increasing. In the region (I), even if radiation source is cut off immediately after ignition (the operating delay time is approximately 70-80 milliseconds), ignition event can shift toward steady burning, being named “self-sustaining ignition (S.S.I.)”. In the region (11), the burning is interrupted by stop- ping laser beam just after ignition, but it is continued only during an irradiation, being named “radiation-assisted ignition (R.A.I.)”. However, in the region (111), ignition and extinction occur alternately while irradiation is going on. The cyclic behavior is called “pulsating ignition (P.I.)” in the present paper. And the phenomenon is apt to occur at both low pressure and low heat flux, being dependent on the formula- tion of propellant, and has been supposed to result from the forming of char layer at surface@). And the region (111) is subdivided into two regions, i.e., (IIIA) and (IIIB), again. The region (IIIA) is the region where a flame is interruptedly formed in gas phase at higher pressures in region (111) and the region (IIIB) being where runaway incandescent reaction at surface occurs cyclically at lower ones. And both (11) and (111) regions are called “Non-S.S.I.” region. No ignition region also is divided into two as follows: one is region (IV) and the other region (V). In region (VI) at much lower pressures and qs over a certain level, the sample begins to decompose and evolve product gases from the surface, but enough gas to establish a flame in gas phase cannot be stored because the diffusion rates of decomposing gases are very high at such lower pressures. Therefore, the exposed surface to radiation gets incandescent gradually and, at the same time, the surface temperature is continuing to rise, but such runaway chemical reaction as is considered to be ignition does not take place at the surface (no ignition with incandescence).

I : Self-Sustaining Ignition (S.S. I.) I1 : Radiation-Assisted Ignition (R.A. I.) 111 : Pulsating Ignition (P. I . ) N : No Ignition with Incandescence

I I I 1

Pressure , P [ tor r ] - Figure 2. Dependence of ignition time for basic propellant on ambient pressures.

There exists another region (V) in which any change hardly appears at surface exposed to radiative heating if heat flux level becomes too small. Such regon as (V) cannot be shown in this figure.

It is noticed that the dependence of ti on ambient pressure is slight in the region (I), but being remarkable in region (11). Especially below 100 torr where diffusion rates of decomposing gases grow higher and it is very hard for a flame to form in gas phase, ignition time depends a great deal on pressure, as can be seen from region (11), and chemical reaction in gas phase would be rate-determinant in ignition.

3.1.2. Addition of CC to AP-80

As shown in Fig. 3(a), propellant samples, AP-80(CC-0.5), containing 0.5 part of CC gave slightly longer ignition time above 100 torr but shorter one below 100 torr as compared

I : Self-sustaining Ignition (S.S.1.) I1 : Radiation-Assisted Ignition (R.A. I.)

30 I \ I , 20 (a) AP-80(CC-0.5) 0 : S.S.I.

\ A : R.A.I. t I1 \. I I

I

\ 1 -

Pressure , P [torr] - Figure 3. Dependence of ignition time for CC added propellant on ambient pressures; (a) AP-80(CC-0.5) and (b) AP-80(CC-4).

with AP-80 samples. The regions (111) and (IV) observed in Fig. 2 disappeared in Fig. 3. Ignition time decreased almost linearly in log-log graphs with increasing pressures in cases of CC addition. For samples to which 4 parts of CC was added, self-sustaining ignition occurred even at pressures as low as 10 torr (Fig. 3b). The observation that ignitability is improved by the addition of CC is in agreement with that in Ref. 14.

3.1.3. Addition of CB to AP-80

As shown in Figs. 4(a) and (b), in contrast with Ref. 14 the addition of CB had the effects of longer ignition times and

132 T. Saito, T. Yamaya, A. Iwama, and T. Kuwahara Propellants, Explosives, Pyrotechnics 10, 129-138 (1985)

I : Self-sustaining Ignition (s.S.1.) I1 : Radiation-Assisted Ignition (R.A. I.) N : No Ignition with Incandescence

I

30 20

10 7 5

3

VI

1 2 - - - 1 * 0.7

E 0.5

+

I 1 I

I I

\

Pressure , P [torr] - Figure 4. Dependence of ignition time for CB added propellant on ambient pressures; (a) AP-80(CB-0.5) and (b) AP-80(CB-4).

poor ignitability, resulting in the shift of the lower pressure limit of self-sustaining ignition to higher pressure. Especially for AP-80(CB-l) samples with one part of carbon black radiation-assisted ignition behavior took place even at the pressure of 100 torr over all the heat fluxes used on the present work.

I : Self-sustaining Ignition (S.S. I.) 11 : Radiation-Assisted lgnition (R.A.1.)

501 I

(a) 500 torr 30

t .4 0 : S.S.I. A : R.A.I.

3 .20f0.11

!i v--- 0.7

0. 1 2 3

1 (b) 100 torr

In this case CB didn’t reveal a role of improving the ignitability as opacifier for C02 laser beam with 10.6 pm wave length, but acted as an inhibitor for ignition at subatmospheric pressures. The addition of CB to AP-80 exterminated the pulsating ignition behavior but, at 20 torr or so, it left the phenomenon that though a flame was not completely extinguished, radiation intensity from it was weakened and recovered alternately.

3.1.4. Ignition behavior on the boundary between (I) and (11) regions

Deluca et al. stated the phenomenon of dynamic extinction for double-base pr~pellants~’~-”); the sustained combustion of them cannot survive the disturbance induced by abrupt termi- nation of the radiation and, when the radiation stops, the propellant is extinguished in much the same way as if it had been rapidly depressurized. However, self-sustaining ignition in the present experiment did not induce the dynamic extinction by rapid deradiation.

On the side of region (11) adjacent to the boundary between (I) and (11) regions under the conditions of p = 100 torr and qs = 4.89 k 0.11 cal/cm2s, basic propellants led to S.S.I. if irradiation lasted for longer duration than about 3.5 s after ignition but they extinguished if not so, as shown in Table 2. On the contrary, at lower pressure than 100 torr the samples extinguished the moment irradiation ceased however long it lasted.

Table 2. Effects of Irradiated Duration after Ignition on Ignition Type on the Boundary between (I) and (11) Regions.

~~ ~ ~~ ~ ~

Incident Ignition Irradiated Ignition Heat Flux Time Duration Type

[caYcm s] f sl [SI

after Ignition

~ ~~ ~ ~

4.87 1.19 0.10 R.A.I. 4.78 1.29 0.14 R.A.I. 4.97 1.20 1.35 R.A.I. 5.00 1.14 2.41 R.A.I. 4.81 1.24 3.56 S.S.I.

t t

4.89*0.15

0 1 2 3

(c) 50 torr

t I t

3.14f0.15

w-1 4.90k0.19 Figure 5. Dependence of ignition time on CC quantity at various pressures;

0 1 2 3 4 (a) 500 torr, (b) 100 torr and CC Parts Added to AP-80 [part]- (c) 50 torr.


3.2. Effects of additional quantity on ignition time

Effects of Additives on the Ignition of AP-Based Propellants 133

Ambient Gas : NZ Heating Rate : 20K/min DTA Sensitivity : & 250 UV

Sample Wetght : 4.0mg

0 ._ E kl 5 :: 1 w

t

I 3.2.1. Copper chromite addition 1 Sample :, AP only

500 torr

100 torr

Figures 5(a), (b) and (c) represent the variation of ignition time with additional quantity of CC to AP-80 at 500 torr, 100 torr and 50 torr, respectively. At the pressure of 500 torr for all the heat fluxes used, after ignition time reached maximum value at 0.5 part addition of CC to AP-80, ti monotoni- cally decreased with increasing CC amount, at last becoming the nearly same value at 4 parts addition of CC as that of AP- 80. Subsequently, the above trend still survived at 100 torr for the case with larger qs and, however, for smaller q, at the same pressure and for lower pressures than 100 torr, ignition time diminished down to that at 2 parts of CC as the quantity of CC increased, remaining nearly constant after then. It is certain that CC addition makes the self-sustaining ignitability better and ignition time shorter below 100 torr.

3.2.2. Carbon black addition

50 torr 1- As can be seen from Figs. 6(a) and (b), roughly speaking, ignition time above 100 torr increased with increasing the amount of CB, followed by saturating at about two parts of carbon black. It is evident from Fig. 6(c) at 70 torr that the addition of CB to AP-80 makes the self-sustaining ignitability worse except for AP-80(CB-1) at lower heat flux. This fact means that carbon black doesn't play a role of catalyst in the ignition at subatmospheric pressures.

$

- 3.3. Curves of differential thermal analysis (DTA) 100 200 3 3 400 500 C

Temperature ["C] - )O

Figure 7. DTA curves for AP only at various pressures.

tives. Figure 7 shows the DTA curves of only ammonium perchlorate (AP) at various pressures. At subatmospheric pressures there existed only one exothermic peak at almost 320 "C which probably corresponds to "low-temperature decomposition" of AP, being followed by an endothermic

3.3.1. Copper chromite addition

A sample weight of 5.0 mg and a DTA sensitivity of +_ 250 p v were used in all DTA measurements except for the case of only AP. The endotherm at about 240 "C in all ther- mograms shows the crystalline phase change of AP which occurs entirely independently of ambient pressures and addi-

Figure 6. Dependence of ignition time on CB quantity at various pressures; (a) 500 torr, (b) 100 ton and (c) 70 torr.


Ambient Gas : Nz Heating Rate : 2OK/min DTA Sensitivity : f 250pV

Sample Weight : 5.0mg

.- 0 E B 5 G

T c i .- E b 5 -0

u

1

Figure 8. DTA curves for AP-80 at various pressures.

peak appearing due to the sublimation of AP("). With decreasing pressures, the temperature pointing out the endotherm lowered and the peak area became wider.

As shown in Fig. 8 for AP-80 of basic propellant, the exothermic peak at ca. 320 "C which grew sharper than that for only AP remained nearly constant with respect to pressures except for near-vacuum, followed by the second maximum one at about 360 "C above 100 torr. Below 100 torr the latter deteriorated, becoming smaller than the former with decreasing pressures, followed by the growing endothermic peak due to the sublimation of AP, and at about 390 "C the third small exothermic peak appeared. When pressures decreased, the first peak became smaller but still existed even nearly in vacuum and the second exothermic peak, however, was buried in the endotherms, the thermogram approaching to that of AP alone. As mentioned above, below atmosphere these three exothermic peaks chiefly appeared in DTA curves of AP-80 type samples. Therefore, these were named lower-, middle- and higher-temperature exothermic peaks, respectively. Though maximum peak temperature over each pressure scarcely changed with decreasing pressures, the steepness of the peak weakened especially below 100 torr. The fact seems qualitatively to explain the behavior that the ignitability of AP-80 got bad as ambient pressures decreased in ignition experiments.

In Fig. 9 for AP-80(CC-0.5) samples, the maximum sharp exotherm following the lower-temperature peak with slightly

Ambient Gas : Nz Heating Rate : 20K/min DTA Sensitivity : f 250pV


c .- E 5 f X U.

T t- I .- E k f

I ;ample : AP-80(CI

i00 torr

.OO torr

50 torr

1 200 3 Temperature ['C] -

Figure 9. DTA curves for AP-8O(CC-0.5) at various pressures.

lower peak height compared to that of AP-80, appeared at almost 400 "C at 500 torr. As the sublimation of AP included in a propellant became severe with decreasing pressures, the maximum exothermic peak was made weaker and weaker. Below 70 torr an endothermic peak came out immediately before above maximum exothermic one. Therefore, it is evident in comparison with Fig. 8 that the maximum exothermic peak is equal to the third, higher-temperature exothermic one promoted by copper chromite, appearing under 200 torr for AP-80. At the same time considerable appearance of the endotherm below 70 torr for AP-80(CC-0.5) may be in agreement with the fact that AP-80(CC-0.5) samples lost self-sustaining ignitability for higher heat fluxes in ignition experiments. Since the higher-temperature exotherm was stimulated with increasing the quantity of addition of CC to AP-80, the endotherm was overwhelmed by the exotherm and couldn't be represented on the DTA curves of AP-80(CC-4) as shown in Fig. 10. In particular, at pressures contained in self-sustaining ignition region for AP-80 and all the propellant samples including CC each maximum peak temperature at which the sharp exotherm might trigger ignition due to condensed-phase or heterogeneous surface reaction, diminished with decreasing pressures but, nevertheless, ignition time became slightly longer. It seems to be partly because of the decline of sharp- ness of exothermic peaks. But it can be suggested that self- sustaining ignitability is mainly controlled by the severeness of exotherms related to condensed-phase or heterogeneous sur-

Propellants, Explosives, Pyrotechnics 10, 129-138 (1985) Effects of Additives on the Ignition of AP-Based Propellants 135

Ambient Gas : Nz Heating Rate : POK/min DTA Sensitivity : f250uV

Sample Weight : 5.0mg -

~

jample

500 torr

100 tori

3 torr

1 ;

4- I 1 300 4

Temperature CI - Figure 10. DTA curves for AP-80(CC-4) at various pressures.

face chemical reactions appearing on DTA curves but that ignition time doesn't always respond to maximum exothermic peak temperature based on them.

On the other hand, in radiation-assisted ignition region, as the exotherms based on condensed-phase or heterogeneous surface reactions become smaller and, in addition, gas-phase diffusion rate is larger on account of low pressure, enough amount of reactant gases to establish a flame cannot be present at local sites in the gas phase, producing considerable pressure dependency.


In contrast with CC addition, the addition of 0.5 part of CB to AP-80 at 500 torr didn't represent lower-temperature exothermic peak and made dull middle- and higher-temperature peaks appear. As pressures lowered, the maximum middle-temperature peak was remained but the higher-temperature one was exterminated as shown in Fig. 11. From Fig. 12 for AP-80(CB-4) it was found that the peaks were not steep over all pressures. It is responsible for CB addition making ignition time longer and ignitability worse that CB addition made the exotherms worse. And so CB didn't exert an effect of an absorber to radiation but played a role of deteriorating exotherms as negative catalyzer.

Ambient Gas : Nz Heating Rate : ZOK/min DTA Sensitivity : f250uV


I I

Sample

100 torr

!OO torr

50 torr -

1 2 Temperature ['Cl - O f

Figure 11. DTA curves for AP-80(CB-0.5) at various pressures.

3.4. Elyrects of added amount on DTA curves

3.4.1. Copper chromite addition

Figures 13(a) and (b) show the change of DTA curves with added amount of CC to AP-80 at 500 torr and 50 torr, respectively. Though the addition of 0.5 part of CC made a peak temperature transfer toward higher one, it was on the down- ward trend with further addition of CC in Fig. 13(a), being in agreement with results of Ref. 19. It can be found at 50 torr in Fig. 13(b) that the addition of 0.5 part of CC promotes the small third peak and exterminates the second one for AP-80 and apparently an endotherm manifests itself, but further addition of CC develops higher-temperature peak and exterminates the endotherm, the maximum peak temperatur lower- ing with increasing added quantity of CC. In spite of being apparently inferior to AP-80 on exotherm in Fig. 13(b), AP- 80(CC-0.5) has appreciably shorter ignition time than that of AP-80 for similar heat fluxes. Therefore, that may mean copper chromite promotes gas phase reactions at 50 torr.


It could be seen from Figs. 11,12 and 14 that the increase of the amount of CB at first sight had similar tendency to that of


Ambient Gas : NZ Heating Rate : 2OK/min DTA Sensitivity : f 2 5 0 w V


Temperature [“C] - Figure 12. DTA curves for AP-80(CB-4) at various pressures.

CC on the behavior of DTA in self-sustaining ignition region but CB addition made exotherms duller with increasing amount of CB at all pressures. However, systematic change in DTA curves with CB quantity could not be recognized with respect to CB addition.

3.5. Relation between ignition time and maximum exothermic peak temperature

Maximum exothermic peak temperatures at which ignition event can be assumed to be stimulated in terms of condensed- phase or heterogeneous surface reactions are plotted against added parts of CC in Fig. 15. The trend of two curves at 500 torr and 50 torr is very similar to that of Figs. 5(a) and (c), respectively, and it can be seen that the lower the exothermic peak temperature falls, the shorter the ignition time becomes for fixed pressures. Consequently, being contrary to Ref. 20, under the condition of fixed pressures, copper chromite promoted exothermic chemical reactions in solid phase or heterogeneous ones at surface and the increasing of CC amount made the maximum exothermic peak temperatures lower. On the other hand, for fixed addition parts of CC, maximum exothermic peak temperatures decreased with decreasing ambient pressures but, as shown in Figs. 2 and 3, ignition times increased with them. Therefore, “the exother-

Ambient Gas : Nz Heating Rate : 20K/m1n DTA Sensitivlty : f 2 5 0 p V

Sample Weight : 5.0mg -

PressL

AP-80 c_

: 500 to

?/

AP-80(CC-4) Y D 200 ‘z

Temperature [‘C] - Figure 13a, 500 torr. Dependance of DTA curves on CC quantity of various pressures.

mic peak temperatures being lower” doesn’t mean “the shorter ignition times”.

In view of above mentioned facts, it can be suggested that copper chromite contributes to the exothermic reactions not only in condensed-phase but also partly in gas phase, but that the determinant reaction in ignition might not be condensed- phase or heterogeneous reactions but gas phase ones.

4. Conclusions

The ignition characteristics of AP-based propellants exposed to C02 laser heating are divided into six regions: (I) self-sustaining ignition, (11) radiation-assisted ignition, (111) pulsating ignition, which further is subdivided into (a) pulsating ignition in gas phase and (b) pulsating ignition at solid surface, (IV) no ignition with incandescence, and (V) no ignition.


Ambient Gas : NZ Heating Rate : 20K/m1n DTA Sensitivity : f 2 5 0 p V


Effects of Additives on the Ignition of AP-Based Propellants 137

t

Ambient Gas : NZ Heating Rate : 20K/rnin DTA S isitivity : f 25OpV


I Pressure : 500 tc

AP-80

3 200 :

I, ~

0 Temperature [ T I -

Figure 13b, 50 torr. Dependence of DTA curves on CC quantity at various pressures.

Ignition time of basic propellant (AP-80) depends little on pressures above 100 torr in region (I), but depends pro- gressively on pressures with decreasing pressures.

Increasing the amount of addition of copper chromite diminishes the dependence on pressures and extends the region of self-sustaining ignition, i.e., improves ignitability, and CC plays likely a role of ignition catalyst especially below 100 torr and acts as a positive catalyst for the exothermic reactions in condensed-phase or heterogenous ones, and may affect partly the gas phase reactions, too.

On the other hand, carbon black has a negative effect on ignition behavior at subatmospheric pressures, making exothermic peaks very dull in DTA curves.

In comparison of ignition times with DTA curves, the present results suggest that condensed-phase or heterogeneous chemical reactions seem to control self-sustaining ignitability but that gas phase reactions may be rate-determinant in ignition at subatmospheric pressures.

Temperature ['C] - Figure 14. Dependence of DTA curves on CB quantity at 500 ton.

5. References

(1) E. W. Price, H. H. Bradley Jr., G. L. Dehority, and M. M.

(2) A. K. Kulkarni, M. Kumar, and K. K. Kuo, AIAA Paper No.

(3) T. Saito and A. Iwama, Proceedings of 10th Annual Conference, Fraunhofer-Institut fiir Treib- und Explosivstoffe (ICT), 1979,

(4) M. Harayama, T. Saito, and A. Iwama, Combust. Flame 52 (l),

(5) K. Kuratani, Aeronautical Research Institute, University of Tokyo, Report No. 312, 1962.

(6) P. W. M. Jacobs and A. Russel-Jones, 11th Symposium (Internu- donaf) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1966, pp. 457-462 [Proc.].

(7) S. H. Inami, W. A. Rosser Jr., and H. Wise, Combust. Flame 12

(8) S. H. Inami, Y. Rajapakse, R. Shaw, andH. Wise, Combust.

Ibiricu, AIAA J . 4 (9), 3153-1181 (1966).

80-1210 (1980).

pp. 127-151.

81-83 (1983).

(2), 41-44 (1968).

Flame 17, 189-196 (1971).

138 T. Saito, T. Yamaya, A. Iwama, and T. Kuwahara

0 : S.S.I. A : R.A.I.

I I I I 1 2 3 4

CC Parts Added to A P - 8 0 [part] - Figure 15. Effects of CC quantity and ambient pressures on maximum exothermic peak temperature.

Propellants, Explosives, Pyrotechnics 10,129-138 (1985)

H. Wise, S. H. Inami, and L. McCulley, Combust. Flame I1

K. Kuratani, Aeronautical Research Institute, University of Tokyo, Report No. 373, 1962. J. B. Levy and R. Friedman, 8th Symposium (International} on Combustion, Williams and Wilkins, 1962, pp. 613-672, [Proc.]. M. W. Evans, R. B. Beyer, and L. McCulley, J. Chem. Phys. 40

A. D. Baer and N. W. Ryan, AIAA J . 3 (S), 884-889 (1965). L. J. Shannon, AIAA J. 8 (2), 346-353 (1970). L. Deluca, L. H. Caveny, T. J. Ohlemiller, andM. Summerfield, AIAA J. 14 (7), 940-946 (1976). L. Deluca, T. J. Ohlemiller, L. H. Caveny, and M. Summerfield,

T. J. Ohlemiller, L. H. Caveny, L. Deluca, and M. Summerfield, 14th Symposium (International) on Combustion, The Combus- tion Institute, 1973, pp. 1297-1307, [Proc.]. S . Morisaki and K. Komamiya, Thermochimica Acta, 12,

R. H. W. Waesche and J. Wenograd, ICRPGIAIAA 2nd Solid Propulsion Conference, Preprint Volume, 1967. C. U. Pittman Jr., AZAA J. 7 (2), 328-334 (1969).

(12), 483-488 (1967).

(9), 2431-2438 (1964).

AIAA J. 14 (8), 1111-1117 (1976).

239-251 (1975).

(Received February 7, 1985; Ms 3/85)

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Effects of Additives on the Ignition of AP-Based Propellants at subatmospheric pressures