Secondly, transpirant flow should be programmed to match the heat flux. If this is not done, the transpiration systems can- not be considered self-adjusting for a variety of conditions.

"MICROTRANSPIRATION", SELF-CONTAINED PROTECTION FROM TRANSIENT ENVIRONMENTS ABOVE 6000°F

W.L. Schalliol and N.A. Hooton The Bendix Corporation Bendix Products Division

South Bend, Indiana

Abstract

A self-contained, thermally protective mechanism is de- scribed and discussed. The mechanism is activated by a cata- lyst which operates within a composite material composed of a refractory metal matrix and an internally ablating oxide.

Physicochemical conversion of the oxide from the solid to the vapor state involves several energy absorption processes. The effusing gases resulting from these processes provide a protec- tive film on the exposed surfaces. The potential of the mech- anism, results of full-scale testing of tungsten-base compos-

ites with current solid propellants, and future applications are discussed.

Introduction

The advancing technology of rockets, missiles, and space vehicles has succeeded in bringing to the fore the importance of materials engineering. More and more frequently new appli- cations cannot be made successful without matching the time- temperature capabilities of materials with the operational time- temperature requirements. Typical applications requiring this type of matching are: nose cones, leading edges, combustion chambers, and nozzle throats. Steurer ( 1 ), Pellini and Harris

( 2 ), and King ( 3 ), among others, have reported on the basic ma- terials and types of protection systems with the greatest poten- tial for a given application.

For exposures of a few minutes and environmental tempera- tures in the 5000-6000°F range, heat sink systems with or with- out film cooling protection are adequate for homogeneous materi- als such as tungsten. At increased transient heat fluxes (up to four minutes at 1000-10,000 Btu/ft -sec) the environmental temperatures may exceed the melting points of all known materi- als (2) and protective systems which absorb thermal energy by

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SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

ablation, sublimation, or transpiration must be utilized. These mechanisms make use of the heats of change of state for energy absorption. Ablation systems utilize the state changes involv- ed while carrying materials from the solid, through the liquid, to the gaseous state. Subliming systems involve materials which will pass directly from the solid to the gaseous state. Tran- spiring systems utilize the heat absorption of a second phase passing through a skeleton matrix. A significant problem which arises while attempting to carry materials through several changes of state is that of retaining the liquid phase until vaporization has occurred and its heat absorption potential fully utilized. Fillers are added to ablative systems in an attempt to accomplish this end. Sublimation does not present a problem since the liquid state does not exist. A well de- vised transpiration system can minimize this problem due to the capillary forces within the channels whereby the skeleton re- tains the liquid phase.

Transpiration, as it is currently interpreted, presents several potential drawbacks to versatile operation. First, the extremely small passages or capillaries which are needed to dis- tribute the transpirant throughout the structure must be kept open at all times. This is necessary to prevent loss of cool- ing effects and resultant hot spots which lead to melting.

Secondly, transpirant flow should be programmed to match the heat flux. If this is not done, the transpiration systems can- not be considered self-adjusting for a variety of conditions.

The above problems have been largely surmounted by a re- cent discovery at Bendix Products Division of a mechanism which is termed "Microtranspiration

. This mechanism, operating in a tungsten composite, occurs in microscopic channels throughout the material. The presence of a catalyst within the material provides a means to open these channels during the exposure to high heat flux and to trigger the physicochemical energy absorp- tion processes, which include reactions and state-changes of a transpirant material. The rate of gas production within the structure is determined primarily by the heat flux to the struc- ture. Therefore, it may be said, that the Microtranspiration mechanism is self-controlling. The workability of this mech- anism has been demonstrated by numerous no-erosion, sub-scale, and full-scale firings of solid propellant rocket motors uti- lizing these composites in the nozzle throat.

Development of the Hypothesis

Microtranspiration effects were discovered during a ma- terials development program at Bendix which was initiated in 1958 and was designed to improve the strength-weight ratio and increase the operating temperature limitation of rocket nozzle inserts. Composites formed from tungsten and refractory oxides

190

wer e evaluated in sub-scale liquid and s o l id motors and found to be superior to pure tungsten. Early in 1960, f u l l - s c a le f i r› ings of a second-stage Minutema n motor were successfully com›

pleted with zero erosion of a tungsten-beryllium oxide throat insert in a Bendix-designed n o z z l e.

A p o s t - f i r i ng evaluation of the second-stage insert r e›

vealed micropores on the f r o n t - f a ce surface which had been ex›

posed to the hot combustion gases. The l e ft portion of Figure 1 show s a bisected throat i n s e r t. The black dots are represen› t a t i ve of micropores found on the curved front-face surface of tungsten composites after each f i r i n g. A rectangular portion of the insert cross-section was mounted in p l a s t ic and then deeply etched. The microstrueture is shown on the r i g h t. Note the retention of the f r o n t - f a ce structural i n t e g r i t y. This r e›

tained structure is responsible for zero-erosion f i r i n g s. Ad›

jacent to the front face is a permeable zone which extends in a rather uniform manne r to a maximu m depth of 0.150 i n. Back›

ing t h is up is the original composite structure which continues to the back f a c e. Magnification of the enlargement in Figure 1 is indicated by the 0.150 i n. l i ne below the photomicrograph.

Discovery of the l o ss of the oxide from only the front face of the composite gave r i se to conjecture that a s e l f - c o n› tained transpiration mechanism was operative. Since the l o ss from the composite occurred only from the microcells of ceramic in the material microstructure, and escape of t h is ceramic was through micropores on the front-face surface, t h is proposed mechanis m was termed "Microtranspiration".

To further study the oxide l o s s, a throat insert was i n›

f i l t r a t ed after f i r i n g , w i th molten copper. It was then s e c t i o n› ed and i ts microstructure examined. Figure 2 compares the r e›

s u l ts of attempted i n f i l t r a t i on of a throat insert before and after f i r i n g. The white matrix is tungsten, the grey-black areas ceramic, and the hatched constituent is copper. It can be seen that before f i r i ng no i n f i l t r a t i on of copper occurs0

After f i r i n g, the copper occupies many of the areas previously occupied by the oxide. This oxide l o ss occurs to a rather u n i›

form depth along the surfaces exposed to the hot gases. Linear analysis of many microstructures of i n s e r t s, i n f i l t r a t ed after f i r i n g, revealed that the tungsten volume constituent remains nearly constant from the front face through the permeable zone to the back f a c e. The ceramic constituent decreases in volume near the front face and then abruptly returns to the normal composition. This indicates that the oxide release occurs in a r e l a t i v e ly narrow band at any given instant during f i r i n g.

A review of nozzle f i r i n gs and experimental work revealed several important p o i n t s: (a) The micropores were never found after f i r i ng on homogeneou s materials such as tungsten but were always found on tungsten composites, (b) the l o ss of oxide is r e l a t i v e ly uniform for a fixed insert composition f i r ed in the

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SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

Figure 1. Sectional View of Throat Insert. ENLARGED VIEW

BACK FACE- ORIGINAL STRUCTURE- -FRONT FACF

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H SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

Figure 2. Oxide Loss from a Tungsten Composite.

AFTER FIRING BEFORE FIRING

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SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

sam e motor, (c) reports from several f u l l - s c a le f i r i n gs i n d i› cated that the Bendix tungsten composites showed l e ss interface carbide formation than did pure tungsten, when both were in con›

tact with a graphite back-up material. Laboratory t e s ts have show n that the depth of t h is carbide formation is dependent up›

on temperature and t h is smaller depth of carbide is interpreted as resulting from lower back-face temperature. This indication of a lower temperature was supported by measurement s made dur›

ing f i r i n g s, (d) a tungsten-aluminum oxide composite showed e v i› dence of oxide loss from the surface of the material when heat›

ed in a furnace at 4000 F. On the colder walls of the furnace, a few inches from the heated specimen, whiskers of AI2O3 were found, indicating that vapor species were released. These data further supported our hypothesis that the transpirant was e f f u› sing as a gas through the front-face surface, absorbing energy en route, and providing a protective f i l m.

Th e means by which the tungsten composite provides the mechanica l retention of the molten material during vaporization is seen in Figure 3. This figure compares the ablation charac›

t e r i s t i cs of an externally ablating material and an internally ablating material. In the homogeneou s material shown on the l e ft (4), the metal or ceramic begins to melt on the surface and the exhaust gas stream sweeps away a portion of liquid par›

t i c l es before the heat of vaporization is completely u t i l i z e d. Fo r applications such as rocket nozzle throats, where zero e r o›

sion is d e s i r a b l e, the mechanism provided by the microtranspir- ing composite shown on the r i g h t, provides for retention of the original surface and allows for complete u t i l i z a t i on of the transpirant heat capacity.

Th e early work in the Bendix composite development program wa s done with tungsten-beryllium oxide composites. The b e r y l›

lium oxide was considered desirable because of i ts high melting point, high s p e c i f ic heat, and r e l a t i v e ly high conductivity. Th e hypothesis of Microtranspiration, however, indicated that the melting point should be of secondary importance to the c a›

pacity of the transpirant to absorb heat during phase changes and d i s s o c i a t i o n. Aluminum oxide then qualified as a potential transpirant because of i ts high heat of dissociation even though i ts melting point is about 1000F lower than that of beryllium oxide. Rocket f i r i ng t e s ts using tungsten-aluminum oxide com›

posites supported the hypothesis.

Potential of the Mechanism

Fo r a theoretical evaluation of the quantitative capaci› t i es of the composites to absorb heat, the end products of the reactions which occur during f i r i ng must be known . From t h is information, along with the heats of state-change, d i s s o c i a t i o n,

194

H SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

Figure 3. Ablation Characteristics of Homogeneou s and Microtranspiring Composite Materials.

COMPOSIT E MATERIA L HOMOGENEOU S

MATERIA L

(EXTERNAL ABLATION) (INTERNAL ABLATION)

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SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

and reaction, the heat absorption capacities of the composites can be calculated. The work done by Inghram and Drowart (5) and Drcwart, et al (6) indicated that aluminum oxide in a tung›

sten Knudsen c e ll heated to 4300F yielded numerou s molecular species which could be identified with a mass spectrometer. These data, extrapolated to 5500F and shown in Figure ¸, give the relationship between the high temperature vapor species in the tungsten-aluminum oxide system. At 4000F the mole p e r›

centages of the two major vapor species are reported (6) as f o l l o w s: `1-¸9 and 0 - 3 5.

Th e t o t al pressures of a ll s p e c i e s, even at 5500 F, is no t s u f f i c i e nt to allow them to effuse against the high p r e s›

sure existing along the front f a c e, so the pressure of the vapo r species must somehow be increased. This increase is a c›

complished by the presence of a catalyst which triggers the r e›

lease of these gases and provides s u f f i c i e nt t o t al internal pressure for the gases to e f f u s e. Without the catalyst no Microtranspiration occurs. For proprietary reasons, it is not possible at t h is time to divulge the exact nature of the c a t a› l y st material. This requirement for a s p e c i f ic combination of high heat f l ux with a two-phase composite containing the nec›

essary c a t a l y s t, is probably why t h is mechanism has not been reported previously.

Based on the vapor species indicated in Figure 4, a com^

parison of the heat absorption capacity of tungsten with three tungsten composites is shown in Figure 5. The s o l id l i n es com›

pare the composites of 20 volume percent transpirant and the dashed lines represent the potential with AO percent t r a n s p i r› a n t. Both types of oxide composites have been made, but f u l l- scale testing to date has been confined to 20 percent oxide com›

p o s i t e s. It can readily be seen that potential heat absorption capacities of greater than A00 percent that of tungsten are i n›

dicated when these oxide composites are heated from room tem›

perature to 6150 F, (melting point of tungsten).

Th e tungsten-copper composite is fabricated by i n f i l t r a› ting a porous tungsten body with a copper a l l o y. One problem is that the maximu m volume percent copper to yield a usable part was found to be about 20 percent. Another problem with the tungsten-copper composite is the low melting point and high f l u i d i ty of the copper which results in l o ss of liquid t r a n s p i r› an t before heat of vaporization is completely u t i l i z e d. Because of the limited potential of the tungsten-copper composite, it wa s discarded in favor of the tungsten-oxide materials. Other advantages gained through the use of oxide transpirants a r e:

(a) that the physicochemical changes associated with these o x›

ides occur at higher temperatures, thereby making greater use of the matrix heat sink potential before internal ablation b e›

g i n s, and (b) the higher internal operating temperatures achiev› ed with oxide transpirants reduces the heat f l ux to the throat

196

Figure 4. High T e m p e r a t u re Vapor Species in the W - A i ^ O ^ S y s t e m.

P A R T I AL P R E S S U RE ( A T M O S P H E R E S)

SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

TEMPERATUR E fF )

L6l

5 0 0 0 4 5 0 0 4 0 0 0

3 5 0 0

3 0 0 0

Figure 5. C o m p a r i s on of Heat Absorption Capacity from 7 0 to 6 1 5 0 F.

SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

HEAT ABSORPTION POTENTIAL BTU/IN3

961

40% TRANSPLANT- -

20%TRANSPIRANT -

i n s e r t, extending operating time. Therefore, the material oper›

ates in that desirable temperature range in which it retains adequate physical properties and yet u t i l i z es much of i ts i n›

herent heat sink capacity.

Based on heat absorption c a p a c i t i e s, computations were mad e comparing the front-face temperatures of a tungsten-alumi› num oxide composite with tungsten in the throat of a nozzle. A s shown in Figure 6, the hypothetical extreme environment to which the 0.800 inch thick throat section was exposed, c o n s i s t›

ed of a gas temperature of 7000F and a heat transfer c o e f f i› cient of 2000 B t u / f t² hr F. Within U seconds, the front-face surface of tungsten would have reached i ts melting point of 6150 F. The 40 volume percent aluminum oxide composite would hav e a front-face surface temperature of only 4700F at U s e c›

onds , and 64 seconds would be required for the surface to reach 6 1 5 0 ^. The ˜ of 1500F shown on Figure 6 indicates the potential of the Microtranspiration mechanism in a tungsten- oxide composite to provide thermal protection against 7000F propellants.

Th e experimental r e s u l ts of V a s s a l l o, et al (7) and Lapple, et al (8) using model systems have demonstrated the f e a s i b i l i ty of internal ablation as a means of thermal protec› t i o n. It is f e lt that Microtranspiration v e r i f i es in practice the experimental work performed by these i n v e s t i g a t o r s.

A s the gases are formed internally and effused through the structure, they pass into the boundary layer which e x i s ts between the main stream of combustion gases and the throat i n›

sert f a c e. The effused gases enter into this boundary layer in i ts lowest v e l o c i ty region and thus r e l a t i v e ly small mass ef› fusion rates are required to produce s i g n i f i c a nt thickening. Th e existence of the effused gases in the boundary layer w i ll create two e f f e c t s: (a) an e f f e c t i ve thermal shielding of the surface of the i n s e r t, and (b) a chemistry change of the gas f i lm immediately adjacent to the i n s e r t. Past studies by Gross, et al (2) and current investigations by V a s s a l l o, et al (7) and Maloof (10) have indicated the potential thermal shielding e f f e ct of gases transpired into the boundary l a y e r. While a quantitative value for the heat f l ux shielding of the effused gas in t h is system is not now a v a i l a b l e, it is f e lt that t h is factor is contributing s i g n i f i c a n t ly to the success of the Microtranspiration mechanism . The a l t e r a t i on of the boundary f i lm chemistry e f f e ct can be described primarily as a d i l u t i on and shielding e f f e ct whereby p o t e n t i a l ly corrosive elements in the combustion gases may be prevented from reaching the surface of the i n s e r t, thereby reducing oxidation, corrosion, e t c. Curren t studies are underway to evaluate t h is portion of the mechanism .

Th e gas evolution and cooling f i lm protection of Micro›

transpiration are s e l f - c o n t r o l l i ng in that at high heat f l ux 199

P U RE W y . M E L T I NG P O I N T- W

^ W - 4 0 % A I

0

MELJiNG_P01NT h

6 A S - 7 0 0 0 *F -

h G A S - 2 0 00 B T U / FT H R *F q<>=27 B T U / I N? S EC

W A LL T H I C K N E S S - 0 - 8 0 0 IN-

Figure 6. Throat M a t e r i a ls C o m p a r i s on in Hypothetical E x t r e me Environment.

SURFAC E TEMPERATUR E F

00 2

T I M E S E C O N DS

6 0 0 0- 5 0 0 0- 4 0 0 0

3 0 0 0- 2 0 0 0- 1 0 0 0-

0

0 10 20 30 40 50 60 70

rates more gases are released, giving rise to more cooling and film protection thereby reducing surface temperatures. The cooling is statically performed so that no auxiliary pumping systems are required. This assures maximum reliability for the protective system.

While pure tungsten appears generally satisfactory for current propellant temperatures, other materials such as the tungsten-oxide composites will be required for operation at 7000°-8000°F. Microtranspiring tungsten composites have been successfully fired with many currently known solid propellants.

A summary of the firings is shown in Table 1. While zero ero- sion has been found in all cases, some problems have been en- countered in certain designs. Care must be exercised to take into account properties such as the thermal shock resistance of the composite materials. Some insert designs have success- fully overcome this problem.

Future Applications

The mechanism is applicable to materials other than tung- sten-base composites and should extend the range of usefulness for many materials to above the melting point of any constitu- ent. The self-cooled nature of this class of materials means that composites can be used in transient environments of high temperatures since the structure itself will not reach these high temperatures. This assures that strength properties avail- able in the moderate temperature range can be retained at ele- vated temperatures for limited periods. The composites, there- fore, should be more resistant to mechanical erosion than pure tungsten because the wall operating temperatures are lower.

It is felt that Microtranspiration provides the key for material systems operating in the 7000-8000°F range of transient conditions. Studies are continuing at Bendix Products Division to obtain more information on this mechanism and its application for transient high heat flux environments.

Conclusions

Based upon information obtained to date regarding the mechanism of Microtranspiration and its effects, some conclu- sions may be drawn.

a. A material has been developed which exhibits an internal ablation mechanism proposed by theoretical studies of other investigators.

b. The mechanism, operating within a tungsten-oxide composite material, has been successfully tested in several, full- scale rocket nozzles exposed to current solid propellants.

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SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

20 2

Table 1. Solid Propellant Firings of Tungsten C o m p o s i t es

MOTOR T E S T S O U R C E EROSION RATES

MATERIA L EVALU › ATIO N

BENDIX , THIOKOL , HERCULE S (MINUTE - MA N PROPELLANT )

ZER O PROPELLAN T

CHEC K

GRAN D CENTRAL , R0HM8HAAS(U P TO 900 PSI- 6 2 0 0 F )

ZER O MINUT E MAN (FULL

SCALE ) 1st STAG E 2nd STAG E 3rd STAG E

THIOKO L

AEROJE T 81THIOKOL HERCULE S

ZER O

ZER O

ZER O

2 0 5

c. The mechanism is not limited to refractory metal compos­

ite systems, but can, by judicious choice of materials and fabrication methods, be extended to other matrix- transpirant systems.

d. The increased energy absorption potential, without in­

crease in weight (with regard to pure tungsten), of the current tungsten-oxide composite should make it appli­

cable for use in transient environments in the 8000°F range.

Acknowledgements

The writers wish to express their gratitude to Dr. G. C.

Kuczynski, University of Notre Dame, Prof. A. W. Allen, Univer­

sity of Illinois, and D. C. Thoman, Bendix Products Division, for their assistance in the analysis of the mechanism and its effects.

The work reported in this paper was performed concurrent­

ly with AF/BMD contract #AF 04(6¸7)-229 on "Minuteman Movable Nozzle Thrust Vector Control System." Since no material de­

velopment was included in this contract, all work was funded by Bendix Products Division. Some of the materials developed during the program were used in full-scale movable nozzle firings.

References

1. Steurer, W.H., "Criteria in the Selection of Materials for High Temperature Structures," ABMA DV-TN-67-58, July 1958.

2. Pellini, W.S. and W.J. Harris, Jr., "Flight in the Ther- mosphere, I - Material Requirements for Thermal Protec­

tion Systems," Metal Progress, vol. 7 7, no. 3, 1960, æ 69•

3. King, H.A., "Five Ways to Control High Heat Flux," Materi­

als in Design Engineering, vol. 5 3, no. 2, 1961, æ 1 0 1.

¸ . Pellini, W.S. and W.J. Harris, Jr. "Flight in the Ther- mosphere, II - Materials for Nonmelting Heat Sink Sys­

tems," Metal Progress, vol. 7 7, no. ¸, 1960, æ 115•

5. Inghram, M.G. and J. Drowart, "Mass Spectrometry Applied to High Temperature Chemistry," Proceedings of an Inter­

national Symposium on High Temperature Technology, Asilomar, California, McGraw Hill, 1959•

6. Drowart, J., G. DeMaria, R.P. Burns, and M.G. Inghram,

SIXTH SYMPOSIUM ON BALLISTIC MISSILE AND AEROSPACE TECHNOLOGY

204

"Thermodynamic Study of

Using a Mass Spectometer,"

J. Chem. Physics, vol. 32, 1960, ρ 1366.

7. Vassallo, F.A., H.G. Camnitz, and H.P. Kirchner, "Study of Thermal Radiation Within Solids and Study of Internal­

ly Ablating Composites," WADD Tech. Report 60-697, Sep­

tember 1960, Preprint Copy.

8. Lapple, C.E., A.P. Brady, and D.L. Chamberlain, Jr.,

"Research on the Mechanism of Ablation of Polymeric Materials," A.S.D. Tech. Report 61-20A, June 1961, Preprint Copy.

9· Gross, J.F., D.J. Masson, and C. Glazley, Jr., "General Characteristics of Binary Boundary Layers with Applica­

tion to Sublimation Cooling," The Rsnd Corporation, ρ 1371, August 1958 Revision.

10. Maloof, S.R., "Development of Ultra-High Temperature Tung­

sten-Base Composites for Rocket Nozzle Applications,"

American Rocket Society Paper 1573-60, December 1960.