space resources WILLIAM H. BOWMAN' RICHARD M. LAWRENCE
for teachers
8011 Stole University
Munde, Indiana 47306
Ablative Materials for High-Temperature Thermal Protection of Space Vehicles
The problem of protecting spacecraft from high t,emperatures encountered during a mission has resulted in the development of several types of thermal protection systems. The flame temperature in the combustion chamber of a liquid propellant rocket, for example, may range from about 3000" C for a fuel-oxidant pair such as hydrogen and oxygen to about 4000" C for some of the more exotic propellants such as fluorine and oxygen. These temperatures are well above the melting point of metals commonly used as structural materials (1). Therefore, the walls of combustion chambers and exhaust nozzles must be protected. Spacecraft must also be protected from high temperature pulses produced by aerodynamic heating, especially during the reentry phase of a mission. A spacecraft falling to Earth from space must dissipate its kinetic energy. By making the shape of the reentry vehicle very blunt, a large portion (about 99%) of this energy can be transferred to the atmosphere. The remainder is absorbed by the vehicle as heat and is sufficient in amount to produce a severe increase in temperature. Protection of the reentry vehicle and its contents thus represents another application for high-temperature thermal protect,ionsystems. One approach is to coat the surfaces to be protected with a substance which acts as a thermal insulator. To be suitable as a thermal insulator, a substance should exhibit a high capacity for heat absorption or reject,ion, low t,hermal conductivit,y, and a low density (8). In addition, space vehicle applications often require that the insulator be able to withstand the mechanical stresses associated with high velocity, turbulent gas flows. This latter requirement is especially critical in the case of insulators used to protect the walls of the combustion chamber, t,hroat, and nozzle of a rocket engine. Erosion of the insulator from these walls, particularly in the region of the throat, can seriously alter the geometry and thus the performance of the engine.
As a class, ablative materials presently find the widest application as thermal insulators for rocket engines and reentry vehicles. Ablalion (5,4) is the systematic sacrifice of surface material through endothermic processes, such as sublimation and decomposition, to protect inner structures from a high heat flux. The success of ablative materials depends on their ability to dispose of large amounts of heat with only a sthall amount of material loss (5). Ablation may be visualized in a simplified way as follows
This article is one of the series of articles based on resource units in Lawrence, R. M., and Bowmen, W. H., "Space Resources for Teachers: Chemistry," NASAEP-87, 1971, available through the Superintendent of Documents, Government Printing Office, Washington, D.C. 20402 ($2.50). Request reprints of this article from Dr. Lawrence. 'Present address: Laboratory of Biochemistry, Metropolitan Life, 1 Madison Avenue, New York, N.Y. 10010.
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Journal of Chemical Education
The surface temperature of the ahlator begins to increme at a rate determined by such parameters as the magnitude of the incident heat flux and the specific heat of the ablative material. The low thermal conductivity of the ablative material effectively concentrates the absorbed heat in the surface region, thereby pmtecting subsurface regions from significant temperature increases. As the surface material reaches a sufficiently high temperature, the endothermic processes of decomposition and phase transition remove the surface layer and expose fresh ablative material. Repetition of this sequence may occur until no more ablative material remains.
Some materials, particularly the composite organic plastics, form a char layer during ablation. As material in the pyrolysis zone undergoes decomposition, liberated gases cause the formation of foam in the surface layer (primary char). As the gases undergo further decomposition and ultimately escape, the foam hardens leaving a porous, carbonaceous char layer (secondary char). This is illustrated in the figure. Ablators provide heat protection through several mechanisms. In addition to retarding the flow of heat into subsurface structures by absorption processes, ablative materials also dispose of incident heat by rejection processes (6). For example, the countercurrent flow set up by the escaping gaseous decomposition LIBERATED GASES HEAT INPUT
A
-8
C
Schematic representation of char formation during oblation. Heat incident on the ablotor (A1 c a u s e decomposition of the surface material, liberation of gores, and the formation of foam ( 8 ) . Addition01 heat hardens the foam and decamporer deeper layerr in the oblotor ICI.
products effectively blocks out a portion of the incident heat and prevents it from reaching the surface materials. The magnitude of this convective blockage (4) is determined a t least in part by the heat capacity of the escaping gases. Ablative materials which generate gases of high specific heat, such as hydrogen, are preferred. Radiation of heat from the high-temperature surface of the ablative material also provides a means for dissipating some of the incident heat load. The char layer formed by some ablative materials is very effective in this respect (6). Because of its porous nature the char layer behaves as an insulator; consequently, its surface reaches very high temperatures and reradiates a substantial fraction of the imposed heat load (7). Presently, the substances that are used most extensively as ablators are composite, reinforced charforming organic polymers. To fa~ilit~ate char formation the polymer should have a high degree of crosslinking, that is, it should be of the thermosetting rather than of the thermoplastic type (8). Pyrolysis of any polymer involves a competition between chain elimination reactions and chain cleavage reactions. Char formation requires that the rate of chain elimination reactions be greater than that of chain cleavagereactions. While this condition must be met both for linear and cross-linked polymers, theoretical calculations (8) indicate that the ratio of elimination to cleavage necessary for char formation is considerably greater for a linear polymer than for a cross-linked polymer of similar size and composition. A typical composite ablative material consists of a char-forming resinous matrix containing a gasgenerating component such as nylon and a reinforcing material such as silica, carbon, or graphite in the form of fibers or cloth (4, 5). Thermosetting resins of the phenolic novolac type have found wide application as the char-forming matrix. The term novolac refers to the linear condensation product of phenol and formaldehyde. This thermoplastic material is then converted to an infusible cross-linked polymer by addition groups. A comof a source of methylene (-CH2-) monly employed procedure is to mix the solid novolac resin with hexamethylenetetramine and cure a t 15& 180" C. Nylon, a linear condensation polymer based on the reaction of adipic acid and hexamethylenediamine, may be added to the novolac as a gas generating com-
ponent. Nylon has a melting point of about 260" C and during pyrolysis undergoes extensive chain cleavage with consequent formation of large quantities of gaseous decomposition products. Little or no carbonaceous char is produced from the nylon. The reinforcing material of an ablator serves the primary function of helping to anchor the char layer to the uncharred ablative material. In addition, it may provide sites for the deposition of pyrolytic graphite and thereby enhance the radiative efficiency of the char layer. I t may also undergo endothermic reactions with decomposition products of the resinous matrix. For example, it has been suggested (8) that silica may react with the deposited graphite to form silicon carbide SiOds)
+ 3C (graphite)
-
SiC(s)
+ 2CO(g)
H D=
+ 151 kcal
Both the amount and type of reinforcing material may aflect ablative performance (6). Charring resin ablators containing up to 35% by weight of silica fiber behave similarly to non-reinforced systems. When the silica content reaches 50% or more by weight, however, a fused silica skin forms over the surface during ablation. The ablative behavior of such a system is less efficient than the non-reinforced system and appears to be controlled primarily by surface vaporization or melting of the reinforcing material. Similarly, when low melting reinforcements such as fiberglass are employed, rapid melting of the surface may also cause poor ablative performance. Lilerafure Ciled ( 1 ) Y o o m , C. W.. i n "MannedSpaoecraft: Engineering Design and O p e r s tion," (Editors: Panssn, P. E.. FLOE=,M. A,, A N D S M ~ T N.~ F.) , Fairohild Publiohtions. Inc., New York. 1964, p. 277. (2) F r s a ~ nA., . KINNA,M. A,. KUBEK.F. J.. A N D BARNET, F. R., "The Performance of Selected Plastic Materials in a High Temperature Environment," U.S. Naval Ordinance Laboratory. White Oak. Maryland. 196%. pp. 1-3. (31 HINNA.P. A. C.. "A survey and Evaluation of Ablation Phenomena:' General Dynamioa Corn., Pomana. Calif.. 1964, no. 4-8. and O p e r s (41 E m , R. 8.. in "Menned Spsceoraft: ~ngineering~esign tion." (Editors: P u n a m , P. E., Fnom, M. A,. A N D SMITH,N. F.) Fairohild Publicationa. Inc.. New Y o r k . 1964, pp. 92-06. (5) ROBEBTB. L., i n "Materials for Space Operatione," NASA SP-27, 1962,
pp. 23-30.
( 6 ) R ~ c ~ * n o T. s . E.. AND RAXTER.J. R.. "Ablative Coatings for Short Term Thermal Protection:' T N CPD-57, Department of Supply. Australian Defence Scientific Service, Weapon; Researoh Establishment, Salisbury. South Auatrslia. 1963. p. 1. (7) A ~ o ~ n s o R n . A,, in "Structures for Space Operations," NASA SP-28, qac.7 " v . "*-"". L , C -""-, (8) M C A L ~ ~ I ~ TL., E RBOLOER. , J.. MCCAFFERI,E., ROY.P.. WARD,F., A N D WALKER,Jn.. A. C., "Behavior of Pure and Reinforced Charring Polymars during Ablation under Hrpervelooit~&-Entry Conditions:' Research and Advanocd Development Division, Avco Corp.. Wilmington, Mass., 1963, pp. 1-14.
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Volume 48, Number 10, October 1971
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