Ablation of ceramic and metallic heat shields

Abstract

Simplified thermochemical ablation models a r e developed f o r s e v e r a l c e r a m i c and metallic heat-shield m a t e r i a l s , potentially useful a s highperformance r e e n t r y nose tips. The models a r e b a s e d on phase equilibrium a t the ablating s u r f a c e and on the simplified film-coefficient approach f o r unity Lewis-Semenov number. Limited t e s t data from the Avco Model 500 a r c and elsewhere a r e compared with the predictions based on equilibrium thermochemistry, and discrepancies a r e disc u s s e d and explained. The thermochemical mode l s a r e used to compare the ablative performance of these m a t e r i a l s i n a typical high-performance nosetip environment, and the m o s t attractive m a te r i a l s , from this standpoint, a r e identified.Nomenclature specific heat of heat-shield m a t e r i a l (BTU/lbm OR) constant defined by Equation ( 3 5 ) (BTU/lbm O R ) d i a m e t e r of t e s t specimen ( f t ) fraction of silicon vaporized standard f r e e energy of formation of specie I a t surface t e m p e r a t u r e ( B T U / m o l OR) h e a t -t r a n s f e r coefficient ( l b m / s e c ft2) h e a t -t r a n s f e r coefficient in the absence of m a s s t r a n s f e r ( l b m / s e c ft2) f r e e -s t r e a m recovery enthalpy ( B T U / l b m ) enthalpy of jet p e r unit m a s s of a i r ( B T U / l b m ) enthalpy of heat-shield m a t e r i a l a t s u r f a c e t e m p e r a t u r e (BTU/lSm) enthalpy of f r e e -s t r e a m gas a t wall t e mp e r a t u r e with a l l elements in their r e f e rence s t a t e s ( B T U / l b m ) enthalpy of specie I a t surface t e m p e r a t u r e ( B T U / l b m ) enthalpy of heat-shield m a t e r i a l a t i t s initial t e m p e r a t u r e ( B T U / l b m ) energy absorbed by chemical reactions and phase changes a t the ablating s u rface p e r unit m a s s of heat shield ( B T U / lbm ) heat of fusion of tungsten ( B T U / l b m ) energy absorbed by chemical reactions per unit m a s s of tungsten oxidized ( B T U / l b m ) heat of formation of specie I a t surface t e m p e r a t u r e ( B T U / m o l OR) t h e r m a l conductivity (BTU/ h r ft O R ) equilibrium constant f o r reaction R2 (atm4) equilibrium constant for reaction R7 equilibrium constant ( a t m ) , defined by Equation ( 5 4 ) m a s s fraction of specie I a t the ablating wall m a s s fraction of I atoms in the f r e e s t r e a m m a s s of f r e e s t r e a m reacting with heat shield ( l b m / m o l ) m a s s of heat shield reacting with f r e e s t r e a m ( l b m / m o l ) m a s s removal r a t e ( l b m / s e c ft2) diffusion-limited tungsten oxidation r a t e 2 ( l b m l s e c ft ) reaction-limited tungsten oxidation r a t e ( l b m / s e c ft2) tungsten oxidation r a t e ( l b m / s e c f t Z ) production r a t e of specie I a t the ablat-2 ing surface ( l b m l s e c ft ) molecular weight of n o n t r a n s f e r r e d gas ( l b m / m o l ) formula weight of I (lbm/ml>l) Downloaded by CORNELL UNIVERSITY on July 30, 2015 | http://arc.aiaa.org | tantalum-c...