F,22 • DISSOCIATION EFFECTS 



pression behind shock waves or friction in the boundary layer) becomes 

 sufficiently high, the air components partially dissociate, and the compo- 

 sition of the new air will be entirely different from that at low temper- 

 ature conditions. For example, at a speed of about Mach 20 at 100,000 

 feet altitude, the composition of the air behind a normal shock would be 



12 16 



Mach number Mc 



20 



24 



Fig. F,22e. Stagnation temperature behind a normal shock for air. 



approximately 50 per cent atomic nitrogen, 24 per cent molecular nitro- 

 gen, and 26 per cent atomic oxygen, compared to 78 per cent molecular 

 nitrogen and 21 per cent molecular oxygen at low temperature. The 

 degree of dissociation increases with decrease in pressure ; hence the result- 

 ing composition of the air is a function of pressure as well as temperature. 

 An important effect of dissociation is an increase in specific heat 



( 421 ) 



