On page 94 of Ignition! an equation for "Frozen equilibrium calculation":

$$ c = \left[ 2H/M \right]^{1/2} \left[ 1- \left( {P_e \over P_c} \right) ^{R/C_p} \right]^{1/2} $$

All quantities are explained, except for $C_p$ - it only appears in the sentence "$\gamma$ is the ratio of specific heats, $C_p/C_v$ of the chamber gases" in a prior equation, but without explaining which of these two is specific heat of what. Then it's later referenced frequently, being an essential parameter, but always only by the $C_p$ symbol.

This is baffling especially in light of the page 99 quote: "As for the solids, $C$, $Al_2 O_3$,and $B_2 O_3$, their $R/C_p$ is precisely zero." Since $R$ is a constant, that would imply infinite $C_p$ - an infinite specific heat? what the heck? What is that quantity?


1 Answer 1


Cp is the "specific heat at constant pressure", just as Cv is the "specific heat at constant volume". You can see the derivation here. The values are stated for a given ideal gas.

Since these values are related to ideal gases, they have no meaning for solids, perhaps leading to Clark's comment on page 99. R has no meaning for a solid either.

A table of example values of the specific heats, from here.

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  • $\begingroup$ So what exactly happens that solids end up with $R/C_p = 0$? $\endgroup$
    – SF.
    Jan 18, 2017 at 14:35
  • 1
    $\begingroup$ The terms mean nothing when applied to solid material, they are only defined for ideal gases. If the combustion products of the solids could be treated as ideal gases, they could have values for specific heats and R. $\endgroup$ Jan 18, 2017 at 14:47

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