  The Noble-Abel (NA) equation of state (EOS) is widely used in interior
ballistics of guns as well as rocket propulsion computations. Its simplicity
and accuracy are key points for intensive computations with hyperbolic
two-phase flow models considered in interior ballistics codes. An alternative
is examined in the present contribution through a first-order virial (VO1)
equation of state. Appropriate methods for the determination of related
parameters, such as specific gas constant, covolume and condensed material
energy for both formulations (NA and VO1) are presented. Combination of closed
bomb vessel experiments and thermochemical code computations are needed. An
extended VO1 EOS with temperature dependent specific heat is examined. Then
extension to multiple reactive materials is addressed. Examples are examined
for each formulation (NA and VO1) and comparisons are done with the
Becker-Kistiakowsky-Wilson (BKW) EOS as reference. Several conclusions emerged.
First, consideration of specific heat temperature dependance in interior
ballistics of guns computations appeared insignificant. Second, VO1 appeared
more accurate than NA, particularly when gas density comes out of the range
used for the EOS parameters determination. Last, regarding mixtures of
condensed reactive materials, producing burnt gas mixtures, NA appeared again
less accurate than VO1. However, its formulation is explicit, while VO1
requires numerical solving of a non-linear equation, with consequences on
computational cost.
