I'm looking at the BMW book† and see that equation 7.5-7 gives $\alpha_0 = \theta = \theta_g + \lambda_E$ as the equation for right ascension (for some launch site at east longitude $\lambda_E$). $\theta_g$ is Greenwich sidereal time at launch, presumably as an angle.
I know that SPICE contains a routine
et2lst which can be used with longitude 0 to get Greenwich sidereal time in h/m/s, but how can I use SPICE to calculate GST in radians instead?
It looks like there's an equation for it, but if SPICE has already implemented this, I'd rather not reinvent the wheel.
†Fundamentals of Astrodynamics, Roger R. Bate, Donald D. Mueller, Jerry E. White