A rocket propulsion concept called microwave thermal propulsion consists of ground based microwaves beamed and focused on the underbelly of a rocket to heat a propellant which is flowing through suitably positioned microwave absorbent heat exchanger walls. The goal here is to heat the propellant up to a stagnation temperature as high as 2500 K without having to carry the source of heat. Assuming a weak dependence on k and that the same hardware is used so that CF 1.7 applies to all propellants of interest, calculate and compare the ideal Is for the following cold gas propellants: methane, helium, hydrogen and nitrogen.