An analytical model has been developed to describe the operation of simple and concentric gas‐loaded heat pipes and to assess the extent of deviations from ideal behavior due to diffusion, viscosity, and sonic flow. The model predicts, with reasonable accuracy, the start‐up power, the maximum heat transport, and the thermal regulation (=&Dgr; density/&Dgr; input power). Experimental results will be presented for Li/He heat pipes. This approximate analytical model should enable spectroscopic and kinetic heat pipe users to design and operate heat pipes optimally without extensive and costly computer solutions of the full Navier‐Stokes equations. The model is based on approximate solutions of diffusion/convection equations, in which the convective velocity distribution of a nearly ideal hat pipe is assumed to be identical to that of an ideal heat pipe. The vapor is treated as a one‐dimensional compressible fluid. Among the more important results are (1) The start‐up powerQSU, defined as the power required to bring the metal vapor density to 95% ofn0, the stagnation density, at the exit of the heated zone (or adiabatic zone if present) is virtually independent ofn0. (2) For a heat pipe whose thermal losses are dominated by radiation, the start‐up power varies asT9/40, whereT0, the stagnation temperature, is defined byn(T0)=n0. (3) The sonic flow limitQCFmay be approximated asQCF= 1/2hfgAn0c(T0), wherehfgis the heat of vaporization per atom,Ais the cross‐sectional area, andc(T0) is the speed of sound atT0. (4) The thermal regulation properties of a concentric heat pipe may be approximated from the thermal regulation properties of simple heat pipes.