Heat rejection: Atmospheric vs vacuum?

[MaverickSawyer]

I'm trying to do a practice burn of my new NTR-based propulsion section for a deep-space vessel I'm working on, but I'm not sure if it's giving accurate results. Primarily, does the atmosphere impact engine heat buildup in any way (such as convection, or reducing the thrust)?

Edited August 8, 2015 by MaverickSawyer

[MaverickSawyer]

Okay, a couple of suborbital test flights with the engines in question have revealed that yes, there is a DRAMATIC difference in heat buildup. The atmospheric tests are highly misleading, as there seems to be some sort of convective heat loss off the engines. So, test your engines/engine units (if you're docking it to the ship) on a suborbital flight, with a minimum ignition point for the engines somewhere above 25km altitude.