One of the most commonly overlooked factors (by amateurs) in the search for new space drives is the problem of heat dissipation. EVERY machine must dissipate heat in order to operate.

An example, below, refers to photovoltaic cells. These must get hot in use, and the heat dissipated into space by radiators. Concentrating the light with lightweight mirrors does not eliminate the problem of heat disposal.

Fission or fusion drives will also have a formidable heat dissipation problem. In space, convective cooling is impossible; only radiative cooling will work. The giant cooling towers of Earth-bound reactors need to be replaced by huge radiators.

Since thermodynamic efficiency is INVERSELY proportional to the absolute temperature, T, and thermal radiation goes as T4, the smaller the surface area of the radiator, the higher the temperature, and the lower the thermodynamic efficiency. This applies to photovoltaic as well as other types of power generation. To obtain reasonably low T, very large radiators are required. Since the radiators must contain channels for cooling fluid, and since leakage must be kept very low, the radiators must be rather massive. (Exception: photovoltaic converters which radiate directly from their shaded side.)

The result is that, within the inner solar system (inside the asteroid belt), photovoltaic powered mass ejection is the most effective per unit weight. The massive radiators for nulcear electric propulsion would be much heavier than the photovoltaic arrays.

Direct heating of hydrogen by a nuclear reactor gets around this restriction partially, since most of the heat is dumped in the exhaust stream itself. However, the specific impulse of this method is rather low when compared with ionic propulsion.

The moral of the story is clear: Heat production requires heat dissipation in space as well as on Earth.