Knowledge Bank

One potential game-changing technology for future crewed missions to Mars is Nuclear Thermal Propulsion (NTP). NTP rocket engines are characteristically designed with higher specific impulse, a measure of engine efficiency, than traditional chemical rocket engines. However, NTP engines typically experience non-uniform heating profiles because each fuel element does not generate the same amount of heat over the cross-section of the engine core and heat generated near the center of the core can be more difficult to dissipate than the heat generated near the edge. These effects can lower the overall efficiency. This study defines the causes of non-uniform heating, with a focus on the impact to specific impulse, as a first step towards addressing this problem. Previous investigations examined heat generation profiles from specific engine designs, which required extensive neutronics calculations and simulations to achieve appropriate accuracy. Instead, an alternate method to the intensive neutronics calculations is to utilize mathematical distribution models of heating profiles. This work generates normal, bimodal, and skew distributed heating profiles, to compare the impact various distributions have on the specific impulse. For each heating profile, correlation equations between heating factor standard deviation and engine specific impulse were defined from the completed computations. The resulting analysis finds that for a typical NERVA based engine, a normally distributed heating profile with a 0.05 standard deviation in heating factors can cause the specific impulse to drop to 800s from 900s. This significant drop corresponds to a costly 5%-10% increase in the overall mass of propellant required for a typical Opposition Class or Conjunction Class Mission to Mars. Developing a method which quickly estimates the impact of non-uniform heating is necessary for preliminary engine design and will lead to improved strategies to address this issue, such as mass flow orificing or fuel loading.