HYDROGEN RECOMBINATION KINETICS AND NUCLEAR THERMAL ROCKET PERFORMANCE PREDICTION

The rate constants for the hydrogen three-body collisional recombinati on reaction with atomic and molecular hydrogen acting as third bodies have been determined by numerous investigators during the past 30 yr, but these rates exhibit significant scatter. The discrepancies in the rate constants determined by different investigators are as great as t wo orders of magnitude in the temperature range of interest for nuclea r thermal rocket (NTR) operation, namely, 2000-3300 K. The impact of t his scatter on our ability to predict the specific impulse (I(sp)) del ivered by a 30-klbf NTR has been determined for chamber pressures and temperatures from, respectively, 20-1000 psia and 2700-3300 K. The var iation in I(sp) produced by using the different rate constants is as g reat as 10%, or 100 s. This variation also obscures the influence of c hamber pressure on I(sp); using fast kinetics, low pressures yield sig nificantly improved performance, while using slow or nominal kinetics, the pressure dependence of I(sp) is negligible. Because the flow comp osition freezes at very small area ratios, optimization of the nozzle contour in the near-throat region maximizes recombination. Vibrational relaxation is found to produce negligible losses in I(sp).