Fuzzy Thrust Staging Kinematics for the NERVA Strap-On Boosters

Abstract

NERVA small space launcher under development in Romania by Politehnica University is based on the solution of hard take-off with three strep-on SRM boosters. Due to the very high thrust enhancement a lift-off loading of more than 40 g-s appears, which is quite challenging. The main challenge is connected with the possible uneven thrust and burn duration of the three SRE-s of the first stage, which we call fuzzy thrust. Experimental data show variations of up to 0.5 seconds in the duration of the powered phase of SRE-s, from a total of 3.0 seconds of a mean burn. Yet undetermined variations in the total ignition delay between individual motors are a stronger concern. While for the unique booster of the standard SA-2 vehicle the ignition delay has no impact, when those engines are forced to work in parallel the individual differences become catastrophic. Severe imbalance between the two lateral engines may occur, ending in a potentially severe damage of the launching rail, possible loss of flight stability along the boost phase and damage of the second stage structure. The problems regarding the efficiency of the propulsion system that accompany this project are considered and solutions are proposed for the high thrust augmentation of the booster stage. Cryogenic vs storable propellants are studied for thrust augmentation of the boosters.