Propulsion propellant control on the launch safety criterion

V.P.Ivanov

V.A.Trapeznikov Institute of Control Problems, RAS

65, Profsoyuznaya St., Moscow, 117997, Russia

Improvement of the power characteristics of the liquid-propellant boosters is a traditional challenge to their designers.љ To overcome it means to increase to the limit the mass of the launched load or the insertion height for the given booster launching mass.љ Improvement of the efficiency of available propellant capacity occupies an important place among the existing methods of increasing the booster power characteristics such as choice of the most efficient propellant, improvement of the liquid-propellant engine, or optimization of the insertion trajectory.

Physically, these methods are based on the notion of the guaranteed propellant reserve which implies that, besides the amount of the working propellant which is fully used in the calculated nominal conditions of insertion, there should be a guaranteed reserve in each rocket tank to compensate the random disturbances which influence the process of propellant consumption and the rocket trajectory. Specification of the guaranteed propellant reserve results in a lower nominal rocket "lifting power."

The guaranteed propellant reserve could be substantially reduced to an acceptable level of at most 0.5% of the mass of working propellant by installing on the booster special systems known under the common name of Propellant Consumption Control (PCC) systems.

Additionally, if the so-called full propellant use is realized on the booster, then the PCC system may be required to determine the instant of propellant exhaustion in the stage tanks.

By the full propellant use is meant a method of its consumption where at the end of stage operation the unused remainders of the propellant components are reduced to the least required values defined by the conditions for trouble-free engine shutdown.

Improvement of the booster power characteristics under full consumption of the propellant of the lower stages is due to the fact that the guaranteed reserves are made mostly on the basis of the condition for compensating at the last stage the stage-total action of random disturbances on the rocket trajectory coordinates. As the result, the required guaranteed propellant reserve of the end stage turns out to be smaller than the total guaranteed reserve allocated separately to each stage.

Upon launching the multistage boosters with liquid-propellant engines, the launching safety depends substantially on the choice of the instant of shutting down the power unit of each booster stage so as to ensure the accident-free engine shut down with the least unused remainders of the propellant components.љ Errors in shutting down the engine can result in its explosion or inadmissible losses in power leading to launch failure.

We consider two problems.љ The first problem lies in maximizing the booster efficiency characterized by the value of the phantom velocity reached by the booster at the end of the final stage.љ Minimization of the unused reserve of propellant and complete exhaustion of the propellant of the booster lower stage by controlling use of the propellant component solves this problem. The second problem lies in maximizing the safety criterion љby controlling the shutdown instant of the lower-stage engine on the basis of information about the booster phantom velocity and measurements of the propellant component reserves.