Comparison of a Booster-Ram Rocket Combination with a Two-Stage Rocket for an Acceleration Mission

Abstract

In recent time spacecraft launchers always have been rockets. In order to save costs some proposals have been made to make the launcher reusable. In this case weight is increasing. For these launchers often air-breathing engines are proposed because of their less propellant weight, but structure weight increases again. There have been made many calculations, and the optimum depends on many parameters and assumptions, for example on the number of launches needed.

In this paper a launcher is investigated being a combination of a rocket booster with a second-stage ram rocket, but the launcher shall not be reusable. So the ram rocket must be built simple, inexpensive and in an extreme lightweight construction. This is possible, because combustion time is short, less than 1/2 min, as will be shown. In the calculations a fixed total starting mass and a fixed specific total energy at propellant cut-off of the ram rocket are assumed, and the engine is optimized to minimum structure and propellant mass of the whole launcher (booster and ram rocket), that means to a maximum so-called ‘payload’ of the launcher. Calculations are compared to those with a single-stage and with a two-stage rocket launcher. To simplify calculations, always a vertical climb is assumed for the launcher, that means worst conditions for the ramjet.

The result of the calculation is a gain in ‘payload’ of the launcher up to 35 p.c., if the second-stage rocket of the launcher is replaced by a ram rocket. But as the ranges in which the ram rocket has sufficient performance are rather narrow, parameters of the engine must be optimized. To get into orbit, upper-rocket stages must be added.