# Why do launch trajectories end downwards

## TL;DR

This paper analytically determines the optimal thrust profile for a launcher upper stage to minimize fuel consumption, revealing that the final point is necessarily at an apside, especially for perigee injections.

## Contribution

It provides a closed-form analytical solution for the optimal thrust direction and profile in the minimum-fuel problem using maximum principle techniques.

## Key findings

- Final point at an apside for optimal trajectories
- Closed-loop thrust direction control derived
- Application to geostationary transfer orbit case

## Abstract

The problem of finding the optimal thrust profile of a launcher upper stage is analyzed. The engine is assumed to be continuously thrusting, following either a linear or a bilevel parametric profile, until reaching the targeted coplanar orbit. The minimum-fuel problem is analytically solved using the maximum principle. A closed-loop solution for the thrust direction is derived and the final point is found to be necessarily at an apside, reached downwards in the case of a perigee injection. The optimal control problem reduces to a nonlinear problem with only the thrust profile parameters as unknowns. An application case targeting a geostationary transfer orbit illustrates the solution method.

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Source: https://tomesphere.com/paper/1706.01044