Integrated Rocket-Back Guidance and a Comparative Rocket-Back Study

Much effort has been put into developing technologies for next generation re-usable launch vehicles. Many re-usable launch vehicles include a booster stage that is designed to return to a landing site that is near the launch site, after it has released the upper-stage, which continues to orbit. One of the major methods for turning the booster around is to execute a “rocket-back” maneuver which uses a rocket to reverse the velocity vector and start the vehicle back towards the runway. Although several general approaches for the rocket-back are possible, one of the most common is to use a pitch up and over that puts the vehicle on a high arcing, exo-atmospheric trajectory, thus allowing significant range to be covered during the ballistic coast portion, prior to atmospheric re-entry. The fuel reserve needed to turn the booster stage around will usually be minimal, in order to provide as much energy to the upper stage as possible. For this reason, the trajectory will often have a steep flight path angle on re-entry. Thus the re-entry trajectory is low energy and amenable to the low-energy re-entry (LOER) guidance approach previously developed by the authors. This paper consists of two parts. In the first part, an approach for autonomous guidance during the rocket-back maneuver is present. The method solves an optimal control problem and by keeping the problem planar, reduces it to a one parameter solution. The aforementioned LOER guidance is embedded in the solution process leading to Integrated Rocket-back and LOER (IRL) guidance. By integrating the two, it is ensured that all vehicle and touchdown constraints will be met by the full trajectory to touchdown. In the second half of the paper, a comparison will be made between the current rocket-back approach and alternate approaches to the pitch-over problem.