Launch Vehicle Guidance for Low Energy Re-entry

Much effort has been put into developing technologies for next generation re-usable launch vehicles. Fully re-usable launch vehicles include a booster stage that is designed to land, usually near the launch site, after it has released the upper-stage, which continues to orbit. 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, once the booster stage has completed a rocket-back maneuver, it will typically be at a high altitude (exo-atmospheric) but with a low horizontal velocity and a correspondingly steep flight path angle on re-entry. Traditional re-entry guidance is designed for vehicles with a high velocity, shallow flight path angle, and with strong constraints on heating, and thus these traditional approaches are not appropriate for a low energy re-entry. We have developed a set of guidance algorithms that will successfully guide a vehicle to landing starting from a Low Energy Re-entry (LOER) condition. The guidance algorithms are based on acquiring an Equilibrium Steep Glideslope that ensures the vehicle can achieve near optimal range performance. The guidance problem can be successfully solved for a wide range of initial conditions, including more traditional initial conditions that would occur at the end of a High Energy Re-entry (HIER) from orbit. Thus, the guidance approach we have developed can be used as a more robust version of Terminal Area Energy Management (TAEM) guidance, as well as for LOER and has been tested for a wide range of vehicles, including the Space Shuttle and vehicles with a wide variety of L/D capability.