SHERPA: a responsive multi-mission space tug

Abstract

The shuttle expendable rocket for payload augmentation (SHERPA) is a rocket-powered orbital transfer system for small satellites. It is currently being developed by the Air Force Research Laboratory Space Vehicles Directorate (AFRL/VS) to support the need of the Department of Defense (DoD) space test program (STP) to deploy satellite payloads to higher orbits. SHERPA, a space tug, mass of 56 kilograms and is capable of boosting a 125-kg satellite payload from an orbit of 350 kilometers to 700 kilometers. The STP utilizes a variety of systems to fly its payloads. Although the current plan for the SHERPA flight demonstration is to use the canister for all payload ejections (CAPE), opportunities exist to also launch as a secondary payload aboard an expendable launch vehicle (ELV). The SHERPA team, consisting of AFRL/VS and several small business innovative research (SBIR) contractors, is designing the core technologies for this effort to be as modular as possible to allow use of these individual technologies in other applications. The planned demonstration SHERPA fly as a secondary payload and deploy after the space shuttle has completed its primary mission. After SHERPA and its satellite payload have separated from the Shuttle, it loiters on orbit until it is adequately separated from the shuttle and international space station (ISS). The SHERPA orient the payload stack and fire its propulsion system to enter a Hohmann transfer orbit. This boost requires a total change in velocity of 240 meters per second (m/s) and includes a one-degree plane change for collision avoidance. After the stack has coasted to the final mission orbit apogee, the propulsion system fires again, inserting the satellite payload into its final mission orbit. SHERPA will then separate itself from the satellite payload and fire a collision/clearance avoidance maneuver (C/CAM), ensuring it is clear of the satellite and guaranteeing the pressurized propulsion tanks are emptied as much as possible before re-entry. After this final maneuver, SHERPA is in its lowest possible orbit and ready for rapid re-entry.