SHERPA: a responsive multi-mission space tug
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.
Development of a Light-Weight, Reliable, Booster System for SHELS-Launched Payloads
Small satellite missions are often used to support low-cost space missions demonstrating new technologies. An economical source of low-cost space lift is to fly these satellites as secondary payloads aboard the Space Shuttle. The Shuttle has accommodations for flying these payloads using the Shuttle Hitchhiker Experiment Launch System (SHELS). While the relative costs for a Shuttle launch are at least an order of magnitude below the cost of a dedicated Expendable Launch Vehicle (ELV), final orbit altitude selection is limited to Shuttle mission goals. The Air Force Space Test Program (STP) is responsible for flying the Space Experiments Review Board (SERB) recommended experiments on a level-of-effort basis. Low-cost space lift is crucial to maximizing the number of SERB payloads that STP can support. Unfortunately, the typical Shuttle orbit does not provide a high enough orbit to guarantee the oneyear orbital lifetime required to meet STP mission objectives. A low-cost, autonomous STP Transfer Upper stage, Guided (TUG) that can boost an STP payload from a typical Shuttle orbit to a higher, longer duration orbit would allow STP to take advantage of the low-cost space lift provided by the Shuttle and still meet their mission requirements. The Air Force Research Laboratory Space Vehicles Directorate (AFRL/VS) is pursuing a solution to fulfill STP’s satellite lifting requirements by developing a low-cost, lightweight, reliable, strap-on propulsion module using several Small Business Innovative Research (SBIR) contracts focused on various parts of the TUG system. The Shuttle Expendable Rocket for Payload Augmentation (SHERPA) program will integrate all of these SBIR programs to meet the STP TUG requirement. The TUG system would be composed of several technologies being developed or already developed by AFRL/VS such as separation systems, guidance systems, propulsion modules, and modular bus architecture. The TUG would be re-startable for multiple orbit changes, station keeping, or deorbiting at the completion of a mission. Three versions of the TUG are envisioned. The first is a simple propulsion module that uses the satellite's Attitude Control System (ACS) and Guidance, Navigation, and Control (GN&C) to provide stack guidance. The second is a fully autonomous TUG that lifts the payload to the higher orbit as cargo, separates from the payload, and then accomplishes a collision avoidance maneuver and propellant burn after payload separation. The third configuration is an autonomous TUG with a long duration module that allow experiments to use the TUG's ACS, GN&C, and power systems in the intended final orbit. There are many challenges in the development of this vehicle. The most difficult of these is meeting the man-rating requirements of the Shuttle. All critical systems must have triple redundancy to ensure that the system does not threaten the Shuttle, its crew, or its mission. Another complication is producing a structure that meets the strict mass and volume restrictions of the SHELS system. Integration is als o a challenge, as many contractors and technologies are brought together under this program.
Development of a low-cost, modular, reliable, Shuttle-launched payload booster
The Space Shuttle is an inexpensive, reliable vehicle for small satellite launch for the Air Force Space Test Program (STP). Unfortunately, typical Shuttle orbits have a limited lifetime due to their low altitudes. One way to allow the STP to take advantage of the low Shuttle space lift cost is to employ a small propulsion module to boost the STP satellite from the relatively low Shuttle orbit to a higher orbit. Current Propulsion Module (PM) technologies are expensive and have difficulties conforming to the man-rating requirements of the Shuttle. The Space Vehicles Directorate of the Air Force Research Laboratory (AFRL/VS) seeks to solve this problem by developing PM technologies that will lower costs, improve performance, and increase the reliability of these systems, facilitating approval to fly on the Shuttle. The AFRL/VS will fund the development of several innovative technology efforts to design this proposed propulsion system to meet the STP requirements. It must be a lightweight, maneuverable vehicle that also meets Shuttle safety requirements. This system will also be compatible with current Space Shuttle Hitchhiker Experiment Launch System (SHELS) configuration for mounting in the Space Shuttle cargo bay, i.e. weight and volume requirements. The PM will also have a re-startable motor and appropriate avionics to provide flexible orbit transfer support.