1. Field of the Invention
The invention relates generally to systems and methods to deliver crew, cargo and other logistics services to space platforms. More particularly, the invention relates to systems and methods for automated rendezvous and docking of a spacecraft to a space platform.
2. Related Art
The Space Shuttle has provided a majority of the assembly and logistics service support to the International Space Station (ISS). The shuttles are expensive to operate, and they place personnel at risk even when a primary payload does not otherwise require a crew. The Russian Soyuz and Progress spacecraft are also used for crew and cargo transport, respectively, to the ISS, including the transfer of consumables such as propellant. Both of these systems have a lengthy heritage and have driven some of the design of the ISS they now support. The European and Japanese space agency transfer vehicles currently in development will be capable of transporting cargo to and from the ISS as well. The development of these systems has helped to identify opportunities to improve on-orbit operations.
A spacecraft is designed to fulfill a set of requirements which reflect mission objectives as well as many constraints. The spacecraft systems are designed to carry out the mission and support the needed cargo. While there are general requirements, detailed requirements are honed to meet specific mission requirements and other considerations are made with regard to performance capabilities, flight environments, safety, mechanical and electrical interfaces, guidelines, standards, regulations, flight rules, schedule, and budget, for example. A number of alternatives may meet the requirements and the systems and subsystems are designed and sized to best satisfy the requirements. The resulting structures and components are modeled, simulated and constructed. Each of these individually, and integrated as a system, must pass through various verification, validation, qualification, certification and acceptance tests. The process is typically long and expensive. The complexity is compounded by the number of systems, their interactions with each other and with other systems and spacecraft. Even the smallest of changes at an inopportune point in the process is potentially very expensive. That being said, large sums are spent to save even larger sums or to mitigate risks.
The cost of spacecraft and their missions is unavoidably tied to the delivering mass to orbit. So, the mass of the spacecraft and its various components is of great concern. The spacecraft and their cargo become high-value assets, and the risks to these mitigated to the greatest degree possible by various logistical measures which, in turn, make the systems and procedures even more expensive to undertake. The result is that simplifications to spacecraft and their missions can greatly affect (reduce) their cost.
One such simplification is to relieve a spacecraft of some part of its functionality or mission, for example, by placing that responsibility with another system or spacecraft. If the other system or spacecraft is reusable, the expense associated with that part of functionality or mission can be amortized over a number of spacecraft or missions; on-orbit operations is one area in which this can be done. The orbital maneuvering, and automated rendezvous and docking (AR&D) functions are candidates for incorporation into a reusable spacecraft.
Transfer vehicles such as the Orbital Maneuvering Vehicle (OMV) have been proposed. The OMV is used to change the orbital plane of a second spacecraft, or to boost the second spacecraft into a higher orbit. This allows the second spacecraft to be built with much smaller propulsive systems designed primarily for attitude control (reaction control systems). The OMV stays in orbit and is able to provide its services to several spacecraft. The result is that the engines used for the orbital changes need only be launched into orbit once, not with each spacecraft. The OMV must carry enough propellant for all of its missions, but the systems associated with carrying the propellant (the “overhead”) are amortized over all of the missions. Also, additional reserve propellant is usually carried by a spacecraft to enable the spacecraft to handle a worst-case scenario. That reserve is likely never used so the cost of putting it into orbit is “wasted”. The OMV is able to use that reserve propellant for another mission.
Attempts are being made to simplify the process of mating visiting vehicles such as logistics spacecraft to the ISS. One such attempt involves the free-flying capture of a visiting vehicle with a subsequent berthing rather than relying on the visiting vehicle to dock directly with the ISS. When docking, a visiting vehicle must safely align itself with a docking port and drive into it with enough force to properly mate with the docking mechanism but not so much as to damage the ISS or force it “out of control”. A berthing operation requires the visiting vehicle to rendezvous with the ISS and to move to within reach of the ISS's robotic arm subject to a number of constraints. The robotic arm then “grabs” and pulls the visiting vehicle to a docking port. The free-flying capture and berthing procedure, however, has proven to be more problematic than hoped and better means for docking visiting vehicles are being explored.
In the early 1990's, a tug was proposed for transferring a supply spacecraft from its orbit to the proximity of the International Space Station (ISS). The features of the proposed system did not justify its manufacture and it was never built. A tug travels to the supply spacecraft and grabs or mates with it. The tug then “pushes” the supply spacecraft to the ISS. The supply spacecraft is then docked. The supply spacecraft is driven into a compatible and available docking port where the supply spacecraft's docking mechanism mates directly with the ISS. The tug is then free to go or may remain to remove the supply spacecraft.
The visiting vehicle must have an ISS compatible docking mechanism and also have appropriate fixtures for the tug to attach to the supply spacecraft. In the case of a supply vehicle needing to perform a fuel transfer, the appropriate ISS port would be a port with a Common Berthing Mechanism (CBM) or the Russian Probe and Cone Mechanism (RPCM).
The ISS is also equipped with an Androgynous Peripheral Attachment System (APAS) which is used by the Shuttles. The ISS is equipped with 2 such mechanisms. It is notable that the both the CBM and APAS are no longer manufactured and that a limited number are available to be used on supply spacecraft. The preservation of these mechanisms is highly desirable. After resupply, the current roster of supply spacecraft deorbit and burn up in the atmosphere (e.g., HTV, ATV, Progress), hence the mechanism for docking with the ISS is lost. A Crew Exploration Vehicle would deorbit and be recovered as are other manned spacecraft such as the Shuttle or Soyuz. However, the deorbit operations may require the docking mechanism be jettisoned.