EVA/Robotic Servicing in the Commercial Space Era

TitleEVA/Robotic Servicing in the Commercial Space Era
Publication TypeConference Paper
Year of Publication2015
AuthorsAkin DL
Conference Name45th International Conference on Environmental Systems
Date Published07/2015

With the start of commercial crew flight to International Space Station (ISS) in 2017, US-supplied human access to low Earth orbit (LEO) will again be available upon need, ideally for significantly less than the cost of a shuttle flight. Besides performing crew rotation to the ISS, commercial crew vehicles may be capable of a wide variety of missions previously accomplished by the shuttle program. Chief among these is in-orbit spacecraft servicing, which was performed to great effect throughout the three decades of shuttle flight operations. This paper examines the requirements for EVA/robotic collaborative servicing in the commercial crew era, chosen from the three vehicles initially in the NASA Commercial Crew competition (Boeing CST100, SpaceX Dragon, and Sierra Nevada Dream Chaser). All these vehicles offer the basic ability to transport crew and some cargo to the target servicing site in LEO, performing rendezvous and proximity operations as part of their basic design requirements. Initial versions of these vehicles lack some of the features that made the shuttle an ideal servicing platform, including large internal and external payload capability (in terms of both size and volume), a grappling manipulator, and EVA support via an airlock and logistics for multiple EVA sessions per flight. For each vehicle under consideration, accommodations were conceptualized and implemented in a solid modeling program to verify dimensions, kinematics, reach envelopes, and other necessary metrics for operational feasibility. Since none of these vehicles are currently equipped with an airlock, options for adding an airlock module were assessed for each system. Similarly, robotic system concepts were developed for each vehicle, based on the constraints of unpressurized cargo capacity for each design. Although three vehicles were originally in contention, this paper focuses primarily on the SpaceX Dragon and the SNC Dream Chaser vehicles. The Boeing CST100 details available to date do not have sufficient detail in the spacecraft’s service/propulsion module to allow assessment of potential volumes for servicing-related systems, and the operational scenarios for the two capsule designs are similar enough to justify focusing on the better-documented Dragon. The Dream Chaser, while not picked for further development under the NASA Commercial Crew program, is an interesting counterpoint to the capsule designs due to the complications of developing EVA and robotic systems concepts compatible with the strict mold line restrictions of a high-lift aerodynamic vehicle. Both vehicles provide adequate, if challenging, volumes for unpressurized cargo. Dragon has the “trunk” adapter behind the entry vehicle, which was designed from the outset for unpressurized storage. While Dream Chaser did not have planned external accommodations, the launch vehicle interface structure (LVIS) provides sufficient volume and attach points for basing robotic systems internal to that structure. While the Dragon trunk provides a significantly larger and more accommodating launch volume for robotic systems, a grappling manipulator would be required to “walk out” of the internal volume of the trunk. The grappling manipulators for both vehicles require additional degrees of freedom to stow in the allotted volume, but unstow to produce a 5-6 m reach capability when deployed. The Dragon version has the additional advantage of direct visibility from the Dragon viewing windows, while Dream Chaser internal control will of necessity be based on video camera feeds. The paper considers various approaches to providing airlocks for each vehicle. Dream Chaser can accommodate an internal airlock, although intrusions into the nominal aft pressurized volume due to external systems and tankage severely limited internal airlock dimensions. A design was also developed for a rigid external airlock, which also served as a mounting location for one or two 5-6 m serving arms. Although there may be options for an internal airlock in Dragon, it was felt to impose too many restrictions on the internal layout, so all airlock options for that vehicle to date were based on an inflatable airlock module.