Feature: ARRM Prototype Legs Undergo Redesign and Fabrication
Detail of CRS concept demonstrator leg segments, joints, actuators, and sensors in Langley’s Technology Demonstrator Lab.
The latest design iteration is complete and fabrication underway at NASA’s Langley Research Center of a soon to be operational set of the Contact and Restraint Subsystem (CRS) Project’s prototype “legs”, which are being designed and developed for attachment to the Asteroid Redirect Robotic Mission’s (ARRM) capture module.
When looking at conceptual drawings of the ARRM capture module fitted with its CRS, one is immediately reminded of Erector Set structures and how those models could be changed, individualized, reformulated, and eventually become anything the imagination might draw forth.
Far from being a toy, ARRM’s capture module will be part of a human exploration mission involving in-space interaction with an asteroid boulder in the mid-2020s. The mission will provide systems and operational experience required for several key science areas: human exploration of Mars; demonstrating an advanced solar electric propulsion system; enhancing detection, tracking and characterization of near-Earth asteroids, enabling strategies to defend our home planet; and expanding our knowledge of small celestial bodies to possibly mine asteroids for resources enabling commercial and exploration needs.
Scott Belbin, a senior mechanical engineer with Langley’s Engineering Directorate, is technical lead for design and development of the CRS. Belbin worked analyzing alternatives to the so-called “capture bag” concept wherein a small (about 10 meters) free-floating asteroid would be captured in a deployable bag for redirection to lunar orbit, intended for astronaut extra-vehicular activity. He was subsequently asked to lead the boulder capture-module design effort during the comparison study between the original capture bag concept and what was eventually chosen, the mission that will land a robotic spacecraft on the surface of a larger near-Earth asteroid and collect a boulder of scientific interest.
During a tour of Langley’s fabrication shop, John Teter overviewed the redesign efforts currently underway. Here he demonstrates how and where the actuator nut mount will be attached to the leg sections.
“After a series of initial ideas and methods of landing and capturing the boulder were explored, the CRS was eventually settled upon as being able to meet the required criteria,” says Belbin. “As part of the capture module, we’ll be designing, fabricating and delivering engineering development units, and flight versions of the CRS, to Goddard for integration into the capture module, an effort being managed by the Flight Projects Directorate.”
Belbin says the CRS constitutes the legs of the spacecraft because they first act as landing gear, attenuating the vehicle’s momentum and bringing it to rest on a surface without using thrusters. The legs then push the vehicle and boulder off of the surface, again without thrusters, to an altitude where it is safe to thrust again.
“The asteroid’s surface is expected to be covered with regolith—fine, sharp particles created in the collisional processes asteroids experience—that can coat solar arrays and degrade performance or get into mechanical joints causing premature failure,” explains Belbin. “Once spaceborne, the CRS restrains the boulder, like your legs around a beach ball, which frees up the robotic arms to conduct science and sample preparation on the trip to lunar orbit.”
One improvement over the CRS demonstrator—made of sheet metal not matching the stiffness of the flight design—is that the prototype legs are welded tube frames, a better parallel to the flight design. Coupled with actuators of similar layout as the flight actuators, the modified prototype legs will allow the team to verify
“Once verified, we’ll be able to run many more boulder shape and size cases than is possible with our physical representation,” says Belbin. “We’re also adding dampers and load cells to explore the various options we have for descent and contact, ranging from a failsafe unactuated landing (stiff-legged with and without dampers), open-loop control (so-called pre-canned motion programming) and force-feedback closed-loop control.”
Based on operational assessments with the earlier concept demonstrator legs, changes underway have a lot to do with the nuts and bolts of things, but also include going from a functioning two-dimensional approach, i.e., from a two-leg flat-floor concept demonstrator, to an operational three-dimensional three-leg flight system concept approach. Getting there calls for modification of a set of welded metal legs that were previously constructed as part of a pathfinder fabrication activity. For this design iteration, the project is taking an existing set of legs constructed for static display and manual manipulation and is transforming these legs into an operational system outfitted with electromechanical actuators, leg segment position sensors, and segment force sensors.
“The model’s construction is more complicated each time, and even gets changed during an iteration’s development,” says John Teter, a mechanical engineer instrumental in the design upgrades and also with Langley’s Engineering Directorate. “We take a model, evaluate test capabilities, equipment and overall functionality, and then identify refinements. Eventually you come to what you need to actually go to the asteroid.”
The base attaching the legs to the capture module is being constructed of a greater thickness material than originally planned for this iteration after some warping occurred when welding the sections together. The base itself curved, causing the pivot-tube holes to move out of alignment with one another. Base fabrication designs were adjusted from a half-inch thickness to a full inch. Machining the flat side of the base to precision measurements ensures the most accurate fit.
Great detail went into redesigning the joints because a different geometry exists for each link, and the joints are the location for encoders, the electrical sensors reading joint angle during motion, which provide feedback for actuators. These data are used to control the capture module’s positioning during a capture procedure.
Teter says it is particularly important that the force readings are correct. “Given that there is a range of possible compressive strength expectations for boulders that may be collected from an asteroid, with the low end of hardness being closer to chalk, we don’t want to use too much force restraining a boulder after collection or it maysimply crumble.”
Other specific changes being addressed include actuator adaptors and associated mounts, the base bracket pad eye, and incorporating spherical bearings for all joints.
Once completed, the three operational prototype legs will be attached to an adapter structure currently under design that will simulate the capture module deck interface. Going forward, this will then become a test bed to support system-level operational command and control scheme development. Another potential use is as a pathfinder to provide early-on assessment of landing and ascent capability at Langley’s Landing and Dynamics Impact Range (LanDIR) gantry facility.