Dava J. Newman

Flexibility in system design and implications for aerospace systems

The purpose of this paper is to review the concept of flexibility as discussed in various fields of investigations, to extract its characteristic features, and to explore its implications in the case of aerospace system design. In order to discuss any subject matter clearly, it is necessary to begin with a clear set of definitions. Indeed much can be gained through careful and consistent definitions of terms alone. Flexibility however is a word rich with ambiguity. While it is being increasingly used in various fields, few attempts have been made to formally define, quantify, and propose ways for achieving flexibility. This paper proposes to fill in part of this gap by synthesizing a clear and consistent definition of flexibility. It will do so by reviewing the usage of the term in various fields of inquiries, and show that it is indeed possible to clearly and unambiguously characterize flexibility, and to disentangle it from closely related concepts.

Flexibility and the value of on-orbit servicing: New customer-centric perspective

A new customer-centric perspective on on-orbit servicing, where the value of on-orbit servicing is studied independently from its cost, is proposed. A framework is developed that captures the value of e exibility provided by on-orbit servicing to space systems. Several options are made available to space missions through on-orbit servicing, such as the option to service for life extension or to upgrade, that need not be set before launch; they can be exercised after the spacecraft has been deployed, depending on how events unfold (market changes, new military contingency, etc. ). It is argued that only by accounting for this e exibility that the true value of on-orbit servicing can beevaluated. Applications of this framework to both nonproe t and commercial systems areprovided that demonstrate the usefulness of this new perspective on on-orbit servicing.

Spacecraft Design Lifetime

A general discussion of issues that drive and limit spacecraft design lifetime is presented. The effects of varying the spacecraft lifetime requirement on different subsystems are explored, and typical spacecraft mass andcost profiles are deduced. Quantitative analyses confirm that the design lifetime is a key requirement in sizing various subsystems and significantly affects the spacecraft mass and cost to initial operating capability. The analysis introduces a formally defined economic metric, the cost per operational day, to help guide the specification of the design lifetime requirement. Preliminary results suggest that other factors should also be taken into account in specifying the design lifetime, namely, the loss of value resulting from technology obsolescence as well as the volatility of the market the system is serving in the case of a commercial satellite.

Altered astronaut lower limb and mass center kinematics in downward jumping following space flight

Abstract Astronauts exposed to the microgravity conditions encountered during space flight exhibit postural and gait instabilities upon return to earth that could impair critical postflight performance. The aim of the present study was to determine the effects of microgravity exposure on astronauts’ performance of two-footed jump landings. Nine astronauts from several Space Shuttle missions were tested both preflight and postflight with a series of voluntary, two-footed downward hops from a 30-cm-high step. A video-based, three-dimensional motion-analysis system permitted calculation of body segment positions and joint angular displacements. Phase-plane plots of knee, hip, and ankle angular velocities compared with the corresponding joint angles were used to describe the lower limb kinematics during jump landings. The position of the whole-body center of mass (COM) was also estimated in the sagittal plane using an eight-segment body model. Four of nine subjects exhibited expanded phase-plane portraits postflight, with significant increases in peak joint flexion angles and flexion rates following space flight. In contrast, two subjects showed significant contractions of their phase-plane portraits postflight and three subjects showed insignificant overall changes after space flight. Analysis of the vertical COM motion generally supported the joint angle results. Subjects with expanded joint angle phase-plane portraits postflight exhibited larger downward deviations of the COM and longer times from impact to peak deflection, as well as lower upward recovery velocities. Subjects with postflight joint angle phase-plane contraction demonstrated opposite effects in the COM motion. The joint kinematics results indicated the existence of two contrasting response modes due to microgravity exposure. Most subjects exhibited “compliant“ impact absorption postflight, consistent with decreased limb stiffness and damping, and a reduction in the bandwidth of the postural control system. Fewer subjects showed “stiff“ behavior after space flight, where contractions in the phase-plane portraits pointed to an increase in control bandwidth. The changes appeared to result from adaptive modifications in the control of lower limb impedance. A simple 2nd-order model of the vertical COM motion indicated that changes in the effective vertical stiffness of the legs can predict key features of the postflight performance. Compliant responses may reflect inflight adaptation due to altered demands on the postural control system in microgravity, while stiff behavior may result from overcompensation postflight for the presumed reduction in limb stiffness inflight.

Heuristic Evaluation of a Web-based Interface for Internet Telemedicine

A low-cost usability engineering methodology (heuristic evaluation combined with small-scale expert assessment) is examined in the context of the design and development of a Web-based telemedicine system. Six experts - three human-computer interaction (HCI) experts and three medical-content experts - examined the Spacebridge to Russia Web site for usability. The HCI experts identified 52 interface problems using a set of ten usability criteria or heuristics; these problems ranged in severity from cosmetic to a major failure. The content experts completed a series of six simple tasks while describing their actions. The usage difficulties were related to the HCI problems identified and were primarily characterized by a mismatch of the designer model and the content expert model. This heuristic/usage methodology can provide an incremental benefit in a variety of other design activities. It is suggested herein that the combined heuristic/usage methodology should be included as a standard design component of dynamic telemedicine systems.

Technical developments of functional electrical stimulation to correct drop foot: Sensing, actuation and control strategies

This work presents a review on the technological advancements over the last decades of functional electrical stimulation based neuroprostheses to correct drop foot. Functional electrical stimulation is a technique that has been put into practice for several years now, and has been shown to functionally restore and rehabilitate individuals with movement disorders, such as stroke, multiple sclerosis and traumatic brain injury, among others. The purpose of this technical review is to bring together information from a variety of sources and shed light on the fields most important challenges, to help in identifying new research directions. The review covers the main causes of drop foot and its associated gait implications, along with several functional electrical stimulation-based neuroprostheses used to correct it, developed within academia and currently available in the market. These systems are thoroughly analyzed and discussed with particular emphasis on actuation, sensing and control of open- and closed-loop architectures. In the last part of this work, recommendations on future research directions are suggested.

Geologic Traverse Planning for Planetary EVA

We describe a process for planning planetary extravehicular activity (EVA) traverses. It enables predictive and parametric analysis of planned traverses, and would improve uncertainty management and realtime replanning of traverses by space-suited astronauts. Using a traverse from the Apollo 14 mission as a case study we show how the same traverse might have benefited from our new approach to EVA planning. Finally, we discuss operational implementation challenges based on our experiences in the development of digital tools for geologic mapping, and on past experimentation with the NASA class III ground suit.

Partial weight suspension: a novel murine model for investigating adaptation to reduced musculoskeletal loading

We developed a new model of hypodynamic loading to support mice in chronic conditions of partial weight bearing, enabling simulations of reduced gravity environments and related clinical conditions. The novel hardware allows for reduced loading between 10 and 80% of normal body weight on all four limbs and enables characteristic quadrupedal locomotion. Ten-week-old female BALB/cByJ mice were supported for 21 days under Mars-analog suspension (38% weight bearing) and compared with age-matched and jacketed (100% weight bearing) controls. After an initial adaptation, weight gain did not differ between groups, suggesting low levels of animal stress. Relative to age-matched controls, mice exposed to Mars-analog loading had significantly lower muscle mass (-23% gastrocnemius wet mass, P < 0.0001); trabecular and cortical bone morphology (i.e., trabecular bone volume: -24% at the distal femur, and cortical thickness: -11% at the femoral midshaft, both P < 0.001); and biomechanical properties of the femoral midshaft (i.e., -27% ultimate moment, P < 0.001). Bone formation indexes were decreased compared with age-matched full-weight-bearing mice, whereas resorption parameters were largely unchanged. Singly housed, full-weight-bearing controls with forelimb jackets were largely similar to age-matched, group-housed controls, although a few variables differed and warrant further investigation. Altogether, these data provide strong rationale for use of our new model of partial weight bearing to further explore the musculoskeletal response to reduced loading environments.

Low Spring Index NiTi Coil Actuators for Use in Active Compression Garments

This paper describes the modeling, development, and testing of low spring index nickel titanium (NiTi) coil actuators designed for use in wearable compression garments, and presents a prototype tourniquet system using these actuators. NiTi coil actuators produce both large forces (>1 N) and large recoverable displacements (>100% length) that are well suited for compression garment design. Thermomechanical coil models are presented that describe temperature and force as a function of nondimensionalized coil geometry, extensional strain, and applied voltage. These models suggest that low spring index coils maximize activation force, and an analytical model is presented to predict garment counter-pressure based on actuator architecture. Several low spring index (C= 3.08) coils were manufactured, annealed, and tested to assess their detwinning and activation characteristics. Results suggest both annealing and applied stress affect activation thresholds. Actuator force increases both with extensional strain and applied voltage up to 7.24 N. A first-generation compression tourniquet system using integrated actuators with direct voltage control of applied pressure is presented, demonstrating >70% increase in applied pressure during activation. This approach enables new, dynamic garments with controllable activation and low effort donning and doffing, with applications ranging from healthcare solutions to advanced space suit design.

Dynamic Understanding of Human-Skin Movement and Strain-Field Analysis

Understanding the skins material properties and natural motion is critical to a myriad of applications from tissue engineering to spacesuits. While there is an extensive understanding of human skin properties based on active tensile testing, both in vitro and in vivo, there is a little current knowledge of the strains experienced by skin during natural movements. Using a motion capture system, we have developed a new technique to measure skin movement and strain around the knee during a squatting motion. With these new data, we are also able to calculate the local direction of lines of nonextension, or contours of the skin that remain a constant length during motion, lines of minimum extension, and lines of minimum compression.