previous day
next day
all days

View: session overviewtalk overview

07:30-08:30Continental Breakfast

Continental breakfast available at Palmer Commons.

08:30-09:00 Session 1: Welcome

Conference Opening with welcome by Incoming Dean of Engineering at the University of Michigan, Prof. Alec Gallimore

09:00-09:45 Session 2: Keynote
Unique Results and Lessons Learned from the TSS Missions
SPEAKER: Nobie Stone

ABSTRACT. The Tethered Satellite System (TSS) Program was a binational collaboration between NASA and the Italian space agency, ASI. NASA developed the Shuttle-based deployer and the 20 km long conducting tether. ASI developed a satellite especially designed for tethered deployment. Science Investigations were funded by both agencies. The TSS program resulted in two Space Shuttle missions, the TSS-1 mission carried on STS 46 in 1993; and a reflight mission, TSS-1R, carried on STS 75 in 1996. The goal of these missions was to elucidate the electrodynamics of a long, conducting tether moving in earth orbit through the geomagnetic field and ionospheric plasma. The TSS carried seven coordinated hardware investigations that provided in situ measurements of the effects of tether electrodynamics on the tether system and on its environmental ionospheric plasma. The physical, in situ measurements were supported by two ground-based electromagnetic wave investigations that were designed to observe TSS-induced perturbations at the bottom of magnetic field lines crossed by TSS; and theoretical investigations of tether dynamics and tether electrodynamics. The first objective of this presentation is to highlight technical findings enabled by the TSS missions, which were significant. TSS achieved a number of “firsts” and produced unique scientific data that proved the ability of electrodynamic tether systems to pro¬duce electrical power for spacecraft operations; and have fundamentally expanded our general understanding of spacecraft-space plasma interactions. The second objective is to address the technical issues suffered by TSS that eliminated the opportunity to perform of a number of planned experiments designed to address specific aspects of the physics of electrodynamic tethers in space. Some of these issues and the subsequent lessons learned have a broad range of application that extends beyond the specifics of space tethers to space missions in general.

09:45-10:45 Session 3: Electrodynamic Tether Propulsion (Missions and Mission Concepts)
Mission Overview of an Electrodynamic Tether Experiment on the H-II Transfer Vehicle (Invited)
SPEAKER: unknown

ABSTRACT. An on-orbit demonstration experiment of an electrodynamic tether (EDT) is planned in Japan Aerospace Exploration Agency (JAXA). This mission is called KITE (Konotori Integrated Tether Experiment) since the experiment is conducted on the H-II transfer vehicle (HTV), which is called “Konotori” in Japanese. KITE is the first step toward the development of active debris removal (ADR) systems using EDTs for remediating the debris problem. EDTs have many advantages that make them promising candidates for deorbit propulsion systems for ADR, including the absence of consumables, low electric power requirements, the absence of thrust vectoring, and easy attachment to debris. These advantages could save manufacturing and operation cost of the ADR system, and lowering the cost is one of the most important indices for realizing the continuous ADR activities in future. The primary objective of KITE is to demonstrate the key EDT technologies for ADR such as a net-shaped bare tether possessing high tolerance to small-debris impact and a field emission cathode (FEC) as a small and simple electron source. The 700-m-length bare tether, which is deployed from the HTV body toward the zenith, collects electrons from the ambient space plasma, and the FEC on the HTV emits 10-mA-level electrons into the plasma. This collector–emitter combination can provide complete propellant-free deorbit propulsion for ADR. The mission will commence after the HTV leaves the international space station, and the planned mission period is 7 days before the re-entry of the HTV. An outline of the sequence is the tether deployment, measurement of tether dynamics and tether voltage, measurement of HTV electrical potential with and without FEC operation, measurement of EDT current–voltage characteristics at various operating conditions, autonomous EDT operation, measurement of change in tether vibration amplitude by EDT thrust, and tether severing at the termination of the mission. Devices and instruments for KITE are to be installed on various locations on the HTV. The major components are the tether, reels for tether housing and braking, end-mass, release mechanism of the end-mass, camera for tether dynamics observation, FEC, electrical potential monitor, magnetic sensor, and data handling unit/power control unit. The flight components are currently under development preparing for the launch of the HTV-6 planned in late 2016.

An Update on EDDE, the ElectroDynamic Delivery Express (Invited)
SPEAKER: unknown

ABSTRACT. "EDDE" (the ElectroDynamic Delivery Express) is a new kind of non-rocket space vehicle. It is solar-powered, propellantless, and persistently maneuverable throughout low earth orbit. EDDE consists mostly of a reinforced aluminum foil tape to collect and conduct electrons, plus solar arrays to drive this current. Hot wires emit electrons back into the ambient plasma. The maneuver force comes from the tape current crossing geomagnetic field lines. The ambient plasma closing the current loop sees an opposite reaction force. EDDE slowly rotates end-over-end to stiffen it. This allows sustained high thrust without dynamic instability. Rotation also improves agility, by allowing a wider range of thrust directions normal to both the EDDE tape and the magnetic field. EDDE is modular and typically weighs 30 to 80 kg for most missions. Air drag sets a minimum altitude near ISS altitude (350-420 km). There is no hard ceiling, but thrust decreases at high altitude. A key benefit of EDDE to secondary payloads is “custom orbits without dedicated launch." The benefit to launch providers is making surplus LEO payload capacity relevant to a much wider range of secondary payloads. EDDE’s total orbit change capability far exceeds what is needed for any single orbit change in LEO. After releasing its payloads, EDDE can inspect failed satellites in multiple orbits, and image impact features and other visible anomalies. With suitable interfaces, EDDE endmasses can also capture payloads. This lets EDDE be a “LEO taxi” that customers rent rather than buy. One frequent customer may be satellite service vehicles, which usually need far larger orbit changes between satellites in LEO than in GEO. EDDE can also rendezvous with and capture ton-class orbital debris in nets. It can then drag it down to short-lived orbits below ISS, or collect it in tethered assemblies at less congested altitudes, for later recycling and/or targeted deorbit. This paper describes EDDE design, components, and operations, the applications discussed above, and flight test plans.

REVIEW: The PROPEL Electrodynamic Tether Demonstration Mission
SPEAKER: unknown

ABSTRACT. The PROPEL (“Propulsion using Electrodynamics”) flight demonstration mission concept will demonstrate the safe operation of an electrodynamic tether (EDT) for generating thrust, which will allow the propulsion system to overcome the limitations of the rocket equation. The mission concept has been developed by a team of government, industry, and academia partners led by NASA Marshall Space Flight Center (MSFC). PROPEL has two primary goals: (1) to demonstrate capability of EDT technology to provide robust and safe, near-propellantless propulsion for orbit-raising, de-orbit, plane change, and station keeping, as well as to perform orbital power harvesting and formation flight; and (2) to fully characterize and validate the performance of an integrated EDT propulsion system, qualifying it for infusion into future multiple satellite platforms and missions with minimum modification.

PROPEL is being designed to be a versatile electrodynamic-tether system for multiple end users and to be flexible with respect to platform. To explore a range of possible implementation configurations, driven primarily by cost and launch vehicle considerations, other mission concept designs are being pursued such that a reduced system can be demonstrated should a flight opportunity be identified. The implementation options being explored include a comprehensive mission design for PROPEL with a mission duration of six months; a space demonstration mission concept design with configuration of a pair of tethered satellites, one of which is the Japanese H-II Transfer Vehicle; and an ESPA-based system.

10:45-11:05Coffee Break
11:05-12:45 Session 4: Electrodynamic Tether Propulsion
REVIEW: Analysis of Electrodynamic Tethers for Orbit Maneuvering, Deorbit, and Power Generation (Invited)
SPEAKER: Robert Hoyt

ABSTRACT. Electrodynamic tether (EDT) propulsion can enable space missions to escape the limitations of the rocket equation by providing both extremely high specific impulse and high thrust-to-power levels for maneuvering and stationkeeping within low Earth orbit (LEO). This capability could be of particular benefit for orbital maintenance and maneuvering of large systems in LEO, such as the International Space Station (ISS) or orbital cargo tugs, where electrodynamic propulsion could dramatically reduce or even eliminate the recurring cost associated with reboost propellant supply. EDTs can also provide a low-mass solution for end-of-life disposal of LEO spacecraft. Additionally, EDTs can be used for power generation by converting orbital energy into electrical power. In this presentation, we will present results of analyses performed using detailed, physics-based simulation tools to evaluate EDT performance for several mission applications, including an orbital-transfer tug, de-orbit modules, and an 'orbital battery.'

Electrodynamic Tether Propulsion Technology for Active Debris Removal
SPEAKER: Kentaro Iki

ABSTRACT. The increase in the orbital debris number has been becoming a serious problem for human space activities. One of the effective strategies to suppress space debris growth is an active removal  of already existing large debris in the crowded low earth orbits. Toward the realization of cost-effective active debris removal (ADR) in the lower earth orbits, the Japan Aerospace Exploration Agency (JAXA) has been conducting research and development of electrodynamic tether (EDT) as a very promising candidate of propulsion for deorbiting in ADR. EDT is an advanced propulsion system which can generate sufficient thrust for orbital transfers without the need for propellant by utilizing interactions between Earth’s magnetic field and currents through the tether. This paper describes the fundamentals of the electrodynamic tether including its advantages and disadvantages for ADR. In addition, for the preparation of ADR system demonstration, we introduce the characteristics of the key EDT system components, and the results of the numerical simulations of tether deployment and de-orbit by EDT thrust using our system.

[brief biographical sketch] Mr. Iki is currently a researcher of JAXA. His research includes Electrodynamic tether technology for active debris removal. He is also a project member of the Kounotori Integrated Tether Experiments (KITE) project, which is planned for demonstration of EDT toward the realization of debris removal by JAXA.

LESSONS LEARNED: Development of the PROSEDS Electrodynamic Tether Mission
SPEAKER: unknown

ABSTRACT. The Propulsive Small Expendable Deployer System (ProSEDS) space experiment was ready to fly as a secondary payload on a Delta–II expendable launch vehicle in late March 2003. Concerns raised by the International Space Station after the February 2003 Columbia shuttle accident resulted in the delay of the launch of ProSEDS. Issues associated with both the delayed launch date and a change in starting altitude resulted in the ultimate cancellation of the mission. ProSEDS was intended to deploy a tether (5-km bare wire plus 10-km non-conducting Dyneema) from a Delta–II second stage to achieve adequate electrodynamic drag thrust that would lower the orbit of the system over days—as opposed to months due to atmospheric drag. The experiment was also designed to utilize the tether-generated current to demonstrate the ability to generate spacecraft power. Considerable effort and testing went into developing the ProSEDS system by a dedicated team. Throughout this effort, important technological issues were identified and addressed and here we review many of the lessons learned and some of the important technical issues and hurdles that had to be addressed to successfully prepare for flight. It is intended that this information will be of use for future tether mission and experiment designers.

REVIEW: Investigating the Potential of Miniature Electrodynamic Tethers to Provide Propulsion to Picosatellites and Femtosatellites
SPEAKER: unknown

ABSTRACT. The next generation of satellites may be as small as modern day smartphones. Satellites at this scale are called picosatellites (100 g–1 kg) and femtosatellites (<100 g), or “picosats” and “femtosats” for short. Due to their small size and weight, they can be many times less expensive to launch into orbit than traditional large spacecraft. As a result, picosats and femtosats can enable unique missions requiring large numbers of spacecraft distributed in space and/or time. However, these extremely small satellites tend to fall out of orbit quickly, with lifetimes ranging from hours to months, which limits their use. Also, modern “smartphone”-sized picosats and femtosats lack coordination and control, so they would behave more like an uncontrolled swarm of “space junk” rather than a coordinated, useful formation.

Electrodynamic tethers (EDTs) are an advanced spacecraft propulsion technology capable of unlocking the potential of picosats and femtosats. In this paper, I will share progress on trade studies that explore the feasibility of using short EDTs for propulsion of pico- and femtosats. Additionally, I will describe the Miniature Tether Electrodynamics Experiment (MiTEE) space mission being developed at the University of Michigan to test the fundamental concept of miniature EDTs. I will also discuss potential applications of this novel tether concept.

Comparison of technologies for de-orbiting spacecraft from Low-Earth-Orbit at end of mission

ABSTRACT. An analytical comparison of four technologies for de-orbiting spacecraft from Low-Earth-Orbit at end of mission is presented. Basic formulae based on simple physical models of key figures of merit for each device are derived. Active devices - rockets and electrical thrusters - and passive technologies - drag augmentation devices and electro-dynamic tethers - are considered. A basic figure of merit is the de-orbit device-to-spacecraft mass ratio, which is, in general, a function of environmental variables, technology development parameters and de-orbit time. For typical state-of-art values, equal de-orbit time, middle inclination and initial altitude 750 km, it is shown that tethers are about one and two orders of magnitude lighter than active technologies and drag augmentation devices, respectively; a tether needs a few percent mass-ratio for a de-orbit time of couple of weeks. For high inclination, the performance drop of the tether system is moderate: mass ratio and de-orbit time increase by factors of 2 and 4, respectively. Beyond collision risk with other spacecraft and system mass considerations - probably main driving factors for de-orbit space technologies -, the analysis addresses other important constraints, like de-orbit time, system scalability, manoeuver capability, reliability, simplicity, attitude control requirement, and re-entry and multi-mission capability (de-orbit and re-boost) issues. They are used to construct an evaluation matrix and make a critical assessment of the four technologies as function of spacecraft mass and initial orbit (altitude and inclination). Emphasis is made on electrodynamic tethers, including the latest progresses advanced in the FP7/Space project BETs. The superiority of tape tethers as compared to round and multi-line tethers in terms of deorbit mission performance is highlighted, as well as the importance of an optimal geometry selection, i.e. tape length, width, and thickness, as function of spacecraft mass and initial orbit. Tether system configuration, deployment and dynamical issues, including a simple passive way to mitigate the well-known dynamical instability of electrodynamic tethers, are also discussed.


Lunch provided in the Palmer Commons.

14:00-14:45 Session 5: Keynote
The Long Slog toward Technology Flight Demonstration – One PI’s Journey and Lessons Learned
SPEAKER: Les Johnson

ABSTRACT. Demonstrating a new technology in space is more than rocket science. The process isn’t well defined, thus requiring tenacity and sometimes involving just plain luck. A good, technically feasible idea is essential; one that aligns with a stated mission need is highly desirable. If the alignment doesn’t exist, then you need to create it and socialize it among the stakeholders. Next comes the formation of the team, writing a winning proposal and being selected for funding. Then arrives the final part: developing the spacecraft hardware and delivering for launch – but not so fast. A project isn’t really ‘sold’ and successful until it has flown, returned all the possible data, and all of the project’s results are published. (A ‘lesson learned’ from personal experience having worked a flight project that was canceled one month before launch!)

14:45-16:05 Session 6: Momentum Exchange Tether Propulsion and Space Tugs
REVIEW: Analysis of Momentum Exchange Tethers for NEA Capture, Debris Removal, and Translunar Injection
SPEAKER: unknown

ABSTRACT. We present results of studies funded by two different NASA Innovative Advanced Concepts (NIAC) and NASA SBIR efforts that evaluated the value proposition for using momentum exchange tethers in place of traditional rocket-based propulsion. The NanoTHOR effort developed a concept for using a simple tether system to enable a vehicle in geostationary transfer orbit to toss a nanosatellite to a lunar transfer trajectory. The WRANGLER effort evaluated the feasibility of using a tethered nanosatellite to capture and de-spin a massive near Earth asteroid. In each effort, we have used detailed, physics-based simulation tools to develop and evaluate methods for controlling deployment and/or retrieval of momentum exchange tethers to accomplish complex, large delta-V maneuvers that traditionally would be performed using propellant-based thrusters. We will present results of these simulations, and discuss quantitative value proposition analyses of the tether methods relative to traditional propulsion systems.

Modeling of Active Tether Systems for Planetary Exploration

ABSTRACT. Extreme planetary environments represent the next frontier for in-situ robotic space exploration. Missions for exploration will be followed by robotic missions for exploitation, and by manned missions. All these missions have one common problem: highly irregular topography, heterogeneous surface properties (soft, hard), harsh, extreme environments, where temperature, radiation, and other factors make the missions inconceivable at present. Furthermore, the diverse geologic sites require versatile in-situ science that can adapt to the local geology and environmental conditions. An Active Tether System (ATS) shows much promise to enable new types of missions with lower risk sampling operations (being far away from the surface), higher rate of science data quality and return (samples with stratigraphy, sub-surface samples), and much more agility (sampling operations can be repeated multiple times at multiple locations without landing). In a multistage architecture, the ATS becomes highly scalable, and represents an advantage over existing asset deployment and sample capture mission operations because it has the potential of further decoupling the end-effector operation from the spacecraft operation during the target interaction phase, thus enabling many new missions. One example of application could be the following: imagine approaching an asteroid and being able to reach the surface to deliver an asset or to collect a sample without ever having to land. By phase-transitioning its material characteristics in a multi-segmented boom, a long appendage (up to a few hundred meters long) changes its shape and its compliance actively to conform to any surface irregularity in any body of the Solar System. By eliminating complex proximity operations near the surface, this intelligent system is the new way to interact at-a-distance with primitive bodies and bring pristine soil samples back to Earth. In this paper, we investigate the modeling aspects of a tether system, which, through changing equilibrium phases in the material, is able to change its compliance in response to different external stimuli. The paper approaches this complex problem sequentially. The first step is the static and dynamic characterization of the component behavior of an ATS element. Some phase-transitioning materials that are considered are piezoelectric materials, electro-rheological materials, electro-active polymers, photo-strictive and magneto-strictive materials. The second step is to investigate the system-level behavior under closed loop control, which is dependent on the scenario of application. To achieve the full potential of distributed actuation, it is necessary to develop models that characterize the hysteretic nonlinearities inherent in the constituent materials, as well a distributed sensing methodology. We have investigated models that quantify the nonlinearities and hysteresis inherent to phase transition, each in formulations suitable for subsequent control design. These models involve first-order, nonlinear ordinary differential equations and require few parameters that are readily identifiable from measurements, hence we have selected to use these differential models in our analysis. To investigate the system-level implications of using this concept, a multibody dynamics simulation of the system behavior of the entire vehicle during sample captures has been developed and tested in a simulation environment. More details will be discussed in the paper.

Study of Dynamical Stability of Tethered Systems during Space Tug Maneuvers
SPEAKER: unknown

ABSTRACT. The dynamics of a space tether system composed of one active spacecraft, an uncontrolled large debris (e.g., a defunct satellite), and a visco-elastic tether connecting the two bodies is investigated in this paper. The active spacecraft is assumed to be equipped with a propulsive system for carrying out a tug maneuver that forces the orbital decay of the debris. The dynamical stability and the eigenfrequencies of the tethered system under the action of the thrust are investigated with both numerical and analytical models. A more complex numerical lumped-masses model provides the reference to validate the results hailing from the simplified models. Simplified models of orbital decay, tether, and debris attitude motions were derived using the Clohessy-Wiltshire equations. The results obtained with the simplified models fit very well those from the lumped-masses model for a wide range of initial conditions. Thanks to the analytical models two resonance conditions were found, both of them affecting the attitude dynamics of the debris, that could represent a serious issue for the safety of the tug maneuver. Also, an instability mechanism that could induce the dual mass system to rotate around its center of mass under certain conditions was identified. These findings make possible to pinpoint the set of initial conditions of the tethered system at the beginning of the thrust event that provides a dynamically stable tug maneuver for different configurations of the system (e.g., low/high thrust, stiff/elastic tethers).

16:05-16:25Coffee Break
16:25-17:45 Session 7: Formation Flying and Enabled Missions
Study of tether- and wing-based balloon guidance system for extra-terrestrial exploration
SPEAKER: unknown

ABSTRACT. Abstract: Exploration of the surface of planets and moons, such as Mars or Titan, have the potential to help scientists unlock the secrets of the universe. So far, extraterrestrial missions have utilized probes in orbit and rovers on the surface to explore new areas of planets and moons. While these platforms return valuable data, large expanses of terrain remain unexplored due to their limitations. Such limits include restrained range of travel and degradation due to weather and dust for rovers, and limited on-board instrument resolution and ground coverage for orbiting probes. A tether/balloon-based system, as described below, combines greater access with lower altitude to enable more flexible and detailed investigations of planetary surfaces.

One platform which shows promise for exploring large areas on other worlds is a scientific balloon tethered to a sail consisting of a wing and a rudder. This concept, initially proposed by NASA Goddard Space Flight Center and Global Aerospace Corporation in the 1990’s and early 2000’s, utilizes a system whereby the balloon motion is influenced by the wind and the lift force resultant from the aerodynamics of the sail, tethered kilometers below. By varying the angle of attack of the sail, significant control of the system may be obtained. This controllable platform has a significant advantage over previous balloon missions, such as the Vega 1 and Vega 2 balloons used to explore Venus’s atmosphere, because random wind currents are no longer the sole driver of the direction of the balloon.

This paper will present an overview of various planets and moons capable of being explored by such a system and recommendations for the balloon and sail system best-suited for each planetary body. Next, the work presents a series of questions which have yet to be answered in the open literature regarding the design and testing of such a system to date. Finally, the work details the ongoing terrestrial experiments (e.g. terrestrial atmospheric flight tests) being conducted to answer these questions by the North Carolina State University Engineering Mechanics and Space Systems Laboratory, as well as future work currently being planned.

Biographical sketch: Christopher D. Yoder is currently a graduate student pursuing a degree in Aerospace Engineering at North Carolina State University in Raleigh, North Carolina. He works in the Engineering Mechanics and Space Systems Laboratory (EMSSL) under the direction of Dr. Andre P. Mazzoleni pursuing research interests regarding tether modeling as it relates to tethered systems, both within and above the atmosphere. Christopher currently holds a dual degree in Mechanical Engineering and Physics from the University of North Carolina at Charlotte. During his education to date, Christopher has gained experience using Matlab to run numerical simulations of dynamic systems while also becoming a Certified LabView Associate Developer for his use of LabView for data collection during experiments.

Rationale and Design for a Manned Partial-Gravity Research Facility

ABSTRACT. Since before the space age there has been periodic interest in manned space facilities that rotate to provide artificial gravity. Early interest was based on a lack of knowledge about the effects of sustained microgravity on astronauts. Current interest is based on multiple negative health trends in microgravity that give no sign of leveling off, even after many months. Diet, exercise, and drug “countermeasures” can reduce these trends, but none have been able to stop any of them.

Studies of artificial gravity usually assume full earth gravity, for deep-space expeditions or free-space settlements. The focus here is very different: the biological effects of sustained Mars and Moon gravity levels on humans and supporting ecosystems. Surprisingly, all 13 bodies in our solar system with surface gravity of 0.09 to 2.3X earth gravity cluster near Earth, Mars, and Moon gravity levels. Five of the 8 planets have 88-110% of earth gravity, Mercury and Mars both have 38% of earth gravity, and the 6 largest moons in our solar system all have 13-18% of earth gravity. The 8 bodies with ~Moon or Mars gravity are not all easily habitable, but they do include most of the interesting candidates for exploration or settlement with substantial gravity.

We already know the effects of earth gravity. The main value of partial-gravity biology research is to affordably determine the effects of sustained Moon and Mars gravity levels on humans. A suitable facility also allows follow-on research on ecosystems we will need, to live sustainably in partial gravity. This paper discusses rationales and research tasks, presents design concepts for a large “rotating dumbbell” facility, and describes precursor tests needed to resolve key questions like allowable rotation rate and hence dumbbell length. The paper explains why rotating-room tests seem unable to resolve this question.

A rotating dumbbell may also acquire other roles. Vehicles visiting such a facility can berth at a hub at the CG, or they can use “trapeze capture” at the end of a tether hanging out from either endmass. This lets visitors be captured from low-perigee orbits. Extending the tether to greater length at the end of a visit allows targeted passive deorbit and net facility momentum recovery. Tethered capture plus release can raise each visiting vehicle’s payload by ~10%. This may make such a facility enough cheaper to visit that it becomes the preferred transport node in low earth orbit. Easy targeted passive deorbit also enables frequent sample return to earth. This can make the facility attractive for all iterative research that benefits from frequent sample return, including low-gravity biology and materials research that can be done at the central hub.

A partial-gravity research facility might be a commercial venture supporting a large international crew focused mostly on human and other biological research. It can shed light on what kind of future we may have on the 8 bodies with gravity near that of the Moon or Mars. It can also tell us the implications of using partial gravity in rotating free-space settlements.

Tethered CubeSat Mission Investigating System Dynamics and Interferometric Techniques
SPEAKER: unknown

ABSTRACT. Several experiments involving tethered satellite systems have been conducted to date, most notably: the TSS and TSS-1R experiments (NASA) and MAST (Stanford/Tethers Unlimited Inc.). In an attempt to continue this important research, the Engineering Mechanics and Space Systems Laboratory at North Carolina State University is developing a 3U CubeSat mission called PACKSAT (PArallax CubeSat Kinetics SATellite) in which three 1U CubeSats will be separated by tethers in order to perform various tether-based experiments. The purpose of this paper is to describe the mission design and discuss the value of results that may be obtained from the experiments.

The primary science objectives of PACKSAT are to investigate the dynamics of three CubeSats connected by space tethers, and conduct science experiments which take advantage of the fact that the CubeSats will be separated from each other (enabling interferometric techniques), and yet be in the same orbit (due to the tethers). Interferometry is the practice of using two or more widely separated telescopes to observe a source with greater resolution than can be accomplished by a single telescope. The vehicle will incorporate a novel, miniaturized radio interferometer to demonstrate acquisition of high resolution astronomical data in the ultra-long wavelength (ULW) portion of the electromagnetic spectrum. Successful implementation of this technology can pave the way for research into ULW astronomy which is essentially the last unexplored area in the field of astronomy. In regards to the tether dynamics investigation, PACKSAT will perform direct testing of tether deployment, tether assisted gravity gradient stabilization, and tethered formation flying. Tether deployment is one of the most important areas where more data is needed as this is the phase where the majority of tethered missions experience some kind of failure. There are a large number of complex dynamical responses predicted in theory associated with the deployment of an orbiting tethered system. Therefore, it is imperative to conduct an experiment that provides data on what dynamic responses actually occur. The information that can be gleaned from such experiments is vital to continued development of tethered space systems, and when the PACKSAT experiment is launched in 2017 (anticipated) the goal is to successfully obtain and publish this data.

Simulation of a Tethered Microgravity Robot Pair and Validation on a Planar Air Bearing

ABSTRACT. A software model has been developed to simulate the on-orbit dynamics of a dual-mass tethered system where one or both of the tethered spacecraft are able to produce propulsive thrust. The software simulates translations and rotations of both spacecraft, with the visco-elastic tether being simulated as a lumped-mass model. Thanks to this last feature, tether longitudinal and lateral modes of vibration and tether tension can be accurately assessed. Also, the way the spacecraft motion responds to sudden tether tension spikes can be studied in detail. The code enables the simulation of different scenarios, including space tug missions for deorbit maneuvers in a debris mitigation context and general-purpose tethered formation flight missions. This study aims to validate the software through a representative test campaign performed with the MIT Synchronized Position Hold Engage and Reorient Experimental Satellites (SPHERES) planar air bearing system. Results obtained with the numerical simulator are compared with data from direct measurements in different testing setups. The studied cases take into account different initial conditions of the spacecraft velocities and relative attitudes, and thrust forces. Data analysis is presented comparing the results of the simulations with direct measurements of acceleration and Azimuth rate of the two bodies in the planar air bearing test facility using a Nylon tether. Plans for conducting a microgravity test campaign using the SPHERES satellites aboard the International Space Station are also being scheduled in the near future in order to further validate the simulation using data from the relevant operational environment of extended microgravity with full six degree of freedom (per body) motion.