. SALSSA: SPACE ASSEMBLY OF LARGE STRUCTURAL SYSTEM ARCHITECTURES

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BUILDING THE FIRST SPACEPORT IN LOW EARTH ORBIT

 
 

 
 
 
 
 
 
 

In-Space Robotic Manufacturing and Assembly

An overview of preparations for the construction of Space Station Freedom (SSF) is presented. The video includes footage of astronauts testing materials for erectable structures in space both in the Shuttle bay while in orbit and in a neutral buoyancy tank at McDonald Douglas' Underwater Test Facility. Also shown are footage of robot systems that will assist the astronauts in building SSF, a computer simulation of an Orbiting Maneuvering Vehicle, solar dynamic mirrors that will power SSF, and mockups of the living quarters of the SSF.

If a satellite suffers a mechanical or electronic failure or runs out of fuel, it is abandoned because no space agency has the capability to service satellites in orbit. Scientists at the West Virginia University Robotic Technology Center are trying to solve part of that problem. VOA's George Putic reports. Originally published at - http://www.voanews.com/a/regular-refu...

SALSSA:  SPACE  ASSEMBLY  OF  LARGE  STRUCTURAL  SYSTEM  ARCHITECTURES 

Having a solid capability for in-space assembly of spacecrafts or space systems will enhance performance of space missions, while could reduce theirs costs. This is exactly the purpose of SALSSA, a concept for Space Assembly of Large Spacecraft Structural System Architectures.

Until today and, except for the construction of the International Space Station  (ISS), spacecrafts was transported in orbit as an integrated unit using a single launch, while limited theirs mass and sizes. Then, its design must responded to the mass of the chosen launch vehicle, the volume of its payload shroud and the loads imposed by its environment.

Once in space, systems as solar arrays, radiators and antennas are deployed to achieve an operational configuration. An example of that is the James Webb Space Telescope (JWST) with its 6.5 meters of primary mirror. This latter size is the upper limit of the aperture that can be achieved for a single-launch telescope using deployable structure and mechanisms.

James Webb Space Telescope (JWST) Launched from the Rocket Ariane 5

The risk of failure become from the rise of number of deployable mechanisms and systems, while decrease spacecraft's mission reliability. Although an on-orbit servicing  and repair capability would help to mitigate those effects, a more innovative and potentially less costly approach is to incorporate a in-space assembly (ISA). Similar to the one used for the ISS assembly, this concept could leverage a variety of launch vehicles and multiple launches available.

Robotic Matter

For in-space robotic operations, the capacity to manipulate large masses was present with the Shuttle Remote Manipulator System (SRMS) and the Space Station Remote Manipulator System (SSRMS), but the SRMS is no longer available and the SSRMS is limited to a single facility, in a specific orbit, with many restrictions imposed by NASA-ISS requirements.

Also, externally located on the ISS, the smaller and multi-armed dexterous manipulator, the Dextre robot, is currently supporting Goddart Space Flight Center (GSFC) satellite servicing experiments.

Then, the SRMS, SSRMS and Dextre are traditional manipulator architectures, consisting of lightweight booms connected by massive rotary joints that account for 85 to 90 percent of the manipulator mass and compliance.

In 2015, the NASA Game Changing Development Program (GCDP), a part of the Space Technology Mission Directorate (STMD), financed a small study to investigate and define the technologies needed to enable Space Assembly of Large Structural System Architectures or SALSSA. Its goal is to enable a new integrated space assembly paradigm that substantially improves the performance, while lowering the cost and risk of future missions.

Concept of Assembly/ Construction, Servicing/ Repair, Repurposing/ Refurbishing/ Recycling, and In-situ Manufacturing

Its Authors believe that, with this infrastructure it will be possible to assemble/disassemble spacecrafts or space components by joining them through a variety of methods. In the same way, make reparation, upgrading and enhancing spacecrafts or system to enabling them to continue to be functional or improving its functions. To do that, old components are replaced by new ones or repairing.

Modular components could also be improving, upgrading, reconfiguring and reusing, as well as, if desired, redefining the purpose of its system.

Once the architecture is in space at the spacecraft operational location (Lagrange points, example), manufacturing feedstock could be provided by launch from Earth, Moon or asteroids to make direct In-Situ Resource Utilization (ISRU) or recycling.

As see, each key capability area requires a supporting set of common or overlapping Technology Elements, while will generally be cross-cutting to all of the others capacities. For example, a robotic system that would assemble a spacecraft could also be used for servicing and repairs, reconfiguring that one at a later time or building a completely different vehicle.

The mean fact about this Study show that, the technologies needed to enable these keys can be grouped under the Modularity, autonomous Operations, Manipulation systems, Metrology and Verification and on-site Infrastructure. 

NASA-Mission: Megawatt Class Solar Electric Tug

Here, the challenges come from the spacecraft bus and its two large solar array wings that, each supply 250 kw to 500 kw of power to the ion engines. To achieve the large area required in a single deployable system, the arrays must be designed with sufficient structural stiffness to meet a frequency of 0.1 hertz, and a strength that can sustain a 0.1 g acceleration during boost using a chemical stage.

With the In-Space Assembly Concept, the solar array wings will be composed of a backbone truss, onto which solar array modules (20 kw to 30 kw each) can be attached. These trusses are sized so that the wings meet the spacecraft stiffness and strength requirements.

Made by a single fold-deployable square of 4-longeron that has a hinge joints and telescoping members in the diagonals, the backbone truss sill be assembly by robotics. To do so, it use a long reach manipulator to deploy a pair of truss bays sequentially. That will followed by a Intelligent Precision Jigging Robots (IPJRs), who set the geometry of the truss bays and weld the joints to achieve the structural integrity.

Example of megawatt class solar electric tug concept

As see, motors, mechanisms and latches associated with conventional deployable trusses are eliminates, while reduce mass, complexity and increases reliability.

A simple structural interface is pre-integrated at evenly spaced locations on the truss, where the the solar array modules can be attached. A long-reach manipulator would position a solar array module near the truss interface, while is grapple by a set of IPJRs, located and oriented at perpendicular position to the center of the truss face.

At this latter point, the IPJRs would then hold the solar array module in place while another manipulator holding an electron beam (E-Beam) tool welds the structural connection.

With this concept, the solar array modules could be removed for replacement or upgraded at a later date because welding is a reversible joining process, providing by an extension wiring bundle with a modular electrical-data interface. The manipulator plugs this into the pre-integrated central electrical-data wiring harness that runs down the interior of the backbone truss. Solar array module can be attached to each of the wing backbone trusses similarly in a repetitive process.  

NASA-Mission: Large Next-Generation Space Telescope

Here, the challenges is to maintain the high mirror surface precision and stability required for the large diameter main aperture. And, be able to deploy the large-expanse Sunshield, providing systems for refueling, mirror's maintenance and scientific instruments replacement and upgrade.

With the Orbit Assembly Concept, the primary aperture would be assembled from a series of integrated modules including support truss, mirrors, mirror mounting and control interfaces, power/data wiring, etc.

These modules would be sized such that they can be packaging in the payload of the launch vehicle fairing. Each module will include a deployable truss, mirrors, electronics, as well as power/data wiring. All will be pre-integrated and the mirror surface precision set before launch.

Example of large space telescope with sunshield

As modules are orbited, they could be aggregated at an assembly site anywhere in space, while could be could be a platform or a spacecraft bus that has robotic capabilities. That means equipped with long-reach grappling and manipulation, dexterous manipulation, precision jigging, and joining capacity such mechanical fastening, E-Beam welding or bonding.

The long-reach manipulator(s) and jigging robots would deploy each integrated telescope module and complete any required joining operations. As each module is deployed, the long-reach manipulator, in coordination with the jigging robots, would position and precisely align each module. A limited number of modular interfaces would be used for module-to-module joining with the goal being to use a simple mechanical connector in conjunction with aseparate data/power connector. After the jigging robots precisely align the modules, a long-reach manipulator with an appropriate joining end effector would complete the mechanical/welded/bonded joints.

NASA-Mission: Repairable, Replaceable, and Reusable Modules for Evolvable Mars Campaign (EMC)

The EMC have to be reusable and modulate. The challenge is to incorporate that into this concept and make things that would benefit its architectures, while simultaneously achieving the goal of developing multi-use and evoluable space infrastructure. One major area the architecture studies are currently focused are concepts for a reusable propulsion module, while an hybrid architecture using chemical and solar electric systems in a single stage fully fueled on Earth departure.

Primary, the major focus to date has been on two approaches considered about the propellant or fuel resupply for the EMC. The first one is the possibility to use a tanker vehicle to refuel the spacecraft, as see below. And, for other possibility, the empty tanks are simply replacing by new full ones.

For both concepts, many challenges concern the approach, the rendezvous and docking of servicing spacecraft, the connecting/disconnecting fuel lines by robotics and ensuring its integrity, as well as performing operations in a timely manner to meet mission departure dates.

Except for the reusable propulsion module, the On-Orbit Assembly Concept, the focus is on the refueling option. After the dock of the tanker vehicle to the Mars spacecraft, fluid's lines must be connected to the appropriate tanks and their integrity verified. When transfered, fluid lines ares disconnected and, other repair tasks performed.

Versions of these operations have been performed in space during the Orbital Express mission and on the ISS with the GSFC satellite servicing experiments.

For the latter case of tank replacement, the servicing vehicle must dock to the Mars spacecraft with its two new full tanks, while the used tanks must be disconnected and staged/stored. Every operations are performed autonomously and by robotics.

At the left, an example of a Mars mission spacecraft being refueled

Benefits of SALSSA for the Launch to Orbit

Normally, for every launch, deployable system, mass and volume are large constraints to spacecraft/mission and, may comes very costly to private and public customers.

With SALSSA Implementation, modules are launched individually, aggregated and assembled on orbit. This capacity reduce and optimize launch cost for mission, relieve geometric constraints on spacecraft modules allowing better performance of the final system. That mean, this increase its size, power level, as well as aperture area.

Module Aggregation

In the past, assembly of the ISS was relied on the use of the Space Shuttle and its long-reach robotic arm and EVA). This latter capability is not used anymore and nothing else exist.

SALSSA Implementation: For mission applications where the final spacecraft will be assembled from multiple modules launched separately, the modules must be aggregated at the assembly site. This would include the new module/spacecraft rendezvousing and berthing with the assembled spacecraft.

Benefits from SALSSA Implementation: As the amount of ISA increases, permanent supporting infrastructure might be established where aggregation would occur. This infrastructure would allow for the staging, pre-positioning and safe storage of arriving modules. The infrastructure could also serve as the permanent location for the assembly support systems such as long-reach manipulators, jigging robots, joining systems and measurement/metrology systems.

Deployable Modules

Traditional Implementation: Spacecraft are assembled on the ground and launched as an integrated unit. Spacecraft can have a variety of deployable subsystems, such as solar arrays, thermal radiators, sun shields, and antennas/reflectors. The deployable systems can require very complex operations to transition from the packaged to deployed state due to packaging constraints. The deployable systems also tend to have a large number of joints, latches, motors, springs, and other mechanisms that add mass, compliance and mission risk. Many deployable beam/mast concepts also require a heavy deployment canister. Packaging constraints can also severely limit the final spacecraft size and mission performance.

SALSSA Implementation: Deployable modules change the paradigm of spacecraft design because they allow the designer to take advantage of both pre-integration and ISA to optimize a particular mission. This versatility is demonstrated in the examples of module concepts developed for the Megawatt Solar Electric Tug and the Large Space Telescope.

The Large Solar Arrays for the Tug consist of a simple deployable backbone truss module that has periodic integration sites for deployable solar array wing modules. The primary aperture for the Large Space Telescope consists of a deployable support truss with integrated hexagonal-panel mirror segments that are pre-integrated (includes all of the power, electronics, and mirror positioning / control / actuation hardware).

For both applications, no deployment motors, canisters, or springs are necessary; a robotic manipulator is used to deploy the modules.

Small jigging robots could set and maintain the final structural precision while a long-reach manipulator would: lock/weld/bond any deployment hinges/joints, deploy the necessary backbone structure, and attach payload modules to that structure at appropriate stages of deployment.

Benefits from SALSSA Implementation: A Module will be defined taking into account factors such as; the mission application, ground-test facility capabilities, technology readiness of sub-module components (mirror segment size for telescope, as an example), mass, and volume capability of the launch vehicle. Modules can be defined based on optimizing the spacecraft performance and/or to minimize cost, mission risk, mass, or some combination of metrics.

Benefits of the SALSSA approach include: reduced module structural mass, simplified module system design and integration, reduced cost and complexity, increase in system and mission reliability, and reusing the robotic infrastructure to assemble spacecraft for other missions.

Modular Spacecraft Design and Design for Assembly

Traditional Implementation: Not applicable, launched as single integrated system.

SALSSA Implementation: Performance and mission requirements are optimized at the spacecraft level and then, the spacecraft is divided into individual modules where the modules are sized to meet launch mass/volume/packaging requirements. The modules could also be optimized such that they can fit into existing ground-based test facilities. In some cases, the mission spacecraft could be optimized to use pre-existing modules that have already been developed, such as solar arrays or mirror segments.

For example, a very large aperture telescope could be assembled using the mirror segments already developed for JWST. By doing this, a very large aperture could be achieved without requiring new investment in mirror technology and the resulting expense and time that would incur.

Benefits from SALSSA Implementation: Ground testing cost and complexity can be dramatically reduced when a very large space system, that only has to function in zero-g, does not have to undergo full system level testing in one-g. Mission integrity can be assured by testing and verifying each individual module on the ground before flight, and by having the capability on orbit to perform servicing and repair functions. Reductions in total spacecraft mass and complexity can also be accrued because the entire spacecraft does not have to be designed to survive launch loads. It is likely much easier to design for launch loads in an individual module than for an entire spacecraft. Cost is reduced by using the same launch integration design for multiple modules. The performance constraints (diameter/area of a telescope primary aperture for example) are eliminated using the SALSSA approach. Launch costs can be minimized by shopping for the launch vehicle that has the lowest price in terms of dollars/pound to orbit. Mission cost can be reduced when multiple modules are fabricated (such as solar arrays) for a particular spacecraft. Further cost savings accrue when new missions begin to incorporate off-the-shelf heritage modules into their design. The modular approach can reduce mission risk and cost by allowing a spare module (as opposed to an entire spacecraft) to be built and launched and used in place of one that has failed. Finally, the modular approach allows for a spacecraft to be assembled and operated at an initial performance level, and then be upgraded incrementally in the future by adding modules.

Reconfigurable Systems

Conventional Implementation: Not applicable, launched as single integrated system.

SALSSA Implementation: A key approach for enabling reconfiguration is to have a limited number (standard set) of reversible structural and utility, etc. joints at module interfaces; include adjustability in structural connections to achieve desired geometry and precision in the final assembled structure; and implement new joining methods into the connectors which could be mechanical connection, welded connection, bonded connection, etc.

Benefits of SALSSA Implementation: Reversible connections allow damaged modules to be replaced. As new capabilities are developed, old modules can be replaced with new ones having either higher/better performance, or new and different functions. These connections would also allow spacecraft to be taken apart and have modules repurposed for other uses or to serve in other systems. This attribute has the potential to reduce mission risk and cost, and increase mission life.

Autonomous Robotic Assembly Systems and Operations

Conventional Implementation: Not applicable, launched as single integrated system.

SALSSA Implementation: Use a robotic assembly infrastructure that consists of the robotic hardware, such as long-reach manipulators, IPJRs, tools and end effectors, and autonomous systems to control the robots. Simple, small, reusable IPJRs would grapple, manipulate and set the precision between modules during structural joining. 

Lightweight general-purpose long-reach manipulators (with appropriate tools and end effectors) would be used to reposition IPJRs, manipulate and position modules for assembly, deploy modules, and make utility connections, etc. Since the robotic operations are likely to take place at many different locations in space, supervised autonomy will be the desired control mode. Planning and surveying systems that use vision and knowledge of the final spacecraft specifications will guide the IPJRs in setting module-to-module precision and aid the general-purpose robots in manipulation and joining operations. Metrology systems will perform final validation of the spacecraft geometry and configuration, while maintaining the ability to make any final adjustments before a spacecraft is released and begins mission operations.

Benefits of SALSSA Approach: Autonomous robotic operations will allow assembly to take place at a location that is best for the mission. For example, a large aperture space telescope could be assembled at its operational location at a Lagrangian point. Many space systems would no longer need a propulsion system for orbital transfer, but would only require what is necessary for station-keeping, pointing and slewing, etc., resulting in a reduction in the mission spacecraft mass, complexity, and cost.

The robotic infrastructure would be reusable, so that no mission had to pay exclusively for the infrastructure design, development, manufacture, and launch. The robotic infrastructure could also be mobile so that repair, servicing and upgrade services could be called “on demand” by a mission.

Source: Space Assembly of Large structural System Architectures - SALSSA, John t. Dorsey, Judith J. Watson, NASA Langley Research Center, Hampton, VA 23681

Requiring several launches over many years, the ISS was assembled with a relatively small number of very large and massive modules and components. The components were positioned and berthed tele-robotically in orbit, and, after, permanent mechanical and utility line connections were completed by astronauts in Extra Vehicular Activity (EVA). With the retirement of the space shuttle, that assembly capability ended. 

Historic Experiences

Source: Commercial Application of In-Space Assembly, John Lymer and Al. from Space Systems Loral, Sean Doughtery from MDA US Systems, Bill Doggett and Al. from NASA, Langley Research Center, Hampton, VA 23681, USA

LaRC 2-inch erectable node cluster consisting of erectable joints attached to a central node

In-Space Assembly HistoryIn the 1980’s and 1990’s considerable effort at NASA was directed at developing ISA techniques including several NASA flight experiments including Space Transportation System (STS) 37, 49 and 61B.6 A primary focus of that work was to investigate efficient assembly of a space station, in particular the Space Station Freedom. One promising construction method extensively developed was referred to as erectable assembly. Erectable truss structures, which can be assembled component-by-component on orbit, package very efficiently for launch. Once on orbit, the pieces would be assembled to form the truss structure by an astronaut extra vehicular activity (EVA) using efficient assembly line-type techniques and translation aids, such as the mobile transporter, that were used to locate the astronauts for assembly as shown during neutral buoyancy tests (as show below).

14m radiometer support structure with 7 panels installed

The space suits worn  by EVA astronauts are bulky and the internal pressure causes the suits to be stiff. The stiffness induced in the spacesuit gloves limits the dexterity of the astronaut’s hand operations and also requires the astronaut to exert force to open or close the glove from its neutral position. The erectable joint design and the 2-inch size were optimized to minimize astronaut hand fatigue.

In 1992, on Space Shuttle flight STS 49, the LaRC 2-inch erectable joints were flown as partof the Assembly of Station by EVA Methods (ASEM) experiment, where a portion of the 5-meter erectable truss was assembled in the cargo bay of the Space Shuttle on orbit. Later, the joint was scaled to approximately 1-inch for use in the assembly of a 14m radiometer primary support structure.

ASEM flight experiment, STS 49