In the history of spaceflight, almost all spacecraft have been manufactured and assembled on the ground, then integrated into a launch vehicle for delivery into orbit. This approach imposes significant limitations on the size, volume, and design of payloads that can be accommodated within the fairing of a single launch vehicle. In particular, fairing diameter limitations restrict the size and number of instruments that can be fielded in orbit for science and national security missions. Current manufacturing and technological limitations are evident in the construction of antennas and mirrors that have to be deployed from a single launch with a single satellite.
To overcome this challenge two technologies are being pursued and On-orbit assembly that refers to aggregation onto a platform of ready-made structures (that were manufactured either on the ground or on-orbit) and On-orbit manufacturing, which is the fabrication of structures (including 3D printing techniques).
On-orbit assembly or In-Space Assembly (OOS / ISA)
On-orbit assembly can be defined as the aggregation onto an orbiting platform of ready-made structures that are manufactured on the ground (they could also be manufactured on-orbit for On-orbit manufacturing). Also known as In Space assembly ISA and defined as the assembly activities completed in the target orbit and extraterrestrial space (such as: Low Earth Orbit, Geostationary Orbit, Cis-Lunar Space, Mars Orbit, Mars Surface, Lunar Space and interstellar space), which is to assemble modules in space in order to form a larger functional element or to recombine one or more structures after separation.
In this process, these modules can be combined using their own power and propellant, or they can be assembled from separate spacecraft. According to the complexity of the assembly, the lowest complexity ISA task just involves mating between elements, i.e. two or more independent spacecraft are assembled into a larger space structure and assembled in space with the lowest complexity.
The more complex task would be Modular assembly, that is docking and assembly of cabins or modules to make it an independent spacecraft or to expand and reconstruct its functions. The most complex assembly is Assembly from parts: Assembly from initial parts to components needs to go through multiple assembly stages, which is similar to building a computer from the power supply, heat sink and other parts, such as building a truss from a structural member, or installing a mirror segment and alignment on a structural frame mechanism.
At a conceptual level, on-orbit assembly offers a number of advantages that may enable and enhance various types of space missions. One is the ability to deploy structures that cannot be launched from Earth because of constraints imposed by launch vehicle fairing size and shape. It is obviously unrealistic to use traditional launch methods for structures with large volumes (diameter greater than 15 m or size greater than 20 m, such as large space telescopes, large space habitats). In-space assembly can enable the construction of much larger orbital structures, for instance for setting up more sensitive radar imaging or for obtaining much higher speed of communications by assembling bigger and more sensitive antennas.
Other advantages include an ability to achieve increased flexibility and resilience of spacecraft assets enabled by assembly involving additions, replacements and technology updates of payloads onto a compliant, orbiting platform. An ability to create structures that cannot be created on Earth at all because of constraints imposed by terrestrial gravity.
Design of spacecraft built on the ground requires all the components of each spacecraft to be hardened (ruggedized) to withstand the harsh launch environment, which includes severe vibrations, acoustics, acceleration loads, and thermal loads. The hardening processes impose penalties in terms of mass and size that ultimately limit payload capabilities and increase launch costs.
At the same time, another consequence of this approach is technology obsolescence due to the long-term ground construction and verification process. On-orbit assembly leads to cost savings by carrying more useful mass — less packaging material (structure) for ruggedization, and less platform material and through reduction in the number and intensity of ground-based tests of space-bound spacecraft and subsystems.
Other possible applications of in-space assembly include artificial gravity or asteroid redirect vehicles, space transportation hubs, space telescopes, in-situ resource utilization for construction, solar electric power and propulsion, sun shields, and atmospheric deccelerators. In the future, robotic construction capabilities could also be applied to create space infrastructure, standing structures such as refueling depots, in-space manufacturing facilities, space-tourism complexes, and asteroid mining stations. Developing a strong and sustainable way of ISA will become the key to human space exploration.
In the last two decades, robotized On Orbit or In-Space Assembly (OOS / ISA) missions have increasingly gained importance. Every technology comes with a challenge and so does on-orbit satellite servicing. And though space construction eliminates the challenge of carrying structures via spacecraft, the supplies and machinery necessary for a build still need to be transported from Earth. The heavy machinery we would typically use for construction projects, like carry deck cranes and scissor lifts, aren’t outfitted to work anywhere other than Earth. Standard design techniques and rules of architecture no longer apply without gravity in the picture.
All of the tools and supplies required need to be small enough to fit on a rocket and strong enough that they won’t break. One building challenge unique to outer space is making something that can withstand the extreme environments of space. One such issue is the need for material that can withstand the bombardment of radiation. Other issues, like being strong enough to exit the Earth’s atmosphere, but light enough so that it doesn’t hinder takeoff must also be addressed.
Many on-orbit assembly capabilities face space environment challenges, such as microgravity, atomic oxygen (in LEO), radiation and micrometeoroid impacts. It may therefore be necessary to deploy a protective shell structure in orbit, in which assembly can proceed free from many of these environmental concerns. For LEO operations, there is also the continual significant variation in the lighting environment caused by going in and out of eclipse, which may negatively impact vision-based operations.
The key technologies of ISA
Many of the technologies and processes required for on-orbit assembly are being developed actively in the areas of on orbit inspection and on-orbit servicing of spacecraft. OOS addresses the maintenance of space systems in orbit, including repairing and refueling, using technologies that can be extended to on-orbit robotic assembly.
On-orbit assembly of spacecraft will require development of a number of technologies and processes involving sensing, robotics, automation and modular interfaces between payloads and platforms. According to the different assembly tools used, ISA technologies of the space structure can be divided into: 1) manual assembly by astronauts with assembly assistance, and 2) autonomous assembly by space robots.
The manual assembly technologies by astronaut represented by the United States and Russia, mainly use space shuttle or spacecraft to send astronauts into space, and then complete the assembly task through astronauts wearing extravehicular space suits. The technologies of the United States and Russia have become mature. In addition, since the 1970s, on-orbit servicing by astronauts, such as maintenance of Skylab and the Solar Maximum Mission satellite, the ISS and the Hubble Space Telescope (HST) have laid the foundation for the development of space assembly technology.
An analysis of the potential applications shows that the demanded cross-cutting capabilities are robotic assembly, standardized interfaces, modular design with high stiffness, deployable subsystems, and docking and berthing.
The improved technologies are now making it feasible for robotic spacecraft to dock with other spacecraft to handle station keeping or maneuvering, or even perform repairs. Some of these include a advanced sensor suite for careful rendezvous and proximity operations, specialized toolkit with robotic arms for docking, interacting with, and manipulating a client, and software for managing semi-autonomous servicing tasks.
With the development of ISA, in addition to space remote control robots, researchers began to work on fully autonomous space robot systems for autonomous assembly. Tele-robotic missions are impacted by communication latencies and therefore require tasks such as rendezvous and docking to be entirely automated.
When not supervised by an astronaut, on-orbit assembly requires a robotic system with high reliability and a high degree of ‘trust’ between human and robot. These require the availability of dexterous robot systems capable of performing complex assembly tasks. The space autonomous assembly robot includes onboard robotics, free-flying assembly agents, and persistent assembly platforms,
Interoperability of different systems
System interoperability refers to the ability of two systems to work normally on the interface, including hardware and software interoperability. In order to accommodate visiting spacecraft and accommodate interaction with other associated systems, standard protocols and ports are required.
The concept of modular spacecraft embodies the idea of standard interface and division of functional units. The module division of the spacecraft is the first step of the modular design task, and it is also an extremely critical step. The modular design of the components expands the initial design space, improves the reusability and sustainability of the system, and reduces development costs and mission risks. The design of the module should be easy for the robot to manipulate and assemble, and it should be adjustable to meet the design accuracy and stability requirements.
When designing the module, it is necessary to standardize the mechanical structure, shape, size, installation direction, installation position and installation accuracy of the module interface. Similar to the “plug-and-play” architecture of the computer industry, the modular design must be used in conjunction with standard interfaces in order to realize all the functions of on-orbit upgrade.
The connection technology of assembly and deployment structure has been widely studied. The more traditional method of assembling space structures uses mechanical joints, which are designed to be compatible with astronauts or robots. In order to reduce maintenance costs and increase service life, certain structural units should have reversible standard interfaces, electrical and fluid connections, and the ability to connect auxiliary tools. What’s more, they should have the ability to disconnect structural, electrical and fluid connections without damaging other system components.
Crews in space need special tools, even for basic construction tasks. These tools need to be easy to use while wearing large gloves and able to withstand the harsh environment of outer space. Two of the most versatile tools for in-space manufacturing are:
Pistol Grip Tool
The pistol grip tool is a cordless drill that was specifically designed for use in space. It’s designed to prevent hand fatigue and is lubricated using dry film, as liquid lubricants can cause a tool to seize up.
It’s attached computer screen allows the user to adjust drill speed and torque. It’s been heavily used for repairs on both the ISS and the Hubble Space Telescope, making it one of the staple tools of NASA’s space kit.
What would construction in space be without a space crane? Created in Canada and sponsored by the Canadian Space Agency, the Canadian Space Crane (otherwise known as the Candarm) is a little more sophisticated than cranes on Earth. Just like a regular crane, it’s able to reach, handle and attach objects much easier than a human can. In fact, the Canadarm was so successful that the CSA built another, much bigger version called the Canadarm2. The Canadarm2 is now attached to the ISS and can move like an inchworm with greater flexibility and an arm span that stretches the entire length of the ISS. With the production of the Lunar Gateway, Canada has pledged to build another of these tools, aptly named Canadarm3, to help with large-scale work, repairs and spacewalks.
In order to enable the robot to assemble and transport the structure, the power and signal lines between the robot and the astronaut must be secured. Autonomous robot technologies will provide greater operational processing capabilities in harsh space environments to expand the capabilities of astronauts.
Facing the future challenges and goals of space robots, it is urgent to develop the advanced technology of space robots, especially to integrate artificial intelligence, develop wireless communication or power transmission, multi-modal perception human-robot interaction, etc.
|Dexterous end effector manipulation technology
|• Robot parts: Composite materials and lightweight metal manufacturing technology; Design and modeling tools of advanced actuator; Absolute position sensing; Ability to process small-sized components; Radiation-resistant electronic components
• Dexterity: Control a large number of degrees of freedom (DOF); Advanced multi-modal control system; Real-time external force response; High-resolution sensor array to sense contact
• Grab and remote control: Dynamically rotating or freely drifting objects
• Assembling different structures: Light weight structures; High strength structures; High stiffness structures; Structures with high dimensional stability; Structures with micro-stable joints
|Robot collaborative operation technology
|• Mobile operation: Coordinated movement and force control of the whole system, and integration of positioning and force control
• Collaborative operation: Large-degree-of-freedom system coordination; Force control problems; Multi-point contact problems; Superimposed human-robot interaction modes on robust systems
|Autonomy and mobility requirements
|• High-precision sensing and multi-pattern recognition technology: 3D sensing; State estimation; Force and tactile sensing; Target identification and measurement
|Human-robot interaction technology
|• Multimodal interaction: Effective use of multiple sensory modes, multiple display modes and multiple communication channels to enhance situational awareness and achieve more efficient and user-friendly interactions (such as virtual environment can be combined with interactive 3D computer graphics)
• Distributed collaboration and coordination: Using an interactive architecture to support human-system coordination, communication, and collaboration
• Remote interaction: Reduce the impact of latency on interactive control
For space robots that use a teleoperation system based on human-robot interaction, effective use of multiple sensory modes, multiple display methods and communication channels can enhance situational awareness.
Future trends: Modular reconfigurable spacecraft to achieve autonomous assembly
Compared with the traditional spacecraft, modular reconfigurable spacecraft has more advantages in design, manufacturing, deployment and usage. It will become a new type of spacecraft that can respond to the operational requirements more quickly, flexibly, reliably and less cost-effectively. Many large structures, such as large telescopes, large (megawatt) solar cell arrays, can benefit from the in-space assembly of lightweight structural elements and modular units.
The NASA Automated Reconfigurable Mission Adaptive Digital Assembly Systems (ARMADAS) mission will develop and demonstrate the automatic assembly of “digital materials” and structures. In the future, Mars surface vehicle, moon outpost, planetary exploration space fleet realized by reconfigurable spacecraft can be used for deep space exploration
In addition, for large structures composed of modular components, reconfigurable space assembly robots also have a broad development prospect. The reconfigurable robot can be reconstructed or decomposed according to the needs to optimize the shape. Its operation is more flexible, fault adaptability is strong, launch cost is low, and it can better meet the assembly requirements of large space structure.
On-orbit manufacturing has recently been demonstrated on the International Space Station (ISS) through additive manufacturing, sometimes referred to as three-dimensional (3D) printing, of small components, such as plastic tools.
3-D Printers: The Future of Construction
Of all of the many useful tools and machines being invented for space construction, the 3-D printer may very well be the innovation that plays the largest role. With the ability to generate anything from small nuts and bolts to large habitats, the 3-D printer’s universal functionality makes it a technological game-changer for structural advancement in space. That said, space still presents challenges that make 3-D printing difficult. One of the biggest issues is that of microgravity, which is the absence of gravity and the appearance of weightlessness in space.
Gravity is what allows the layers of 3-D printed material to “stick” as they’re printed and dried. Without the forces ensuring that objects lay properly as they’re being printed, it’s more common for 3-D printed objects to be defective. Another printer problem caused by zero gravity conditions is structural and design inaccuracy. The slightest mistake in a design or a hiccup during printing can cause an item to be structurally compromised, which in turn can be a recipe for disaster when printing essential tools in the life-or-death vacuum of space.
In an effort to solve these zero gravity obstacles, a company called Made In Space partnered with NASA to develop a special 3-D printer called the Additive Manufacturing Facility (AMF). The AMF uses a method developed by engineers in the United States’ Small Business Innovation Research program for the 3-D Printing in Zero-G Experiment. The experiment allowed them to develop a 3D printer that can print just as reliably as a 3-D printer on Earth despite the absence of gravity. The AMF printer was shipped to the ISS in 2014 for testing. After printing out over 200 tools and assets, the AMF was officially deemed a success. With a functional space printer at the ready, crews can print not just small tools and supplies but entire living structures. On Earth, 3-D printed homes are already a reality, and now, the same types of structures can be 3-D printed in space as well.
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