A large unfurlable antenna undergoing tests at a Harris facility in Florida

Satellites unfold in space: building bigger structures in low-earth orbit

Image credit: nasa

How can we assemble bigger structures in orbit?

Even those with only a passing knowledge of the ‘Star Trek’ universe recognise its addiction to scale: the spaceships are the size of ocean liners and built in space docks, the space stations are like small towns and it’s taken for granted that construction on such a scale is the norm.

Today’s reality is on a much smaller scale. The largest and most complex structure ever built in space is the International Space Station (ISS): it took a decade to complete and still houses only half a dozen astronauts and their kit. Is the conquest of space going to be done floating in tin cans or can we find ways to build bigger spaces in space?

It is difficult to avoid the fact that size is important. The volume of habitable space aboard a space station governs directly the number of crew and consumables, such as oxygen and food, that can be accommodated and the amount of equipment installed. The size of the solar arrays governs the power available to the station and the size of the radiators determines the amount of waste heat that can be radiated: there is no conduction or convection in space to help the process.

The problem the space industry faces is that there is no known way to launch large, ready-built structures from the Earth. The answers lie with techniques being developed for construction in orbit.

Historically, the largest single module launched as part of a space station was the converted Saturn V third stage that formed the Orbital Workshop of Nasa’s Skylab station, deployed to low Earth orbit (LEO) in May 1973. The former propellant tank provided a habitable volume of just over 300 cubic metres, which compares well with the ISS’s largest module, Destiny, at 106m3. The addition of an airlock module, a docking adapter and the Apollo Telescope Mount - designed for solar observations - took Skylab’s volume to 360m3.

Considering the four decades that separate the two stations, the long defunct Skylab looks pretty impressive. Its volume was more than a third of the ISS’s 930m3.

The comparison does more to highlight the importance of the launch vehicle. The lifting power of the Saturn V was governed by the need to launch two Apollo spacecraft - an orbiter and a lander - all the way to the Moon. It is a capability that no longer exists. The performance of conventional satellite launchers and the payload bay of the Space Shuttle limited the mass and size of the individual modules. This ISS had to be constructed over many years as a kit of parts carried gradually into orbit. A key part of that kit - delivered to orbit in several parts - forms the backbone of the station. The Integrated Truss Assembly supports the modules, solar arrays and radiator panels, and determines the station’s minor dimension of 74m which, along with an array span of 110m, makes the ISS about the size of a professional football pitch (68m x 105m).

In terms of overall size and mass - at some 420 tonnes - the ISS is unlikely to be beaten any time soon. Even with an extension of its operational lifetime to 2024, the entire assembly could be retired well before a replacement is even approved, let alone built. Nevertheless, much of the technology required to build larger orbiting structures is already available or under development. If pursued, the resulting structures could be in orbit sooner than we think.

The first step towards these larger structures would be to augment the ‘kit of parts’ construction philosophy with methods that rely on building big things from relatively small elements, analogous to building a skyscraper out of steel beams bolted together.

This was the basis of a late-1970s development by Grumman Aerospace, builder of the Apollo lunar module. The company designed a device called the Beam Builder to weld together long beams in space in a more or less continuous process.

The open-framed aluminium beams would form the basis of truss assemblies that could be joined together to form the primary structure for a space station, ‘space dock’ or solar power satellite. Grumman built a prototype for Nasa, but the concept was never used operationally. Instead the trusses were built on Earth and shipped into orbit readymade on a Space Shuttle.

Another approach is to pack the module into a small space and unfurl or expand it in space. The approach lay behind the TransHab module, a Kevlar-based, multi-layer inflatable structure designed in the late 1990s for the ISS programme. The technology was later developed by Bigelow Aerospace and eventually launched to the ISS - in the form of the Bigelow Expandable Activity Module - in April 2016 (E&T Aug/Sep 2016). The company plans to launch a much larger version - the B330 space station with a volume of 330m3 - by the end of the decade.

The unfurling option has the longest heritage among space projects, largely due to the success of commercial communications satellites. Ever since their introduction in the 1960s, the size and capability of these satellites has grown as a result of two deployable items: solar arrays and parabolic antennas.

To obtain more DC power for more capable communications payloads, it was necessary to deploy ever larger solar array wings. And, in order to concentrate RF power on the ground to support simpler domestic receivers, it was necessary to deploy larger antennas that can focus narrower beams.

Florida-based Harris Corporation has worked on unfurlable antennas since the early 1960s, when constraints on space were even more stringent. In the early decades of the satellite age, launch-vehicle fairings were limited to a diameter of about 3m. The solution was the unfurlable antenna, at first sight an unlikely-to-work package of spring-loaded rods and visually-transparent wire mesh furled like an umbrella.

When the company supplied two 5m-diameter unfurlables for each of the Nasa Tracking and Data Relay (TDRS) satellites of the early 1980s, the concept was proved in orbit. David Alexander, senior business development manager for space antennas at Harris Corporation, says the company’s unfurlable mesh antennas have accrued “more than 800 years of mission life, including 27 years for TDRS-1 from 1983 to 2010”.

Today, the company supplies far more complex unfurlable antennas up to 22m in diameter - large enough to cover a couple of tennis courts. Examples are the LightSquared (now Ligado) SkyTerra-1 and MexSat-2 satellites, launched in November 2010 and October 2015 respectively.

In orbit, the antennas are deployed by DC motors, making it possible to stop and reel the antenna parts back in if the deployment hits a snag. Strain gauges and motor-current readings keep a close eye on the unfurling process.

Alexander explains: “One of the key features of our deployment scheme is controlled repeatable deployment. No strain is built up in the mechanism prior to deployment, so there’s no effect on the satellite’s attitude control, and we know the precise position of the elements during deployment.”

The facilities in which the antennas are constructed are huge, especially considering that several antennas might be in different stages of construction at once. The difficulty comes in simulating the weightless environment here on Earth, which cannot be done effectively; the solution is to “measure the antennas face up and face down”, says Alexander.

How large can such an unfurlable antenna go? Alexander says Harris has “done studies for a 35m antenna operating at 35GHz” and considered the design to be feasible. In the early 1980s the firm built a “50-metre demonstration segment” for a Nasa deep-space communications system study. “The objective system was 100m in diameter,” he says, but no space-qualified hardware was built.

The next step is to assemble structures in space from building blocks. How do we put them together? Forty years after Grumman’s Beam Builder was proposed, the space industry has much more experience of in-orbit construction processes; the development of additive manufacturing, or 3D printing, is continuing rapidly; and robotics has advanced - for both tele-operated and autonomous applications.

Space robotics technology is arguably already more advanced than most of us realise. It has gone largely unnoticed because robotic manoeuvres cannot compete in media terms with live spacewalks. An example is Nasa’s anthropomorphic Robonaut 2, which reached the ISS in 2011. The robot, with its name often and unsurprisingly shortened to R2 - though with no D2 suffix - tweets at intervals from the space station, with some help from the Nasa human team.

Meanwhile, the more mundanely named Mobile Servicing System (MSS) has been instrumental in building the ISS since its launch to the formative station in 2001. At the risk of abbreviation overload, it is worth noting that the MSS comprises three main parts named in classic Nasa style: the Mobile Base System (MBS), Space Station Remote Manipulator System (SSRMS) and Special Purpose Dextrous Manipulator (SPDM).

The MBS is effectively a moveable work platform that serves as a base for the other two elements and can move them, on a miniature railway system, from one end of the Integrated Truss Assembly that forms the backbone of the ISS to the other. The remote manipulator is a ‘robot arm’ similar to that carried by the space shuttles - named informally for its nation of manufacture as Canadarm 2.

Canadarm 2 is longer than that of the shuttles and double-ended, a design that lets it ‘walk’ from site to site by grabbing hold of station fixtures. The dextrous manipulator, usually called Dextre, has two smaller robot arms for precision handling tasks. It also has its own lights, video equipment and a fully-equipped tool station.

The Spidernaut could make the space robot even more mobile. Currently under development at Nasa’s Johnson Space Center as part of its Extra-Vehicular Robotics (EVR) programme, the platform uses its eight legs to spread the weight of large payloads. Nasa says the design would make it possible to transport structural materials “across an extensive solar array or mirrors across a telescope without significant structural loading”. The legs can be fitted with different shoes to suit a range of applications. The spider metaphor goes further. The agency says derivatives of the Spidernaut could deploy “a ‘web’ of space tethers to cross structural spans”.

The spider concept goes beyond Nasa. Tethers Unlimited (TUI), a prospective space manufacturing company founded back in 1994 to “develop products based upon space tether technologies”, is currently pushing a concept for additive manufacturing and assembly called SpiderFab. According to founder and CEO Robert Hoyt, the concept includes “a mobile SpiderFab Bot” that uses two “specialised spinneret fabrication tools”.

One spinneret would be used to make high-performance composite tubes or trusses on site; the other would join the elements together. The latter “adapts 3D printing techniques”, says Hoyt, “to create optimised, high-strength bonds between the structural elements”.

An artist’s impression shows a spider-like array of manipulator arms constructing the skeleton of a large antenna dish. In another development, TUI appears to have reinvented the Beam Builder, rebranding it the ‘Trusselator’, and is “currently implementing the first step in the SpiderFab architecture” under contract to Nasa.

When ‘Star Trek’ sprang onto our TV screens in the 1960s, the idea of a space station the size of a football pitch was science fiction; so was the idea of a pocket-sized communicator that received signals from an orbiting spacecraft. Today, the disruptive technologies of additive manufacturing, autonomous robotics and artificial intelligence, coupled with cheaper access to space from companies such as SpaceX, look set to change the architecture of space infrastructure.

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