Nasa successfully inflated its first expandable space station module in May in an attempt to find cheaper ways to put equipment and people into orbit. How will blow-up modules change space?
It took just a matter of hours to move into position but caution dictated it would take almost a month before the first astronauts could venture inside the Bigelow Expandable Activity Module (Beam). In early April, the long arm of the manipulator on board the International Space Station (ISS) pulled the Beam from the SpaceX Dragon cargo capsule carrying it and moved it into position the same day. But Nasa waited until late May before pumping air into Beam to make it ready for inspection.
At a pre-launch press conference held at Kennedy Space Center, Jason Crusan, director of Nasa’s advanced exploration systems division, had explained that everything would be “done slowly” because Nasa had “never tried an expandable at ISS before”. Perhaps the audience had expectations a little quicker than an inflation procedure that allowed air into the module one second at a time, taking more than nine hours to complete, but that’s the difference between space fact and fiction.
The mass issue
So what is the point of using expandable modules? In common with anything sent into space, it’s all about mass. Mass costs money when it comes to buying a launch.
Arguably the most challenging aspect of developing a space-based infrastructure is lifting the hardware out of the Earth’s gravity well. So the engineers examine anything they design to see if they can reduce its mass. The standard solutions are to limit the mass of supporting structures, use materials with higher strength-to-mass ratios (such as composites) and reduce the component count wherever possible.
The basic structures of habitable space station modules present big problems. They must be large enough to provide a useful working environment but also strong enough to protect crews from space radiation and impacts from micrometeoroids and orbital debris. The results so far are based on cramped metal cylinders.
One answer is to launch a module that is sufficiently lightweight and compact to fit within a launch vehicle fairing (typically constrained to about 5m in diameter) and expand it in orbit. This is the philosophy behind Beam, which was supplied by Bigelow Aerospace, a company founded by hotel-chain entrepreneur Robert Bigelow in 1999 to develop expandable modules.
But how well does Beam reflect the mantra of ‘more volume for less mass’? Nasa’s Destiny module, a conventional ‘metal tube’, has a volume of 106m3 for a mass of about 14,000kg. Beam’s specifications are 16m3 and 1,400kg, which is 15 per cent of the volume for 10 per cent of the mass – an improvement but hardly stunning.
Crusan explained, however, that because of the unknowns Beam has “a four times safety factor”, compared with 1.5 to 2.0 for existing ISS modules, which means that “Beam is not optimised for mass”. Nevertheless, in terms of its strength-to-mass ratio, it has “almost an order of magnitude advantage”, he added.
Because Beam is small, it’s clear that its metallic elements, such as the docking collar that links it the ISS, account for a disproportionately high percentage of the mass budget compared with larger modules. What this means in practical terms is that expandable technology is better for bigger modules. Going larger is exactly what Bigelow has in mind.
Bigelow Aerospace’s experience in space inflatables dates back to 2006 and 2007, when its Genesis I and II free-flying modules were launched by Russian Dnepr rockets; they both operated for more than two years and remain in orbit. With volumes of 11.5m3 they were smaller even than Beam, but Beam is attached to a manned space station and had to jump all Nasa’s safety hurdles to get there.
Bigelow himself has long been frustrated by the lack of launch options for his hardware and it is only the move at Nasa towards commercial station supply capsules and operators such as SpaceX that has allowed him to launch a module to the ISS now. “It’s important to us that Nasa has the confidence in our little company to do this”, he stated politely at the KSC press conference, but he knows better than anyone that it’s been an uphill battle.
Expandable structures have been in Nasa’s portfolio since the early years of the Space Age. In fact, one of the Agency’s inaugural projects upon its foundation in 1958 was Echo, which saw two spherical, aluminium-coated Mylar balloons (30m and 41m in diameter) placed in low Earth orbit (LEO). These inflatable structures were used, in 1960 and 1964 respectively, as ‘passive satellites’ to reflect transmitted radio waves to earthbound receivers.
Many proposals followed, but it was not until the industrialisation of composite materials such as Kevlar – otherwise used for bullet-proof vests – that expandable structures for crew habitation became practicable. Thus, in 1997, Nasa began development work on an ISS module called TransHab (a contraction of Transit Habitation that referenced the original application of housing Mars-bound astronauts).
When the project was cancelled in 2000 without any hardware yet having made it into space, Bigelow Aerospace negotiated the patent rights, built a plant in north Las Vegas and embarked upon a development programme that has so far seen three of its modules in orbit.
As Bigelow himself pointed out, the relationship with Nasa has been one of “insight rather than oversight”. Arguably, it has been symbiotic in that Bigelow Aerospace would not have the technology without Nasa, and Nasa would not have an expandable module at the ISS without Bigelow.
Proof of concept
Despite the importance of Beam for both parties, neither is hyping its potential: it’s a fairly basic test facility and its specified two-year design lifetime will be relatively uneventful if everything goes to plan. Crusan described Beam simply as “a testbed for structural, thermal and radiation performance” and a chance to confirm “behaviour models for long-term durability” of expandable habitats.
He also took the opportunity to address why Nasa prefers the term ‘expandable’ over ‘inflatable’: it’s all about structure. The term inflatable implies “something like a balloon that doesn’t have a structure in and of itself”, he explained, whereas an expandable structure “retains its rigidity even after a reduction in air pressure”. This may seem like semantics, but an astronaut would rather be inside an expandable module than an inflatable if it was punctured by a chunk of space debris – because it wouldn’t simply pop like a balloon.
For safety reasons, the module’s expansion was initiated and controlled by the astronauts. The deployment sequence involved the pyrotechnic release of straps constraining the module’s size for launch and initial pressurisation was achieved by allowing station air to enter the module and gradually expand the structure; air tanks in the module itself would later bring it up to full station pressure.
Asked about contingency plans should anything go wrong, Crusan explained that the module could be jettisoned using the same robotic arm that berthed it: “...but that’s Plan F”. Bigelow himself insisted he was not worried about the hull at all. “My focus is on seals,” he said, referring to the “interface between metallic systems and soft-good systems”. Although Beam has no windows, Bigelow is aware that if a leak occurs anywhere in his designs it will be around bulkheads, hatches and windows.
The Beam module itself has two metal bulkheads, a central aluminium structure and a multi-layered skin similar in design to a spacesuit. From the inside out, it features an air barrier or ‘bladder’, an internal restraint that forms the primary structural loadbearing element, and a micro-meteoroid and orbital debris (MMOD) shield. This is covered in a multilayer thermal insulation blanket and a woven silica-fibre layer known as Beta cloth.
The MMOD is designed to protect the restraint and gas bladder and avoid debris penetrations that could lead to depressurisation and has already been proven by the Genesis modules. According to Bigelow, hypervelocity tests have demonstrated protection “superior to that of the traditional aluminum-can designs” and even in the unlikely event of a penetration, the Beam – being expandable rather than inflatable – would “leak slowly instead of bursting”.
As for that other astro-bête noir, radiation, Bigelow says that expandables offer better protection than aluminium modules because the latter “create a scattering effect”, producing “damaging secondary radiation”. Beam will hopefully provide data to back this assertion.
With the module fully expanded, astronauts were able to enter in June to place sensors inside and then withdraw to allow an uninterrupted ‘quiet period’ for data collection. According to Crusan, they are expected to visit the module three or four times a year to retrieve data from the drives, and so on, but there are “no restrictions on crew entry”. It has been suggested that, because Beam is essentially ‘soft’, it will be quieter than the rest of the station with its cooled equipment racks and air transfer fans.
But Beam is just a starting point for Bigelow. The nub of his business case for space, he said, is “based on volume”, which is why he is developing the B330 space station with a volume of 330m3. “The biggest ISS module is about 110m3”, said Bigelow, “so you’re getting three times that per launch”.
While Beam relies entirely on the ISS, the B330 is designed to be a standalone space station with its own avionics, propulsion and life support systems. Showing a graphic of two B330s linked together to form a 660m3 station, Bigelow revealed plans to have them in LEO “by 2020”. Asked whether the station would be for Nasa or a commercial user, he was non-committal, saying only that “we’re working on a combination of things”.
What he does make clear is a business case based on “sharing time and volume, which means not having to write a large cheque to buy the entire thing”. So it seems that, in future, the way to get ‘more space in space’ will be more like renting a timeshare than designing, building and launching your own space station. Arguably, that’s even more ‘out there’ than the expandable modules themselves.