LITTLETON, CO. – The popular saying that “space is hard” may need a redefinition, given the increasing use of soft goods to create inflatable structures, from airlocks to habitats for future missions to the Moon and Mars.
At Lockheed Martin's Waterton Canyon facility, the company is testing inflatable structures that offer advantages over all-metal structures. Pressurization and depressurization evaluations of an inflatable airlock unit were conducted on August 14.
Inflatable technology is an increasingly popular idea that is being adopted and tested by several companies, including Max Space and its expandable habitat architecture. There is also inflatable work underway at Sierra Space on a large integrated flexible environment habitat. Sierra recently performed an explosion test from its inflatable module, announcing on July 25 that the habitat withstood air pressure higher than NASA requires.
One attractive aspect of inflatable space structures, explained Uy Duong, chief engineer for commercial civil space habitability at Lockheed Martin, is that the underlying technology is highly scalable. Plus, inflatable structures offer greater volume with less mass. That also means a larger habitable volume can be sent into space, packed inside the fairing of modest-sized launchers, said Rowan Palmer, a systems engineer on Lockheed Martin Space’s soft goods and habitability team.
Duong added that soft goods also perform much better than metallic ones when it comes to shielding against radiation and thermal conditions.
In the Aug. 14 test, the inflatable airlock design was put through multiple gas-in and gas-out cycles — essentially, inflations and deflations with enough nitrogen gas to pressurize the airlock to the point where it becomes as stiff as steel — to assess the extent to which its Vectran material deforms over time, a process called creep. Knowing how creep affects a Vectran structure will allow Lockheed Martin to properly assess its operational life potential. Test engineers here have also put subscale soft-goods habitat designs through their paces, deliberately blowing them up to highlight their robust nature and determine their pressure thresholds.
Vectran is a high-performance liquid crystal polymer fiber that is particularly strong, stiff and stable, Duong explained. “It is very similar to Kevlar in terms of strength, however, it actually has better creep resistance. It can withstand extreme environments much better than Kevlar,” he said.
Lockheed Martin is conducting inflatable structure work as part of NASA's Next Space Technologies for Exploration Partnerships (NextSTEP) program, a joint public-private initiative.
Safety factors
“We’re doing cyclic load testing, pressurizing up to 14 (pounds per square inch), the operating pressure of an airlock, and then depressurizing,” Palmer said. A pressure of around 14 PSI is similar to the atmospheric pressure experienced on Earth and the International Space Station.
That cycle repeats itself several times, Palmer said. Space Newswith the aim of evaluating the behaviour of the Vectran soft material layers and how they fuse with the metal core of the inflatable structure as a complete system.
“NASA has a standard of four times the safety factor as a minimum for soft goods. Our burst testing showed that we exceeded that value, with a six times higher safety factor,” Palmer said. By comparing the creep test data to the maximum burst pressure, the result yields the life of the unit for the mission with a quantified safety factor, he said.
“The key outcome of any type of test like this is to gather as much data as possible,” Palmer said. During the test, internal indicators showed critical information about pressure, the timing of certain events, and the behavior of the structure overall.
Palmer declined to share certain proprietary aspects of the inflatable manufacturing process.
Range of opportunities
Duong said the new safety data will allow Lockheed Martin to test larger designs than originally planned. These could include cryogenic fuel storage units, pressurized tunnels to connect habitable modules and other structures to support crews on the Moon and Mars. “There is a wide range of opportunities for us to apply this technology,” he said.
Palmer said the new inflatable structures destined for space can be designed, built and tested in much shorter timeframes than large metal structures, which require long lead times for qualification testing. He described a “test as if you were flying” mentality and “a development process that allows us to move pretty quickly.”
It took three months to build and prepare the airlock unit and other inflatable designs for testing, Palmer said.
Scaling up
A central reason for the rapid manufacturing of inflatable units is that everything is done on site, said Tyler Muma, director of soft goods technology at Lockheed Martin.
Vectran raw materials supplied by the supplier are converted into in-house configurations, allowing the company to closely control production, “so that all layers will work as a seamless system,” Muma said.
Muma added that the specific strengths and widths of raw straps and cords used to manufacture inflatable units are made under controlled conditions, so that a repeatable and accurate result can be achieved with each build.
Smaller development units, such as the airlock-type structure, can be built in the Waterton Canyon area. But Muma said Lockheed Martin is investing in a full-scale capability to manufacture large inflatable units at NASA's Stennis Space Center in southern Mississippi.
“We will have more sewing machines, other equipment and a larger table that will allow us to make full-scale habitats. Things we would see in low Earth orbit or on the lunar surface,” Muma said.
