For years now, Sierra Space has been working on inflatable space habitats. The future goal is to launch these modules and then inflate them once in orbit, effectively launching massive space station segments in a single mission, and avoiding the issue of fairing size limitations.
Over the past few years, we’ve seen them conduct a bunch of different tests on its inflatable LIFE habitat. This included pressure tests on both scaled-down versions and eventually full-size test articles. Recently, just in the last few weeks, the company began conducting hypervelocity impact trials.
Impact Testing
Once inflated in Earth orbit, one major concern is the possibility of space debris or micrometeoroids impacting the habitat. In order for it to be rated for human use, it not only needs to withstand a possible impact but also have quite a bit of margin. Starting late last month in April, Sierra Space first announced some of the testing they had been undergoing. Specifically, in a statement, they reported that the company had “recently conducted successful hypervelocity impact trials at NASA’s White Sands Test Facility in New Mexico, to optimize the structural integrity of Sierra Space’s Large Integrated Flexible Environment (LIFE®) habitat.”
They also provided some images and video of the actual testing. While not a lot, it does highlight the impact and the forces they are up against and need to withstand. For testing, they used a two-stage light gas gun. These guns use gunpowder and highly compressed hydrogen to accelerate projectiles at speeds up to 27,500 feet per second (8400 meters per second) to simulate impacts of particles on spacecraft and satellite materials and components. Testing is also conducted in a near-vacuum chamber to simulate space conditions.
At the end of the barrel was the test material. For these tests, it was comprised mainly of various ballistic fabrics and micrometeoroid and orbital debris (MMOD) layers. At the very end of these test layers is what they call the rear wall, which is one of the final layers before the air barrier and interior of the habitat. In other words, they were making sure this layer wasn’t punctured, as it’s one of the last lines of defense.
Looking at the actual footage, you can see how the initial ballistic fabrics take the brunt of the projectile’s force, with it going right through them. After it goes through multiple layers, it’s not only fragmented but also loses the majority of its velocity and is stopped before damaging the rear wall. Teams explained how they were working to find a balance between strength and weight. They were quoted saying, “If you’re never seeing a failure, you’re certainly over-engineered. So in the beginning when we were seeing failures, it’s pretty good to understand that, because now we can strategically add mass and ariel density to get to a point where we’re right on the line of passing all of our safety requirements, but we didn’t add too much mass to the system” he said.
Looking at a diagram of the layers that make up this structure, you can see that the majority are MMOD, and are meant specifically for protecting the inner bladder. That being said, each layer has its own purpose.
First, you have the restraint layer for LIFE, constructed of high-strength “softgoods” materials, which are sewn and woven fabrics – primarily Vectran – that become rigid structures when pressurized. They point out that under normal operating pressure, the Vectran softgoods materials become 5x stronger than steel, exceeding station lifetime performance safety factors. The restraint layer is complemented by a bladder allowing controlled inflation and pressurization to ultimate burst pressure test failure. Together, this is meant to create a structure strong enough to withstand micrometeoroid impacts and other projectiles.
Focusing back on the recent tests, they clarified that “The impact trials were conducted in two phases. The first grouping of shots varied the softgoods materials while keeping gun parameters constant, simulating MMOD impacts to directly compare how each material performed. After identifying the most promising materials, the team adjusted gun parameters to develop an equation characterizing the efficacy and performance of the selected shield stack. During the tests, 40 experimental shots were fired toward the materials to confirm the configuration selection. Once the team had established a strong but mass-efficient shield configuration, 19 additional shots were discharged at the material. These efforts were critical to mitigate future risks posed by MMOD—tiny, high-speed particles that can cause significant damage to spacecraft and habitats in orbit.”
After testing, the Vice President at Sierra Space was quoted saying, “Our innovative space station technology drives scientific discovery and fuels a low-Earth orbit economy. This collaboration with NASA advances our efforts to develop a shield that protects against micrometeoroids and space debris, bringing us closer to launching the LIFE habitat into orbit and readying our technology for repeat and long-duration space missions.”
Full Certification
At this point, the company is getting closer to eventually sending one of these inflatable habitats into Earth orbit. The certification process with NASA is thorough and takes time, but it’s almost complete. Late last year, they completed a sixth successful stress test, and the fourth Ultimate Burst Pressure (UBP) test, for its LIFE 10 commercial space station technology. This test, in particular, was the final UBP test that Sierra Space needed to perform on LIFE 10 to fulfill Factor of Safety (FOS) recommendations ahead of certifying the structure for human habitation.
For these burst tests, they pressurize the test article and continue to increase the pressure until it gives in, recording the results along the way. This most recent test ruptured at the highest pressure yet, 255 psi, and was the highest loading to date of any test article in the three-year restraint layer certification test campaign. The 255 psi failure point exceeds any guideline for restraint layer capability recommended by NASA in all applications and environments. As a standalone product line, this test proved that the LIFE 10 restraint layer surpassed NASA’s 4x factor of safety recommendation in both LEO and lunar environments.
In the future, Sierra Space sees applications beyond just Earth orbit, but even habitats on the surface of the Moon. In terms of design, the first product in the roadmap is a large, three-story structure that is 27 feet (8.2 meters) in diameter. They point out that this module can comfortably sleep four astronauts, with additional room for science experiments, exercise equipment, a medical center, and Astro Garden® system, which can grow fresh produce for astronauts on long-duration space missions.
In the coming years, the company wants to iterate on larger designs. A 5000 cubic-meter version, packaged inside a nine-meter rocket fairing, for example, would surpass the internal volume of multiple International Space Stations in a single launch. In other words, assuming this initial LIFE module works as intended, they will continue to iterate and make it even larger. This process starts with LIFE 10 and goes up to LIFE 5000, the biggest variant.
Sierra Space is quoted saying, “In a LEO environment where the maximum internal pressure of the module will resemble that of Earth at 15.2 psi, the LIFE 10’s factor of safety is greater than 16x. In a lunar environment where – due to different operational needs – the internal pressure is lower (around 10.8 psi), LIFE 10’s restraint layer has an impressive 23x factor of safety. With such high margins, Sierra Space is concluding the UBP portion of the LIFE 10 test campaign, solidifying Sierra Space’s position as the industry leader in commercial space station development,” they said.
Looking at the roadmap, LIFE 10 is just a stepping stone with LIFE 285 expected to be one of the first real modules. LIFE 10 is a one-third scale version of the company’s LIFE® 285 habitat, which inflates to the size of a three-story apartment building on orbit. Sierra Space has conducted two UBP tests on LIFE 285-scale modules in the past two years.
For a lot of the testing that’s taken place, particularly with pressure tests, a large metal plate can be seen. In a different burst test, the test article once again included two four-foot by four-foot blanking plates – metallic structures inserted into the softgoods shell to emulate a future design component, such as a window, robotic arm, or antenna attachment point. They were 50 pounds lighter than the ones used in the first full-scale test and designed to accommodate larger windows. In order to get accurate test results, they include these plates to ensure they aren’t a weak point for the inflatable.
In reference to some of the work the company has been completing on the LIFE Module, the VP was quoted as saying, “No other company is moving at the speed of Sierra Space to develop actual hardware, stress-tested at full scale, and demonstrate repeatability. We’ve taken a softgoods system that very few companies around the world have been able to design, and now we have consistent, back-to-back results. A second successful full-scale test is an absolute game-changer. We now know it’s possible to equal or surpass the total habitable volume of the entire International Space Station, in a single launch.”
With future projects like Orbital Reef, Sierra Space is hoping to have these modules ready and significantly increase the internal volume available for science, habitation, etc. Something we can look forward to seeing in the future.
Conclusion
Recently, Sierra Space conducted some of the first hypervelocity impact trials on its LIFE Habitat. During these tests, they tried different materials and gun parameters to try and find the sweet spot between weight and strength.