XCOR Fluoropolymer Composite Material (Nonburnite™)

XCOR is developing a non-flammable, high-strength, lightweight composite material that is suitable for making liquid oxygen tanks for space flight service. We began development of this composite because XCOR’s vehicles, as well as our client’s vehicles, require lightweight, long-service life LOX tanks that are capable of withstanding hundreds of ambient and cryogenic temperature cycles, have a low thermal expansion coefficient, and are inherently LOX compatible.

In April 2005, XCOR received a contract from NASA for a “Long-Life, Light Weight Oxidation Resistant Cryogen Tank program.  This program was carried out over a 12 month period, and consisted of fabrication and testing of an oxidation resistant cryogen-compatible material and cylinder. This Phase 1 effort involved maturing the fluoropolymer composite material suitable for use as tank inner skin material, and fabrication and testing of sample coupons. After the skin coupons demonstrated properties suitable for tank fabrication, a series of sandwich coupons consisting of a skin-foam-skin construction were fabricated and tested. Once both skin and sandwich coupons became satisfactory, we developed a process for fabricating cylindrical test tanks from the new material. These tanks were then pressure cycled and cryogenically cycled to demonstrate adequate life of the material system.

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XCOR’s fluoropolymer composite material (Nonburnite), right, does not burn in an oxygen enriched atmosphere, even with direct flame applied.  On the left is a standard aerospace graphite epoxy, which continues to burn without a direct ignition source.

Testing included coefficient of thermal expansion (CTE), tensile (stress-strain), porosity, three-point flexural bend, scanning-electron microscopy (SEM) analysis, and flammability hazards analysis performed by NASA’s White Sands Test Facility’s (WSTF) in accordance with WSTF methods where XCOR’s fluoropolymer composite coupons did not burn when exposed to an 100% oxygen-enriched environment, and when they were immersed in liquid oxygen and held at 60 psi in a pure oxygen atmosphere.The composites currently used by NASA are made of flammable resins and fibers, and have not been accepted as suitable for LOX tank construction. Existing LOX tank materials are aluminum (most applications) and stainless steel (Atlas and Centaur vehicle stages).  XCOR began the development of this material system because the proposed resin and fiber materials were already accepted for use in NASA oxygen systems, and because we projected that the high strain to failure of the thermoplastic fluoropolymer resins would lead to low inherent microcracking properties. The principal benefit of this material over graphite/epoxy is its non-combustibility, and, compared to aluminum, it has lower density, lower coefficient of thermal expansion (CTE), and higher strength.

Some other potential applications that we foresee include main and secondary propulsion tanks, use in expendable launch vehicles, integrated tank -aeroshell structures in reusable lifting body and winged vehicles, and sunlight and radiation resistant habitat structures.

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Tank cylinder of Nonburnite for cryogenic testing

It is inherently resistant to microcracking (a porosity problem that in the past has required a liner on epoxy matrix materials used in cryogenic applications) due to the high strain-to-failure of the resin options.There are four distinct advantages in using this material for these applications:

  1. It has a low coefficient of thermal expansion (CTE), which allows it to be part of, rather than suspended within, a complex vehicle’s primary structure. This allows significant weight savings in some types of vehicles.
  2. The foam serves as thermal insulation, as well as structure, in a skin-foam-skin composite, which allows long missions without vehicle mass penalty for additional thermal insulation.

It has inherent combustion resistance, which allows its use on manned systems.

These features, along with the high strength-to-weight ratio typical of composites, make it an enabling technology for building a lighter, cheaper, more robust, space infrastructure, and to do more with any given transportation system.

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XCOR’s ‘Nonburnite’ fluoropolymer composite material (white bottom layer) subjected to stress testing in an Instron machine.

XCOR’s efforts demonstrated that it is possible to create a fire resistant structural composite that can replace aluminum. We are confident that this material can be used for numerous applications, in particular manned space flight hardware.   We have trademarked the material name as “Nonburnite” with the U.S. Patent Office.