April 1 - 2019

Elon Musk made the decision to use stainless steel instead of carbon fiber/epoxy materials for his Starship.

Stainless steel properties:

• Very ductile, very tough. No fracture issues
• that at cryogenic temperatures, the strength is boosted by 50 percent.
• it’s obviously cheap, it’s obviously fast.
• It has a high melting point. 1600 degrees Fahrenheit.

Elon Musk also proposed a transpirational heat shield:

“You just need, essentially, [a stainless-steel sandwich]. You flow either fuel or water in between the sandwich layer, and then you have [very tiny] perforations on the outside and you essentially bleed water [or fuel] through them … to cool the windward side of the rocket.” – SpaceX CEO Elon Musk.

All in all, this would appear to be the perfect application for Hexaflex. 

Hexaflex can be fabricated from a continuous sheet of stainless steel that has been die cut with a repeating geometric design, creased and folded concertina style in upon itself to make a double sided core material that is flexible, able to vent, exhibits good bend and shear strength and has a large surface bonding area rendering it suitable in the construction of lightweight sandwich panels.

A transpirational heat shield must be able:

• to efficiently join inner and outer shell surfaces with stringers

• to provide a clear interstitial space, allowing fluids to flow through its structure
• to be formed into compound curvatures
• to serve double duty as structure

Expanding on the concept of thermal protection systems,  Hexaflex panels can be arranged in multi-layers or plies in order to create storage for different gasses or  liquids.

This blog is dedicated to the advancement of a new type of honeycomb core which we have named "Hexaflex". As you can guess from the name, this core is based on a hexagonal lattice framework, and exhibits distinct advantages over the more traditional kinds of industrial honeycomb.  

Up until relatively recently, we had found it a challenge to be able to manufacture Hexaflex.  With the more recent developments in 3D Printing, what was probably not even possible before, has now been produced as an attainable, and multi-functional, construction element, that shows us a viable, alternate method of manufacturing into the next millennia. 

Scan to the bottom of this blog for recent updates.

BACKGROUND

The rising demand for new materials with higher strength to weight ratios has created a dramatic growth in sandwich composite technology. Sandwich construction employs a lightweight core that has a flexural strength and flexural modulus far exceeding that of the skin laminates alone. The most common type of core material is honeycomb, which is used primarily in the aerospace industry.

The main disadvantage of honeycomb type cores used in the Aircraft and Aerospace industries is that of delamination which can cause a catastrophic failure of the vehicle. This is caused primarily by the failure of the epoxy adhesives to maintain a bond between the facing skins and the core because of the very small bonding area that honeycomb cores edges offer.

This is further exacerbated by the fact that honeycomb type cores create pockets of trapped air within the closed cells of the core when the skins are attached. The air pressure experienced at high altitudes is much lower than the trapped air within the cells with the result that the skin is pushed away from the inner core by the air pressure. Ingress of water into already partially delaminated cells at high altitude freeze into ice particles which expand and force the skin to separate from the core. Eventually after many cyclic operations the skin will delaminate. Additionally, lightning strikes can cause entrapped moisture within the core to immediately turn to steam with catastrophic results to the integral strength of the panels.

The Aerospace Industry remains the greatest consumer of honeycomb materials, whether for civil aircraft, military jets, helicopters, aero-engines or the newer space satellite and launchers.

The Director of NASA’s Marshall Space Flight Center once stated that “our (composite) technology has not yet advanced to the point that we can successfully develop a new reusable launch vehicle that substantially improves safety, reliability and affordability.” He was referring to the failure of the composite fuel tank panels of the NASA X-33 Reusable Space Vehicle due in part to honeycomb delamination.