Resilient Rockets

But now scientists at NASA's Marshall Space Flight Center (MSFC) have developed a solution to this conundrum. Using an advanced "vacuum plasma" technique to spray both the coating and the liner onto a mold, building them up slowly layer by layer, they have learned to build a rocket engine combustion chamber that can withstand more than a hundred firings in a row without the slightest sign of roughening or blistering.

"It seems to be basically impervious to the wear-and-tear of repeated use, at least within the limits of our testing," says Richard Holmes, an engineer at MSFC. That trait makes a rocket built with this new technique an excellent candidate for an ultra-low maintenance space ferry or for a next-generation launch vehicle, Holmes says. The technique may also have use in improving race car engine performance and increasing the efficiency of normal automobile engines.

A rocket combustion liner (left) being made by spraying material onto a mandrel with a hot plasma (right).

The advantage of this new technique is not found in something that it adds, but in something it takes away: the sharp boundary between the coating and the combustion liner. As the micron-thin layers of material are sprayed onto the mold, the ratio of coating to liner material is slowly varied from 100% coating to a 50-50 mix to 100% liner, creating a gradual transition between the two. This way, the stresses caused by heat-induced expansion get distributed over the whole transitional zone instead of focusing at the sharp boundary. The result: one awfully durable rocket liner.

The coating itself is a metallic alloy of nickel, chromium, aluminum, and yttrium, abbreviated as NiCrAlY (which Holmes pronounces "ny-CRAL-ly"). In addition to the protection this mixture provides against weathering, it's also an excellent insulator against heat, reducing the temperature of the rocket combustion liner during firing by 200°F

It's this combination of heat insulation and resistance to wear that holds promise for race car and automobile engines, Holmes says. Race car engines run at much higher RPMs than street cars - over 12,000 RPM is not unusual, versus a 6,000 to 8,000 RPM redline in most cars. These high RPMs lead to greater wear-and-tear, and race car engines must be overhauled frequently. A NiCrAlY coating with zirconia (a ceramic thermal barrier coating) applied by the vacuum plasma technique could improve the life of the piston heads and block heads of these engines.