Clarifications and corrections on hydrogen embrittlement, contrails and tanks
September 2024
AIAA senior member Douglas Dobbin, an aircraft designer and hydrogen engineer at WHA International, an engineering consultancy in New Mexico, took issue with several points in the May commentary, “Fast action on climate change means picking the right mitigations,” by Dennis Bushnell, a futurist and former NASA scientist. On the topic of hydrogen embrittlement and aircraft, Dobbin is correct that the article erred when it stated that “any” steel that came into contact with hydrogen would become embrittled. Dobbin pointed out that “hydrogen embrittlement is real in some materials, but there are many steels available that are not embrittled by hydrogen.” We have updated the online version of the article to make the broader point that embrittlement must be considered when choosing a metal.
Also, Dobbin felt that the piece oversimplified the effects of contrails on the climate when it said that contrails merge to form “blankets of cirrus clouds that reflect the outgoing infrared energy back to the planet.” Dobbin asked: “Are we to believe that these clouds are black on top and white on the bottom?” On this point, nothing in the article was meant to convey that cirrus clouds are entirely absorptive in one direction and reflective in the other. The thin cirrus clouds formed from contrails have been shown to have a net warming effect.
Dobbin also disagreed with Bushnell’s view that storing hydrogen aboard an aircraft requires “noncylindrical tanks,” and he said that “hydrogen is best stored in cylindrical tanks.” In response, Bushnell told us he is aware that cylindrical and noncylindrical tanks have appeared in various designs, but he believes that spherical tanks, in particular, will prove to be best for storing liquid hydrogen because they are better at preventing heat from entering the tank.
Recalling another AI demonstrator
The May article “AI in the cockpit” about DARPA’s dogfighting trials with the X-62A VISTA reminded Frank Hurley, AIAA Associate Fellow emeritus, of a NASA program he encountered in the 1980s while serving as the agency’s deputy chief scientist:
“NASA developed the Adaptive Maneuvering Logic computer programs to represent an opponent aircraft in one-on-one dogfight training in a simulator. AML was ‘adaptive’ in the sense that the algorithms could be updated, but not in that the software ‘learned’ and improved itself. Then as now, it was not divulged which pilot won most often in the man-vs-machine contests, but NASA did say AML proved a worthy adversary.
“It occurred to some of us that putting AML in an airframe that could exceed a human pilot’s limit of about 8 gs would be near unbeatable. It turned out that two shipsets of a very capable airframe were available: the Air Force-NASA Highly Maneuverable Aircraft Technology (HiMAT) demonstrators built by Rockwell. Less than half the size of a conventional jet fighter and dropped from a B-52, the remotely piloted HiMAT had a number of closed-loop flight control features, including smoothly sustaining an 8-g turn at Mach 0.9. After a couple of dozen (not problem-free) flight hours, the demonstration flight objectives had generally been achieved.
“I traveled to NASA Dryden in California [now NASA Armstrong] to propose an AML+HiMAT demonstration. Alas, the idea was not greeted with enthusiasm. The HiMAT flights were extremely demanding and stressful for the demo team, mostly due to the anxiety over the possibility of crashes — more accurately, of media sensationalism should a crash occur, although no pilots would be at risk and two airframes were in hand.
“I have often thought that DoD and NASA should proudly and aggressively publicize having ‘tested to destruction’ unmanned X-aircraft in order to capture crash performance data as well as flight control innovations. Instead, we tend to be deterred by our own culture.”
Frank Hurley, AIAA Associate Fellow emeritus
Chapel Hill, North Carolina
