Technological evolution often requires decades of incubation and advancement before large scale commercial adoption is achieved. First discovered as a discrete substance by Henry Cavendish in the late 1700s, hydrogen has followed a circuitous path of discovery and application in a variety of fields.
Its primary large scale commercial use was in the petroleum and chemical industry where it's still a critical element of fossil fuel upgrading processes. Various other industrial processes - including applications as wide ranging as food preparation and semiconductors - use hydrogen.
Although it's been demonstrated in nearly every type of internal combustion engine as a replacement for fossil fuels, it's primary use for power and propulsion (until recently) has been in the aerospace industry.
Liquid Hydrogen
Liquid hydrogen (LH2) has been in routine and continuous use in the space program since the early 1960’s. However, many are not aware that its roots in aerospace trace much further back in aviation to the initial jet engine research and development in the late 1930’s; and later with successful flight demonstrations of a liquid hydrogen fueled jet engine in the mid-1950’s [1].
Initial jet engine testing done by the German's in 1937 used hydrogen in part due to its ease of ignition and high flame speed. First used on a 250 pound thrust (lbf) jet engine operating at 10,000 rpm (and later on a 989 lbf jet engine), hydrogen proved to be an ideal fuel for this new propulsion technology.
About twenty years later, Pratt & Whitney Aircraft developed a jet engine with an afterburner that operated on liquid hydrogen. The project was started in 1956 and resulted in a 4700 lbf jet engine intended for a supersonic reconnaissance aircraft under development by Lockheed. The engine was a success, but the aircraft concept was cancelled in favor of the Blackbird SR-7 development.
Aircraft Flight Testing with LH2
Liquid hydrogen was eventually tested successfully in a series of B-57 flights at the NASA Lewis Research Center from 1956 through 1959. The aircraft was modified with an LH2 tank under one wing; a helium pressurant tank under the other wing; and a heat exchanger to vaporize and warm the hydrogen prior to engine injection.
No modifications were made to the Curtiss Wright J-65 turbojet engines that typically operated on JP-4 (kerosene) fuel. Multiple in-flight tests involved taking off with JP-4 and then switching to hydrogen in one of the two engines during flight to demonstrate various operational conditions.
Three successful flight test campaigns were completed with 38 transitions from JP-4 to hydrogen that thoroughly demonstrated the feasiblity of using LH2 for jet aircraft. In parallel with the flight tests, wind tunnel and fixed engine tests were also performed. The hydrogen jet engines were found to significantly outperform their JP-4 counterparts in terms of engine mass, thrust, stable operation, and fuel consumption.
Beyond Aircraft
Paradoxically, aviation did not become the primary use case for LH2 despite these early successes. However, it did set the stage for LH2 use in rockets and future space vehicles. More on that in the next post.
As a final thought, it is interesting to note that electric cars have followed a similar path. First introduced by Thomas Edison circa 1913, they were initially unable to compete with internal combustion engines. Now both technologies are aggressively competing to overtake fossil-fueled aircraft and vehicles in the marketplace.
References
