"Zip" Fuels: the F-101 and Research on Borane Fuels

Last week saw the celebration of the 61st anniversary of the first flight of the F-101 Voodoo, on 29 September 1954.  Ship No.1, 53-2418, had a very interesting career after initial flight testing.  She was soon "bailed" to General Electric for tests with their new J79 engine, then under final stages of development for the Convair B-58A Hustler and Lockheed F-104A Starfighter.  Due in part to the persistent compressor stalls encountered with the Pratt & Whitney J57-P-13 engines, the J79 garnered considerable interest from McDonnell for developed versions of the Voodoo.  While it did not offer the fuel efficiency of the standard Pratt & Whitney engine, the J79 produced more thrust from a lighter engine and, most importantly, was free of compressor stalls, even with the initial Type I inlets that had caused so much grief for both the Air Force and McDonnell from its first flight.  But as the J79 began to mature into an established engine, General Electric was already at work on a very advanced derivative, the much larger J93 engine intended for the North American B-70 Valkyrie bomber.  Initial flight testing would take place with the J93-GE-3 engine, using hydrocarbon fuel.  Six of these engines would propel the massive stainless steel bomber to speeds of over Mach 3.  But design work proceeded on another version, the J93-GE-5, using a completely new, synthetic type of fuel that was so volatile it did not exist in nature.  Rather than using petroleum-based hydrocarbon fuel, the new fuel was composed of a class of chemicals called boranes.  Ground testing, in cooperation with the NACA, began in the mid-1950s using a modified J47 engine as a testbed.  But by 1957, the time had come to begin planning for flight testing of modified engines with the new fuel.  Given that the J93 was essentially a scaled-up J79 engine, the twin-engine General Electric Voodoo was selected for those historic tests.  Below is the rest of the story:

Ship No. 1,  bailed to General Electric and carrying modified YJ79 engines, conducting the first-ever powered flight using high-energy borane “zip” fuel on 28 September 1958. Gerald Balzer Collection, Greater St. Louis Air & Space Museum.

Pursuant to the 21 June 1955 request for a study of the J79 engine for the Voodoo, Ship No. 1, 53-2418, was bailed to General Electric in 1956 and modified with a pair of YJ79 engines in place of its Pratt & Whitney J57s.  The keel area was reworked to accommodate the new engines and the intakes were modified with longer ramps.  53-2418 first flew with theYJ79 installation on 3 November 1956 with an initial fit of two YJ79 Phase 0 engines.  Later that year, the NF-101A was modified with improved YJ79 Phase I engines modified with the Basket Burner Test Package and the Parker afterburner selector valve.  During its service with G.E., the aircraft also flew with YJ79-GE-3 and YJ79-GE-7 engines.  In 1958, 53-2418 became the first aircraft to flight test exotic borane-based high-energy “zip” fuel (HEF). This was done as an adjunct to the General Electric J93-GE-5 program.  The J93 was an enlarged derivative of the J79 intended for use with the B-70 Valkyrie and F-108 Rapier.  The kerosene-fueled J93-GE-3 made it to the hardware stage and was extensively flight-tested on the XB-70.  Waiting on the drawing boards was the J93-GE-5 engine, substituting borane compounds for traditional hydrocarbon compounds as fuel.

Instrument panel of 53-2418 on 2 October 1958.  The master switch for the borane fuel supply can be seen on the upper right, marked "HEF". Gerald Balzer Collection, Greater St. Louis Air & Space Museum.

Borane fuels were the focus of a great deal of research in the 1950s, with hundreds of millions of dollars quietly spent on their development.  These materials reacted with oxygen like traditional hydrocarbon fuels, but offered nearly twice the energy for the same weight of fuel.  In theory, an airplane using these fuels would need to expend less fuel to produce a given amount of thrust, resulting in greatly increased range.  This made borane fuel an extremely attractive option for the B-70, a six-engine, Mach 3-capable aircraft that required intercontinental range without refueling.  However, there were very serious drawbacks to the use of boranes as a practical fuel.  Besides being extremely toxic, boranes are also extremely reactive.  The same qualities that make them an excellent fuel also explain why they do not exist in nature—chemically, they are very unstable.  Boranes have to be synthesized, a very expensive process.  Diborane, the basic building block of all other borane compounds, is a gas that combusts simply on exposure to air at normal temperatures and pressures.  Pentaborane, a liquid, spontaneously combusts at temperatures above 78°F and was itself too unsafe to use as a practical fuel.  Decaborane, which is stable at normal operating temperatures, received serious attention from researchers as both a jet and rocket fuel, but is a solid at room temperature.  Decaborane could be added to the air mixture of a turbine engine as a fine dust or mixed with a hydrocarbon liquid such as benzene.  Unfortunately, the combustion products of borane fuels form a highly corrosive mixture of boric anhydride and water as well as extremely refractory deposits of boron carbide. (Slightly less hard than diamond, boron carbide is presently used as an industrial abrasive and has been used for cockpit armor on the Ling-Temco-Vought A-7D Corsair II).  Injecting borane fuel into the combustion chambers of a turbojet would soon result in a wrecked engine.

In order to be usable, boranes had to be injected into the afterburner section, well behind the more delicate engine components.  This was the arrangement planned for the J93-GE-5.  Borane fuels were flight-tested with this configuration for the first time on 28 September 1958, using the NF-101A with modified J79 engines as the test bed.  The flight tests themselves were successful, but the borate deposits left in the nozzle and afterburner section seriously degraded the usable life of the engines.  Also, the borane fuel produced a great deal of smoke, enough for some to judge it as being impractical and to be potentially unsafe during takeoff.  Despite the promise of borane fuels, their benefits were greatly outweighed by safety concerns and by production and maintenance costs.  Although not an important consideration in the late 1950s, widespread use of these fuels with their acidic combustion products could have also posed significant environmental problems.  One borane compound, triethylborane (TEB) found use as a catalyst for the ignition of the high-flashpoint JP-7 fuel of the SR-71.  The Department of Defense abandoned work on the J93-GE-5 in 1959, in large part due to the results of the flight test work performed by the NF-101A.  However, general research on borane fuels continued for some time afterwards.

By May of 1959, the NF-101A had had its J79 engines removed before the open house at Edwards AFB that year.  Thus, the unwanted and little-heralded Voodoo played an integral part in what had once been thought one of the most important and promising research programs of the Cold War.  Fortunately, this historic aircraft has been preserved.  After the J79 installation was removed, 53-2418 was moved in 1960 to Amarillo AFB Technical Training Center and used as a hydraulics trainer until placed on stands outside of the base as a “gate guard”.  With the closure of the base in 1970, the aircraft was later sold as excess property and purchased by Mr. Dennis E. Kelsey in February 1975.  After residing on the grounds of Bell Helicopter Co., 53-2418 was issued FAA Number N9250Z on 7 April 1976 and moved to Pueblo, Colorado on 11 January 1977.   At this writing, 53-2418 has been moved from Pueblo and after extensive restoration work has been on display at the Evergreen Aviation and Space Museum in McMinniville, Oregon since 2013.