Utilization of High Energy Fuel
Boron-containing materials offer high promise as high-energy liquid propellants for military and perhaps also civilian use
All Aircraft Fuels are materials from which energy can be released in a controlled manner to provide for propulsion. Different applications make different demands on fuels, and so the criteria for choosing a fuel may vary widely. J.R. Cracknell, writing in the March 15, 1957, issue of Flight, said that the ideal fuel should possess the following characteristics:
High Energy content per unit weight
High energy content per unit volume
“Unfortunately for most of the commonly used fuels, these requirements tend to be mutually exclusive. Today, however, a new class of ‘exotic fuels’ encompasses all these requirements.”
Mr. Cracknell pointed out that “the first aviation fuels were hydrocarbons, and with comparatively minor modifications they still hold the field today. They have given good service and appear to meet all the requirements. If this be the case, why the insentive to change and why the interest in new high-energy fuels, especially those of the light-weight elements?”
He also said that “for the civilian operator there is at present no incentive, but for the military every last iota of efficiency and performance counts. With the increasing efficiency of miniaturized nuclear weapons, the weight of the fuel load becomes a critical factor in performance. Conversely, the defense must at all costs ensure that every bomber is intercepted well before it reaches the target, and that every interception will result in a certain kill. Thus, each is straining to the utmost in the performance race and today, with the greatly reduced number of bombers now needed to deliver a crippling blow, even the rarest and most expensive fuels must be considered, if they will give the required performance.”
One of the most important properties of a fuel for an air-breathing power plant is its B.t.u. per pound valve, compounds of the lighter weight elements having higher heats of combustion than the hydrocarbon fuels. Liquid hydrogen immediately stands out, with its very high treat of combustion. However, its extremely low density would require a large storage system for a power plant and this, of course, would necessitate a large refrigeration or insulation system. Beryllium in nearly all of its compounds is extremely toxic and also is in short supply. For these reasons it was decided not to investigate these materials.
The most promising area for research, therefore, appeared to be the boranes (compounds of boron and hydrogen). These boranes, although of a lower heat of combustion per pound than liquid hydrogen, nevertheless possess much higher heats of combustion than the hydrocarbon fuels. In fact pentaborane is about 60% better than JP-4, a commonly used hydrocarbon fuel.
In view of Olin Mathieson’s long interest in explosives and ammunition, as well as the success achieved in developing and producing hydrazine, it was only natural that the corporation should be interested in a high-energy fuels program. It therefore accepted a contract with the Navy Bureau of Aeronautics in 1952 on the ZIP project to prepare high-energy liquid fuels. The goal was to develop a liquid fuel which would perform substantially better than JP-4, the hydrocarbon currently in use.
For the reasons outlined, the field of boron chemistry was chosen as a promising area for development of new fuels.
Boron was discovered as a separate element in 1808. The next step in the development of its chemistry was the work in Germany to prepare the boron hydrides, characterize them, and establish some of their properties. The American effort started in 1942 with extensive studies on the chemistry of the boron hydrides. In 1947 a survey was made by the British, which encompassed studies of boron materials to be used as ramjet fuels. At the same time American effort started on combustion studies and the use of boron hydrides in rocket motors, ramjets, and air-breathing engines. The ZIP project started in 1952 and intensive studies have been in progress ever since that time. A breakthrough has been made and what are now known as the HEF fuels are being synthesized. These materials have been produced in test quantities and have lived up to expectations of high density, low toxicity, and safe handling in addition to a high energy content.
In general, the manufacturing process being used to prepare HEF fuels consists of treating boron-containing ores in such a manner that they are converted to useful intermediate boron compounds. Subsequent reactions of these intermediates convert them to the final product. The entire operation is a completely integrated process, requiring only the starting boron chemicals and a small amount of other materials for chemical make-up to compensate for minor losses. By-products of certain steps in the process are recycled for use as reactants in some earlier processing steps.
Although the entire process is completely integrated and is currently in use for the production of HEF fuels, Olin Mathieson is continuing to investigate alternative routes for certain specific operations, with the view of lowering costs and obtaining higher yields of products and conversions of reactants. To illustrate, the company now has three competitive routes for the conversion of the boron-containing ores. In addition, every step of the process has one or more competitive and economically feasible alternative routes that could be used as the occasion demanded.
From the standpoint of use in liquid propellants, the boron-containing materials evoking the most interest are pentaborane and decaborane.
The replacement of hydrogen by an alkyl group results in a lowered heating value for the resultant compound.
Diborane, the simplest of all boranes, can be prepared by reaction of lithium hydride with boron trifluoride etherate. The product, being a gas, is conveniently removed from the reaction medium. The higher boranes, pentaborane and decaborane, are prepared in several ways. The route most usually followed is through diborane. Somple application of heat is sufficient to convert the diborane to pentaborane and decaborane.
The HEF fuels are believed to meet the requirements for furute high speed missile and aircraft applications. Continued research and development are expected to keep pace with the growing demand for superior performance high-energy fuels in the liquid propellant field. The day may not be far off when these fuels will be available for civilian as well as military uses.
Down the road from our Research and Manufacturing facility was a series of dull grey “army style” buildings set aside from the plant as a top-secret facility, which was, in typical armed forces style, secured with armed guards. Little was known that the work inside did not justify this level of security. The aura, which hung over this facility, was equivalent to the Manhattan Project. Thus this ill conceived idea was that “High Energy Fuels” would provide a tremendous strategic psychological, advantage over the Russians, and thus be instrumental in shifting the balance of power in the Cold War to the west.
The ultimate fate of this plant and research facility and all its associated expense, was that nothing developed there ever would be of national security interest and the cost and difficulties associated with manufacturing the organoborons’ were so high they could never be a commercial success. I have been reminded of a story, in 1959, by a British reporter who wrote “the cost of 1 gallon of this fuel cost as much as an entire British car”
Reference: Borax to Boranes – A collection of papers comprising the Symposium – From Borax to Boranes, presented before the Division of Inorganic Chemistry at the 133rd National Meeting of the American Chemical Society, San Francisco, Calif., April 1958, together with three papers from the 135th ACS Meeting in Boston, Mass., April 1959.
Donald Martin of Olin Mathieson→ wrote the introduction which covers diborane, pentaborane, and decaborane and their properties and the processing the boron ores (from Boron California in the Mojave Desert) sodium tetraborate→ boric acid→ boron trifluoride → sodium borohydride and conversion to diborane and on to higher boranes followed by subsequent alkylation to higher organoboranes. Boron Trichloride may be substituted for boron trifluoride
Pioneer Personalities in Borane Chemistry
Development and Present Status of the Borax Industry
Production of Ammonium Pentaborate and Boric Oxide from Borax
Preparation and Chemistry of Elementary Boron
Structure and Polymorphism in Elemental Boron
High Temperature Chemistry of the Binary Compounds of Boron
Preparation of Diborane
Tetraborane, a Review
Mechanisms of Isotopic Exchanges in the Boron Hydrides
Kinetics and Mechanism for Acid-Base Reactions Involving Boranes
Kinetics and Equilibria in the Alkylation of Diborane
Explosive Oxidation of Boranes
Mass Spectrometry in Boron Chemistry
Infared Spectrometry in Boron Chemistry
Interaction of Boranes
Unstable Intermediates in the Pyrolysis of Diborane
Conversion of Diborane to Higher Molecular Weight Boranes in the Presence of Certain Heterogeneous Catalysts
Impact of Recent Developments in Boron Chemistry on Some Scientific and Engineering Problems
Metal Boron Hydrides
Unusual Salts Derived from the Boron Hydrides
Structures of ***********
Synthesis of Boron-Carbon Ring Compounds
Research on Boron Polymers
Reference: Soap Perfumery and Cosmetics, Author: Maurice Schofield, Pages 286-288, March 1959. BORAX – Rockets into the News