Zip fuel
Encyclopedia
Zip fuel, also known as high energy fuel (HEF), is any member of a family of jet fuel
s containing additives in the form of hydro-boron compounds, or borane
s. Zip fuels offered higher power density than conventional fuels, helping extend the range of jet aircraft, a major problem for the military planners in the 1950s. A number of aircraft were designed to make use of zip, including the XB-70 Valkyrie
, XF-108 Rapier
, as well as the BOMARC
, and even the nuclear powered aircraft program. In testing, the fuels proved to have several serious problems and the entire effort was eventually canceled in 1959. It was later claimed that the Blackstar
spaceplane
uses zip fuel, but Blackstar is almost certainly mythical.
. However, gaseous hydrogen has very low density; liquified hydrogen has higher density but is complex and expensive to store. When combined with other elements, like carbon, the hydrogen can be rendered into the easily burnable hydrocarbon
fuels. Other elements, like aluminum and beryllium
have even higher energy content than carbon, but do not mix well to form a stable fuel that can be easily burned.
Of all the low-mass elements, boron has the combination of high energy, low weight and wide availability that makes it interesting as a potential fuel. Boranes have a high specific energy
, about 70,000 kJ/kg (30,000 BTU/lb). This compares favorably to a typical kerosene
-based fuel, such as JP-4
or RP-1
, which provides about 42,000 kJ/kg (18,000 BTU/lb). They are not suitable for burning as a fuel on their own, however, as they are often prone to self-ignition in contact with air, making them dangerous to handle.
When mixed with conventional jet fuels, however, they add to the energy content while becoming somewhat more stable. In general terms, boron-enhanced fuels offer up to 140% the energy density of plain JP-4 in terms of both weight and volume. In the US a whole family of fuels were investigated, and generally referred to by the names they were assigned during the Air Force's Project HEF: HEF-1 (ethyldiborane), HEF-2 (propylpentaborane), HEF-3 (ethyldecaborane), HEF-4 (methyldecaborane), and HEF-5 (ethylacetylenedecaborane).
All zip fuels also have a number of disadvantages. For one, the fuel is toxic, as is its exhaust. This was of little concern "in flight", but a major concern for ground crews expected to service the aircraft. Additionally, the fuels burn to create solids that are both "sticky" and corrosive. In particular, boron oxides are chemically reactive (and thus toxic), and boron carbide is mechanically "sharp". This caused serious problems for turbine
blades in jet engines, where the exhaust built up on the blades and reduced their effectiveness and sometimes caused catastrophic failure of the engine. Finally, the exhaust plume is filled with particulates, like coal smoke, allowing an aircraft to be spotted visually at long range.
In the end, the problem of burning HEF throughout the entire engine proved impossible to solve. Removing the buildup was difficult, and the wear on the metals it caused was something that materials science was unable to address. It was possible to burn it with relative ease in an afterburner
, but this would only be effective on aircraft that used an afterburner for extended periods of time. Combined with the high cost of producing the fuel and the toxicity issues, the value of zip fuel was seriously eroded. In 1959 the Air Force eventually gave up and canceled the program, although some small-scale work as a rocket fuel continued. This too proved to be a dead-end; the solid boron oxides in the combustion products interfered with the expected thermodynamics, and the thrust advantages could not be realized.
rocket-related Project HERMES in the late 1940s, the U.S. Navy
Bureau of Aeronautics
's Project ZIP in 1952, and the U.S. Air Force
's Project HEF (High Energy Fuels) in 1955. For much of the 1950s, zip fuels were considered to be the "next big thing" and considerable amounts of money were poured into these projects in an effort to bring them into service. The Navy's name stuck, and all of the boronated fuels became known as "zip fuels", although the Air Force's naming for the fuels themselves became common.
The main thrust of the Air Force's program was based on HEF-3, which seemed to be the most likely candidate for quick introduction. HEF became part of the WS-110 efforts to build a new long-range bomber to replace the B-52 Stratofortress
with a design able to dash at speeds up to Mach 2. The initial designs from Boeing
and North American Aviation
(NAA) both used conventional fuels for takeoff and cruise, switching to HEF during the high-speed dash, burning it only in their afterburner sections. This avoided both of the main problems with HEF; by burning it only in the afterburners the problem with buildup on the turbine was eliminated, and since the afterburners were only used for takeoff and high-speed flight, the problems with the toxic exhaust were greatly reduced.
When the initial designs proved to be too expensive to justify their relatively small performance improvement, both returned to the drawing board and came up with new designs that flew at supersonic speeds for most of a combat mission. These designs were based around new engines designed for sustained high-speed flight, with the NAA B-70 Valkyrie and General Electric J93
progressing to the prototype stage. In these cases the afterburners were used for a longer period, maximizing the benefits of HEF. There were plans to introduce a later version of the J93 that would burn HEF-4 throughout. Meanwhile there were also studies on using HEF-3 in the BOMARC ramjets, as well as studies about carrying it on the U.S. Navy's aircraft carrier
fleet to power future aircraft, but these programs both died out.
As the problems were proving intractable, the Air Force canceled their program in 1959, and interest in zip basically disappeared. By this point the only design still considering using HEF was the XB-70 and its J93. NAA and General Electric
responded by redesigning the engine to run on a new higher-density form of jet fuel, JP-6, and filling one of the two bomb bays with a new fuel tank. In doing so the range was dramatically reduced from about 7700 nautical miles (14,260.4 km) to 5500 nautical miles (10,186 km). This reduced the selection of targets that could be attacked from the United States and required in-flight refueling for every mission profile, one more problem that led to the project's eventual re-direction as a purely experimental aircraft.
It is estimated that the U.S. spent about $1 billion on the program, in 2001 inflation-adjusted dollars. At least five HEF production plants were built in the U.S., and two workers were killed in an explosion that destroyed one plant in New York. Most of the program was classified Top Secret
while being carried out, but both the U.S. and Soviet Union
independently declassified their research in 1964. In spite of classification, the topic was widely covered in the press.
One potentially lasting relic of the HEF program is an abandoned dirt airfield outside Boron, California
. Marked on USGS topographical maps as "Air Force Plant #72", nothing but the airstrip and a water tank were even built on the site. It is speculated that this would have been a factory for HEF fuel, using the large borax
deposits nearby (giving the town its name), where it could be easily shipped to Edwards Air Force Base
.
Jet fuel
Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is clear to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1 which are produced to a standardized international specification...
s containing additives in the form of hydro-boron compounds, or borane
Borane
In chemistry, a borane is a chemical compound of boron and hydrogen. The boranes comprise a large group of compounds with the generic formulae of BxHy. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently. The parent member BH3 is called...
s. Zip fuels offered higher power density than conventional fuels, helping extend the range of jet aircraft, a major problem for the military planners in the 1950s. A number of aircraft were designed to make use of zip, including the XB-70 Valkyrie
XB-70 Valkyrie
The North American Aviation XB-70 Valkyrie was the prototype version of the proposed B-70 nuclear-armed deep-penetration strategic bomber for the United States Air Force's Strategic Air Command...
, XF-108 Rapier
XF-108 Rapier
The North American XF-108 Rapier was a proposed long-range, high-speed interceptor aircraft designed by North American Aviation. Initiated to defend the United States from supersonic Soviet bombers, the program, initially called NA-257, was cancelled in 1959 due to a shortage of funds and the...
, as well as the BOMARC
Bomarc Missile Program
The CIM-10 Bomarc was the only surface-to-air missile ever deployed by the United States Air Force. All other U.S. land-based SAMs were and are under the control of the United States Army....
, and even the nuclear powered aircraft program. In testing, the fuels proved to have several serious problems and the entire effort was eventually canceled in 1959. It was later claimed that the Blackstar
Blackstar (spaceplane)
Blackstar is the reported codename of a secret United States orbital spaceplane system. The possible existence of the Blackstar program was reported in March 2006 by Aviation Week & Space Technology magazine; the magazine reported that the program had been underway since at least the early 1990s,...
spaceplane
Spaceplane
A spaceplane is a vehicle that operates as an aircraft in Earth's atmosphere, as well as a spacecraft when it is in space. It combines features of an aircraft and a spacecraft, which can be thought of as an aircraft that can endure and maneuver in the vacuum of space or likewise a spacecraft that...
uses zip fuel, but Blackstar is almost certainly mythical.
Description
The ultimate fuel from a mass-to-performance perspective is hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. However, gaseous hydrogen has very low density; liquified hydrogen has higher density but is complex and expensive to store. When combined with other elements, like carbon, the hydrogen can be rendered into the easily burnable hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
fuels. Other elements, like aluminum and beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
have even higher energy content than carbon, but do not mix well to form a stable fuel that can be easily burned.
Of all the low-mass elements, boron has the combination of high energy, low weight and wide availability that makes it interesting as a potential fuel. Boranes have a high specific energy
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
, about 70,000 kJ/kg (30,000 BTU/lb). This compares favorably to a typical kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
-based fuel, such as JP-4
JP-4
JP-4, or JP4 was a jet fuel, specified in 1951 by the U.S. government . It was a 50-50 kerosene-gasoline blend. It has lower flash point than JP-1, but was preferred because of its greater availability. It was the primary U.S. Air Force jet fuel between 1951 and 1995. Its NATO code is F-40...
or RP-1
RP-1
RP-1 is a highly refined form of kerosene outwardly similar to jet fuel, used as a rocket fuel. Although having a lower specific impulse than liquid hydrogen , RP-1 is cheaper, can be stored at room temperature, is far less of an explosive hazard and is far denser...
, which provides about 42,000 kJ/kg (18,000 BTU/lb). They are not suitable for burning as a fuel on their own, however, as they are often prone to self-ignition in contact with air, making them dangerous to handle.
When mixed with conventional jet fuels, however, they add to the energy content while becoming somewhat more stable. In general terms, boron-enhanced fuels offer up to 140% the energy density of plain JP-4 in terms of both weight and volume. In the US a whole family of fuels were investigated, and generally referred to by the names they were assigned during the Air Force's Project HEF: HEF-1 (ethyldiborane), HEF-2 (propylpentaborane), HEF-3 (ethyldecaborane), HEF-4 (methyldecaborane), and HEF-5 (ethylacetylenedecaborane).
All zip fuels also have a number of disadvantages. For one, the fuel is toxic, as is its exhaust. This was of little concern "in flight", but a major concern for ground crews expected to service the aircraft. Additionally, the fuels burn to create solids that are both "sticky" and corrosive. In particular, boron oxides are chemically reactive (and thus toxic), and boron carbide is mechanically "sharp". This caused serious problems for turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
blades in jet engines, where the exhaust built up on the blades and reduced their effectiveness and sometimes caused catastrophic failure of the engine. Finally, the exhaust plume is filled with particulates, like coal smoke, allowing an aircraft to be spotted visually at long range.
In the end, the problem of burning HEF throughout the entire engine proved impossible to solve. Removing the buildup was difficult, and the wear on the metals it caused was something that materials science was unable to address. It was possible to burn it with relative ease in an afterburner
AfterBurner
The AfterBurner is a lighting solution for the Game Boy Advance system that was created by Triton-Labs.Originally, portablemonopoly.net was a website created to petition Nintendo to put some kind of light in their Game Boy Advance system...
, but this would only be effective on aircraft that used an afterburner for extended periods of time. Combined with the high cost of producing the fuel and the toxicity issues, the value of zip fuel was seriously eroded. In 1959 the Air Force eventually gave up and canceled the program, although some small-scale work as a rocket fuel continued. This too proved to be a dead-end; the solid boron oxides in the combustion products interfered with the expected thermodynamics, and the thrust advantages could not be realized.
History
Several studies were made into boronated fuels over the years, starting with the U.S. Army'sUnited States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...
rocket-related Project HERMES in the late 1940s, the U.S. Navy
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...
Bureau of Aeronautics
Bureau of Aeronautics
The Bureau of Aeronautics was the U.S. Navy's material-support organization for Naval Aviation from 1921 to 1959. The bureau had "cognizance" for the design, procurement, and support of Naval aircraft and related systems...
's Project ZIP in 1952, and the U.S. Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
's Project HEF (High Energy Fuels) in 1955. For much of the 1950s, zip fuels were considered to be the "next big thing" and considerable amounts of money were poured into these projects in an effort to bring them into service. The Navy's name stuck, and all of the boronated fuels became known as "zip fuels", although the Air Force's naming for the fuels themselves became common.
The main thrust of the Air Force's program was based on HEF-3, which seemed to be the most likely candidate for quick introduction. HEF became part of the WS-110 efforts to build a new long-range bomber to replace the B-52 Stratofortress
B-52 Stratofortress
The Boeing B-52 Stratofortress is a long-range, subsonic, jet-powered strategic bomber operated by the United States Air Force since the 1950s. The B-52 was designed and built by Boeing, who have continued to provide maintainence and upgrades to the aircraft in service...
with a design able to dash at speeds up to Mach 2. The initial designs from Boeing
Boeing
The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...
and North American Aviation
North American Aviation
North American Aviation was a major US aerospace manufacturer, responsible for a number of historic aircraft, including the T-6 Texan trainer, the P-51 Mustang fighter, the B-25 Mitchell bomber, the F-86 Sabre jet fighter, the X-15 rocket plane, and the XB-70, as well as Apollo Command and Service...
(NAA) both used conventional fuels for takeoff and cruise, switching to HEF during the high-speed dash, burning it only in their afterburner sections. This avoided both of the main problems with HEF; by burning it only in the afterburners the problem with buildup on the turbine was eliminated, and since the afterburners were only used for takeoff and high-speed flight, the problems with the toxic exhaust were greatly reduced.
When the initial designs proved to be too expensive to justify their relatively small performance improvement, both returned to the drawing board and came up with new designs that flew at supersonic speeds for most of a combat mission. These designs were based around new engines designed for sustained high-speed flight, with the NAA B-70 Valkyrie and General Electric J93
General Electric YJ93
-See also:-External links:...
progressing to the prototype stage. In these cases the afterburners were used for a longer period, maximizing the benefits of HEF. There were plans to introduce a later version of the J93 that would burn HEF-4 throughout. Meanwhile there were also studies on using HEF-3 in the BOMARC ramjets, as well as studies about carrying it on the U.S. Navy's aircraft carrier
Aircraft carrier
An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...
fleet to power future aircraft, but these programs both died out.
As the problems were proving intractable, the Air Force canceled their program in 1959, and interest in zip basically disappeared. By this point the only design still considering using HEF was the XB-70 and its J93. NAA and General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
responded by redesigning the engine to run on a new higher-density form of jet fuel, JP-6, and filling one of the two bomb bays with a new fuel tank. In doing so the range was dramatically reduced from about 7700 nautical miles (14,260.4 km) to 5500 nautical miles (10,186 km). This reduced the selection of targets that could be attacked from the United States and required in-flight refueling for every mission profile, one more problem that led to the project's eventual re-direction as a purely experimental aircraft.
It is estimated that the U.S. spent about $1 billion on the program, in 2001 inflation-adjusted dollars. At least five HEF production plants were built in the U.S., and two workers were killed in an explosion that destroyed one plant in New York. Most of the program was classified Top Secret
Classified information
Classified information is sensitive information to which access is restricted by law or regulation to particular groups of persons. A formal security clearance is required to handle classified documents or access classified data. The clearance process requires a satisfactory background investigation...
while being carried out, but both the U.S. and Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
independently declassified their research in 1964. In spite of classification, the topic was widely covered in the press.
One potentially lasting relic of the HEF program is an abandoned dirt airfield outside Boron, California
Boron, California
Boron is a census-designated place in Kern County, California, United States. Boron is located east-southeast of Castle Butte, at an elevation of 2467 feet . The population was 2,253 at the 2010 census, up from 2,025 at the 2000 census...
. Marked on USGS topographical maps as "Air Force Plant #72", nothing but the airstrip and a water tank were even built on the site. It is speculated that this would have been a factory for HEF fuel, using the large borax
Borax
Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...
deposits nearby (giving the town its name), where it could be easily shipped to Edwards Air Force Base
Edwards Air Force Base
Edwards Air Force Base is a United States Air Force base located on the border of Kern County, Los Angeles County, and San Bernardino County, California, in the Antelope Valley. It is southwest of the central business district of North Edwards, California and due east of Rosamond.It is named in...
.