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`JOURNAL OF PROPUISION AND POWER
`Vol. 19, No. 6, November—December 2003
`
`(I)
`
`CrossMark
`“have.
`
`Liquid Fuels and Propellants for Aerospace Propulsion:
`1903-2003
`
`U.S. Air Force Research Laboratory, Wright—Patterson Air Force Base, Ohio 45433-7103
`
`Tim Edwards
`
`Introduction
`
`AJOR increases in liquid-fueled propulsion perfomtance
`have occurred in the past 100 years. The Wright brothers first
`flew on 14 December 1903 with an engine that generated slightly
`more than 130 lb of thrust for flights that ranged from 120 to 850 fi
`(Ref. 1). Contrast that with current aircraft such as the Boeing 777,
`which can fly 8000 n miles, equipped with engines from the GE90
`and PW4000 series that generate over 100,000 lb of thrust. The
`Wright brothers probably consumed less than a gallon of gasoline
`in that first day of flight tests. In 1997, airlines consumed an average
`of 177 million gal/day of jet fuel worldwide.“ Robert Goddard’s
`first flight of a liquid rocket on 16 March 1926 reached an altitude of
`4] ft and landed 184 ft from the launch point (see Ref. 3). Again, the
`contrast with current vehicles such as the space shuttle, which can
`lift over 50,000 lb to 200+ miles into low Earth orbit,‘ is dramatic.
`The intent of this paper is to describe the evolution of liquid fuels
`for aircraft and rockets as the engines and vehicles they fuel have
`undergone these significant increases in performance.
`
`For liquid propellant rockets, the key propellant-related param-
`eters are specific impulse In, (thrust divided by propellant mass
`flow rate) and propellant density. Of course the key difference be-
`tween aircraft engines and rocket engines is the rocket requirement
`to carry an oxidizer onboard the vehicle.The propellant-drivenparts
`of the I", parameter are the propellant (fuel plus oxidizer) heats of
`formation and the propellantstoichiometry.5'6 In general, I
`is pro-
`portional to the square root of the ratio of the flame temperature to
`the combustion-productmolecular weight, so that In, is maximized
`by high flame temperatures and low-molecular weight combustion
`products. A key parameter for both rockets and airbreathing mis-
`siles is the propellant density. For a given application,the figure of
`merit for the propellants is I“, X density‘, where a is an exponent
`that varies for different applications,but is typically between 0.2 and
`1 (Ref. 7). In general, density is most important for the first stage of
`multistage vehicles. Although the cost of the propellants is a small
`fraction of the launch cost, the operationscost and ease of handling
`of propellants is a key concern, particularly for cryogenic and toxic
`storable propellants. For ballistic missile applications (where the
`rocket must remain fueled for long-periods of time), noncryogenic
`(storable) propellants are used to avoid the difficulties of long~term
`storage of cryogenic propellants.
`
`Engine Figures of Merit
`To understand the evolution of fuels and propellants, it is help-
`ful to introduce the fuel properties that drive engine performance.
`For reciprocating, for example, Otto cycle, engines, the key en-
`Brief Note on Terminology
`gine parameters are horsepower/weight ratio (primarily driven by
`Fuels based on petroleum distillates have incorporated a host of
`compression ratio) and specific fuel consumption (fuel mass flow
`somewhat nebuloustenns based in the terminologyofthe early refin-
`divided by engine thrust or horsepower). As described later, the
`ing industry. Early petroleum refineries were primarily distilleries,
`maximum compression ratio is typically limited by the tendency
`with products (fractions) differentiated by boiling point. Thus, a
`of aviation gasoline to autoignite prematurely, as characterized by
`octane number. For gas turbine (Brayton cycle) engines, the most
`reader may come across unfamiliar terms such as kerosene, naph-
`tha, gas oil, heavy or light fraction, sweet or sour crude, straight-
`important engine parametersare thrust/weight ratio and specific fuel
`run, hydrotreated, etc. Table l
`is an attempt to list the common
`consumption. The fuel parameters most relevant to gas turbine en-
`gine and vehicle performanceare the heat of combustion on a mass
`terms for petroleum fractions.2"‘” There is considerable overlap
`between categories, as well as considerable disagreement about the
`basis in air and the density. Through the well—known Breguet range
`equation, these two parameters directly affect the range of the air-
`definition of a particular generic category. For example, one refer-
`ence gives a definitionof kerosene as “a refined petroleumdistillate
`craft. For aircraft today, the cost of the fuel is also a key parameter.
`There are many other fuel parameters that are important to reliable
`that has a flash point of 25 C (77 F),”“ in contrast to the boiling
`engine operationrelating to fuel composition,volati lity, combustion
`range definitions in Table 1. In general, as the boiling temperature
`increases, the molecular weight and density increase and the va-
`performance, stability, and contaminants.The role of these proper-
`por pressure decreases. Thus, one might describe the upper boiling
`ties in gas turbine engine performance is a key part of the historical
`discussion to follow.
`range of a fraction as the heavy or residual end of the fraction.
`
`
` Associate Fellow of AIAA.
`
`Tim Edwards is the Senior Scientist of the Fuels Branch in the US. Air Force Research Laboratory's Propulsion
`Directoratd'l‘urhine Engine Div’sion. He has 20 years of experience in research in fuek and propellants, working
`at both of the Propulsion Directorate's sites at Edwards Air Force Base (no relation) and Wright—Patterson
`Air Force Base (WPAFB). His interests include properties and applications of hydroarbon fuels/propellants and
`endothermicfuels, in support ofadvanwd propulsionprograms for the Department ofDefense and NASA. Edwards
`manages the in-house basic research program in advanced fuels, sponsored by the Air Fome Office of Sdentilic
`Research and assists in coordinating the on-site research occurring at the National Aerospace Fuels Research
`Complex at WPAFB for the Wraitile Affordable Advanced Turbine Engine program. His early work was focused
`on spectroscopic measurementsofsolidpropellant flames at pressure. He could be cons'dered a “native"ofWPAFB,
`having been born in the base hospth while his father worked in the Fuels and Propellants Branch there in the late
`19505. He received a PhD. in chemical engineering from the University of California at Berkeley in 1983. He is an
`
`Received 17 April 2003: revision received 15 August 2003: accepted for publication 29 Augua 2003.This material is declared a work oftbe U.S. Govemmem
`and is not subject to copyright protection in the United States. Copies of this paper may be made for personal or internal use. on condition that the copier
`pay the $10.00 per—copy fee to the Copyright Clearance Center. Inc.. 222 Rosewood Drive, Danvers. MA 01923: include the code 0748-1465803 $10.00 in
`correspondence with the CCC.
`‘Data rise available at httpJ/wwwchevron.com/prodserv/fiielslbulletinl. 1089
`
`UTC-2010.001
`
`GE v. UTC
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`Trial IPR2016-01301
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`

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`1090
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`EDWARN
`
`Table I Petroleum distillate terminology u "
`
`'Iypiarl average Most prevalent
`Approximate
`boiling range, °C urban number
`carbon nunber
`
`Fractions
`Generic Ira-ms
`Gasoline
`Naphtha
`Kerosene
`Fuel oil/gas oil
`Specific products
`Avga
`Motor gasoline
`(mow)
`All“! diesel
`Jet fuel
`
`<200
`150—250
`m3“)
`>275
`
`45—145
`30-200
`
`“-350
`150-265
`
`C1
`C1
`
`Cm
`C”
`
`CH,-CH2-CH,-CH,-CH2-()Ha-CH3
`nomratparaflh: oheptane
`
`(EHJ
`tljl-t'
`CH,-C-CH,-CH-CH3
`I
`CH3
`
`iso-paratlin: iso-octane
`
`CH7=CHvCH7-CH,-CH,
`olefin: 1-pentene
`
`aromatics: benzene
`CH3
`
`/CH
`
`H23 ’
`|
`
`\CHa
`|
`
`nzc\ CH/CHZ
`2
`
`cyclopataftin: methyl cyclohexane
`
`fig. 2 Classes and examples of hydrocarbons found in hydrocar-
`bon fuels; research octane nmnba's: isooctane: lot], n-lreptane=0,
`l-penture: TI, methyl cyclobexane (MCH): 7|, and benzene = 123.
`
`the gasoline to autoignite ahead of the spark (preignition)or before
`the arrival of the flame front (knocking, pinging) was a key limiting
`factor in aircraftengineperforrnance.'lhis is the same knockingthat
`occurs in automobile engines today, but it is much more destructive
`and dangerous in aircraft engines. During the World War I period,
`various researchers (notably Ricardo’s group in England) identified
`this autoignitionphenomenon and began to identify the fuel’s char-
`acteristics that controlled it. The hydrocarbon constituents of fuels
`can be divided into five classes of compounds (Fig. 2): paralfins,
`isoparatfins, cycloparalins (naphthenes),olefins, and aromatics. It
`was found that aromatics such as benzene (benzol in older literature
`was a crude mixture of aromatics consisting primarily of benzene)
`were less prone to knocking and thus, avgas produced from crude
`oilsrich in aromatics were betterperformersthanfuels that were pri-
`marily parafiinic. These differences became evident as the United
`States entered World War I and began supplying gasolines fi'om
`quite different sources than typically encountered in Europe. The
`more-paralinic U.S. gasolines performed relatively poorly in the
`advanced European engines such as the French Gnome. A key de-
`velopment in understanding avgas performance came in the 1920s
`with the standardizationof test methods and engines to determinea
`the tendency of a fire] to knock. This standardization work was led
`by the Cooperative Fuel Research (CFR) committee in the United
`States, composed of representatives from the engine and petroleum
`refining industries. The tendency of a fuel to knock was character-
`ized by its octane rating, developed by Edgar, which compared the
`tendencyofa given gasolineto krtockto that ofa referencefuel com-
`posed of a mixture of n-heptane (octane number= 0) and isooctane
`(octane number: 100). On this scale, the Wright brother‘s 1903
`fuel has been estimated to be 38 octane,”-" with their 1910 fuel
`estimated as 58 octane (Ref. 18). By 1932, industry had generally
`adopted a smndardized method for knock rating, using an engine
`designed by H. L. Homing of the Waukesha Motor Company for
`the CFR committee, and Edgar’s fuel standards. Note that the octane
`scale ranks fuels quite differently titan the cetane scale for fuels for
`dieselengine applications. In acompression-ignitionengine such as
`a diesel, fuels that ignite easily are desirable. Thus, cetane (hexade-
`cane, Golly) has a cetane number of 100, whereas the isoparaflin
`heptametlrylnonane has a cetane rating of 15. Note the inversion
`of the scales: Aromatics and isoparatfins rank high on the octane
`scale and low on the cetane scale, whereas the reverse is true of
`n paraflins.
`In a reciprocating spark-ignition engine, the fuel and air are
`mixed, injected into a cylinder, compressed, and then ignited. The
`development of higher octane avgas was driven by the engine per-
`forrnanceincreasesobtainablevia urrbocharginglsuperchargingand
`increased compression ratios.“ 'Drrbocharging is accomplished by
`compressing engine inlet air using a turbine driven by hot engine
`exhaust gas. Supercharging is the same air compression accom-
`plished by a shaft-mounted turbine. 'l‘urbochargingsupercharging
`
`UTC—2010.002
`
`C3
`C5
`
`Cm
`C”
`
`;
`1
`Jet A
`w
`Areas F
`HP-I
`
`I il
`
`f
`f
`.
`l
`i
`7I
`
`3
`
`60
`
`50
`
`40
`
`59
`o
`f» 30
`3
`20
`
`10
`
`
`
`.
`
`.
`I;
`<
`»
`_
`‘
`4567891011121314151617
`Carbon number
`
`Fig. 1 Carbon numberdistributionfor various hydrocarbon fuels and
`propellants.
`
`low- (mercaptan) sulfur-content products are described as sweet
`(in contrast to high-sulfurproducts being described as sotrr). A fuel
`that is created primarily by distillation is straight run, whereas a
`fuel subjected to the two most common treatments to reduce sulfur
`and other impurities would be called either hydrotreated or Merox
`treated. The original product of the petroleum refining industry was
`lamp fuel; hence, Whittle’s use of illuminating kerosene in his gas
`turbine engine. Aviation gasolineobviouslyfallsin the generic gaso-
`line fraction, whereas mostjet fuels and liquid hydrocarbon propel-
`lants can be described genericallyas kerosene. These definitions are
`shown in Fig. l, where the carbon number distribution for aviation
`gasoline (avgas), jet fuel (let A), and rocket fuel (RP-l) is illus-
`trated. The carbon number distribution in Fig. l is interpreted as,
`for example, avgas consisting of over 50% (by weight) molecules
`that contain 8 carbon atoms. Auto diesel fuel falls mostly into the
`kerosene range, whereas some heavier, for example, marine, diesel
`fuels can be described as gaslfuel oil or residual fuel. Currently, the
`United States consumes roughly twice as much gasoline as diesel
`andjet fuel combined, whereas otherparts of the world have a more
`even distribution.
`
`Airbreatiting Aviation Fuels
`This discussion on aviation fuels relies very heavily on several
`significant references.”"‘ The interestedreader is stronglyencotn-
`aged to seek out these excellentdocuments,and the referencescited
`therein, for additionaldetails that are beyond the scope ofthis paper.
`
`Aviation gasoline (avgas)
`In 1903, the Wright brothers flew, quite naturally, on motor gaso-
`line. They built their own engine, one of many innovations in their
`aircraft. Their four-cylinder water-cooled engine weighed 180 lb
`(83 kg) and developed 12 hp (9 kW). They used “several cans of
`Standard Oil motor gasoline from a nearby boatyard.’"1 This engine
`was quite an advance over the steam engines used by some of their
`competitors.As submquentaircraftenginesevolved,the tendencyof
`
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`EDWARIE
`
`109]
`
`I
`
`1980
`
`1997
`
`
`
`g 1::
`.5:
`as
`'-.
`§ 8
`-'
`g =
`103
`1':
`0
`w
`o 8’
`.
`E
`m;
`E
`
`o53%— ‘
`°
`iE
`a5
`1959
`
`r: :lavlation gasolineg (1e:l)fuelI turbine
`
`25 ~
`
`.
`
`
`
`'5
`
`dramatically increase the maximum speed and altitude that can be
`achieved, when compared to an engine where the intake air is at
`ambient pressure. The compression ratio is the ratio of the com—
`bustion chamber volrnne at bottom dead center (maximum) and top
`dead center (rninimurn). Higher compression ratios enable smaller
`engines for a given power output, in other words, increased engine
`power/volume ratios. The maximum compression ratio and maxi—
`mum amount of inlet air pressurization are both limited by knock-
`ing/fuel autoignition. A tremendous advance in fuel performance
`came from the investigations of Midgeley and Boyd, working for
`Charles Kettering in the Dayton Fmgineering laboratory (Delco) in
`Dayton, Ohio. From an exhaustive study of knock promoters and
`preventers emerged the remarkable additive tetraethyl lead (TEL)
`in December 1921. This additive enabled the production of large
`quantities of higher octane avgas. It was found that the sensitivity
`of avgas to lead varied widely, with aromatic avgas less sensitive
`to knock retardation by lead (as compared to more paraflinic gaso-
`lines). The problem of deposition of solid lead oxides on exhaust
`valves was solvedby the addition ofethylenedibromide (EDB) with
`the TEL. The EDB reacted with the lead in the combustionchamber,
`forming volatile lead bromide compounds.
`Fuel refiners and enginemanufacturersengagedina gameoftech—
`nological leap frog in the late 1920s and 1930s, as improvements
`in engines led to requirements for improved fuels (and vice versa).
`The first avgas specification that included octane number perfor-
`mance requirements was issued by the 0.8. Army for 87 octane in
`1930”“ A significant milestone came in 1934, when the Power-
`plant Branch ofthe US. Army Air Corps at McCook (later Wright)
`Field in Dayton issued the specification X3575 for 100]] 30 octane
`avgas.This specificationcame aboutfrom the work at McCook Field
`(Dayton, Ohio) led by Heron, which demonstrated the significant
`gains in perfomrance from higher octane fuels. The first number in
`the specification is the lean octane rating and the second is the rich
`(high-power) rating. Octane ratings above 100 were characterized
`by performance numbers, with the reference fuels being mixtures
`of isooctane and TEL. For example, 3—cmJ THJgal of isooctane
`yields a rich performance number (PN) of 145. The PN scale is
`roughly linearly related to maximtnn engine output power; in other
`words, a fuel with a 145 PN will yield 45% more knock—limited
`output power than a 100 PN (=100 octane) fuel. Alternatively, an
`engine rated at 1500 hp on 145 PN avgas may only develop 600 hp
`on 80 octane avgas.“s The 100/130 octane specification (AN-F-28 in
`1942) allowed up to 3—cm3 ofTEUgal, but still required high octane
`base gasoline. The productionofthis fuel challengedrefiners for the
`next 10 years and lead to the developmentofalkylationand catalytic
`cracking processes. The availability of 100 octane avgas from the
`United States is widely acknowledgedas one ofthe key contributors
`to the British victory in the Battle of Britain against a numerically
`superiorfoe during World War I]. The productionofavgas increased
`from 54 million gal/year in 1932 to more than 25 million gal/day at
`the end of World War II. The unprecedented industry/govemment
`cooperation needed to surmount the challenges posed by this enor-
`mous expansionin refining capacity has been describedby Heron."
`The quest for more powerfulengines led to 108/135 and l 1511 45 av-
`gas specifications in the 19405. The rather chaotic pace of gasoline
`and engine development led to the creation of 12 separate grades
`of gasoline by 1945, often distinguishedby dye.‘9 In that year, the
`American Society for Testing and Materials (AS'I'M) codified the
`grades of avgas in specification D 615: 80/87 (red) with 0.5-cm3
`TEJgaL91/96 (blue,4.64:rn’lgal), 100/130 (green,4—crn31gal), and
`1 15/145 (purple, 4.6-crn3lgal).
`As gas turbine-powered aircraft became predominant following
`World War II, the 10W] 30 and 80/87 octane specifications became
`the primary ones used. Civilian airlines had generally resisted the
`use of 100 octane fuel before the war because of cost factors. How-
`ever, improvements in fuels and engines enabled greater altitudes
`of operation,which had many operational benefits. The availability
`of improved fuels has been credited as the biggest contribution of
`World War II to the commercial airline industry?" Once the air-
`line industry adopted the gas turbine engine in the late 1950s,2| the
`use ofaviation gasoline was confined to the relatively small general
`
`1951
`
`1970
`
`Fig. 3 Trends in US. aviation fuel consumption."22
`
`aviation market. An illustrationof these market changes can be seen
`in the fuel consumption numbers in Fig. 3.2'22
`By the 19805, various forces had reduced the available grades of
`avgas to essentially one: 100LL (low lead), cunently specified un-
`der (ASTM) specification D 910 as containing a maximum of 0.56
`gll TEL maximum. 'Iype certification can be obtained from the air-
`craft manufacturer for the use of unleaded motor gasoline. The use
`of (unleaded) automotive gasoline in aircraft is controversial.“23
`Aside from the lead issue, there are differences in volatility, addi-
`tives and blending components (especiallyoxygenates),and quality
`control.
`Thus, the development of high-performanceavgas in many ways
`paced the development of reciprocating aircraft engines. The rela—
`tionship betweenjet fuels and gas turbine engines was not as close,
`however. Even in 1949, it was anticipated that improvements in
`gas turbine engine performance would be much less sensitive to
`improvements in fuel.”
`
`'lirrhine Engine l-‘uels
`The earlypioneersin gas turbine developmenLWhittle in England
`and Von Ohain in Germany, faced a wide variety ofoptionsin choos-
`ing a fuel for gas turbines. Whittle had considered diesel fuel, but
`ended up choosing illuminating kerosene because of an expected
`requirement for a lower freeze point than that available with diesel
`(see Ref. 15). In contrast, Von Ohain originally demonstrated his
`turbine engine with hydrogen, but vehicle considerations led to a
`switch to liquid fuel (see Refs. 12 and 21). The world's first turbojet—
`powered flight was made on 27 August 1939 in a Heinkel 178 air-
`craft buming avgas. The first flight of the Whittle engine occlnred
`on 15 May 1941 in a Gloster Meteor aircraft using kerosene as
`the fuel. Despite their head start in turbojet engine development,
`Germany did not decide until 1943 to producejet-poweredaircraft.
`One of the arguments for development at that time was Germany’s
`shortage of high octane fuel and that the jet engine could run on
`diesel frrel.l2
`Most of thejet engines developed before the end of World War II
`utilized conventional kerosene as a fuel. The first jet fuel specifi-
`cation was directorate of engine research and development (DERD
`2482),publishedin Englandin 1 947." As enginesand specifications
`developed, it became apparent that several fuel properties were key
`to bounding the envelope of jet fuel characteristics. mgr-altitude
`operation meant fuel freeze point required attention. However, the
`lower the freeze point, the lower the fraction of crude oil that was
`suitable, so that freeze point had to be balanced against availability.
`Higher fuel volatility/vapor pressure aided vaporization-controlled
`engine perfonnancereqlrirementssuchas altituderelight,which had
`to be traded against boilofl' and entrainment losses from fuel tanks
`at altitude (as well as safety concerns from explosive mixtures in
`tank vapor spaces)“ In the United States, IP— 1 , JP-2, and JP—3 were
`unsuccessful attempts to balance the conflicting requirements of
`volatility, freeze point, and availabilitybost.” TWO fuels emerged
`in the late 1940s and early 1950s from this chaotic situation: a
`wide—cut naphtha]kerosenemixture called JP-4 in the United States
`(MIL-F-5624 in 1950) and a kerosene fuel with a —50°C (—58°F)
`freeze point (DERD-2494 in England and Jet A—l in ASTM D- 1655
`
`UTC—2010.003
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`

`

`1092
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`EDWARN
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`sonic flight leads inevitably to regenerative cooling of (at least) the
`combustor. The heat loads and fuel flows are such that high lev-
`els of fuel heat sink are required. This heat sink can be obtained
`from sensible heating of high-heat capacity fuels such as liquid hy-
`drogen or the use of endothermic (hydrocarbon) fuels. For many
`applications, hydrocarbon fuels are preferred due to their greater
`density" and ease of handling. Endothennic fuels achieve heat sink
`by deliberate reactions of the bulk fuel, such as dehydrogenationor
`crackingfu'” Engine conceptsfor hypersonic aircraft are currently
`under development in several programs. Potential future develop-
`ments in this area will be discussed in the Future Trends section.
`The petroleum shortages of the 1970s led to the search for do-
`mestic sources of liquid transportation fuels. large United States
`reserves of coal and oil shale (and Canadian reserves of arr sands)
`spurred the development of conversion processes to produce fuels
`from these non-petroleumsources. In the 19808, programs were ini-
`tiated to derrronstrate the suitability of fuel derived from shale,”‘“
`coal,” and tar sands.‘0 Engine testing and flight demonstrationsof
`shale-derived IP-4 indicated no deleterious effects resulting fi'om
`the use of shale4lerivedfuel. The success of this program indicated
`that the JP-4 specification was restrictive enough to provide ade-
`quate fuel, regardlessofthe hydrocarbon source. Jet fuels produced
`from synthesis gas (C0 + H2) via Fischer—Tropsch (F—T) mchnol-
`ogy are currently being studied for their suitability for aircraft (see
`Refs. 41 and 42). The synthesis gas can be produced from coal, nat-
`ural gas, or other carboncontainingmaterials. A 50/50 mixture of
`petroleum-derivedJet A-1 and isoparaflinic kerosene derived from
`coal is being delivered to aircraft in South Africa. A thorough study
`by Southwest Research Institute demonstrated that the 5(Y50 mix-
`ture properties fell well within the Jet A-l specification range and
`should have no impact on engine operation.“42 The composition
`differences between the two fuels are shown in Figs. 4 and 5. Lu-
`bricity and elastomer compatibility issues for use of the pure F—T
`fuel are currently being addressed.
`World-widejet fuel consumptionwas 177milliongallda in 1997,
`with about 40% of the consumption in the United States. US. jet
`fuel consumption is predominately(~90%) by commercial aircraft.
`
`in the United States). This freeze point was arrived at through a sig—
`nificant research effort. ASTM D—1655 also specified let A with a
`—40°C (—40°F) freeze point. The Jet A—l freeze point was changed
`to —47°C in the late 1970s to increase availability.” Civil avia-
`tion currently uses Jet A—l (or its equivalent) throughout the world,
`except for domestic carriers in the United States, who use let A.
`Military aircratt used 1P4 until converting to lP—8 in the 19805. JP—
`8 (M1L-T-83133)is essentially Jet A—l with three military-speciied
`additives (as described later). The conversion to 1P—8 occurred pri—
`marily to improve the safety of aircraft, although the single fuel for
`the battlefield concept (and the similarity ofjet fuel to diesel fuel)
`is centered on the use of aviation kerosene in all US. Air Force and
`US. Army aircraftand ground vehicles. A similar process is occur-
`ring in the US. Navy, where the large variety of liquid fuels have
`compressed down tojust two, JP-S (for aircrafl) and F—76 diesel for
`all other liquid fuel requirements.
`The history of the evolution of conventional,widely availablejet
`fuels from the late 1950s to the present is mainly the story of the
`evolutionof test methodsand fuel additives to maintain the integrity
`of the jet fuel supply and to improve safety and correct operational
`problems. Becausetheirimportance.specificationsltest methodsand
`additives are discussed separately later.
`Specialty fuels were developed for various applicationsthrough-
`out the second—half of the 20th century. In the early 1950s, lP—S
`(included in MlL-F—S624) was developed. JP-S is a high-flash—point
`(60°Cll 40"F) aviation kerosene used onboard US. Navy ships to
`enhance safety. The development of higher Mach aircraft led to
`several specialty fuels. As flight velocity increases, aerodynamic
`heating leads to larger amounts of heat being rejected to the fuel,
`both in the tanks and in the engine, leading to vapor pressure and
`thermal stability concerns. The cutotfpoint between the use ofcon-
`ventionalJet A- lIJP-8 fuels and speciallyproducedfuels is between
`Mach 2.2 and 3. Thus, the Mach 2.2 Concorde uses .Iet A—l , whereas
`the Mach 2-3 XB-70 and SR-71 used specialty fuels. JP-6 (MIL-F-
`25656) was a low-volatility kerosene developed for the Mach 2+
`XB-70.” The Mach 3 811-71 required JP—7 (MlL-T-38219), a low-
`volatilitylhigh thermal stability, highly processed (low sulfur and
`aromatics) kerosene?” The U-2 high-altitude reconnaissanceair—
`craft requiredbottr improvedtherrnal stabilityand lower freeze point
`in its fuel (JP-TS, MlL-T-25524) because of its high-altitude, long-
`duration cruise. These specialty fuels gave higher performancethan
`conventional aviation kerosenes, at the expense of higher fuel and
`logistical costs (JP-7 and JP-TS are roughly three times the cost of
`JP—8 and Jet A-l). The accepted operational temperature limits of
`these various fuels are approximately 163°C (325”F) for Jet AIJet
`A—lIJP-8IJP-5, 219°C (425°F) for JP-TS, and 288°C (550°F) for
`JP—7 (Ref. 26).
`Russian jet fuels underwent a parallel evolution throughout this
`period?” In most areas, cru'rent Russian fuels T'S—l and RT“ and
`Russian specifications (GOST 10227) are interchangeable with let
`A- lIJP-8. The rrrain difference between fuels TS-l and RI‘ is in the
`area of thermal stability: 'l'S-l is a straight-run fuel, whereas RT is
`hydrotreated. By comparison with Jet A—1IJP—8, TS-l and RT are
`lighter (have a lower initial boiling point and 10% recovery point
`in distillation) and have a conespondingly lower flash point and
`freeze point. Thus, worldwide there are three major specifications
`in civil use: ASTM D 1655, British Defence Standard (Def Stan)
`91—91 (successorto DERD 2494), and 6081' 10227. International
`oil companies have created the Joint Check List to standardizejet
`fuel deliveries worldwide under let A— lIDef Stan 91 —91 . The Inter—
`nationalAirTransportAssociationhasalso issued guidancematerial
`for its members codifying the Jet AIJet A- llT‘S-l specifications.
`Pustyrev discussestwo specialty Russian fuels specified in COST
`1 2308: T-8V, a higherdensitylhigher flash-pointkeroseneand T-6, a
`high—densitykerosene (specific gr'avity0.84 vs 0.8 forJet A-1/JP—8),
`which has no commercial or military counterpart in Europe or the
`United Suites." U.S. AirForce prograrnsin the 1980s demonstrated
`the production of fuels similar to T—6 (Refs. 29 and 30), but no
`specification was published in the absence of user requirements.
`Beyond Mach 5, flight speeds are considered hypersonic (vs su-
`personic). The high-heat loads encountered by vehicles in hyper—
`
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`fig. 4 Compositiontlistribution for South African petroleum-derived
`Jet A-l (Rd. 41).
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`Fig. 5 Composition distribution for isoparaflinic kerosene produced
`from coal-derived synthesis gas by F—T procus."
`
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`cuu /
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`Fig. 7 Jet fud additives.
`
`In 1954, the US. Government began using commercial pipelines
`to transport aircraft fuel to US. Air Force facilities. To combat the
`excessive corrosion in the ground fuel systems, and to reduce the
`carryoverofcorrosionproducts into aircraft fuel systems, corrosion
`inhibitoradditiverequirementswere addedto specificationsforboth
`avgas and turbine engine fuels. A specification (MIL-I-25017) was
`issrredin 1954for corrosioninhibitoradditives. These additivesalso
`act to improve fuel lubricity(lubricatingcapability)and are typically
`added at approximately 20 ppm.
`Water contamination in aviation fuels has always been a serious
`problem. In liquid form, water can cause temporary fiameout in the
`engine, but in solid form (ice), it can block filters and fuel lines
`and completely stop the flow of fuel to the engine. In the l940s
`and 19505, free, undissolved water in fuel was suspected as the
`cause of many in-flight incidents and accidents. A major research
`and development program was initiated to solve the water-in-fuel
`problem as a result of a B-52 crash in 1958. One objective was
`the development of a fuel system icing inhibitor (FSII). The FSII
`was to be added to the fuel, but would preferentially migrate to
`any free water present and act as an antifreae. The current icing
`inhibitorused is diethyleneglycol monomethyletherat a maximum
`concentration of 0.1 5 vol%. The FSll also acts as a biocide.
`Because of their low electrical conductivity, aviation fuels can
`build up a static electrical charge, especially during fueling. Dis-
`charge ofthis builtup charge in areas where flammable fuel/air mix-
`tures exist, for example, fuel tanks, has been a problem. A solution
`is use of static dissipator additives (SDA) in the fuel. Octel Stadis
`450 is the only currently approved SDA for turbine engine fuels.”
`Typical concentrations of 0.5—2.0 mgll increase the fuel conductiv-
`ity to between200and600pSlm. Asshownin Table 2,theminimrun
`conductivity allowed in JP-8 is 150 pSIm. The main difference be-
`tween commercial .let A-l fuel and military IP—8 fuel is the specified
`presence of corrosion inhibitor, PSI], and SDA.
`Antioxidantadditivesare added to turbine engine fuels and other
`petroleum productsto prevent the formation of gums and peroxides
`during storage by reducing the formation of free radicals in the fuel.
`Peroxides are deleteriousto thermal oxidative stability, possibly be-
`ingprecursorsto theformationofdeposits.Peroxidesalsoattackfuel
`tank polysulfide sealantsand otherfuel systemelastomers. The most
`common antioxidantsare hinderedphenols, exemplified by 2,6—di—
`tert—butyl—4—methylphenol[butylatedhydroxy—toluene(BHT), also
`used in food]. Normal antioxi