26th INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES

AN ASSESSMENT OF THE POTENTIAL OF HYDROGENFUELLED LARGE LONG-RANGE TRANSPORT AIRCRAFT

D. VerstraeteCranfield University, School of Engineering

Keywords: hydrogen fuel, preliminary design, long-range transport aircraft

Abstract

The rapid growth of air traffic prohibits the reduc-tion of the absolute levels of CO2 by aviation, de-spite the increasingly clear need to do so. Alter-native fuels like hydrogen could therefore allowa sustainable growth of air traffic. However, theadoption of hydrogen as an aviation fuel posesseveral technical challenges and an assessment ofits economic potential is thus in order. In this pa-per this assessment is made for a large long rangetransport aircraft.

After a brief description of the routine used toperform the preliminary design of both keroseneand hydrogen fuelled aircraft, a case study on alarge long range transport aircraft is reported. Forboth fuels the wing area and aspect ratio leadingto minimal direct operating costs are determinedand the influence of the price level of the fuel isgiven. It is shown that hydrogen might have a sig-nificant economical potential as the fuel price fora given amount of energy can be twice as high forhydrogen without leading to an increase in oper-ational costs.

1 Introduction

Civil aviation faces a mounting conflict betweenthe rapid growth of traffic demand and the in-creasingly clear need to reduce the emissions ofgreenhouse gases, particularly CO2, into the at-mosphere. Even though advances in aeronauticaltechnology have lead and will lead to a significantimprovement in the energy efficiency of transportaircraft, there is no realistic prospect that such

gains in efficiency will be sufficient to compen-sate for the anticipated traffic growth as far asgreenhouse gas emissions is concerned. Nor is itlikely that aviation will be accorded a privilegedexemption from the general need to reduce CO2emissions.

As an energy carrier which could be producedfrom any primary energy source - especially fromrenewables or nuclear energy as this would leadto a completely CO2 free chain from "well towing" - the adoption of hydrogen would protectsustainable long term growth of air traffic andalso improve the reliability of the energy supplyto the aviation sector. As multiple technologiescan be used to produce the hydrogen fuel, a morestable price would normally be obtained as well.Increases in production costs through one spe-cific technology could namely be counteracted byswitching to another primary energy source. Thiswould lead to a much more stable economical en-vironment for the complete aviation sector, in bigcontrast with the current situation where airlinersare facing the daily challenge of continuously ris-ing kerosene fuel prices, a situation not likely tochange in the future seen the dwindling resourcesof fossil fuels and the rising demand.

However, the use of hydrogen as an aviationfuel entails several technical challenges on theaircraft level. Even though hydrogen fuel con-tains, weight for weight, 2.8 times more energythan kerosene, a significant part of this advantagewill be eaten up by the weight of the complex fuelsystem, specifically the large, bulky and heavycryogenic fuel tanks. After all, to store the sameamount of energy, liquid hydrogen needs a vol-

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D. VERSTRAETE

ume about 4 times bigger than kerosene. In addi-tion, the tanks must be spherical or cylindrical inshape to allow for insulation requirements, whichresults in unusual aircraft configurations with thefuel tanks inside the fuselage.

This paper assesses the economical and oper-ational potential of hydrogen for large long rangetransport aircraft. In a first section, the routinedeveloped for the preliminary design of keroseneand hydrogen fuelled aircraft is briefly described,focussing on the adaptations needed when usinghydrogen fuel. As the tanks are a critical compo-nent of the hydrogen fuelled aircraft, the next sec-tion is devoted to their design and sizing. Onceall models used are established, a case study isrun on a 380 passenger 7500 nm transport air-craft.

2 Description of the preliminary design rou-tine for hydrogen fueled aircraft

As one of the main goals of this work is an as-sessment of the technical and economical poten-tial of hydrogen fuelled long-range transport air-craft, in a first step a routine was developed forthe preliminary design of kerosene fuelled air-craft. The structure of this routine is given inFig. 1. As shown on the figure, input is requiredfor the engine fuel consumption during the differ-ent flight phases and to define the mission range,initial cruising altitude, cruise Mach number andnumber of passengers. The diameter of the fuse-lage and the length of the cabin need to be givenas well to set up a design calculation. Based onthese input parameters, the different modules ofthe routine are called to perform the preliminarydesign calculations.

More details on the different modules and theadopted correlations for the calculations of themass of the components, the drag and lift of theaircraft, the performance requirements and thedirect operating cost calculations can be found inref. [13] and [14] where some results for largelong range passenger aircraft case studies aregiven too.

Here, only the changes in the method to al-low the designs of the hydrogen fuelled aircraft

Fig. 1 The structure of the routine

will be commented upon. As the hydrogen fuelwill be stored in large tanks inside the fuselage,its weight and size need to be reassessed. Thesame applies to the wing as the wing root bend-ing moment is no longer relieved by the weightof the fuel stored in it. As the hydrogen tanksmight entail an increased aircraft acquisition andmaintenance cost, the direct operating cost calcu-lations are also reviewed.

2.1 Changes to the aircraft fuselage

The liquid hydrogen fuel has to be stored in tanksinside the fuselage (ref. [1], [3] and [9]), so boththe geometry and the weight of the fuselage needto be adapted when changing from kerosene tohydrogen designs. As the fuselage length willdepend on the amount of fuel needed for the mis-sion, an iteration on the length is required.

The fuselage weight for the kerosene aircraftis calculated as the average of the weights ob-tained with the correlations from references [2]and [4]. Equation 1 gives the correlation fromreference [2]:

Wf us = KeKlgSwet

!10+1.2d +0.0019

nWdim

h1.7

"

(1)where d and h are the fuselage diameter andheight [m], n is the ultimate load factor [–], Wdimthe fuselage dimensioning mass [kg] and Ke andKlg correction factors for the engine respectivelymain landing gear location [–].

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An assessment of the potential of hydrogen fuelled large long-range transport aircraft

Both adopted correlations use the wetted areaof the fuselage to determine its weight, so theeffect of the increase in fuselage size is alreadyimplicitly included. However, the reinforcementsneeded to accomodate the tanks will increase thefuselage weight even more. According to ref. [3],this can be accounted for by an increase of theweight by 6%.

For longitudinal balancing and to ensure aproper location of the aircraft centre of gravity,two tanks are adopted in this design. One tankis positioned in the front section of the fuselage,in between cockpit and cabin. The second tankis located aft of the cabin in the tailcone of theaircraft. Following ref. [3], the front tank is takenslightly smaller than the aft tank, storing 40 % ofthe total fuel.

2.2 Changes to the aircraft wing

The cryogenic liquid hydrogen fuel is not storedin the wing, so its weight will not reduce thewing root bending moment as is the case with thekerosene fuel. The structural weight of the wingwill therefore slightly increase, keeping all otherinfluential parameters constant.

Some of the correlations for wing weight cal-culations found in literature take the effect of thefuel weight on the bending moment into accountby a so-called bending relief factor. The correla-tions from references [2], [4] and [5] are adoptedin this work as all three allow to take the absenceof the fuel in the wing into account. The averageof the 3 methods is used as the final wing weight.

2.3 Changes in direct operating costs

The change to hydrogen fuel represents a verysignificant one-time capital investment cost toadapt the complete infrastructure of all airportsin the world. In this work, this cost of changingthe infrastructure will not be considered to allowa comparison of kerosene and hydrogen fuelledaircraft on a fair and equal basis. Therefore onlythe direct operating costs will be used in the as-sessment.

Direct operating costs are those costs associ-ated with flying the plane and its maintenance.

They comprise the costs associated with the ini-tial capital investment when buying the airplane(depreciation, insurrance), the cost of operatingthe plane (fuel, crew) as well as the maintenancecost. Direct operating costs are normally used inthe preliminary design phase to identify the opti-mal size and configuration of the aircraft as theyonly consider the operational aspects of the totalcosts.

Apart from the hydrogen fuel storage anddelivery system, the hydrogen fuelled aircraftare similar to their conventional kerosene-basedcounterparts. The main changes in aircraft directoperating costs will thus stem from the integra-tion of the cryogenic fuel tanks in the aircraft. Asit is hard to predict the influence of these tankson the overall aircraft price, the acquisition andmaintenance cost of the airframe (aircraft less en-gines) will be varied for the different case studies.An increase of 10,50 and 100 % of the airframeacquisition and maintenance cost is used to as-sess its influence on the overall direct operatingcost as shown in the case studies of section 4.

3 Design of the LH2 tanks

The design and development of the fuel tanks andtheir thermal protection systems to contain cryo-genic liquid hydrogen in a satisfactory manneris regarded as one of the crucial technical chal-lenges confronting use of LH2 in operational air-craft (ref. [1], [3] and [9]). After all, this type oftank currently does not exist as cryogenic LH2tanks for space applications are designed for avery short tank lifetime and a limited number ofcycles. Automotive applications are, on the otherhand, much less influenced by weight limitations,which leads to relatively heavy designs that arenot suited for aeronautical applications.

To minimize the weight of the tanks, integraltanks, where the tank wall forms a part of the ba-sic airframe structure, are adopted. They namelylead to a lower tank weight and a smaller over-all tank volume compared to non-integral tanks(ref. [1] and [3]). As the fuel is stored as a cryo-genic liquid at roughly 20 K a good tank insu-lation is critical to the success of the tank de-

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D. VERSTRAETE

sign. Based on extensive studies by NASA and inthe Cryoplane project (ref. [3] respectively [1]),two different families of insulations are adopted:a foam insulation and a multi-layer insulation.Foam insulations will namely be slightly thickerdue to the lower thermal performance but theyonly require a single tank wall and are inherentlycheaper than a multi-layer insulation. The lat-ter consists out of alternating layers of double-aluminzed mylar reflective sheets and low con-ductivity spacer material, operated at vacuumlevel below 13 mPa (ref. [6], [7], [9] and [10])and are therefore more expensive to build andmaintain. The need for a vacuum operation fur-thermore entails an operational risk for the air-craft. A vacuum break would after all lead to arapid boil-off of the liquid hydrogen. For the casestudy in this paper only foam insulations will beanalysed.

During the flight the pressure inside the tankwill fluctuate as a consequence of the fuel con-sumption of the engines and the boil-off of liquiddue to the heat input in the tank. A minimumtank pressure, slightly above atmospheric pres-sure needs to be set to prevent air leaking into thetank. For safety reasons, a venting pressure alsohas to be set. In case of a degradation of the insu-lation, the tank pressure will namely rise signifi-cantly and a venting valve has to be foreseen forsafety reasons. The value adopted for this ventingpressure is a compromise between the tank wallweight and the insulation thickness and weight.After all, if a low venting pressure is set, a thickinsulation will be required to limit the heat inputin the tank.

The pressure variation for a homogeneouscryogenic tank can be calculated through an in-tegration of the following formula (ref. [8]):

dPdt

=!V

#Q! mout · hlg

$x+

"g

"l!"g

%&

where P is the pressure in the tank [Pa], t the time[s], Q the heat input in the tank [W ], mout the fuelflow [kg/s], hlg the heat of vaporization at thecurrent pressure [J/kg], x the quality of the two-phase mixture [!], which is the ratio of vapourmass to liquid mass in the tank and "g and "l the

density of the gas and liquid phase at the currentpressure [kg/m3]. The so-called energy deriva-tive ! ([Pa/(J/m3)]) represents the pressure risedue to an energy input in a given volume for thetwo-phase mixture.

As some level of stratification will occur dur-ing the flight, a factor of 2 is added to the ho-mogeneous pressure rise rate, following ref. [8].The fluid convection due to the acceleration andvibration levels of the plane and the conductiv-ity of the tank wall will namely result in a moreor less homogeneously mixed tank so the level ofstratification will be relatively small (ref. [1] and[3]).

Figures 2 and 3 show the influence of theventing pressure for the tanks of the aircraft un-der consideration. For each venting pressure theminimal insulation thickness to prevent ventingduring the flights is determined. As shown a sig-nificant variation in tank wall weight is obtained.Below the optimum, the tank weight increasesdue to the need of a thicker insulation, whichfurthermore leads to a slightly longer tank for agiven mass of fuel as the outer diameter of thetank is set to the fuselage diameter. For ventingpressures higher than the optimum pressure, theweight increases as the tank wall has to be de-signed for higher burst pressures.

Fig. 2 The weights of the two tanks forpolyurethane and rohacell foam

The figures furthermore show that the vent-

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An assessment of the potential of hydrogen fuelled large long-range transport aircraft

Fig. 3 The length of the two tanks forpolyurethane and rohacell foam

ing pressure leading to a minimal tank length ishigher than the pressure leading to a mimimumweight (2 respectively 1.75 bar). As the changein tank length is relatively small for this partic-ular applicaton, a venting pressure of 1.75 bar isused for the case study of this paper.

4 Case Study: 380 passengers long rangetransport aircraft

New passenger transport aircraft design stud-ies are usually preceded by an extensive marketstudy to determine the design mission for the air-craft. Usually a ’family’ approach is adopted tomitigate the financial risks involved with the de-cision to start a new design and possible futurestretch and shrink versions of the baseline fuse-lage are taken into account. As the market studyis without the scope of this work, a typical singledeck long range aircraft specification is adoptedfrom ref. [11].

An aircraft housing 380 passengers in a threeclass layout with a design range of 7500 nm atMach 0.84 is withheld for the case study underinvestigation. Besides the basic 3 class layout astretched version carrying 454 passengers in thesame 3 class layout and the respective single classhigh density version will also be considered inthe design of the fuselage. The initial cruising al-titude is set to 35000 ft and a stepped cruise with

steps of 2000 ft is allowed. The reserve fuel iscalculated using a diversion mission with a cruiserange of 200 nm and a hold phase as shown onFigure 4

Fig. 4 The mission profile for international flights

For the current study only a four engined ver-sion of the aircraft is considered. A turbofan witha BPR at take-off of around 8.4, similar to theGE90 is considered and its data is obtained withthe Gasturb software. More data on the engineand its performance can be found in ref. [12].

Below, first the baseline kerosene version isdetermined. After sizing the fuselage a wingparametric study is performed to determine theaspect ratio and wing area that lead to minimaldirect operating costs. Once the kerosene fuelledaircraft is sized, the hydrogen fuelled version isdesigned using a similar approach and the influ-ence of the increased acquisition cost is assessed.In a final subsection, the direct operating costs forboth fuels are compared.

4.1 Baseline kerosene version

For the kerosene version a single deck fuselagelayout is adopted. For the range of passengersconsidered only a 3-4-3 seating arrangement ispossible for the economy class. A 2-5-2 arrange-ment would namely lead to a stretch version thatexceeds the 80 m airport constraint. With this

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D. VERSTRAETE

3-4-3 seating a diameter of 6.95 m is obtainedfor the cylindrical mid section of the fuselage.The corresponding first and business class seat-ings are 2-2-2 respectively 2-3-2. The baselinefuselage is 68.48 m long and can accomodate 550passengers in the single class high density layout.

The wing area and aspect ratio have a signifi-cant impact on the aerodynamic efficiency of theaircraft and thus also on fuel consumption anddirect operating costs. A wing parametric studyto determine the optimum combination of bothparameters is therefore executed using the directoperating costs of the aircraft as the figure ofmerit. Figure 5 shows the direct operating costsfor different wing designs for a fuel price of 0.8US$ per gallon. The wing area is varied between300 and 450 m2 whereas aspect ratios are takenbetween 8 and 12. The colours on the figure indi-cate contour levels of DOC. The black lines indi-cate approach speeds in knots while the full whiteline indicates the limit of volume available in thewing to store the fuel. On the left and above thefull white line the wing is too small to store allthe fuel. The white dashed line on the other handshows a limit on the wing shape to avoid buffetonset during the flight (ref. [4]). Again the areato the left and above the line needs to be excludedfrom the design space.

Fig. 5 DOC per seat mile for 0.8 US$ / gallon

Based on the above mentioned limits, the fi-

nal wing design point is selected:

Sw = 400m2 ARw = 8.5

This point is indicated by the white dot on Figure5. Table 1 summarizes the most important char-acteristics of the airplane whereas figure 6 showsthe fuel weight for the design mission under con-sideration.

Fig. 6 The fuel weight for the design mission [kg]

As the direct operating costs are used as thefigure of merit an investigation on the influenceof the adopted price level on the choice of the de-sign point is in order. The fuel price for keroseneis varied from about 0.4 US$ / gallon to around2.6 US$ / gallon to anticipate the rising keroseneprices. As shown on Figure 7 the assumed pricefor the fuel does not change the shapes of the di-rect operating costs considerably. It just alters thelevel of the total costs and the relative contribu-tion from the fuel.

4.2 Baseline hydrogen version

To select the baseline hydrogen fuelled transportaircraft, a similar approach is used as for thekerosene fuelled version. However, seen the pres-ence of the fuel tanks inside the aircraft fuselage,its size can no longer be fixed before setting up allcalculations. After all, the total fuselage lengthwill be determined by the size of the fuel tanksand thus by the amount of fuel to be stored for

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An assessment of the potential of hydrogen fuelled large long-range transport aircraft

Fig. 7 DOC per seat mile for 2.6 US$ per gallon

the mission. Instead of fixing the overall lengthonce the cabin has been designed, one can onlyfix the cabin and the cockpit length. The fuselagelength is then determined iteratively based on thesize of the fuel tanks.

As the fuel tanks inside the fuselage will leadto a longer fuselage than for a kerosene fuelledaircraft, a double deck version has to be consid-ered seen the requirement of a strech version. Inthe design under consideration, the upper deckhouses all the business class passengers in thefront of the deck. The first class is on the otherhand situated directly below the business classon the lower deck of the fuselage as this willallow an easier embarking of first and businessclass passengers. The remaining part of bothdecks then houses the economy class passengers.Both first and business class have a 2-2-2 seatingwhereas the economy class seats are arranged ina 2-4-2 respectively 3-4-3 accomodation for theupper and lower deck. This leads to a fuselagediameter of 7.65 m and a fuselage length withouttanks of 47.35 m (cabin, cockpit and part of tailcone)

As was also the case for the kerosene fueledaircraft, a wing parametric study is executed tofix the overall characteristics of the plane. Theresulting direct operating costs of this paramet-ric study are given on Fig. 8. Obviously, as thefuel is no longer stored in the wing, the wing size

is no longer limited by a volume requirement sothis line is omitted from the figure. All otherlines have the same meaning as on Fig. 5. Asshown in the caption of the figure, the fuel priceis no longer given in US$ / gallon but in US$ /MJ instead. The values however correspond for akerosene lower heating value of 43.1 MJ/kg.

Fig. 8 DOC per seat mile for 0.006 US$ / MJ

As can be seen from the figure, the followingdesign point, indicated by a white spot has beenchosen for the hydrogen baseline aircraft:

Sw = 280m2 ARw = 11

A relatively big margin to the buffet limit fromref. [4] has been adopted. After all, the fuel inthe wing acts as a damper on the aerodynami-cally excited vibrations, so its absence will leadto a higher susceptibility of the wing to flutter.In a later stage of the work, more detailed calcu-lations will be made to substantiate the adoptedmargin. Table 1 summarizes the design point datafor the hydrogen fuelled aircraft. The direct oper-ating costs are given for the same price per energycontent of the fuel as for the kerosene fuelled air-craft.

As shown in the table, the liquid hydrogenfuelled aircraft has more or less the same oper-ating empty and landing weight as its kerosenecounterpart, despite the much smaller wing area.However, the low fuel load leads to a much lowertake-off weight for the aircraft. The wing of the

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D. VERSTRAETE

Table 1 Design point for kerosene and hydrogenfuelled aircraft

Sw [m2] 400 280ARw [!!] 8.5 11.0WTO [kg] 320760 205280

WF [kg] 156110 40240WOE [kg] 128450 128840

WL [kg] 172820 167720DOC [US$ c /pax/nm] 7.03 6.01

hydrogen fuelled aircraft also has a higher aspectratio than for kerosene fuel.

4.3 Influence of an increased aircraft acqui-sition and maintenance cost

As indicated previously, the change in the fuelstorage and delivery system might change thecost of acquisition and maintenance for thehydrogen-fuelled aircraft and possibly also thewing size and aspect ratio leading to the minimaldirect operating cost. Figures 9 and 10 show theDOC contours for an increase in acquisition andmaintenance cost of 50 respectively 100 % .

Fig. 9 DOC per seat mile for a 50 % increase inacquisition and maintenance cost

The figures show that the shape of the contourlines remains more or less the same up to 50% in-creased cost. At a 100 % higher acquisition andmaintenance cost, the contour lines are however

slightly different due to the increased importanceof the aircraft takeoff gross weight on the DOC.However, it seems to be highly unlikely that thechange in the fuel delivery and storage systemwould entail the doubling of the price of the air-craft.

Fig. 10 DOC per seat mile for a 100 % increasein acquisition and maintenance cost

4.4 Equivalent hydrogen price for same mis-sion direct operating costs

To allow a correct assessment of the economicpotential of hydrogen fuelled large long rangetransport aircraft, the price of both fuels is variedand the resulting direct operating costs are calcu-lated. As indicated previously, the optimal pointis not affected by the value assumed for the fuelprice. All calculations can thus be done for a sin-gle wing design. For each fuel, the optimum pre-viously determined is adopted. Figure 11 givesthe results of this calculation.

The figure clearly shows that a significantlyhigher price can be paid for the hydrogen fuel,based on the amount of energy stored. At a priceof about 80 cents per gallon of kerosene (0.006$/MJ) the price of hydrogen can be almost twicethe kerosene price to obtain similar direct oper-ating costs. If a 50 % increase in acquisitioncost is assumed the price of hydrogen can still beabout 10% higher. As the curves diverge, the ex-tra money that can be paid for the hydrogen fuel

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An assessment of the potential of hydrogen fuelled large long-range transport aircraft

Fig. 11 Comparison of DOC for different fuel prices

(based on an equal amount of energy) increasesfor increasing kerosene prices. This shows thatthe potential of hydrogen will increase with time.

5 Conclusion and future work

Seen the increasing attention paid to the emis-sions of greenhouse gases into the atmosphere,alternative fuels like hydrogen which could pro-vide a CO2 free flight become more and moreinteresting. However, the change to hydrogenas an aircraft fuel poses several technical chal-lenges and its performance and economical po-tential needs to be assessed.

After a brief description of the routine used toperform the preliminary design of both keroseneand hydrogen fuelled aircraft, a case study on alarge long range transport aircraft is reported. Forboth fuels the wing area and aspect ratio leadingto minimal direct operating costs are determinedand the influence of the price level of the fuel isgiven. It is shown that hydrogen might have a sig-nificant economical potential as the fuel price fora given amount of energy can be twice as high forhydrogen without leading to an increase in oper-ational costs.

As only conventional engines are used for thecase study under investigation, the potential ofhydrogen might even be bigger than shown here.After all, the heat sink of the cryogenically storedfuel can be exploited to reduce the fuel consump-

tion of the engines. In the future, unconventionalengines using heat exchangers will therefor besimulated.

References

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drogen fuelled aircraft - system analysis, TTR2.2.9, 2000.

[12] D. Verstraete, P. Hendrick, P. Pilidis & K. Rams-den, Hydrogen as an (aero) gas turbine fuel,ISABE-2005-1212, Munich, Germany, 2005.

[13] D. Verstraete and P. Hendrick, A preliminaryaircraft design platform for the assessment ofadvanced engine technologies, 7th NationalCongress on Theoretical and Applied Mechan-ics, Mons, Belgium, 2006.

[14] D. Verstraete, P. Hendrick, K. Ramsden andP. Pilidis, An aircraft preliminary designplatform for the assessment of advanced en-gine and aircraft technologies, 2nd EuropeanConference for AeroSpace Sciences, EUCASS1.02.01, Brussels, Belgium, 2007.

The authors confirm that they, and/or their company orinstitution, hold copyright on all of the original mate-rial included in their paper. They also confirm theyhave obtained permission, from the copyright holderof any third party material included in their paper, topublish it as part of their paper. The authors grant fullpermission for the publication and distribution of theirpaper as part of the ICAS2008 proceedings or as indi-vidual off-prints from the proceedings.

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