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aero-online.org June 2013

Boeing’s Seven-Crew CST-100

Progress in Space

Turboelectric Propulsion

18 aero-online.org Aerospace Engineering, June 2013

What’s Online

Addressing limitations of composite/metallic airframestructure optimization

Given the growing adoption of complex composites inwing box and other airframe structures — as in the Boeing787, Airbus 350, and Bombardier Learjet 85 and CSeries —analysis is now a highly complicated exercise involving layupsin some areas that are nearly 100 plies thick. Read more atwww.sae.org/mags/aem/11858.

Tracking the health of aircraft electrical generatorsResearchers have come up with basic modules of a pro-

posed diagnostic and health-management system architec-ture based upon data-driven algorithms. Read more atwww.sae.org/mags/aem/11662.

NASA Langley explores nanomaterials for next-genstructures

Researchers at NASA's Langley Research Center in Hamp-ton, VA are looking beyond the current state-of-the-art light-weight material — carbon-fiber composites — to promisingnanostructured materials; namely, carbon nanotube compos-ites. Read more at www.sae.org/mags/aem/11794.

Non-orthogonal rotary axes featured in automated fiberplacement machine

Electroimpact says that with this approach, the lay-down ratecan remain high over high-contour surfaces because the linearaxes never reverse direction during a change in compaction axisnormality. Read more at www.sae.org/mags/aem/11865.

New temperature-indicating paints for turbinesResearchers have developed a thermal-history paint — a ce-

ramic-based coating — that is faster, more robust, and non-toxic. Past temperature exposures can be determined whenthe coated component has cooled to room temperature. Readmore at www.sae.org/mags/aem/11936.

Top Articles

Nonferrous furnace capabilityWallwork Heat Treatment has commissioned a new 800-kg-

capacity furnace for the processing of aluminum and magne-sium products. The furnace provides variable heat treatmentfor cast, forged, wrought, or fabricated components from

single items of upto 2 m3 to batchesof components assmall as 0.5-mm di-ameter. The firm’score business re-mains the thermalprocessing and sur-face coating ofhigh-value compo-

nents, including those for the aerospace and motorsport indus-tries. Read more at www.sae.org/mags/aem/11952.

Horizontal machining centerThe next-generation Horizontal Center Nexus (HCN) 4000-III

horizontal machining center from Mazak Corp. includes per-formance enhancements that provide increased productivityand cost effectiveness, especially when it is used as a go-to work-horse machine for high-volume part-processing operations. Theenhancements include those that shorten part cycle times andallow the center to handle a wider variety of part sizes and moreparts per setup. Read more at www.sae.org/mags/aem/11951.

I/O panel meterThe Ashcroft Model DM61 digital panel meter does more than

just display the analog or Modbus RTU (remote terminal unit)digital signal from a sensing device with an output. Equippedwith a variety of electronic features, this I/O device allows theuser to configure the display to read out in the required engi-neering unit, recall minimum and maximum readings, and sethigh-low alarms. Read more at www.sae.org/mags/aem/11950.

Arbitrary waveform generatorsTektronix’s AWG 70000 Series arbitrary waveform generators

provide up to 50 GS/s sample rate, 16 GS of waveform memory,and 10-bit vertical resolution, making them suited for high-speed testing and advanced research applications. The devicesproduce fast, clean signals that can be routed through a receiveror other device under test for long periods of time for compre-hensive testing. Read more at www.sae.org/mags/aem/11949.

Multitasking turn/mill centerMethods Machine Tools’ Nakamura-Tome NTJ-100 Multi-

tasking Turn/Mill Center offers fast tool changes compared toa full-tool-spindle ATC machine. A large tool capacity — up to54 tool stations for turning and 24 tool stations for millingtools — reduces setup time. For faster cycle times, the newcenter has two high-rigidity turrets, each with a Y-axis, 3.5” onthe upper and 2.6” on the lower, aiding pinch turn and pinchmill operations. Read more at www.sae.org/mags/aem/11948.

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Aerospace Engineering, June 2013 19Free Info at http://info.hotims.com/45604-781

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Editorial

For the firsttime since the ini-tial 1998 releaseof the Report Cardfor America’s In-frastructure by theAmerican Societyof Civil Engineers(ASCE), in themost recent Re-port Card released in March 2013, theoverall grade for the nation’s infrastruc-ture improved from four years ago.That’s the good news. The bad news isthat it improved all the way to a D+.The worse news is that aviation didn’teven do that good, contributing a D tothe overall infrastructure GPA. (For therecord, the highest-ranking score was aB- for solid waste.)

In an era when, from school kids toscientists, there is much energy and in-terest in the “next frontier” of aircraftand air travel that includes everythingfrom UAVs to the commercialization ofspace travel and space tourism (for moreon the latter, check out the feature “Fill-ing the Space” on page 24), it could ap-pear that our industry is looking to getits driver’s license before it has mas-tered its tricycle.

While not meaning to downplay thedifficulties of managing the NationalAerospace System (NAS) — according tothe FAA, there are around 7,000 aircraftin the air over the U.S. at any giventime — we didn’t go from 0 to 7,000overnight.

To be sure, the human race has beendealing with solid waste much longerthan it has been dealing with 7,000 air-craft in the air. But, wouldn’t thechanges that have allowed for extremeadvances in aircraft technologies overthe past couple of decades also have al-lowed for the advancement of controland coordination of said aircraft?

Without even bringing up the addedcomplexity of managing the NAS withUAVs possibly gathering traffic dataamidst spacecraft possibly blasting offfor Disneyland-Mars, how about con-sidering the added complexity of, oh,say, 10% of air traffic controllers beingfurloughed on any given day.

While there were certainly some whoclaimed that the sky was indeed fallingthe day that sequestration hit the FAAin late April, in all honesty, it really did-n’t look all that different from any otherday in the East Coast skies. That is tosay, there were plenty of delays, some“over an hour.”

Let me say that to this traveler, just anhour delay trying to get into or out ofNewark or Philadelphia or La Guardia orJFK is the equivalent of what Christmasmust feel to a five-year old child. In fact,it honestly seems sometimes that I’vespent more time trying to get into or outof those airports in the past year than Iever spent in kindergarten.

There are many more problems withthe management of the NAS than a 10%staff reduction. Yet, some have been ar-guing there are better ways for the FAAto cut the $600 million it needs as re-quired by the sequester than furloughs,such as cutting money out of theNextGen (Next Generation Air Trans-portation System) budget.

Now, if there’s truly anything thatwill lead to an even lower grade on thenext ASCE Report Card, it would be tak-ing money away from the alreadypainfully delayed NextGen program. Ifanything, the continued developmentand implementation of NextGen needsto be expedited, not stalled further.

According to the Reason Foundation,closing over 100 targeted air traffic facil-ities would generate approximately $1.7billion in one-time savings, and, goingforward, would contribute to productiv-ity gains and reduced maintenance andfacility costs by $1 billion annually.

Most important, according to Rea-son, “The days of air traffic controllersneeding to be right below specific por-tions of the airspace are over. Today'stechnology allows air traffic controllersto guide planes from anywhere.”

Logical, technologically sound solu-tions are available to solve airspacemanagement issues. All we need now issomeone licensed and knowledgeablein the driver’s seat.

Jean L. BrogeManaging Editor

Towering Decisions

Thomas J. DrozdaDirector of Programs& Product [email protected] JostEditorial DirectorJean L. BrogeManaging EditorLindsay BrookeSenior EditorPatrick PonticelAssociate EditorRyan GehmAssociate EditorMatt MonaghanAssistant Editor

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Technology Update

Hitachi Metals Reduces Rare-Earth Dysprosium in Electric-Motor Magnets

In response to highprices and supply risks fordysprosium and otherrare-earth elements usedin neodymium sinteredmagnets for hybrid andelectric vehicle (HEV/EV)traction motors, electricpower steering, and otheractuators and electric mo-tors, Hitachi Metals Amer-ica (HMA) has developedan alternative solutionthat reduces dependenceon such rare earths.

The Purchase, NY-basedcompany’s reduced-dyspro-sium NEOMAX series Nd-Fe-B sintered magnets areenabled by DDMagic, a dys-prosium vapor depositiondiffusion technology. HMAfirst applied DDMagic tech-nology in 2009 for factory automationand extended it to the NEOMAX seriesfor HEV applications starting in 2011.

Dysprosium (Dy), one of the most ex-pensive heavy rare-earth elements, isused in neodymium sintered magnetsto improve temperature resistance. De-spite its performance-boosting charac-teristics, Dy has a significant downside:it’s currently sourced from a singlecountry — China — which leads to sup-ply shortages and erratic pricing as de-mand increases.

“The main objectives of our decisionto develop these new NEOMAX mag-nets are product differentiation basedon reduced-Dy magnet technology toachieve high performance; reduction ofDy volume, which had a big impact inproduction cost; and lowering the riskof depending on one supply source,”explained Koshi Okamoto, ExecutiveDirector, Corporate Business Develop-ment, Hitachi Metals America, Ltd.“Cost saving compared with traditionalsintered magnets depends on the mar-ket price of dysprosium, which has fluc-tuated very much in the past and ishard to predict.”

DDMagic is a method for thermallydiffusing Dy vapor over a magnet’s sur-face. The technology mitigates the fall

in remanence (Br) caused by Dy, ac-cording to HMA. Dy deposition diffu-sion not only reduces Dy usage, butalso increases heat-resistant tempera-ture and coercivity (HcJ). Coercivity isimproved by 320 kA/m (4 kOe) or more(effect differs by magnet shape), and Brcan be raised more than 40 mT com-pared with conventional materials of

the same coercivity, thecompany claims. Thesefeatures can contribute toa lighter, more compactmotor.

“There are other heavyrare-earth elements besidesdysprosium, such as ter-bium (Tb), which is morerare and more expensive,”said Okamoto. “Magnetsthat are so-called Dy-freesometimes may includeother heavy rare elementssuch as Tb, which is ad-versely more expensive andis sourced solely by China.Our DDMagic technology-based reduced-Dy NEO-MAX magnets really re-duce heavy rare elementsas a whole.”

HMA has recently devel-oped new technologies to even furtherreduce dysprosium use, which Okamotoreferred to as “one of our top-priorityprojects.” However, he could not dis-close any more technical details aboutthis next-generation magnet.

“We think that the trends are for re-duced-Dy magnets and even for Dy-freemagnets,” he said.

The company is currently running asample evaluation program with keycustomers, and expects full commer-cialization of the NEOMAX series withnew Dy-reduction technologies in 2014.

HMA’s main target segment for NEO-MAX magnets is currently automotive,“mainly for EV/HEV including off-high-way applications,” Okamoto shared.But he noted that aerospace applica-tions for actuators and electric motorsare a possibility as well.

NMF series sintered ferrite magnetshave long been manufactured at HMA’ssubsidiary in North Carolina for NorthAmerican and European customers. InApril 2013, Hitachi Metals will beginNorth American production of theNEOMAX series Nd-Fe-B sintered mag-nets at its NC subsidiary to meet in-creasing demands.

This article was written by Ryan Gehm,Associate Editor.

Hitachi Metals’ reduced-dysprosium NEOMAX series Nd-Fe-B sintered magnetsare suited for HEV/EV traction motors and other actuators and electric motors invarious applications.

Hitachi Metals’ DDMagic is a method for ther-mally diffusing Dy vapor over a magnet’s sur-face. The technology not only reduces Dyusage, but also increases heat-resistant temper-ature and coercivity.

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Persuading politicians to fund big-scale space projects is more diffi-cult today than ever, as witnessedby the scores of advanced spaceplaneand expendable launcher projects thathave fallen by the wayside over the last30 years.

But access to space is still vital assatellites keep the global economymoving and enable seamless real-timefinancial trading and communicationat a level of reliability and accuracythat was unthinkable three decadesago, and plenty of nations still want todo it. Apart from the established majorspace players in the U.S., Europe, andRussia, China, Japan, and India alsohave ambitious national space pro-grams.

The biggest change in a national ap-proach to space policy, however, camefrom the U.S. The trend started in 1984when the Commercial Space LaunchAct abandoned the NASA monopoly onspace launchers and opened up the op-portunity for private operators to de-velop expendable launch vehicles. Sixyears later, President George Bush fol-lowed up with further legislation to en-able commercial companies to supplyNASA with launch services.

Transitions in SpaceThe last launch and recovery of a

NASA space shuttle occurred in 2011.This 1980s-era space vehicle had origi-nally been intended as the first of a newgeneration of space vehicles that would

offer a more frequent, high-capacityspace transportation capability.

It was supposed to be more afford-able over a lifetime of operation thanreliance on expendable heavy launchvehicles. It was certainly flexible in op-

Filling the SpaceA look at the changing face of mannedspaceflight and the commercialization oforbital access.

by Richard Gardner, European Editor

The SpaceX Dragon new-generation space

transport vehicle.

Detail of the unique Burt Rutan design of wing feathering on SpaceShip 2, which allows the space-plane to return in a controlled, low-speed glide back to Earth. (Richard Gardner)

Minimizing component weight with advanced engineering,technology and materials. Without sacrificing performance.It’s what we do. More at altair.com/lightweight.

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Filling the Space


eration, with its abilities to dock inspace with other manned vehicles,place large satellites in orbit, and serv-ice and repair others. Its huge cargo bay,equipped with a Canadian-supplied re-

mote operating arm, looked like theshape of the future.

But in reality, the space shuttle fleet be-came very costly to maintain with its largeand complex vehicle, and substantial

launch and support infrastructure. TheEuropean Space Agency, formed in 1975,looked at its own smaller shuttle, knownas Hermes. This was to be launched froma new heavy-lift launcher, Ariane V.

As the weight and cost of the French-led Hermes project grew, the vehicle ca-pacity for carrying astronauts or cargogot smaller and smaller until the con-cept was abandoned. However, the Ari-ane V launcher continued as a vehicleto lift large satellites into high orbit,and over the last 20 years, has provedto be a most reliable launcher, with 52consecutive launch successes.

With the prospect of NASA being un-able to fund a shuttle follow-on of itsown, a less ambitious series of projectsbegan to gain momentum. In manyways, they looked like a throwback tothe early Apollo-mission Earth orbit andsupport capsules. Yet they incorporatedthe latest materials and control technol-ogy and were modest in scope, thus look-ing realistically affordable compared toearlier proposals. Most importantly, theyfocused on commercial innovation,breaking away from the traditional pathof reliance on state-funded space pro-grams. The revolution had arrived.

Partnering for SuccessMany leading decision-makers in the

space sector believe that the key to sus-tainable space transportation is reliable,safe, and affordable vehicles that can beused over and over again in much thesame way that we accept and use globalair transport today. This is, of course, eas-ier said than done.

But, the constant evolution of spacetechnology has enabled new launchersand transit vehicles to be designed todeliver genuinely reusable componentsthat should help avoid the runawaycost escalation that killed so many pre-vious major national space programs.Partnerships between space agenciesand space companies, and betweencompanies, are now starting to showhow this can be done, though as safetyis so essential with manned missions,the development, test, and evaluationprocess cannot be rushed.

NASA remains the national championin the U.S. for space implementation,but as a part of its more outward policy

The commercially developed Skylon project features disruptive technology in its hybrid engine andwill offer single-vehicle takeoff-to-landing access to space. (Reaction Engines)

The European Automated Transfer Vehicle (ATV) is a space cargo carrier, and is to be combined withthe Orion capsule for extended ISS support capability. (Richard Gardner)

Aerospace Engineering, June 2013 27Free Info at http://info.hotims.com/45604-787


toward commercial partnerships, it has focused on a smallnumber of specific company initiatives to develop new spacevehicles that could provide continuing transport to the Inter-national Space Station.

There are clearly good pro spects for some of these programsto provide manned access to more challenging tasks later inthe next decade, such as a return to the Moon, or as a build-ing block in a more comprehensive Mars mission later still. Inaddition to new space vehicles, the U.S. space vision playingout over the next decade includes the modernization of thelaunch infrastructure at the Kennedy Space Center.

In place of the shuttle will be more conventional-lookingboo ster rockets that will carry a transport capsule that willreturn by parachute. The booster rocket stages are intendedto be recoverable and reusable. The new Space Lift Systemwill be suitable for extended human exploration missions.Initially it will have a lifting capability of 70 tons, but laterversions will be almost double, lifting 130 tons. The first SLSlaunch and full test is due in 2014, with a long-durationflight planned for 2017.

A SpaceX Falcon 9 rocket lifts off at Cape Canaveral this past March, carry-ing a Dragon capsule filled with cargo. The capsule was making its thirdtrip to the ISS, following a demonstration flight in May 2012 and the firstresupply mission in October 2012.

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Filling the Space

NASA chose the Lockheed Martin Orion project capsule for a pre-2020 launch.Orion’s design was optimized to carry crew into space for International Space Sta-tion (ISS) changeovers or to sustain longer space missions beyond low-Earth orbit.Now Europe is joining the U.S. by combining ESA’s ATV support vehicle with Orionfor greater cargo capacity and extended mission duration.

Options in Space NASA has signed contracts for commercial companies to design, build, and test

commercial spacecraft that will be safe and certifiable for carrying NASA astronautsinto space. Through these contracts, there is constant feedback on how the compet-ing designs are likely to meet the NASA requirements.Apart from the Orion program, selected space transport projects include

SpaceX's Dragon, Boeing's CST-100, and Sierra Nevada's Dream Chaser. The latteris the only spaceplane proposal within this NASA initiative — the others beingspace capsules. NASA has awarded contracts worth $3.5 billion for at least 20 re-plenishment missions to the ISS.

The Dragon made history in 2012 as the first private spacecraft to visit and dockwith the ISS. This vehicle is designed for crew transportation and can carry up toseven crew/passengers. More testing is required before humans will fly on a Dragonmission, but work is progressing on the plan to undertake the first manned orbitalmission in 2015.

The company’s other big program is the Falcon re-usable launcher family pow-ered by the Merlin 1-D rocket. The Falcon 9 uses nine rockets. SpaceX is currentlyundergoing vertical takeoff/vertical landing tests in the form of one modified Fal-

con Stage 1 rocket, known as the Grasshopper, to evaluateand refine its performance. It is an essential part of an ambi-tious scheme to make the whole Falcon launch vehicle recov-erable. Beyond the standard Falcon 9 will be an even largerFalcon Heavy rocket that is claimed to offer twice the launchcapacity (in weight) of any existing launcher.

Elon Musk, CEO and Chief Designer at SpaceX, said that theintention was to recover the entire rocket and be able to re-launch the whole vehicle quickly. This has long been a dreamfor space planners, but so far, the successful recovery and reuseof the whole space vehicle, and all its launchers, has been elu-sive. Maybe this aim is now getting closer. If so, the overall costof commercial as well as exploratory science-based space mis-sions could be seriously reduced.

Like the Dragon, Boeing’s CST-100 crew space transporta-tion capsule is reusable and can carry up to seven crew. It isintended to have a life of at least 10 missions, and use acombination of parachutes and air bags to return to Earth.The first test is due in 2015. Detailed capsule design is cur-rently being evaluated, with attention to the displays andcontrols, carried out at the company’s Houston Product Sup-port Center.

Sierra Nevada’s Dream Chaser is the only winged space-plane in NASA’s current evaluation program. At the end oflast year, landing gear tests were carried out on the engineer-ing flight-test vehicle. Launch will be on top of a boosterrocket, but after the mission, recovery will be on a runway.Engine tests continue, including work on the reaction controlpropulsion system that will steer the vehicle in space andmake small adjustments in directional thrust when dockingwith the ISS or other future stations.

Blue Origin offers another capsule space vehicle designed tocarry up to four astronauts. A successful pad escape test was


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achieved in October 2012 at the com-pany’s West Texas launch site, and thevehicle flew up to 2,307 feet under ac-tive thrust vector control before makinga parachute descent to a soft landing. Asa part of the re-usable booster configura-tion tests, a rocket chamber engine

thrust firing also took place last October.The 100,000-pound thrust BE-3 engineis a liquid oxygen design.

Space LandingsCommercialization of space is moving

beyond the established national space

agencies. Bigelow Aerospace has beenproposing a huge space station, the BA-330, featuring multiple linked inflatablemanned units that could be used tohouse space laboratories or act as an or-bital hotel, where paying space touristscould spend some time enjoying ex-

tended periods in the weightless con-ditions while enjoying spectacularviews of Earth. The privatized Russianspace sector is proposing a new spacestation called the Excalibur Almaz.

Back down on Earth in New Mex-ico, future space tourists will soonget their first taste of space travelwhen they check in at the impressiveSpaceport America terminal for aflight aboard a Virgin Galactic Space-Ship Two spaceplane. This, morethan any other to date, is a space ac-cess project on a sufficiently grandscale to generate widespread publicinterest when the commercial flightsbegin later this year.

The vision and enterprise of SirRichard Branson, combined with theinnovation and engineering genius ofBurt Rutan, are set to transform howwe think of space travel. No longer aprospect available only to the fittestof the fit, with a track record as a testpilot or space scientist, the VirginGalactic vision aims to cut the cost ofspace access in a meaningful way overtime, just as aviation has become aglobal enabler when once it was avail-able only to the super-rich.

It is thought that by 2020 the costof a sub-orbital tourist trip into spacecould be down to $50,000, andmaybe $10,000 by 2030. More sig-nificantly perhaps is that companiessuch as Virgin Galactic and SpaceXare looking at reducing the cost ofdelivering one pound of cargo intospace to around $1,000.

XCOR Aerospace’s Lynx is asmaller spaceplane that takes off andlands from a normal runway at Cu-racao. It is intended to start flighttests in 2013. First bookings havebeen taken. The first generation oflucky space tourists will no doubt bethe super-rich, but the cost of spaceaccess will most likely come tum-bling down as more efficient vehi-cles arrive in service.


Filling the Space

This image was taken during a series of wind tunnel tests for Blue Origin's Space Vehicle at LockheedMartin's High Speed Wind Tunnel Facility in Dallas. The vehicle's biconic shape is designed to providemore cross-range and interior volume than a traditional capsule, and weigh less than a winged vehicle.

Sierra Nevada’s Dream Chaser full-scale test vehicle is lifted by a helicopter to verify proper aerodynam-ic flight performance. Data from this test will provide an opportunity to evaluate and prove hardware,facilities, and ground operations in preparation for approach and landing tests scheduled for later thisyear

Aerospace Engineering, June 2013 aero-online.org 33

Turboelectric propulsion is a revo-lutionary system in which thepropulsor (fan) is not connectedto the turbine through a mechanicalconnection; thus, both components canbe set at their individual optimumspeeds. Also, turboelectric propulsionmakes use of distributed fans, using sev-eral small fans in place of the traditionallarge propulsors on the wings or sidesof the fuselage.

Distributed fans are usually placed onthe top side of the fuselage of a blendedwing body aircraft to take advantage ofboundary layer ingestion that increasesfan performance. The smaller fans can

run at faster speeds than their largercounterpart in a turbofan engine.

By applying this new propulsion sys-tem concept, NASA has estimated thatfuel burn could be reduced by 10%, alsocontributing to reduced harmful emis-sions. In addition, this architecturecould be modified to allow for the useof alternative fuels such as hydrogen.The system can also aid in the reduc-tion of noise by using the fuselage toshield the noise from the ground.

To implement this system, manychallenges must be overcome. Newtechnologies will have to be imple-mented into the architecture to create

this system. Most of these technologiesare still in the experimental stages; con-sequently, data is limited.

Since these technologies are not com-pletely developed, there is uncertaintyin their weights and performance. Theincreased number of components addscomplexity to the system, and the in-teractions between components are notyet understood.

One of the primary components ofthe turboelectric architecture that needsto be designed is the electrical distribu-tion system. A recent study by scientistsat Georgia Institute of Technology fo-cused on the design of the two primarycomponents of the power distributionsystem: the power distribution cablesand power converters.

System ArchitectureThe size and performance of the elec-

trical distribution is dependent on theentire turboelectric propulsion architec-ture since it will be responsible forpower flow between components. Thepower output and power requirementsof each element will be important fac-tors in the design of the electrical distri-bution system. The placement of thecomponents will also be important be-cause it will determine the length of thecables in the system.

The aircraft selected for this study wasNASA’s N3-X concept vehicle. The N3-Xwas conceived as a blended wing bodyaircraft that can carry approximately300 passengers at a range of 7500 nauti-cal miles, cruise speed of Mach 0.84, andcruise altitude of 30,000 feet. This air-

Distribution of PowerWhen designing the electrical distribution system for a turboelectric architecture, it is imperative to select technologies that will provide the highest efficiency and stabilityfor the propulsion system.

Researchers from Georgia Institute of Technology considered the NASA concept vehicle, N3-X, forits study of turboelectric propulsion systems.

Gas turbine

Generators

ThermalManagement

ElectricalDistribution

Motors

Fans

Mechanical Connection

Electrical Connection

Coolant Flow

Schematic of a turboelectric propulsion system architecture. The direction of the arrows shows whichcomponent is the source of energy and which component receives it (at the arrow head). The bluearrows are in both directions for each line because the coolant will circulate through the system.

Distribution of Power


craft has a distributed propulsion sys-tem consisting of 14 fans driven by elec-tric motors that span the upper body ofthe aircraft. The gas turbines and gener-ators are located at the wing tips so theyreceive undisturbed air and supply abending moment for the wings.

This concept aircraft was selected be-cause some preliminary design hadbeen conducted for its turboelectricpropulsion system. Components suchas the generators and motors had beensized, and preliminary weight and per-formance values had been published.Sizing the electrical distribution systemwould further contribute to the prelim-inary sizing information for the overallN3-X propulsion system.

A turboelectric propulsion system con-sists of six primary components: gas tur-bine, superconducting generator, distri-bution system, thermal-managementsystem, superconducting motors, andfans.

The primary task of the electric distri-bution system is to deliver power from

the generator to the motors for the fans.Also, the electrical distribution systemmust be cooled. Therefore, coolant willbe delivered from a thermal-manage-ment system.

In turn, the electrical distribution sys-tem must deliver power to the thermal-management system. In this study thefocus was on delivering power from thegenerator to the loads. The cables andpower converters were designed for themaximum power draw by the system.For the NASA N3-X, the expected valuefor the maximum power load is 40 MW.

Power Distribution Cables The power distribution cables are re-

sponsible for delivering power betweencomponents. A number of designchoices must be made to determine thesize and performance of the cables.

One of the initial decisions thatmust be made is whether the power willbe distributed in ac or dc. Although acpower is traditionally used, in high-power applications such as turboelec-

tric propulsion, dc has the potential tooutperform ac for several reasons.

First, the bus may require the use ofsuperconducting cables, which have vir-tually no losses when operating with dcpower; ac superconducting cables havelosses and will force a fixed ratio be-tween the generator and motor. Thatfeature inhibits being able to operatethe generator and motor independentlyto achieve the best efficiency.

Second, batteries and other storagedevices can easily be added to the busto provide additional back-up power.

Lastly, if any load is capable of pro-ducing regenerated power, the powercan easily be returned to the bus.

Although many advantages for dc dis-tribution exist, some issues have to beaddressed, particularly system stabilityand power quality. The system must beable to remain stable under transientloads. One way to address these con-cerns is through converter design.

With the proper design, converterscan help stabilize the bus by properlymitigating power while maintaininghigh power quality. The next decisionto be made is whether to use conven-tional cables or superconducting cables.

Conventional Cable Design The first decision that must be made

in the design of the conventional cableis determining its structure. Most con-ventional cables have a cylindrical corethat acts as a conductor. Conductor ma-terial is an important design choice aswell. The material must have a suffi-cient current density to transmit andaccommodate the power load whilemaintaining a reasonable weight andcost. The most popular choices for thistype of cable are copper and aluminum.Copper is the most widely used becauseof its good current density, good con-ductivity, and relatively low cost.

In addition to the conductor, threeprotective layers must be added to thestructure to complete the cable: a di-electric layer, a magnetic shield, and acooling sleeve.

The purpose of the dielectric layer isto resist the potential between the wireand the magnetic shield. Its thicknessis selected based on the system nomi-nal voltage. Many standards for the se-

Cable design choices.

Stabilizing Layer

Superconductor

Substrate

IntermediateLayer

ProtectiveLayer

For superconducting cable, cylindrical wires do not carry the current. Instead, they are made into"tapes." Shown is the YBCO tape structure.

Transmission AC DC

Type Conventional Superconducting

Material Copper Aluminum YBCO BSCCO

Structure Single-core 2-core 3-core Single-core 2-core 3-core

Coolant Water Air Oil Hydrogen Nitrogen

Aerospace Engineering, June 2013 aero-online.org 35

lection of the dielectric thickness areavailable.

The AEIC (Association of Edison Illu-minating Companies) standards are themost widely used in North America andwere used to determine dielectric thick-ness in this study. XLPE (cross-linkedpolyethylene) was selected for the insu-lation for the cable due to its long-termreliability.

The purpose of the magnetic shield isto protect the wire from the magneticfield induced by the current flow, so thethickness of this layer is dependent onthe maximum current flow through thewire. This layer is often made out ofaluminum.

For this architecture, a cooling sleevewas necessary to maintain a safe tempera-ture for the wire. The maximum allow-able temperature is an important con-straint to include in the design so thatexcess heat does not damage the cable orany surrounding electronics such as thepower converters. Also, in situationswhere the cable may be in close proxim-ity to fuel lines, the designer must ensurethat the heat expelled by the cable willnot ignite the fuel.

Superconducting Cable Design For a material to be considered “su-

perconducting,” it must distribute twoproperties: zero electrical resistance andperfect diamagnetism when cooledbelow a critical temperature, usuallycryogenic (below 123 K).

Zero resistance means that there areno losses when transmitting current.Perfect diamagnetism means that thematerial does not permit an externallyapplied magnetic field to penetrate intoits interior.

In this application, dc transmissionwas chosen since there are some lossesin superconducting cables when trans-mitting ac power. Also, high-tempera-ture superconducting (HTS) dc cablesonly carry real power. There is no reac-tive power, so there will be no signifi-cant derating of the cables — meaningthat the cables will not lose their abilityto transmit their full power rating.

The structure of a superconductingcable is quite different from a conven-tional cable. There is no cylindricalwire that carries the current; in -

stead, superconduct-ing wires are made in“tapes.” To make thecable in a cylindricalshape, these tape arewrapped around aform er. The liquidcool ant can f lowthrough the center ofthe former, adjacenttubes, or in an outerlayer surroundingthe HTS tape. On theouter side of the su-perconducting tape,there is electrical andmagnetic shieldingas in the case of theconvectional cable.

To determine the size of the wire, thefollowing parameters must be deter-mined: HTS tape material, HTS tapethickness, HTS tape winding pitch, di-electric thickness, coolant type, formerdiameter, and outer cable diameter.

Based on current research, the bestsuperconducting material for this appli-cation is yttrium barium copper oxide(YBCO). It is the most cost-effective op-tion for the current-carrying capacityrequired for this application. Studieshave shown that YBCO tapes have beencreated that have a critical current den-sity of 1.4-1.5 MA/cm2. The current-car-rying capacity of these wires is about400 A/cm of wire width. When thewires are transmitting dc, the amount

of power that the wire can carry is pro-portional to the volume of the wire.

As in the case of the conventionalcable, electrical insulation will beneeded. The amount of shieldingneeded depends on the cable dimen-sion of the former and HTS tape, thesystem nominal voltage, and the designstress of the insulation. Along with theelectrical tape and insulation, the cablewill also include a thermal-manage-ment system to keep the superconduct-ing tape under the critical temperature.

System Design When designing the electrical distri-

bution system for a turboelectric archi-tecture, it is important to consider ways

To make the cable into a cylindrical shape, the tapes are wrapped around a former, as shown in thesimple HTS cable configuration.

Outline of the basic steps of the algorithm for HTS cable sizing. Thefirst step is to determine the number of cores.

ThermalProtection

CoolantSupply

ColdDielectric

Former

HTS Tape

CoolantReturn

Set Number Of Cores

Set Critical Current Density

Determine Critical Current

DetermineShieldingThickness

Calculate Conductor Size

Set Voltage

Calculate Coolant Channel

Thicknesses

Determine Pumping Power

Required

Ensure CriticalMagnetic Field

Is Not Exceeded

Propulsion Feature


that the other components in the tur-boelectric system may affect the per-formance of the electrical distributionsystem. Therefore, the basic architec-ture of the system must be determinedto identify interactions between theelectrical distribution system and othersystem components. The electrical dis-tribution itself will consist of manycomponents.

The type of power transmission ca-bles selected for electrical distributionwill have a significant impact on the ef-ficiency of the system. Within an air-craft, a limited amount of space is avail-able to place power transmission cables,and their weights will have an impacton the performance of the aircraft;

therefore, minimizing the size of thecables is crucial.

Due to the high power loads on theaircraft, superconducting cables are thelogical choice for power transmission.Compared to conventional copper ca-bles, superconducting cables can trans-mit power with a significant reductionin cable thickness and weight.

Also, by using dc distribution, thepower losses during transmission canbe minimized. Of course, using super-conducting cables will present theunique challenge of maintaining cryo-genic temperatures throughout thelength of the cable. The cooling of thissystem can be tied into the thermalmanagement needed for the supercon-

ducting generators and motors that theturboelectric architecture requires.

Another concern is that even withthe use of superconducting cables, thebus voltage must be much higher thanvoltages present on aircraft today.Safety must be carefully consideredwhen designing a system of this typeto prevent problems such as electricalfires.

The type of power converters selectedalso affects the performance of the sys-tem, having a large impact on the per-formance of the entire turboelectric sys-tem since they will control the powerflow between the dc electrical bus andeach component.

Power converters are required tomaintain stability on the dc bus withthe best possible efficiency. The greatestamount of stability and efficiency canbe achieved through the use of activelycontrolled converters. Although activepower converters can maintain stabilityduring transient events fairly well, theresponses of the converter must be care-fully examined to ensure that voltagesand currents throughout the systemstay within specified limits.

The efficiency of actively controlledconverters is very high. However, thereare some losses associated with conduc-tion and switching. These losses can beminimized by operating the convertersat cryogenic temperatures.

Conduction losses will be less sincethe resistance of the materials used inthe converter will be smaller. Further-more, the switching frequency can beset much higher in a cryogenic con-verter to further improve efficiency.

The results of this study can assist inthe selection of technologies for theelectrical distribution that will providethe highest efficiency and stability forthe propulsion system. In addition, thecable sizing results and converter mod-eling approach can contribute to esti-mating the size and performance of theelectrical distribution system, and theycan provide insight on the performanceof the entire turboelectric propulsionsystem.

This article is based on SAE Interna-tional technical paper 2012-01-2180 byAngela Lowe and Dimitri Mavris, GeorgiaInstitute of Technology.

Three-Phase Diode

Line-Commutated Controlled

Force-Commutated Three-Phase Controlled

Star RectifierCircuit

Diode Bridge

Three-Phase Half-Wave

Six-Pulse or Double Star

Double Star Rectifier with

Interphase Connection

Graetz Bridge

Voltage Source

Current Source

VOC

DPC

Power converter type is a critical choice in turboelectric propulsion systems. Rectifiers convert ac intodc power; shown are the choices that must be in during rectifier design.

Distribution of Power

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