Frontiers of the 'responsible imaginable' in - CAFE Foundation

Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc.

THE 1998 AIAA DRYDEN LECTURE

FRONTIERS OF THE "RESPONSIBLY IMAGINABLE" IN (CIVILIAN) AERONAUTICS

Dennis M. Bushnell*National Aeronautics and Space Administration

Langley Research CenterHampton, Virginia 23681-0001

ABSTRACT

Paper discusses future alternatives to currently deployedsystems which could provide revolutionaryimprovements in metrics applicable to civilianaeronautics with, in many cases, significant military"spinoff." Specific missions addressed include subsonictransports, supersonic transports, personal aircraft, andspace access. These alternative systems and conceptsare in many cases updates to known "end point" designsenabled by recent and envisaged advancements inelectronics, communications, computing,CFD/nonlinear aerodynamics and "Designer FluidMechanics"-- in conjunction with a design approachemploying extensive synergistic interactions betweenpropulsion, aerodynamics and structures.

INTRODUCTION

The last 50 years of aeronautics have been trulyrevolutionary. In the developed world, train and shiplong haul passenger traffic has been replaced by aviationand aviation has assumed a dominant role in warfare.The list of revolutionary technological developmentsduring this period includes large swept wing near-sonictransports of the B47-707 genre, supersonic cruise andfighter aircraft, turbojets and ramjets, high strengthaluminums and composites, and a vast array ofavionics. Much of this progress occurred under thedominant metric of higher, faster and larger is better andwas a continuation of aeronautical development trendssince the early 1900's.

Today and for the foreseeable future, the metrics, andhence the nature of desired technological improvementsare "different." These "new" (civilian) metrics includeAFFORDABILrTY (initial, life cycle), productivity(aircraft, air space), safety and the environment (noise,pollution). Driving these metrics are global economiccompetition (exacerbated by the demise of the "coldwar"), an increasing demand for air travel in the shorter

term, increasingly stringent environmental regulationsand the emerging competition from the ongoingtelecommunications revolution(s) wherein businesstravel in particular may become increasingly replaced by"virtual" interpersonal interaction via (eventually) 3-Dtechnology such as holographic projection, virtualreality immersion, etc. Estimates indicate a 40 percentbusiness travel reduction over the next 20 years due tothe "teletravel" revolution with consequent (non-trivial)reductions (the order of 1000 units) in CTOL transportproduction (e.g., references 1 to 5). Such a reduction inbusiness air travel would leave recreational travel thedominant market sector, a sector which is extremelyprice sensitive, placing a further premium upon costreduction technologies.

Corresponding updated military metrics are also lead byAFFORD ABILITY, followed by responsiveness,flexibility, lethality, survivability, logistic support andstandardization/interoperability. The present paperplaces major emphasis upon the civilian sector, withobvious military "spinoff possibilities" noted.

The current approaches to satisfying these metricsalmost universally involve incremental/evolutionarytechnological improvements to the existing paradigmscoupled with revolutionary reductions in design cycletime and "manufacturability" improvements in thecontext of an "integrated product team." What areconspicuous by their absence are any major attempts tosatisfy these metrics via the complementary approach ofinventing, developing, and deploying farther term/advanced technologies, in particular advancedconfigurations or systems, with revolutionaryperformance improvements. While traditionallyperformance improvements have been used to enhancespeed or reduce fuel consumption, they can obviouslyalso be employed to address the present metrics ofcost/part count/weight, productivity, safety and theenvironment, where advanced performance can have

*Chief Scientist, Fellow AIAA 1American Institute of Aeronautics and Astronautics

Copyright © 1998 by the American Institute of Aeronautics andAstronautics, Inc. No Copyright is asserted in the United Statesunder Title 17, U.S. Code. U.S. Government has a royalty-free licenseto exercise all rights under the copyright claimed herein forGovernmental purposes. All other rights are reserved by thecopyright owner.


truly dramatic payoffs. Also, much of the majoraeronautical improvement actually fielded over the past40 years, particularly in the long haul arena, has beenthe result of propulsion technology, primarily higherbypass and turbine inlet temperature, which providedmuch of the technology for a near trippling of seatmiles per gallon. Studies and suggestions for advancedconfiguration concepts (e.g., references 6-11) have, ingeneral, not been investigated in depth norimplemented. In fact, vehicle long term strategicplanning/research in the civilian aeronautical arena hasbeen exceedingly sparse for decades. This has not goneunnoticed and calls for renewed longer term researchefforts have come from both Dan Goldin, the NASAAdministrator, and Congress "Focus NASA'sAeronautics on Revolutionary New Concepts" (reference12). The military has been much more proactive in thisregard (e.g., references 13,14 and the previous"forecast" studies-e.g., reference 15).

The purpose of the present paper is to discuss exampleadvanced concept approaches, across the speed range andfor space access, which may provide revolutionarychanges and opportunities in civilian aeronautics in thefuture. The context of this advanced conceptsdiscussion is that there are no "magic bullets," i.e.,concepts which require no R&D, have no problems,require no research and provide guaranteed (huge)benefits. All of these approaches require considerablework to move them through the two filters which existbetween an idea and a deployed system. The first ofthese filters is a technical one which asks the question"does it work." The second, and immensely important,filter is technological and addresses the issue of whetherthe concept makes sense in the "real world" when all the"illities" are considered. Also, most of these conceptsare not new (see reference 16), simply worthy of beingreaddressed in the context of new metrics,missions/requirements, available technology levels andimplementation ideas. A central theme is the use ofserious synergies between propulsion, aerodynamic andstructural systems (see reference 17). Many of theconcepts discussed herein devolved from a series ofunpublished workshops and studies convened by thepresent author at NASA Langley in the late 80's-early90's specifically to (re)address advanced concepts invarious mission areas (CTOL, HSCT, hypersonicairbreathing propulsion and personal aircraft).

A major "bottom line" suggested by this

(re)examination of advanced aeronautical concepts is thataeronautics is far from operating near the limits of whatcan be accomplished either physically or economically—i.e., the conventional wisdom (which is unfortunatelyapproaching a "self-fulfilling prophecy") thataeronautics is a "plateau"/"mature" industry isemphatically not correct. There is such a wealth ofalternative concepts, each of them offering trulyrevolutionary possibilities, that the adaptation of evenone or a limited number could lead to a "Renaissance"in aeronautics (see also reference 18).

DESIGNER FLUID MECHANICS

Most of the configuration concepts discussed herein relyon a set of technologies developed extensively over thelast 30 years which can be collectively termed "DesignerFluid Mechanics." These technologies include laminarflow control (reference 19), mixing enhancement (e.g.,reference 20), separated flow control (references 18 and22), vortex control (reference 23), turbulence control(e.g., reference 24), anti-noise, favorable waveinterference (reference 25), and even "designer fluids."In most cases, these have been taken to the flight teststage and beyond and are thus ready, in variousmanifestations, for inclusion in, and to provide enablingtechnology/ capability for, synergistic advanced aircraftconcepts.

Consider, for example, the currently applicable aircraftmetrics-productivity, safety, environment andaffordability. These would be greatly enhanced bymajor simultaneous reductions in wake vortex hazard(reference 23), drag-due-to-lift (references 26-34) andfriction drag (references 35,36). The Designer FluidMechanics literature just cited offers an extensive arrayof alternative reduction approaches for each metric/flowphenomena. If the lists of the various approaches are"merged" and "simultaneous solutions" sought, theresultant configurational implications strongly suggestseveral revolutionary configurational alternatives, forexample, to the current 707-DC8 CTOL transportparadigm. This serves to illustrate the thoughtprocesses followed to develop the advancedconfigurations described in the following sections.

ALTERNATIVE SUBSONIC LONG HAULTRANSPORT CONFIGURATIONS

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Strut/Truss-Braced Wings with Wing-Tip EnginePfenninger has long advocated strut bracing to improvethe performance of conventional transports (e.g.,references 37 and 38, see also references 39-44). Theresulting (bending, torsion) structural benefits allowreduced wing thickness and sweep, resulting in atremendously enhanced and easily maintained (reducedsensitivity to roughness/insect remains/ice clouds,reduced cross flow) extent of natural-to-easily forcedlow drag laminar flow, as well as increased span. Thelatter allowed a reduction in wing chord, furtherenhancing the extent of laminar flow, as well asenhanced takeoff and climb performance and reducedvortex hazard. Plenninger's designs for such aircraftyielded L/D values in the 40's, over twice currentlevels. The concept was not, however, adoptedprimarily because the extensive wing span did not fitthe FAA "80 meter box" for airport gate compatibilityand disbelief that a transonic strut braced wing could bedesigned with acceptable shock drag (and obtain laminarflow on the strut/truss). Obviously strut-bracing isroutinely employed on low(er) speed aircraft. Thelatter objection is probably not valid in light of today'sCFD capabilities. "We build what we can compute"(reference 45) and we have been too long constrained inaircraft design to linear theory and consequent "linearthinking."

The span of a strut braced configuration can probablybe reduced to the 80 meter requirement and the overallperformance retained if an alternative approach isemployed for major drag-due-to-lift reduction-wing tipengine placement (also enabled by strut/truss bracing).Whitcomb (reference 46) and others (e.g., references 47to 55) have shown that up to the order of 50 percentDDL reductions are obtainable using this approach,which probably requires circulation control in theempennage region and utilization of thrust vectoring onall engines to handle the "engine out" problem. ThePfenninger approach of wing sweep/thicknessreductions and consequent natural-to-easily forced winglaminar flow as enabled by strut bracing is alsoretained.

Synergistic propulsion-aerodynamic interactionThe use of tip engines for aerodynamic drag-due-to-liftand wake vortex reduction is part of an overallparadigm shift in aircraft design where a configurationalconcept is sought in the context of an "openthermodynamic system," i.e., synergistic use is made

of the energy added by the propulsion system.Historically, aerodynamic theory is almost totallypredicated upon analysis within a "closed system" (noenergy added within the control volume). By necessity,as speed is increased, increasing use has been made offavorable propulsive interactions—wing propulsive pre-compression/engine nacelle favorable interference lift atsupersonic speeds and, at hypersonic conditions, theentire undersurface of the body is an integral part of theengine flow path for the airbreather case. However,little "first principles" work is available regardingfavorable interactions in the subsonic case.

A theoretical construct for aerodynamic optimization inan open system is not yet extant, but there are severaldiscrete examples, in addition to the whig-tip enginecase, where synergistic airframe/ aerodynamic-propulsion integration has been studied and in somecases even applied. These include circulation controlwing flow which offers up to a factor of 4 increase inCL compared to conventional flaps and slats withtremendous reductions to "part count" (reference 56),wake/boundary layer ingestion into the propulsionsystem [order of 15 percent increase in propulsionefficiency (e.g., reference 57)] including synergisticinteraction with an LFC suction system (reference 58);thrust vectoring for control (e.g., X-31, "tail-lessfighters, etc.) and, hypersonically, for lift enhancementand thrust requirement reduction; utilization of aleading edge region LFC suction system for high liftduring takeoff; the ejector wing for improved structuraland aero efficiency (reference 59); wing tip injection forwake vortex and drag-due-to-lift reduction and myriadpropulsive-augmented high lift schemes. An additionalmajor opportunity for synergistic propulsion-aerodynamic interaction is the "Goldschmied" wing (orbody), a takeoff on the Griffith wing (reference 60).The basic concept is to position the propulsive inlet inthe recompression region on the wing/afterbody toeffectively place "sinks" inside the body and convertmuch of the balance of the afterbody into a stagnationregion instead of having a rear stagnation "point."Estimates indicate up to a 50 percent "cancellation" ofthe body friction drag is possible via this "staticpressure thrust" approach (references 61-65). Anadditional, but to this point unevaluated, approach isuse of inflatable surfaces to provide "missionadaptation." A particularly attractive application ofthis would be tailoring of the afterbody region ofhypersonic cruise vehicles during acceleration through



the transonic speed regime to alleviate the tremendoustransonic "drag pinch" problem on configurationsdesigned for efficient hypersonic cruise/acceleration/airbreathing.

Double FuselageConventionally, double fuselage/multi-body aircrafthave been employed to provide span-load distributionand accrue the associated structural weight benefits(reduced wing bending moment) without going all theway to a "blended wing body'Vspanloader configurationi.e., providing such benefits via "conventional" (e.g.,"comfortable") technology. Total aircraft drag is alsoreduced, primarily due to favorable effects on drag-due-to-lift (e.g., references 9 and 66-70).

An advanced double fuselage approach could attempt todelete the conventional outer wing panels and onlyretain a, largely unswept/long chord, whig sectionbetween the fuselages. This requires prodigious drag-due-to-lift reduction, a requirement which can beaddressed via design of the fuselages as wing-tip "endplates" and the individual fuselage empennage as"winglets," i.e., the tails become thrusting surfaces inthe presence of the wing vorticitv wrapping around thefuselage(s).

For this case, the "midwing" can become the site of thegear (to allow use of conventional runways), engines"buried" at the rear of the wing to accrue the benefits of"boundary layer ingestion" and extensive (natural/suction) laminar flow. The major payoff would accruefrom making the fuselages detachable/interchangeableto provide a civilian "sky-train" with enhancedproductivity. The midwing portion which does all the"flying" could be in the air nearly "around the clock"with freighter and/or passenger modules, thereby nearlydoubling the productivity/duty cycle and "return oninvestment" (reference 71). Such an approach wouldallow a restructuring of the airline capital investment,with the airlines "owning" their fuselages and leasingthem from a "rent-a-wing" company. Obviously,military versions could have cargo, troop, and refuelingfuselages-providing a quantum jump in militaryflexibility and productivity.

Biplane/Ring WingAnother alternative configuration is a "back-to-the-future" relook at a long-haul biplane (reference 72, seealso references 9,73 and 74). Recent work (reference

72) indicates a wide spectrum of benefits for such anapproach, again in the context of present-to-futureadvances in CFD, materials, controls, "Designer FluidMechanics," etc. technologies. Some of theseestimated benefits are major-60 percent reduction inwing weight, order of 30 percent increase in L/D andreduced vortex hazard. The related ring whig (e.g.,references 75 and 76) is also worth revisiting.

JIJMBO AIRCRAFT

As this paper is written, the requirement for, andinterest in, "Jumbo Aircraft" (>600 PAX) is in a stateof uncertainty. Airbus is proceeding with an enlarged"conventional" aircraft study/development, Boeing hasplaced its corresponding effort on the "back burner" andthe fate of a developing Douglas aircraft blended wingbody effort is dependent upon the outcome of theproposed Boeing-McDonnell Douglas "merger." Thereis little doubt that a larger size/updated version of the747 would have greatly improved performance (e.g.,reference 77). At some level the major players (U.S.,Europeans, Russians) are studying the technology for ajumbo aircraft in the 800+ PAX range which is,"different"-some variant of the spanloader or "blendedwing body" (BWB). From reference 78, "machineswith more than 1,000 seats will have to be designed.For such machines, the unstable flying wing, quiteprobably equipped with a canard, will be the imposedsolution." (Bernard Ziegler, Senior Vice President,Engineering, Airbus). Jumbo options aside fromconventional and BWB include multi-body and wing-in-ground (WIG) effect The multi-body is a viablecandidate but the WIG probably is not. Study of theextensive Russian work in the WIG arena indicatesseveral nontrivial problem areas for the WIG vis-a-visthe long haul transport mission-operation near thesurface in high density air engendering a high draglevel, structural and propulsive weight inefficienciesassociated with water impact and takeof f thrustrequirements and safety problems associated withoperation at the extremely low altitudes (fractions ofthe wing chord) required to attain appreciable groundeffect benefits on "cruise" L/D.

The success of a "deployed version," the B-2 bomber,has renewed interest, worldwide, hi spanloader/blendedwing-body aircraft The major performance benefits ofsuch aircraft are required to address the potential "killerissues" for jumbo aircraft-noise and vortex hazard



engendered by their great weight. It is not clear,although jumbo aircraft will obviously carry morepassengers individually, whether airport passengerthroughput will go up or down with the introduction ofjumbos—more PAX per aircraft but perhaps lessaircraft/unit time in terms of takeoffs/landings due towake vortex/noise/passenger handling delays.

Obvious benefits of spanloader aircraft include largeincreases in L/D (due primarily to the demise offuselage wetted area/skin friction) and reduction inempty and gross takeoff weight. The design approach"puts the lift where the load is" for a requisite sizeaircraft with a physical wing thickness sufficient toallow passenger seating within the wing. Thetechnological challenges (as well as the opportunities)of an 800+ PAX long haul B WB transport aretremendous but at least thus far evidently "workable."These challenges include thick wing sections (possiblywith hybrid LFC and synergistic ("Goldschmied")propulsion integration), non-circular pressure vessels,stability and control, emergency passenger egress, ridequality, airport compatibility and very high Reynoldsnumber aerodynamics-both high lift and cruise. TheB WB design inherently contains a surfeit of internalvolume and is therefore highly conducive to enhancedrange, cargo operation and passenger comfort In regardto the latter benefit, "sleeper" versions of the B WBcould provide very interesting competition to theHSCT for trans-Pacific routes in terms of enhancedcomfort/lower price versus shorter transit time/higherprice. The USAF "New World Vistas" study (reference14) specifically called out the BWB approach as anexcellent candidate to provide enhanced "global reach"airlift capability—in conjunction with precision (GPS-guided) delivery pallets as an alternative to the vehicledesign decrements and vulnerability of landing "intheater." The large payload/volume and extraordinaryrange of BWB transports also provides extraordinarycapability for high capacity paratroop drops, cruisemissile or UCAV carriage/launch, "AWACS" missionsand "aerial replenishment" References 9 and 82-87provide an entre into the BWB literature, reference 88describes a very different Russian spanloader approach.

THE CHANNEL WING

The channel whig is an interesting example of anadvanced configuration which was somewhat ahead ofits time. The concept was originally developed,

essentially empirically, in the 40's and early 50's as aSTOL light aircraft and several versions were flown,this is not just a "paper airplane." The essentialtechnology consists of a semicircular airfoil "channel"underneath, and surrounding on the underside,"standoff wing mounted engine nacelles such that theengine-induced airflow produces sizable lift on the wing"channel" at zero-to-low forward speed, providingdramatic STOL capability. The developers of theconfiguration claimed, but there was never anysatisfactory "proof that the approach was capable ofnear VTOL performance. Development of this concepthas been essentially dormant since the late 50's exceptfor Soviet research-Antonov produced an advancedprototype-demonstrator in the late 80's termed the AN-181 (reference 89).

The original 40's-to-50's era research on the channelwing was, by necessity, highly empirical and resourceconstrained (see references 90-97). The conceptprovides a classic example of an approach worthrevisiting with updated tools and technology toascertain the extent to which its STOL performancecould approach VTOL. In particular, the incorporationof circulation control in the classical sense (viablowing immediately upstream and above the trailingedge), when combined with the engine-induced flowover the wing, should further augment the liftingcapability of the configuration. Additional technologyupdates/approaches of possible interest include (power-on) CFD, flow separation avoidance and control, winglaminar flow for cruise performance and inboard strut-bracing along with the all important controls issues.

The potential for such an updated channel wingapproach to address the "V-22 niche" should bedetermined as the channel wing could possiblyincorporate very interesting lifting capabilities with afairly high cruise speed at reduced weight/cost comparedto the V-22 (with its engine rotation requirements andassociated systems penalties).

HIGH SPEED CIVIL TRANSPORT

The increasing importance of "Pacific Run" air traveland the requisite long transpacific flight times for thecurrent subsonic transports have renewed interest in aMach 2 class "high speed civil transport." Such adevice is nominally a "flying fuel tank" to a muchgreater extent than the subsonic case, due to the



addition of appreciable wave drag. This large fuelfraction (order of one half to two thirds of gross takeoffweight) makes the designs exceedingly sensitive to draglevel and drag reduction. A mere 1 percent dragreduction can yield a IS Klb reduction in take offweight. To first order the cruise drag is nearly equallypartitioned between skin friction, volume wave dragand wave/vortex drag-due-to-lift. Large drag reductionsare potentially available via "Designer FluidMechanics," e.g., laminar flow control, favorable waveinterference and flow separation control as well asconfiguration tailoring to produce an elongated liftingline (references 25 and 99). The following advancedconfiguration concepts provide enhanced performancealternatives to the conventional "double-delta"planforms. Drag reductions theoretically attainableinclude up to 70+ percent of the skin friction vialaminar flow control and a very large fraction of thevolume wave drag via favorable wave interference.

Parasol WingThis is an old approach wherein reflections of thefuselage nose shock provides favorable interference liftand subsequent afterbody region thrust (references 4,99-102). Estimated L/D improvements are in the rangeof 25 to 30 percent Advanced technologies required toaccrue these benefits include flow separation control forthe shock-boundary layer interaction regions andfluidics or variable physical geometry to work the "off-design" performance issues. The reduction in volumewave drag may allow reduced wing sweep andconsequent greatly enhanced LFC application.

Strut-Braced "Extreme Arrow"Pfenninger has also advocated an externally strut-bracedHSCT with truly revolutionary cruise performance-anL/D of order 20, over twice that of the best of thecurrent approaches (references 103 and 104). The strutbracing allows use of an extreme arrow wing planformwith minimal wave drag-due-to-lift, increased aspectratio and extensive laminar flow ("controlled"). Mid-wing fuel canisters are used to provide favorable waveinterference and load alleviation with extensive"natural" laminar flow on both the fuel canisters andthe fuselage.

Other Alternative HSCT ApproachesNorthrup has studied a "reverse delta" configuration forpurposes of obtaining extensive regions of "natural"laminar flow on the wing (reference 105, see also

reference 106). "Novel" general concepts withapplication across the configurational spectrum includeuse of flow separation control at cruise to allow fullexploitation of inviscid design precepts (reference 25).The current design approach entails investigation ofadvanced (primarily inviscid flow-driven) aero conceptsin the wind tunnel to determine flow separationboundaries in terms of vehicle attitude and geometry.The potential performance of the advanced concept isthen degraded accordingly. Utilization of "designerfluid mechanics" technology in a proactive manner forcruise should allow recovery of this flow separation-induced performance loss. Potential benefits includeenhanced wing leading edge thrust, increased uppersurface lift, increased fuselage lift/camber (reduced waveDDL) and enhanced performance of favorable waveinterference (via shock-B.L. separation control) as wellas "bird-like" flight for enhanced ah- systemproductivity/noise reduction at lower speeds.

Another "alternative" is to consider a "multi-stage"aircraft (e.g., references 25 and 107). In-flight refuelingand some type of "take-off assistance" are obviousways to "multi-stage," but what is specificallysuggested herein is that since the HSCT is a "flyingfuel tank" the aircraft that lands is very different/lighterweight than the vehicle at takeoff. Therefore, theheavy fuel/noise control/high lift devices and gear (gearweight approaches fuselage weight for an HSCT)required for the "takeoff' condition could perhaps bepositioned toward the rear of the craft and"detached'V'flown back" once airborne. The vehicle isthus not burdened throughout its flight by apparatusuniquely required for mission initiation only.

If economics or environmental considerations dictate alower altitude/lower speed (« 50 Kft, M =1.5) than anobvious "approach of choice" would be the R.T. Jonesyawed whig (references 108-110) which is capable,from reference 108, of doubling speed and increasingPAX load-out 25 percent vis-a-vis the 747 foressentially the same fuel burn.

Another exceedingly interesting but at this pointhighly speculative approach to HSCT (and also CTOL)optimization is through wave drag reduction via"plasma Control"/ electro-gas-dynamics. Theconventional shock-plasma Interaction literature isclustered about two "end points" in terms of parameterspace-planetary (collision-less) bow shock formation in

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the solar wind and inertial-confined fusion. Ofparticular interest in the former case is Venus, whichhas no magnetosphere and is therefore subject toelectro-gas-dynamics as opposed to magneto-electro-gasdynamics phenomena (e.g., reference 111).Examination of the physics of shock-plasmainteractions (c.f., reference 112) indicates that, in theabsence of a magnetic field ion-acoustic waves areexpected to be dominant. Research in Russia (e.g.,reference 113) and the U.S. (reference 114) indicatesunexpectedly large experimental effects for a "newarena" of shock-plasma interaction—weakly ionizedplasmas at conventional gas dynamic conditions(continum flow, supersonic/hypersonic Machnumbers). Typical requisite experimental conditionsare 1012 electrons/cc, but relatively high electrontemperature (vis-a-vis ion/neutral temperature) andplacement of electrons well ahead of the unmodifiedbow shock. Conventional wisdom (e.g., reference 115)would indicate that such weak ionization should havelittle-to-no effect on the neutral gas dynamics.However, the observations indicate virtual replacementof the usual speed of sound by a higher speed wavesystem the order of the ion-acoustic velocity andconsequent reduction in effective Mach number andbody drag. The reducio-ad-absurdium implications aretruely revolutionary-an increase in drag rise Machnumber from 0(.85) to 0(2+), HSCT's which aresimilar in configuration to CTOL transports and CTOLtransports with greatly increased wing thickness/reducedsweep, etc. The keys to such leaps are the verificationof the available observations, understanding of theoperative physics and obvious developmental aspectsassociated with high temperature electron generationand placement in and ahead of the vehicle flow field.

That an increase in sound speed can reduce wave draghas long been known (e.g., references 116 and 117),but heating per se is simply not an energeticallyefficient approach to obtaining such a sound speedincrease/drag reduction. What is "new" is thepossibility of employing new physics (e.g.,coulombic-induced communication-ion-acousticwaves) and the exceedingly low power levels which theexisting data base suggests are required. Some type of"amplification system" may be operative to achievesuch dramatic influences upon gross flow field featuresfrom such a small energy input/low ionization fraction.A current line of investigation in that regard involvesdynamic processes including ion-acoustic instabilities

(e.g., reference 118).

PERSONAL AIRCRAFT-THE VTOL"CONVERTICAR"

The developed nations entered the 1900's with atransportation system (for people) centered upon thehorse, the railroad and the steamship, with associatedtravel times the order of hours-to-days/weeks,depending upon distance. In the closing years of thesame century, the automobile has long supplanted thehorse and the fixed whig aircraft has nearly driven therailroads and steamship companies from the long haulpassenger business. Travel times have shrunk tominutes-to-hours. In the process of supplanting oldertransportation systems, these newer approaches havehad a profound influence upon the structure of modemsocieties. In the U.S., cities have expanded out of 18thcentury seaports and 19th century railheads, wheremuch of the developed region was by necessity withinwalking distance of the transportation terminals, intotremendous suburbs with attendant reductions incrowding/increased opportunity for individual homeownership etc., etc. The existing transportationsystem fulfills a variety of purposes, including travelto and from work and stores, and for various business,service and pleasure related activities.

This portion of the present report centers upon futurepossibilities/options for a specific portion of thetransportation spectrum, short-to-moderate range,nominally from 10's to 100's of miles. The currentdominant transportation mode for this mission is theautomobile, which, possibly more than any othersingle technical achievement, has enabled the currentlife style enjoyed by the developed nations. In thisprocess, the auto has created massive safety problems(order of 40,000 deaths/year due to highway accidents)and has been responsible for the expenditure of trulyprodigious sums on roads, bridges, along withpollution-induced health and material degradationremediation. The current status of the autoinfrastructure is that we continue to clear and pavemore of the watershed, contributing to air pollution,flooding, desiccation, the formation of heat islands andwildlife habitat degradation. Also, the average triptime is increasing due to suburbian expansion andincreased congestion, causing non-trivial changes infamily life as travelers attempt to utilize non-traditionaltime slots, or suffer long/nonproductive commutes. In



the U.S. the interstate highway system is (finally)finished and is already clearly overburdened and in needof very expensive repairs and expansion, particularly inthe urban areas.

Society cannot, easily or otherwise, continue to bearthe costs imposed by almost sole reliance upon theautomobile for short-to-intermediate passengertransport, alternatives are necessary for the future—bothfor the developed societies and those that desire to/aredeveloping. Probably the most commonly advocatedalternatives involve some form of mass transit, whichhave, along with tremendous capital costs, severalother drawbacks such as passenger wait time, weatherexposure and lack of privacy, security, pride ofownership and personal stowage. Additional drawbacksare the fact that they are not portal-to-portal and there isno guarantee of having a seat, as well as an inherentassumption regarding increased populationdensity/concentration. Undoubtedly, the future mix ofshort-to-intermediate transport systems will includeboth mass transit and automobiles of some variety,probably operated on "intelligent" highways toimprove safety and throughput/trip time (reference119). The "smart highway" alternative is, however,expensive and very limited in terms of growth potential(=0[factorof2]).

There is, however, both a need and an opportunity toinclude in the transportation mix a personal air vehiclewhich would provide, percentage-wise, the sameincrease in speed (compared to the auto in traffic), asthe auto provided over the horse. Personal airtransportation usable by "everyone" is bothrevolutionary and the next logical step in thedevelopment of human infrastructure and corporalcommunication. The increased speed of such acapability, along with the greatly reduced capitalrequirements in terms of highways/bridges, etc., shouldallow significant increases in the quality of life as wellas reduced state and national public works budgets.Specific benefits include distribution of the populationover a much larger area allowing a more peaceful/lessdamaging co-existence of man and nature, along withimproved transportation safety. The "vision" is ofmultilevel highways in the sky, controlled andmonitored by inexpensive electronics as opposed tonarrow, single level, exceedingly expensive "ribbons ofconcrete" (e.g., reference 120). Such airsystems/vehicles could also obviously be used for

longer haul, as are automobiles today. The variouswait times associated with commercial air travel, alongwith the inefficiencies in terms of transit time of thehub and spoke system mitigate in favor of reducedoverall trip time for slower, but more direct, travel viapersonal aircraft (compared to the "faster" commercialjet). Various options exist for personal aircraftsystems. The discussion herein will address one suchoption, an automatic VTOL-converticar, and attempt todefend that particular recommendation.

Certain requirements/desirements are common to anypersonal transportation vehicle/system. These includeshort transit time/high speed, direct portal-to-portal,privacy and security, constant availability, personalstowage and a suitability for use by the "non-pilot."The latter necessitates from the outset that an obvious(and probably attainable) goal should be an automaticpersonal air transport system, automatic with respect tonavigation (e.g., references 121-123), air traffic controland operation. The technology to accomplish this iseither currently employed by/for the long haul airtransport application, or in the research/applicationpipeline, thanks to the microchip "electronicsrevolution" and includes GPS, personal and othercommunication satellites (44 broadband/mobilecommunication satellite systems "on the books'Vinprogress) and the military investments in RPV's,AAV's, UAV's, UTA's, UCAV's, etc. Suchautomatic operation could provide vastly improvedsafety, as the preponderance (70 percent to 80 percent)of air transport accidents have historically been due to"human error." In addition, it makes personal airvehicle transportation available to the general public,as opposed to the few who have the opportunity,wealth, and physical characteristics to become pilots,as well as reducing the unit cost by an order ofmagnitude or more due to the concomitant vastincreases in production rate/market.

Conventional wisdom holds that, to be successful, analternative transportation system must be not onlyfaster, but also relatively inexpensive. The costsinvolved in any system include acquisition, operation,maintenance, and depreciation. To be competitive withthe automobile a personal VTOL-converticar shouldhave an acquisition cost in the vicinity of a qualityautomobile. Although in terms of the currenthelicopter industry, this is a ridiculous target, theadvantages of a production run of millions instead of



hundreds, along with a recent offering of a single seathelo for $30K (references 124 and 125) and a two-place"gyroplane" for $20K, all at small production runsmakes the outlook to achieve such a goal possible ifnot probable (see reference 126). Operational costsinclude fuel, insurance, parking fees, etc., and need notbe greater than the auto. Maintenance is considerablygreater for present helos than for autos, and thereforethis issue would have to be addressed in any personalhelo technology development program.

All-weather operation is also a requirement, the sameall-weather capability one now has in an automobile,which is by no means absolute. Extremely heavy rain,extreme winds, ice and snow will all either slow orstop the auto, and similar restrictions will probablyhold for the personal helo. Obviously the evolving"detect and avoid" technology could be utilized by thepersonal helo (either on or off board) to increase safetyvis-a-vis extreme weather. In terms of speed and range,the helo must provide a significant speed advantage orit is simply not viable. As compared to a fixed wingpersonal aircraft, the helo speed advantage is much lessvis-a-vis the auto, but at a nominal factor of 4 (for thetraffic case) still sufficient We are currently spendingsignificant sums to gain a factor of 2+ in the highspeed civil transport program (vis-a-vis subsonictransports). Another key issue is rider acceptance interms of acoustics, vibration, ride quality, andreliability/safety. All of these technical areas willrequire further work, although the helo community hasmade significant strides in these already andconsiderable further gains/technological advances are inthe pipeline. A final major set of issues involvecommunity acceptance in terms of acoustics anddowndrafts during near surface operations. Again, morework is needed, but these could be addressed byoperational as well as technological approaches.Previous approaches to the "personal helicopter" havemainly considered existing machines as opposed to theadvanced technology/ farther term vision discussedherein (e.g., references 127-129). There have been,however, calls for such an approach (references 130,131).

Over the years, particularly since the 1930's, there havebeen suggestions, and in some cases strident calls, forthe development and marketing of personal aircraftAlthough "general aviation" has made considerableadvances, the "aircraft for the masses" never really

caught on for a variety of reasons, mainly involvingCOST, requisite technology readiness and an absoluterequirement that the "operator" be a "pilot" e.g., non-automatic operation. History is replete with examplesof concepts which are good ideas and which keepresurfacing until the technology base is ready. Anobvious example is the gas turbine engine. Since thelast personal aircraft campaign in the late 40's-50's,major strides have occurred in several enablingtechnologies. These include light weight miniature,inexpensive and tremendously capable electronics/computing, lightweight composite materials with"nearly infinite" fatigue life, computational fluidmechanics, smart-to-brilliant materials/skins, flowcontrol of several types and active controls/loadalleviation. Such advances significantly change thepersonal aircraft discussion, particularly for the helo."The helicopter looks, 35 to 40 years after itsinvention, to be poised in the position the fixed wingaircraft were in the late 40's and early 50's, again 40years after the first flights were being made" (reference132). In particular, the personal helicopter wouldprofit from much of the sizable investment made inmilitary machine research, albeit the civilianapplication is in many ways less severe in terms of"rough usage" etc. This is again directly analogous tothe fixed wing situation where the 707 class oftransport aircraft profited immensely from/was enabledby, the military investments in swept wing/jetpropelled bombers/tankers/transports.

Key helo-specific technologies either available or in thepipeline include composite blades with 10,000 hourfatigue life, the hingeless-bearingless rotor with lowdrag hub, automatic health monitoring to allowsignificant reductions in maintenance costs, anti-vibration and anti-noise for enhanced rider comfort,automatic piloting and navigation/nap-of-the-Earthoperation, and composite structure and smart skins forflow and load control (see, for example, references 133-140).

There are several "systems level" issues and criticalchoices regarding the personal aircraft which served askey discriminators in the selection of the particularpersonal aircraft discussed herein, a helo-converticar.The first such issue is whether the personal aircraft(either "fixed" or rotary wing) should be a separate airvehicle, or a "converticar," i.e., a combinationautomobile and air vehicle capable of economically



performing both missions. Economics and utilitystrongly favor the "converticar" option. There arenumerous elements common to both the air and groundvehicles, such as passenger compartments, engines,etc. and therefore, if it is technically feasible to reducethe weight of an auto to what is reasonable for an airvehicle, then a single device should be considerablymore economical (initial cost as well as maintenance-wise) than buying and maintaining two separatevehicles, particularly when one considers the presentcost of autos. Simplex estimates of the flight-specificcomponent weights indicate a value of less than 1000pounds, indicating that, with shared utilization ofcommon systems such as the engine, the "all-up"weight of the converticar could be in the (reasonable)range of 3000+ pounds. From an operationalviewpoint, usage as well as maintenance-wise, a singlevehicle should be much more convenient, obviating theneed for a "rent-a-car" in the vacinity of one'sdestination. Once the converticar option is selected,some decision/recommendation has to be maderegarding the provision for the "air-unique"components, particularly the lift-producing surfaceswhich require, for reasonable levels of drag-due-to-lift,non-trivial span/aspect ratio. Options include towed"trailored" wings (utilized in early versions of theconverticar), fixed wings of inherently low aspect ratiofor "readability" (reference 141), airport "rent-a-wing"concessions where the wings are attached prior to, andremoved at the conclusion of, flight, andtelescoping/folding wings. The present author favorsthe telescoping/folding option as offering the bestcompromise between convenience and performance.

The next critical choice is between conventionaV'fixedwing" operation and a VTOL device. An essentialdifference is that the fixed wing machine/operationrequires an airport. There are many thousands of GAairports in the U.S. and one would have to begin andend the air portion of the trip at one of these. In theopinion of the present author, this is simply toorestrictive and contravenes several of the fundamentalpurposes of the personal air vehicle such asindependence of/reduced requirement for large civilworks, portal-to-portal transportation, and access toremote sites (remote from roads, etc.).

The VTOL option would allow development/usage ofcurrently undeveloped nations/regions at a fraction ofthe cost of the roads/bridges, etc. usually required for

such development, and at much less disruption to theenvironment (reference 142). The estimated off-shoremarket for such a device is the order of $.5T/year.Conversion from ground to flight and back again for ahelo-converticar requires only a relatively hard surfacewith a diameter the order of 25 ft, something whichcould be placed at intervals alongside the existinghighway system to provide convenient ground-to-air"merging" away from existing builtup housing areas tominimize acoustic/downdraft etc., influences upon thepopulation. Further advantages of the helo include theprovision for both lift and propulsion in a single deviceduring air operation and ATC "margin" (in the event ofan ATC conflict the vehicles involved could "hover" orlocally land while the problem is addressed/resolved).

Another major option involves the extent to which theoperation in the air mode should be automatic asopposed to pilot/human derived. While sport modelscould be somewhat human-controlled (within theconfines of the ATC/safety regulations) the optimalsolution is clear. The portion of the populationphysiologically capable of becoming pilots is not largeand there is considerable cost and time involved indoing so, most accidents are due to pilot error (reference129), and the ATC system requires, for the largenumbers ultimately envisaged, automatic operation.Therefore, a user-orientated personal air capabilityshould, ultimately, be automatic in operation as wellas navigation and ATC, as already suggested herein.

A personal transportation machine capable of bothground and (VTOL) air operation could be anautomobile with an 1C engine (reference 144), probablyinitially a two-seater and at least somewhat pilot-controlled, which is light enough to also fly and whichhas built into its roof an erectable low drag, large taper(reference 145) rotatable hub with a diameter consistentwith the vehicle width containing the order of four ormore telescoping/ folding rotor blades. In addition, arear deck vertical fin is required within which is a,perhaps electrically driven, tail rotor. Alternativeapproaches include circulation control on the"afterbody" or a tandem/counter-rotating rotor system.

As stated several times in this discussion, the centralissue is COST (see the quote from Henry Ford inreference 146) and usability. As a result oftechnological advances in several areas, many of themmomentous, and the tremendous requirement/market for



such an affordable/user-friendly capability, the issue ofpersonal air transportation should be revisited. Theprobable course of development for personal airtransportation is parallel to that of the automobile inthe early 1900's. The initial machines were expensive("rich man's toys") with many impediments to theiroperation such as poor roads, noise sensitivity and lawswhich were in many cases "anti-automobile." Onceindustrialists (e.g., Henry Ford) addressed the problemvia "design to cost/PRICE," simplicity (any color aslong as it's black) and mass production, the pricedropped drastically and the resulting widespreadsales/utilization of the product revolutionized, in manyways, our entire society (see also references 147 and148). The key to such a mass market/affordability forthe VTOL converticar is automatic operation via alargely existing infrastructure and technology base.Militarized versions would provide the armed forceswith a (robotic/automatic) flying jeep capable ofmeeting the battlefield mobility requirements for the"Army after next" (beyond "Force 21"). Such an"automatic" VTOL-capable machine would alsoprovide, in a "future world" of increasing prevalent tele-commuting and "electronic cottages" affordable/roboticdelivery of requisite food supplies for the carbon-basedinhabitants as well as goods ordered on the"net'Vshopping channels.

SPACE ACCESS

There are two disparate space access missions withfundamentally different but overlapping metrics--inexpensive transportation of "pounds to orbit" and"space warfare." The former mission is vital toeconomical/ viable space utilization (environmentalmonitoring, navigation, communications, naturalresource observation, space manufacturing,astro/solar/planetary physics, planetary explorationetc.) and is currently being worked by NASA via theX-33 and X-34 programs. The space warfare missionwas the subject of the NASP program of the late 80'sand early 90's and the continuing/increasingrequirement for a military Transatmospheric Vehicle(TAVy'space plane" is clearly stated in severalrecent/ongoing Air Force planning studies (e.g.,Spacecast 2020, Air Force 2025, New World Vistas).The rationale for a military space plane includes battlespace awareness, global precision strike and spacecontrol. The present writer suggests that the differentmetrics/requirements for these two missions may

perhaps best be satisfied by disparate approaches~anadvanced rocket for inexpensive space access and anadvanced airbreather which could uniquely provide therequired flexibility for the military space planemissions. The increased dry weight/cost associatedwith airbreathing engines makes the hypersonicairbreathing option problematical for the "pounds toorbit" mission but its capability for self ferry, enhancedcross range, orbital plane change, recall, enhancedlaunch window, in-atmospheric abort/assured payloadreturn, dispersed launch sites, orbit/deorbit/reorbit, etc.uniquely satisfy various space warfighting metrics (seereferences 149-152).

Considering first the "pounds to orbit" mission forboth civilian and military applications, over the years alarge number of studies have "mixed and matched" thevarious multitudinous technical/technological spaceaccess options available, focusing on (significantly)reduced cost—with no obvious "clear winningcombination." The following suggested advanced rocketinvolves emerging technologies not yet included inthese previous systems studies with the intent ofaddressing the prime metric—affordabili ty. Thesuggested technological elements of a space accessadvanced rocket include propulsion, fuel and aerocomponents. The suggested primary propulsion cycleis a pulse-detonation wave rocket engine, a technologywhich is successfully emerging from several years ofPhase 2 SBIR research projects. Potential benefitsinclude lightweight, order of 15 percent Isp increase(constant volume as opposed to constant pressurecombustion) and up to a factor of 40 reduction inpressure level requirement for the turbine feed pumpswith attendant major reliability and cost benefitscompared to current (i.e., SSME) practice/requirements(e.g., references 153-157). Significant additional thrustup to the Mach 6 regime is available via a lightweightexhaust region shroud/ejector designed using NASP-level and beyond mixing augmentation technology.Estimates indicate the addition of the ejector couldsignificantly increase payload fractions (e.g., references158-161). The suggested advanced fuel is solidhydrogen (with imbedded Boron lattice) underdevelopment in the Phillips Laboratory HEDMProgram (reference 162). In addition to the expecteddensification benefits, the Air Force estimates up to a25 percent fuel Isp increase. Finally, to obviate aero-control-dictated ballast and "packaging" requirementsrecourse could be made to "designer aerodynamics"



wherein dynamic pneumatic vortex generation isutilized to provide stable/controlled flight in lieu of/inaddition to conventional fins and "static margin" (e.g.,reference 163), i.e., application of the NASA/USAFHARV forebody vortex control technology for fighteraircraft and the X-29 (unstable vehicle) controlapproaches) to the (unstable) rocket control problem.Optimization of the base ejector approach suggests a"different" configuration-an annular/hollow cylindricalgeometry with extremely (structural/cost) efficienttorroidal fuel/oxidizer tanks and both "internal" andexternal air entrainment into the ejector propulsionstream.

Considering next the military-specific space-accessrequirements, the suggested airbreathingcycles/technologies for the TAV/space warfare missionhave as their "core" (up to March=10) the"conventional" NASP approach of (with increasingMach number) low speed system/RAMJET/diffusive-burning scramjet A key finding from the NASP effortwas the criticality of the high Mach number (M>10)propulsive performance. In particular, the internallosses/penalties (friction, heat transfer, shock, mixing,weight, disassociation) of the diffusive burningscramjet in the high Mach number regime are notconsistent with a viable machine.

Therefore, two new cycles are suggested for this M>10speed regime. For 10<M< =15 Langley has begunworking a "PMSIC" (premixed shock inducedcombustion) approach which features forebody fuelinjection/mixing, greatly reduced combustorlength/losses/weight and an Isp increase the order of 25percent (e.g., references 164-168). For the "pullup"into orbit portion of the flight path, a lox-augmentedscramjet, essentially a high Mach number RBCC issuggested which utilizes flow-path embedded linearrocket motors to pressurize the combustor/allowairbreathing to higher altitudes (reference 149) andprovide shear for enhanced mixing and species fornozzle catalysis. The key to a viable PMSIC engine isthe avoidance of "pre-ignition" upstream of thecombustor in the forebody premixing region alongwith simultaneous forebody injection/mixingoptimization for penetration, fuel addition thrust,injection-induced compression and forebody skinfriction/heat transfer reduction.

In addition to these basic cycles, other technologies

could be employed to enhance airbreathing propulsiveperformance at high Mach number-thrust vectoring fortrim and drag reduction, multiple shock inlet, nozzleflow catalysis to reduce the thrust loss associated withspecies "freezing," fuel thrust via (highly) heated fuelinjection and forebody boundary layer transition delayfor flow path friction/heat transfer loss reduction.

With the exception of limited application of fuelheating and transition delay these technologies/cycleswere not employed in the various NASP design cycles.Their utilization should enable development of aviable/affordable "warfighting" space plane. Anadditional requirement for such a development is a"T&E" engine test facility in the Mach 10 to 16 rangewith the requisite large size/long run time needed forhypervelocity engine "certification" in terms of theinteracting system elements-combustion, aero-integration, aero-thermal, aero-thermal elasticity,materials/ structures, coatings, seals, controls,operability, etc. Promising facility research for thisrequirement is underway at Princeton University andMSB, Inc. in Butte, Montana. The current "best bet"approach is a combination of "cold" very high pressureair storage/supersonic flow establishment withsubsequent (serial) energy addition to a moving streamvia e-beams (or MW/lasers) and subsequent MHDacceleration (reference!69). The R&D engine testrequirements can be met viable a detonation driverupgrade to the NASA Hypulse facility (reference 170).The design driver for both of these facilities is the hightotal pressure required to simulate the forebody (postbow shock) flow for the PMSIC class of engines (e.g.,simulation of "combustor entrance" conditions is notsufficient).

Our problem in space access is emphatically not adearth of promising technological possibilities, thereare many more than mentioned herein. We areemphatically not up against physical/physics barriersin terms of major performance improvementapproaches for system weightreduction/affordability/flexibility, etc.

CONCLUSIONS

Advanced configuration and "frontier" aeronautics isadmittedly long term and high risk (by definition) butconstitutes, in the opinion of the author, the onlyapproach available which can seriously address the



productivity, cost, safety and environmental metrics ina truly meaningful manner.

1. Contained within the discussion herein areseveral relatively novel general approaches/concepts.These include, for example, transport aircraft designedand optimized in an open thermodynamic system(utilization of extensive propulsive/ aerodynamic/structural synergisms) and utilization of flow control atcruise and otherwise to accrue full iiiviscid performancebenefits.

2. A suggestion is made for development ofautomatic personal aircraft operation (via GPS, cornssatellite constellations and RPV/AAV/UCAV, etc.technology) enabling a large vehicle production run andan affordable revolution in personal mobility (VTOL"converticar") and many aspects of our culture/economy($.5T/year to $lT/year market).

3. A multitude of updated/modified alternativeconfigurations are suggested for both subsonic andsupersonic long haul which have revolutionarypotential. The wealth of available possibilities/approaches greatly increases the probability that one ormore of these could "work" in the "real world." Theresource level currently expended in the area of advancedconcepts/configurations is, and has long been,"subcritical" in terms of providing adequate evaluationseven of the potential of the ideas currently extant.

4. Suggestions are made for advanced (insome cases revolutionary) approaches to space access-for both the "pounds-to-orbit" and space warfightingmissions.

5. In the opinion of the present author, weshould return to the invention/development andutilization of advanced concepts as a method ofworking affordability (along with the current designcycle and manufacturability/"process" approaches).

ACKNOWLEDGEMENT-The author is, and willforever be greatly indebted to the many in and out ofaeronautics who have had the courage, foresight,knowledge and creativity to seriously consider thefarther term possibilities in aviation. Their work isabsolutely crucial to the health andcompetitiveness of our industry and national defenseand, increasingly, to the enrichment of the humanexperience.

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