s&tr july 1994

Review&

University of California • Lawrence Livermore National Laboratory

■

July 1994

■ Pathfinder: On the Way to Solar Rechargable Aircraft

■ Flight Test of the ASTRID Rocket

Review&

University of California • Lawrence Livermore National Laboratory

■ July 1994

■ Pathfinder: On the Way to Solar Rechargable Aircraft

■ Flight Test of the ASTRID Rocket

Nonprofit Org.U. S. Postage

PAIDLivermore, Ca.Permit No. 154

Technical Information DepartmentLawrence Livermore National LaboratoryUniversity of CaliforniaLivermore, California 94551

About the CoverPathfinder, serving as our flying testbed for

designing a solar-rechargeable airplane, embodiestechnologies and engineering practices essential forachieving “virtually eternal” flight at high altitude.Derived from an airframe that flew nearly a decadeago, the Pathfinder expresses today’s advances inelectric motors, solar arrays, and power conditioning.Last fall and winter, the Pathfinder completed alow-altitude flight test series at the Rogers DryLake Bed, Edwards Air Force Base in California,and is being enhanced to attempt a high-altitudeseries. For more information, see the article on p. 1.

About the Journal

The Lawrence Livermore National Laboratory, operated by the University of California for the UnitedStates Department of Energy, was established in 1952 to do research on nuclear weapons and magneticfusion energy. Since then, in response to new national needs, we have added other major programs,including technology transfer, laser science (fusion, isotope separation, materials processing), biology andbiotechnology, environmental research and remediation, arms control and nonproliferation, advanceddefense technology, and applied energy technology. These programs, in turn, require research in basicscientific disciplines, including chemistry and materials science, computing science and technology,engineering, and physics. The Laboratory also carries out a variety of projects for other federal agencies.

Energy and Technology Review is published monthly to report on unclassified work in all our programs.Please address any correspondence concerning Energy and Technology Review (including name and addresschanges) to Mail Stop L-3, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94551,or telephone (510) 422-4859.Prepared for DOE under contract

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Pathfinder and the Development of Solar Rechargeable Aircraft 1For many purposes, unmanned, solar-rechargeable, high-altitude aircraft flying continuously for multiple weeks offer a low-cost, desirable alternative to satellites. The Laboratory and its industrial partners have evolved such an aircraft concept, and the Pathfinder serves as our flying testbed. Our project draws upon our nation’s leaders of electric vehicles and solar-powered aircraft innovation.

ASTRID Rocket Flight Test 11The successful flight test of our new, lightweight propulsion system paves the way for application of this technology by the aerospace industry.

Abstracts 18

Review&■ July 1994

SCIENTIFIC EDITOR

William A. Bookless

PUBLICATION EDITORS

Lori McElroy, Lauren de Vore

WRITERS

Kevin Gleason, Robert D. Kirvel

DESIGNER

Kathryn Tinsley

ART PRODUCTION

Treva Carey

COMPOSITOR

Louisa Cardoza

PROOFREADER

Catherine Williams

This document was prepared as an accountof work sponsored by an agency of the UnitedStates Government. Neither the United StatesGovernment nor the University of California norany of their employees makes any warranty,express or implied, or assumes any legal liabilityor responsibility for the accuracy, completeness,or usefulness of any information, apparatus,product, or process disclosed, or represents thatits use would not infringe privately owned rights.Reference herein to any specific commercialproduct, process, or service by trade name,trademark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement,recommendation, or favoring by the United StatesGovernment or the University of California. Theviews and opinions of authors expressed hereindo not necessarily state or reflect those of theUnited States Government or the University ofCalifornia and shall not be used for advertising orproduct endorsement purposes.

Printed in the United States of AmericaAvailable from

National Technical Information ServiceU.S. Department of Commerce

5285 Port Royal RoadSpringfield, Virginia 22161

UCRL-52000-94-7Distribution Category UC-700

July 1994

ATELLITES have shrunk theworld to the size of the proverbial

global village. They track weatherand the traffic patterns of ships andaircraft, and monitor our environment.Defense satellites provide high-resolution images of objects on theground to protect our troops andallies. Telecommunication satelliteshave interwoven business sectors,corporations, and markets intoglobal networks.

Nevertheless, orbiting satellitesmay not be the best choice for allapplications requiring a high vantagepoint. Satellites and their payloadsare expensive, and launching them

or months) and can roam virtuallyanywhere.

Relocatable Satellites in theAtmosphere

High-altitude, long-endurance,unmanned air vehicles are less costlythan satellites in every way. They takeoff and land rather than having to belaunched. They can vary their flightpaths like any other aircraft, providingcontinuous local coverage and thenrelocating hundreds of kilometers ina single day. An airplane powered bythe sun during the day and by an on-board rechargeable energy-storage

by rocket is expensive and risky.They must operate in the extremeconditions of space, bombarded byradiation and with no airflow to cooltheir electronics. Only valuable, long-term missions would seem to justifythe expense and risk of a satellite.Even then, satellites are not alwaysthe best choice. They typicallycannot hop to a new orbit, and someuses, such as local and globalcommunications, require a largenumber of satellites to ensureadequate coverage. There is clearlya large potential role for high-altitude,atmospheric vehicles that can stayaloft for very long periods (weeks

Pathfinder and theDevelopment of SolarRechargeable Aircraft

Recent flight tests of the Pathfinder (shown above) demonstrated thepotential benefits of solar-rechargeable aircraft. Since the Pathfinder is nonpolluting and very smooth flying, it offers a unique platform for

high-altitude surveillance and atmospheric sensing.

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system at night can, in principle, stayaloft for weeks or months at a time.Because the energy is renewable (see Figure 1), only the wear ofcomponents limits flight duration.Equipped with the proper sensors orcameras, a single aircraft flying at analtitude of 20 to 25 km could scansome 200,000 km2 at a time (this areais roughly equivalent to the size ofNebraska). These aircraft couldperform a variety of environmental,commercial, and scientific services:• Spot the beginning and monitor theevolution of natural disasters on allscales, from hurricanes and riverfloods to forest fires.• Monitor pollution, toxic gas releases,and effluents from volcanic eruptions.• Monitor the spread of an oil spill toaid containment and cleanup efforts.• Monitor agricultural conditions

(crop health, blight, pestilence, soilerosion, and irrigation) and helpmeasure harvests.• Map resources such as minerals,oil, and geothermal power.• Monitor oceanic thermal currents.• Measure high-altitude ozone levelsand other atmospheric phenomenafor studies of global warming.

For military applications, suchplanes could be sent aloft over areasof known or suspected hostileactivity to perform high-resolutionsurveillance or to work as “aerialmines.” That is, they could detect thelaunch and ascent of a hostile theaterballistic missile, such as a SCUD,and release a small, agile, high-velocity interceptor to smash into itwhile it is still over enemy territory.1Such planes could have been used togreat effect in the Gulf War.

A Flying Wing Design

The first steps toward developinga solar rechargeable airplane weretaken in the early 1980s, with thedevelopment of HALSOL (high-altitude solar vehicle). AeroVironment,Inc., of Monrovia, California, designed,built, and tested an experimentalprototype aircraft.2 The goal of theunmanned plane was to fly day andnight, at high altitudes (above 20 km),for very long periods in temperateand tropical latitudes (i.e., too farfrom either pole to capitalize on itsseasonal 24-hr sunlight). Such anairplane would have to collect at leasttwice the solar energy needed fordaytime flight and store the surplusfor use at night.

Compared with internal-combustionaircraft, solar-powered aircraft requirea large area of wing per unit weightof the craft (i.e., a “low wing loading”).Moreover, the higher the desiredoperating altitude, the more power isneeded to sustain flight. Therefore,the wing area must be increased inorder to collect more sunlight.

Conditions at the desired operatingaltitudes required an aircraft withvery large wings—too large for aconventional cantilever design, inwhich the wing is projected from alarge central mass, the fuselage. Thenew aircraft required a design in whichthe solar collection area could beincreased without forcing an increasein the thickness of the spar (the mainstructural element of the wing). Theweight is distributed as evenly aspossible across the wingspan (i.e., it is a “span-loaded” design). Thus,HALSOL was a flying wing,resembling a 30 m ¥ 2.6 m plankflying sideways. For a description of the operating conditions at highaltitudes, see the box on p. 8.

The 200-kg HALSOL made nineflights under battery power (to avoidrisking expensive solar arrays during

Solar Rechargeable Aircraft E&TR July 1994

2

Power management

unit

PayloadPropulsion

Charge

Night discharge

Solar array

Energy storage

unit

Collela Figure 1

Figure 1. A simplified schematic of solar-rechargeable, round-the-clock flight. Less than halfthe sunlight striking the solar cells powers the motors; a larger fraction is stored on board.As the solar energy striking the cells falls below a certain minimum, the aircraftÕs power-management unit begins drawing on the stored energy for the ÒnightÓ portion of the flight.

early experimental flights) over atwo-month period in 1983. The testsvalidated the plane’s aerodynamicand structural properties, itsmechanical performance, and itsflight behavior. However, availablebatteries had inadequate specificenergy (energy available per unitweight) for nighttime use, and the morepromising alternative—namely, fuelcells—was an immature technology.The HALSOL program was terminatedand the vehicle was mothballed in 1983.

Pathfinder

The next steps followed nearly10 years later, in 1992, when LLNLteamed with AeroVironment to identifyaffordable current and near-termtechnologies that could be engineeredinto the design of a solar rechargeableaircraft. To do so quickly and withmodest resources, we had to capitalizeon the lessons learned during thedevelopment of HALSOL.

Under the sponsorship of theBallistic Missile Defense Organization,we obtained the mothballed vehicle,upgraded its airframe (as describedunder “Technology Advances”), andequipped it with today’s technologyfor propulsion and vehicle control.The metamorphosed aircraft wasrenamed Pathfinder, and it serves asour flying testbed for critically testingfactors affecting solar rechargeableflight at high altitude. Figure 2 showsPathfinder at rest and in flight.

Like HALSOL, Pathfinder spans30 m and has a 2.6 m chord (thedistance from the wing’s leading edgeto its trailing edge). The wing’s sparis a hollow tube made of carbon fibercomposite. The wing is assembledfrom five modules for flexibility inflight conditions and for ease ofground transport, and carries eightmotors.

If, for the sake of simplicity, weimagine Pathfinder flying a steadycourse in broad daylight at some 20 km

altitude, we can give a basic descriptionof what Pathfinder does: it gatherselectric current from the solar cellson its wings and converts it to therotary motion of the propellers (withoccasional adjustments of the elevatorsas needed). This simple descriptionmasks an engineering challenge, whichrequires a sophisticated control-systemdesign. External conditions at the solarpanels, such as the intensity and angleof the sunlight and the density andtemperature of the air, determine theamount of available electrical energy.

Air density and velocity also determinewhat power the motors require tomaintain a steady course.

A human pilot negotiates amongthese conditions by reading instrumentsthat give the needed information andthen, on the basis of training andexperience, manipulating the controlsto adjust the plane’s behavior. In anunmanned vehicle, an integratedsystem of sensors and electroniccontrols performs all the tasks ofcollecting and measuring information,interpreting it, and feeding the correct

E&TR July 1994 Solar Rechargeable Aircraft

3

(a)

(b)

Figure 2. (a) Pathfinder on Rogers Dry Lake Bed before flight. The dark areas are the solarcells. Compare the slight droop of the wing with its position in (b), which shows Pathfinderin flight (altitude ~60 m). The upward curve of the wing results from lift, and this curvature of the wing puts its structural components in the condition of least load. However fragilePathfinder might look, its flexibility and light weight allow it to withstand high wind gusts asit climbs to high altitude.

amount of power to each motor. In thenear decade between the mothballingof HALSOL and the fitting ofPathfinder, all the technologies thatcontribute to such a control systemadvanced significantly.

Technology AdvancesIn addition to the sensors and

electronic controls, solar-photovoltaic-array and motor technologies advancedconsiderably. AeroVironment, byvirtue of its involvement with GeneralMotors on solar-powered and electricautomobile projects (called SunRaycerand Impact, respectively), becameexpert in designing electric motorsand power-control systems.Electronic component technologiesleapt forward even more strikingly.The following descriptions highlightthe ways in which Pathfinder wasmodified from its predecessor.

Structural Design. Pathfinder’swing surfaces are skinned with newstructural polymers that are stronger

and more resistant to the sun’sultraviolet light. Although the aircraftis deceptively flimsy in appearance,it can withstand 4-

g shocks on landingand 5-g loads in flight. It is alsodesigned to handle the strong guststhat it might encounter near a jetstream. The span-loaded designevenly distributes gust loads; whenit encounters turbulence at highaltitude, it responds much like an airmattress riding the ocean’s waves.

Solar Cells. The silicon solar cellsare extremely thin and lightweight, yetare affordable. Compared to the bestcells available ten years ago, today’scells have two to five times the specificpower (power available per unitweight) and a unit cost (dollars perunit of available power) that is nearlya factor of ten lower. During its low-altitude flight testing, the wing wasequipped with nearly 19 m2 of solarcells (up to 60 m2 is possible) that aremore efficient and lighter. More than90% of the area of the solar arrays

collects light; less than 10% is areabetween cells or structural support.

Figure 3 shows a wing section withsolar arrays installed. Siemens SolarIndustries and LLNL teamed toproduce the modules at very low costfor early flight testing on Pathfinder.Later in the program, LLNL developedand flew even lighter-weight modulesand also developed a technique forlaminating modules to ease theirhandling and increase protection.

Motor Design. Because movingparts wear more rapidly in the partialvacuum at high altitudes and mayfail, we employed two solutions:redundancy and extreme simplicity.Pathfinder’s 8 propeller pods and 26elevators provide enough redundancyto ensure adequate propulsion andcontrol for recovering the aircraft ifsome parts fail. On the other hand,we radically redesigned the motorand propeller assembly to reducethe number of moving parts.

Because the brushes in conventionalelectric motors arc and wear rapidlyat high altitude, Pathfinder useslightweight, brushless DC motors withrare-earth, permanent magnets torotate the propeller. This electronicallycommutated design eliminated morethan 100 moving parts from eachpropeller pod, leaving only threemoving parts: two bearings and alightweight spider armature on whichthe propeller is mounted. (LLNL madethe motor housings and spiderarmatures.) The new design alsoreduces the drag-inducing frontal areaof the motor cowling. Figure 4 showsthe Pathfinder motor assembled anddisassembled.

To improve cooling efficiency,we moved the heat-producing powercomponents to the outside of a nickel-plated aluminum motor housing andradially splayed the cooling fins.

Propellers. A plane mustnegotiate flight environments thatvary in combinations of wind force,


4

Figure 3. A wing section of Pathfinder in the shop after nine solar arrays were installed.

air temperature, and air density,while undergoing differentperformance demands, such asclimbing, accelerating, and levelcruising. Conventional propeller-driven planes adapt at least partiallyby using variable-pitch propellers.(The blade can change its angle to thedirection of its travel, its pitch; at oneextreme, it can cut through the aircleanly like a knife edge or, at theother extreme, present a broad surfacelike a Ping-Pong paddle being swung.)

The HALSOL motors had variable-pitch propellers, but the Pathfindermotors use fixed-pitch propellers.The pitch is set before takeoff to whatis optimal at the aircraft’s cruisingaltitude, and motor speed compensatesfor the pitch deficiencies at otheraltitudes. This change to fixed pitchallowed us to significantly reduce thenumber of parts, as described above,by eliminating the gearbox andcontrol mechanisms to vary pitchand the complex pitch-control analogcircuit. The new fixed-pitch, ground-adjustable propellers are lighter,stronger, and more reliable thantheir variable-pitch predecessors.

Flight TestingBetween October 1993 and

January 1994, the Pathfinder, remotelycontrolled from a ground station, flew10 low-altitude test flights using bothbattery and solar power. These flighttests took place at Rogers Dry LakeBed, NASA Dryden Flight Center,in California.

All technology upgrades werevalidated. We quantitatively assessedthe controllability of the airplanethrough precise measurements of lift,velocity, power, and angle of attack,and are using the collected data torefine the dynamic control model.The flight series met all objectives,along with a few bonuses:• One flight was exclusively solarpowered.

• We obtained high-resolution imagesfrom an on-board camera that wasmounted directly to the airframe,demonstrating the remarkably lowlevels of vibration of the aircraft duringflight.• We measured the dust levels abovethe Rogers Dry Lake Bed using asensitive particulate counter. Becausethe aircraft is nonpolluting, it will be anideal platform for many environmentalmeasurements.

After the entire available wing area—60 m2 of the 75 m2 total (15 m2 mustbe free of cells to keep the wing near

the leading edge aerodynamicallyclean)—is covered with solar panelscurrently being fabricated, the planewill be able to ascend to more than20 km. Simulations indicate thatPathfinder could stay aloft for severaldays above the Arctic Circle duringthe Arctic summer, even without anenergy-storage system.

Future Work: Storing andRecycling Energy

We have analyzed and designed arechargeable energy-storage system


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Figure 4. Two viewsof a Pathfinder motor:with propellerassembly (a) andwithout (b), showingits internalcomponents and the radial cooling fins.There are only threemoving parts in theentire assembly, ascompared to morethan 100 in theprevious generationdesign.

(a)

(b)

that makes night flight possible.The next stage in the developmentprogram would be to build this system,install it in the aircraft, and test thesystem’s performance and durabilityat altitude. Rechargeable batterytechnology falls far short of our needs,but an energy storage system basedon fuel-cell technology could meetour requirement for high specificenergy. Figure 5 gives a schematicoverview of a solar-rechargeablepropulsion system.

Fuel CellsA battery and a fuel cell both store

energy in the form of reactants andproduce power through electrochemicalreactions in cells. Unlike a battery,which is self-contained (all itscomponents for storing energy andproducing power are within a sealedvolume), the fuel cell stores one orboth of its reactants externally. Theenergy capacity of a system using afuel cell can be made larger simplyby storing more fuel.

The fuel cell brings hydrogen andoxygen gases together in a controlledreaction to produce electrical powerand water during nighttime. Duringdaylight hours, electrolytic cellsrecharge the system by decomposingthe same water (by electrolysis) intohydrogen and oxygen gases that arethen stored at high pressure in tanksin the wing. The reactants are recycledeach day.

This type of system is called aregenerative fuel cell. The system canbe designed to use separate electro-chemical cells to create electricity andto electrolyze water, or to use the samecells to perform both functions (at avery small sacrifice in efficiency).The second type is a reversible systemcalled a unitized regenerative fuel cell.Figure 6 shows a conceptual designof this type of fuel cell.

Weight ConsiderationsThe weight of the entire energy-

storage system is critical because itrepresents from a third to nearly half

the total weight of the aircraft andpayload. Since a unitized regenerativefuel cell uses the same componentsfor creating electricity and forelectrolysis, it can be significantlylighter than a system using separatefuel and electrolytic cells. It also hassimpler plumbing and thermalmanagement requirements.

Combining a small fuel cell stackwith large-volume, lightweight tanksshould yield high specific energy andenergy capacity. Although unitizedregenerative fuel cells requireadditional development time andcost, the suggested benefits of lessstorage-system weight andcomplexity and greater reliabilityargue in their favor. However,prototyping the energy-storagesystem with discrete components(i.e., separate electrolyzer and fuelcell stacks) allows the earliestopportunity to develop and test, in alaboratory setting, the hardware andsoftware related to the energy andpower-management system.


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Figure 5. Aschematic of thepropulsion system ofa solar-rechargeableaircraft. Figure 6shows thecomponents of theenergy-storagesystem.

Master controller

Power conditioning

Electrolyzer

Energy-storage system

Motor

Motor drive electronics

Motor drive electronics

Motor

Solar array Solar array

Current sense

More motors,

solar arrays

More motors, solar arrays

Current sense

DC power bus

Command Command

Collela Figure 6

Possible DesignThe LLNL /AeroVironment team

concluded that the reactant gases(hydrogen and oxygen) can be storedunder pressure within the hollowwing spar. However, a permeationbarrier is needed because the fibercomposites that make up the spar are fairly permeable to pressurizedhydrogen gas. The conventional

barrier of thin-walled aluminumwould add unwanted weight to thespar. Therefore, we have pursued the development of a bladder linerconsisting of a sandwich of metal-coated polymer membranesthat has extremely low loss rates ofpressurized hydrogen and oxygengases. One or two bladders (one eachfor hydrogen and oxygen) could be

installed in each spar segment. Usinga polymer bladder instead of analuminum liner could reduce theweight by more than 20 kg, and usingthe spar as tankage may save anadditional 20 kg. (Forty kilograms isa sizable percentage of the ~100-kgpayload capacity.) Thus, any savingsin vehicle weight can be traded forincreased payload weight.


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Figure 6. Rendering of a generic unitized regenerative fuel cell subsystem, including storage of reactant gases in the wing spar.

Coolant flow

H2 phase separator and water tank

Membrane stack

Water tank

Current (in and out)

Hollow spar

Valves and pumps

Controller

Propulsion and payload

Power conditioning

Solar arrays on top of wing

Collela Fig. 07

E&TR 7/94 31252 TJC

H2 gas O2 gasSputtered, metal-coated, laminated, polymer bladder to contain pressurized gas


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High-Altitude Solar Flight: The Balancing ActAs a solar-powered aircraft gains altitude, its

environment gradually changes in ways that significantlyaffect its need for power and the amount of poweravailable. The intensity of sunlight increases with altitude,and air temperature and density decrease. With no lossesto atmospheric absorption and scattering, sunlight in spaceis about 35% more intense than at Earth’s surface. Airtemperature averages about 15°C at sea level and drops to –45°C at 30 km. The electric power available from thesolar panels is nearly proportional to the intensity ofsunlight, and is inversely proportional to the temperatureof the solar cells.

The figure to the left shows the factors that must bebalanced for high-altitude, solar-powered flight. As shownin (a), solar intensity increases with altitude. However, theever-thinning atmosphere supplies progressively lessairflow to cool the solar cells (b), which produce lesspower as their temperatures increase (c).

Thus, as the aircraft rises through the dense butgradually thinning lower atmosphere, from sea level to12 km, increasing sunlight intensity and gradually coolerair produce a rapid increase in electrical power availablefrom the solar panels. This power is about 45% higher at12 km than at sea level. However, at altitudes above 12 km,power from the panels gradually decreases as paneltemperature increases; the thinner air provides less flowover the panel surface to cool the cells. Electric powerfrom crystalline silicon cells decreases 0.5% for everydegree Celsius in temperature rise.

The decreased air density not only causes a drop insolar-cell efficiency, it requires more power from themotors to maintain sufficient lift (the upward force on awing created by the greater air pressure under the wingthan above it). For example, a wing needs about 400%more power to maintain level flight at an altitude of 20 kmthan it requires to cruise at sea level, and 550% at 25 km.At the same time, the electrical power available from thepanels has increased less than 45% since the aircraft leftsea level, and is slowly decreasing with additional altitude.

This combination of factors leads to a fairly inflexiblealtitude ceiling, where the rapidly increasing demand for power exceeds the relatively fixed electrical poweravailable from the wing. Factors such as the plane’s totalweight, the total area of the solar panels, and the efficiencyof the solar cells fix the altitude where this occurs.Pathfinder’s ceiling occurs at 20 to 25 km.

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To have adequate power tomaintain lift in the thin upperatmosphere while recharging theenergy-storage system, a plane musthave a larger solar collection areathan Pathfinder’s. Pathfinder iscapable of reaching a peak altitude of20 to 25 km, but it can cruise there onlyduring daylight conditions. A solar-rechargeable aircraft will require twicePathfinder’s wingspan to operatecontinuously at 20 km or higher whilecarrying a 100-kg-class or heavierpayload and the energy-storage system.Doubling the wingspan to 60 m whilekeeping the same 2.6 m chord willdouble the solar collection area. (Stilllarger wing spans are possible but maybe unwieldy for ground handling.)

Summary

Some assignments, both civilianand military, are better performed byunmanned, long-endurance, solar-rechargeable aircraft high in theatmosphere than by satellites in low-Earth orbits. These aircraft wouldhave more flexible flight paths andwould be far less costly than satellitesto design, build, and operate. LLNLhas been working for the Departmentof Defense’s Ballistic MissileDefense Organization to developunmanned aircraft to protect ourmilitary forces and our allies fromattack by theater ballistic missiles.

In collaboration withAeroVironment, Inc., we developeda lightweight flying wing, calledPathfinder, that uses present-daytechnologies in photovoltaics, powerelectronics, aerodynamics, andguidance and control. Because of itslight weight, 30-m wingspan, andintended high peak altitude (20–25 km), Pathfinder is span loaded;

that is, the weight of all componentsis distributed as evenly as possibleacross the five modules that make upits wing. Flight tests have proven thesoundness of the concepts andcomponents.

We have analyzed and designed arechargeable energy-storage systemthat makes night flight possible, thusmaking flight durations of monthspossible. The technology, which isbased on fuel cells, has been developedfor other applications. Our challengeis to fit the application to the constraintsof a span-loaded aircraft operating athigh altitudes. The design mustminimize total system weight whiledistributing the weight of thecomponents as uniformly as practicalacross the span. The preferredapproach is a unitized regenerativefuel cell system, in which the fuel-cell components that produce electricpower and water also perform thereverse process of electrolysis, breakingthe water into gaseous hydrogen andoxygen for storage. However, a systemwith separate components, thoughheavier, may offer a nearer-term,more-affordable option.

Although Pathfinder is capable ofreaching a peak altitude of 20 to 25 km,it cannot cruise there in the absenceof sunlight. An aircraft capable ofperforming day/night missions at thataltitude, especially with the addedweight of the energy-storage system,must have larger dimensions thanPathfinder for more solar-collectioncapability. LLNL, together withAeroVironment, is designing such aplane—a high-altitude, extremelylong-endurance, solar-rechargeableairplane. We are teaming withindustry to identify affordable near-term technologies that could beapplied to reduce weight, increasereliability, and maintain adequate

efficiency of components. The tasksahead involve applying good system-integration practices to ensure lowwingloading and high reliabilityneeded for a multiweek mission, and to develop an aircraft that canbe handled readily on the groundand in the air.

Work funded by the Department of Defense’sBallistic Missile Defense Organization.

Key Words: fuel cells—rechargeable, regenerative,unitized regenerative; HALSOL; high-altitudeunmanned flight; Pathfinder; solar aircraft.

Notes1. This concept motivated the RAPTOR/TALON

Program at LLNL. RAPTOR stands forResponsive Aircraft Program for TheaterOperations; TALON stands for TheaterApplication Launch On Notice. The BallisticMissile Defense Organization within theDepartment of Defense put LLNL in charge of the program.

2. AeroVironment, Inc., established preeminencein the field of solar flight in 1980, when itsGossamer Penguin became the first mannedsolar-powered plane to fly, and again in 1981,when its Solar Challenger made aviation historyby safely ferrying its pilot 262 km, from Paris,France, to Kent, England, at altitudes exceeding3 km.


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For further information contactNicholas J. Colella (510) 423-8452(LLNL)/ (412) 268-6537 (CarnegieMellon University) or Gordon S.Wenneker (510) 422-0110.

OR more than 5 years, theLaboratory has been developing

a new kind of liquid rocket propulsionsystem with support from theDepartment of Defense’s BallisticMissile Defense Organization. Ourlightweight propulsion technology,which won an R&D 100 award in1992, features miniature pumps thatcan react to thrust changes within amillisecond to meet a range ofvehicle-control requirements. Thethrust-on-demand, pumped-propulsiontechnology for spacecraft is describedin more detail in the March 1993 issueof

Energy and Technology Review.This year, we flight tested the specialengine for the first time with the

required so that the fuel tanks canremain thin-walled and lightweight.Pumps would give the high pressuresthat are desired without the addedweight needed for high-pressure tanks.

Small rocket systems now operateby adding a high-pressure inert gasto pressurize the propellant tank to slightly above thruster pressure.All spacecraft to date—includingcommunication satellites and planetaryspacecraft such as Viking, Voyager,Magellan, Galileo, Clementine, andthe Mars Observer—use propellanttanks that operate at a higher pressurethan the thrust chambers. Such apressure-fed system is simple andgenerally reliable, but, once again,

ground launch of the world’s smallestpump-fed rocket.

Why a New Propulsion System?

The performance of small liquidpropulsion systems for spacecraft hasalways been limited by the absenceof pumps. Small propulsion systemsthat run their tanks and engine atabout the same pressure, using nopumps in the process, represent acompromise. Their performance isconstrained because designers haveto settle on an intermediate valuebetween the high pressure requiredso that thrusters can be small andlightweight and the low pressure

ASTRID Rocket Flight Test

Our recent ground launch of the world’s smallest pump-fed rocketdeveloped at LLNL shows that our new technology can propel a miniaturerocket fast enough to intercept theater ballistic missiles. We now envisioncost-effective uses for the new propulsion system in commercial aerospace

vehicles, exploration of the planets, and defense applications.

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the performance is limited. Inparticular, tanks designed for standardspacecraft pressures of about 2 MPa(300 psi) can store and deliver 10 to20 times their own mass in propellant.Thrusters designed to be fed from suchmoderately pressurized tanks typicallycan lift 10 to 20 times their own weight.

In contrast, our new approach—aunique propulsion system that isdesigned around pairs of reciprocatingpumps that stroke alternately(Figure 1)—allows spacecraft to go faster and farther with less totalweight than was previously possible.

Low-pressure tanks compatible withour technology can hold about 50 timestheir own mass in propellant, and ourengine (including pumps and high-pressure thrusters) can deliver thrustequal to 50 times its own weight.Figure 2 compares ASTRID withexamples of existing rocket-propulsionsystems. In addition to the weight oftanks and engines, this graph takesinto account accessory propulsionhardware, which is found on bothASTRID and pressure-fed spacecraft. Our advance in rocketry means thatit is now possible to plan a variety of

missions in space with smaller andlighter propulsion packages thanwere feasible before.

About the ASTRID Vehicle

We just completed a year-longproject, which culminated in the firstflight test of our new rocket engine.Many of the propulsion componentswe tested in the rocket evolved overa 5-year development effort that hadits origins in the Brilliant Pebblesprogram, which focused on newminiature interceptors. Owing in

ASTRID Flight Test E&TR July 1994

12

Ground pressurant fill valve

Warm gas manifold

Warm gas regulator

Spin baffle

Warm gas check valve

Titanium tank for hydrazine

fuel

Conduits through

tank

Thruster valve cable

Quad-piston pumps

FilterLiquid filter

Gas generator valve

Liquid regulator

Filter

Gas generators

Warm gas tube for thruster valve pilot stages

4-channel valve

Hydrazine thrusters

Whitehead Figure 4

Figure 1. Our newthrust-on-demandpropulsion systemuses fourreciprocating free-piston pumps andmonopropellanthydrazine. The quad-piston pumpset(inset) delivers itsown mass (365 g) in hydrazine eachsecond at a pressureof 7 MPa (1000 psi).

part to this history, our project wasnamed the advanced, single-stagetechnology, rapid-insertiondemonstration, or ASTRID for short.

We built the ASTRID rocket(Figure 3) to demonstrate thefeasibility of a small, high-velocityinterceptor vehicle that would alsobe equipped with a navigation systemand side-mounted thrusters for steering.Such interceptors were intended tobe carried aloft and launched fromunmanned, high-altitude aircraftcalled RAPTORs, which were being developed in parallel by theLaboratory’s Theater Missile DefenseProgram to safeguard our military,our allies, and their population centersfrom hostile attacks by theater ballisticmissiles. (See the article beginningon p. 1 for a description of one ofthe RAPTOR prototypes, calledPathfinder.)

The ASTRID project culminatedin a successful demonstration of theability of our new propulsion systemto function in atmospheric flight. Tominimize risks, we kept the ASTRIDvehicle quite simple for the flight test.For example, we built a monopropellantmachine using components alreadytested. Monopropellant systems aresimpler and require fewer parts thanthe pumped bipropellant (fuel plusoxidizer) system ultimately envisioned.For added simplicity and reduced cost,we opted for a fin-guided, roll-stabilizedvehicle and did not employ activeflight control.

We built two complete vehicles.Vehicle assembly and most of theparts fabrication was done at LLNL;other parts were purchased. We usedthe first vehicle for 10- and 30-secondstatic fire tests on the ground inNovember and December 1993. Thesecond ASTRID rocket—with itsinnovative pumped-propulsiontechnology—was launched fromVandenberg Air Force Base on themorning of February 4, 1994.

As shown in Figure 4, the testrocket was 1.9 m long and had adiameter of 0.16 m. The empty massof the ASTRID flight vehicle was8.25 kg. Of this mass, the lightweightpropulsion components weighed less

than 3 kg. The airframe was basicallybuilt around the lightweight, 15.3-literfuel tank made from a titanium alloy.We designed the tank with a largesafety margin for pressure (about afactor of 4) and avoided 90% of the

E&TR July 1994 ASTRID Flight Test

13

Whitehead New Figure E&TR 7/94

31252 TJC

Propellant weight/propulsion-hardware weight

S-II, stage 2 Saturn V

Delta stage 1

0.11001010.1

Thru

st/p

ropu

lsio

n-ha

rdw

are

wei

ght

Clementine maneuvering system

Propulsion with reciprocating pumps Launch vehicle propulsion with turbopump engines Spacecraft propulsion with pressurized tanks

1

10

100

ASTRID with full tank and full vacuum thrust

S-IC (first stage of Apollo's Saturn V)

Clementine attitude-control system

Near-Earth asteroid rendezvous

Miniature Seeker Technology Integration (MSTI) 3 and 4

Space shuttle, full external tank and 3 engines in vacuum

ASTRID as flown (sea level thrust)

Figure 2. ASTRID compared with examples of existing rocket propulsion systems. Bothpropellant weight and thrust are normalized to total propulsion hardware weight (includingtanks and engines) because rocket systems must provide both propellant storage and thrustwith as little hardware as possible. Note that pressure-fed spacecraft systems are generallyfound in the lower portion of the graph, whereas high-performance, launch-vehiclepropulsion systems (fed by turbopumps) fall toward the upper right. ASTRID lies near theupper right of the graph, which indicates that the LLNL piston pumps can support launch-vehicle performance capability on a small scale. ASTRID’s 2.6 kg of propulsion hardwarestored 12.7 kg of propellant before the flight, and delivered 440 N of thrust.

pressure safety documentationnormally required for spacecrafttanks. Prior to flight, we loaded thefuel tank with 12.7 kg of industry-standard monopropellant fuel, high-purity hydrazine (N2H4).

The ASTRID avionics werehoused in the fiberglass nose of therocket. These components consistedof sensors for vibration, acceleration,airspeed, and system pressures; acontroller to demonstrate the thrust-on-demand capability of the propulsionsystem; and an encoder and transmitterto relay the in-flight data.

The Launch and Flight

At first, we proposed to makeASTRID an inertially guided vehicle.

However, to reduce the cost andcomplexity associated with flightelectronics, we decided to eliminateactive control altogether. This decisionenabled us to focus on our primarytest objective: to demonstrate thepropulsion technology in theconditions of free flight. In a parallelactivity at the Nevada Test Site, theengineering issues related to activelyguiding an agile, small interceptorwere being addressed.

We launched the vehicle, as shownin Figure 5, under essentially windlessconditions from an 18.3-m-long (60-ft)rail that was set at an angle of 80 degfrom horizontal. After the rocket leftthe rail (Figure 6), fixed fins on theaft section provided aerodynamicstability and guided the rocket in a

gravity-turn trajectory. The four finswere canted to induce roll—that is, torotate the vehicle just after it left therail. Through roll-rate stabilization,any slight asymmetries associatedwith aerodynamics, thrust, structure,or mass distribution are averaged out.

During the 1-minute flight, ourengine enabled the ASTRID vehicleto soar 2 km up and to reach nearlythe speed of sound (Mach 1, whichis 300 meters per second) beforesplashing down in the Pacific Oceanabout 8 km downrange. The testclearly demonstrated the ability ofour new propulsion system tofunction in atmospheric flight.

The speed attained was limitedbecause ASTRID was launched fromsea level for this experiment. Avehicle flying in this densest part ofthe atmosphere is subjected to severeaerodynamic drag. Had the launchtaken place in the upper atmospherefrom an aircraft such as RAPTOR,ASTRID would have gone about sixtimes as fast (in excess of 2 kilometersper second).

Results

From the ASTRID flight, weobtained a large quantity of data,including measurements of axialacceleration, thrust level, vibration,trajectory, velocity, airspeed, and rollrate. However, the primary objectiveof ASTRID was to demonstrate howreciprocating rocket pumps wouldperform in flight and to make twokey pressure measurements. Thesemeasurements indicated that thepumps boosted the pressure of thefuel delivered to the thrusters, asexpected, to more than 10 timestank pressure throughout thepowered flight.

An obvious concern related tousing pumps is vibration. Amongother things, vibration could adversely


14

Figure 3. The pump-fedASTRID rocket.Standing next to the rocket prior tosunrise before launchat Vandenberg AirForce Base is theinventor of thepropulsion system,John C. Whitehead.

affect sensitive inertial guidanceinstruments that might be used infuture applications. We found thatvibration both along and transverseto the axis of the ASTRID vehiclewas not unusually high duringlaunch, and the magnitude of

vibration peaks decreased duringfree flight.

Following ignition, the rocket wasreleased after a short holding time of8.3 seconds. The rail ascent itselftook 1.7 seconds, and the poweredflight time from the rail top was

27.7 seconds. Data fromaccelerometers clearly indicate that the thrusters operated for 37.6 seconds.

Operation and shutdown of thepropulsion system performed asexpected, with no major problems or


15

Figure 4. The 1.9-m-long,pump-fed ASTRIDrocket. The fiberglassnose houses avionicsfor sensing, control,and transmitting databack to the ground.The forward section ofthe vehicle, betweennose cone and tank,is an aluminum shellcontaining pressuretransducers andhardware for fillingthe fuel tank. Thetitanium fuel tankoccupies just abouthalf of the vehicle’slength and serves as the center body.The componentsassociated withpropulsion occupyonly about half of theaft skirt. The four fins,made of lightweightKevlar, provideaerodynamicstability.

Fiberglass nose cone

Avionics deck and computer

Filler port

Fuel tank

Propulsion components

0.16 m

Gas generator (1 of 2)

4-port valve

Aft skin panel (1 of 4)

Fin with foil (1 of 4)

Drain line and valve

Thrusters (2 of 4)

1.9 m

surprises occurring during the flighttest. Our preliminary analysis of thedata shows that the thrust level wasinitially just over 440 N and wasabout 420 N during ascent on the railand flight. This thrust is within thepredicted range. The maximum rollrate of 4.1 revolutions per second(rps) also compares well with thedesign rate of 4.5 rps.

Collaborations and Future Plans

The ASTRID flight test was a trulycollaborative effort involving more

than 40 LLNL technicians, engineers,and scientists and a well-coordinatedteam of outside collaborators. Thisgroup built the rocket, developed thedata-acquisition systems, performedall fielding operations, and analyzedthe data. Organizations that madecritical contributions to developingand testing the pump-fed rocketinclude:• Olin Aerospace Company(formerly Rocket ResearchCompany).• Moog, Inc.• Ball Aerospace Company.

• Johns Hopkins Applied PhysicsLaboratory (APL).• Vandenberg Air Force Base.• Catto Aircraft Inc.

We envision a variety of potentialuses for the miniature propulsionsystem we have developed andsuccessfully flight tested. Forexample, we have worked for morethan six years with Olin AerospaceCompany, whose engineersdeveloped the thrusters for ASTRID.We will explore the possibility ofcommercializing our technology withthis company and others for possibleuse on spacecraft and launch vehicleupper stages.

Our propulsion system can beused in an upper stage to boost asatellite from low Earth orbit (at analtitude of a few hundred kilometers,where the Space Shuttle circles Earth)to geosynchronous orbit (at an altitudeof about 35,000 km above Earth, wherea satellite remains “stationary” overone geographic location). Once inorbit, the system can be used to makehard maneuvers and adjust the orbit.

Our new technology providesmore propulsive capability per unitmass of propulsion system hardwarethan any other means available todayfor small liquid systems. In particular,our pumped propulsion can makemissions to the moon and planets muchmore economical than in the past.

A mission to make a soft Marslanding and retrieve rock and soilsamples would benefit greatly fromthe high performance, low mass, andthrust-on-demand features of thissystem. The precision-throttlingcapability conferred by the responsivepumps would facilitate control of thedescent vehicle’s landing. Then, afterlanding and sample collection, avehicle would need to be launchedfor the return flight to Earth. Ournew technology provides the requiredlaunch-vehicle performance on asmall scale.


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Figure 5. ASTRIDascends the launchrail on February 4,1994. The glow of thefour thrust chamberscan be seen, but theplume is invisiblebecause hydrazinefuel decomposescleanly and at a lowertemperature thanother rocketpropellants.

On another front, several ongoingdefense programs can benefit fromour technology, particularly those thatrequire small defensive missiles thatare highly maneuverable. We plan toexplore these and other potentialapplications for the new pumped-propulsion technology in the future.

Work funded by the Pentagon’s Ballistic MissileDefense Organization. BMDO is the successor tothe Strategic Defense Initiative Organization.

Key words: advanced single-stage technology rapidinsertion demonstration (ASTRID); RAPTOR;rocket propulsion.


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Figure 6. Sequence of three photos taken of the ASTRID flight test. At t = 1.7 seconds after launch, ASTRID leaves the launch rail atVandenberg Air Force Base on its way to spashdown about a minute later in the Pacific Ocean.

For informationcontact John C.Whitehead (510) 423-4847, Lee C. Pittenger(510) 422-9909, orNicholas J. Colella(510) 423-8452/(412) 268-6537.

Abstracts E&TR July 1994

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Abstracts Pathfinder and the Development of Solar Rechargeable AircraftFor many military, civilian, and commercial applications, unmanned aircraft

flying at high altitude with long endurance offer advantages over satellites.These advantages include substantially lower costs for design, construction,and launch. Such aircraft may function as reusable, relocatable, geostationarysatellites operating within the atmosphere. We are working for the BallisticMissile Defense Organization to develop unmanned aircraft platforms forprotecting our military forces and our allies from attack by theater ballisticmissiles. A team of engineers from AeroVironment, Inc., and LLNL havedesigned and recently completed flight testing of a solar-powered airplanecalled Pathfinder. The Pathfinder is a flying testbed for proving keytechnologies, critical system integration approaches, and flight control issuesessential to achieving solar rechargeable flight at high altitude. Fuel celltechnology has been demonstrated to the point that unitized rechargeable fuelcells are possible. Such an energy-storage system weighs less than a third theweight of the best rechargeable batteries available. The next step would be tobuild a plane like Pathfinder that has wing dimensions that can accommodatethe weight of the rechargeable energy-storage system while enabling it tooperate continuously at altitudes (20 to 25 km).Contact: Nicholas J. Colella (510) 423-8452 (LLNL)/ (412) 268-6537 (Carnegie Mellon University) or Gordon S. Wenneker (510) 422-0110.

ASTRID Rocket Flight TestOn February 4, 1994, we successfully flight tested the ASTRID rocket from

Vandenberg Air Force Base. The technology for this rocket originated in theBrilliant Pebbles program and represents a five-year development effort. Thisrocket demonstrated how our new pumped-propulsion technology—whichreduced the total effective engine mass by more than one half and cut the tankmass to one fifth previous requirements—would perform in atmospheric flight.This demonstration paves the way for potential cost-effective uses of the newpropulsion system in commercial aerospace vehicles, exploration of theplanets, and defense applications.Contact: John C. Whitehead (510) 423-4847, Lee C. Pittenger (510) 422-9909, or Nicholas J. Colella(510) 423-8452 (LLNL)/ (412) 268-6537 (Carnegie Mellon University).

s&tr july 1994

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