An remotely piloted plane that set an altitude record two years ago broke apart during a test flight today and crashed into the Pacific Ocean, according to NASA officials:


The Helios Prototype solar electric plane crashed some distance off Kauai inside the test area of the US Navy's Pacific Missile Range Facility at Barking Sands.  The news was released by the National Aeronautics and Space Administration.


Helios was a $15 million dollar, solar-electric project.  She was propeller-driven and had a wingspan of 247 feet.  Described by some as more like a flying wing than a conventional plane.  Helios reached an altitude of 96,500 feet during a flight in 2001 also from Barking Sands. The roughly 18 mile altitude, was considered by NASA to be a record for a propeller powered winged aircraft.  It was designed for atmospheric science and imaging missions as well as relaying telecommunications up to 100,000 feet.




The $15 million dollar solar-electric, Helios airplane


This flight was to test the plane's fuel cell system.  According to Alan Brown, a spokesman for NASA's Dryden Flight research Centre, Edwards, California, USA, Helios was 30 minutes into the flight at around 8,000 feet west of Kauai, when the aircaraft broke up over water. The cause of the crash is unknown.  NASA is said to be forming an accident investigation team.


Helios had been flying under the guidance of ground-based mission controllers for Aero Vironment Inc. of Monrovia, California USA, the plane's builder and operator.  It was one of several remotely piloted aircraft whose technological development NASA has sponsored.


The prototype, powered by solar cells during the day and by fuel cells at night.  This technology is similar to that employed in Solar Navigator   A spokesman said: NASA intends to develop another Helios aircraft, calling it "technology worth pursuing."



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The Helios Prototype is the latest and largest example of a slow-flying ultralight flying wing designed for high-altitude, long-duration Earth science or telecommunications relay missions. A follow-on to the Pathfinder and Pathfinder-Plus solar aircraft, the Helios Prototype soared to 96,863 feet altitude in August 2001, setting a new world record for sustained altitude by winged aircraft, powered only by energy from the sun.


Developed by AeroVironment, Inc., of Monrovia, Calif., under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) project, the unique craft was designed to demonstrate two key missions: the ability to reach and sustain horizontal flight near 100,000 feet altitude on a single-day flight, and to maintain flight above 50,000 feet altitude for almost two days, the latter mission with the aid of an experimental fuel cell-based supplemental electrical system now in development.


The Helios Prototype is an enlarged version of the Centurion flying wing that flew a series of test flights at Dryden in late 1998. The craft has a wingspan of 247 feet, 41 feet greater than the Centurion, 2 1/2 times that of the Pathfinder flying wing, and longer than the wingspans of either the Boeing 747 jetliner or Lockheed C-5 transport aircraft.


The remotely piloted Helios Prototype first flew during a series of low-altitude checkout and development flights on battery power in late 1999 over Rogers Dry Lake adjacent to NASA's Dryden Flight Research Center in the Southern California desert.


In upgrading the Centurion to the Helios Prototype configuration, AeroVironment added a sixth wing section, a fifth landing gear pod and a differential Global Positioning Satellite (GPS) system to improve navigation, among other improvements. The additional wingspan increased the area available for installation of solar cells and improved aerodynamic efficiency, allowing the Helios Prototype to fly higher, longer and with a larger payload than the smaller craft.


During 2000, more than 62,000 bi-facial silicon solar cells were mounted on the upper surface of Helios' wing. Produced by SunPower, Inc., these solar arrays convert about 19 percent of the solar energy they receive into electrical current and can produce up to 35 kw at high noon on a summer day.


The second milestone established by NASA for its development Š a long-endurance demonstration flight of almost two days and nights Š required development of a supplemental electrical power system to provide power at night when the solar arrays are unable to produce electricity. AeroVironment developed an experimental fuel cell-based electrical energy system combining advanced automotive fuel cell components with proprietary control technology designed for the harsh environment above 50,000 feet altitude.


The first version of this system combines gaseous hydrogen from two pressurized tanks mounted on Helios' outboard wing sections with compressed oxygen from the atmosphere via a series of proton-exchange membrane fuel cell "stacks" mounted in the central landing gear pod. The system produces more than 15 kW of direct-current electricity to power Helios' motors and operating systems, with the only by-product being water vapor and heat. The system will increase the Helios Prototype's flight weight by about 800 lb to about 2,400 lb.


Two other versions of the system are contemplated: One, employing liquid hydrogen, would enable the Helios to fly for up to two weeks in the stratosphere anywhere around the Earth, not limited to temperate or equatorial latitudes. Another version, a closed or "regenerative" system, uses water, a fuel cell, and an electrolyzer to form a system similar in function to a rechargeable or "secondary" battery, but with much greater efficiency than the best rechargeable battery systems.


A production version of the Helios with the regenerative fuel cell system is of interest to NASA for environmental science, the military and AeroVironment for various roles, primarily as a stratospheric telecommunications relay platform. With other system reliability improvements, production versions of the Helios are expected to fly missions lasting months at a time, becoming true "atmospheric satellites."





The Economist:  Projects UAVs Like Helios May Soon Be Doing Work of Satellites  07-17-2003


HYPERSONIC drone aircraft that could bomb any target in the world from a base in America recently grabbed headlines in the Guardian and other newspapers. But fast and flashy is no substitute for slow and steady. UAVs (unmanned aerial vehicles) that can loiter over the same spot for months are likely to be of more lasting military and commercial significance. They are also hard to build. The most advanced prototype of such a UAV, a solar-powered craft called Helios, was destroyed on June 26th when it crashed into the Pacific. The cause of the crash is still unknown, although turbulence is thought to have been a factor.


Helios was built by AeroVironment, a Californian company that specialises in innovative engineering. (Its founder, Paul MacCready, built the first human-powered aeroplane.) Unfortunately, the recent bad luck of NASA, which manages the programme, seems to have rubbed off—and the speed with which an accident investigation team was assembled is testimony to NASA's current investigative fervour. Unlike the ill-fated space shuttle Columbia, however, Helios was exactly the sort of programme that NASA should be funding—an unmanned craft that is pushing technology to its limits.


The ultimate aim is to create a pilotless aircraft that can loiter over a particular spot on the ground, at an altitude of around 20km, for up to six months. This would allow the craft to serve as a "geostationary" communications relay, substituting for satellites that now do the job. Being closer to the ground than such satellites (which are in orbit almost 36,000km away), means that the transmitters and receivers involved could be less powerful, and more information could be transmitted at far less cost. Hovering UAVs could also serve as sentries, watching over a country's coastline for smugglers and terrorists, and as military reconnaissance platforms. And both communications and reconnaissance UAVs would be significantly cheaper to build and launch than their satellite equivalents.




But there are still many hurdles to be jumped before solar-powered UAVs can loiter for months at a time. Most observers estimate that such flight durations are at least a decade away. When it crashed, Helios was preparing for a flight of only 40 hours. Nonetheless, AeroVironment is planning to start a commercial, UAV-based communications relay service within three years, according to Stuart Hindle, a vice-president of both AeroVironment and its subsidiary, SkyTower, which is devoted to the communications project.


By that time, Mr Hindle says, SkyTower should be able to make flights of several weeks' duration over the wealthy markets of North America and Europe. (Longer flights will be possible over countries near the equator, because they are sunnier and there is less wind.) This, he says, is long enough to make it possible to provide a broadband service that would be cost-competitive with today's land and satellite-based systems. A single UAV could provide connections of at least five gigabits per second to around 200,000 subscribers, and a rotating fleet of them would provide continuous coverage.


Although broadband communications may be the first UAV technology to go commercial, the craft are also appealing as "virtual" mobile-phone towers, which could provide extra capacity when a crowd migrates to a normally under-populated area (as in the recent Glastonbury music festival, when more than 100,000 people flocked to a field in the British countryside). Indeed, SkyTower successfully tested such a system in 2002, in co-operation with Japan's Ministry of Telecommunications. Armed forces around the world are also interested. UAV-based systems could provide a battlefield with temporary communications coverage.


Long-duration UAVs depend on sunlight. All the existing models are pulled around the sky by electric motors, and the electricity to do the pulling has to come from somewhere. However, blanketing a craft's wings with photo-cells that convert sunlight into electricity is not sufficient, as night inevitably follows day. The most important trick is to find an electricity-storage system that is light and efficient enough to power a craft through the hours of darkness. If that system can be fully re-charged during the day, then a UAV could fly indefinitely. 


The problem is that re-chargeable systems tend to be too heavy. To solve this, AeroVironment has turned to fuel cells, which work by combining hydrogen and oxygen to produce water and electricity. The most appropriate sort of fuel cell, called a closed-cycle cell, uses electricity from solar panels to break its water up into hydrogen and oxygen during the day—in effect, re-charging itself. But closed-cycle systems are, for the moment, too heavy to be feasible in windy, sunless high latitudes. So Helios and its immediate successors will rely on open-cycle systems that bring hydrogen with them and take oxygen from the air. Such fuel cells can, however, power a UAV for only a couple of weeks, as the hydrogen they carry eventually runs out.



According to Chris Kelleher, a project manager at QinetiQ, a British defence contractor that is building a UAV called the Zephyr 3, another option worth examining is super-efficient batteries, probably derivatives of the lithium-based batteries used in mobile phones. As Mr Kelleher points out, the distinction between batteries and fuel cells is a bit blurred: both rely on combining chemicals to produce electricity.


One way to avoid the problem altogether, says Dyke Weatherington, an official responsible for UAV planning at America's Department of Defence, is to turn to nuclear power. Like that on satellites, this would rely not on nuclear fission, but on heat generated by radioactive decay, which would be converted into electricity by devices called thermocouples.


Some people in the American defence establishment see this as the ideal solution, he says, though he recognises that it is fraught with environmental and political risks. And unlike Mr Hindle and Mr Kelleher, who seem convinced that propellers are the best way to pull UAVs through the thin air at high altitudes, Mr Weatherington reckons that the next five years may also see the development of new sorts of low-fuel-consumption jet engine.


The main competition to fixed-wing UAVs will come from unmanned airships. Although airships would be less useful for reconnaissance missions that require mobility, they might serve as communications relays. Indeed, QinetiQ is combining the two, after a fashion. In a few weeks' time Zephyr 3 will be lifted to a height of 9km by a helium-filled balloon, before being launched on a mission intended to break Helios's record for high-altitude flight, which stands at 29km. (The balloon will then follow Zephyr 3 up, so the mission could also break the altitude record for a manned balloon.)


As Mr Kelleher points out, airship technology is less mature than that of UAVs. Although the buoyancy of a bag full of helium allows an airship to carry a larger payload, that bag is also buffeted by the winds of the stratosphere, which means that an airship has to have a more powerful propulsion system. Leland Wight, a programme manager for high-altitude airships at Boeing, says that the biggest engineering challenge is to get the balloon, which is only partially inflated at ground level, up to its operational altitude without its getting tangled up.


One benefit of the greater carrying capacity of airships is that they will be able to use less exotic power sources than fixed-wing UAVs. Mr Wight says that Boeing is planning to employ an internal-combustion engine powered by a special fuel to supplement the solar cells. In competition with Lockheed Martin and a third team led by Aeros, an airship company based in California, Boeing is developing a prototype for America's Missile Defence Agency. This should fly in 2006. Mr Wight says Boeing is especially interested in the possibility of using airships as communications relays. But, if SkyTower's plans go well, UAVs will enter the market a lot sooner than airships.





SCIENTISTS are searching for cleaner ways to power vehicles and to make better use of domestic energy resources. The fuel cell, an electrochemical device that converts the chemical energy of a fuel directly to usable energy without combustion, is one of the most promising of these new technologies. Running on hydrogen fuel and oxygen from the air, a 50-kilowatt fuel cell can power a lightweight car without creating any undesirable tailpipe emissions.

If the fuel cell is designed to operate also in reverse as an electrolyzer, then electricity can be used to convert the water back into hydrogen and oxygen. (See Figure 1.) This dual-function system is known as a reversible or unitized regenerative fuel cell (URFC). Lighter than a separate electrolyzer and generator, a URFC is an excellent energy source in situations where weight is a concern.

Weight was a critical issue in 1991 when scientists at Lawrence Livermore National Laboratory and AeroVironment of Monrovia, California, began looking at energy storage options for an unmanned, solar-powered aircraft to be used for high-altitude surveillance, communications, and atmospheric sensing as part of the Strategic Defense Initiative. Called Pathfinder, the aircraft set an altitude record for solar-powered flight in 1995, flying to 15,400 meters (50,500 feet) and remaining aloft for about 11 hours. Pathfinder's successor, Helios, will remain aloft for many days and nights. For that aircraft, storage devices were studied that would provide the most energy at the lowest weight, i.e., the highest energy density. The team looked at flywheels, supercapacitors, various chemical batteries, and hydrogen- oxygen regenerative fuel cells. The regenerative fuel cell, coupled with lightweight hydrogen storage, had by far the highest energy density--about 450 watt-hours per kilogram--ten times that of lead-acid batteries and more than twice that forecast for any chemical batteries.

The Prototype

Fuel cells have been used since the 1960s when they supplied on-board power for the Gemini and Apollo spacecraft. Today, fuel cells are being used for Space Shuttle on-board power, power plants, and a variety of experimental vehicles. However, none of these applications uses the URFC because early experience did not uncover the usefulness of the reversible technology, and little research had been funded. Recent results of Livermore research indicate otherwise, based on more thorough systems engineering and improved membrane technology.

Challenged by a lack of information on the technology, Livermore physicist Fred Mitlitsky was determined to uncover just how to make the combination of technologies work. Mitlitsky continued in 1994 with a little funding from NASA for development of Helios and from the Department of Energy for leveling peak and intermittent power usage with sources such as solar cells or wind turbines. (See Figure 2.)




Figure 2. Unitized regenerative fuel cells will someday find a multitude of applications.  URFCs are ideal for cars, solar powered aircraft, energy storage, propulsion in satelites and micro spacecraft and load leveling at remote power sources such as wind turbines and solar cells.

The 50-watt prototype that Mitlitsky's team developed is a single proton-exchange membrane cell (a polymer that passes protons) modified to operate reversibly as a URFC. It uses bifunctional electrodes (oxidation and reduction electrodes that reverse roles when switching from charge to discharge, as with a rechargeable battery) and cathode-feed electrolysis (water is fed from the hydrogen side of the cell). By November 1996, the prototype had operated for 1,700 ten-minute charge-discharge cycles, and degradation was less than a few percent at the highest current densities.1

Testing will continue in a variety of forms. Larger, more powerful prototypes will be created by increasing the size of the membrane and by stacking multiple fuel cells. For use on Helios, a prototype will likely provide 2 to 5 kilowatts running on a 24-hour charge-discharge cycle. As funding becomes available, prototypes may also be tested for other uses. A lunar rover, for example, would require cycles of about 29 days.

URFC-Powered Electrical Vehicles

In a 1994 study for automotive applications, Livermore and the Hamilton Standard Division of United Technologies studied URFCs. They found that compared with battery-powered systems, the URFC is lighter and provides a driving range comparable to gasoline-powered vehicles. Over the life of a vehicle, they found the URFC would be more cost effective because it does not require replacement.2

In the electrolysis (charging) mode, electrical power from a residential or commercial charging station supplies energy to produce hydrogen by electrolyzing water. The URFC-powered car can also recoup hydrogen and oxygen when the driver brakes or descends a hill. This regenerative braking feature increases the vehicle's range by about 10% and could replenish a low-pressure (1.4-megapascal or 200-psi) oxygen tank about the size of a football.

In the fuel-cell (discharge) mode, stored hydrogen is combined with air to generate electrical power. The URFC can also be supercharged by operating from an oxygen tank instead of atmospheric oxygen to accommodate peak power demands such as entering a freeway. Supercharging allows the driver to accelerate the vehicle at a rate comparable to that of a vehicle powered by an internal-combustion engine.

The URFC in an automobile must produce ten times the power of the Helios prototype, or about 50 kilowatts. A car idling requires just a few kilowatts, highway cruising about 10 kilowatts, and hill climbing about 40 kilowatts. But acceleration onto a highway or passing another vehicle demands short bursts of 60 to 100 kilowatts. For this, the URFC's supercharging feature supplies the additional power. A URFC-powered car must be able to store hydrogen fuel on board, but existing tank systems are relatively heavy, reducing the car's efficiency or range. Under the Partnership for a New Generation of Vehicles, a government-industry consortium dedicated to developing high-mileage cars, the Ford Corporation provided funding to LLNL, EDO Corporation, and Aero Tec Laboratories for development of a lightweight hydrogen storage tank (a pressure vessel). 


The team combined a carbon fiber tank with a laminated, metalized, polymeric bladder (much like the ones that hold beverages sold in boxes) to produce a hydrogen pressure vessel that is lighter and less expensive than conventional hydrogen tanks. Equally important, its performance factor--a function of burst pressure, internal volume, and tank weight--is about 30% higher than that of comparable carbon-fiber hydrogen storage tanks. In tests where cars with pressurized carbon-fiber storage tanks were dropped from heights or crashed at high speeds, the cars generally were demolished while the tanks still held all of their pressure - an effective indicator of tank safety. Unlike other hydrogen-fueled vehicles whose refueling needs depend entirely on commercial suppliers, the URFC-powered vehicle carries most of its hydrogen infrastructure on board.3 But even a highly efficient URFC-powered vehicle needs periodic refueling. 


Until a network of commercial hydrogen suppliers is developed, an overnight recharge of a small car at home would generate enough energy for about a 240-kilometer (150-mile) driving range, exceeding the range of recently released electrical vehicles. With the infrastructure in place, a 5-minute fill up of a 35-megapascal (5,000-psi) hydrogen tank would give a 580-kilometer (360-mile) range. Commercial development of unitized regenerative fuel cells for use in automobiles is perhaps 5 to 10 years away. With their long life, low maintenance requirements, and good performance, URFCs hold the promise of someday supplying clean, quiet, efficient energy for many uses. 




Figure 1. The electrochemistry of a unitized regenerative fuel cell.  In the fuel cell mode, a proton-exchange mebrane combines oxgen and hydrogen to create water. When the cell reverses operation to act as an electrolyzer, electricity and water are combined to create oxygen and hydrogen.


Fuel Cell Control


For years, fuel cell technology has been touted as the power technology of the future. It seems, however, that the future is still ahead of us as regards this technology. Fuel cells are, according to many futurists, going to replace the internal combustion engine, and sharply reduce our dependence on petroleum and natural gas as fuels. This will, in turn, help us to preserve our environment and work to improve our quality of life. Anybody who lives in the Los Angeles basin will certainly be awaiting this development.


General Motors, Ballard Power, General Electric and others are developing fuel cell technology for earthbound applications. Ballard has announced their first practical fuel cell product, and General Motors, according to Ian Jakupca from Analex, plans to have the first production version of a fuel cell powered automobile in the market by the end of 2004. Also for 2004, Ford has scheduled a fleet version of the Focus to be powered by a fuel cell. Giner Electrochemical Systems (Newton, MA) is working for NASA at Glenn Research Center in Ohio to develop fuel cell technology for flightline and space applications.


NASA is working on regenerative fuel cells for the Environmental Research and Atmospheric Sensor Technology (ERAST) project. The goal of the project is to be able to produce an unmanned aerial vehicle capable of continuous flight for six months at altitudes beyond 60,000 feet, carrying a payload. Very clearly, ERAST vehicles could replace many telecommunications satellites at an incredibly economical cost, since they can be flown down for repairs and flown back to the station.

Regenerative fuel cells are integrated energy systems that incorporate a fuel cell (electrochemical device to convert hydrogen and oxygen into electricity and water) and an electrolyzer (electrochemical device to convert electricity and water into hydrogen and oxygen). In earthbound systems, oxygen can be delivered from the air, but in ERAST systems and in future space vehicles, oxygen must come from the electrolyzed water.


Fuel Cell Basics

Simply, how does a Proton Exchange Membrane (PEM) fuel cell work? Hydrogen is passed over an anode, and split into H+ ions and electrons. The electrons are forced into a circuit, and made to do work, while the H+ ions migrate through a proton exchange membrane. At the cathode, O2 is combined with the H+ and the electrons to form water. The electrolyzer operates in a similar fashion. Water passes an anode and splits into H+ and O- ions, and electrons. The O- ions make O2 gas and bubble out into storage. The H+ ions migrate through the membrane, and at the cathode, the H+ ions and electrons combine to form H2 gas and it bubbles out into storage, where the fuel cell can use it.


The points of danger in a regenerative fuel cell system are the hot, wet gas coming out of the electrolyzer and going into the fuel cell, and the coolant going into the fuel cell stack. The fuel cell must be kept cool enough so that the components aren’t destroyed, yet warm enough to sustain the electrochemical reaction. The coolant of choice is water. The water must be deionized, and have very low conductivity. Higher conductivity water indicates the presence of dissolved solids, which may plate out on the cooling coils inside the fuel cell, degrading performance and eventually causing failure.


Flow must be measured, too. Water flow in the cooling system must be adjusted to maintain the proper fuel cell stack temperature, and any fluctuation needs to be alarmed. Gas flow throughout the system must also be measured, since stagnant flow conditions indicate real problems. Temperature and pressure must also be measured, both in the coolant and in the fuel cell itself. Humidity must be controlled in the gas feed to the fuel cell. And, of course, the output of the fuel cell (current and especially voltages) must also be measured.


So, what looks like a simple problem requires a fairly long list of sensors, and a control system to monitor those sensors and control the process. conductivity, flow, pressure, temperature, humidity, voltage, and current, and derived values for efficiency and contamination, are required to control the process.


Off the Shelf

NASA tries to use commercial off-the-shelf (COTS) products whenever they can, so all of the sensors NASA is using for the ERAST fuel cell projects are just that, from vendors such as Thornton, Vaisala, Dynaload, Lambda, Hastings, Setra, Tescom and Omega, as well as a PC-based data acquisition and control system from National Instruments. In August of 2001, the Helios Unmanned Aerial Vehicle broke the record for sustained horizontal flight at altitude by flying at 96,500 feet. This is the type of flight profile needed for flying in the Martian atmosphere. The wings of Helios are covered with solar cells, which operate the plane, and the electrolyzer (storing hydrogen and oxygen during the day) and fuel cell operates the plane at night. 
So measuring flow and a few other variables may someday allow us to replace telecommunications satellites inexpensively, and even fly across the face of Mars.






1. F. Mitlitsky, B. Myers, and A. H. Weisberg, Lightweight Pressure Vessels and Unitized Regenerative Fuel Cells, LLNL, Livermore, California, UCRL-JC-125220 (November 1966). Presented at the 1996 Fuel Cell Seminar, San Diego, California, November 17-20, 1996.

2. F. Mitlitsky, N. J. Colella, and B. Myers, Unitized Regenerative Fuel Cells for Solar Rechargeable Aircraft and Zero Emission Vehicles, LLNL, Livermore, California, UCRL-JC-117130 (September 1994). Presented at the 1994 Fuel Cell Seminar, Orlando, Florida, November 28-December 1, 1994.

3. "Getting along without Gasoline--The Move to Hydrogen Fuel," Science & Technology Review, UCRL-52000-96-3 (March 1996), pp. 28-31.




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