(c)l999 American Institute of Aeronautics & Astronautics

A99-33313

BALLOON TRAJJKTORY CONTROL

Kim M. Aaron’, Matthew K. Heun’, Ke? T. Neck’ Global Aerospace Corporation

P.O. Box 93305 Pasadena, CA 9 1109-3305

Abstract

A method of controlling trajectories of balloon systems is presented. This approach, for which a patent is pending, involves suspending a wing-like aerodynamic surface on a long tether (several kilometers) below a balloon system. The wing is hung with its wingspan essentially vertical. The wind velocity (both direction and magnitude) at the altitude of the trajectory control device will usually be different from the wind velocity at the altitude of the balloon. The wing exploits this relative wind to generate a lift force that acts in a predominantly horizontal direction. This force is transmitted via the tether to the balloon and causes the balloon to drift relative to the air in which it is floating. The direction and magnitude of this lift force can be controlled over a wide range by varying the angle-of- attack. The Trajectory Control System (TCS) enables the balloon to avoid hazards, reach targets, steer around avoidance countries and select convenient landing zones. The balloon could be thought of as acting like the keel of a sailboat, and the wing as the sail. Because this device sails in the stratosphere, we call it a StratoSailm TCS. No longer will balloons be at the mercy of the winds.

De&n Conced

The StratoSailTM TCS being developed by Global Aerospace Corporation is a new concept. No known devices have used or even suggested the use of a Iift- generating device, such as a wing, suspended on a long tether, to effect trajectory control of a balloon.

Figure 1 illustrates one concept for a balloon trajectory control system. The StratoSailm TCS basically consists of a wing on end connected to a rudder and a counterweight all located on a boom and suspended from a tether up to 15 km below the balloon to take advantage of the variation in wind velocity with altitude. The wing generates a horizontal lift force that can be directed over a wide range of angles. This force,

* Senior Engineer, Global Aerospace Corporation. t President, Global Aerospace Corporation. ’ Copyright 0 1999 by Global Aerospace Corporation.

Published by the American Institute of Aeronautics and Astronautics, Inc. with permission.

AIAA-993865

transmitted to the balloon by the tether, alters the balloon’s path. The TCS is scaleable over a very wide range of sizes. Obviously the magnitude of the trajectory control will depend upon the relative sizes of the balloon and the wing, coupled with the ratio of air densities and the magnitude of the wind velocity difference between the two altitudes.

Rudder

etweight

Figure 1 StratoSaiP Trajectory Control System

The initial development of the StratoSailm TCS is being funded by a phase I Small Business Innovative Research (SBIR) grant. This particular funded effort is geared towards providing trajectory control capability to the NASA Ultra Long Duration Balloon (ULDB) program. Although the potential application is clearly much broader, much of the initial sizing is based on a ULDB class balloon system.

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(c)l999 American Institute of Aeronautics & Astronautics

The ULDB system is planned to operate at altitudes near 35 km. The air density is less than 1% of the sea level value. Thus, the balloon needs to be very large to displace enough weight of air to support the payload. For a balloon it is very important that all systems be designed to keep weight to a minimum. The orientation of the wing on end naturally lends itself to lightweight construction techniques because the weight is concentrated close to the load-bearing elements. The density of the air 15 krn below the balloon is about a factor of ten greater than at the balloon altitude. Since $he aerodynamic force scales directly with air density, the wing area can be reduced in direct proportion. There will be an obvious tradeoff between size (weight) and control authority. A device can be made large enough to generate an appreciable force to a ULDB-sized system. ;Yet it is also possible to scale the system down to a size that could operate with a small weather radiosonde type balloon. I Wind Variation with Altitude I ifhere. are significant variations in the wind speed and drrectron with altitude. Figure 2 displays the mean /zonal wind profiles for two routinely used balloon jlaunch sites and seasons. Significant relative winds will Abe available for TCS operation. For example, there is a 126 m/s relative wind between 35 and 20 km altitude for ‘the January Alice Springs latitude and an 11 m/s relative wind for the Fairbanks latitude during July. /Adding in the expected meridional wind components are expected to further increase these relative wind /velocity estimates. The direction of the wind is not overly important because the magnitude and direction of the lift force can be varied over a substantial range by controlling the angle of attack of the wing, much like the ability of sailboats to travel in many different directions in the same wind.

10 - _ Source: Randel, W.J.,

-40 -30 -20 -10 0 10 20 Mean Zonal Wind, mk

j Figure 2 Sample Zonal Wind Profiles

&stem DescriDtion

As shown in figure 1, the main lift-generating element is shown as a wing comprising a leading edge spar, a trailing edge spar, and a several ribs. The leading edge spar, trailing edge spar and ribs support the skin. The wing is attached to a boom. A counterweight is attached to the front end of the boom and a rudder and rudder actuator are attached to the back end of the boom. A control module is mounted at a convenient location on the device. The control module includes a power source (not shown), if required, comprising a battery and a solar panel. A yoke connects the boom to a tether. The upper end of the tether is attached to a winch (not shown) mounted on the gondola of the balloon. Figure 3 is a schematic that illustrates the overall system geometry. The tether would be much longer than shown here.

i : 2 Figure 3 Schematic of Overall System

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(c)l999 American Institute of Aeronautics & Astronautics

The tether is unwound from a spool using a winch mounted on the balloon gondola. The winch may be powered or it may simply provide a passive means of lowering the TCS at an acceptable rate, with the weight of the TCS serving to pull out the tether. In the latter case, the TCS may be discarded at the end of the flight by severing the tether and providing a parachute for the device to control its rate of descent. If the winch is powered, it may be used to raise or lower the TCS in altitude to reach favorable winds or to restow the device.

Depending upon the needs of the particular balloon flight, the control module may receive its commands from the balloon gondola by radio, or other communication means. Alternatively, the control module may be preprogrammed prior to launch of the balloon system.

Ooeration of StratoSaiP TCS

Figure 4 Simulated Uncontrolled Free-Flight Trajectory with Actual January 1983 Wind Field; Launch from Alice Springs

Figure 5 Controlled Trajectory with Wind Field Identical to Fig. 4 using 1 m/s Control Authority

Figures 4 and 5 illustrate the potential benefit of a balloon trajectory control system. Figure 4 shows a trajectory that would have been followed by a balloon launched from Alice Springs in January of 1983.

Figure 5 shows the trajectory the balloon system would have followed if it had had a TCS system generating a mere 1 m/s lateral velocity of the balloon with respect to the winds in which it was floating. This particular set of wind data was selected because it showed a dramatic departure from more typical trajectories. Yet even with such an extreme case, a relatively small amount of trajectory control could easily negate the undesirable wandering of the balloon. For typical conditions, a greater drift velocity is predicted using the StratoSaiP TCS. It is clear that even a very modest amount of control acting over long periods of time can yield dramatic improvements to the trajectory, negating the undesirable latitude excursions.

Benefits

One of the advantages of the TCS is that it can be operated in different modes with more or less complexity depending on the desired degree of trajectory control. For example, if the purpose is simply to provide a bias airfiow past the supporting balloon to sweep away contaminants to improve the performance of sensitive instruments, then the rudder could be set at a faed angle before the flight. This fixed angle could be selected based on a desired relative velocity coupled with prior knowledge of the expected winds at the altitudes of the balloon and the wing.

For the planned ULDB float altitudes, the prevailing winds typically are in a generally easterly or westerly direction depending on the season. An ultra long duration balloon may go around the Earth several times. If the desire is to force a general drift towards the pole of the Earth (or perhaps away from the pole and toward the equator) then again the angle could be preset before launch based on the known prevailing winds and the desired drift direction.

A more complex control scheme, perhaps under autonomous control, could command the wing to “tack” downwind across the wind. The wing would traverse a long zigzag pattern across the average flight path. This would increase the relative wind speed of the wing and therefore the maximum aerodynamic force too. This approach could provide significantly greater control over the trajectory direction, but would require a more complex set of control algorithms.

Since it is likely that a NASA ULDB payload (located in the gondola) will have a command link (e.g. radio) to remote operators, another command link (perhaps a wire in the tether) between the gondola and the control module could be used.to relay directions to the TCS. Alternatively, the TCS could be controlled by a separate, direct command link from the ground.

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he advantages of the TCS are that it uses very little dower to operate and can therefore operate at night. It can be made of very light weight materials and generates more force than a comparably sized drag device while providing a much wider range of control force direction. The direction of the control force can be changed fairly rapidly. Because it operates at a lower &.itude where the air density is greater, the device can be smaller. Some balloons currently seek altitudes with favorable winds to effect trajectory control. With a TCS, the balloon can remain at constant altitude. And, with knowledge of the wind field, the performance of the TCS can be enhanced by using more sophisticated predictive control algorithms. The use of wind field knowledge to enhance trajectory control is discussed in +3ther paper at this conference.’

Recent Testing

!A dynamically-scaled model of the StratoSaiP TCS iWing Assembly (TWA) was recently tested in natural /winds at El Mirage dry lake in California. The radio- ~coptrolled model was constructed at a scale of 1:7.5 usmg available model aircraft parts. The model was suspended from a tethered blimp using a fifty-meter lKevl.ar line. The tether was short relative to scaling requirements, but the design reduced blimp height to meet FAA regulations. A longer tether would have been

1 preferred. The five-meter-long support blimp was restrained by three guy lines disposed at 120” intervals to hold the tether suspension point at an essentially fixed point above the ground despite changing winds.

) The model TWA was operated roughly 10 to 20 m 1 above the ground, where it could easily be observed and photographed. A small camera mounted on the TWA was used to acquire airborne images as well as to

observe a small tell-tale used as an angle-of-attack meter. The testing confirmed many aspects about the operation of the StratoSaiP TCS. We are currently analyzing the test results and will report them at a later date.

I (c)l999 American Institute of Aeronautics & Astronautics

Figure 7 TWA Model in Flight

Related Prior Work

Ever since balloons were first conceived, people have sought to control their trajectories. History books show pictures of balloons with sails and oars. Since a balloon drifts with the local winds, there is essentially no relative wind that can be used to inflate a sail and generate lift. Still, the intent and desire is clear: people wanted to control where their balloon would go. Early on, hot air balloonists discovered that by changing altitude one could reach different, sometimes more favorable wind directions. Today, hot air balloonists compete to fly set patterns across the ground using winds aloft. Altitude change however comes with a cost. In the case of hot air balloons, the cost is fuel to heat the air in the envelope. In the case of light gas balloons it’s the venting and loss of the buoyant gas itself or the dropping of ballast mass, neither of which are inexhaustible. Also, without detailed wind information with a resolution of about 100 m, controlling trajectory by varying altitude is often a hit or miss type of endeavor.

i ‘g Fl ure 6 Scale Model TCS Wing Assembly

Airships and powered blimps use. engines to turn propellers to propel themselves. The power and fuel required limit the duration of such an approach and at very high altitudes, the large required propeller size becomes impractical. Some naturally-shaped balloons have also flown using propellers driven by engines suspended on tethers. The power required to overcome

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(c)l999 American Institute of Aeronautics & Astronautics

the drag of the balloon is significant and extended operation is not feasible. The StratoSailTM trajectory control ‘system approach circumvents many of these drawbacks.

One potential application for trajectory control is scientific balloon flights for which mission planners are concerned with problems of overflight of non- cooperative countries and of flight termination in places where payloads cannot be recovered easily. Balloon trajectory control could also be applied to sport balloonists, “around the world” balloon challengers, and planetary exploration.

Although drag chutes on tethers have been proposed to alter the trajectory of balloons, no known devices have used or even suggested the use of a lift-generating device, such as a wing, suspended on a long tether, to effect trajectory control of a balloon. However, a few methods have been used to control the trajectory of lighter-than-air vehicles (LTA).

Propeller-driven airships obviously control their trajectories. However, the attainable altitude and payload mass for airships are quite restricted in comparison to free balloons. Free balloons carrying science instruments typically drift freely in the prevailing wind at the operating altitude. In many cases, launch of such balloons must be delayed until forecast winds are projected to carry the balloon system into a region of interest, or away from a forbidden zone. Frequently, such balloon flights must be terminated prematurely to avoid flying over countries that have not given permission, or to ensure that the payload descends into an appropriate landing site, or to avoid endangering densely populated regions. The ability to provide even a small amount of trajectory control could eliminate these reasons to terminate the flight early.

Propulsion

Several studies have been performed of concepts to propel lighter-than-air (LTA) vehicles. Naturally- shaped balloons driven by propellers suspended on relative short tethers have been investigated.2*3 Both papers refer to studies at Goodyear Aerospace Corporation in which naturally shaped balloons with a propeller and power plant suspended on a tether were tested in flight. These tests indicated that the operation of such devices was feasible, although some instability was noted. If further development had occurred, operation of such devices would have been limited to a couple of days, according to the summary papers, due to the large propulsive energy required. The papers also discussed the difficulty of operating engines at high altitude. Air breathing engines require several

stages of supercharging to increase the air density to the point that it will burn efficiently with fuel. In addition, both .dombustion engines and electric engines suffer from the difficulty of rejecting the substantial waste heat to prevent overheating in the low density atmosphere.

Another propeller driven LTA vehicle, POBAL-S, a high altitude airship, was designed to operate at an altitude of 2 1 km for a period of about a week.4

Both the propeller-driven balloon and the propeller- driven airship described above were designed to stationkeep. That is, they were design to maintain the position of the LTA system above a specific point on the ground. This requires them to fly at a relative speed equal to the wind speed at the operating altitude. Because the winds aloft can have very high speeds in the range 15-50 m/s (50- 150 tvs), this leads to the very large power requirements.

Drag

A drag device, such as a parachute, can be deployed at a significant altitude below a balloon where the winds will usually be blowing in a different direction.’ This approach can be used to generate a force that will cause the balloon to move relative to the surrounding air. However, the direction of the force is restricted essentially to the direction the wind is blowing at the altitude of the parachute. It is possible to use a winch to raise or lower the parachute to altitudes with different wind directions. This can take a significant amount of time to change the direction of the force. Also, it requires a significant amount of power to raise such a device fighting both gravity and the aerodynamic drag. In addition good knowledge of the wind distribution with altitude is required.

Altitude Change (Earth hot air snort balloons. Venus balloon. Mars)

A less ambitious approach suggested many times in the past to control the trajectory of a balloon is to select an altitude at which the wind is moving in a favorable direction (or closest to a desired direction, at least). This is the main trajectory control technique used by sport balloonists with either hot-air balloons or helium balloons.

Selecting altitudes at which the balloon will float in order to select different drift directions also has many drawbacks. First, some means of controlling altitude must be provided. Hot air sport balloons raise or lower the temperature of the lifting gas to adjust altitude. Helium balloons tend to alternate the dropping of

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ballast weight and the venting of lifting gas. This use of consumables ultimately limits the duration of the mission and carrying the ballast in the first place reduces the weight available to the payload. A further

‘drawback is that many balloon-borne science ~ instruments (especially astronomy and astrophysics experiments) need to be above most of the atmosphere (99%) and cannot acquire high quality data at lower altitudes. Furthermore, good knowledge is needed of

‘the wind at different altitudes in order to select an appropriate altitude. Such detailed knowledge is usually unavailable during the flight.

~ Another concept for balloon altitude control is the so- ~ called “anchor” balloon which utilizes a main helium balloon envelope and a sturdy futed volume air bag which is alternately filled or emptied of high pressure ambient air in order to change buoyancy of the whole system. Such a balloon was employed by Richard Branson’s Earthwinds, an early “around the world challenge” attempt. In a different version of the same concept, the French have also used this technique to fly meteorological balloons at constant pressure (isobars) altitudes using a ballonet inside the main helium instead of a separate bag.6

Some disadvantages of pressurized air ballast concepts are the power required to pump air into the high pressure bladder, the difficulty of pumping gas at very low pressures, and the additional mass of the pumping equipment and the anchor balloon or bladder.

Another innovative altitude control approach was developed at the Jet Propulsion Laboratory called ALICE (ALtItude Control Experiment).’ This device sought to develop an altitude-control mechanism that would not require ballast. The approach used two balloons, really one balloon filled with helium, and a second bag filled with a much denser refrigerant. At low altitudes, the refrigerant was a gas. At high altitude, the refrigerant condensed into a liquid. Thus, above a certain altitude, because the displaced volume of air decreased by the volume of gas that condensed, the overall buoyancy of the system decreased. As the system descended into warmer air at lower altitude, the liquid vaporized again, thus expanding to displace a large volume of air. At this point, the buoyancy exceeded the weight and the system ascended again. The system naturally cycled over a large range of altitudes without discharging helium or ballast. By trapping the liquid in a pressure vessel and releasing it back into its sealed envelope, it would be possible to control the altitude to some extent. The time scales involved for each altitude cycle is a few hours. This altitude control scheme has a major disadvantage in that it does not work in the stratosphere; it requires the

(c)l999 American Institute of Aeronautics & Astronautics

particular variation of temperature and pressure that can only be found in the troposphere. A similar variation also exists on Venus and other planets and moons for which this system was conveived.

Altitude change to control a balloon’s trajectory has one inherent drawback, namely, the altitude of the balloon changes. For some balloon science missions (including many proposed ULDB experiments) such altitude change is unacceptable, especially for certain astrophysics and astronomy payloads where a lower altitude can significantly degrade or destroy scientific data collection due to the increased atmosphere above the balloon.

Winches and Tethers

Winches have been developed to raise and lower balloon

H) ayloads up to several kilometers below balloons. *gP1o*ll Significant mass and power requirements can be necessary depending on the winching rate and the supported weight. If the weight of the TCS is low (< 100 kg) and the required time to raise or lower the TCS is long (~24 hours), mass and gondola power requirements may be quite acceptable. If there is no raising requirement, winch power may not be required; in fact a brake may be all that is necessary.

Summary and Conclusions

The StratoSaiP Balloon Trajectory Control System being developed by global Aerospace Corporation has been described. The device uses a wing on end to generate a lateral lift force, which is used to alter the path of the balloon system. The wing assembly is suspended on a long tether to take advantage of the wind variations with altitude. This approach has been contrasted with previous approaches and found to be quite promising. Recent testing has confirmed the expected behavior of the approach, although fiuther development is required to develop the full system.

Acknowledgement

The development of the StratoSaiP Trajectory Control System is funded by the NASA SBIR program. This support is gratefully acknowledged. The scale model testing of the StratoSaiP TCS was supported using Global Aerospace Corporation internal research and development (IR&D) funding.

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