Raven Industries' Final Report to the U.S. Navy
Made available through the kind courtesy of the Balloon Historical Society and Christine Kalakuka

This is the exact report Ed Yost presented to the Office of Naval Research in 1963, at the conclusion of a program that resulted in the invention of the modern hot-air balloon.  In 2001, Balloon Excelsior / Balloon Publishing (Christine Kalakuka and Brent Stockwell) honored the dedication of the Ed Yost marker at Bruning Field, Nebraska (pictured above), by producing, with Ed Yost's permission, a booklet containing this report.  Now, The Early Years of Sport Ballooning is pleased to present the text and images of the Yost report, as presented in that booklet (©Balloon Publishing Company and P.E. Yost, 2001), courtesy of the Balloon Historical Society.


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Final Report:

One Man Hot-Air Balloon System Development


for

Office of Naval Research


Report No. 1863

1 November 1963

Prepared by:


P. E. Yost
Program Manager

Raven Industries, Inc.
Box 916
Sioux Falls, South Dakota



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Index


I.  Introduction............................1

II. Preliminary Preparations................3
 1. Balloon.................................3
 2. Heating System.........................11
 3. Flight Instrumentation.................14
 4. Gondola................................15
 5. Ground Inflation.......................16

III. Balloon Testing Program...............17

IV. Conclusions............................40



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2. Design, construct and test a hot-air balloon system to meet the following optimum requirements:

A. The balloon to carry one man.
B. A fuel supply to provide a flight duration of three hours.
C. The unit to be capable of flying at the 10,000 foot level.
D. The entire system to be reusable.

3. Develop an inflation and launching system that would require a minimum of personnel.

4. Keep the size and weight of the complete balloon assembly to a minimum.



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II. PRELIMINARY PREPARATIONS

After reviewing the data which had been accumulated from previous studies and experiments, it became apparent that additional knowledge and information was required prior to the assembly of a flight test vehicle. For the purpose of discussion, we will subdivide the complete balloon system into five general topics. These are:

     the balloon,
     heating system,
     flight instrumentation,
     gondola and
     ground inflater.

1. Balloon

A. Fabric

An investigation of available materials which would be suitable for use in the manufacturing of the balloon envelope was begun. The ultimate material should be capable of withstanding high temperatures (250F) for extended periods of time, have a high tensile strength, be tear resistant, light in weight, easily fabricated and relatively inexpensive.

Plastic films, some of which were impregnated with scrim reinforcement, were analyzed first. These materials could satisfy many of the requirements but were not ideally suited to simple fabrication techniques, difficulty was encountered in distributing load stresses into the envelope evenly, and the


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life expectancy of the material under hard usage was inadequate.

It should be noted that previous experiments indicated that the fuel consumption could be greatly reduced (25-30%) If the interior of the envelope was a reflective surface. (See Figure 1). A vacuum coating of aluminum on the smooth plastic films described above could provide a suitable material for some future program requiring an extended duration flight profile. The aluminized coating on the balloon envelope also provided a tremendous target for radar detection which in some cases might not be desirable.

The next area of investigation was lightweight woven fabrics. Materials woven from Dacron, Nylon or Orlon thread seemed to present the most desirable characteristics. Various weaving mills and firms dealing in assorted fabrics were confronted with the problem of providing a suitable lightweight material.

It had been established that the rudimentary material should have a minimum tensile strength of thirty pounds per inch both in weave and warp and have a weight below one ounce per square yard.



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Figure 1: Methods of Reducing Heat Loss & Fuel Consumption (click image to enlarge)


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Several of the fabric weaving mills were willing to manufacture a material to meet our specifications but the price was prohibitive.

One of the firms contacted, the Chase-Foster Company (a laminating firm) suggested the investigation of a nylon fabric designated as Flare-cloth. Samples of this material were obtained and found to be acceptable in strength and weight. The fabric had a tensile strength of forty pounds per inch and a weight of .84 ounce per square yard. The only obstacle that presented itself was the excess porosity of the material. As a temporary solution to this problem it was decided to laminate a thin film of Mylar plastic (.0005 inch thickness) on one side of the fabric. The balloon would be constructed in such a manner that the Mylar film would be on the inside of the envelope. Fifteen hundred lineal yards (enough for two balloons) of 52/54 inch wide material was ordered. Upon its arrival, the weight of the laminated fabric was found to be 1.1 ounces per square yard and quite suitable for a flight test vehicle.

B. Shape

Most of the modern gas balloons (helium and hydrogen filled) are designed and manufactured having a shape which minimizes the stresses horizontally around the equator of the envelope.



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Since the balloon in this case would utilize heated air and combustion gases for the lifting medium, a new shape concept was required. To derive this shape, a special polyethylene balloon was manufactured. This balloon, if filled with helium, would have a diameter of thirty-nine feet and a nominal volume of 27,000 cubic feet. It was constructed using a standard gore length of 61.5 feet and with a surplus of fabric around and parallel to the equator.

An inflation test using a propane burner for heat was scheduled so as to obtain design, lift and fuel consumption data. In order to provide an environment of calm conditions for the experiment, the equipment was transported to the Hippodrome at the Minnesota State Fairgrounds. The balloon was inflated and erected by means of a Herman-Nelson aircraft heater. The gas burner was then ignited and sufficient heat introduced into the balloon to obtain a gross lift of 450 pounds.

The inflated balloon was then photographed so that a profile view could be recorded (Figure 2). It should be noted on the photograph that excess lateral fabric is evident and that the envelope has assumed a "natural" shape for this normal operating lift-volume ratio.

The photographs were subsequently scrutinized, and charted in


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Figure 2: Polyethylene envelope built to determine
the nylon balloon's shape (click image to enlarge)


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detail so as to provide a balloon design pattern. Mathematical calculations of the data established that a balloon diameter of forty feet and a volume of 30,000 cubic feet had been attained.

C. Manufacture

A flight test balloon was manufactured using the fabric described earlier and a design shape derived from the inflation test. This balloon (see Figure 3) consisted of 32 half gores which were joined by means of two seams applied by a double needle sewing machine.

A rapid means of deflating the balloon to prevent dragging across the terrain during the landing process was necessary. This was accomplished by tailoring the top of the envelope into a cylindrical shape with an opening diameter of nine feet. This venting orifice was held in the closed position during flight by means of a 1,000 pound test nylon line tied tightly around the gathered balloon material at the apex. The nylon line would be severed at landing touchdown by a standard explosive squib line cutting assembly.

At the base of the balloon a circular metal ring having a diameter of twenty-eight inches was installed. This device served as a load transmission member between the envelope fabric and the suspended payload. The burner assembly was mounted in

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Figure 3: Chart showing gore widths of
polyethylene balloon (click image to enlarge)


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the center of this ring in a vertical position so as to inject the flame toward the apex of the balloon.

Since the heat radiating horizontally from the burner might, under some conditions, damage the lower balloon envelope fabric, some system of protection was necessary. As a temporary solution, the lower twelve foot section of the balloon fabric was removed and replaced with a glass cloth. This material had a thickness of .007 inch and was woven from glass yarn with a thread count of 60 x 64 per inch.

2. Heating System
The entire heating system for the balloon was to consist of an ample supply of fuel, suitable fuel containers, an efficient burning mechanism and a means of controlling the heat output or burning rate.

The first step in developing a heating system was the selection of a fuel to be converted into heat energy. This selection was governed by four basic requirements which were of fundamental importance. The fuel must be:

     1. Relatively inexpensive.
     2. Available on a universal basis.
     3. Have a high heat value per pound of fuel.
     4. Be easy to handle and convert to heat.




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Various fuels were examined in relation to the heat energy available per pound of fuel (see Figure 4).

It was determined that some type of hydrocarbon fuel would be the most suitable in meeting all of the requirements in this program.

Three standard available fuels were tested and evaluated during the burner development program. These were gasoline, kerosene and propane - all of which contain an almost equal amount of heat value per pound of fuel.

The merits and disadvantages of each individual fuel may be summarized as follows:

a. Kerosene and gasoline. These fuels are available in most parts of the world and can be carried or stored in lightweight unpressurized tanks. The three outstanding disadvantages are:

1. Some means of pumping or applying pressure is required to force the fuel through the burner.

2. Impurities and carbon particles formed during the vaporization process, restrict the main discharge nozzle of the burner and cause it to malfunction periodically.

3. To ignite the burner it must be preheated (generated) for several minutes in order to attain satisfactory combustion. This complicates the system and causes additional launch delays.

b. Propane. Although the availability of this fuel is not as prevalent as gasoline and kerosene, it has many

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Figure 4: Chart showing heat values of various fuels (click image to enlarge)


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desirable characteristics. Since it is a gas compressed into a liquid state, the vapor pressure can be utilized to transfer the fuel to the burner assembly. Clean burning properties, stability of flame control and instantaneous ignition further enhance its usage. The major disadvantage, although not of prime importance, is the requirement of a special fuel tank. Since the gas is under pressure a tank capable of withstanding the induced stresses must be used.

On the basis of this information, it was decided to use propane for the flight test balloon.

A complete system composed of two fuel tanks, a vernier control valve and a burner device was assembled and attached to the balloon load ring.


3. Flight Instrumentation
Two conventional aircraft instruments, a sensitive altimeter and rate of climb indicator were procured and mounted together in a protective box. The box was then suspended in such a fashion that it was visible to the pilot during flight. Ordinarily these instruments function satisfactorily when mounted in the instrument panel of an airplane although both have an inherent lag error. Since there is no vibration associated with a balloon, the hysteresis in the units must be counteracted by a periodic manually applied jostling.


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It was desirable that the pilot have a continuous visible reference indicator denoting the apex balloon envelope temperature during flight. This instrument could be marked (red lined) in such a manner that the pilot would know when the maximum safe operating temperature had been attained.

A suitable device was not located until the latter part of the flight test program. During the early flights, the envelope was equipped with small temperature indicator plates which changed color at prescribed temperatures. These could be analyzed subsequent to the flight to provide the maximum skin temperature attained at various locations on the balloon envelope.

The balloon crown tie-off squib release assembly (for landing deflation) had two wires extending to the base of the balloon. A standard 2 "D" cell flashlight was modified to provide an electrical power source for activating the release. The pilot would be required to engage two switches simultaneously in order to accomplish this release function.

4. Gondola

Little can be said about the first gondola utilized. Since effort was directed towards simplicity and minimum weight, a 1/2 inch thick plywood board served as the pilots seat. This in turn was


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suspended from the balloon load ring by four lengths of 1/8 inch diameter aircraft cable.

5. Ground Inflater

To render the hot-air balloon airborne it is first necessary to inflate the envelope with heated air by some auxiliary machine. To accomplish this task various portable and lightweight devices were assembled and tried during the developmental program.

The first two inflations were accomplished using a Herman Nelson aircraft heater to introduce warm air into the envelope. Although the balloon was erected in a normal manner, the required time of inflation (30 minutes) was excessive. It was agreed that inflation and launching should not exceed ten minutes.




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III. BALLOON TESTING PROGRAM

The newly manufactured forty foot diameter flight test balloon was transported to Minneapolis, Minnesota to undergo indoor testing under calm air conditions.

The balloon was inflated and sufficient heat introduced to obtain buoyancy with an initial gross weight G311ot included) of 420 pounds.

The first test ran for three hours and was aimed toward teaching the pilot the art of controlling his floating altitude. An accurate measurement of fuel consumption was also obtained. This was found to average 4.8 gallons (23 pounds) of propane per hour.

The weight schedule of the experiment was as follows:

Ambient temperature...................46° F
Balloon envelope......................74
Burner assembly.......................14
Seat, cables and instruments..........9
Steel shot (simulated parachute)......18
Pilot.................................190
Left tank (38 Lbs. fuel)..............57
Right tank (39 Lbs. fuel).............58_
Gross weight..........................420 pounds



The second test was a duplicate of the first and produced almost identical fuel consumption data.




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On the following day the tests were resumed to obtain additional information. A series of thermistors were attached to the balloon envelope at various locations to provide skin temperature data. In addition, a thermistor was attached to a long pole (40 feet) which could be manually moved throughout the envelope to obtain temperature data at various locations. The results from this experiment are shown in Figure 5.

The next indoor inflation was performed to test the lifting power of the balloon with the existing burner assembly. The maximum skin temperature reached on this test was 2200 near the apex of the envelope. The equipment weight schedule was as follows:

Ambient temperature 46.5° F.

Balloon..............................74
Seat, cables and instruments.........9
Burner assembly......................14
Left tank (28 Ibs. fuel).............47
Right tank (38 Ibs. fuel)............57
Pilot................................190
Thermistor probe and wiring..........3
Ballast..............................177
                    Gross weight.....571 pounds

On returning to Sioux Falls, South Dakota, with the balloon and test equipment, a meeting of personnel was conducted to discuss and analyze the status of the vehicle. It was the general opinion that the device was not as sophisticated as desired but was apparently airworthy and flyable. The next step should be a free flight to


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Figure 5: Internal and skin temperature profile (click image to enlarge)


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obtain additional data prior to making any design changes in the equipment.

Since the area surrounding Sioux Falls is somewhat heavily populated and cluttered with obstacles (trees, buildings and power lines) it seemed advisable to conduct the first flight in a more remote area. The abandoned military airport at Bruning, Nebraska, was selected as being suitable since the locality is sparsely settled and few obstructions exist.

A launching crew, composed of four Raven Industries employees and two representatives from the sponsoring agency, transported the balloon to Bruning, Nebraska, on 21 October 1960.

Early the following morning, the assembly was prepared for flight and the inflation commenced. Two Herman Nelson aircraft heaters (the type used during the indoor

experiments) were to be utilized to expedite the inflation. Unfortunately one of these units refused to function so the inflation was accomplished with a single heater.

A slight breeze from the west added further complications to the inflation. The large envelope, partially filled with warm air, was


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difficult to restrain. It became apparent that at least three handling lines should be attached to the envelope near the equator and a more rapid inflation technique developed.

After thirty minutes of feverish activity the envelope was completely filled with warm air and the flight burner ignited.

The following weight schedule prevailed at the launching of the flight.

Raven flight No. 540N    22 October 1960
Field elevation 1544 feet
Temperature 44° F

Balloon..............................74
Burner assembly......................14
Seat, cables and instruments.........10
Pilot (Yost).........................190
Parachute............................18
Left fuel tank.......................51
Right fuel tank......................52
                    Gross weight.....404 pounds

The burner control was set for maximum heat output in order to render the entire system buoyant as rapidly as possible. After five minutes of operation the balloon had not yet attained sufficient lift to become airborne. It was surmised that the cool breeze passing around the envelope was dissipating an excess amount of heat so that flight status could not be attained.


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To rectify this situation the inflation crew walked downwind with the balloon (see Figure 6) for approximately fifty feet at which time buoyancy was attained.

The balloon began a very slow rate of rise (50-100 feet per minute) with the burner operating at its maximum output until reaching a level 500 feet above the terrain. Here it leveled off for approximately ten minutes and then a gradual loss of altitude was experienced. Level flight could not be regained and a landing was made after being airborne twenty-five minutes and traversing a distance of three miles.

Upon landing, the apex squib release was fired by the pilot which in turn opened the nine foot diameter venting orifice for balloon deflation. The expulsion of heated air and subsequent loss of lift required an abnormal amount of time therefor causing the gondola and pilot to drag helplessly across the terrain.

Although this first flight was not an outstanding success, the entire crew was jubilant and eager to introduce the required modifications to improve the system.

Four basic changes were to be incorporated prior to the next flight test mission. These were:



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Figure 6: Raven crew walk the balloon with the
wind as buoyancy builds (click image to enlarge)


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1. The balloon was reinforced at the equator in three locations so as to provide attachment points for handling lines. These could be manually restrained during inflation to prevent the envelope from buffeting excessively in a breeze.

2. The apex opening in the balloon was extended in size from nine to thirteen feet in diameter. This increased the cross sectional area by a factor of two and allowed the balloon to deflate more rapidly. The ensuing reiterative flights have substantiated this opening size as being adequate for a hot air balloon having a volume of 30,000 cubic feet.

3. An investigation into the inadequacy of the flight burner was undertaken with meticulous concern. We had previously demonstrated, during the indoor tests, that sufficient heat was being produced to maintain flight with a substantial overload. A subsequent examination of the flight data and burner assembly yielded the following conclusions:

a. The fuel system was designed in such a manner that vaporized propane was transmitted from the top portion of the fuel tank to the flight burner. When operating under low ambient air temperatures, the fuel would not vaporize rapidly enough to provide adequate heat. This phenomenon was further aggravated by the refrigeration effect induced during vaporization of the fuel.

To counteract this problem a technique was adopted whereby liquid fuel was transmitted to the burner under the pressure induced by gaseous propane in the upper portion of the fuel tank. Here the liquid fuel entered a jacket which surrounded the burner flame. The intense heat was utilized to vaporize the liquid propane for efficient combustion.


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Prolonged ground tests of the new burner assembly were conducted with excellent results.

b. When air passes around the balloon envelope during ground inflation and while making vertical movement in the atmosphere, a reduction in lift is experienced. This is caused by conductive losses as the moving cooler ambient air absorbs some of the heat from the envelope. The flight burner must have the capability of counter-acting this action. Experience gained later provided a "rule-of-thumb" balloon burner requirement. To be adequate, it must have the capability of developing twice the heat required for level flight.

Another item to be noted at this time is the importance of flying the hot air balloon level and at a constant altitude if the fuel supply is to be conserved.

4. Since we were eager to resume the flight testing of the balloon, the development of a refined inflation device was secondary.

A large axial fan blower (capable of delivering 5,000 cubic feet of air per minute) and a commercial weed burner were procured. The fire from the burner was injected into the intake opening of the blower to obtain heated air.

The equipment was now in readiness for the second free flight.

The following objectives were outlined as knowledge to be obtained from the next flight test mission:


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a. A thorough test of the revisions and equipment modifications resulting from flight test #l.

b. To analyze the balloon performance and controllability at higher altitudes.

c. The flight to have a duration of approximately two hours.

It was decided to launch this balloon from the Stratobowl which is located in the mountains of western South Dakota. This site is approximately 4,500 feet above sea level and would afford an opportunity to perform several altitude-lift experiments without becoming airborne.

The equipment and personnel arrived at the flight test area during the afternoon of 10 November 1960 and were joined by two representatives from the sponsoring agency.

Numerous tethered flights were conducted during the following day (see figure 7) to verify the theories of hot-air balloon performance at this altitude. Additional information pertaining to burner operation, fuel consumption and envelope temperature were accumulated.

The following morning (12 November 1960) provided ideal weather conditions for the flight. The inflation required six minutes and a routine launching was accomplished at 0840 MST with the following weight schedule:



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Figure 7: Tethered flight at the Stratobowl,
November 10, 1960 (click image to enlarge)


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Flight 542N                     Temperature 25° F

Balloon..............................76
Burner assembly......................14
Seat, cables and instruments.........16
Left fuel tank.......................60
Right fuel tank......................61
Pilot (Yost).........................190
Parachute............................18
Ballast..............................30
                    Gross weight.....465 lbs.


A four hundred feet per minute rate of rise (average) was experienced until reaching the 7,000 foot level. The pilot then began a gradual reduction of the burner heat output in order to attain a level floating condition. This was accomplished by reducing the fuel flow through the needle. (vernier) control valve. This valve required 22 complete turns of the control handle from off to full open position.

A somewhat level floating altitude was established for twenty minutes during which time the needle control valve required only periodic minor adjustments.

At 0912 MST the burning rate was increased in order to climb to a higher altitude. An average rate of ascent of 300 feet per minute was maintained until reaching the 9,000 foot (MSL) level. Here the pilot reduced the heat (fuel flow to burner) so as to again attain level flight. This adjustment was inadvertently excessive resulting in inadequate heat for buoyancy.




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A rapid descent (600 feet per minute) ensued and the pilot applied full heat in order to initiate a recovery. After losing 1,000 feet of altitude the descent was halted and a rapid rate of rise experienced. The heat again was reduced excessively and another altitude oscillation introduced. These uncontrolled maneuvers lasted for approximately fifteen minutes although diminishing in severity. After regaining level flight control the heat was slightly reduced in order to establish a slow rate of descent. At 1030, after being airborne for one hour and fifty minutes and traversing a distance of 39 miles, a normal landing was accomplished.

Thirty pounds of ballast (see weight schedule) was carried on this flight in order to conduct a deceleration experiment during the landing maneuver. At an altitude 100 feet above the terrain, while descending at 150 feet per minute, the ballast was jettisoned. No appreciable retardation in descent velocity could be detected. This was expected since the ballast dropped was a very small percentage of the overall system weight.

An analysis of the fuel consumed during the flight yielded the following information:

Fuel on board at take off............19.5 gallons (83 Lbs.)
Fuel on board at landing.............5.4 gallons (23 Ibs.)
Total fuel consumed..................14.1 gallons (60 Ibs.)



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The average fuel burned per hour was 7.6 gallons (32.3 pounds) which exceeded the estimated requirement substantially. This was attributed to the oscillating flight profile which required periodic maximum burner output for corrective measures.

As a whole, the flight mission was extremely successful and informative. It had been demonstrated that a rapid inflation could be executed, a prolonged flight attained and a normal landing accomplished.

The attempt to control the climb, descent and level float characteristics of the balloon was only partially successful. This was primarily due to the design of the fuel control valving system. As a corrective measure, a quick operating (from off to full open valve was connected in parallel with the existing needle control valve. The pilot could utilize the quick operating valve to render the burner output to a maximum without disturbing the adjustment of the needle valve.

The next (third) free flight was flown from Joe Foss Field at Sioux Falls, South Dakota on 8 December 1960. Two areas of interest were to be investigated on this experiment. These were:

1. To determine if the revised fuel control system would aid the pilot in maintaining a more level flight profile.

2. The launching to be accomplished during period of light


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surface winds (approximately 10 mph) in order to observe the ventilation heat losses during inflation.

Although the balloon envelope experienced a considerable amount of buffeting as it was being filled, the launching was accomplished routinely. The weight schedule for this event was as follows:

Flight 707-N. Air temperature +5° F. Clear sky. Wind NNE 8 mph.

Balloon..............................76
Burner assembly and controls.........14
Seat and instrumentation.............16
Left tank and fuel...................58
Right tank and fuel..................59
Pilot (Yost).........................190
Parachute............................18_
                    Gross weight.....446

A 400 feet per minute rate of rise was established and maintained until reaching the planned cruising altitude of 4,000 feet. The controls were then adjusted for level flight which was held constant for a one hour period. Next, a reduction in heat output was introduced and a gradual descent begun. The flight was again leveled off at an altitude which was approximately 100 feet above the terrain and held constant until a suitable landing area was selected. The landing was accomplished without incident one hour and fifty minutes after launching.

This flight served the purpose of providing additional launching experience and verified the burner control modification as being


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beneficial for maintaining a more positive flight control.

The experiments conducted heretofore were performed primarily to provide procedure information and performance data. It was now necessary to acquire knowledge relative to the safety aspects of the system in the event of a flight burner malfunction. Since the descent velocity of the balloon without heat was unknown, it was decided to conduct an unmanned flight.

The complete balloon assembly utilized throughout the program was prepared for this test.

A radio transmitter was installed for the purpose of telemetering pressure altitude throughout the flight profile. In order to simulate a normal flight, canvas bags containing steel shot for added weight were attached to the gondola seat.

The operations plan called for a twenty-five minute ascension with the burner operating and then a descent without heat. The balloon (Flight 720-ru3 was launched on 30 January 1961 with a gross system weight of 457 pounds (see Figure 8).

The balloon rose to the 7,000 foot level at which time the nine pounds of fuel was expended. The subsequent cooling and loss of lift introduced a descent rate which did not exceed 1,000 feet per minute at any time.

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Figure 8: Profile of unmanned flight to test flameout at altitude (click image to enlarge)


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Visual observations of the balloon during the descent portion of the flight indicated that the envelope remained full and taut at all times.

Since the descent rate from this test under fully loaded conditions was still appreciably less than a man descending by parachute, an added feature of safety was discovered.

The balloon envelope suffered extensive damage during the landing. Several vertical tears which extended from the base to the apex were acquired due to the rapid change in shape of the entrapped air mass as the ground impact occurred.

A thorough examination of the balloon envelope revealed that additional damage had accrued during the overall program. The glass fabric near the base of the balloon was perforated from handling and a delamination between the Mylar film and nylon fabric was evident in numerous areas. A decision to design and manufacture a completely new balloon system was reached.

The new balloon system (ONR X 38) was completed in mid March 1961 and (Figures 9 and 10) incorporated the following modifications.

1. The envelope shape was identical to the previous balloon although slightly smaller in size. The diameter was reduced from 40 feet to 38 feet which in turn reduced the volume from 30,000 to 28,500 cubic feet.

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Figure 9: Gondola of ONR X 38, completed
in March 1961 (click image to enlarge)

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Figure 10: ONR X 38 in flight. Note "drag skirt" near equator, which
added the vertical drag of an 18' parachute (click image to enlarge)


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2. A drag skirt was attached to the perimeter of the envelope at a point three feet below the equator. This device would induce drag (Figure 10) equivalent to an eighteen foot diameter parachute during a normal or "flame out" descent.

3. The glass fabric at the lower portion of the balloon was eliminated (Figure 10) and the gondola attached to the envelope by means of 1/8 inch diameter aircraft cable.

4. A new lightweight burner and seat assembly was constructed (Figure 9) which provided more comfort and placed the flight controls within easy reach of the pilot.


A test flight of two hours and twenty minutes was conducted at Sioux Falls, South Dakota on 21 March 1961. As a result of the findings from this experiment, the following equipment modifications were required:

1.  The lower six feet of fabric in the balloon had been attached in such a manner that stress concentrations were introduced into the envelope (Figure 10). This section was subsequently removed and reinstalled correctly.

2.  At various times during flight, and especially when climbing or descending, horizontal air movement was experienced. Although this breeze was not severe, it still had enough velocity to bend the burner flame and direct it toward the envelope fabric. As a precautionary measure, a conical


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skirt, having a base opening diameter of six feet, was attached to the balloon envelope (figure 11).

3.  The new burner functioned normally during the portions of the test when level flight or climbing altitudes were assumed. When the heat output was reduced to introduce a balloon descent the fire would extinguish itself. This required the pilot to manually relight the fire a total of twelve times during the flight.

A ground test of the equipment indicated that three changes were needed; (a) the discharge nozzle orifice was oversize and a correction was made, (b) an automatic pressure regulator valve was installed in the fuel line so as to provide a constant fuel pressure to the burner, (c) the fuel vaporization chamber surrounding the burner barrel had split open due to uneven expansion. A new type of vaporization chamber consisting of several coils of stainless steel tubing was fabricated. This modified heat generating system was subjected to three days of ground testing under all ranges of throttle settings.

The balloon was reflown on 3 April 1961 for two hours and ten minutes during which time all of the components functioned properly.

Twenty-six ascensions were made during the period beginning 3 April 1961 and ending 19 October 1961. The average time aloft on each flight was two hours and ten minutes.


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The two balloons had flown a total of thirty-two free flights and numerous captive tests. Sixty-five hours and twenty-five minutes of flight time had been logged during the development and flight test program.


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IV.  CONCLUSIONS

At the completion of this program in October 1961, all of the basic requirements had been met. A low-weight manned flight system capable of safely carrying one man for three hours, at altitudes up to 10,000 feet had been repeatedly flown.

Inflations and launchings in winds of more than 10 mph and temperatures as low as +50~ had been successfully accomplished. Operations at altitudes of 4500 ft. MSL had been demonstrated. Ground support equipment had been developed so that inflation, under light wind conditions, could be carried out in less than ten minutes. Controls, burners and instrumentation adequate to operate the system had been developed.

As a result of the work done under this contract, the feasibility of using hot air balloons for sustained flight was demonstrated. By the application of modern materials, design techniques and instrumentation, an aerostat which had been known for more than 175 years was resurrected and shown to have substantial value in modern research and operations. The limits of duration and lifting capacity appear to far exceed that developed under this contract - the low cost, simplicity of operation, and


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logistic advantage which hot air balloons provide, when compared with light-gas balloons, are most impressive. Within the altitude limits where hot air balloons may be used, these modern Montgolfier systems may become an important tool for atmospheric research and low-level lifting and transport tasks.

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Figure 11: ONR X 38 in finished form, with wind-deflecting skirt
and redistributed envelope stresses (click image to enlarge)

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