Tandem Ballooning with Doongara

Earlier this year we developed a reusable non-pyrotechnic line cutter, Doongara, for use on traditional, latex balloon flight trains and tandem ballooning. By adding Doongara in a 2-balloon tandem flight train, we demonstrated advanced flight profiles that vastly increase a payload’s time at altitude. Depending on the amount of helium in the tandem configuration’s two balloons, we can perform stepped vertical ascents, constant rate descents, and even approach neutral buoyancy for some types of payloads.

Doongara Cut-Down Device

Burst
Latex Balloon at Burst

The original intended use for Doongara was to separate balloon remnants after burst by cutting the line connecting the parachute and balloon. Doongara permits a user to program the cutting of a line based on any combination of the following triggers:

  • Elapsed time since power-on
  • Pressure altitude reached during ascent
  • Detection of a rapid descent after balloon burst

With Doongara’s three (3) programmable settings, a number of new applications are possible for traditional latex balloon flight trains, such as:

After Separation.JPG
Payload Separated From Balloon
  • Cutting away balloon remnants after burst.
    • Balloon remnants can tangle with the parachute and payload, potentially decreasing parachute performance and obscuring cameras.
  • Terminating the flight before burst.
    • The flight can terminate at a specific pressure altitude below burst resulting in a closer correlation of the actual landing site with the predicted. This is useful for ballooning in geographically constrained areas near mountains or large bodies of water.
  • Separating a secondary payload, such as a glider or sonde at a specific pressure altitude.

Tandem Balloon Configuration

Tandem Ballooning Diagram
Small Tandem Balloon Flight Train

A tandem configuration is simply the use of two latex balloons to loft a single payload. As seen in the “Small Tandem Balloon Demo” figure, we refer to one of the balloons as the “Tow” balloon and the other as the “Float” or “Main” balloon. Depending on application and type of flight desired, these balloons differ in size and the amount of helium filled during inflation.

For reference, a conventional high-altitude balloon contains one quantity of helium that can be simplified as serving two different purposes:

  1. Lift the balloon and payload so it is neutrally buoyant just off the ground. The payload is just floating and not going up or down. We call this the neutral-lift helium.
  2. Accelerate the balloon and payload upward so that it ascends at a fairly constant velocity. Typical ascent rates for a high-altitude latex balloon are about 5 m/s. We call this the free-lift helium.

Thus, all the helium in a conventional balloon is simply the total of the neutral-lift helium and the free-lift helium. As happens with traditional latex balloons ascending at 5 m/s, the balloon’s size increases until failure.

In a tandem configuration, we can divide the two quantities of helium (the neutral-lift component and free-lift component) into two separate balloons. We put the neutral component in the Float/Main Balloon, and the free-lift component in the Tow Balloon.

Using two balloons does make the entire system heavier and there is more drag generated using two balloons, so the above comparison of helium quantities going from a traditional balloon to a tandem balloon configuration is a simplification for the purpose of discussion.

Combining Doongara with Tandem Ballooning

Because we now have the ability to release the Tow Balloon, and the free-lift gas it contains, we can quickly change the overall buoyancy of the system. An instantaneous change in buoyancy resulting from the release of the Tow Balloon permits the payload and remaining Float Balloon to ascend, descend, or float, depending on how much helium was initially loaded into the Float Balloon during inflation.

An actual flight example, again consider the “Small Tandem Balloon Demo” figure showing a tandem configuration just after launch.  One Doongara, labeled the “Tow Cut-Away”, was connected inline with the Tow Balloon. The other Doongara, labeled the “Float Cut-Down”, was connected inline with the Float Balloon. The objective of this flight was to have the tandem balloon ascend to 9000 m, at which point the Tow Balloon is released by the Tow Cut-Away Doongara.  The Tow Cut-Away Doongara was set to release at a pressure reading corresponding to the 9000 m altitude. We filled the Float Balloon with slightly more helium than the neutral-lift amount in order to make sure it would ascend until eventual burst. A timer-enabled Doongara was connected to the Float Balloon for termination redundancy.

Stepped Ascent
Tandem Balloon Stepped Vertical Ascent

Due to errors during the helium fill, the ascent rate to 9000 m was lower than expected, at about 3.5 m/s. When the Doongara sensed a pressure altitude corresponding to about 9000 m, it severed the line connecting the Tow Balloon to the payload. Shortly thereafter, the ascent rate dropped to about 1.5 m/s. The Float Balloon and payload continued to rise until burst. This flight’s stepped vertical ascent profile permits a payload to rapidly rise to a specific altitude before slowing down through a region of interest.

The Pro’s & Con’s of Tandem Ballooning

For researchers, payload developers, and advanced latex ballooning enthusiasts, tandem ballooning with Doongara offers a new and relatively simple means of controlling buoyancy. The tandem technique offers both a variety of new advantages and some distinct disadvantages one should consider.

Advantages

Tandem ballooning:

  • Permits new flight profilesComparison of Conventional and Tandem Flight Paths
    • Stepped Vertical
    • Constant Rate Descending
    • Pseudo-float
  • Quickly reduces ascent speed
  • Increases payload exposure at altitude
  • Decreases pendulum motion during ascent and at altitude
  • Increases balloon burst altitude
  • Supports heavier payloads
  • Allows for uncontaminated air sampling during constant-rate descents

Disadvantages

  • Uses more helium due to increased drag and balloon weight
  • Increases flight costs by using two balloons
  • Requires more planning and more accurate weigh-off
  • Complicates and prolongs inflation

Learning More About Tandem Ballooning

Since our early test flights with small tandem balloon configurations, we’ve developed better tools and components to make this ballooning technique easier. We also recently used a tandem configuration with Boomerang in flying an 8 kg scientific payload in India. New 3D-printed balloon neck nozzles enable more accurate helium loading during the inflation process while a calculator helps users determine weigh-off weights for the Tow and Float Balloons. These are included in the Doongara Purchase Package we offer. Contact us for a quote.

Interested in the scientific applications of constant velocity descent flight profiles? This is a very recent article:

Controlled weather balloon ascents and descents for atmospheric research and climate monitoring

Want to know more about how a tandem balloon system can be used to float a payload, and balloon material dynamics that make it possible? These two articles are a good start:

A new paradigm in space based experiments using rubber balloons

Study of the properties of cosmic rays and solar X-ray flares by balloon borne experiments

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