Balloon, Tether, and Cutdown

Secondary camera view of the cutdown pyro at the moment of firing. The main tether is already recoiling upward like a spring. The primary 360 camera is visible at top center.


up, up, and away

In principle, the concept is simple: Tie the HAPP to a balloon and let it fly. In reality, it took some serious engineering to create a system that gets the job done while providing high reliability and low weight.



Lift is provided by a helium-filled weather balloon. At launch the balloon is almost 3 meters in diameter. It takes over 12,500 liters of helium to fly –  a little more than 3 medium-sized tanks. The balloon expands as it travels higher in the atmosphere, eventually reaching its burst diameter of approximately 7 meters as the HAPP approaches target altitude. 

Bridle attached to balloon neck

Weather balloons can be purchased in a range of sizes. The HAPP uses a 3000 gram balloon, which refers to the amount of latex rubber contained in the balloon's skin. 3000 grams is the largest size readily available, and in fact it defines the upper bound on the total allowable mass for the HAPP.

The HAPP is suspended underneath the balloon using high-strength Kevlar string. This abrasive string can easily cut through latex if tied directly to the balloon's neck, so instead the string is affixed to a simple bridle. The bridle consists of a small steel ring and serrated carbon fiber plates that function as clamps.



The lower end of the Kevlar string is attached to the HAPP at its apex. Here we need a robust mount, but we also need a place to fix the primary 360 camera, as the apex is the only place where the camera can have an unobstructed view.  Unfortunately, the apex is also the worst place to add any weight, as it raises the center of gravity and makes the HAPP aerodynamically unstable. The mount must be strong but also extremely lightweight.

FEA simulation of balloon tether mount under high stress (left); Mount in assembled position after cutdown pyros were fired (right)

The final design is the result of multiple iterations. We used finite element analysis to verify the mount's strength while gradually slimming it down. The mount is fabricated from high-strength 7075-T6 aluminum alloy and weighs only 89 grams.

The mount has a safety factor of 10 for static loads. This high safety factor is necessary to ensure the mount can withstand violent accelerations as the HAPP flies through turbulent air.

The 360 camera is attached to the mount using a standard bolt that inserts upwards through the mount base. After the camera is attached, the base is inserted into the top of the HAPP's main strut and bolted into place.



Once the HAPP reaches the target altitude, we need to sever the Kevlar string. We might also need to bring the HAPP down earlier for a variety of reasons such as floating out of range. Whatever the reason, we need a method to execute the cutdown. This is accomplished with a pyrotechnic guillotine. 

Prior to launch, the string is threaded through the guillotine before tying onto the mount. In fact there are two identical guillotines to ensure redundancy.

Cutdown guillotine in lab tests; hard to see it up in the air!

The guillotine is a small aluminum cylinder that contains an electronic igniter, a charge of black powder, and a steel ram. When it's time for cutdown, the flight computer energizes the igniter, which detonates the powder and drives the ram through the string, severing it cleanly.