Most cutdowns are light and placed between the balloon and the parachute. This seems logical, but the biggest issue with this arrangement is the weight of the cutdown and the size of the parachute. The bigger the parachute, the more likely there will not be an issue.
In recent times I have had great issues with the balloon not exploding cleanly. Five balloons this year have had massive twisting issues and that alone can collapse a parachute as the cord between the payload and the parachute twists and causes issues as it tends to make the cord shorter as it twists up.
One balloon burst without shedding any material and with the 1.6kg balloon and the weight of the cutdown, there was nearly 2Kg of mass pulling the top of the parachute well to the side. The payload hit the ground at about 60kph (35mph). This damaged one of the trackers and almost left the payload without any way of locating it. Luckily a second tracker half survived and we were able to locate it even though it was not sending GPS co-ordinates.
The picture above shows the result: That is my good friend Todd Hampson helping us track down the lost payload. It was transmitting, but with no GPS location. We resorted to directional location and a “beep every 20 seconds. We recovered the payload. 2 faulty trackers and we still found it. It pays to be prepared. Note, I question the element spacing on this antenna. It is made from PVC tubing and fittings and uses roll-up tape measure elements. You can fold it up into a slightly bigger PVC tube or a canvas tube. You need big attenuators like 60dB and 120dB to insert inline as you get closer to the transmitter. You also need a radio that has a metal case to shield direct signals from getting into the radio and bypassing the antenna.
One remedy is to use some small swivels. They are simple and can be found in fishing shops. This will help with twist on a rapid spin. You will be amazed at how fast a payload can spin when the parachute is fouled. The video below shows the fouled cute payload and its initial spin and the final impact with the ground
As I said, most payloads are placed above the parachute. My recommendation is below the chute. This increases the stable loading and decreases any unstable loading that can collapse the chute. The picture below shows how and independent cutdown should be placed. A longer line may be needed if the payload and the parachute are further apart and the payload needs connectivity to the link to the ground. I suggest a swivel and a short line to the parachute as well as a long thin multi-stranded line to the cutdown. After all any damage to the cutdown wire will be not an issue after the payload is on the way down.
There are many other ways to provide this service, but they are often less than ideal. This is the most stable way of providing the system. I suggest that the cutdown box or bubble wrap be taped to the nylon cord to prevent it from swinging around.
The small piece of Nichrome wire needs to be either twisted or threaded through the nylon. I prefer threading as it means the Nichrome wire is insulated and the heating is efficient. Other prefer w spiral around the thread. Either way, it is important that any swivel placed inline is below the cutdown box or above the cutdown Nichrome wire. Very important.
The reason is that any twist on the way up will cause issues and this is more likely if the swivel is between the cutdown Nichrome wire and the box with the electronics.
I would love to hearmore about your results. Let me know.