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Date: 4th December 2011, 7:45am - 8:45am
Location: Denzil Joyce Oval, NSW, Australia
Conditions:  Calm 0 -10km/h wind, warm, blue skies
Team Members at Event: John K, Paul K, Jordan K, PK and GK

As we look to potentially open parachutes at higher speeds, we are trying different ways to reduce the shock on the parachute and the rocket. Today we tested a couple of simple techniques to reef a parachute so that it does not snap open.

The first technique used a reefing ring threaded onto the shroud lines, and the second technique simply put a few twists in the shroud lines themselves.

Experiment Setup

The reefing experiments used the following configuration:

Looking at the parachute in flight

In order to get a better sense of how the reefing works in flight we decided to mount a camera on the shock cord where it attaches to the rocket. We wanted the camera to follow the parachute from the time it deployed to the time it fully inflated, and then continue to follow it until landing. Initially the parachute deploys sideways and then swings to the back of the rocket where it inflates, and then swings out to the side again when it has reached terminal velocity. Just mounting the camera directly to the shock cord would have introduced a lot of roll and would have been hard to see anything. So we attached the camera to a little Coriflute platform suspended between a triangle made out of another cord and attached at two points to the rocket. This eliminates the roll but still allows us to mostly follow the parachute.

We also filmed the rocket from the ground with zoom both in HD and slow motion to try to get a better idea of the process from a different perspective.

Flight Day

We arrived at our local park at 7:45am, with beautiful blue skies and almost no wind. We had the three boys with us, and now being older they are a lot more help in carrying gear and setting up. We had the first rocket on the pad within 10 minutes of arriving at the park.

Reefing Ring

The first flight was with the reefing ring and the rocket was only filled to 100psi. The park is fairly small and so the rocket can drift a long way if we pump up the rocket too much. The flight went well and the reefing ring appeared to also work well. We had set the deploy delay to 3 seconds which was right around apogee as we didn't know what may happen and so we wanted to give the parachute maximum amount of time to open.

We flew the same configuration again on the next flight, and the parachute worked well again. On the third flight we increased the pressure to 110psi and set the deploy delay to 4 seconds to get the parachute to open a bit later while the rocket was coming down. Again, the reefing ring appeared to work.

When we later looked at the on board and ground videos you could see that the reefing ring did stop the parachute from snapping open and nicely ballooned out before fully inflating. It was also evident that the ring only made it half way down the shroud lines on all three flights. This was enough for the parachute to be almost fully inflated and bring the rocket down safely. The ring wasn't being caught on anything as one video showed that as soon as rocket touch down the ring slid down the rest of the way.

The ring's position on the shroud lines is determined by a number of factors including the friction between the ring and the lines and the amount of force being exerted on the shroud lines by the canopy

Twisting the lines

This was always going to be an interesting one because you normally try to stop tangles. The key assumption here was that the parachute will want to unwind itself when a force is applied to the shock cord and shroud lines. Of course aerodynamics could also want to twist the parachute in the other direction and twist up the shroud lines even more.

As a first test we only twisted the lines 5 full revolutions. This was perhaps less than ideal for effectively reefing the parachute but it was a toss up whether we wanted to potentially crash the rocket or not. The flight went well again and both ground and onboard video showed that the twisted lines worked a little bit to reef the parachute but not as effectively as the reefing ring. The shroud lines did untwist all the way so the shroud lines were fully separated - which was good.

We repeated the experiment again with a second flight, and again the parachute deployed well and fully inflated. We suspect that some of the twist in the shroud lines was transferred to the shock cord on opening and hence untwisted faster than expected.

As a control, the sixth flight of the day was a normally packed parachute without any reefing arrangement so we could compare the other techniques to it. You could clearly see that the parachuted snapped open quickly and you could also hear the pop of the parachute. Something you didn't really hear with the two reefing methods.

Here is a highlights video from the day:

We packed up after the 6th flight as the wind started picking up and was blowing towards the road. We used the simple break wire trigger for the Servo Timer II on all the flights as we were using lower pressures and restricted nozzles. The trigger worked really well on all the flights. A good day all round and happy to get all rockets back without damage.

Next we'll need to try to deploy the parachute at higher speeds to see what happens - both on the way up and on the way down.

When we were testing the reefing ring the night before by swinging the parachute around, we noticed that it did not work at all if the parachute had a hole in it. The reefing ring will only work if the entire canopy is closed.

Further Reading

• http://www.thefintels.com/aer/reef.htm
• http://www.pcprg.com/inflate.htm

Flight Details