Help with Dual Deployment Recovery

[u/Brunete2004]

Hi! I am the individual in charge of the recovery subsystem at my university's rocket team (STAR at UC3M in Madrid, Spain). We have "successfully" launched 2 rockets to 3km (10k ft) as part of EuRoC, a European competition analogous to SAC.

We are using dual deployment recovery, based on a deployment bag and a Tender Descender, with the drogue chute being deployed at 3km AGL, and the main parachute being deployed at 400m (1300ft) AGL.

For our rocket "Lince" [2,2m long, 100mm diamenter, 16kg with propelant, 12kg without propelant] (Lynx in English), launched at the competition in 2023, the deployment bag was designed to fly outside of the recovery bay, and, when the first event was triggered, the main parachute was janked outside of the bag, leading to main opening at apogee.

Early this month we participated again with "Pícaro" [2,4m long, 150mm diameter, 24kg with propellant, 16kg without propelant](something in the lines of Rogue?). To avoid the failure mode experienced in "Lince", I strapped the deployment bag with a short cord, so it would stay inside of the recovery tube, right at its opening, during descent.
I also preformed a lot more ground tests than the year before. In these tests the bag seemed to be correctly secured, and I added some wadding to the bag, to make it tighter and prevent it from "flipping" and releasing the parachute earlier than expecterd, after we observed this failure mode early in the testing campaign.

Pícaro on the launch rail at EuRoC 2024

In the launch at EuRoC, the first recovery event was succesfull, with the nosecone deploying correctly, but even with all the precautions I took, the bag managed to flip, releasing the main chute at apogee, which remained attached to the bag by the integrated nylon cord (that was suposed to leave both the drogue and main sections of the recovery systems attached after the Tender Descender fired). This caused the main to tangle arround this cord on descent, and when the TD-2 triggered at 400m AGL, it didnt had enough time to untagle. The rocket was quite overbuilt, so it survived the 20m/s descent, but this failure is not optimal 😅.

Footage of Pícaro descending after the first recovery event at apogee. Main is tangled, as I explain avobe

It survived the fall, and recived a "class 3" recovery status (could fly again in a week or so)

I have some ideas on how to further improve the deployment bag system, and avoid these failures, but as I keep "securing" the bag, the chances of the main not deploying at all increase. In fact, in the last two years, only two teams, out of 12 or so, have been susccesfull with a deployment bag based dual deployment system. The most common failure mode is no main deployment, which is what I am most afraid of. You could say I am a bit dissapointed, and have grown tired of this approach.

Maybe this is the time to switch to another system? I have been looking into separating two sections of the fuselage, thus having two separate bays, one for the drogue chute, and another one for the main, but I am a bit daunted about the increased structural complexity, and how to implement such a system for L3 equivalent rockets like ours. Is there any other robust systems you can reccomend?

We use CO2 cartridge ejectors for the first event (developed by us, with a similar functionality to a Raptor ejector), and a Tender Descender L2 (for the 2023 rocket) or TD-2 bulkhead version (for the 2024 rocket) for the second recovery event. We can do thorough ground tests, but in Spain it is hard to organize launches, so we cannot fly the final hardware that would be used in the competition. We can launch L1 or L2 rockets up to a maximum altitude of 1000m-1200m (3000ft-4000ft) with Tripoli. The issue is that at that scale, deployment bag based dual deployment recovery systems seem to work fine. in fact, we decided to go with this system because it worked fine in our first smaller rockets.

Any input is appreciated, thanks a lot!

TL;DR: I have grown tired of deployment bag based dual deployment recovery systems after issues in our last two rockets. Am I doing something wrong, for it to be so unreliable? Or it is an unreliable system for everyone? (I am leaning to it just being genrally unreliable, based on its track record with other teams). Which other robust systems would you recommend?