For the first discussion, a lot boils down to whether you plan to bring the nosecone down on its own parachute or whether you plan to bring the entire system down on one large main. It certainly seems like the “traditional” method is to separate the nose when the main charge goes off and bring the nose down on one main parachute and the booster/payload bay down on another main parachute. This method has worked for countless flights and would certainly be a good choice.

I felt like two separate parachute recovery systems added complexity and a lot of moving parts. I discussed the issue with Teddy Chernok from Onebadhawk (Teddy built the bridle and shock cords for me) and he recommended a single recovery system and parachute. In order to do this, you really need to follow the system from the apex of the parachute to the anchors on the MMT and ensure there are no weak points in the system. That includes any hardware, such as swivels and quick links.

It seems that conventional wisdom is to use a 50 G event as a planning factor. That seems to be based on high velocity minimum diameter flights. Even on an O Motor, this rocket will not reach speeds that can generate more than a 13 G event, but, in the pursuit of overkill, I chose to target a 40 G event as the goal. Assuming the max recovery weight of the rocket will be about 80 pounds (on an Aerotech N motor), that means the whole system needs to be able to handle around 3200 pounds of “shock” force. It should also be noted that the force will not be the same across all components. For example, the only parts of the system that will be subject to the full weight of the rocket are the main parachute and the attach point for the main parachute. By the time we get down to the booster, the only weight on that part of the system will be the booster itself, so it will be significantly less force. For simplicity, I targeted 3000 pounds for all components of the system.

Working from the booster up:

• Calculating the amount of force the centering ring/MMT assembly can handle is a bit difficult. You would have to take into account the tensile and flexural strengths of the fiberglass used, the epoxy strength, the hardware “grip” points, etc. Suffice to say, I am comfortable that the procedures I used would be sufficient to handle a high G event.
• The U-bolts I used on the upper CR are 3/8”-16 316 Stainless Steel U-Bolts from McMaster-Carr. The weak point on these U-Bolts is the threads. The strength rating is derived from a straight pull on the U-Bolt until the threads strip out. This weak point can be mitigated using steel plates and multiple nuts. I also epoxied the plates/bolts to both sided of the CR. The threads are rated to 1,200 pounds of straight pull force. Using the mitigations discussed, I estimated that the bolts would be good for at least 1600 pounds each and, since the force is spread over two U-Bolts, I am comfortable the U-Bolts will handle 3000 pounds of force.
• Next up is the bridle, shock cords and the swivels. Teddy used 7/16” tubular Kevlar and a 3/8” swivel. Both these are rated for shock loads well over 5000 pounds, so we are good there.
• The quick links are a common point of failure. I have seen a lot of people compromise their recovery systems with weak quick links. For this build, I am using McMaster_Carr 3/8” thick quick links rated for 3,900 pounds each.
• The main shock cord is attached to the Ubolt on the nosecone bulk plate and then the main parachute is also directly attached to that U-Bolt. This attach point did generate some discussion with my TAPs. It is difficult to calculate the strength of the U-Bolt here because the strength rating is based on the threads, but, in this case, the force applied will be sideways on the U-bolt, so the force on the threads will be minimal. So, we need the tensile strength of the steel bar itself, which I could not find. However, if we look at other products using the same steel and same 3/8” thick diameter (such as the quick links) we see strength ratings for many thousands of pounds. So, in this case, while I did not have a specific strength rating, I felt confident that the 3/8” thick steel bar would be more than sufficient to handle the 3000+ shock load of a high G event.
• Finally, the main parachute. I could not find any manufacturer that gave an indication of the maximum shock load their parachutes could handle, but I had to make an assumption that if the parachute was rated to bring down a 100 pound rocket safely, they would manufacture it to handle a high G stress load (or, we would probably see a lot of discussion on rocket forums about parachute failures). For the Cert 3 XXL, the one concern I had was the swivel that came attached. It was rated for a 1500 pound shock load. That seemed very low to me and dangerously close to the possible shock load this rocket could encounter, considering that swivel would have to be able to handle the weight of the entire rocket. So, I had Teddy cut it off and replace it with a 5000 pound rated swivel and a sewn Kevlar strap.

At the end of the day, I was comfortable that I had thought through the possible shock loads and capabilities of the components of the recovery system.