I'm going to resurrect this thread for a related HED question, but first some feedback on the above. We flew "configuration B" with the drogue pulling out the main via the apex and it worked flawlessly every time (n = 4?) for a large chute. As stated above, I think the trick is to make sure the decent rate of the drogue carrying the nose+line is slower than the main carrying the rocket. So the drogue/nose always stays above the main chute.
QUESTION 2: What is the best HED configuration for a large and heavy rocket?
For a bi-prop project we were testing a number of "pyro-less" designs using springs and latches over the last 18 months. Ultimately, all of the designs proved to have too many risk or failure points, not to mention that the weight to umph to complexity ratios were way off. To reliably separate a nose cone on a large vehicle that might "pyro late" after apogee, you need a few hundred pounds of pressure behind the nose. It is very difficult to get that with springs. Springs will work in a "perfectly timed apogee" situation, but not reliably early or late. So we are pivoting back to pyros.
I naively thought that reverting back to pyros would be easy with an obvious simple design, but I've now reviewed a few dozen student designed rockets and forum posts and there is a broad array of "best practices". Our design point is a HED dual deploy for a 180 lb. six inch diameter rocket going to no more than 30K feet. The rocket/tanks are all aluminum, but the upper airframe holding all the recovery will be fiberglass.
DESIGN 1 - TINDER DESCENDER: My first design assumed a bulkhead on the nose, nose pyros to eject the nose and drogue at apogee, and then a heavy Tinder Descender anchored to the base/bulkhead of the upper airframe. The main pyro would release the TD and the main chute gets pulled to freedom. I still like this design, but if the TD is anchored to the base of the airframe there is risk of the line/quick link getting stuck by an impacted chute. You could move the TD up high above the main, using a riser tether, but then the TD and the associated pyro wires are flailing around in the wind (no bueno for me). This setup also requires a "donut avBay" in the aft recovery airframe, since the main eye bolt is centered on the lower bulkhead.
DESIGN 2 - NOSE AVBAY: I am now leaning towards a traditional avBay coupled into the nose and into the upper airframe (like a large Wildman HED kit). The inside of the nose would hold the drogue and drogue line. At apogee they would separate leaving the nose AvBay in place. The main would fire the other side and the avBay would separate and free the main chute. I like the simplicity of this design, as well as maintaining a traditional avBay that isn't crammed in the bottom of the airframe. My concern with this design is the amount of pull on the avBay when the drogue deploys. I know there are lots of ways to minimize the shock, but late opens happen. It would require a lot of shear pins on a large heavy rocket. This is a risk, but worse case the shear pins separate, the main comes out early and it is a very long walk (um, drive).
DESIGN 3+ ALTERNATIVES: We've seen lots of variations and alternatives. We have one design that builds a free floating shelf below the nose and the shelf gets ejected with the drogue. There are 2-3 other ways to use a Tinder Descender, like have it hold down the chute bag and use a pilot chute on the bag to pull the main once released.
I'd love to get feedback from anyone that has done a large or heavy HED deployment. What approach did you use? How did it perform? We are looking for an approach that is easy, highly repeatable, and reliable for student groups to use.
Thanks,
Mike
