GRAPHITE NOZZLE

AKA: Shipping Delays Cause Minor Design Change

The single-use design of Aerotech nozzles may be cost-effective to include in a kit, but when ordering from RCS, the availability of a particular nozzle needed for the motor case may not be available due to shortages or other delays related to their business. Going with the original throat diameter of 0.734 will allow for adjustment, but RCS was showing shipping delays of 4-6 weeks. I placed two orders: one in early April for a majority of the consumables and two blue thunder 38mm grains, and in May I ordered a medusa and single throat nozzle for the 98mm. So far, none has shown up. I reached out to Rocket Bits after coming across one of their postings in the Tripoli group on social media to see if they could replicate the single throat design out of graphite. The single-use design is more than adequate when made out of the proper materials, but unfortunately, if made out of graphite, the flange most likely would not have been able to contain the pressure and would have failed, as Chad Holm advised me.

Original style nozzle as used in the 98mm Aerotech hybrid.

What Chad did was machine it so it was similar to a snap ring style nozzle. It now fits entirely inside the motor case, although I lose a little bit of the third grain; unless someone has a damaged Aerotech 4-grain case that I could cut down to accommodate the new nozzle design for a full 3-grain motor they would be willing to part with rather cheaply. The best part about the graphite nozzle is it is reusable, of course, thus keeping with the theme of low cost with minimal tooling for the hybrid experiments. The nozzle rests against the aft motor flange with 3 O-ring grooves; paraffin has a regression rate that is 3 times that of composite propellant, so it may be a bit of overkill, but better safe than sorry.

This design protects the nozzle from impact damage, since the original style would protrude from the rocket’s end. I could probably have gotten away with doubling the flange thickness only if the nozzle were recessed. A medusa, of course, would also be great, but I was thinking about having one printed from 304 stainless using laser metal sintering; the only thing I could find allowing steel alloy nozzles in research motors was for sugar-based propellant, and I did not really have enough time to petition the Balls safety committee for consideration, so graphite it is. Many thanks to Chad for getting it to me quickly. The lesson here is that the small vendors may be more responsive and quicker than our traditional suppliers.