Claybaugh 6-inch Rocket, Post-Flight Inspection

by Bill Claybaugh, RRS.ORG


EDITOR’S NOTE: This is a continuation of the reporting from the 10-16-2021 flight of the 6-inch rocket design, built and flown by RRS member, Bill Claybaugh.


Post-Flight Motor Inspection

Recovery of the spent motor hardware allowed a detailed disassembly and inspection of the parts.  This revealed several useful observations:

Motor Tube

The recovered Motor Tube showed a dent just above the fins that was deep enough to have caused a pressure failure if it had been present while the motor was operating; we thus conclude that the dent occurred during or post impact.

Localized dent in the aluminum case, likely resulting from impact after burnout

Bulkhead

Inspection of the Forward Bulkhead showed it to be in good condition with no evidence of any gas leaks above the two O-rings.  The bottom of the Bulkhead showed some damage to the fiberglass heat shield from the ground impact of the rocket but showed plenty of fiberglass heat shield remaining after the about eight second burn.  The “nose” of the ignitor assembly remained in place in contrast to previous tests where this part had shattered upon ignition; the change to a steel “gun barrel” liner for the initiator appears to have resolved this issue.

The forward side of the bulkhead showing no leakage or damage.
Aft side of the bulkhead showing damage to fiberglass heatshield.

Fins

The four fins were intact and largely undamaged; they appear suitable for reuse in future flight vehicles.  Checking with a 0.002” feeler gauge showed there was no gap between the “nose” of any of the fins and the motor tube.  A further check using backlighting confirmed that there were no visible gaps between the fins and the motor tube at any location along the fin edges.

Nozzle

The graphite nozzle insert had broken free of its aluminum shell on impact; it was damaged at the exit end and is not suitable for reuse. The aluminum shell showed signs of erosion at the very top of the nozzle.  This area was covered by a ring-shaped fiberglass heat shield that was not present upon disassembly.  This suggests that the heat shield was fully consumed by hot gas erosion during motor operation; a thicker heat shield is evidently appropriate in future nozzles.

The titanium nozzle extension was undamaged and is suitable for reuse in future nozzles of the same design.

Nozzle was damaged in the impact.

Fin Can

The internal “Fin Can” showed some evidence of blow by of the O-ring that normally sits between the Fin Can and the phenolic liner at the base of the propellant grain.  No hot gas erosion was evident in the aluminum structure or in the O-ring, but soot was found on the downstream side of the O-ring.  If this O-ring were breeched, hot gas could—in principle—circulate between the liner and the motor wall; thus, this is a potentially significant issue.  Mitigating against circulation is the use of high temperature grease between the liner and the motor wall. There was no evidence of any soot or hot gas circulation along the interior of the motor wall. Likewise, there was no evidence of any hot gas leak between the fin can and the motor wall.  With minor refurbishment, the fin can does appear suitable for reuse excepting the potential change to two O-rings between the liner and the fin can.

Some “blow by” transient leakage past the seals was evident.
Opposite side of the fin can shows same pattern of the “blow by”.

Phenolic Liner

The propellant grain liner was partially consumed at the forward and bottom ends where the liner is exposed to hot gas for the full eight second duration of the burn.  There was no evidence of any hot gas contact with the motor tube wall and we thus conclude that the existing liner is of sufficient thickness to handle the current eight second burn time.

Conclusions

Based on this inspection it appears some minor redesign of the nozzle top heat shield is required.  It may likewise be prudent to replace the single O-ring used between the internal Fin Can and the phenolic liner with two O-rings.  The rest of the vehicle hardware appears to be in good shape and does not seem to require any design changes.

The lack of gap between the fins and the motor wall appears to rule out the possibility of part of the belly-band having become trapped on one of the fins and causing the unexplained turn to the Northeast.  The cause of that turn remains a mystery.


Claybaugh 6-inch Rocket, Notes on Propellant Processes

Bill Claybaugh, Reaction Research Society


EDITOR’S NOTE: This article may be revised or expanded at a later date. As part of the second of three reports on this topic, this is a brief paper on the increased propellant density available from using IDP with some mention of the importance of post-mixing shaking (vibration) and vacuum-based degassing.


Two changes were made to the propellant for the 6-inch flight vehicle, as compared to the previous static test motor: one chemical, the other process-related.  These two changes resulted in an increase in the flight motor’s solid propellant grain density.

The previous static test motor propellant used DOA (Di-Octyl Adipate) as the plasticizer.  For this mixture, we substituted IDP (Iso-Decyl Pelargonate) on a 1:1 basis. This change in plasticizer resulted in a noticeably less viscous mixture whereas previously the mix had been a “thick and sandy” wet solid that did not slump. This new mixture while also still “thick and sandy” was noticeably given to slumping when moved from the mixer to bowls for compacting into the motor.

Previously, the propellant had been put under a vacuum for ten minutes between final mixing and the beginning of packing the wet propellant into the motor.  This process had no noticeable effect on density compared to the previous mixes which did not use vacuum degassing.

For this mix, vacuum was limited to five minutes but was applied at the same time as the mixing bowl and contents were strapped to a shaker table that vibrated the wet propellant mix both vertically and in one horizontal plane.  When the vacuum cover was removed from the bowl, the mix showed obvious signs of degassing, including both numerous surface “craters” as well as an about one-half inch gap between the propellant mix and the walls of the mixing bowl.

Electric powered shaker table for degassing batches of solud propellant mixtures

Upon completion of packing the propellant into the motor it became clear that density had been increased. The total propellant load was expected to be just over 51 lbm. but was clearly higher because we had much less surplus propellant mix left after casting than expected.

Weighing of the motor following curing and post-processing confirmed the suspicion of the previous afternoon that the net propellant mass was 54.2 lbm for a density of 0.0593 pounds-mass (lbm) per cubic inch, an about 5% gain over the previous 0.0564 lbm / cu. inch.

We thus concluded that while applying vacuum after mixing but before casting has little effect on density; vacuum with shaking does result in some degassing of the propellant mix when combined with using IDP for reduced viscosity.  We also note that propellant density remains about 3% below the theoretical 0.061 lbm / cu. inch that could be realized when mixing under vacuum rather than only applying vacuum and shaking after mixing.  Given the very high cost of vacuum mixing equipment and the impracticality of using such equipment in the field, there is a relatively small gain that could be achieved compared to using the present method. We conclude that post-mixing processing under vacuum with shaking is a lower cost alternative that provides some gain compared to open-air propellant mixing without degassing.

View of the finished propellant grain from the head-end.
View of the propellant grain from the aft-end showing the four-finocyl grain design.

March 2021 Virtual Meeting

by Keith Yoerg (RRS Secretary)


The latest meeting of the Reaction Research Society took place this past Friday, March 12th and had 20 attendees (who came & went at different times) – including a guest presenter. Society president Osvaldo began discussion by informing the group that USC has requested a launch of their 6″ booster rocket in April, before leaving the meeting to go on a Home Depot supply run.

Screenshot of discussion during the monthly meeting

GUEST PRESENTATION – GEORGIA TECH YELLOW JACKET SPACE PROGRAM

Sam Kim from the Georgia Tech Yellow Jacket Space Program made a presentation on their mission to be the first collegiate team to send a liquid rocket to Karman Line. The team conducted the first hot fire of an 800 pound-force LOX-kerosene engine in November 2019 (pictured below). This engine will support a sub-scale rocket which will be used to prove and test methods on this student-designed, -machined, and -assembled project.

November 2019 static fire of the Georgia Tech YJ-1S rocket

The team has planned a launch for the sub-scale rocket in October to an apogee of 5,000 ft. An 11-second burn static fire of this engine is expected in April. The team conducts testing out of the DeKalb-Peachtree Airport, and plans to make launches from the Spaceport Camden in southeast Georgia.

BILL CLAYBAUGH’S 6″ ROCKET

Bill Claybaugh presented a number of hardware pieces related to a 6″ rocket which he hopes to launch this year. The rocket will fly on a 6″-diameter, 60″-long motor producing 1,350 lbf of thrust with an 8.3-second burn time. This design is optimized to be used as the second stage in a rocket boosted by a 9″-diameter motor, but the impending flight is only the 6″ second stage. Bill mentioned that he has a test flight planed for the 9″ first stage some time within the next year. The combined 2-stage rocket will have to fly out of a national range because of the expected altitude of 120 statute miles.

Bill showing off the avionics package on his rocket

The hardware that Bill C. presented from his rocket included the tapered fins, bulkhead assemblies, and a section of the 40″ avionics payload which will be mostly contained within the nosecone. In addition, Bill provided insight on the FAA paperwork that he is currently completing for the flight of the 6″ rocket. The FAA form 7711-2 he has been working on is the same one used for airshows, and requires a supplemental that covers both class 2 & 3 amateur rockets. The final hurdle is the “splash pattern” Monte Carlo analysis of over 1,000 launches with varying launch angles, wind, and other launch parameters to determine the probability of landing in a populated area. He expects the launch will be deemed safe, and plans to submit the paperwork soon.

PLANS FOR THE UPCOMING MTA EVENT – SAT. MAR 20

Several members stated their intentions to join the launch & work event at the MTA site next Saturday, March 20th. Work on welding the new plate on the vertical test stand, clearing brush in the launch/firing areas, and other site maintenance may take place if time and equipment availability allow. There has also been discussion of taking an inventory of the working order of some Society equipment (such as the PA system) stored at the site.

Bill Inman plans to once again bring out his solar-powered steam rocket, Solar Cat, which uses mirrors to heat water with sunlight. Bill has been a fixture at MTA events the past several months, perfecting the design and procedures so that he won’t be caught unable to launch because of a minor oversight like untested equipment or cables that are too short. Although all of his equipment is flight-ready, MTA launches are always at the mercy of the Mojave desert weather. Wind, rain, and sunlight permitting – we hope to see the Solar Cat take to the skies next weekend!

Early stages of Dimitri’s water bottle rocket module, which is now assembled & ready for testing

Wolfram Blume is vaccinated (an impediment to his attendance at last month’s event) and ready for another launch attempt of the booster stage on his rocket, Gas Guzzler. In the final version the upper stage will include a gas-powered ramjet, but this flight will be flown with water instead. The goal of this test is to measure drag & acceleration, particularly during separation of the booster stage, which will help inform the final design parameters. We’re excited to see this launch, and expect it will be a fun one to watch!

Keith Yoerg will bring his model rockets and multi-pad wire rail launcher to test out deployment boxes and high-power solid rockets to test LoRa GPS trackers as a cheap rocket tracker. Dimitri is finishing work on a his hybrid & water rocket launch controllers (pictured above), both of which should be ready for testing on Saturday as well. It was once again agreed that a grill-out should take place, which is quickly becoming an MTA event tradition.

WIRELESS LAUNCH CONTROLLERS

The last topic stimulated a great discussion on the use of wireless launch controllers, with many members providing thoughts and opinions. Richard Dierking presented the commercially available Wilson F/X wireless control box, which consists of the firing box shown in the photo below and 2 wireless modules which run on 12V gel cell batteries. This entire system cost him around $900, and larger versions of this system have been used by the Rocketry Organization of California (ROC) and Friends of Amateur Rocketry (FAR) for launches of high-power solid rockets.

Richard Dierking showing a Wilson F/X wireless control box

Dimitri expressed his comfort and trust in the Cobra wireless firing system, which he has used many times as the Pyro Op for million-dollar shots in movies and television. The Cobra system uses 64-bit encryption (it was suggested that the Wilson F/X system uses 32-bit) and the only issues Dimitri reported was when attempting to fire directly near an ultra high-speed camera. Richard stated that he would look into getting someone from the Wilson F/X company to attend a future RRS meeting to describe that system in further detail.

Most members expressed cautious optimism about the potential of using wireless launch controllers at RRS events, though it was re-iterated that the Pyro Op in charge has the final say in what firing systems may be used at any event. The consensus best path forward was progressing slowly by starting with LED lights & low energy firings like model rockets. The aim is to build experience with and knowledge of these systems to determine if they can be safely used for more energetic firings. Richard & Dimitri plan to bring the Cobra and Wilson F/X systems up to the MTA event next weekend, where (with the permission of the Pyro Op in charge) they will be tested safely on a small scale.

NEXT MONTHLY MEETING

The next RRS monthly meeting will be held virtually on Friday, April 9th at 7:30 pm pacific time. Current members will receive an invite via e-mail the week of the meeting. Non-members can request an invitation by sending an email to:

secretary@rrs.org

The Executive Council has committed to an additional monthly meeting moving forward to address administrative matters. Members who would like to discuss an admin topic in detail can request attendance at a Council meeting by sending an email to the Secretary at the address above.