MTA Launch Event, 2021-11-20

by Dave Nordling, Reaction Research Society


The UCLA Project Prometheus held a static fire event at the RRS MTA for two of their latest designs of their liquid rocket engine. The pyrotechnic operator in charge was Osvaldo Tarditti with Dimitri Timohovich and myself as apprentices for these two static fire operations. This was a liquid ethanol and oxygen engine of the same 1500 lbf design used in prior years. There was a change in the injector pattern and a new ablative liner was used in the first of two engines.

UCLA positions their equipment and makes final checks before inspection from the pyrotechnic operator.

UCLA had come to the MTA on the prior afternoon to begin their setup with plans to be ready for the first of two hotfires when the pyrotechnic operator was to arrive that day. UCLA was in fact ready and after a short review of all plumbing and changes made since last year’s testing followed by the basic safety briefing to all attendees the tanking operations began.

During the pandemic, UCLA had a long pause without access to their laboratory. This time allowed the team to collaborate remotely and consider improving their testing rig which was deployed at the MTA for the first time.

The first engine hotfire had a few delays from the igniter failing to light in the last seconds of the count. The count was recycled with the same result. After the avionics team corrected the problem and the oxidizer supply was replenished, UCLA returned to their countdown and had a generally successful hotfire. The test ran the whole duration but the chamber internal wall ablative liner seemed to not be sufficient and a breach of the chamber jacket was seen.

Chamber ruptured on the first engine at the end of the burn after the ablative wall expired.

After purging the engine and safing the ground test system, UCLA waited for the engine to cool. Photos were taken of the post-test conditions and we all took a break for lunch before swapping engines for the second of two planned tests.

The second engine installed and ready.

The second engine had the old ablative liner material and went full duration without any obvious trouble. Also, the second engine used a small solid motor on a 3D-printed clamp-on mount which worked well. Similarly the engine was purged and allowed to cool before its removal for inspection back at the university. UCLA will likely examine the igniter firing circuit and system before their next engine firing or flight.

Second hotfire went full duration.
Group photo at the end of a successful day.

The team was very proud of the progress made and the data gathered will be very useful in anchoring their next flight vehicle’s performance. UCLA intends to surpass 30,000 feet with this next flight to claim the FAR-MARS prize. UCLA is still the current record holder at 22,000 feet from last year’s flight. Vehicle dry weight reductions in this year’s design and minor improvements to other vehicle systems could make the difference in claiming the prize.

The sun setting after a pleasant afternoon at the RRS MTA.

The old blockhouse had it’s roof replaced two weeks ago thanks to Dimitri Timohovich and other RRS members who lended a hand. Trimming of the roof beams was finished and the blockhouse was used for the first time with UCLA’s liquid rocket static fire.

As UCLA was packing up to depart the MTA, we used the time to build another wire launcher rail for model rockets in upcoming school events with LAPD CSP. Dimitri and his son, Max, launched a few volleys of some water rockets using a special system using an air compressor and solenoid firing box built for remote charging of nitrous oxide based hybrid motors. The system worked well and it was great fun.

Dimitri Timohovich reloads a water rocket based from Smartwater one-liter plastic bottles.
Under his father’s supervision, Max Timohovich prepares to launch the next volley of water rockets in the last hour of sunlight.

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.