by Dave Nordling, President, Reaction Research Society
The University of Southern California (USC) Rocket Propulsion Laboratory (RPL) conducted a series of six propellant sample burns for characterizing their latest mixture. I was the pyrotechnic operator in charge for that day. New member and former USC RPL student, Michael Rouleau, was my apprentice that day.
Testing took place in the horizontal configuration on our repaired pad with the new 3/4” female pattern using a load cell and chamber pressure transmitters reading from the bulkhead in a modular nozzle can configuration.
USC RPL has had several motor failures in recent times which has led this year’s team to try a better known and safer recipe. COVID-19 played a role in creating a knowledge gap. This year’s team hopes to reclaim a success when the full static fire motor is tested at the RRS MTA in a little more than a month.
By Dave Nordling, President, Reaction Research Society
The society had a small work event at the Mojave Test Area on August 4th, 2022. The purpose was limited to starting the build of a new launch pad foundation for Bill Claybaugh’s upcoming large solid motor powered vehciles. The summer heat and tough soil limited progress but it was useful to gauge what the next steps should be. Launch is scheduled for mid-October.
Many thanks to Rushd Julfiker and Joe Dominguez for volunteering their support to Bill on that day. A new and larger launch pad is designed to support Bill’s larger adjustable launch rail system which will be useful to the larger sizes of future rocket projects at the RRS. New developments will be reported in the near future.
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.
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.