Cherry Limeade

Formula

Burn Characteristics 

Mixing Procedure

Pouring Procedure

Mixing Spreed Sheet

Mixing Log

Motor Log

Notes

This formula proved that we were moving in the right direction with our propellant making. We continued to remove unnecessary ingredients that were included in the team's original formula in favor of the basics, and focused more on our mixing process and actually understanding what ingredients were for. The result was a formula that performed drastically better than XB (+32% ISP) and slightly better than OW (+6.5% ISP) while also improving density over OW (+6%) for a direct performance gain of around 13% when compared to OW. We managed to achieve these results while still maintaining an easily pourable mixture by slightly refining our procedures to get the most of our ingredients. The team has always used relatively expensive chemicals for each ingredient (IDP instead of DOA and MDI instead PAPI), but due to poor procedures didn't get the results that we were paying for. This was especially obvious on XB where the propellant was barely pourable at ~78% solids, because the procedures were essentially to dump everything into the mixer and mix for 5 minutes and then "until homogeneous". This was not repeatable due to its subjective nature and produced viscous and poorly-bonded propellant because particles did not have enough time to be coated fully before casting. We completely rewrote our procedures for OW and were able to make a 80.7% solids propellant significantly more pourable by adding ingredients in stages and drastically increasing the amount of time spent mixing and vacuuming. Slight tweaks to the OW procedures were used for CL at 82.5% solids and the resulting propellant was still easily pourable, so it appears that these procedures will scale to even higher solids loading to further increase our propellant's performance.

A subtle change that also increased the motor's performance was the removal of the burn rate catalyst and the 400 AP to slow down the burnrate. 400 AP was originally included for this goal, but we believe that it contributed to the erosivity that we frequently observed with OW because the large particles were easily ripped from the binder and that its removal actually lowered the propellant's burn rate. Though higher burn rates are typically associated with higher ISP, we were able to design our high aspect ratio motor to have tighter cores and thus higher volume loading because it featured lower burn rate propellant and didn't suffer from erosive burning as much. In general, we have found slower burning propellants to be more forgiving and easier to work with in long motors.

As the team prepared to make the qualification P9100 motor, we found that the HTPB that we typically used from RCS that includes HX-752 was temporarily out of stock. We learned that it would not be in stock for months, so we decided to use simple HTPB from CS Rocketry that was left over from the 2016-17 school year. We did a test mix using this HTPB on a single 98mm grain towards the end of first semester and found that the grain had typical density and poured as usual, so we were comfortable with this change. We also discussed whether the lack of HX-752 required recharacterization with Gary Rosenfield at RCS and he said that it did not because our formula did not have large (400u+) AP. We attempted to mix a grain for the motor at the beginning of IAP using the CS Rocketry HTPB but found that the propellant cured very quickly and was too viscous to pour as easily as it did in the past. The resulting grain had poor density (-2.5%) and visible voids, so it was rejected. Fortunately, RCS had restocked by this point so we purchased enough HTPB to make the rest of the motor and the flight motor and decided to do further experimentation to see if we could get the CSR HTPB to work. After the P motor static fire, we did an experimental mix using only the liquid ingredients of CL to see how they behaved and found that the curative interacted very quickly with some ingredient to form clumps that the mixer broke up within a few minutes of mixing. The mixture poured well and took about 2 days to harden fully to a springy rubber. We hypothesized that the clumping was due to the interaction of castor oil and MDI because they have low EW and thus a large number of bonding sites. To verify this theory, another mix was done using a new mixture of liquids including fresh CSR HTPB and no castor oil. We also increased the amount of IDP included to compensate for the lack of HX-752, as it has low viscosity and probably contributed to the lower viscosity we saw with the RCS HTPB. This mixture did not form clumps during mixing and poured well. The cure time was similar to the other mixture and the resulting rubber is similar, but lighter in color and slightly springier. Based on this result and discussion with Gary, we have decided to remove castor oil from our next formula.

Links

https://wikis-test.mit.edu/confluence/display/RocketTeam/Cherry+Limeade