Cycle 2 What we Simulated

Owned by Joel Godard
Last updated: Mar 04, 2023
5 min read

  • Cycle 2 Baseline (clipped delta with cycle 2 masses etc)

    • This is the cycle 2 masses and lengthes and such with the fins from last year as a starting place

  • Cycle 2 sim 1 (first stab at fin size for 2.5 calibers)

    • This is a rough simulation Joel did to get a sense for what the flight parameter's looked like before he messed with stuff a lot

    • Ended up just shortening the fins a hair to 6in.

Wind Design Condition Studies

Fin Shape Testing (cycle 2.0):

Methodology: As there are too many variables to independently tweak by hand all at once and simulate the outcome of each option now that we/Joel has a basic grasp on how stable the rocket is and how different changes to the fins affect the results he will design a fin shape and then run it through a series of simulations to see if the performance of the fin is up to our desired results. If this does not result in an effective fin or Joel thinks there is performance left on the table he will repeat the process. Fin shapes are designed for 2.5 cals of stability at 9mph wind case with FOS on flutter of 1.5

  • Characteristics to be measured:

    • Apogee, lowest stability prior to apogee (>5secs before apogee), lowest stability, highest stability, max velocity,

Tests to be run:

  1. Expected Mass and length:

    1. expected wind, pressure, temperature

    2. 15mph wind with whatever is worse 85th percentile pressure and temperature

    3. 0mph wind with whatever is worse 85th percentile pressure and temperature

    4. Improved surface finish with expected wind, pressure and temperature

  2. Max Mass Condition:

    1. expected wind, pressure, temperature

    2. 15mph wind with whatever is worse 85th percentile pressure and temperature

    3. 0mph wind with whatever is worse 85th percentile pressure and temperature

    4. Improved surface finish with expected wind, pressure and temperature

  3. Min Mass Condition:

    1. expected wind, pressure, temperature

    2. 15mph wind with whatever is worse 85th percentile pressure and temperature

    3. 0mph wind with whatever is worse 85th percentile pressure and temperature

    4. Improved surface finish with expected wind, pressure and temperature

Shape “KOTS 2.0“:

  • Start with the KOTS shape and increase height until stability is correct and then mess with flutter/thickness after

Shape “Tip“:

  • Idea is to start with a KOTS fin and moderately increase the tip chord is the hopes of finding a way to increase the flutter velocity without much increase in thickness. Likely will also require a height increase of the fin.

Shape “Tip with Sweep“:

  • Start with the tip concept and then add a sweep to move the fin CP effect back more in hopes that height can decrease and therefore thickness can decrease.

  • Analyzed without changing the weight and appeared to be the best option however upon doing a weight study found roughly 2.3in^2 increase in area and plate stock is 0.277lbs/in^2 for 3/16 thick meaning adding 0.637lbs in plate stock. Because of our own added material rounded up to 1.25lbs. dropped apogee for 1a case to 28557 which is significantly worse but still better than all other shapes. However lowered stability to 2.3 roughly meaning a size increase which would reduce flutter meaning a thickness increase and even more weight. This will for sure reduce apogee down to similar levels as KOTS 2.0 design. therefore shape was determined against.

If extra time:

Shape “Playing with Fire“:

  • Design to have closer to 2 cals of stability at 9mph and really work on getting the apogee up even with some risk.

Cycles 2.1 and 2.2:

After shape KOTS 2.0 was selected as the best option the propulsion team started refining the final flight engine masses which caused some changes in the mass. This resulted in running two additional sets of sims. Starting with the KOTS 2.0 shape and setting stability at 2.5 with 9mph wind and then running through 1a-d, 2a-d and 3a-d as above in cycle 2.0. The names are kinda confusing to be honest but essentially this was were the final tweaks were happening to get what we wanted.

 

Sensitivity studies on:

  • Other OR sim inputs

    • Not a huge effect for pressure and temp but did weather analysis and used for cycle2.0-2.2 conditions testing.

  • Fin shape with flutter considered

    • Looked at increasing the length of tip chord in order to get better flutter characteristics and there was slight benefit to it but did not end up increasing this due to drag and not hugely increased flutter qualities.

  • Increasing fin thickness vs area for flutter resistance

    • It appears that changing fin area does not affect flutter FOS nearly as much as changing fin thickness so thickness changes will be used for weather studies.

  • Launch rail being shorter if want

    • As the limiting factor on stability is still off rail for most of the simulations we don’t really want to lower the launch rail size but we also don’t really need it longer as we are very close to not having off rail be the worst stability. Launch tower team suggested the shortening was not a high priority so sims were not done.

  • Surface roughness improvemen

    • Ran study on file with cycle 2 masses not quite fully done and apogee went from 29413ft to 31347ft changing from regular paint (2.36mil) to smooth paint (0.787mil). No noticeable impact on stability.

    • More work done in cycle 2.0/2.1/2.2

  • Weight increase due to larger fin area on tip swept back shape

    • Early on the tip swept back shape seemed really good but weight increases not being analyzed meant that a drawback was concealed

    • Analyzed and found roughly 2.3in^2 increase in area and plate stock is 0.277lbs/in^2 for 3/16 thick meaning adding 0.637lbs in plate stock. Because of our own added material rounded up to 1.25lbs. dropped apogee for 1a case to 28557 which is significantly worse than previous and not really an advantage over other shapes.

 

 

 

Four Fin vs. 3 Fin Study:

  • Used the comp sim and set the material of the fins to carbon fiber and figure out how much it thinks those fins weighed

    • This value is 1.5 lbs which seems high but going with it.

  • Subtract that number from the section mass and make a copy of cycle 2.2 1a-c using this new number and see that it makes sense

  • Make a new file with four fins and dial in stability first

  • Then do flutter calcs and thickness back into openrocket and check stability again and iterate.

Results:

  • discovered that four fins is more efficient and leads to higher apogee

  • Optimized shape a lot and ended up with a more swept back shape with really good flight characteristics. Calling it tail stabilizer shape as it looks like the tail stabilizers on the f35 which aren’t really just tail stabilizers but anyway.

  • Checked with airframe team and no major manufacturing concerns for either sweep or four fins so going with it.

  • Created new cycle 2.3 mass sheet and updated based on openrocket estimate of new fin size weight