Sunday, April 5, 2015

Holtville Launch - April 2015

This last Saturday was amazing at Holtville and here's some slow-mo evidence...

I completed three simple flights using only my 3D-printed V2 and all with progressively longer CTI Pro24 reloads.  I didn't risk an altimeter in any of them so all I have is my visual memory:
  • 2G 50F51 Blue Streak:
    • In all three liftoff vids I see some lateral wiggle on the rod so perhaps the integrated rail guide isn't quite long enough.  That's odd though because there seemed to me too much friction in the rail channel so I was planning to reduce the guide's cross section by a few percent.  Dunno.
    • Jittery liftoff above the rod so there's still not quite enough lead in the nose tip.  
    • The altitude was lower than expected and the 10 second delay about 4 seconds too long.  More on this below.
    • The landing was in some rock-laden dirt in the bowl and one of the fin bottoms broke.  It was still attached, however, so I just CA-glued it back on for the second flight.
  • 3G 60F50 Skidmark:
    • Even these tiny 24mm Skidmarks ignite instantly and are impressively loud!  Good titanium times.
    • I had shortened this delay to 8 seconds but that was still about 2 seconds too long.
    • The parachute fouled after eject but, surprisingly, there was no fin damage after landing in our bowl.
  • 6G 140G145 Pink:
    • The liftoff was most expeditious!  I sort of noted some pink shades in the flame from a distance but it's hard to tell unless you're watching slow-mo video of the liftoff.  :)
    • Because of the higher thrust and total impulse I left this third delay at 8 seconds.  The flight still ejected about 2 seconds after apogee and, upon watching that, it finally sank in that the uniform, 0.3mm high, radially oriented bands resulting from 3D-printing induce MUCH drag.  For the next print I'll try 0.2mm layers and that will take several more hours to print but should reduce the drag coefficient somewhat.
    • This flight achieved a surprisingly lofty altitude.  Darrel had packed the parachute for me using the burrito method so it popped cleanly at ejection.  Wind was building a bit so I had to walk to the north runway to retrieve the vessel.
    • Having landed on concrete that same fin tip broke once again and this time the top 1/4" of the nose tip chipped off.  This was OK because testing of this version zero design is a wrap and I'll move it to archive.
Apart from the issues I found over the four total test flights everything else worked surprisingly well.  The structural integrity of the airframe and shock cord mounts proved more than adequate.  I observed no melting from the casing heat or black powder ejection.  But, in addition to the modeling improvements I'd planned previously, I think I also need to edit the following:
  1. Remove the vented portion of the motor mount - this doesn't seem to be necessary for robust motor orientation and it just adds complexity, tail weight, and print time.
  2. I think I'll maintain the rail guide length on version 1 and decrease the cross-sectional size by perhaps 2%.
  3. In the Simplify3D slicer it's possible to vary print parameters by layer height.  As such I think I'll go with 60-70% internal fill from the glass plate to above where the fins meet the airframe to reduce the likelihood of damage upon landing.
  4. I think I'll use the same trick to beef up the nose tip and reduce it's tendency toward damage on concrete impact.
  5. I'll try 0.2mm layer height but this will increase print time on the airframe from ~15 hours to ~21 hours.  Longer print times mean more opportunities for print defects so reprinting would necessarily delay the time to v1.  Fingers crossed.
  6. I just realized I forgot a vent hole but the nosecone didn't pop off during ascent so I just lucked out.  Rather than modeling and printing a hole I think I'll cheat and use a drill bit.  
Onward and upward on the 3Dp tip!


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oddmanrockets said...

Sorry I missed the flights