Thursday, September 15, 2011

Belated: L585 with perfect dual deploy!

My fourth and final flight at Plaster City's June '11 launch was my new 4" fiberglass Extreme DarkStar.  I've sort of resisted the 75mm motor class for a decade but now I'm into it.  I ran some calculations and the cost in $/N•s is identical between 75mm and 98mm reloads (~$0.06-0.10/N•s).  This means the overall cost is lower with identical impulse per $.   Big fan.
     The wind was kicking up big time but I was confident that dual deploy combined with shear pins would bring the rocket down reasonably close.  Unfortunately my keychain video camera died on the K740 flight below so I have no onboard or ground-based video. The pix are still good though.  KO snapped this prep shot and I like it! (ignore my photo credit as that's an artifact of batch export):

Here's the pressure up:

And liftoff... note the excessive wind blowing the plume:

The ascent was arrow-straight but I lost site after apogee ejection because I was using CO2 deployment and a tiny drogue 'chute.  This was also my first time using the Beeline GPS unit because the fiberglass structure doesn't attenuate the signal.  Unfortunately I forgot to tape the unit down to the foam mount so I think it dropped to the bottom of the electronics bay on liftoff as my GPS altitude data was wonky.  I did receive what appeared to be directional information, however, so I started walking NW.  Then I heard a loud pop as the 4g main black powder charge blew at roughly 1200' as expected.  I watched the rest of the descent and it landed only ~1/4 mile NW of the launch site (closer than most flights in low wind!).  Luckily the dragging parachute caught on a bush so no damage was done by subsequent dragging.  Stupid mass convection.  Here's the Raven Altimeter data with a barometric peak altitude of 7,489':

And here's the ARTS2 altimeter data with a barometric peak altitude of 7,233' for an average of 7,361' (1.39 miles) at sub-Mach velocity:

I also ran the ARTS2 motor analysis and the report says it's an L680 (vs. the nominal L585).

Finally the ARTS2 coefficient of drag was roughly 0.5 at peak velocity.

I next plan to fly this rocket on the same propellant type but twice the motor size (M1230) so I expect roughly twice the altitude or ~14,000'.  Fingers crossed for October 8th!

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