I looked at the video from this Wednesday's drop testing of the drogue
Here are the parameters of the drogue as designed:
Arm length 1.5m
Arm width 0.5m
Drag coefficient 0.5 (conservative design value, anticipated value
Desired maximum terminal landing velocity 55 mph.
Thanks to whoever took the video of the last drop there was some good
shots of the landing, and i was able to estimate the landing velocity.
Based on analysis of the drop testing video, the descent velocity on the
test load was 8.15 m/s.
Post-testing measurement gives these figures for the drogue as-built
Arm length 1.4m
Arm width 0.44m
Additionally the drop test load mass was 2.3435 kg.
Putting these facts together, the drop test indicates an actual drag
coefficient for the drogue parachute of 0.62 .
Extrapolating this to a 50 pound rocket at desert altitudes implies a
landing velocity of 25.7 m/s, or 58 mph.
The chief surprise here is what we had already noticed when sewing the
drogue. Ripstop nylon tends to gather when sewn. The canopy cloth was
cut for the design length of 1.5m, but the finished length came out
about 7% short due to material gathering during sewing. The Arm width is
actually 13% too short, this may be due to our sewing the width 1st, and
not controlling the gathering as well during our initial attempts.
Luckily, since a conservative Cd was used in designing the parachute
(about 20% too low), the loss of canopy area is just about compensated
for by the real, higher, Cd and the design meets the requirements fairly
Hopefully if another drogue is sewn the gathering effect can be reduced
by more effective basting.
Otherwise, the results are quite good and overall acceptable :)
PS yes, the video will be up on the wiki as soon as i am somewhere not
using dial-up ;)
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