From Dave's slo mo vid, the pads seem very stable, no rod or base movement
at all. The ones vectoring off of vertical seem to do so at about 10 feet
or so above the pad. I wonder if that is a stability problem. There were
a couple that looked like they went straight but that could be just the
angle of observation.
Kurt
On Mon, Jun 15, 2015 at 1:22 PM, Allen Farrington <allen.farrington@xxxxxx>
wrote:
I would add that perhaps for the Punisher’s launched last weekend that the
rails were TOO LONG.
The speed at which the rocket is stable is reached well before the end of
the rail, therefore the rail is constraining the direction that the rocket
wants to fly (combination of fin alignments and motor thrust vector). The
entire pad tips as the rocket flies up and the longer the rail, the less
side force it takes to create a tip-off angle. If we had some slo-motion
video of this effect it would tell us a lot.
An interesting discussion.
Allen
On Jun 15, 2015, at 11:35 AM, Kurt Gugisberg <kurtgug@xxxxxxxxx> wrote:
Richard, I'm not sure if stiffer rail is really the answer. Last year at
XPRS, there was a pad out there that had a triangulated super structure
that kept the long rail straight and it was on a huge base anchored to the
ground and I think it even had guy wires. I watch for four days as
rockets came off of it and kicked out at an angle just like on our loosey
goosey pads. Asymmetric thrust might be one reason but this pad had a long
rail that should have helped. I'm beginning to think it might have
something to do with the spacing or location of the rail buttons. The fact
that one button is off the rail prior to the second one may allow it to
torque sideways. I wonder if one continues rail guide would solve that.
Kurt
On Mon, Jun 15, 2015 at 9:55 AM, R Dierking <applerocketry@xxxxxxxxxxx>
wrote:
First, I’m wondering how much stiffer a 1515 is than a 1010 rail?
Cross section is 50% more, so approximately double?
One of my last fights over a year ago now (way too long ago) was a K1275R
off one of the back pads using a 1010 rail. The rocket went noticeably
West immediately after leaving the rail and it wasn’t the wind. Luckily,
the rocket landed close to the pad because of the wind during descent. One
time the wind really helped.
There’s always a lot of discussion about rocket stability, but think
about how important a reliable launch pad is. Of course, what engine in
what rocket is very important to how massive/stable the pad must be and how
long the guiding structure should be. Regardless of size, I think there is
some occasional thrust vectoring with AP engines that takes place moments
after ignition that seems to stabilize. What would be the reason that some
rockets left the rail in the Punisher drag race at an angle? I’m curious
if other people believe momentary thrust vectoring occurs?
If you are going over say 10K’ in a location like Lucerne Dry Lake, you
need to be reasonably sure that the rocket is stable (of course), and that
it’s going to fly in the direction you choose.
I’ve been considering launch pad designs for several years, and checked
out all the custom pads at the last two BALLS launches I’ve gone to. Some
are really cool with elaborate pad leg designs and lifting mechanisms but
the ones with a sturdy/stiff launch rail adequately supported along its
entire length seem the best to me. This is my opinion, and I might be a
little jealous of the money some spend on their launch pads.
I’m considering purchasing a 12’ x 5” utility trailer for the base of a
launch pad that would also be a platform for a test stand. The pad would
use the triangular aluminum tower material that seems to be popular for
many large launch pads. A link follows that shows a base for an antenna
tower. * I would like to know if anyone else is interested
in participating in the construction of a large launch pad/test stand. It
would serve as our launch pad/test stand at ROC and FAR launches and be
very useful for XPRS/BALLS.*
*http://www.dipolnet.com/footing_for_aluminum_antenna_tower_E9151.htm
<http://www.dipolnet.com/footing_for_aluminum_antenna_tower_E9151.htm>*
Richard Dierking
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Allen H. Farrington
http://www.allenfarrington.org
818-653-2284
