Much of this is can be resolved with preflight testing of the airframe
which has the dubious task of being stable BOTH Before AND After
reaching aerodynamic stability BEFORE leaving the end of the launch
rail. Simply put if the rocket leaves the rail before it is
aerodynamically stable the Motor thrust vector dominates the departure
angle of the airframe.
Thus any combination of motor thrust, thrust vector and aerodynamic
stability has a specific tower length requirement. If you like to use
big motors on short towers you probably should be ground testing your
motors thrust offset before launch. Your SIM software can tell you at
what altitude/speed your airframe is aerodynamically stable at what
thrust / thrust offset.
In practice longer, taller towers are usually saved for "Added Safety
Margin" with the emphasis placed on the lowest reasonable thrust vector
which then drives the minimum size of the aerodynamic stability surfaces
at launch. I say at launch because some people use drop away boosters
with additional large surfaces that then fall away once airframe speed
allow the smaller lower drag stability surfaces to reliably overcome any
remaining thrust offset.
I have found a simple laser pointer like those sold to sight in rifles
allows me to ground test the thrust offset / motor mount alignment to
ensure it is within an acceptable range for the launch tower length /
height I plan to use for that launch. In fact I re-test that alignment
prior to each flight when working with groups that are launching
anything larger than an "F".
For those of you familiar with telescopes the process is similar to
collimation. You can even build your motor mounts to provide some degree
of pre flight adjustment to get your offset thrust as low as possible
before flight.
On 6/15/2015 11:36 AM, Bryce Chanes (Redacted sender
bandman444@xxxxxxxxx for DMARC) wrote:
Richard.
My observations from last years XPRS when Aeropac had their new "Trans-America Tower" or Uber Uber Rail, (A 30' tall radio tower with a rail on it) and used it to launch the ARLISS projects, was that even with 30' of rail launching an M, in most if not all cases the rockets were sent off in a random direction when they reached the top of the rail, even though they are considerably stable rockets, and the wind was low to non-existent.
I don't know, but to me, having a rocket fly straight off the pad is more than just a more stable rocket and longer/stronger tower.
Very interested.
Bryce Chanes
At Jun 15, 2015, 9:55:44 AM, R Dierkingwrote:
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*
Richard Dierking
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