Excellent analysis…
On Jun 16, 2015, at 8:48 AM, Chris J Kobel <Chris.J.Kobel@xxxxxxxx> wrote:
Here's my quick and dirty take on the trajectory dispersion - YMMV.
I think the issue, as alluded to by Richard and Kurt, is that there can be
some thrust misalignment that allows the rocket to pitch as it leaves the
rail, prior to the fins generating enough corrective force to keep it on a
straight flight path. The Punisher video seems to reinforce this. Our test
group here at The Aerospace Corporation has done some basic research
investigating the extent of thrust misalignment that can occur in small solid
rocket motors. Aerospace was the first ever to burn a solid rocket motor on
a picosatellite in space (PSSC-2, November 2011). That 1-sec burn, using an
Aerotech E28T solid rocket motor, provided ~4 mps delta-V, but also resulted
with a PSSC-2 spin rate of 360 deg/sec due to thrust misalignment relative to
the picosat center of gravity.
Testing we performed in 2012 on small Aerotech motors provided thrust stand
data and additional information depicting the misalignment variation over the
burn duration. The results showed misalignment varied between 0‐4 deg for
the three F22s tested and that the variation was most significant, in terms
of off-angle, during the startup transients and motor tailoff. Subsequent
slug tests measured off‐axis thrust angles that varied from 0.22 deg to 1.44
deg for E28Ts and E23Ts.
In our situation, some assumptions for our Punisher flights:
Torque due to thrust misalignment: Torque = thrust x distance x
sin(angle)
where thrust = 380 lbf, distance (nozzle to CG) = 18 in, and an assumed
thrust misalignment angle = 2 deg, resulting in a torque of ~240 in-lb trying
to pitch the rocket after it leaves the pad.
The correcting torque (calculated conservatively, based on the entire
cross-sectional area of the rocket modeled as a flat plate flying sideways at
a 10 deg angle of attack) would be:
½ r V2 Cd Area * (stability distance) * sin (alpha) where
air density r = .0765 lbm/ft3, Cd =2 (flat plate), V = 85 mph (125 ft/s – 6’
rail exit speed), Area (cross sectional area of 3” x 50” airframe plus two 25
in2 fins= 200 in2 or 1.4 ft2), stability distance of 1 caliber (3 inches),
and an assumed angle of attack of 10 deg. This results in a correcting force
of 52 lbf acting over a perpendicular distance of .52 in, creating a torque
of 27 in-lb trying to correct a thrust misalignment torque of 240 in-lb.
Not gonna happen, not even at much higher angles of attack.
As the motor burns, our data showed that thrust misalignment improves, the
velocity of the rocket increases increasing the aerodynamic correcting force,
and the CG moves forward, increasing the aerodynamic leverage arm, all
increasing the ability of the rocket to fly straight.
My conclusion: our Punisher situation of high thrust motors with minimum
stability margins makes our rockets very susceptible to even small variations
of thrust misalignment right off the pad. I believe that the 6’ rails are
long and stiff enough and that the rail button location and crosswind has a
minimal effect – i.e. they are not the problem. And basically, I don’t know
what affects the amount of thrust misalignment other than luck of the draw.
Chris
From: R Dierking <applerocketry@xxxxxxxxxxx>
To: "roc-chat@xxxxxxxxxxxxx" <roc-chat@xxxxxxxxxxxxx>,
Date: 06/16/2015 07:08 AM
Subject: [roc-chat] Re: [roc-chat] Re: Launch Pads and Interest
Posting for Construction of Large Launch Pad
Sent by: roc-chat-bounce@xxxxxxxxxxxxx
Perhaps there have been similar discussions on other chat groups? Seems we
have just become accustomed to rockets taking weird paths occasionally. But
there has to be a reason. Perhaps other than rocket instability or a
deflecting rail, there’s several things we could do to reduce this from
happening. Like relocating the rail buttons; perhaps it’s not even that
difficult.
For example, I’ve wondered if it would help by rotating the pads 90 degrees
so the usual wind doesn’t push the rocket ‘sideways’ as it travels down the
rail. Does this make sense?
It would be nice to have some information and analysis for rockets that have
‘weird’ liftoffs. Sometimes the answer is right there, it just takes close
attention.
Anyway, for the large launch pad on a trailer, I’m going to start with some
basic ideas and go from there. You must have a massive or well
anchored/stable pad base with a blast deflector that will not move the base
during initial thrust, and a rigid guiding structure. It would be nice to
have a launcher like the ones used for sounding rockets, but this pad must be
mobile. I need to work on the logistics a bit, because presently my trailer
storage spot is not large enough for my camper trailer plus this new utility
trailer. I’ll post back in several months and hopefully will have some
progress.
From: Bryce Chanes <mailto:dmarc-noreply@xxxxxxxxxxxxx>
Sent: Monday, June 15, 2015 11:36 AM
To: roc-chat@xxxxxxxxxxxxx <mailto:roc-chat@xxxxxxxxxxxxx>
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 Dierking 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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