In our testing, we assumed the answer was yes, but we didn't try to
correlate off-axis direction vs C-slot orientation.
Chris
From: Chris Attebery <chrisattebery1971@xxxxxxxxx>
To: roc-chat <roc-chat@xxxxxxxxxxxxx>,
Date: 06/17/2015 01:42 PM
Subject: [roc-chat] Re: Launch Pads and Interest Posting for
Construction of Large Launch Pad
Sent by: roc-chat-bounce@xxxxxxxxxxxxx
The 24mm motors that were tested are all c-slot type motors, right? Don't
you think that they would have asymmetric thrust like a moon burner does
due to the offset core?
On Wed, Jun 17, 2015 at 11:58 AM, Chris J Kobel <Chris.J.Kobel@xxxxxxxx>
wrote:
Dave,
I believe the thrust misalignment is from the motor itself - maybe due to
uneven burning characteristics, nozzle imperfections, uneven surface
finishes, deformations, pressure variations, what-have-you, etc - and
definitely beyond our control. That is why we believe we got different
characteristics (thrust misalignment angles) from several burns of the
same motor type in our single test stand. With the long motors and motor
mounts we were flying, I believe their alignment was more consistent than
the thrust vector provided by the motors. Long towers, larger fins,
increased stability margin, induced spin would all help reduce the
dispersion.
Chris
From: "Dave Peterson" <woshugui@xxxxxxxxxxxxx>
To: <roc-chat@xxxxxxxxxxxxx>,
Date: 06/17/2015 10:12 AM
Subject: [roc-chat] Re: Launch Pads and Interest Posting for
Construction of Large Launch Pad
Sent by: roc-chat-bounce@xxxxxxxxxxxxx
Chris,
Been thinking about your nice analysis over the last day or so. It really
makes the most sense to me for the seemingly random dispersion. It occurs
to me that a significant portion of thrust misalignment could come simply
from the the off-axis tolerance on the motor mount and the tiny amount of
space between the case and tube allowing the motor to rock side to side.
Even if the thrust on the motor itself is perfectly on axis, the placement
of the motor within the rocket is going to have an off axis vector. That
vector is going to be randomly distributed on the different rockets. Other
than tightening up the tolerances and spinning the rocket at a very high
rate coming off the tower, it doesn't deem like there is much that can be
done about it.
-Dave
From: roc-chat-bounce@xxxxxxxxxxxxx [mailto:roc-chat-bounce@xxxxxxxxxxxxx]
On Behalf Of Chris J Kobel
Sent: Tuesday, June 16, 2015 8:49 AM
To: roc-chat@xxxxxxxxxxxxx
Subject: [roc-chat] Re: Launch Pads and Interest Posting for Construction
of Large Launch Pad
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
Sent: Monday, June 15, 2015 11:36 AM
To: 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
Richard Dierking
-- ROC-Chat mailing list roc-chat@xxxxxxxxxxxxx
//www.freelists.org/list/roc-chat
