Yup, this is quite the brain teaser - and we love it. On Monday, I was on the
freeway thinking about this and missed my exit by 10 miles!
And, why do I think that Chris and the other AC guys didn’t just throw up their
arms and say, “well, guess we can’t do it!” So Dave, are you trying to get
Chris on the hook with your last comment: “it doesn’t seem there is much that
can be done about it.” waiting….waiting…. 😊
Chris said that the misalignments were mostly at start and end with the E’s
tested. Perhaps these are the only engines tested by AC because this was a
practical matter and let’s keep that in mind. Some commercial engines might
have a high tendency for thrust misalignments and others not so much depending
on single or multiple throat, grain config, well, lots of things. Ces Aero
supplies engines to the military right? Seems if they don’t have the
misalignment problem on their list of things to test for, this would be a
really good thing to check and hopefully start publishing for commercial
engines.
I’m going to start a new thread with the Subject: Punisher Drag Race and Thrust
Misalignment Discussion
From: Dave Peterson
Sent: Wednesday, June 17, 2015 10:12 AM
To: roc-chat@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
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