1) Undersized fins with under sized motors is a good recipe for a wild
weasel ;)
2) Placing a "Third" launch lug above the "Bottom" lug can significantly
increase the usable length of the launch rail.
On 6/16/2015 10:28 AM, Tom Hanan wrote:
Here , here!!!
But remember when it comes to fins, SIZE Matters ;)
On 6/16/2015 10:21 AM, John Coker wrote:
For normal rockets, you want sufficient speed over the fins for them to be effective in guiding the rocket. Imagine a rocket moving upward through the air. As it starts to tilt off-axis, the side away from the nose tilt shows a larger surface (frontal) area to the wind, which tends to generate a counter force that straightens the rocket.
If the rocket is traveling too slowly, there isn't enough airflow to apply sufficient corrective force. It may help to think of the fins like wings with a minimal stall speed, although the analogy isn't exact since wings are shaped for lift in one direction and rocket fins are symmetrical.
Once the fins are keeping the rocket flying straight, there is no more need for a launch guide and extra length just adds friction.
As an aside, early rocketeers thought four fins were necessary and it wasn't until some adventurous experimentation that they discovered that three were enough.
John
On Tue, Jun 16, 2015 at 10:12 AM, R Dierking <applerocketry@xxxxxxxxxxx <mailto:applerocketry@xxxxxxxxxxx>> wrote:
John: Please run paragraph 3 by me again. Just not so fast.
Thank you.
*From:* john@xxxxxxxx <mailto:john@xxxxxxxx>
*Sent:* Tuesday, June 16, 2015 9:50 AM
*To:* roc-chat@xxxxxxxxxxxxx <mailto:roc-chat@xxxxxxxxxxxxx>
I believe the 6' length comes from standard rod availability. Also, it's a convenient size to manage. Finally, the Blacksky
ProRails were this length.
Most rockets reach sufficient velocity within 4' or so (usable
length of a 6' rail), so only rockets flying on underpowered
motors really need a longer rail. The consensus is that a guide
velocity of 50ft/s (15m/s) is plenty for most rockets.
The only trick when running a simulation to verify that you have
sufficient velocity for an underpowered motor is the usable
length of the rail is less than the total length. You need to
subtract the spacing between your rail guides as well as the
space between the aft rail guide and the absolute bottom of the
rail (within the stand).
RockSim will tell you how far it took to reach sufficient
velocity and I assume OpenRocket does so as well. I generally
look for a rail 2' longer than this distance.
John
On Tue, Jun 16, 2015 at 9:11 AM, R Dierking
<applerocketry@xxxxxxxxxxx <mailto:applerocketry@xxxxxxxxxxx>> wrote:
Wow! I have so much work to do and I started reading this
and I have to admit I don’t give a crap about work any more. And you do this for work Chris! Do ya pinch yourself
occasionally? Even Allen and the SpaceX guys must be a
little jealous.
All kinds of nozzles? Possible solutions? Spin and fire?
I’ve been curious if the 8’ 1010 rails are even worthwhile
and maybe anything over 6’ should be 1515. Or, 4’ long
1010’s? How was 6’ picked as standard anyway?
Seems Chris might know more but he would have to kill us if
he told us. Like how to light a E28T in orbit. 😊
Sorry, not believing to just accept the luck of the draw. You guys have the launch velocity data. Would a 10’ long 1515
significantly decrease the chance of the pitch we witnessed?
*From:* Chris Kobel <mailto:Chris.J.Kobel@xxxxxxxx>
*Sent:* Tuesday, June 16, 2015 8:48 AM
*To:* roc-chat@xxxxxxxxxxxxx <mailto:roc-chat@xxxxxxxxxxxxx>
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 rV^2 C_d Area * (stability distance) * sin (alpha)
where
air density r= .0765 lbm/ft^3 , C_d =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 in^2 fins= 200 in^2 or
1.4 ft^2 ), 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
<mailto:applerocketry@xxxxxxxxxxx>>
To: "roc-chat@xxxxxxxxxxxxx <mailto:roc-chat@xxxxxxxxxxxxx>"
<roc-chat@xxxxxxxxxxxxx <mailto: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
<mailto: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@freelists.org_ <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_*
Richard Dierking
-- ROC-Chat mailing list roc-chat@xxxxxxxxxxxxx
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Regards,
Tom Hanan
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