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
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