Yes, that does sound good; and I’ve done that. But, alignment of those 3 can
be a real bugger. If the center one is off just a little bit, it will bind or
cause a lot of friction. You must have a rail available and check fit before
launch day.
Sent from Windows Mail
From: Tom Hanan
Sent: Tuesday, June 16, 2015 10:33 AM
To: roc-chat@xxxxxxxxxxxxx
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> wrote:
John: Please run paragraph 3 by me again. Just not so fast.
Thank you.
From: john@xxxxxxxx
Sent: Tuesday, June 16, 2015 9:50 AM
To: 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> 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
Sent: Tuesday, June 16, 2015 8:48 AM
To: 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 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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Regards,
Tom Hanan
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