Yes it can, but the shorter the distance between the three lugs the
easier it is.
Many people mistakenly think that the lugs need to be far apart to work.
That is only true if you have a lot of weight at both ends (Motor and
Nose cone). If your nose is light and thus your CG/CP are much lower you
can place your lugs much lower and closer together. But it helps if you
considered that desirable configuration when designing the Rocket ;)
Some rockets, designed to do so, work very well with only 300mm between
the top and bottom lug! The middle lug can be as close as 30% to the
bottom lug with the right CG/CP thus maximizing rail utilization and
thereby supporting the maximum safe initial weight / thrust.
On 6/16/2015 10:43 AM, R Dierking wrote:
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 <mailto:tom.hanan@xxxxxxxxxxxxxxx>
*Sent:* Tuesday, June 16, 2015 10:33 AM
*To:* roc-chat@xxxxxxxxxxxxx <mailto: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 <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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-- Regards,
Tom Hanan
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