Carl:
As you know, both FAR and the MTA are located at the top of a slope; a
slope that the prevailing winds run up when there are winds.
Half a century of observation at the MTA leads me to conclude that the
wind, when blowing from down range, can separate due to the sudden change
in ground contour at the top of that slope and thus create a wind shear at
that location. Hundreds of launch photographs document that shear as
occurring about 100 feet above the surface at the MTA when the winds are
moderate or strong.
For rockets that are minimally aerodynamically stable as they leave the
launcher it strikes me as possible that the residue wind below the shear—or
the un-sheared wind when wind is light and conforming to the surface
contour—is sufficient to explain weathercocking.
I’m looking at building a big launch tower and I sure enough understand the
cost and structural reasons for going with a triangular cross-section
launch tower structure and some sort of “button” to attach the vehicle to
the tower. I am inclined to reject those good reasons and go with a four
rail launch tower that minimizes tip-off and thus avoids torques upon
clearance of the tower.
For vehicles that are not going more than a minimum of 100 ft/sec. as they
clear the launcher, I can only advise one or more strap-on motors or a
boost stage.
Bill
On Fri, Jul 31, 2020 at 6:38 PM Carl Tedesco <ctedesco@xxxxxxxxxxxxxx>
wrote:
I’m sure at least one of the propellant lines makes a 90 degree turn into
the rocket motor. But if this were the case, wouldn’t it continue to torque
the rocket until the rocket gains enough speed to be aerodynamically
stabile? What I have seen is just one abrupt “kick-over” then the rocket
flies true. Full disclosure: We are not always able to locate the launch
lugs at the CG and CP.
--- Carl
*From:* arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> *On
Behalf Of *ken mason
*Sent:* Friday, July 31, 2020 12:22 PM
*To:* arocket@xxxxxxxxxxxxx
*Subject:* [AR] Re: Modeling liquid engine aft body in OpenRocket
Carl, do you have any plumbing runs perpendicular to the flight path where
corriolous might cause a torque moment?
This effect is used in very precise mass flowmeter like the Micromotion
brand,I used the model C with 1/10% accuracy. Model D replaced it.
Ken
On Fri, Jul 31, 2020 at 10:48 AM Carl Tedesco <ctedesco@xxxxxxxxxxxxxx>
wrote:
Bob,
FYI… here is a design for a retractable launch lug. This is for a C-type
rail, but we have modified for the Baxter launcher at FAR which uses a
T-type rail. I have found (and you can sim to verify) that the launch
lugs/rail guides add a "not non-significant" amount of drag to the rocket.
<<...>> <<...>>
Kevin,
I have had "slow" accelerating liquids weathercock by 5-10° even in light
wind conditions off the Baxter rail at FAR. I feel there might be some sort
of "torqueing "effect as the aft rail guide leaves the rail. Can anyone
address this?
--- Carl Tedesco
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> On
Behalf Of Robert Watzlavick
Sent: Wednesday, July 29, 2020 5:42 PM
To: arocket@xxxxxxxxxxxxx; Kevin Ho <rebelwithoutajob@xxxxxxxxxxx>
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket
I was going to follow the FAR guidelines for the Baxter launcher, placing
one at the CG and one at the aft end of the body. I've seen other
references that suggest one at the CG and one at the CP. The skin is very
thin (0.016 inches) so I'm going to have to add stiffeners underneath.
I'd really like to 3D print the rail guides out of nylon but I'm worried
the steel rail will cut through it so I'll make them out of aluminum.
-Bob
On 7/29/20 7:17 PM, Kevin Ho wrote:
IIRC you are using a long rail, which should reduce weathercocking to
a certain extent. Longer rails means higher off-rail speed, which
means that even in a wind gust your relative wind will shift by less
degrees off the nose.
Also a controversial topic, but where are your rail buttons going to
be? That can greatly affect off-rail weathercocking. Placing the aft
button close to the CP can reduce the swing somewhat.