<< With a 4.5 inch fin span, the stability margin starts out at 2.2, then
drops momentarily to 1.1 when it hits the wind, then increases from there. Do
you think 1.1 at low speeds is sufficient margin? >>
Yes. The shift is what Robert was describing, which for your long skinny
rocket is significant.
This effect shows why you should start with a stability margin of 2.2, because
with your low thrust to weight ratio at lift-off with the angle of attack due
to the wind the stability margin will decrease to 1.1.
Note too that jet damping (also included in RASAero II) from the motor thrust
(mdot Ve) provides a damping term as the rocket tries to rotate from the wind.
This can be significant with a high thrust solid rocket motor, but with a lower
thrust liquid less so.
The minimum stability margin of 1.1 with wind should be fine.
As others have pointed out, and your simulations are showing, with wind a
subsonic stability margin of 3.0 will cause too much weather-cocking, which
results in a high horizontal velocity through apogee, which can shred the
recovery system (the drogue parachute, or the main parachute).
So for a low thrust to weight liquid rocket you're kind of in a box. The
zero-wind starting stability margin of around 2.0, dropping down to around 1.0
right off the rail with wind, looks like the sweet spot.
Charles E. (Chuck) RogersRogers Aeroscience
-----Original Message-----
From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx
Sent: Wed, Jul 29, 2020 1:50 pm
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket
When I enable the Rogers Modified Barrowman option, I can see the CP move
forward forward during the sim with a crosswind after it leaves the rail. I'm
trying to balance the stability in crosswind against horizontal velocity at
apogee. With a 4.5 inch fin span, the stability margin starts out at 2.2, then
drops momentarily to 1.1 when it hits the wind, then increases from there. Do
you think 1.1 at low speeds is sufficient margin? At apogee, that translates
into a 75 ft/s horizontal velocity at apogee. Launching downwind should help
but I think there's a bug right now where you can't enter negative launch
angles.
-Bob
On 7/29/20 10:36 AM, (Redacted sender crogers168 for DMARC) wrote:
This type of model is built into RASAero II. The specific method used is the
Jorgensen Viscous Crossflow Method. On the Aero Plots you can print out the
Potential CN (the classic Potential CNalpha slope though zero, Barrowman is one
method) and the Viscous CN (Jorgensen Viscous Crossflow). I used the body
component planform areas and planform area center of areas from the original
publication of Robert's paper. (Special thanks to Robert.)
When you add a non-zero wind in RASAero II, you can see the non-zero angle of
attack occur as the rocket leaves the launch rail, and the CP moves forward.
Charles E. (Chuck) Rogers Rogers Aeroscience
-----Original Message-----
From: Galejs, Robert - 1007 - MITLL <galejs@xxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx <arocket@xxxxxxxxxxxxx>
Sent: Wed, Jul 29, 2020 5:28 am
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket
#yiv6444353481 -- filtered {}#yiv6444353481 filtered {}#yiv6444353481
filtered {}#yiv6444353481 p.yiv6444353481MsoNormal, #yiv6444353481
li.yiv6444353481MsoNormal, #yiv6444353481 div.yiv6444353481MsoNormal
{margin:0in;margin-bottom:.0001pt;font-size:12.0pt;font-family:New
serif;}#yiv6444353481 a:link, #yiv6444353481 span.yiv6444353481MsoHyperlink
{color:blue;text-decoration:underline;}#yiv6444353481 a:visited, #yiv6444353481
span.yiv6444353481MsoHyperlinkFollowed
{color:purple;text-decoration:underline;}#yiv6444353481
p.yiv6444353481MsoListParagraph, #yiv6444353481
li.yiv6444353481MsoListParagraph, #yiv6444353481
div.yiv6444353481MsoListParagraph
{margin-top:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;font-size:12.0pt;font-family:New
serif;}#yiv6444353481 p.yiv6444353481msonormal0, #yiv6444353481
li.yiv6444353481msonormal0, #yiv6444353481 div.yiv6444353481msonormal0
{margin-right:0in;margin-left:0in;font-size:12.0pt;font-family:New
serif;}#yiv6444353481 p.yiv6444353481msonormal, #yiv6444353481
li.yiv6444353481msonormal, #yiv6444353481 div.yiv6444353481msonormal
{margin-right:0in;margin-left:0in;font-size:12.0pt;font-family:New
serif;}#yiv6444353481 p.yiv6444353481msochpdefault, #yiv6444353481
li.yiv6444353481msochpdefault, #yiv6444353481 div.yiv6444353481msochpdefault
{margin-right:0in;margin-left:0in;font-size:12.0pt;font-family:New
serif;}#yiv6444353481 span.yiv6444353481msohyperlink {}#yiv6444353481
span.yiv6444353481emailstyle18 {}#yiv6444353481 p.yiv6444353481msonormal1,
#yiv6444353481 li.yiv6444353481msonormal1, #yiv6444353481
div.yiv6444353481msonormal1
{margin:0in;margin-bottom:.0001pt;font-size:11.0pt;font-family:sans-serif;}#yiv6444353481
span.yiv6444353481msohyperlink1
{color:blue;text-decoration:underline;}#yiv6444353481
span.yiv6444353481emailstyle181
{font-family:sans-serif;color:windowtext;}#yiv6444353481
p.yiv6444353481msochpdefault1, #yiv6444353481 li.yiv6444353481msochpdefault1,
#yiv6444353481 div.yiv6444353481msochpdefault1
{margin-right:0in;margin-left:0in;font-size:10.0pt;font-family:New
serif;}#yiv6444353481 span.yiv6444353481EmailStyle26
{font-family:sans-serif;color:#1F497D;}#yiv6444353481
.yiv6444353481MsoChpDefault {font-family:sans-serif;}#yiv6444353481 filtered
{}#yiv6444353481 div.yiv6444353481WordSection1 {}#yiv6444353481 filtered
{}#yiv6444353481 filtered {}#yiv6444353481 filtered {}#yiv6444353481 filtered
{}#yiv6444353481 filtered {}#yiv6444353481 filtered {}#yiv6444353481 filtered
{}#yiv6444353481 filtered {}#yiv6444353481 filtered {}#yiv6444353481 filtered
{}#yiv6444353481 ol {margin-bottom:0in;}#yiv6444353481 ul
{margin-bottom:0in;}#yiv6444353481 Many years ago, I put together a modest
extension to the simple Barrowman analysis for model rocket stability to look
at stability margin issues. The bottom line is that for long/skinny rockets
you may need much larger stability margins than typically assumed to account
for CP shift with AOA. Conversely, short/fat rockets can likely do with less
margin. Your rocket is relatively long with modest/small fins so I think a
bigger margin would be appropriate. Here’s a reprint in the Apogee Rockets
newsletter…
https://www.apogeerockets.com/education/downloads/Newsletter470.pdf -
Robert From: arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx>
On Behalf Of dmarc-noreply@xxxxxxxxxxxxx
Sent: Monday, July 27, 2020 10:25 PM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket
Robert: The rasaero.com web site should be up, I was on the web site earlier
today and a few minutes ago. The RASAero II software includes corrections to
the Fin Supersonic CNalpha and CP that particularly affect short span fins.
These corrections have not been made for the Fin Subsonic CNalpha and CP
because comparisons with Subsonic wind tunnel data hasn't indicated that they
are needed. In RASAero II we recommend a minimum stability margin of 1.0
calibers Subsonic, and 2.0 Calibers for Supersonic and Hypersonic. In RASAero
II you'll get a warning message if the stability margin falls below these
limits. Until a recent exception, every rocket run on RASAero II with a
stability margin from RASAero II of at least 2.0 calibers for all Supersonic
Mach numbers did not have any stability issues up to and above Mach 3. (The
exception had high-altitude coning above 100K ft.) If you're concerned about
stability, you could have a stability margin of 2.0 calibers for all Subsonic
Mach numbers. Earlier you wrote that you were looking for 2-3 calibers
stability margin. 3 calibers might cause wind induced weather-cocking for a
low thrust liquid rocket. You probably want to have a minimum stability margin
at all Subsonic Mach numbers of 2.0 calibers. Charles E. (Chuck)
Rogers Rogers Aeroscience -----Original Message-----
From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx; Redacted sender crogers168 for DMARC
<dmarc-noreply@xxxxxxxxxxxxx>; ctedesco@xxxxxxxxxxxxxx <ctedesco@xxxxxxxxxxxxxx>
Sent: Mon, Jul 27, 2020 6:49 pm
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket Chuck / Carl,
Thanks for the advice. I did notice that when I added the boattail, the Cp
became very sensitive to fin span. I also tried RasAero 1.0.1.0 (the website
is down currently so I can't try the newest version).
Do you (or anybody else) have a recommended stability margin for a long thin
rocket like this? Max expected velocity is below Mach 1.0. I'm worried about
low-span fins getting lost in the boundary layer - does your code take that
into account?
-Bob On 7/27/20 7:29 PM, (Redacted sender crogers168 for DMARC) wrote:
Carl: The altitude comparisons with flight data, CD comparisons with
wind tunnel data, and CD comparisons with in-flight measured CD, are all on the
RASAero web site ( www.rasaero.com ). The comparisons of the RASAero predicted
altitude with flight data indeed are all for solid rockets. The current
release of the RASAero II software (Version 1.0.2.0) had extensive new
protuberance drag models added. See Pages 24-30 of the RASAero II Version
1.0.2.0 Users Manual. It's been my experience that going over the rocket
in detail and really picking out all of the protuberances can really increase
the altitude prediction accuracy. Rail Guides, Launch Shoes and Launch Lugs
were already included in RASAero II. RASAero II Version 1.0.2.0 added a
Protuberance Input Section with; Streamlined - No Base Drag, Streamlined - With
Base Drag, and multiple Inclined Flat Plates. There are pictures in the Users
Manual showing parts of various rockets and describing which protuberance type
should be used to model the protuberance. Fin Brackets can be modeled using an
inclined flat plate (with the total frontal area the same as the Fin Brackets,
assuming the Fin Brackets have the same plate angle.) Note that the
protuberance drag modeling in RASAero II does not include that part of the
protuberance is actually buried in the boundary layer. All of the protuberance
is included, and thus the RASAero II protuberance drag prediction is
conservative (extra drag). Once you start inputting all of the
protuberances on the rocket, you'll see the 17K ft altitude fall with the
addition of each protuberance getting you closer and closer to the 13.2 K ft
flight data. This protuberance drag modeling is a RASAero II feature not
available in the other rocket flight simulation software. Robert; the
bottom of your rocket has a very complex shape, and is very difficult to model.
Probably the most conservative approach CP-wise is to model the bottom of the
rocket as a Boattail, as you have done in the middle figure. Boattails can be
pretty destabilizing, so if the bottom of your rocket acts as a Boattail, you
could have a marginal CP situation. CP-wise, I'd model it as a Boattail. If
it doesn't act as a Boattail, then you'll have additional CP margin.
Drag (CD)-wise, I'd run the rocket as the bottom figure (full base area), and
then take the first forward facing conical expansion, and add it's frontal area
as a Streamlined - With Base Drag protuberance. Note that you'll have one
RASAero II run to get the CP, and you'll have to keep track of the CP
separately. The other RASAero II run will be with the bottom figure and the
extra protuberance drag, to get the right drag (CD) for the trajectory
simulation. Charles E. (Chuck) Rogers Rogers Aeroscience
-----Original Message-----
From: Carl Tedesco <ctedesco@xxxxxxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx
Sent: Mon, Jul 27, 2020 1:25 pm
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket Robert,
I have no experience with OpenRocket. I use RASAero2. Regarding your fin
mounts… in RASAero they allow you to enter the frontal area of a launch shoe
(think launch lug). Does OpenRocket have an option like this? If so, maybe you
could model the fin mount as additional launch shoe frontal area. It does not
let you tell it where the launch shoe is (i.e. how far aft), so it probably
will not accurately predict CP, but it may be useful for drag (CD). Our
liquid rockets that have performed nominally have never achieved the sim
results. Our last rocket sim’d at ~17 kft but achieved 13.2 kft. This is
probably because the complex liquids have features that don’t make it into the
simplistic sims (like your motor exposed to the free stream). I had always
hoped some college team/student would compare the freeware rocket sims
aerodynamic modeling modules and report on which is the most accurate (hint,
hint if any of my students read this). Chuck Rogers who created RASAero has (or
use to have… I have not checked in a while) some case studies that compare wind
tunnel based aerodynamic data with the data predicted from his software, but I
believe they were all 1950’s-70’s solid rockets which are a lot closer to the
simplistic rockets that can be input and not like the liquid rockets we build.
--- Carl From: arocket-bounce@xxxxxxxxxxxxx
<arocket-bounce@xxxxxxxxxxxxx> On Behalf Of Robert Watzlavick
Sent: Monday, July 27, 2020 11:14 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Modeling liquid engine aft body in OpenRocket I'm using
OpenRocket to predict the Cp of my fin configuration but I could use some
advice with the configuration of the aft body. This is for the rocket using my
250 lbf LOX/kerosene engine. The engine
(http://www.watzlavick.com/robert/rocket/regenChamber3/photos/dsc_0749m.jpg)
sticks out from the aft body without a shroud. I modeled it three ways getting
varying Cp locations. The components are all zero weight so there is a lumped
mass in the fwd section corresponding to the empty weight with the "engine"
located at the aft face of the fwd body tube. The CG shifts about 4 inches
forward as the tanks burn down.
Method 1 - Actual engine geometry, not sure how well OpenRocket handles that:
Method 2 - Modeled as a boat tail:
Method 3 - Straight tube - probably not correct:
The fact that Method 1 and 3 have essentially the same Cp tells me that
OpenRocket probably isn't modeling the actual engine geometry very well. I
could create a shroud for it as the boat tail configuration has higher
performance but I'd rather just leave it as-is as I'm not too worried about
performance for the first flight. Max expected altitude depends on the
propellant load (10 vs. 15 sec) but should be between 10k and 17k ft. I've
seen other liquids that had similar arrangements with the engine hanging out so
how were they modeled for fin placement purposes?
One other issue is the interaction of the fin mounts. The fin mounts are
adjustable on the body and the fins are also adjustable within the fin mounts,
with a slot down the middle:
http://www.watzlavick.com/robert/rocket/rocket1/photos/dsc_8310m.jpg. I ;
finally got access to AeroFinSim and realized the original fin design was
flutter prone (and way too stable). The span of the fins is 5 inches but only
4 inches extend beyond the fin clamps so for flutter purposes, the fin span is
4 inches. However, the fin mounts must contribute something to stability so in
OpenRocket, I made a freeform fin that combines the mounts and fin. The body
diameter is 6 inches. I was targeting 2-3 cal of stability, mainly because the
Cp seems overly sensitive to fin height.
Any advice would be appreciated.
Thanks,
-Bob
