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) RogersRogers 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
#yiv1927528229 #yiv1927528229 -- _filtered {} _filtered {} _filtered
{}#yiv1927528229 #yiv1927528229 p.yiv1927528229MsoNormal, #yiv1927528229
li.yiv1927528229MsoNormal, #yiv1927528229 div.yiv1927528229MsoNormal
{margin:0in;margin-bottom:.0001pt;font-size:12.0pt;font-family:New
serif;}#yiv1927528229 a:link, #yiv1927528229 span.yiv1927528229MsoHyperlink
{color:blue;text-decoration:underline;}#yiv1927528229 a:visited, #yiv1927528229
span.yiv1927528229MsoHyperlinkFollowed
{color:purple;text-decoration:underline;}#yiv1927528229
p.yiv1927528229MsoListParagraph, #yiv1927528229
li.yiv1927528229MsoListParagraph, #yiv1927528229
div.yiv1927528229MsoListParagraph
{margin-top:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;font-size:12.0pt;font-family:New
serif;}#yiv1927528229 p.yiv1927528229msonormal0, #yiv1927528229
li.yiv1927528229msonormal0, #yiv1927528229 div.yiv1927528229msonormal0
{margin-right:0in;margin-left:0in;font-size:12.0pt;font-family:New
serif;}#yiv1927528229 p.yiv1927528229msonormal, #yiv1927528229
li.yiv1927528229msonormal, #yiv1927528229 div.yiv1927528229msonormal
{margin-right:0in;margin-left:0in;font-size:12.0pt;font-family:New
serif;}#yiv1927528229 p.yiv1927528229msochpdefault, #yiv1927528229
li.yiv1927528229msochpdefault, #yiv1927528229 div.yiv1927528229msochpdefault
{margin-right:0in;margin-left:0in;font-size:12.0pt;font-family:New
serif;}#yiv1927528229 span.yiv1927528229msohyperlink {}#yiv1927528229
span.yiv1927528229emailstyle18 {}#yiv1927528229 p.yiv1927528229msonormal1,
#yiv1927528229 li.yiv1927528229msonormal1, #yiv1927528229
div.yiv1927528229msonormal1
{margin:0in;margin-bottom:.0001pt;font-size:11.0pt;font-family:sans-serif;}#yiv1927528229
span.yiv1927528229msohyperlink1
{color:blue;text-decoration:underline;}#yiv1927528229
span.yiv1927528229emailstyle181
{font-family:sans-serif;color:windowtext;}#yiv1927528229
p.yiv1927528229msochpdefault1, #yiv1927528229 li.yiv1927528229msochpdefault1,
#yiv1927528229 div.yiv1927528229msochpdefault1
{margin-right:0in;margin-left:0in;font-size:10.0pt;font-family:New
serif;}#yiv1927528229 span.yiv1927528229EmailStyle26
{font-family:sans-serif;color:#1F497D;}#yiv1927528229
.yiv1927528229MsoChpDefault {font-family:sans-serif;} _filtered
{}#yiv1927528229 div.yiv1927528229WordSection1 {}#yiv1927528229 _filtered {}
_filtered {} _filtered {} _filtered {} _filtered {} _filtered {} _filtered {}
_filtered {} _filtered {} _filtered {}#yiv1927528229 ol
{margin-bottom:0in;}#yiv1927528229 ul {margin-bottom:0in;}#yiv1927528229 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) RogersRogers 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 OpenRocketChuck / 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?
-BobOn 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) RogersRogers 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 OpenRocketRobert, 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
