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 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
#yiv2562201309 -- filtered {}#yiv2562201309 filtered {}#yiv2562201309
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{}#yiv2562201309 div.yiv2562201309WordSection1 {}#yiv2562201309 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
