Robert,
This is not my area of expertise so consider these suggestions to be worth what
you’re paying for them.
Looking at the illustrations you sent through: the fins (to me) look a touch on
the small side for a large heavy rocket that’s expecting a slow acceleration
profile. The issue for me isn’t so much the stability towards the end of the
burn (highest velocity) but more at the start. From my understanding, fins
basically serve 2 primary purposes: (1) is to provide aerodynamic stability by
shifting the Cp aft of the Cg but (2) they keep the rocket pointed in the same
direction. They do that via correcting forces from aero lift on the frontal
face in non zero AOA shifts and drag (normal to the face) on the opposite side
in such conditions.
Obviously (for given atmospheric conditions) the correcting force provided is
proportional to the fin area and speed of the rocket.
Yes, increasing the fin area will likely produce more susceptibility to
flutter, but such can be overcome with fin design without significant mass
penalties. It will also produce more stability margin and hence more leverage
for cross-winds to weathercock the rocket into it at low speeds. Yes, there’s a
trade there, but one thing you can (perhaps) do to offset that is to add some
smaller fins on the opposite side of the Cg to assist offsetting the leverage
the main fins have at the base from cross-winds. Obviously you would size them
to achieve the desired stability margin of the rocket. You could even slide
them on with sliding lugs to allow for varying fin areas for various different
cross-wind conditions.
Anyway… just some ramblings from the peanut gallery.
Carry on…
Troy
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 < <mailto:rocket@xxxxxxxxxxxxxx> rocket@xxxxxxxxxxxxxx>
To: <mailto:arocket@xxxxxxxxxxxxx> arocket@xxxxxxxxxxxxx; Redacted sender
crogers168 for DMARC < <mailto:dmarc-noreply@xxxxxxxxxxxxx>
dmarc-noreply@xxxxxxxxxxxxx>; <mailto:ctedesco@xxxxxxxxxxxxxx>
ctedesco@xxxxxxxxxxxxxx < <mailto: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 ( <http://www.rasaero.com/> 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 <mailto:ctedesco@xxxxxxxxxxxxxx> <ctedesco@xxxxxxxxxxxxxx>
To: <mailto:arocket@xxxxxxxxxxxxx> 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
<http://www.rasaero.com/dl_software_ii.htm> 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: <mailto:arocket-bounce@xxxxxxxxxxxxx> arocket-bounce@xxxxxxxxxxxxx <
<mailto:arocket-bounce@xxxxxxxxxxxxx> arocket-bounce@xxxxxxxxxxxxx> On Behalf
Of Robert Watzlavick
Sent: Monday, July 27, 2020 11:14 AM
To: <mailto:arocket@xxxxxxxxxxxxx> 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>
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>
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
