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 OpenRocket
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div.yiv9327662127WordSection1 {}#yiv9327662127 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