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 <http://www.rasaero.com/dl_software_ii.htm>. 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 <mailto:arocket-bounce@xxxxxxxxxxxxx> <arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx>> *On Behalf Of *Robert Watzlavick
*Sent:* Monday, July 27, 2020 11:14 AM
*To:* arocket@xxxxxxxxxxxxx <mailto: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
