IIRC you are using a long rail, which should reduce weathercocking to a certain
extent. Longer rails means higher off-rail speed, which means that even in a
wind gust your relative wind will shift by less degrees off the nose.
Also a controversial topic, but where are your rail buttons going to be? That
can greatly affect off-rail weathercocking. Placing the aft button close to the
CP can reduce the swing somewhat.
On Jul 29, 2020, at 4:42 PM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx> wrote:
Yeah, I didn't start out with the intention of making the vehicle so long. I
chose the 6 inch body diameter based on the thinned-down medical oxygen tanks
I was using for propellant tanks. Each of the sections between tanks ended
up taking more room than I expected. With a lot of effort (manifolds instead
of tubes), I could have shaved off maybe 2 feet on the length.
I'm not so worried about having to replace a body tube due to zippering from
the drogue but I don't want it to take out anything else that might prevent
the main from deploying.
Thanks for the assistance.
-Bob
On 7/29/20 5:00 PM, (Redacted sender crogers168 for DMARC) wrote:
<< With a 4.5 inch fin span, the stability margin starts out at 2.2, then
drops momentarily to 1.1 when it hits the wind, then increases from there.
Do you think 1.1 at low speeds is sufficient margin? >>
Yes. The shift is what Robert was describing, which for your long skinny
rocket is significant.
This effect shows why you should start with a stability margin of 2.2,
because with your low thrust to weight ratio at lift-off with the angle of
attack due to the wind the stability margin will decrease to 1.1.
Note too that jet damping (also included in RASAero II) from the motor
thrust (mdot Ve) provides a damping term as the rocket tries to rotate from
the wind. This can be significant with a high thrust solid rocket motor,
but with a lower thrust liquid less so.
The minimum stability margin of 1.1 with wind should be fine.
As others have pointed out, and your simulations are showing, with wind a
subsonic stability margin of 3.0 will cause too much weather-cocking, which
results in a high horizontal velocity through apogee, which can shred the
recovery system (the drogue parachute, or the main parachute).
So for a low thrust to weight liquid rocket you're kind of in a box. The
zero-wind starting stability margin of around 2.0, dropping down to around
1.0 right off the rail with wind, looks like the sweet spot.
Charles E. (Chuck) Rogers
Rogers Aeroscience
-----Original Message-----
From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
<mailto:rocket@xxxxxxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
Sent: Wed, Jul 29, 2020 1:50 pm
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket
When I enable the Rogers Modified Barrowman option, I can see the CP move
forward forward during the sim with a crosswind after it leaves the rail.
I'm trying to balance the stability in crosswind against horizontal velocity
at apogee. With a 4.5 inch fin span, the stability margin starts out at
2.2, then drops momentarily to 1.1 when it hits the wind, then increases
from there. Do you think 1.1 at low speeds is sufficient margin? At
apogee, that translates into a 75 ft/s horizontal velocity at apogee.
Launching downwind should help but I think there's a bug right now where you
can't enter negative launch angles.
-Bob
On 7/29/20 10:36 AM, (Redacted sender crogers168 for DMARC) wrote:
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) Rogers
Rogers Aeroscience
-----Original Message-----
From: Galejs, Robert - 1007 - MITLL <galejs@xxxxxxxxxx>
<mailto:galejs@xxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
<arocket@xxxxxxxxxxxxx> <mailto:arocket@xxxxxxxxxxxxx>
Sent: Wed, Jul 29, 2020 5:28 am
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket
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 ;
<https://www.apogeerockets.com/education/downloads/Newsletter470.pdf>
- Robert
From: arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx>
<arocket-bounce@xxxxxxxxxxxxx> <mailto:arocket-bounce@xxxxxxxxxxxxx> On
Behalf Of dmarc-noreply@xxxxxxxxxxxxx <mailto:dmarc-noreply@xxxxxxxxxxxxx>
Sent: Monday, July 27, 2020 10:25 PM
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
Subject: [AR] Re: Modeling liquid engine aft body in OpenRocket
Robert:
The rasaero.com <http://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) Rogers
Rogers Aeroscience
-----Original Message-----
From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx
<mailto:rocket@xxxxxxxxxxxxxx>>
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>; Redacted sender
crogers168 for DMARC <dmarc-noreply@xxxxxxxxxxxxx
<mailto:dmarc-noreply@xxxxxxxxxxxxx>>; ctedesco@xxxxxxxxxxxxxx
<mailto:ctedesco@xxxxxxxxxxxxxx> <ctedesco@xxxxxxxxxxxxxx
<mailto: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 <http://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> <mailto:ctedesco@xxxxxxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx <mailto: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 ;
<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
