It's because RASAero is doing more accurate modeling of CD at high altitude
than Open Rocket. Much more accurate it turns out.
What's happening at high altitude? The Reynolds numbers are getting very low.
When RASAero II Aero Plots is run with the defaults, the CD are calculated at
sea level. If you want to run the CD's at other than sea level, you have to
use the Mach-Alt feature in RASAero, detailed on pages 84-86 in the RASAero II
Users Manual.
When the RASAero II flight simulation is run, the Reynolds number is calculated
at altitude, and at high altitude, 75K-100K ft, and in particular above 100K
ft, the Reynolds numbers get very low. At very low Reynolds numbers, the
friction coefficient gets very high, producing a high CD. It's just like the
very high, steep portion of the CD curve down at Mach 0.01 to Mach 0.10 in the
sea level CD plots, where the Reynolds numbers are very low, and the CD gets
very high.
With the Mach-Alt feature in RASAero II, you can put in the Mach-Altitude for
the trajectory of the rocket, and in the Aero Plots and the Excel output file,
you will get the CD's with the correct low Reynolds numbers at high altitudes.
For high altitudes flights, if you are running the RASAero II flight
simulation, it does this correction for the low Reynolds numbers automatically.
But if you are pulling the CD's and other aero data out of the Aero Plots
Excel output file, you need to use the Mach-Alt feature to make the low
Reynolds number correction.
This is exactly what you do in Missile DATCOM when you want a more accurate CD
prediction, you use the Mach-Altitude input in Missile DATCOM to put in the
Mach-Altitude for the trajectory of the rocket or launch vehicle.
So RASAero II is doing the CD calculations during the flight simulation runs
exactly how it should, and the low Open Rocket CD's at high altitude are
incorrect, they are too low. Matching the CD's as you've burned out and are
coasting through 100K ft is important, this is why RASAero II produces more
accurate altitude predictions for high altitude rockets.
It's not a big effect, when the CD's are very high the dynamic pressure and
drag are very low, but it is a real effect, and an example of the attention to
detail in RASAero II.
You can take the RASAero II flight simulation run for the rocket to get the
Mach-Altitude, so you can figure out the Mach-Altitude points to enter into the
Mach-Alt input screen. (RASAero II linearly interpolates between the
Mach-Altitude points.) Again, see Pages 84-86 in the RASAero II Users Manual.
Then, you can run Aero Plots, and export the Excel file with the Aero Data, and
you will have the correct Reynolds number at each of the Mach points.
When the rocket is run in Open Rocket with RASAero II Aero Data, you of course
will get different Mach-Altitude points, because RASAero II does other things
more accurate than Open Rocket. (For example, RASAero II varies thrust with
altitude using the nozzle exit diameter/exit area. I'd have to confirm, but I
believe Open Rocket does not vary thrust with altitude.) But once you have
used one series of Mach-Altitude points to generate the RASAero II Aero Data,
you will be much closer, and generally you don't need to do further adjustments
of the Mach-Altitude data for subsequent trajectory runs.
Charles E. (Chuck) RogersRogers Aeroscience
-----Original Message-----
From: Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx
Sent: Mon, Dec 14, 2020 2:01 am
Subject: [AR] Re: Calculating CDs (was Re: In praise of Open Rocket)
#yiv8054495142 #yiv8054495142 -- _filtered {} _filtered {}#yiv8054495142
#yiv8054495142 p.yiv8054495142MsoNormal, #yiv8054495142
li.yiv8054495142MsoNormal, #yiv8054495142 div.yiv8054495142MsoNormal
{margin:0cm;margin-bottom:.0001pt;font-size:11.0pt;font-family:sans-serif;}#yiv8054495142
a:link, #yiv8054495142 span.yiv8054495142MsoHyperlink
{color:blue;text-decoration:underline;}#yiv8054495142 a:visited, #yiv8054495142
span.yiv8054495142MsoHyperlinkFollowed
{color:purple;text-decoration:underline;}#yiv8054495142
span.yiv8054495142EmailStyle17
{font-family:sans-serif;color:windowtext;}#yiv8054495142
span.yiv8054495142EmailStyle18
{font-family:sans-serif;color:#1F497D;}#yiv8054495142
.yiv8054495142MsoChpDefault {font-size:10.0pt;} _filtered {}#yiv8054495142
div.yiv8054495142WordSection1 {}#yiv8054495142 Yeah, I’ve just gone back over
some of my high alt sims on RASAero and noticed similar CD profiles, only much
much worse. Troy From: arocket-bounce@xxxxxxxxxxxxx
[mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of mark.spiegl@xxxxxxxxx
Sent: Monday, 14 December 2020 5:18 PM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Calculating CDs (was Re: In praise of Open Rocket) >> Does
the second chart include a parachute deployment? That's a good thought, but
it didn't pan-out when I looked at the data. See attached. The modification
to OpenRocket to import RASAero CD data is finished. It seems to work as
designed... but the crazy numbers coming out of RASAero leads to predictably
bad results. I wonder if angle of attack is changing in the coast phase.
Looking through all the RASAero charts, the only way I see CDs approaching and
exceeding 1.0 is at high angle of attack. However... according to the
simulation output, AOA is zero for the whole flight. Maybe Chuck is printing
the wrong variable???? Dunno. All the high altitude examples supplied with
RASAero show the exact same pattern as QU8K. Stumped (for now) --MCS
Maybe in the next few days I'll try to derive a Mach vs CD table from the data
in the "Flight Simulation" tab instead of the "Aero Plots" tab.
