In the RASAero II software the launch site elevation is used to determine the
pressure altitude of the launch site. The altitude of the rocket above the
ground is added to the pressure altitude of the launch site as an input to an
atmosphere model to give the atmospheric pressure, used for the variation of
thrust with altitude.
The launch site temperature input is used to determine the launch site
temperature altitude. The rocket altitude is added to the launch site
temperature altitude for the atmosphere model to determine the temperature at
altitude, for the speed of sound for the Mach number calculations.
The pressure and temperature at the launch site are used to calculate the
density altitude of the launch site. The altitude of the rocket above the
ground is added to the launch site density altitude as an input to the
atmosphere model, used for the variation of density with altitude.
The best way to do a one-to-one comparison is to use standard day launch site
inputs. As an example, you'd input 0 ft elevation for the launch site (sea
level pressure), 59 deg F launch site temperature (standard day sea level
temperature), and the atmosphere model will give you the atmospheric density at
the launch site which will be the sea level atmospheric density.
As a further example, you can then put in an elevation of 3,000 ft, and make
sure you use the standard day temperature at 3,000 ft, and then you will get an
atmospheric density at the launch site of 3,000 ft. You then can look up the
atmospheric density at altitude from a standard day table.
This is what RASAero II does. I'd have to go through the documentation to see
what Open Rocket does for atmospheric modeling using the launch site inputs.
It might be that Open Rocket assumes the launch site elevation is the launch
site density altitude, and then does the speed of sound modeling separately.
RASAero II uses I believe a more sophisticated approach for atmospheric
modeling. Probably the simplest solution is to make sure the RASAero II launch
site temperature is the standard day temperature at the launch site elevation
altitude. Then the atmospheric models should line up.
Charles E. (Chuck) RogersRogers Aeroscience
-----Original Message-----
From: mark.spiegl@xxxxxxxxx
To: arocket@xxxxxxxxxxxxx
Sent: Sun, Jan 3, 2021 6:45 pm
Subject: [AR] Re: Calculating CDs (was Re: In praise of Open Rocket)
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div.yiv3365368007WordSection1 {}#yiv3365368007 OpenRocket Bug: When the launch
site altitude is 0, OR and RAS agree. However, when the launch site altitude is
non-zero, OR deviates significantly. Example numbers for one of my test cases:
OR launch site 0 -> altitude 90KRAS launch site 0 -> altitude
90KOR launch site 5000 -> altitude 100KRAS launch site 5000 -> altitude 120K
But here's the conundrum with the 5000 ft launch site:At burnout: OR and
RAS Altitude and velocity are identicalDuring coast: CDs identical, Drag
force(lbf) identical, Weirdness: Coast velocity very different, Max
altitude very different It seems if OR had a bug in the base simulator or
atmosphere models, the 0 altitude sims wouldn't work either. Taking
suggestions. :-) --MCS