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Space Shuttle and wing bending moment

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Space Shuttle and wing bending moment

Postby PlayeRom » Thu Nov 05, 2020 3:55 pm

Almost in every launch I get the message: "Wing bending moment approaches safety limits! Watch AoA!" which in realistic model may cause a disaster. And it often happens - I'm launching automatically with the autopilot, so no hands on the stick and with default launch guidance

Image

From the wiki:http://wiki.flightgear.org/Space_Shuttle I found:
Wing bending moment coefficient CBW between -0.019 and 0.019 at max. qbar (modeled)
At max qbar, the wing bending moment is a function of Mach number and AoA. Since Mach number is close to 1.4 in this phase of the flight, this limit basically translates into alpha between -8 degrees and 2 degrees. This can only be achieved if the orbiter is in inverted flight.


So how I understand the problem is incorrect AoA, and I should keep it between -8 and 2 degrees, but for me this happened even on 0.6 degrees of AoA and I don't understand why. Could someon explayin me what is worng, and how to prevent it?

Should I take the control and corrects the autopilot? Why does the autopilot fail to cope with it and lead to dangerous situations?

Is it possible to watch somehow the CBW value which should be between -0.019 and 0.019 at max. qbar?
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Re: Space Shuttle and wing bending moment

Postby GinGin » Thu Nov 05, 2020 4:28 pm

@PlayeRom : you may find this article interesting
https://www.aiaa.org/docs/default-source/uploadedfiles/about-aiaa/history-and-heritage/why_the_wings_stay_on-ehrlich.pdf?sfvrsn=801c62b5_0

Thorsten used it for the fdm and it works really well in the sim with those constraints in mind

When approaching max q , around Mach 1.2 , max aoa allowable is around 2.
So at 0.5 you are fine , just close to the limits like annunciated with the message
In real, it was flown around -4 degrees to have a bit of margin.

You can either take it in manual and relieve a bit the normal load by stopping the pitch down movement ( neutral stick or a slight pull on it ) or leave the ap on as it is just approaching but not exceeding the max wing bending moment .


You can try the dev branch , stage 1 open loop is closer to the real one flown pitch wise with a bit more margin .
You will see there the sweet spot for aoa around -4 degrees at max q.

Something like that

Image
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Re: Space Shuttle and wing bending moment

Postby eatdirt » Thu Nov 05, 2020 5:17 pm

Yep! I suspect you're using latest milestones and you should refer to the Shuttle manual. You may also try to launch with different heading= angles, the Shuttle spends some time rotating after clearing the tower if you don't use the default heading=270), this is potentially delaying the MET at which it passes qbarmax, and this plays a bit.
As Gingin, said, that's a warning, you'll know it when it's not :)

Cheers,
Chris.
Last edited by eatdirt on Tue Nov 10, 2020 9:35 pm, edited 1 time in total.
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Re: Space Shuttle and wing bending moment

Postby Thorsten » Thu Nov 05, 2020 7:07 pm

Could someon explayin me what is worng, and how to prevent it?


Most likely you're using an initial launch location / orientation that's not compatible with what's pre-programmed for the ascent trajectory. As you have discovered the hard way, the error margins are actually rather slim, so you need to tune the parameters of the launch program to your location/orientation/payload if you're not using the standard.
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Re: Space Shuttle and wing bending moment

Postby PlayeRom » Thu Nov 05, 2020 8:44 pm

@GinGin Thanks for the interesting article.

The dev version looks promising, I launched 3 times and no notifications about limits.

@Thorsten
For both versions (official hangar and dev) I used the following position:
Code: Select all
--lat=28.60834
--lon=-80.60405
--heading=0

this is exactly Launch Complex 39A. I guess this (or around) should be a standard place on which the mission.xml file (or default parameters) are based, for launch guidance, I'm right?
I compared the mission.xml from the hangar and the dev version and there are differences. The file from dev version seems to work better for Launch Complex 39A for me.

Thanks for answers
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Re: Space Shuttle and wing bending moment

Postby Thorsten » Fri Nov 06, 2020 7:17 am

It would seem heading=0 might be culprit, the manual instructs you to do 270 degrees and that is what the mission file refers to. :D

Please read the manual and take the information seriously, it will save you lots of pain.

The file from dev version seems to work better for Launch Complex 39A for me.


That may or may not be, but generally understanding what the issue is and being able to address it properly yourself is preferable to randomly swapping partially incompatible files.
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Re: Space Shuttle and wing bending moment

Postby Thorsten » Fri Nov 06, 2020 6:38 pm

@all:

I realize you're (rightfully) proud of our devel branch - but please do not recommend it to users. It is really for people who are familiar with Shuttle operations, don't need any support and are part of the development/testing process.

First, there has been extensive unnoticed breakage of things in the past (for instance you couldn't dock for a while, so using the devel branch to train ISS approach procedures would have led to disappointment). That's not so for the stable branch, there all operations described in the manual have been tested prior to release.

Second, the devel branch contains undocumented features - it may be entirely possible that people are required to perform an operation that's simply not described in the manual. That's not so for the stable branch, the manual is updated to document everything necessary prior to a release.

In the event, the manual for the stable version has a section 'troubleshooting', that section would have contained a section referring to the problem outlined in the thread and offered the correct solution. I assure you from long and painful experience that having supported stable and unsupported devel branches is an eminently sensible concept. :D
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Re: Space Shuttle and wing bending moment

Postby eatdirt » Tue Nov 10, 2020 9:35 pm

agreed on everything, I've edited my post to not let the suggestion to use the devel branch!
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