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Space Shuttle - Development

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Re: Space Shuttle - Development

Postby GinGin » Fri May 24, 2019 3:13 pm

Yes, we needed inertial speed/1000 here, right?


Yes, that is perfect, thanks
And in MM 304, it should be the Earth relative speed ( vtrue-fps in Jsbsim/velocities)

Really nice for the correction, useful for TAL capabilities after MECO :)


The only situation I remember where manual trim was needed was when rudder and elevons fight for lateral control, each trying to cancel each other. I know the kind of situation actually can arise in the simulation, but I find it (luckily...) hard to trigger it


Indeed, me too. Rudder and ailerons trimming in the same way when rudder activated too early due to low speed at high altitude ( like RTLS) to cancel the sideslip, but with the opposite effect actually ( something like that)
Anyway, never encountered that so far.

The only interest I would see could be for manual trimming in pitch in CSS manual phase to release pressure on the stick and the bip trim on the stick that controls a rate of derotation ( 2 or 3 ° s if I remember well) to avoid hard touchdown of the nosewheel.
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Re: Space Shuttle - Development

Postby Thorsten » Fri May 24, 2019 3:17 pm

And in MM 304, it should be the Earth relative speed ( vtrue-fps in Jsbsim/velocities)


Actually, I never found any clear statement - it just says 'Mach', I can't remember seeing anything that says it's vtrue.

Anyway, the indicator whether you see VI or VR is also done, not sure whether you noticed.

The only interest I would see could be for manual trimming in pitch in CSS manual phase to release pressure on the stick and the bip trim on the stick that controls a rate of derotation ( 2 or 3 ° s if I remember well) to avoid hard touchdown of the nosewheel.


Meh - that depends a lot on the physical HW that's plugged in no?
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Re: Space Shuttle - Development

Postby GinGin » Fri May 24, 2019 3:44 pm

Actually, I never found any clear statement - it just says 'Mach', I can't remember seeing anything that says it's vtrue.



The reference for the Speed tape:
And "real "Mach Number below 4

Image



Anyway, the indicator whether you see VI or VR is also done, not sure whether you noticed.


Very nice, I just noticed the VR in mm 304 :)
But speed shown is not the vtrue at the beginning of mm 304 transition ( it adds 1000 fps to the Vi from MM 303, I went from VI 25.8 to VR 26.7 instead of 24.7 ish)
I will push the last dev version

Anyway, I am on my way to White Sands :)
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Re: Space Shuttle - Development

Postby Thorsten » Sun May 26, 2019 12:54 pm

I've now a model for the J2 altitude variations in the course of an orbit in the far zone of orbital targets. It's based on a Fourier series of the altitude variation with the coefficients being altitude and inclination dependent.

The problem is that 'circular orbit' isn't really overly well defined in J2 and J3 gravity - what I've done is to choose an orbital speed to give a circular orbit at zero inclination and used the same speed for other inclinations, but the net result of this is that all variation is upward - so in essence a high inclination looks like getting a small eccentricity. In the case of ISS, there's some 10 mile or so altitude variation.

I've done a quick test and the Lambert routine targets the thing just the same, so it doesn't seem to feel much different operationally.

@GinGin (mostly) - would you be willing to set up some phasing tests whether this is more like you'd expect if I push it? Or should we change the scheme to re-define 'circular' to something that has a smaller variation?
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Re: Space Shuttle - Development

Postby GinGin » Sun May 26, 2019 1:17 pm

Wow, Outstanding Thorsten.

Of course, I will give it thorough test.
If I understand well, to have exact J2/J3 effects for the target , you would need to enter datas for all the different inclinations?

For the test, what we can do is to have an accurate J 2/3 modeling of the target at a set inclination for a circular Orbit ( let's say 51.6 ° for ISS) and make test on that one.
I guess what is important is the relativity between chaser and target for Lambert targeting ( almost on the same orbit), to observe a same variation of the Orbit between chaser and target to have a constant relative delta height like in the NASA plot

Push what you think is the best ( and closest to the real physcical effects), I will test it anyway :)
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Re: Space Shuttle - Development

Postby Thorsten » Sun May 26, 2019 1:52 pm

If I understand well, to have exact J2/J3 effects for the target , you would need to enter datas for all the different inclinations?


Well, fire up LEO targeting, switch J3 and IERS Earth on, then plot altitude vs time of an orbit and try to construct a circular 60 deg inclination orbit - you'll understand the issue. :D
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Re: Space Shuttle - Development

Postby GinGin » Sun May 26, 2019 5:25 pm

Yep, I had that result couple of weeks ago for a circular orbit on equator and J2/IERS influence

Image

It is also the fluctuations I observed for the Shuttle with same parameters.

So if ISS is doing the same kind of non spherical gravity oscillation for a " circular" orbit now, very fine for me, relative height between between chaser and target will be in accordance with the same gravity source

That should really decrease the "strange" effect we saw during final part of rendez vous with the perfect gravity circular ISS orbit while the Shuttle was under J2/IERS influence :)
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Re: Space Shuttle - Development

Postby Thorsten » Mon May 27, 2019 7:47 am

Okay, I realized I made a rather stupid mistake in the analysis, so it's going to be a few more days...
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Re: Space Shuttle - Development

Postby GinGin » Mon May 27, 2019 3:38 pm

Alright, no worries :)

I was looking for a document showing Angle of Attack variation during ascent, or a lift/drag courb for ascent.
East to find for the Entry TAEM etc, but I didn't find anything for the Ascent part.

Do you have maybe something like that in your documentation ?
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Re: Space Shuttle - Development

Postby Thorsten » Tue May 28, 2019 10:08 am

I was looking for a document showing Angle of Attack variation during ascent, or a lift/drag courb for ascent.


AoA variation I have not found - experimentally I'd say it's completely determined by the pitch schedule that's flown - which during stage 1 is a canned profile.

Lift/drag during ascent might be in "Determination of the aerodynamic interference between the space shuttle orbiter, external tank, and solid rocket booster" (that's a rather huge collection of raw wind-tunnel data).

***

I've now pushed an update to the orbital target code to FGData (Nasal/orbital_target.nas) . That should contain a better simulation of the node drift with altitudes different from ISS altitude and also a better altitude variation corresponding to a J3 orbit.

I've briefly tested this by plotting out values and checking the graph against LEO Targeting, but no flight testing. So let me know. I'll try to come up with a mission file for HST soonish, then we can see who manages to fly there first :mrgreen:
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Re: Space Shuttle - Development

Postby GinGin » Tue May 28, 2019 5:21 pm

AoA variation I have not found - experimentally I'd say it's completely determined by the pitch schedule that's flown - which during stage 1 is a canned profile.


Argh, indeed.
It seems impossible to find something about it. Thouhg I found an interesting sum up of data of a direct insertion launch to ISS

https://spaceflightnow.com/shuttle/sts124/fdf/124ascentdata.html

Anyway thanks for the suggested documentations. By googling it, I found very useful links on my interrogations

I was looking to have more informations on the negative Angle of attack ( -2° ish) that was flown to have minimal wing bending, after they found out that the real vs expected q effects would blow the wing out

Image

I found some interesting links about that.

Image

Image



The links:

https://www.aiaa.org/docs/default-source/uploadedfiles/about-aiaa/history-and-heritage/why_the_wings_stay_on-ehrlich.pdf?sfvrsn=801c62b5_0

https://www.nasa.gov/centers/johnson/pdf/584730main_Wings-ch4d-pgs226-241.pdf

Seeing that wing bending moment is taken into account into FG and that you used accurate datas, I guess we would have some similar requirements with load alleviation in FG?
I am curious to know a bit more what is behind the code for wing bending moment, load alleviation etc :)

Ah, maybe you can help me with that question: Do you know the angle of incidence between the wing chord axis and X shuttle body axis? ( Impossible to find that one also)



I've now pushed an update to the orbital target code to FGData (Nasal/orbital_target.nas) . That should contain a better simulation of the node drift with altitudes different from ISS altitude and also a better altitude variation corresponding to a J3 orbit



Very nice, I will try that now, thanks


I'll try to come up with a mission file for HST soonish, then we can see who manages to fly there first



Excellent, I might have my chance here compared to the prox OPS challenge ahah
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Re: Space Shuttle - Development

Postby Thorsten » Tue May 28, 2019 5:42 pm

I am curious to know a bit more what is behind the code for wing bending moment, load alleviation etc


It's basically all based on the Ehrlich paper you quoted and the plot given there - the wing bending moment is computed as a function of AoA and Mach number and compared against the constraints.

Do you know the angle of incidence between the wing chord axis and X shuttle body axis?


Sorry, no.
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Re: Space Shuttle - Development

Postby GinGin » Tue May 28, 2019 6:27 pm

It's basically all based on the Ehrlich paper you quoted and the plot given there - the wing bending moment is computed as a function of AoA and Mach number and compared against the constraints.


Alright, nice. So really close to real ?
Better to stay close to zero AOA or slightly negative close to the max q and throttle bucket :)

Lately, I have a lot of fun flying the Shuttle manually for ascent ( I saw that it is sensitive in pitch and in AOA increasing close to limits)
I am following canned standard NASA profil for first stage of ascent , and for second stage. MECO and TTA are really closed to Real missions one.

I was wondering if there is a place in the auto launch.nas where to put a specific predefinned profile for first stage for auto guidance?


PS: On my way to ISS with the new Orbital target.nas :)
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Re: Space Shuttle - Development

Postby Thorsten » Tue May 28, 2019 6:54 pm

I was wondering if there is a place in the auto launch.nas where to put a specific predefinned profile for first stage for auto guidance?


auto_launch.nas operates based on phases (clear gantry - rotate to launch azimuth - pitch down and climb out - cross max qbar - pitch down further...) - you'd have to alter the targets during the different phases. To some degree they can be made configurable, but if different phases are desired, they'd have to be written.

Better to stay close to zero AOA or slightly negative close to the max q and throttle bucket


Yes, the safe region is actually pretty narrow, it's fairly easy to mess up the pitch schedule and die, I think eatdirt had the problem at some point that the launch scenario required re-designing the thrust bucket to throttle down earlier.
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Re: Space Shuttle - Development

Postby GinGin » Wed May 29, 2019 3:06 pm

auto_launch.nas operates based on phases (clear gantry - rotate to launch azimuth - pitch down and climb out - cross max qbar - pitch down further...) - you'd have to alter the targets during the different phases. To some degree they can be made configurable, but if different phases are desired, they'd have to be written.



Nice, very well written and clear. I am playing with it to have a more pronounced bend during first stage.

I am struggling to find how to limit the rate of pitch used by AP ( I want 0.5 °/s max to avoid goind above 1 or 2 ° of AOA)
When Flying Manually, I don't exceed that value, and AOA stays negative for almost the whole stage ( smoother than AP, around 2 °/s with sometimes instantaneous AOA of 8 ° or more and crack )

So far, I managed to decrease TMECO from 9:30 to 8:40 ish to be closer to real just by having a less vertical first stage :)





Yes, the safe region is actually pretty narrow, it's fairly easy to mess up the pitch schedule and die


Really educative to see it by flying manually. Always something to learn in that sim, entertaining !
It was like flying in a head needle


A couple of questions I had in my head since a long time:
Is it possible to have at game start MET instead of GMT dispayed on CRT 1 through 3?
Is it possible to change somewhere the thrust pourcentage for ascent ( most of missions had a 104 % profile)

I saw something nice :)

Image
Alright, back to the phasing test :)


I am doing it with the old mission scenario ( Inclination 42 °, 2 degrees behind and 10 Nm below, is it ok ?)
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