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Re: Space Shuttle

Postby Thorsten » Sat Apr 23, 2016 6:07 am

We're actually doing a fraction of what we should do for a switch - for instance for most switches we assume they're in working order and that their bus is powered. I've implemented checking for a switch failure for those which have a software override function - but in principle, we shouldn't animate them then, so switch position for the animation needs to be distinct from switch command to the systems model :-)

I think the engines actually go through more or less the full list: Is the controller powered, is the controller switched on, is there helium pressure, is there a valid command path to the controller, is there fuel, is there oxidizer, is the engine itself okay, is the engine gimbaling okay, is there hydraulic pressure for gimbaling?

either by design or luck Thorsten has managed to keep it manageable


If you think about it, JSBSim forces you into a very reasonable design. Consider a typical channel - some input properties are mapped via intermediate chain links to output properties. You can't set either intermediate or output properties by hand or from Nasal because they're overwritten 120 times per frame - so you naturally get a compartmentalization where you only set the input from high-level routines and read out the rest.

The fact that we organize it in channels likewise encourages a natural separation, so we can distinguish software issues (what's the command issues to a thruster) from hardware issues (what's the response of the physical thruster when it gets the command) in a relatively clean way.

So I'd say it's largely to Jon's credit that it's manageable. The problem with my design is that I only know after the fact how I should have done it properly. I'm relatively happy with a few elements, the antenna manager or the CWS or limit check loops, but much else isn't quite as nice as I would have hoped.
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Re: Space Shuttle

Postby Thorsten » Sat Apr 23, 2016 10:29 am

This is seriously weird... two engines out, flying with the wraparound DAP in which the RCS takes over roll control and the last engine TVC provides pitch and yaw.

Image

Feels a bit shaky and isn't quite crisp, but definitely flyable.
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Re: Space Shuttle

Postby wlbragg » Sat Apr 23, 2016 7:53 pm

I was testing the Man Engine cutoff switches shortly after launch and shutdown left and center. I wasn't really paying attention to the data and was bouncing view modes back and forth. I didn't realize I was becoming critical with drag and probably not enough escape velocity, I had a catastrophic event. :cry:

Something worth noting, I was having an extremely hard time getting my throttle from 65% to 90% in time to fire the SRB's. I couldn't do it from my custom scenery, After changing launch locations to the Cape I finally, after several attempts, got the throttle moved from 60% to 90% in the allotted time. Is there another way to trigger this without the need to climb the throttle or maybe even a realism setting that can be reduced? This was caused due to system frame rate or some kind of system rate compromise on my poor, burdened system.
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Re: Space Shuttle

Postby Thorsten » Sun Apr 24, 2016 5:18 am

I was testing the Man Engine cutoff switches shortly after launch and shutdown left and center.


Yikes - that's actually a contingency abort scenario - just try to survive as the Shuttle falls back into the atmosphere and bail out is the goal... I don't think you have even TAL capability with a single engine until over halfway into the flight.

If you experiment, remember you can throttle to 109% by changing the mapping on SPEC 51 - the 9% more thrust will come in handy...

Something worth noting, I was having an extremely hard time getting my throttle from 65% to 90% in time to fire the SRB's.


Keybinding for Ctrl+i (ignition) setting all three engines to full perhaps? It'd be handy to have a Nasal function for ignition anyway, so we can eventually do it automatically after a countdown.
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Re: Space Shuttle

Postby Thorsten » Sun Apr 24, 2016 2:40 pm

After a few tests of the SRB separation button to simulate premature separation - I don't know how to kill thrust of a solid rocket motor in JSBSim. They can't be throttled obviously, trying to set thrust-lbs directly doesn't help either, the rocket engine definition doesn't look like it takes an additional function,... Actually I don't even want to kill the thrust computation (which might still be passed to the ballistic submodel) but the thrust that acts on the Shuttle.

Maybe I need to hack this by introducing an external force just countering the SRB... though that's ugly...
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Re: Space Shuttle

Postby wkitty42 » Sun Apr 24, 2016 2:50 pm

if the SRBs separate early, they still have to finish their burn, don't they? or is this a problem with once they are separated they're like an AI craft and do whatever is coded for them?
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Re: Space Shuttle

Postby Thorsten » Sun Apr 24, 2016 3:03 pm

As I said, they're ballistic submodels after sep. And they do whatever you tell them to. At this point, my concern is telling the FDM that while the rocket engine still produces thrust, it has no attachment point to the Shuttle to the effective thrust for the FDM center point is zero.
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Re: Space Shuttle

Postby wkitty42 » Sun Apr 24, 2016 4:21 pm

ahhh.. sub models is what i was trying to think of... you couldn't switch their FDM to use their CoG compensating for their loss of propellant? i'm guessing that when they are released, any thrust they would have contributed to the shuttle assembly is nulled or zeroed out... it is kinda hard to visualize as an outside observer but something the thoughts of an observer can be helpful in switching thought tracks and looking at something from a different aspect :)
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Re: Space Shuttle

Postby Thorsten » Sun Apr 24, 2016 5:04 pm

you couldn't switch their FDM to use their CoG compensating for their loss of propellant?


The ballistic submodel 'FDM' doesn't affect the Shuttle in the first place - it's fed from the Shuttle's FDM. The problem is nulling the thrust of the SRBs in the Shuttle FDM - because JSBSim doesn't have throttled solids.

It all makes sense the way it's implemented, it just didn't take into account that someone would want to release the SRBs prematurely, in which case we separate 1 FDM into 3 separate ones. That's the real issue here.
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Re: Space Shuttle

Postby Hooray » Sun Apr 24, 2016 5:09 pm

It all makes sense the way it's implemented, it just didn't take into account that someone would want to release the SRBs prematurely, in which case we separate 1 FDM into 3 separate ones. That's the real issue here.

I don't know what you are talking about, or if you are aware of this, but just for the sake of completeness: at some point there used to be support for some kind of child-FDM mode in JSBSim for similar purposes: http://wiki.flightgear.org/Implementing ... Child_FDMs

Presumably, Erik or Jon should know more about whether this may be useful, or just a red herring given the requirements here.
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Re: Space Shuttle

Postby Thorsten » Mon Apr 25, 2016 5:21 pm

Instrumentation for the RTLS abort is slowly coming along. That's somewhat tricky, because OPS 6 is sort of a branch to the otherwise neat sequential ordering of the modes - yet it partially relies on the same bits that are used for a nominal mission - so as of now I still managed to 'get stuck' (enter a mode I couldn't leave) in the avionics.

And I'd not recommend anyone to actually *use* RTLS TRAJ 2 at the moment - it will guide you *somewhere*, but I don't think it's where you want to be..
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Re: Space Shuttle

Postby Thorsten » Tue Apr 26, 2016 10:25 am

For anyone who is sufficiently experienced with launch and entry by now, here's a tutorial how to fly a transatlantic abort landing (TAL).

It's largely possible to do this under auto guidance, but you'll be working under time constraints, so you need to know how to operate the systems in a hurry.

Anyone who manages to fly from Kennedy to Banjul successfully, please let me know :-)
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Re: Space Shuttle

Postby erik » Tue Apr 26, 2016 11:37 am

Hooray wrote in Sun Apr 24, 2016 5:09 pm:Presumably, Erik or Jon should know more about whether this may be useful, or just a red herring given the requirements here.

I think Jon toyed with the idea for a while but it never fully matured.

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Re: Space Shuttle

Postby amalahama » Tue Apr 26, 2016 2:58 pm

Thorsten wrote in Tue Apr 26, 2016 10:25 am:It's largely possible to do this under auto guidance


Roll reversals included? :)

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Re: Space Shuttle

Postby Thorsten » Tue Apr 26, 2016 3:08 pm

Powered phase only, the entry is 'as is'.

The tutorial also isn't 100% faithful to the manual (for instance the fuel dump is split into two phases there) - but the avionics supports also the more complicated procedure.

RTLS is still a bitch... I don't see that one under auto...
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