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Re: Space Shuttle

Postby MIG29pilot » Thu Jul 02, 2015 5:53 pm

How difficult would it be to model the ISS? Something like a space-aircraft carrier scenario.
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Re: Space Shuttle

Postby legoboyvdlp » Thu Jul 02, 2015 8:29 pm

On reentry at an average of 42 AOA, never below 40, I get thermal protection failure at about 2000F and I pitch up to like 40 degrees per second and spin out of control, what did I do wrong this time? I have never succesfully made orbit yet!
Also, when starting at sah KTTS is he arpt, and using -entry version, the computer cant find a deorbit solution...
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Re: Space Shuttle

Postby Thorsten » Fri Jul 03, 2015 5:35 am

Also, when starting at sah KTTS is he arpt, and using -entry version, the computer cant find a deorbit solution...


How's it going to accomplish that? If you init that way you're 120 km directly above the landing site and moving with Mach 26 - do you plan to stop instantaneously and drop down vertically? :-) Take a look at diagrams in the web where the entry interface for a landing is - it's some 5000 miles away from the landing site, out over the pacific ocean.

On reentry at an average of 42 AOA, never below 40, I get thermal protection failure at about 2000F and I pitch up to like 40 degrees per second and spin out of control, what did I do wrong this time?


My educated guess is that you left the ET umbilical door open, hence creating a huge gap in the heat shield through which plasma enters and melts the aft fuselage. Once that's gone, well, you don't have that much control any more.

How difficult would it be to model the ISS? Something like a space-aircraft carrier scenario.


That about is the math you have to encode, and then you have to get it numerically stable.

(You have two objects moving with 8 km/s alongside such that their relative motion is close to zero, but our coordinate system is not the inertial system in which they're moving but co-rotates with Earth since our infrastructure is for a flightsim where we don't need non-rotating inertial reference frames. The rotation is (dependent on latitude) a bit faster than speed of sound, so you need to get all the numbers to some precision and without numerical jitter if you aim at cm/s docking velocities).

Doing a 3d model of the ISS should be easy by comparison.
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Re: Space Shuttle

Postby wlbragg » Fri Jul 03, 2015 5:55 am

Image
We already have the model sitting stationary above KEQA in KS. It is even roughly close to the correct size. :)
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Re: Space Shuttle

Postby Johan G » Fri Jul 03, 2015 9:13 am

Is it possible to have the space station more stable in relation to the shuttle by adding it either as a submodel or as an animated object that is "part" of the Space Shuttle?
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Re: Space Shuttle

Postby Thorsten » Fri Jul 03, 2015 10:29 am

Submodels are out, the code doesn't handle orbital dynamics.

Parts of the model is tricky - you have to move the part to match every translational and rotational movement of the Shuttle - but keeping track of nested translations and rotations is not trivial.
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Re: Space Shuttle

Postby Hooray » Fri Jul 03, 2015 11:14 am

Thorsten wrote in Fri Jul 03, 2015 5:35 am:That about is the math you have to encode, and then you have to get it numerically stable.

(You have two objects moving with 8 km/s alongside such that their relative motion is close to zero, but our coordinate system is not the inertial system in which they're moving but co-rotates with Earth since our infrastructure is for a flightsim where we don't need non-rotating inertial reference frames. The rotation is (dependent on latitude) a bit faster than speed of sound, so you need to get all the numbers to some precision and without numerical jitter if you aim at cm/s docking velocities).

Doing a 3d model of the ISS should be easy by comparison.

In general, this is touching on https://en.wikipedia.org/wiki/Arbitrary ... arithmetic and "bignum" - neither of which is currently supported by any existing C++ SG/FG code, because a flight simulator has very different constraints compared to a space simulator.

With Nasal, all numbers are stored internally as IEEE double-precision values, but the GC would make update rates non-deterministic unfortunately.

Also, see MAKG's comments in a related thread (not about spaceflight obviously):

Subject: Coordinate Systems and Reference Positions
MAKG wrote:While its true that floating point precision is not infinite, IEEE double precision ought to be completely adequate for guiding an aircraft around the world. I see folks overestimating this effect on a weekly basis.

15 digits (decimal) is quite a lot. If the transformations lose significant precision, that's a fault in the transformations.

Note that I've seen loss of precision occur when using unit vector/rotation matrix and quaternion formalisms, when the normalization is not properly maintained. This is a common error, but it's fixed by enforcing unitarity at every step that might change it (this removes a systematic error and replaces it with a random one, far better).


So using XML property rules (mapped to C++ data structures and algorithms) or the corresponding FDM equivalents would probably be better. Otherwise, it would be easier to extend the underlying C++ code accordingly (property rules, submodules or FDM). Unless of course someone wants to come up with a GPGPU solution using pure GLSL... or diretly link in Fortran code as a dedicated SGSubsystem. :lol:
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Re: Space Shuttle

Postby MIG29pilot » Fri Jul 03, 2015 11:40 am

Johan G wrote in Fri Jul 03, 2015 9:13 am:Is it possible to have the space station more stable in relation to the shuttle by adding it either as a submodel or as an animated object that is "part" of the Space Shuttle?

Just do it like a carrier scenario, with a certain speed and altitude.That can be the rudimentary basis for a more complete system latyer on, anyway.
There's one thing, there will need to be some sort of docking arrangement on the shuttle (I am speaking about the code, not the model) How does the Shuttle offload onto the ISS?
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Re: Space Shuttle

Postby Thorsten » Fri Jul 03, 2015 2:30 pm

Just do it like a carrier scenario, with a certain speed and altitude.


Right.

So how's the Shuttle (which is constrained by pretty exact orbital dynamics) supposed to reach it then? The Shuttle can't even compensate a velocity error of 20 m/s in orbit - because if you add 20 m/s, you're going up into a new, higher orbit.

Nothing is actually floating up there - everything is finely balanced between gravitational and centrifugal force. Things also don't follow a compass heading in space, they follow geodesics.

In general, this is touching on https://en.wikipedia.org/wiki/Arbitrary ... arithmetic and "bignum"


No, it's not. The precision of the variables is not the issue, the issue is reaching the necessary precision and long-term stability in the computation. Even floating point precision is good enough to resolve the world on a ~10 m scale (we know that from applying shaders in world coords).

The algorithm needs to be stable against perturbations and self-correcting. Whether it's done in Nasal, xml or C++ is rather irrelevant.
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Re: Space Shuttle

Postby legoboyvdlp » Fri Jul 03, 2015 3:35 pm

Thorsten, when I put my plane in the reentry scenario, I expect it to be those 5000 milesbout. I dont expect it to be above the landing site. So...
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Re: Space Shuttle

Postby MIG29pilot » Fri Jul 03, 2015 4:46 pm

Thorsten wrote in Fri Jul 03, 2015 2:30 pm:So how's the Shuttle (which is constrained by pretty exact orbital dynamics) supposed to reach it then? The Shuttle can't even compensate a velocity error of 20 m/s in orbit - because if you add 20 m/s, you're going up into a new, higher orbit.


Oh, so in real life, the ISS is actively changing relative position to the shuttle to help get to it?
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Re: Space Shuttle

Postby Hooray » Fri Jul 03, 2015 5:20 pm

Whether it's done in Nasal, xml or C++ is rather irrelevant.


Well, I don't know - but I guess that most people around here (without your kind of hardware), will probably not enjoy/appreciate having to do any Nasal-based docking maneuvers at 8 km/sec given typical Nasal GC induced lag - even if you say, that variable precision is not going to be an issue (which seems in line with MAKG's comments quoted above).
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Re: Space Shuttle

Postby Thorsten » Fri Jul 03, 2015 5:42 pm

Well, I don't know - but I guess that most people around here (without your kind of hardware), will probably not enjoy/appreciate having to do any Nasal-based docking maneuvers at 8 km/sec given typical Nasal GC induced lag - even if you say, that variable precision is not going to be an issue (which seems in line with MAKG's comments quoted above).


Nasal delivers updates at framerate, so does xml. Assuming the algorithm delivers the precision, the framerate or regularity of frames is rather irrelevant, the relative motion is 10 cm/ second - I can do this with less than five frames per second easily.

I'm guessing you're not really seeing where the issue is and where it isn't.

Oh, so in real life, the ISS is actively changing relative position to the shuttle to help get to it?


No, in real life both ISS and the Shuttle follow the same laws of orbital mechanics. Which is what you need to do in-sim as well to make any docking possible.

Thorsten, when I put my plane in the reentry scenario, I expect it to be those 5000 milesbout. I dont expect it to be above the landing site.


Um... why?

Usually if you start FG with --airport=XXXX, you expect to be somewhere in the vicinity of XXX, no? Surely being 5000 miles away would not count as even remotely close, and I dare say it'd be counterintuitive that if you specify and airport you're half a world away from it upon init.
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Re: Space Shuttle

Postby legoboyvdlp » Fri Jul 03, 2015 5:54 pm

You see, when one selects entry at airport eg kedw, one would expect the plane to start at the beginning of the entry for KEDW, at least I would...
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Re: Space Shuttle

Postby MIG29pilot » Fri Jul 03, 2015 5:55 pm

Thorsten wrote in Fri Jul 03, 2015 5:42 pm:Usually if you start FG with --airport=XXXX, you expect to be somewhere in the vicinity of XXX, no? Surely being 5000 miles away would not count as even remotely close, and I dare say it'd be counterintuitive that if you specify and airport you're half a world away from it upon init.


@Thorsten, I think what he means is that when he loads up the re-entry scenario, he expects to be on an approach run to the selected airport on init, rather than right over the airport. It's rather counter-intuitive to have an re-entry scenario that puts you bang over your runway instead of ina good position for re-entry and landing at the selected airport.
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