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Adventures in Space

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Adventures in Space

Postby Thorsten » Wed Dec 11, 2013 12:52 pm

Since I'm traveling (again) for the last weeks, I find myself with long evenings in hotel rooms, and I've decided that this time I will use the opportunity to do something I've wanted to do a while - fly a simulated mission with the Space Shuttle manually (i.e. no autopilot, advanced computational tools, just basic orbital mechanics indicators and such...). The Shuttle is a very sedate spacecraft, it turns slowly, it doesn't have too much propellant to do orbital adjustments quickly, and once the SRBs and the tank are gone, the remaining thrust doesn't get you anywhere in a hurry - so this really needs time - I think I spent a grand total 8 hours real time flying (using time fast-forward) doing this.

Tool of simulation is Orbiter 2010, I'll later comment on my impressions on Orbiter vs. FG.


Image

Me blasting off into space - this is the part when the Shuttle has lots of thrust. However, there's not much reserve, it carries one into an orbit, but not further, so the key part is altitude and orbital plane management, which basically starts right after clearing the atmosphere at 70 km. So I precalculated the launch azimuth for ISS manually. Orbiter out of the box doesn't have any visual cue in launch position where the compass directions are, you have to fly horizontally to get good readings, so I needed to download a launch compass tool to get the liftoff roughly correct. Flying the Vostok from FG which comes with the 150 km altitude restriction has taught me pretty much all there is to know about altitude management during launch, so the only challenge was to use an orbital plane intersection indicator and vector thrust during the ramp-up to orbital velocity such that the planes of ISS and STS-101 are really aligned, while keeping an eye on the thrust figure (it shouldn't get above 3g) and vertical speed. I launched with a 15 deg mismatch and managed to get this down to 1.5 deg by the time I had an apogee of 370 km and had to cut thrust and jettison the tank.



Image

Once up, the first task on the way to the apogee was to get the satellite out of the cargo bay. There is a manipulator arm simulated which moves with glacial speeds, so you can watch as it tranquilly unfolds, grapple the satellite and move it out of the cargo bay.

I had launched into a fairly good orbit already with 240 km perigee and 370 km apogee, and since I was trailing ISS by about 1000 km, I decided to leave it that way since that would make me catch up with the station about 5 orbits later, just made the remaining orbital plane adjustment to a deviation of just 0.4 deg and then took a spacewalk. There's an MMU module which allows you to float out and inspect the cargo bay and the satellite - but it's pretty scary, you can't cheat, you actually have to maneuver back to the airlock if you want to go back in! So, inspected the satellite using the MMU, then released the satellite and waited to catch up with ISS, enjoying the view.

Image

That's the workplace, the 3d cockpit of Atlantis. I think the instrumentation is fairly Orbiter-generic, but the layout has the look and feel of the real thing. Especially the visibility is pretty lousy, pretty views from inside to Earth are surprisingly rare if you don't rotate the shuttle to get it. It'd be pretty cool to have the original instrumentation of Atlantis though!

Image

The orbit matching procedure got me after prograde burn to 45 km distance to the ISS, so I picked up the transponder signal from there and 'just' flew over. Of course there's orbital drift, even on the scale of 45 km, and you shouldn't bump into it, not fry it with thrusters so... it took me the better part of 30 minutes to get to a distance of 70 m, by which I then spotted a docking port to hook my spaceship onto.

Then the real bitch started. The docking collar of the Atlantis is in the cargo bay, i.e. you don't have visuals while you fly, it's not aligned with any of the ship's main axes, i.e. every time you rotate to correct attitude the position goes out of alignment, and in any case you have to think in a weird coordinate system and the tolerance is just 10 cm or so. I spent a whole 45 minutes just easing the shuttle cm by cm closer to the airlock (I remember starting just at sunset, and seeing the sun coming over the horizon just when I had made it). I guess that might be pretty realistic though... at least the basic physics is simple enough.

Image

Now the fun part started - when ISS was moving over the Cape, I undocked and did the de-orbit burn. Turns out there's no procedure or checklist published with the shuttle, nor is there a unique opinion on how it should be done in the Orbiter community. Either people use some aerobrake addon module, or cooking recipies (the original procedure NASA uses apparently does not work because the FDM doesn't reproduce reality well enough) or just gut feeling. So I had to calculate a few numbers, particular deceleration figures I need based on current velocity and distance and just made myself a table.

Anyway, de-orbit burn has to start about 12.500 km before the destination, it lasts well till 9.500 km as the thrust is pretty weak, then the Shuttle hits the lower layers of the atmosphere about 2000 km from the landing site which is doable within
structural limits. The picture is taken just when I hit the atmosphere (I was too busy afterwards).

I was always under the impression that the re-entry track is more or less set when you hit the atmosphere, but no - the Atlantis can actually FLY during re-entry. This is incredibly exciting and fun once I had worked out what I need to focus on. Basically the elevator has enough action to control altitude, which in turn controls air density and deceleration force. So you can keep it high up and decelerate with just 0.5 g, or push it down and get a solid 2 g. In addition, below about 4000 m/s, it starts to turn if you roll it to a high bank angle - very slowly, but one can start to adjust the course to base. Or do roll reversals to prolong the track and get additional deceleration. All while red-hot plasma is shooting across the windows and the Earth, seen from 50 km altitude at Mach 20, rushes by.

The last stage comes quickly - once at 1000 m/s it drops rapidly, so I had to manage the entry trajectory and deceleration force to dump me right over the Cape with that velocity - then it falls down like a rock, and if that isn't fast enough there are speedbrakes... but fundamentally it's a glider, a bad one, but still a glider, and nothing anyone who has ever flown gliders in FG couldn't manage easily.

Image

So here we go - STS-101 is back on Earth. And I am a fairly proud shuttle pilot. I gather flying the shuttle is generally regarded as difficult as compared to the fictional spacecraft Orbiter has to offer. And note how much better our terrain is from close-up :-)

Here's for direct comparison the visuals I was able to get with FG and the Vostok-1 (I think from a similar altitude, about 300 km):

Image

Image

As you can see, EarthView isn't really good enough to get the fogging in the atmosphere well, but otherwise the terrain impression is fairly competitive. I wonder how OSGEarth will do - supposedly it can show the whole world, but probably a dedicated planet fogging shader is needed.

So, poking around in the Orbiter community, I can say I understand now much better how good the FG approach is. Having a central repository with the aim of including all scenery addons, planes and other useful work is really a godsend. Orbiter has gazillions of addon modules which all aim at doing similar things, lots of work is simply done twice (thrice,...), some modules are not mutually compatible, different people recommend different modules for the same problem. There is no real documentation on the fine print of how to fly these things - the Vostok-1 in FG comes with all cue sheets, numbers, instruments and instructions you need to fly a complete mission, the Shuttle in Orbiter does most decidedly not. Scenery in FG largely comes from a single source, Orbiter has a host of mutually incompatible planet texture/feature addons. Orbiter has an addon to run via DirectX rendering - lots of other addons require that,... So if anyone feels GIT and the FG central scenery repository isn't working - the alternative is much, much worse!

In terms of handling - Orbiter is good where it's easy - up in space. The atmospheric flight dynamics didn't impress me much. Comparing hover thrusters of the Delta Glider with a thrust vectoring plane like the Harrier, I have to say that the Harrier has the instabilities I'd expect, the DG has not. The shuttle should actually perform the hottest part of the entry with the nose 38 deg above horizon - if you try that in Orbiter, it just bounces back into space. The FDMs are more or less the subsonic dynamics extrapolated to hypersonic velocities.

I really wish there'd be a JSBSim shuttle for FG - JSBSim would rock for flying re-entries! Generally (and perhaps not surprisingly) the flight dynamics of FG blows what I have seen of atmospheric flight in Orbiter out of the water. Well, we are a flight simulation. And I had the impression that Orbiter has more the aim of being educational than realistic - see the generic instrumentation for instance. Anyway - one day I would love to do this in Flightgear!
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Re: Adventures in Space

Postby Gijs » Wed Dec 11, 2013 1:43 pm

I really wish there'd be a JSBSim shuttle for FG

I actually started on one a looong time ago (using NASA's 3D model, see viewtopic.php?p=32000#p32000 ). Coincidence or not, last week I finally managed to do a sucesfull launch, but the (very basic) control and guidance system isn't able to keep her from pitching over after T+100 secs or so. I'll have another look tonight, and see if I can set up a Git repo so you (and others) can have a look.


There's been a shuttle glider FDM in fgdata for years btw (without 3d model), but I don't know how accurate that one is.
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Re: Adventures in Space

Postby Hooray » Wed Dec 11, 2013 2:06 pm

nice writeup

PS: I think Thorsten knows about the jsbsim shuttle:

http://www.mail-archive.com/flightgear- ... 36640.html
jonsberndt wrote:Our space shuttle orbiter model (from many years ago) is incomplete, and
based on limited data from publicly available tech reports. I do have a
"J246" model that is a launch vehicle designed to place heavy payloads into
orbit. That is incomplete, and awaiting some simple guidance for the upper
stage to try and get it into orbit from the launch pad. A couple of years
ago Robin Gerard made a 3D model of that, as well as the pad. It will be
nice to see that fly someday.


Regarding the history of the shuttle FDM in JSBSim, see: http://sourceforge.net/p/jsbsim/mailman ... ?q=shuttle
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Re: Adventures in Space

Postby someguy » Wed Dec 11, 2013 4:38 pm

"Open the pod bay door please, HAL."

Thorsten, you're a madman. In a good way.
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Re: Adventures in Space

Postby Buckaroo » Wed Dec 11, 2013 7:18 pm

I came to Flightgear from the Orbiter world. Spent a lot of time with the Delta Glider, the cargo hauler, and the little cargo manipulator vessel, I think it was called the Dragonfly. It taught me a lot. I was interested in the Apollo Project work, but back in 2006 when I was heavy into Orbiter, those efforts were still young.

Thorsten's experience of documentation and add-ons mirrors mine. Add-ons were scattered projects with little or no coordination between them and lots of replicated effort. I never got involved in tinkering with Orbiter. At the time, there was not much support or encouragement for getting your hands dirty with development. The project didn't discourage it, but I don't recall much of a centralized community for development support. In that sense, there was a huge difference between Orbiter and Flightgear.

It was through a forum discussion of Orbiter that I learned of Flightgear. And to think, back then I thought /Orbiter/ was a blackhole for personal time. Oh how naive I was...

someguy wrote in Wed Dec 11, 2013 4:38 pm:"Open the pod bay door please, HAL."


Actually, you could experience that very thing, or close to it. There used to be an add-on package called "World of 2001", or something like that. I've still got it somewhere. It had the 2001 shuttle, the space station, the moon base, the moon bus, Discovery, pods, pretty much everything. Not bad for the time, I enjoyed messing with it. I wonder if it's still in development.

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Re: Adventures in Space

Postby Mr.Snappy » Wed Dec 11, 2013 8:19 pm

I've wanted to get Orbiter for a long time, but had some problems with it (Wine couldn't handle it). After this topic however, I decided to try again and it works now. :D
I'm writing this from the Atlantis shuttle at 540 km and love it. So thanks to Thorsten for giving the idea. :)
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Re: Adventures in Space

Postby Bjoern » Thu Dec 12, 2013 4:24 pm

Great, now I want to reinstall Kerbal Space Program and do some space stuff, lol


But just like Orbiter, KSP is suffering from mod overload. You can find them all in the official forums though, but they're all so cool that you just don't know where to start.
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