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Author Topic: PrObLeMs with DGIII reentry and landing  (Read 15313 times)

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Offline NASSAC

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18 July 2004, 02:02:06
We have just completed our 2nd NASSAC 30 hour real time mission. The Launch and docking went exactly as planned,
but the EVAs encountered a problem when the second joystick in the docking moduled kept sticking or refused to
work at all. But the hardware problems were easy to work around compared to the software bugs in the reentry and
landing.
  We used the DGIII last year and found the flight characteristics of the new DGIII to be somewhat  problematic.

1. The rentry program p104s40 when activated appears to use too much RCS fuel. It apears to over correct and is in
a state of constent fuel usage. On some test runs we ran out of RCS fuel before we could land. This was
compensated for by delaying the execution until 115KM altitude or latter.

2. It appears to us that any attempt to bank the DGIII to North or South in an attempt to align the appraoch angle
meats with disaster. Unlike the shuttle the DGIII does not appear to tolerate this.

3. When switching to the lower panel to transfer some fuel to RCS mode or for any reason the renetry auto pilot
looses control and the DGIII dips down, increasing hull temperatures. This bug cost us the simulated lives of our five
astronauts. We owe it to them to get these problems fixed before attempting another mission with the DGIII.  

4. The DGIII doesn't glide. Even with the fuel levels below 50% the DGIII slows down fairly fast even when in a 30
degree decent. We find the drag effect to be unrealistic for "glider".

5. When approaching the runway for a runway landing the DGIII seems to loose too much lift when the landing gear
are deployed. If you increase your speed to above 160 knots then your landing is too fast and the gear collapses.
The gear collapse too easily in our opinion. The question we have is how much a factor are the fuel levels at landing?
Should landings be at 20% 30 % or less levels? And will this help?  

:)

John


NASSAC DIRECTOR
John

Offline Arkalius

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Reply #1 - 18 July 2004, 04:10:03
1. You actually ran out of RCS fuel? It is true that the reentry autopilot "cheats" and actually puts out more force with
the RCS thrusters than you could do manually, but even still, you should not run out of RCS fuel unless you were
already low. I usually just hold a 40 degree AOA manually until I dip below 100km, at which point I engage the
autopilot.

2. This is definitely true. The DGIII experiences a severe pitch-down moment at around 25 degrees AOA (basically, if
you get around 25 degrees AOA, there's an extremely strong force pushing the nose down). Usually during these
banking maneuvers, the DGIII will get dangerously close to that AOA and has a very hard time recovering.

3. That you are running out of RCS fuel in the first place is unusual, however this is an obvious bug that I'm sure Dan
will get fixed.

4. What reentry angle are you using? Use the deorbit display on the DGIII computer (press d2 to get there) when
doing your deorbit burn. I find a reentry angle of 0.70 works best if you use a 40 degree AOA descent.

5. Gear will collapse if you hit the ground too fast, not if you are moving too fast when you land. Just make sure your
vertical velocity is greater than -5 m/s or so if you can manage it before landing. Try some practice landings to get the
hang of it.

Hope this helps.


-Arkalius

Offline NASSAC

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Reply #2 - 18 July 2004, 05:21:27
Hello Arkalius,

Thanks for the suggestions. I have run many landing scenarios and I have been able to set the DGIII down without
collapsing the gear but it is very tricky with the loss of lift when you lower the gear. The nose just does not want to
come up for the flare out. A slow and long nearly straight in approach seems best but  has a very unstable feel to it.  
I just think that the flight dynamics need some attention is all. As far as the AOA is concerned I am using a 40 degree
angle of attack with a 5 to 10 degree variation depending on how long or short I may be of the target runway. What
MFD do you use to measure the decent angle per your .70 recommendation? Also what would you suggest for the
distance from the taget base to begin your decent. We currently do a retro burn at 16.4M from our target base
and burn untill our periapsis is  6.400. :)

John

NASSAC DIRECTOR
John

Offline NASSAC

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Reply #3 - 18 July 2004, 05:34:01
I just found this on the Orbiter forum. The following contains some very important detailed instructions that i will
attempt to follow and will get back with the results of the test. I think I may know why I am having some problems
with the DGIII.  John

Author: Pierre_le  <-Msg      15 Posts  Status: Young recruit  Date    07-15-04 04:43

i just manage to make a quite perfect transfer from mir to ksc using this procedure. i do not pretend that is the
ultimate perfect reentry but it worked pretty well for me. feel free to correct my parameters

BaseSyncMFD is required http://koti.mbnet.fi/jarmonik/Orbiter.html

- load "DGIII docked to mir" scenario
- undock, close kone and activate gear oil pressure
- low your alt. to 140km (ped=6510) and circularize it
- use BaseSyncMFD. burn +/- orbit normal to intersect ksc. i suggest to predict sync for the next orbit, not the actual
one
- eventually re-correct orbit to keep alt=140 km ecc=null
- when you are on final approach, head retrograde and do your deorbit burn 11600km away from target to
ped=6380. it's the most sensible point.
- prograde, hlevel. prepare cpu autopilot for reentry pro104spec40, display 3
- eva some crew members
- wait for atmosphere entry. @ 80km alt. engage autopilot (key E).use temporaly 50 AoA to get -100 m.s-1 vspeed,
and then stick to 40. keypad 2 & 8 to correct pitch
- keep eye on vertical speed (surfaceMFD on!) and keep it close to -100 m.s-1. normally you just got to let AoA = 40
and wait. hit F1 and watch flame while going down
- you should not have to care about ship external temp. if alarm is fired it means you fall to fast
- eventually correct base intersect point during descent by banking left or right (keypad 4 & 5)
- when the nose pitch and speed goes lower, disengage autopilot (space key). you must be about 35km alt and ksc
just in front of you. at this point flight using aerodinamyc control, not rcs
- do a nice landing using a -25 descent slope, aim the start of the runway. airbrake to keep ~160m.s-1 airspeed
- @ alt.="three hundred" meters, pull da stick, gear out and touchdown smoothly with a maximal vspeed of -5 m.s-1.

NASSAC DIRECTOR
John

Offline DocHoliday

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Reply #4 - 18 July 2004, 12:00:00
Yup, Pierre described the same procedure in this forum as well..

Recommend you get fully aquainted with the DGIII's instrumentation, specifically the centra MFD which is unique to
the DGII/III. The flight computer supports different displays to be used in different situations. Pressing "D" key and
then a number from 1-8 displays them (check by pressing d1 to d8 to see their functions). When you set your deorbit
burn you can use display 2, which will help you with the periapsis and reentry angle. When you begin the
atmospheric interface, use Display 1 to monitor your descent autopilot and display 3 to monitor your hull
temperature. Check the DGIII FAQ on this forum for various data regarding temperature maximums and dynamic
pressure maximum as well as gear collapse maximum (like AH noted).

Display 3 also notes the current set pitch angle for the descen autopilot, so you need not switch to Display 1.

After Dan fixed the temp bugs, the reentry has become much more easily attainable, so make sure you have the LAST
release of the DGIII. Check here: http://orbiter.dansteph.com/index.php?disp=dgIII

Good luck, pilot :)


~~~

"Mood is a matter of choice. I choose to have fun!" -Vidmarism No 15

Offline AphelionHellion

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Reply #5 - 18 July 2004, 12:02:11
I think that pitch down is "not a bug, it's a feature" :)
At AOAs higher than about 20 degrees, you're essentially falling - in a stall and not producing lift with the wings. This is
great for decelerating, but at below about 25 degrees the DG will "snap" into controlled aerodynamic flight.
I guess if the atmosphere is thin enough at your altitude your nose can keep dropping past 0 degrees and head into
the negative numbers before aerodynamic forces swing it back up.

I second the "It's not really a glider per se" sentiment, though! Heck with 30 percent fuel, I just dump all of it
when I get near the runway, and still the DG refuses to keep any airspeed unless pitched down a lot.
Heck, the Shuttle performs better, and it doesn't have the big blended wing body that the DG has.
I don't really thing the problem is the DG itself but Orbiter, though. It's a space simulator first and a flight simulator
second... or is it fourteenth?  :)  The flight model seems quite a bit simpler than what you'd find in a flight sim.
All due respect to Dr. Schweiger and the Orbiter dev deities, of course :)


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Offline DocHoliday

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Reply #6 - 18 July 2004, 12:09:25
Perhaps it has something to do with the RCS mode switch.. ATM AUTO and the like, that might influence how the
autopilot behaves in atmospheric conditions.. I didn't test that, but it may have an influence.


~~~

"Mood is a matter of choice. I choose to have fun!" -Vidmarism No 15

Ijuin

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Reply #7 - 19 July 2004, 11:18:00
I think you're right about that, Doc. If you have the RCS mode set to rotational mode, it uses the RCS jets for all
maneuvering, whereas if you use the ATM AUTO mode, it will use the elevons and rudders whenever there is enough
dynamic air pressure, thus using no RCS fuel.


Offline DocHoliday

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Reply #8 - 19 July 2004, 11:42:26
Okay, cool. And I think you are safe to leave the reentry pilot disengaged down to 100km or so. That's when dynamic
pressure starts to show itself.. Before that it's a waste of fuel more or less. Even thought I never had this problem.

I have another question tho. Is the 19-ton maximum reentry mass final? Are you difinitelly destroyed if the DG3 weighs more??? Or is it just harder to get down to ground??



Post Edited ( 07-19-04 13:53 )

~~~

"Mood is a matter of choice. I choose to have fun!" -Vidmarism No 15

Offline Arkalius

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Reply #9 - 20 July 2004, 03:09:10
For the landing, try adjusting trim. Use the PgUp and PgDn keys for this. Adjust it so that the ship does not pitch up
or down by itself when not touching the controls. This should make it easier to do the flare before landing. An
airspeed of around 140 m/s is desirable for touchdown. The ship will stall at around 120 m/s though, so be careful.
Touchdown can be tough to do without using the main engines to maintain speed. I usually have to use the engines
during approach to prevent slowing too much. Just make sure to remember to cut them off on touchdown :)

The distance that you should do your deorbit burn is very dependant on your alititude when you do it. The distance
you should do it will change around 2000km between a deorbit burn done at 200km alt, and 250km alt. My
suggestion is find an altitude you like and try doing your deorbit burns from that altitude every time. Trying to guess
where to do it for an unfamiliar altitude can often lead to over or undershooting your base by more than 100km which
will leave you too low on fuel to fly manually.


-Arkalius

Offline NASSAC

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Reply #10 - 20 July 2004, 03:26:36
Ok thanks for all the input and advice. I will put the DGIII to the test. But I also noted another problem and this may
also be due to a mixup in the version I am using. I think I am using the latest version from Dan but now that I think
about what is happening I may have the first release in the main simulator. I think this because test runs on another
simulator did not produce some of the radical behavior that I noted above.

Another probelm we encountered on the reentry was that when we activated the D-3 to see the rentry temperatures
the renetry program would halt. This is what leads me to now believe that part of the problem, and a big part at
that,  may be  due to the use of the wrong version of DGIII. I will check this out and get back to everybody.

John :)

NASSAC DIRECTOR
John

Offline DocHoliday

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Reply #11 - 20 July 2004, 14:19:30
Hmm.. I remember that a few of the releases of DGIII require a fresh Orbiter install or at least a totally fresh DGIII
install. If it's not too much trouble, you might want to separately install Orbier, Orbitersound and latest Dg3. None of
what you say appears on my machine, appart from the "aerodynic" behaviors, which are probably normal.

Cheers,


~~~

"Mood is a matter of choice. I choose to have fun!" -Vidmarism No 15

Offline NASSAC

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Reply #12 - 27 July 2004, 04:27:24
 OK, after upgrading the DGIII files the reentry appears to much more stable and the problem with the program
stopping when the renetry display was activated is gone. But........

1. We have attempted to land the DGIII from a 344Km orbit. WE have attempted to deorbit at a distance of 15,500
KM from KFC with 3800 main fuel in our tanks. We activated auto pilot at 90 KM alt.(on all scenarios) and ended up
450KM short of the runway and out of fuel. We reduced our PED to 6.350

2. We then atempted another reentry this time at 14,000KM from KFC and reduced PED to 6.400. We over shot KFC
by 1,500 Km. IN our atempt to turn the DGIII back to the base our under panel overheated at an altitude of 59Km
and the ship disintergrated.

3. We, then attempted to adjust our third attempt by reorbiting at a distance of 1450 from KFC and reduced our PED
to 6.300. THis appeared to be right on the money but when we attempted to adjust our course at 55 Km and the
center panel overheated and we  disintergrated.

We will continue to adjust our de-orbit burns to bring the DGIII in for a landing, but the inability to redirect our course
correction as we are coming through the atmosphere is problematic.

Any thoughts or recommendations?  

John :)

NASSAC DIRECTOR
John

Offline AphelionHellion

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Reply #13 - 28 July 2004, 03:44:41
Quote
1. We have attempted to land the DGIII from a 344Km orbit. WE have attempted to deorbit at a distance of 15,
500
KM from KFC with 3800 main fuel in our tanks. We activated auto pilot at 90 KM alt.(on all scenarios) and ended up
450KM short of the runway and out of fuel. We reduced our PED to 6.350

2. We then atempted another reentry this time at 14,000KM from KFC and reduced PED to 6.400. We over shot KFC
by 1,500 Km. IN our atempt to turn the DGIII back to the base our under panel overheated at an altitude of 59Km
and the ship disintergrated.

3. We, then attempted to adjust our third attempt by reorbiting at a distance of 1450 from KFC and reduced our PED
to 6.300. THis appeared to be right on the money but when we attempted to adjust our course at 55 Km and the
center panel overheated and we disintergrated.

Wow, you guys are determined to get to Kentucky Fried Chicken as soon as possible! :)
I guess I'd be hungry for some too if I'd been in space eating freeze-dried spaghetti, but I'd at least land at KSC first :)


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Offline DocHoliday

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Reply #14 - 28 July 2004, 08:20:40
:) Yup, we're still working on a reliable reentry and landing procedure. You can influence where you get out of the
atmosphere a lot by adjusting the pitch angle when around 60-70km altitude. Watch the range indicator in Display 3
so you come out near KSC :)


~~~

"Mood is a matter of choice. I choose to have fun!" -Vidmarism No 15

Cupcakus

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Reply #15 - 28 July 2004, 17:27:15
I've never landed completely by glide alone... I get very close now but I have to burn a little near the end to get the
nose up for landing.

It's basically trivial to get the DGIII to KSC with the right tools.  First use the BaseSyncMFD to sync with KSC as close
as possible.  Then use the Reentry MFD to plan your entry angle.  I typically put the Map MFD up and the Reentry MFD,
when the ship is between 130deg and 120deg, retro burn until the decel required on the reentry MFD is about 15,
then use the reentry auto pilot at a 40deg AOA.  If you do it closer to 120deg from base your reentry angle from ISS
will be close to 1.7deg... yikes... but you should be OK, that will level out to 0.7deg or so when the heat really gets on
and the DG may temp alarm, but shouldn't burn up.

Once your speed is down around 4,000k to 3,500k you can pitch down to 25deg AOA still using the auto pilot without
incident, this will cause your VSpd to decrease and your decel to decrease as well.  This will also bounce you back out
of the atmosphere if you're not careful.  I would definetly not lower your AOA less than 25deg, at 20 or less you run a
very real risk of the nose pressure pushing the nose of the DGIII down and you'll get real warm, real fast. Watch your
VSpd in the surface MFD, and if it starts to rise above 0, pitch up to 40deg, or you will glide right out of the
atmosphere and right past KSC, the autopilot sometimes can't pitch up again on it's own, so you will need to help it.
Keep your actual decel just below the required decel and you will be on a course to overshoot KSC, which is good
because you will need the extra energy to turn and line up with the runway.

It takes a lot of practice to deorbit safely and effectively, but the Reentry MFD and the BaseSync MFD really help in
getting you close.  It is likely you won't make it your first time to KSC, but you will definetly be close enough to only
have to use a little fuel to land.


Offline NASSAC

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Reply #16 - 29 July 2004, 05:39:25
Thanks Cupcakus,

For responding to our posting on reentry.  We will follow your instructions. I have never used the SyncMFD for rentry
to a base.  I will study the process and try it and get back to you with the resuts.

As for A.H  and  his silly response to our "KFC" typo instead of "KSC" I hope someday to return the favor.

John

NASSAC DIRECTOR
John

Offline Pierre_le

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Reply #17 - 29 July 2004, 06:06:03
here is the translated reentry procedure that i've posted on french forum. these parameters was successfully tested
many times by
myself using "dgiii docked to iss" scenario. enjoy and dont forget to check out the little video

-starting at alt. 360km. undock

-sync your route with basesyncMFD to intersect KSC using +/- normal burns.

-circularize orbit ecc=0 to keep alt. = 360km

-critical point -> do the deorbit burn preciselly 13900 km away from target, until you reach ReA 1.70 on the onboard
display 2 of the glider.

-correct your base intersect point with basesyncMFD. it has changed due to velocity variation in the previous step.
don't forget this point.

-hlevel, prepare AoA 40 (pro104) and engage it at alt. = 80km. the target distance must be 2100km at this instant.

-let the glider climb down, dont touch anything! wait, drink coffee or beer :drink:, even if alarm is fired. the external
temp could be close to alarm threshold but dont panic everything's ok.

-engage manual control (space bar) @ airspeed = M3.5 using aerodynamic surfaces. your are now at vertical of the
base.

-aim the closest runway with descent slope about -25/-30 . at "three hundred" pull the stick. keep ~240 m.s-1
airspeed while going down. use airbrake with care.
take out the gears as late as possible cause they will make you loose a lot of speed once deployed.
don't fly under 120ms.-s or you will stall.

-touchdown smoothly the runway ~-3 m.s-1 vspeed.
using motors or hovers during the reentry/landing process is absolutly forbidden ;) !

you need about 16 minutes of real life time do complete the manoeuver starting from ISS ending stopped on ksc
runway

final landing video. sorry for low quality, next time i'll choose a different codec/bitrate
http://cobalt.homeunix.net/~pierre/dl/mirror/?file=deltagliderIII_glide_landing.avi
divx5.1+mp3

---------------------

res gesta per excellentiam

Offline AphelionHellion

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Reply #18 - 29 July 2004, 20:47:34
NASSAC: Thanks, I appreciate it :)
Hey, being silly is what I do :beer:


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Offline AphelionHellion

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Reply #19 - 29 July 2004, 20:53:11
Pierre: Wow, looks good! I'll try it as soon as possible! :gift:
One question -  only 16 minutes from ISS to ground? 8o Perhaps I didn't read that right, but it always takes me a lot
longer, even when I use a somewhat suicidal glideslope. However, perhaps I'm porpoising more than I'd realized - I
don't always use the DGIII auto-AOA.


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Offline NASSAC

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Reply #20 - 30 July 2004, 03:09:02
Thanks Pierrre,

  I appreciate you sharing your experience and knowledge with us. We will set the landing up as you have suggested
and let you know how we make out.


John

A.H. - You don't do silly you are silly, there is a difference, so being silly is what you are, at least in my book. But
enough time wasted on this silliness. :) :) :) :)

NASSAC DIRECTOR
John

Offline NASSAC

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Reply #21 - 30 July 2004, 03:20:09
A.H.

 By the the way "proposing" is not spelled -porposing and glide slope is two words, I guess now you can consider the
favor returned. "It's easy to make typo's isn't it? :)

NASSAC DIRECTOR
John

Offline DocHoliday

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Reply #22 - 30 July 2004, 07:50:10
HEy freespace, my bet is on NASSAC, when these two have a showdown :) who'd you bet on? :)


~~~

"Mood is a matter of choice. I choose to have fun!" -Vidmarism No 15

Offline AphelionHellion

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Reply #23 - 30 July 2004, 08:22:33
NASSAC: You're right that glide-slope is normally either two seperate words or hyphenated. Shame on me :)
 But I was referring to "porpoising" in the aeronautic sense, as in oscillating in pitch up and down like a porpoise or
dolphin leaping out of the water. It's probably not in most dictionaries, but I've seen the term used in aviation
publications.

Seriously though, I was joking. No offense was intended! I've read the NASSAC project page and it looks like an
outstanding and innovative program.
And yes, I am silly; but I "do silly" as well :beer: We may play Orbiter because it's educational, but I believe we do things that are educational because learning is fun. Orbiter isn't going to help us balance our checkbooks or repair our cars, but it's as fun and challenging (in my opinion, at least) as most any "conventional" video game. So please pardon my occasion- er... ok, constant silliness.

:friend: Truce?  :)



Post Edited ( 07-30-04 08:27 )

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Offline DocHoliday

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Reply #24 - 30 July 2004, 08:27:43
Damn, was looking forward to a good verbal fight, you people are so peaceful :) I'll take my business to the official
forum then :)


~~~

"Mood is a matter of choice. I choose to have fun!" -Vidmarism No 15