See site in english Voir le site en francais
Website skin:
home  download  forum  link  contact

Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with username, password and session length

Author Topic: PrObLeMs with DGIII reentry and landing  (Read 15312 times)

0 Members and 1 Guest are viewing this topic.

Offline AphelionHellion

  • Sr. Member
  • ****
  • Posts: 382
  • Karma: 0
Reply #25 - 30 July 2004, 09:54:45
Doc: *rofl* Well sorrrrry I couldn't be more entertainingly mean :turning:  Perhaps I could make up some questionable
sexual practices to accuse everyone of? :) Muhahahahaha!
:hot: The only people I let have it are those who are racist, bigoted or hateful or nasty, and only intentionally so.
Fortunately, all the folks I've met so far on the Orbiter forums have been insightful, helpful, kind and mature - except
for Freespace2dotcom, of course.
( Just kidding FS! :friend: )

:doubt:Hey wait just a minute! I get it! You're trying to catch me being abusive so you can ban me! Thaaaaat's your
angle ehhh? Trying to get rid of ol' A.H. huh? Well it's not going to be quite that easy! I'm on to you, mister! :rant:


< [yellow]C[/yellow]arpe [yellow]N[/yellow]octem! >

Offline Atom

  • Legend
  • ******
  • Posts: 1099
  • Karma: 0
Reply #26 - 30 July 2004, 12:34:28
No, Doc is not that evil. (I  think) and you obviously haven't been to the off-topic forum on M6 (main forum).
There are quite a few political and other subjects brought up which end in flame wars. But I haven't been on there
recently so they may have made the moderation stricter. :)

Plus a good verbal fight would have been funny.



Intel Pentium 4 630 3Ghz|1024mb 400mhz DDR RAM|ASUS P5P800-VM|Nvidia GeForce 6200 256mb|Creative Sound Blaster Pro Value!|Windows XP SP2

Offline DocHoliday

  • Legend
  • ******
  • Posts: 2475
  • Karma: 2
Reply #27 - 30 July 2004, 12:49:03
And on that note, let's resume this debate in the GUPT as I have already done, so we don't pollute the good stuff
that's going on here about reentry procedures.


~~~

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

Offline AphelionHellion

  • Sr. Member
  • ****
  • Posts: 382
  • Karma: 0
Reply #28 - 30 July 2004, 19:22:59
Good plan, Doc :)
Speaking of reentry procedures, would it be considered cheating to use the DGIII's retro or hover engines while in the
atmosphere and decelerating?
Obviously the space shuttle doesn't have that luxury, but considering the fact that the DG is fictional anyway...

I recall that while using the DGII, I'd use the hover engines to control rate of descent, so I didn't drop too far into the
atmosphere at too high a velocity. That was before the reentry AOA autopilot, though :)


< [yellow]C[/yellow]arpe [yellow]N[/yellow]octem! >

Offline AphelionHellion

  • Sr. Member
  • ****
  • Posts: 382
  • Karma: 0
Reply #29 - 30 July 2004, 19:36:01
Oh! Speaking of shuttle landings, has anyone here used the Shuttle training aircraft 3.2?
Hats off to the guys who designed that add-on. I've seen it around for awhile but just today decided to try it 8o It's
great!
One disconcerting thing, though; even in shuttle sim mode with the spoilers deployed and reverse thrust engaged,
this thing still glides much better than the deltaglider :turning:


< [yellow]C[/yellow]arpe [yellow]N[/yellow]octem! >

Offline C3PO

  • Full Member
  • ***
  • Posts: 159
  • Karma: 0
Reply #30 - 30 July 2004, 21:59:18
Maybe it should change name to Deltaplunger :)

One thing that makes the DG hard to deorbit is the fact that the drag INcreases when you lower the nose(from 30-40
to 15-20 degrees AoA)
This makes it very hard to recover from a low energy state.

The huge pitch-down force around 10-15 AoA is also a weird thing given the delta wing plan form. It reminds me of a
delta winged RC-glider I tried years ago. The problem then was that the CoG was way too much forward. Maybe this
could be reason the DG bahaves in this way.

And a stall speed around 400 km/h is a bit on the stiff side 8o

Also the large increments of the PRO104SPEC40 program makes it hard to maintain a stable rate of descent.  Whe I
reenter the NASA Shuttles I use small variations in roll to mantain tha rate of descent. The 30 degree increment would
not be acurate enough for this way of control. And the Autopilot isn't able to hold the AoA once you get down below
25-30 deg.

With the right control it's quite easy to deorbit if you have some points to aim for. In the Shuttle you compensate for
low energy by lowering the nose 2-3 deg and vice versa. Then you use the roll to keep a steady rate of descent.
When the v3.+ Shuttles were released I used a real STS deorbit plate as a guide to find the best "points" to aim for
during the descent.

Now we just have to find some good values for the DGIII. But maybe it's better to wait for the next DGIII/Orbiter
release, in case the behaviour changes in the new version. (It takes quite a bit of deorbiting to find the optimal
reentry)

MAN! I can't wait to have all the default Orbiter ships even close to the DGIII level of detail. WOW!
thanx a 1E12 Dan :wor:

C3PO



Offline NASSAC

  • Full Member
  • ***
  • Posts: 123
  • Karma: 0
Reply #31 - 31 July 2004, 04:17:35
A.H.
 Let's put all the silliness behind us and keep the topic on point. I am not offended I never get mad but I always get
even. I am very competitive.  I  like the Orbiter forums because of its serious, kind and mature content let's keep it
that way. :):)


John

NASSAC DIRECTOR
John

Offline AphelionHellion

  • Sr. Member
  • ****
  • Posts: 382
  • Karma: 0
Reply #32 - 31 July 2004, 04:44:50
NASSAC: Agreed :)

Regarding DGIII reentries: it seems to me that among all these variables, it might be useful to find a constant or two,
if one actually exists. I've been trying to figure out if there's an optimum slope to aim for, or if this in fact varies based
on speed, altitude, and the orbit you're entering from. The DGIII's extra HUD is very useful for this sort of thing (stats
that the default orbiter displays don't give you) so I've been focusing on things like glide slope and mach number that
you won't find on the default displays. But given the deceleration and increasing atmospheric density during a full
reentry, I question whether any single number, be it glide slope or m/s vertical speed or any such thing, is going to
stay at all constant :doubt: It'd be great if we could find one, though!
The best I hope I can do is to try and find a good combination of rate of descent and speed at any specific given
altitude, I guess?  :???:


< [yellow]C[/yellow]arpe [yellow]N[/yellow]octem! >

Offline C3PO

  • Full Member
  • ***
  • Posts: 159
  • Karma: 0
Reply #33 - 31 July 2004, 12:12:30
Quote
AphelionHellion wrote:

Regarding DGIII reentries: it seems to me that among all these variables, it might be useful to find a constant or two,
if one actually exists.

Actualy there is a constant you should aim for during any reentry. KEAS or knots equivalent air speed. (or maybe
m/sEAS in the case of orbiter)

The only place I've seen it is on Duncan's reentryMFD. http://www.orbitermars.co.uk/reentry.zip

But it isn't going to be any help in getting closer to the landing site.

I've found that the easiest way to deorbit is to have some kind of "points" to aim for as you fly down the reentry. But
it requires a LOT of tests to find the optimal reentry path. The kind of points I find most useful is a set of
DistanceToBase/Speed values. If you are a bit slow, you just lower the nose 2-3 deg and vice versa. This works
perfect with the Shuttles, but it might be a little tricky in a DGIII.

It looks like people have been so afraid of bouncing of the atmosphere, that the DG falls like a Steinway if you don't
keep it level almost all the way down.

Has the DG's flight model been changed since the 031217 patch?

C3PO



Offline DocHoliday

  • Legend
  • ******
  • Posts: 2475
  • Karma: 2
Reply #34 - 02 August 2004, 08:56:21
Interesting debate. The speed constant is one variable you could fix, the second one I would propose would be the
vertical speed. This should make sure that your temperature limits are under control. Lastly, has anyone seriously
made use of the projected distance indicator. I think it's in Display 3 and tells you how far off the DG should touch
ground if nothing changes? That piece of information should be useful but it keeps changing making it rather useless,
but combined to reference distance points it could be used to measure one's progress.

Still as there are some many interlinked variables I prefer to keep the WHOLE reentry phase fixed in terms of not
changing pitch or anything and prefer to vary the altitude and timing of the retroburn.. Realistically speaking, I don't
think I pilots can do much while in the reentry phase, with all the G-force bearing down on them. Small corrections
yes, but drasting measures to correct a mistake in the initial setup, no.. :)

As I said once before, I can't really imagine how the Shuttle crew manages to do those S-turns designed to bleed off
speed but not increase the drag. The forces must shift drastically.

Cheers,


~~~

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

Offline C3PO

  • Full Member
  • ***
  • Posts: 159
  • Karma: 0
Reply #35 - 03 August 2004, 03:15:24
Quote
DocHoliday wrote:
the second one I would propose would be the vertical speed. This should make sure that your temperature limits are
under control.

That's actually well spotted :) I use the Vvel as the primary data to control the descent, and I have no truble at all
getting the Shuttle on the runway. But the DG still has me in a spin, literaly 8o

This is a reentry plate from STS-101:

And a short explanation:
Across this card you have the vehicle's inertial velocity in 1000's of feet per second (again, roughly the mach
number of the vehicle), the reference 'alpha' or angle of attack, the range to the landing site (in nautical miles), the
height (in thousands of feet), the rate of change of altitude (in feet per second) and the reference roll angle of the
vehicle (either to the left or right).  Since the shuttle has to maintain the high angle of attack in order to prevent
burning the tail off during reentry, the vehicle will perform the rolls in order to reduce lift and get down into the thicker
air such that the required amount of drag is achieved that will slow the vehicle properly.


As you can see the Vvel is pretty constant.

Quote
DocHoliday wrote:
Realistically speaking, I don't think I pilots can do much while in the reentry phase, with all the G-force bearing down
on them. Small corrections yes, but drasting measures to correct a mistake in the initial setup, no.. :)

Actualy real reentries have to have some correction because the top of the atmosphere isn't flat like in Orbiter. Even if
the corretions are "small" they have a huge impact on how close you get to the intended landing site. You can try
calculating what a 0.1 degree error is after you've traveled 3,000 km :)
I read a paper on STS emergency reentries, and from what I understood the Shuttle can make quite large corrections
for example skipping out of the "thicker" altitudes if the entry slope was too high.
 
Quote
DocHoliday wrote:
As I said once before, I can't really imagine how the Shuttle crew manages to do those S-turns designed to bleed off
speed but not increase the drag. The forces must shift drastically.

I don't know what you mean by shifting forces. The AoA is constant at 40 deg (+/- 3 deg to correct for high/low
energy) The trick is to keep the side slip as close to zero as possible. The Shuttle doesn't produce lift in the normal
way in the upper atmosphere. It just deflects the oncoming air downwards (40 deg AoA) To control the rate of
descent it does the infamous roll-reversals that deflects the air sideways too.
Remember that reentry capsules (Apollo, Soyuz etc.) use the same kind of "deflection control" during the descent,
despite having no wings or airfoils of any kind.
Because the 40 deg AoA is way past the stall AoA, small changes in pitch do not affect the drag very much, it just
changes the diraction of the reflected air. I had some good pictures explaining the effect, but I can't find them right
now:bug:

The DG has several problems with this kind of descent. First of all the DG is extremely stable (want's to pitch/yaw
towards the vel vector), but the Shuttle is very unstable (it needs the computers to keep it at the right attitude)
Second the DG looses the "lift" very fast as the speed is reduced (even when the deflected air gets denser) My guess
is that the DG is producing lift in the normal way at 40 deg AoA, and lift produces induced drag. But if you use the
deflection method the AoA doesn't change the drag very much (just try to fly a kite, and you'll see what I mean)

I'll never get beond my "Pilot" status if I keep writing these long posts:wall:
:)

C3PO



Offline NASSAC

  • Full Member
  • ***
  • Posts: 123
  • Karma: 0
Reply #36 - 03 August 2004, 11:56:50
Pierre_Le wrote "sync your route with basesyncMFD to intersect KSC using +/- normal burns"

1. Pierre is the basesyncMFD a special MFD or are you talking about the MAPMFD?

2. I raised my orbit after undocking to 360KM and then corrected ECC to 0.

3. I then used a Orbit +-Normal burn to get my orbit to pass over KSC but I was less than 13,900 away from KSC so I
did not do my reentry burn. I will now wait for my next orbit and again do an orbit alignment burn to get my Orbit to
again pass over KSC when I am more than 13,900 KM from KSC.

4. I am a little uncertain about which MFD's I should use for rentry. HSI, GPC, RENTRY and the BasesyncMFD?? I am
looking for instructions on how to use the reentry MFD.

NASSAC DIRECTOR
John

Offline DocHoliday

  • Legend
  • ******
  • Posts: 2475
  • Karma: 2
Reply #37 - 03 August 2004, 12:11:05
I don't think there are any instructions on Reentry MFD coz it's beta beta.. What I do know is that you should watch
the horizontal indicator at the bottom. It shows in which direction the landing site will be once you're near it. Similar
to Surface MFD compass but I think it shows the heading at the time you reach the spaceport, so you can correct
your path to intercept it nicely.

As for BaseSyncMFD - get it here http://koti.mbnet.fi/jarmonik/Orbiter.html (bottom of the page). The manual is
included but it's pretty straight forward. Use Direct approach and Closest passage parameters. I think that should
work best.

Cheers,


~~~

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

Offline C3PO

  • Full Member
  • ***
  • Posts: 159
  • Karma: 0
Reply #38 - 03 August 2004, 12:48:40
I believe the indicator at the bottom of ReentryMFD is the difference between direction to base and the direction of
the velocity vector, not heading. The heading isn't very useful at high AoA if you have any kind of bank angle.

The other main thing on the ReentryMFD is the required acc and actual acc. As long as you keep those values close
together, and not too high, you should be on the right path.


Offline DocHoliday

  • Legend
  • ******
  • Posts: 2475
  • Karma: 2
Reply #39 - 03 August 2004, 12:59:46
Quote
I believe the indicator at the bottom of ReentryMFD is the difference between direction to base and the
direction of the velocity vector, not heading.
Thank you, that's what I meant, but had obvious difficulty explaining it :)


~~~

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

Offline Pierre_le

  • Sr. Member
  • ****
  • Posts: 384
  • Karma: 0
Reply #40 - 03 August 2004, 17:39:09
NASSAC: you must install basesyncMFD, that is an additional mfd that can be found here
http://koti.mbnet.fi/jarmonik/Orbiter.html

set target to "cape canaveral" and keep default closest passage mode.  then do you align burns to project your
intersection to the next orbit, not the current one.
dont forget to keep ecc=0, +/- normals burn alter orbit stability so you do the base alignement before orbit
stabilisation, its simpler

use DGIII internal display 2 as explained to read you reentry angle

you are close to the thruth

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

res gesta per excellentiam

Offline C3PO

  • Full Member
  • ***
  • Posts: 159
  • Karma: 0
Reply #41 - 04 August 2004, 00:06:14
After several attempts to reenter the DGIII I've found out that the 40 deg AoA isn't a good attitude for this craft. It's
impossible to control the vertical speed because the DG doesn't have enough lift to maintain the 0.70 deg slope.
I tried several PeD hights and used Pro104Spec40 with no bank at all, but it never got above -37 m/s vertical velocity.
This means that no energy went into changing the direction, so there is no way to correct any error in the alignment
to the target base.

I know it's possible to align with the base before the reentry burn, and delay the burn so you "fall" out of the sky
directly over the landing site, but a futuristic space-plane like this should realy have more control over the reentry
then the americans first attempt designed in the '70s, right?

I'm going to try if I can do any better using 35 deg AoA, but that's dangerously close to that pitch-down force.

Maybe we should start a petition to convince Dan to make the increments in the autopilot smaller ;)

C3PO



Offline DanSteph

  • Administrator
  • Legend
  • *****
  • Posts: 15407
  • Karma: 256
  • Hein, quoi !?
    • FsPassengers
Reply #42 - 04 August 2004, 00:49:52
No spacecraft in the world would maintain a reentry angle, it's normal
but it's true that adjustment are needed for this part of the DGIII
they where planed but life don't allow me anymore to work so much
on the DGIII yet...

One must eat uh ? ;)

Dan


Offline Pierre_le

  • Sr. Member
  • ****
  • Posts: 384
  • Karma: 0
Reply #43 - 04 August 2004, 00:57:01
c3p0 you never need to bank to correct trajectory with a basesyncMFD approach
you can set the AoA value whatever you want using pro104specXX where XX is your initial angle. it is true that aoa
increment is 5° but no critical need to fine tune it more

i encourage you to use a 40° reentry interface, or sometimes 39° to fall a bit slower depending of your initials deorbit
parameters and feeling


i see some altitudes "floors" during a descent

80km - init interface
65km - flames :-)
50km - start high density atm, highest temp and high G decc.. your vspeed goes temporaly low. wait
30km - if you need to glide some km to the base, do it now! below this alt. you will be unable to save energy efficiently
10km - be care to not fly above M3 during manual pitch down or you will burn
0m  - you can release the stick dude

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

res gesta per excellentiam

Offline C3PO

  • Full Member
  • ***
  • Posts: 159
  • Karma: 0
Reply #44 - 04 August 2004, 02:01:09
Quote
DanSteph wrote:
........ but life don't allow me anymore to work so much
on the DGIII yet...

One must eat uh ? ;)

Hehe...... no worries, Dan. If there's one thing I've learnt in my years in Orbiter, it's that good things come to those
who wait. :) Do you know if Martin has planed any change in the atmospheric model in the next version?

Pierre_le:
I know how to make the DGIII fall out of the clouds over the runway, but it's not as much fun as "piloting" it
there ;)

C3PO



Offline Pierre_le

  • Sr. Member
  • ****
  • Posts: 384
  • Karma: 0
Reply #45 - 04 August 2004, 11:08:12
i've just flown 2 touch & go round trip between ksc & iss and yeah it's a great success (applause now)

i add some very little changes to my reentry procedure posted earlier. read carefully

- still from "iss" altitude of 360km, you got to do deorbit burn exactly 13650km away from target, to get a rea
of 1.70° on display 2.
- immediatly after this deorbit burn and the second base route fine correction, empty the whole content of the main
fuel tank to 0%. just press the dump button on DG bottom panel and wait for
atmosphere contact. now your ship will be lighter and external temp will be less hot during decceleration.
don't dump rcs fuel or you will get serious problems.

an advice if you didnt made the deorbit burn exactly at 360km altitude. start to check the "alt." indicator in
basesyncMFD while you are about 500km from target.  it predict your altitude while flying at vertical of the base
(dst=0). try keep this alt = 25km or less to ensure good landing conditions. change aoa setting to 45° to break more
(init alt>360km) or 35° do glide longer (init alt<360km). yes, basesync rulez

don't forget to tune ILS/HSI com frequencies in 134.20 & 132.60 during descent. HSI MFD is usefull to make a perfect
final runway alignment

you are back on runway, well done dude. call the gas truck, fill the tank and take off for new adventures. :drink:

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

res gesta per excellentiam

Offline NASSAC

  • Full Member
  • ***
  • Posts: 123
  • Karma: 0
Reply #46 - 04 August 2004, 11:24:32
Thanks, Doc, Cp30 and Pierre. This is the kind of feed back that I have been looking for. I searched for the Basesync
MFD but came up empty. I appreciate all of you sharing your knowledge with us recruits.  I have learned a lot since I
started using Orbiter. I guess this is exactly what Martin had envisioned. I feel like I owe a lot to people like Martin
and Dan. I only hope that in the future I can make a meaningful contribution as well.

Dan wrote, "One must eat uh?"  Hey Dan,  Thanks again for all the work you have already done on DGIII. From what I
have seen of your work and what you have shared of your personal life, I think your priorities are in order--Family-
Work-Personal Time-------Orbiter.

John :) :)

NASSAC DIRECTOR
John

Offline DocHoliday

  • Legend
  • ******
  • Posts: 2475
  • Karma: 2
Reply #47 - 04 August 2004, 11:36:57
Hey NASSAC, Pierre posted a link to BaseSyncMFD a few post above. You should get it there. Bottom of page.

As for priorities, I am more Family - Personal time (orbiter) - Work :)) we only work to eat, not eat to work :) but
that's me.


~~~

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

Offline C3PO

  • Full Member
  • ***
  • Posts: 159
  • Karma: 0
Reply #48 - 04 August 2004, 15:37:57
Quote
Pierre_le wrote:
- still from "iss" altitude of 360km, you got to do deorbit burn exactly 13650km away from target, to
get a rea
of 1.70° on display 2.


I just tried that method. Those numbers result in a PeD of -50km and a 4G reentry. 8o

It's more like a ballistic then a controlled reentry that's more suitable for a capsule then a lifting body design. There is
very little margin for error in Vspeed and NO margin for direction. The stopping distance is so short that you have to
consentrate on slowing down without burning up, instead of possitioning you'r self for the final approach.

I hope the DG's aerodynamics get a small overhaul after the next release of Orbiter. Untill then we have to make do
with the short stopping distance caused by the lack of lift.

PS:
Pierre_le: If you try a reentry in a v3.5 Shuttle using shuttle_gpc_mfd_v2.0 you can see the difference.

C3PO



Offline Pierre_le

  • Sr. Member
  • ****
  • Posts: 384
  • Karma: 0
Reply #49 - 06 August 2004, 18:48:48
here are my reentry flight datas. still iss->ksc trip, without touching anything between deorbit-burn and manual
control close to runway.



note that the data recorder seems to not handle time compression changes, and sample data on pc clock time basis
instead of sim time.

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

res gesta per excellentiam