Flight Question Unable to hold AoA while re-entering

[thedopefishlives]

Hello Orbinauts.

So I've been working on my Earth atmosphere re-entries with the DGIV before moving on to the more technically difficult Mars re-entry. I've been trying to use Basesync and Aerobrake MFD's to guide me down to Cape Canaveral, but at some point during the re-entry, every time, I lose the ability to hold my AoA and my nose plummets, which totally throws off my re-entry curve. I've tried a variety of parameters for the de-orbit burn, but all that does is change the timing. Part of the problem is that all the re-entry threads I've read suggest an AoA of 40 degrees, with 43 degrees as a maximum; I've never been able to get AerobrakeMFD to show me hitting my target with less than a 45 degree AoA. What am I missing?

 

[Hielor]

Use the DGIV's AOA hold autopilot.

 

[Tommy]

As pointed out above, use the DGIV's re-entry autopilot (PRO104SPECnn, where nn is the desired AoA). Once engaged, the numpad can be used to alter the AoA or bank as desired. It is impossible to hold a high AoA using just the aerodynamic controls alone.

Also, if you need a 45 degree AoA, you are either de-orbiting a bit late, or your Re-entry Angle is too low. The DGIV likes a ReA of about 1 to 1.2, this can be seen on the panel's little display (press the "d" key, then the "2" key to see the ReA.

 

[Wrangler]

Hello Orbinauts.

So I've been working on my Earth atmosphere re-entries with the DGIV before moving on to the more technically difficult Mars re-entry. I've been trying to use Basesync and Aerobrake MFD's to guide me down to Cape Canaveral, but at some point during the re-entry, every time, I lose the ability to hold my AoA and my nose plummets, which totally throws off my re-entry curve. I've tried a variety of parameters for the de-orbit burn, but all that does is change the timing. Part of the problem is that all the re-entry threads I've read suggest an AoA of 40 degrees, with 43 degrees as a maximum; I've never been able to get AerobrakeMFD to show me hitting my target with less than a 45 degree AoA. What am I missing?

Ok I know exactly where you are. I was there a few weeks ago myself. It's those darned astronuats at NASA. They make it look so easy.

Anyway, since you are intelligent enough to be using DGIV (I was trying to do it with the stock Atlantis... I know...Dumb!), as mention by others here, use the PRO104SPPEC<angle>. Works like a charm. And by Autopilot, it just means that the computer is managing the AOA. You still have to decide the pitch and roll angles. So you are still flying this thing. This is not AutoFCS.

As for the high AOA, well I do go that high myself during the first minute or so , right after EI. I also induce a high bank at the same time to drop me to a more denser part of the atmosphere, while zeroing out any cross-range that I have. I can then see if I can reduce the AOA and still manage the approach. Also, when you set the AOA at 40 degrees, don't worry if the AerobrakeMFD shows you overshooting, especially before the EI. If you have about 45-50 degrees of Anticipation angle, you can burn off any excess speed doing the S-turns in the atmosphere. I was observing a reentry of one of the David413's shuttles, with the AutoFCS, and before the EI, AerobrakeMFD showed the vessel overshooting KSC and crashing somewhere in JAPAN!. After EI, AutoFCS does a few high AOA/high banking manuevers and gets rid of all the excess crossrange and downrange.

So deep breath and keep trying. :lol:

 

[thedopefishlives]

I figured out, at least, that part of the problem was that I was using Aerobrake's AoA autopilot and was attempting to adjust the angle manually, seeing as how it doesn't have an adjustment on it. As a result, when I would go to make an adjustment in certain phases of the re-entry, it would not be able to recover the angle and would fall. Next question: Using a re-entry profile of 1 degree angle, 50 degrees anticipation, and 80k altitude, for smaller values (like the typical 40 degrees) of AoA, Aerobrake shows a wavy, up-and-down re-entry profile. Now I understand that this is a perfectly valid re-entry technique and that basically anything that gets me on the ground without burning me up or crushing me to death is OK. I would prefer, however, to learn how to do a smoother re-entry.

On another topic, I don't understand how to enable the re-entry display on the DG. When I press the "d" key, it opens and closes the radiator.

 

[dgatsoulis]

I would prefer, however, to learn how to do a smoother re-entry.

Try a more aggressive reentry with a PeA ~50 km and AoA ~42°.

On another topic, I don't understand how to enable the re-entry display on the DG. When I press the "d" key, it opens and closes the radiator.

The stock DG doesn't have a reentry display.

 

[RisingFury]

If you are supposed to hold 45° AOA with a DGIV on reentry, then your reentry burns are too late. Do your burn several thousand kilometers further away from your target for a shallower reentry. You'll spend more time in the atmosphere and your AOA will be lower. DGIV can hold a reentry AOA from about 28 to 43°.

 

[Ripley]

...On another topic, I don't understand how to enable the re-entry display on the DG. When I press the "d" key, it opens and closes the radiator.

Are we all talking about Dansteph's DGIV-2?

"d" is the key for the radiator of the stock DG.

 

[thedopefishlives]

The stock DG doesn't have a reentry display.

Let me guess, it doesn't have the nice AoA autopilot, either.

 

[agentgonzo]

Try a more aggressive reentry with a PeA ~50 km and AoA ~42°.

Indeed - if you are having a very 'up-and-down' reentry on the AerobrakeMFD display, what is happening is that your AoA is generating a lot of lift. As you drop from higher altitudes to lower ones, the atmosphere gets thicker and your AoA generates a lot of lift and throws you back up out of the thicker atmosphere again (the traditional 'bouncing off the atmosphere' phrase). This will happen multiple times until you've bled off enough speed to drop down into the thicker atmosphere without bouncing up again.

If you raise your AoA as you encounter the thicker atmosphere, you will not get as much lift and so won't bounce back up again quite so drastically and have a smoother (and much faster) reentry.

FYI, one of the problems with a real-world 'bouncing' trajectory that you're getting here is that vessel will repeatedly get hot/cold/hot/cold as it bounces up/down/up/down on your profile. It's a lot easier to predict how the thermal protection system on the vessel will behave on a smooth gradual temperature curve than one that repeatedly bounces hot/cold all the time and less will hopefully go wrong.

 

[thedopefishlives]

Okay, orbinauts, first of all, all y'all are awesome. I found where I've been going wrong: I referred to the "stock DG" as the DGIV, because that's what Orbiter calls it. I went and downloaded dansteph's DGIV-2, and I immediately noticed that the 40 degree AoA showed a clean, smooth re-entry profile. So the final question is, has anyone actually found a good way to land the stock DG without it bouncing along the atmosphere for a half orbit on the one hand, or coming in at such an extreme AoA that it can't hold the angle, on the other?

 

[dgatsoulis]

Okay, orbinauts, first of all, all y'all are awesome. I found where I've been going wrong: I referred to the "stock DG" as the DGIV, because that's what Orbiter calls it.

Orbiter calls the stock DG as DeltaGlider, while DanSteph's DGIV is called DeltaGliderIV.

I went and downloaded dansteph's DGIV-2, and I immediately noticed that the 40 degree AoA showed a clean, smooth re-entry profile. So the final question is, has anyone actually found a good way to land the stock DG without it bouncing along the atmosphere for a half orbit on the one hand, or coming in at such an extreme AoA that it can't hold the angle, on the other?

It's the same as chess. First learn the annotation and the basic "moves". (So that we can converse using the same reference points).
Then we can talk about tactics. (And at a higher level strategy).

The stock DG has a very different L/D (Lift/Drag) profile than the DGIV and eventhough there is a "damage" simulator, it's not as sensitive as the DGIV's.

I'm not sure about the tactic you are following, (first learn how to use a more complicated craft than the stock DG), but it may be as usefull as any I've seen.
My advise is this: For every scenario you load, use the -available- autopilots in one, don't use them in the other. This should help you get an intuitive "feel" of how a spacecraft should behave during the various phases of a flight.

 

[Wrangler]

Okay, orbinauts, first of all, all y'all are awesome. I found where I've been going wrong: I referred to the "stock DG" as the DGIV, because that's what Orbiter calls it. I went and downloaded dansteph's DGIV-2, and I immediately noticed that the 40 degree AoA showed a clean, smooth re-entry profile. So the final question is, has anyone actually found a good way to land the stock DG without it bouncing along the atmosphere for a half orbit on the one hand, or coming in at such an extreme AoA that it can't hold the angle, on the other?

Well... I am still a noob in many respects, so "Been there and done that"...lol

I kinda agree with dgatsoulis. You need to understand each stage of the de-orbit, reentry and landing on a single frame of reference. Dan's DG-IV is a forgiving spacecraft, with some neat tools. Use it for a few landings. Use the PRO105 reentry program, which takes over the AOA management. *Observe* it in action. See how it controls the L/D ratios and manages the approach, as you change the lateral trajectory. Then give it a try with PRO104SPEC40.

As for the Aerobrake's AOA and Bank hold options, they leave alot to be desired. The passive components are great, but for some reason it just can't hold the AOA steady. You have use a lot of RCS to keep things steady (not a good thing on a Space shuttle, where fuel is not a luxury). Still, I have managed to perform a reentry with the Stock DG, but it was not pretty, and well, I had to cheat during the final approach to the SLF. :shifty: Had to do a couple of burst on the main engine, as I was gonna fall short. Still looking for a decent AOA hold program for the stock DG. Let me know if you find anything.


In the mean time, start practicing on DGIV-2. Use this http://www.orbiter-forum.com/showthread.php?t=18671 . This is a very good tutorial, if you are just looking for instructions on what to do when. If you are bit more geeky (like moi), and want to know WHY do those steps, you will then have to read around the web. I recommend the orbiter Wiki as a start.

Good Luck.

 

[Ripley]

Don"t forget that AerobrakeMFD relies on .ld files that contain lift and drag data. Each ship has its own file.
I don't remember if the MFD already comes with the ld file for the stock DG.
Anyway you can create it by flying in the atmosphere and performing a 360 degrrees loop.
Then press Shift S to save it. Right or left shift, according to where you opened the MFD.

 

[Cras]

I think it does....
I think it comes with those files for the DG, DGIV, and the XR-series

 

[Wrangler]

Don"t forget that AerobrakeMFD relies on .ld files that contain lift and drag data. Each ship has its own file.
I don't remember if the MFD already comes with the ld file for the stock DG.
Anyway you can create it by flying in the atmosphere and performing a 360 degrrees loop.
Then press Shift S to save it. Right or left shift, according to where you opened the MFD.

Is this an AerobrakeMFD feature? I didn't find it in the documentation.

 

[Loru]

AeroBrake.pdf page 9 section 5 (located in doc folder)

 

[agentgonzo]

I'm not certain, but I think that if AerobrakeMFD can't find the .ld file it will attempt to create it on the next run so that it can be used forever more.

 

[Ripley]

Mmm...Some time ago I had a chat wit gp (AerobrakeMFD's father) on FOI forum, and he didn't mention that at all.
He asked for some .ld files to include them with the next (at that time) release.

 

[thedopefishlives]

The latest release does have .ld files for the stock DG, as well as DGIV, DGXR1, the XR2 and the XR5.