Flight Question Needing assistance with Delta-Glider re-entry

[blixel]

I have been experimenting with Orbiter for about 6 months. I feel like I have a pretty solid understanding of the fundamentals. I have learned how to use several of the MFD's by reading the PDF's and watching tutorials. So far, I am only comfortable with the default Delta-Glider.

My question is regarding re-entering Earth's atmosphere and landing back at Cape Canaveral. Specifically, for some odd reason - when I re-enter, I don't have use of the flight controls. (e.g. 4/6 banking doesn't work at all. And yes, I have turned off RCS mode.)

Otherwise I think I have worked out the basics of landing back on Earth. I turned on Groundtrack on the Map MFD in order to track where my Delta-Glider will be over the Earth. Then, while in orbit, I use Normal or anti-Normal to adjust my Orbit so that I intersect with Cape Canaveral. At a distance of about 2,500nm from Cape Canaveral, I burn retograde until my predicted orbit stops at Cape Canaveral. (This is just guess work on my part. I've done the burn at various distances.)

Once my predicted orbit stops at Cape Canaveral, I then turn prograde and level out the horizon. I then turn off the autopilot, switch to Rotation mode and use the thrusters to keep the Delta-Glider oriented in the direction of flight.

This all works very well.

However, once I am back in the atmosphere, I turn off RCS mode and have no control of the Delta-Glider. At that point there's nothing I can do except watch myself crash.

I've gone through this scenario a few times (fortunately I have a scenario saved just after reaching MECO after launch ... so I can practice re-entry using that scenario.) But I don't know why I lose the ability to control my Delta-Glider between the time I launch, and 1 orbit around the planet.

I have learned how to fly all the way to the moon and land. I am real comfortable with that whole process now. (Which means I have at least a basic understanding of the Sync Orbit MFD, the Align Plane MFD, the Orbit MFD, the COM/NAV & VOR/VTOL MFD, the Transfer MFD, and the Map MFD.) But of course, landing on the moon uses RCS Mode / Hover engines ... so this issue was never a problem when learning to fly to the moon.

Do the flight surface controls "freeze over" while in Orbit? Do I have to turn on a heater to warm them up? Or maybe my re-entry is too steep and causing the flight surface controls to "burn up"?

I'm at a complete loss on this one.

 

[Wishbone]

1. Turn RCS on.
2. Check if AF Ctrl switch is ON. (top right switch in the internal view)
3. Do an earlier re-entry burn.
4. Read tutorials on re-entry (the most recent one here was written up by Grover).

 

[Fabri91]

If I remember correctly, the DG has a separate switch next to the RCS one to control the control surfaces. It might be a really silly question, since I'm maybe mistaking that from the XR vessels, but have you turned the control surfaces back on?

 

[blixel]

1. Turn RCS on.

Hmmm... on? RCS Mode is off for the ride up to Orbit. It is turned on after launch for use during Orbital flight, but don't I want to turn it off for re-entry?

2. Check if AF Ctrl switch is ON. (top right switch in the internal view)

AF Ctrl is one of the controls that I don't ever toggle. It is on at the time of launch, it's on during Orbital flight, and it's still on when I'm getting ready to re-enter. (I'm not sure what condition would require changing it?)

3. Do an earlier re-entry burn.

Noted. I've just been guessing as to when to start the re-entry burn. If there is an MFD that takes the guess work out of it, I've yet to find it. I've been working on learning to re-enter from the Deorbiting directions in the Orbiter.pdf guide. (Which don't give you much to go on.)

4. Read tutorials on re-entry (the most recent one here was written up by Grover).

Noted. I always look at the existing tutorials and scan the forums for similar topics before starting a new thread. So far, I haven't located any good posts for Deorbitting / re-entry to the Earth using the default Delta-Glider, but I'll keep looking.

 

[Urwumpe]

Hmmm... on? RCS Mode is off for the ride up to Orbit. It is turned on after launch for use during Orbital flight, but don't I want to turn it off for re-entry?

Until you are below 30 km again, you want RCS.

AF Ctrl is one of the controls that I don't ever toggle. It is on at the time of launch, it's on during Orbital flight, and it's still on when I'm getting ready to re-enter. (I'm not sure what condition would require changing it?)

AF Ctrl manages how Orbiter translates your input to aerodynamic control surface deflections. If this is OFF, you have only RCS for controlling your attitude. if this is in ELEV, you have only elevator control, you can only change pitch without RCS (useful for the early phases of reentry, when pitch is the only axis that you need to control well). ON enables all the magic.

 

[blixel]

Until you are below 30 km again, you want RCS.

Ah, right. That makes sense. And that's what I've been doing. When you said turn RCS on ... I just misunderstood what you meant. RCS is already on when I'm in Orbit. I leave it on through the first part of re-entry, and then turn it off when I get into the atmosphere.

AF Ctrl manages how Orbiter translates your input to aerodynamic control surface deflections. If this is OFF, you have only RCS for controlling your attitude. if this is in ELEV, you have only elevator control, you can only change pitch without RCS (useful for the early phases of reentry, when pitch is the only axis that you need to control well). ON enables all the magic.

I'm guessing this control is there for cosmetic purposes? Off the top of my head, I can't think of a reason to adjust AF Ctrl in Orbiter. Turning it off while in Orbit has no impact on the experience. And having it off when it's needed is obviously not good ... so I can't think of a situation where it really does any good to adjust it. (Might as well leave it ON at all times.)

Is it just one of those check list items? Things that you do to enhance the realism ... like external lights? But has no impact on the simulation experience?

 

[Tommy]

If you are performing the de-orbit burn at only 2.5M from the target, and burning until the flight path ends at the target, then you are reducing your velocity to very close to zero. Then you level out, so you are re-entering almost straight down - and belly first. This could cause the lack of control since there is almost no forward velocity - so no air is moving parallel to the wings.

Try de-orbiting much further away, at least 1/3 orbit before the target. Burn until the PeA is < 50k, or until the flightpath ends at your target, your choice. Then, regulate the rate of braking by changing the altitude - lower altitude = faster braking.

 

[orb]

If there is an MFD that takes the guess work out of it, I've yet to find it.

BaseSyncMFD - to download, scroll down to Miscellaneous section on jarmonik's IMFD page.

 

[Mantis]

Try Grover's tutorial - follow it step by step and you WILL re-enter and land safely. I did it on my first try following this: http://orbiter-forum.com/showthread.php?t=18671

 

[blixel]

If you are performing the de-orbit burn at only 2.5M from the target, and burning until the flight path ends at the target, then you are reducing your velocity to very close to zero. Then you level out, so you are re-entering almost straight down - and belly first. This could cause the lack of control since there is almost no forward velocity - so no air is moving parallel to the wings.

I think this was a big part of my problem. I was trying to "cram" the Delta-Glider into the atmosphere to get the velocity lowered in too short of a distance. (I didn't realize I was being so aggressive with the angle.) It was causing the red warning light to go off ... which I now take to mean that I was "dead" ... which is probably why I lost control of the flight system. (Perhaps if I turned off Damage and failure simulation, then that wouldn't have been an issue. But I kind of prefer "dying" if I do it wrong.)

Try de-orbiting much further away, at least 1/3 orbit before the target. Burn until the PeA is < 50k, or until the flightpath ends at your target, your choice. Then, regulate the rate of braking by changing the altitude - lower altitude = faster braking.

I tried deorbitting a couple times from much further out ... 7M and 10M. In both cases, I still overshot Cape Canaveral ... but I managed to slip into the atmosphere gently enough not to set off the red warning light. (So I finally had control of the Delta Glider when I got below 30km.)

So I guess now it's just a matter of figuring out how to calculate all the variables. Seems like this is going to be even harder than landing on the moon. I actually figured landing on Earth would be easier since I have quite a bit of Flight Sim experience. I figured if I could get the darn vessel back down into the atmosphere without burning up, then landing wouldn't be too bad. (I know how to use ILS and VOR towers in flight simulators.)

But calculating how to get down out of Orbit and in the relative vicinity of a given target is, apparently, going to be much more challenging than I originally anticipated.

Jeez ... you'd think this was rocket science or something.

 

[Tommy]

I didn't think of it, but yes it is possible to "break" a DG by overstressing the wings. If you look at the bottom of the 2D panel (you may have to scroll the panel to see it) there is a "Wing Load" gauge. Pulling excessive G forces will "break" the wings and control surfaces won't respond any more. You can dig into the atmosphere more than should be possible - if you keep the AoA low. The wings can be damaged by forces along the vessels "up/down" axis - but not the "fore/aft" axis.

Atmospheric re-entry is one of the hardest things to do in Orbiter - even with a heat-proof DG. The stock DG doesn't have an autopilot to maintain the high AoA (typically 35 to 40 degrees) used in aerobraking the shuttle, or the DGIV or XR series add-ons. You need to shed a LOT of velocity, and it takes practice to learn what speed you should have for a given distance to target. As a hint, the last 2 km/s go pretty quickly - you'll want to be within 1M of the target by the time you are down to that velocity. As I said, you can control the decelleration rate by changing your altitude, and the stock DG can shed speed pretty quickly if you force it lower. I suggest using elevator trim rather than the stick - this will help prevent over-controlling the vessel and causing high G loads.

Don't be afraid to de-orbit early - you can circumnavigate the planet after the de-orbit burn if you keep the altitude high (around 65k - 70k to start, dropping down slowly as the velocity reduces). If you seem to be coming up short, gain altitude and "skip" a little bit.

If you need to slow down quicker, sometimes it helps to use S-Turns - this allows you to use neutral trim (actually couple clicks downward trim may be better) and use a 90 degree roll angle to "knife" down into the atmosphere, and use slight variations to the roll angle to control the descent rate without overloading the wings. You'll need to reverse the roll from time to time to keep yourself headed toward the target - make sure you have the trim at -2 clicks when doing this.

 

[lyndell1974]

After I align my orbit to Cape Canaveral I turn retrograde and perform the burn half an orbit before Cape Canaveral. I burn until the periapsis drops to 60K, turn prograde, and level the wings to horizontal with the surface. The map won't show the marker to be right over Cape Canaveral (I believe I read somewhere that is due to the map not taking the atmospheric effects into play) but after re-entry I usually end up close enough to see the runway. Just my 2 cents.

 

[Fabri91]

What you might want to use also is the [ame="http://www.orbithangar.com/searchid.php?ID=2139"]AerobrakeMFD[/ame]. It's imho essential for precise reentries, as it predicts your impact point based on your attitude and speed.