Flight Question Inverted reentry with DGIV

[laukejas]

Hello,

As I'm doing the free-return from moon with DGIV on Orbiter 2010P1, I decided to make direct reentry. My speed at periapsis is about 11.02km/s. I think the only way to reenter is reenter inverted. But I can't seem to get knack of it. I burn up. Here's what I do:

1. Set Periapsis to 70km,
2. Roll inverted with AoA of 30, trim up, select control - air surfaces + RCS (with F8 method)
3. Endure until periapsis is reached
4. Try to lower the AoA to produce negative lift, so I can make my vertical speed near zero.

Problem is, that in the 4th step, if I try to lower my AoA, atmosphere "catches" my aircraft, and pulls out of stall. My AoA becomes near zero, and I can't stall again, there isn't enough force from ailerons and RCS even together. Then my vertical speed drops rapidly, and in about 10 seconds, I fry up.
If I'm careful, and don't lower AoA, so atmosphere won't pull DG out of stall, I bounce off the atmosphere, and in the upper altitudes, where heat is no more a factor, I can't produce enough negative lift.

If only I could slip back into stall, I think I could manage it. But once my AoA drops below 25, nose is pulled towards velocity vector, and I can do nothing but watch this "DESTROYED" sign.

I just made my 11th attempt with same scenario. Still burn up. Very dangerous, that inverted reentry is.

What am I doing wrong here?

 

[Mister Mxyzptlk]

Inverted when you can afford it, S-turns when you can't. Are you using Aerobrake MFD?

 

[agentgonzo]

What altitude are you at when you drop out of the stall? If you set the perigee to 70km then begin your inverted reentry, the aerodynamics will pull your perigee lower. I normally target a little higher than that so by the time I reach perigee it's about 70km

Also s-turns (high bank attitude) as mentioned if you still can't do this

 

[laukejas]

Yes, I'm doing it with Aerobrake MFD. But what if these S turns won't bleed of enough speed?

I'm pulled out of stall at about 65-68km... What should I do if I'm pulled out?

 

[agentgonzo]

If you're pulled out of a stall, just go with it. At present, your altitude is being lowered too much by the lift (downwards) of your wings. The problem is not the lack of stall, but the altitude lost because of it. If you change your bank so that it is high, most of this lift will cause a lateral accelecation, rather than a vertical acceleration and you won't lose as much altitude. You can tune your bank angle to keep the vertical speed zero. This lateral acceleration will change your course over the earth though, so after a while, change your bank from being 'heading left' to 'heading right' (or vice versa) to negate this lateral change. This is the S-turns.

You won't lose speed as much as with a higher AoA, but at least you won't die

 

[Rtyh-12]

70 km? That's quite low, try to make it higher.

 

[RisingFury]

Increase AOA to as high as it will go. The higher your AOA, the lower the heating is with DGIV. If you do it right, then you won't need negative lift - you'll lose enough velocity when flying through the atmosphere. Higher AOA also creates more drag, which should help you out.

I've done many re-entries from geostationary orbit and some from the Moon. Also, don't rely on the data from Orbit MFD. The periapsis altitude tents to change as you get near Earth. It can change up to 10 km...

 

[markl316]

I use 71 kilometers. Also an AOA of 30 degrees is WAY to low. Start at 50, and GRADUALLY decrease it as you come in. You should bottom out at 62 kilometers. PM me if you have any more questions.

 

[laukejas]

Thank you for your replies,

I have tried higher AOA. But then I just bounce of the atmosphere, and my reentry fails. If I try to lower AOA, I get fried.

Tried higher PeA - I bounce off.

 

[Tommy]

As you have discovered, it's nearly impossible to maintain the correct AoA in the DGIV (also, in stock DG or any vessel that doesn't support COG shifting) when trying a high AoA inverted re-entry.

You can use a low AoA inverted re-entry using just the elevator trim to maintain the desired altitude. Since you don't have much drag at low AoA's, expect to make about a full orbit during the re-entry - and up to an hour in the atmosphere.

The best way to do a low AoA re-entry is to set the elevator trim at "full upward trim", and enter at a 90 degree bank. This way you can use small roll angle changes to manage your vertical speed, and can quickly change from a "downward" lift vector to an "upward" lift vector. Set the trim to about 1 "click" of downward trim when performing roll reversals to prevent a disasterous dive during the reversal.

 

[Grover]

i have a method for re-entry, nice and efficient (though it requires some patience):

just set PeA to 71Km (for that speed, you might struggle) then roll over and fly at AoA between 0* and 10*

this is because in a delta-wing (ive tried it a little bit) maximum lift occurs at 10* AoA, and you need a lot of lift generated to hold your altitude down

now, dont hold your AoA, keep manual control and keep the velocity ball (or vertical speed and acceleration) close to 0

if youre coming from the moon, i find that 72Km is best to start at, which means you'll probably start "level" flight at 71Km. this puts the CR2s hull temperatures into the yellows, but as long as they stay there, thats fine

as you slow down, you can afford to go deeper in, and it speeds things up, but if you go too far, and your temps hit the red, quickly go up a km or two to get them back into yellows at least, go too high though and youre on a one way trip outta the atmosphere, and an XR2 doesnt have the life support to hold you up there for a week while you come back

you might be able to save yourself with a burn towards the planet, but this is a last resort, because it costs a lot of fuel, which you might not have

its not the only way, and probably not the fastest, but its possible

good luck!