Flight Question Turn upright from inverted reentry

Darkriser

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Hi all...

Recently, I've been practicing the inverted reentry using XR-5 and I love it.
Now I'm able to slow down without burning up when returning from Moon and Mars.
What I still can't do properly is turning back upright once I've slowed down enough and I'm not able to keep the Alt.
What I do:
  • just before reentry I set-up the Attitude AP to 180dg bank, 80dg AoA and engage it
  • when the VS rises to around -150m/s, I quickly lower the AoA to 30dg to decrease VACC and once around 0, increase the AoA as needed
  • keep adjusting AoA to descend, slow down and not to burn up
  • from experience I know that once my SPD drops to M20, it's too late because I can't keep VS from rapid decrease anymore
  • so around M22-23 it's time to turn back and continue in 'normal reentry'
During the inverted reentry I have the Att AP engaged.
Once ready to flip over, I disengage the AP and start banking manually for a 180dg flip.
I succeed maybe in 1 of 5-10 attempts as I don't have any given procedure to follow to perform the flip.
Vast number of attempts end up by 'HEAT DAMAGE' that occur during the flip.
Success is always matter of luck....which doesn't seem a good approach to space flights :eek:

Do you have any hints/procedure to follow in order to master the flip without damage?
Many thanks.....
 

GLS

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Control your vertical speed (and thus drag) with bank angle instead of AOA.
 

Darkriser

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Control your vertical speed (and thus drag) with bank angle instead of AOA.
Of course, once I'm no longer in an inverted reentry, I do exactly that.
My problem is that I have problems to 'exit the inverted reentry' and continue a normal one (where I can control my VS by changing bank angle).
 

GLS

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What is the "inverted reentry", bank = 180º? Nothing special about that, other than the lift vector is pointing down. When you're happy with the descent rate and want it to level off, bank to 90º (lift vector to the side) and keep playing with it... unless there is a control issue that prevents bank control.
 

Marijn

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80 deg AoA seems a lot to me. I don't think that makes it any easier. But regardless of that, I think you shouldn't roll 180 deg. What I do is rolling about 100 degrees when vertical speed is still high at about -500m/s. Then a little bit of lift is pointing upwards, helping to reduce the vertical speed to not burn up. When the vertical speed is about -150, I roll about 20 degrees back to point a little bit of lift downwards again to maintain about -50m/s vertical speed. From there you can do the usual roll reversals.

If you share a scenario with the approach set up, I see if I can make a video.
 

Darkriser

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I'm attaching the scenario and also recording of my procedure until M20 (didn't try to roll, just wanted to show the entry+slow-down).
  • until VS: -150 -> do nothing
  • at VS: -150 -> set AoA to 30dg
  • watch VACC to keep between -1 and 1 (to keep Temp in reasonable limits)
Pls, ignore the Warning - LOX is low since this is a return flight from Mars.
(if I missed some files, let me know, pls)
 

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  • entry.scn.zip
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  • entry_rec.zip
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Marijn

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I had a few attempts at it. With the technique I am more familiar with, about 35 deg AoA initially, I can safely balance the vertical speed to about -50m/s. But then it doesn't seem possible anymore to not overshoot the base. I would correct that normally by adjusting the de-orbit parameters (angle, anticipation and altitude). Do you know which values you used?
 

Darkriser

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I had a few attempts at it. With the technique I am more familiar with, about 35 deg AoA initially, I can safely balance the vertical speed to about -50m/s. But then it doesn't seem possible anymore to not overshoot the base. I would correct that normally by adjusting the de-orbit parameters (angle, anticipation and altitude). Do you know which values you used?
Sure :)

Alt: 62.00k
Ant: 50.00dg
ReA: 5.5dg
Ang: -0.31dg

Scenario attached....
 

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Marijn

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Ok. I am usually setting lower ReA values, like 1.5 degrees, although it seems harder to find a solution with IMFD's Base Approach program when using a lower value. The scenario posted above is already too close to earth to change this.

But I think you should experiment with 1.5 degrees. You'll hit the atmosphere with less vertical speed. Then it's easier to do the roll.
 

Linguofreak

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Of course, once I'm no longer in an inverted reentry, I do exactly that.
My problem is that I have problems to 'exit the inverted reentry' and continue a normal one (where I can control my VS by changing bank angle).

You can reduce bank from 180 to 170, then to 160... until you reach 90, at which point you're on the boundary between inverted and upright. 180 gives max downward lift, angles between 180 and 90 give progressively less negative lift until your lift is entirely sideways at 90, then angles less than 90 give progressively more positive lift.
 
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