Could need some basic help with reentry

[Tommy]

Don't start Aerobrake MFD untill your alt (from your target body, such as Earth of Mars) is less than 20M. Also note that Aerobrake doesn't consider atmospheric effects untill you're under 200k (at least on earth) - above that it show a simple ballistic trajectory.

 

[Hoggorm]

Hi again,

The last few weeks I've practised docking, changing orbits, and I've even flown to the moon, and landed on the base there. However, I've tried, and failed many times, but I'm still unable to get a good reentry...

I've read the "Concepts for atmospheric reentry" tutorial by ar81 and done a lot of searching the web without me getting any smarter... Mostly I've tried "Mission 3 - Deorbit" found in the Checklists folder and the Delta Glider. I've tried using the AeroBrake MFD, but I am still confused... After undocking with Mir, I turn retrograde and bring up the AeroBrake MFD. I then burn retro until the "impact pointer" is very close to my target base Cape Canaveral. I then turn prograde and either:

1 - Keep AoA 0 deg and wait until I reach the atmosphere
2 - Pitch for AoA 40 deg approx and wait.

But in both cases I end up with skipping out in space again and land in Africa...

Once again I have to ask if someone could be so nice to learn me, step by step preferably, how I can get a good reentry - and hitting the base I'm looking for.

What values am I looking for in the MFD? What steps am I missing?

Or - is there any tutorials or replays that I've missed? I have not seen any replays for example...

Obviously I'm doing something wrong with the AeroBrake MFD

Thanks,

 

[Urwumpe]

Don't forget you also need to bank, usually when your vertical speed during reentry is zero.

 

[Tommy]

You should be doing your de-orbit burn when you are near the opposite side of the planet from the base you want to land at. Over 200k alt, AerobrakeMFD doesn't consider the atmosphere and wings. Under 200k it will show your path, but assumes you will hold your pitch and bank steady throughout the re-entry. The easiest way to to it is once you are under 200k Alt, pitch up to about 40 degrees AOA. Use AerobrakeMFD's mode that shows the world map, like the MapMFD does. adjust your pitch and bank untill the green line ends at Cape Canaveral. The upper right corner of the MFD shows a more detailed target. Once the green path ends at Cape Canaveral, engage the Autopilots on AerobrakeMFD (the <AOA> and <BNK> butons on the right side). If you read the AerobrakeMFD manual, it tells how you can make small adjustments to keep you on track. I think its SHIFT-2 and SHIFT-8 for AOA and SHIFT-4 and SHIFT-6 for bank. If you have AeobrakeMFD in the right MFD you must use the right shift key. If it's in the left MFD, use the left shift key. Use the numbers on the num-pad. You shouldn't need to make big adjustments once you are on target.

 

[Hoggorm]

Urwumpe,

Thanks! You answered one question I forgot to ask. Until now I have just guessed when to bank. I'll try your method next time!

Tommy,

Thank you! I'll try this also! Correct me if I'm wrong, but it should not be any difference if I pitch to 40 deg AoA just after the de-orbit burn, or if I do this at 200 km altitude?

Anyway - I first did the burn until the impact point in AeroBrake MDF is close to the base. Then I turn prograde and pitch to 40 deg AoA. While I pitch, my impact point tends to go far beyond the base, and I will be landing in Africa. It seems like the AoA should have been more or less so that my ship does not produce lift and climbs again. I'll experiment a bit with your hints and tips. Thank you.

 

[Urwumpe]

Urwumpe,

Thanks! You answered one question I forgot to ask. Until now I have just guessed when to bank. I'll try your method next time!

Just remember this rule of thumb: Your descent speed defines your G-load and reentry heating. If you want to slow down at a constant rate, you need to fly at the right descend velocity, requiring only small corrections all the time.

You leave the AOA constant, but control the direction of your lift vector with the bank angle - and that way you can change your descend speed.

Doing this first bank at 0 m/s vertical speed has the advantage, that you can use the bank angle first for just achieving level flight (Not = wings level and nose horizontal, but level flight as not changing altitude) and that way get a feeling for the flight dynamics of your spacecraft.

 

[Hoggorm]

Tommy,

I just tried the tips you gave me to see how it all worked out. I did the burn on the opposite side of the earth for just a few seconds until the impact pointer ended approximately on Cape Canaveral, then I waited for 200 km altitude over the Pacific where I turned prograde and pitched to AoA 40 deg. AeroBrake then told me I would be landing in Africa again, so I pitched further up to about 52 deg AoA before the MFD told me I would hit somewhere close to the cape. Then I keep 52 deg AoA, just checking that the impact pointer remains at the Cape, and it does. Then, at about 70 km altitude, I start to climb again... Why might that happen you think?

I then try Urwumpe's hint and start a left bank. I am above 0 m/s (actually climbing quite fast), and this makes the DG go all over the place... As I still have no aerodynamic forces on my ailerons and rudder, the bank did not turn out well...

Just wanted to hear if anyone might have a comment on what I did wrong this time? In the meantime, I'll be in the sim, trying once more

Thanks!

 

[ar81]

Then, at about 70 km altitude, I start to climb again... Why might that happen you think?

Climbing again is a sign of bouncing against atmosphere because of lift.
Did you start your deorbit at low altitude (barely aove 150 km)?
If your entry was too steep you will go too fast and it might produce more lift.

I would advise you to try Delta Glider IV first.
Stock Delta Glider has too much lift.

 

[Hoggorm]

Climbing again is a sign of bouncing against atmosphere because of lift.
Did you start your deorbit at low altitude (barely aove 150 km)?
If your entry was too steep you will go too fast and it might produce more lift.

I would advise you to try Delta Glider IV first.
Stock Delta Glider has too much lift.

No... I started at the same altitude as Space Station Mir (about 370 km?). I've tried to follow the tutorial that comes with the Orbiter Manual, trying to modify it a bit with the AeroBrake MFD and do the de-orbit burn on the oposite side of Cape Canaveral.

I'm returning to the sim...

 

[Urwumpe]

http://www.orbiterwiki.org/wiki/Intuitive_Atmospheric_Entry

 

[Tommy]

Bouncing a bit is normal when using Aerobrake. As long as AerbrakeMFD shows your endpoint at Cape Canaveral you can allow some bouncing, just hold the AOA as is and the ship will come back down.

Also, if you are flying the stock DG, it's aerodynamics aren't really correct for a realistc reentry profile. Many people use Delta Tweak, [ame="http://www.orbithangar.com/searchid.php?ID=2648"]DeltaTweak 0.2[/ame] , or an add-on such as the DGIV or XR series.