Hitting the atmosphere properly.

penlu

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I cannot do reentries. When coming down from the ISS or Mir, I always fall short of the base I'm trying to land at. When aerobraking from the moon, I burn up. Can someone tell me what my periapsis should be when coming down from the ISS in a DGIV? Assume the burn happens 180 degrees from the base. Periapsis for aerobraking from the moon would be good too. And explain plane change maneuvers with assistance from the atmosphere. Thanks!
 

Kaito

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AerobrakeMFD.

From what i've heard, you should lower your PeA to about 80km when coming from ISS
 

Jarvitä

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There was a DGIV surface base distance/deorbit burn specs/AOA chart floating around the internet a few months ago I'm sure...
 

Scarecrow

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When re-entering from the ISS, or another low orbit, put your PeA at 50km. From the Moon, 60km works well, but be sure to fly upside down if you don't want to skip out of the atmosphere, so that you use your lift to counteract the tendency to rise after your get to your periapsis. Then turn heads up again when your ApA is where you want it, or (if you're planning to land) in the atmosphere.

For a plane change in the atmosphere, just turn sideways, so your lift vector applies force to the side.
 

RisingFury

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AerobrakeMFD.

From what i've heard, you should lower your PeA to about 80km when coming from ISS


I first lower my orbit to below 200 km, then lower my PeA to about 10 km... I do the burn around 16 000 km from the base.
 

Agra Barecasco

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Aerorake MFD

Actually there is serious problem between arrive at the base accurately and not to get burned in the re-entry.
If you use aerobrake MFD, you can manage your AoA to create a trajectory (atmospherical trajectory) that makes your vessel hit right at the base with small speed.
The trouble is when you are setting your atmospherical trajectory by AoA with the MFD, you still have to watch your falling speed and temperature.
If you fall too fast (greater than -150 m/s), the temp. will increase insanely fast that you won't have the time to recover your vessel attitude to safe position.
Small AoA means you want longer glide trajectory, small falling speed (or even positive, you go higher), and small risk to be killed in the sky. but if you keep this, you'll overshoot the base.
High AoA means you want to braking in the air, increase falling speed, shorter trajectory, increase the risk.
So the the key is to keep you vessel temp. is safe, by letting the trajectory overshoot the base first, after your speed is lower, use high AoA to shorten the trajectory. Keep watching your falling speed. Some times you need to alter your AoA form high to low (vice versa) frequently during the re-entry to keep a safe and accurate flight.

The most important factor of surface heating is your tangential and falling speed. So, you will fail an aerobrake maneuver just because your tangential speed is to high.
 

Tommy

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If you're using Aerobrake MFD, from LEO (ISS or lower), do the de-orbit burn opposite the target base, and drop the Pe to about 48k to 50k (or when the green line ends at your base in map view). Remember that AerobrakeMFD doesn't consider atmospheric effects until you are below 200k, so don't worry about setting your AoA untill you are lower than that. While you may have to make some adjustments to your AoA on the way down, keep them small, and unless you are very hot, always use the setting that leaves you short of the base not long. Once you're down to about 1500k velocity the main risk is over, and you can lower the AoA a bit to get the range needed to make base. As pointed out earlier, if your vertical speed is more than 150m/s decending, you'll be in trouble. 80m/s is best for the DGIV at a high (40 degrees or so) AoA.

For a TEI from the moon, 70k is much safer than 60K. A DGIV will burn up at about 64.3 k altitude at 7.5 m/s velocity. Re-entry speed on a lunar direct return is over 10k m/s, so a 60k Pe is almost certain death. Over 70K altitude the air gets to thin, you won't get enough lift (inverted) to keep you from rising back out of the atmosphere.
For a tutorial (annotated flight recording and PDF manual) for a direct re-entry from the moon in an XR2 (very similar flight profile and heat tolerance to the DGIV) see the following:
[ame="http://www.orbithangar.com/searchid.php?ID=3660"]Home Direct[/ame]

Requires the XR2 Ravenstar. While IMFD and AerobrakeMFD are used in the flight, they aren't required for playback.

For aerodynamic plane changes, see this thread:
http://orbiter-forum.com/showthread.php?t=6129
 

Interceptor

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Hey tommy. Can you please show a pic of what you mean by when the green line ends at your base, on aerobrake. Thanks
 
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Kaito

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Another thing to consider: Weight. The lighter your vehicle, the steeper re-entry angles it can sustain. So, if your high on fuel, just dump a fair amount. If you're good, you should only use a couple percent for some corrections. Or, if you're REALLY good, you can dead-stick it the whole way :p (i've come close to that...used about 2% fuel to keep my speed up)
 

Arrowstar

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That technique is how the KSC to WideAwake run is done so fast. By shedding all the vessel's weight, it allows us to re-enter at speeds and angles never thought possible... :)
 

Tommy

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Hey tommy. Can you please show a pic of what you mean by when the green line ends at your base, on aerobrake. Thanks

It's on the mode/view that looks like MapMFD. I'll try to post a picture tonight (about 12 to 14 hours from now), not at home right now.
 

Linguofreak

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Yeah. That map mode is way too well hidden. I didn't even know it existed until I saw it in a playback.
 

Tommy

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Sorry for the delay on this screenshot, had to drive over two hours in freezing rain last night and was all used up when I got home.

Anyway, in AerobrakeMFD hit the <PG> button until you are in map/graph mode, then hit the <PRJ> button until you see the world map. NOTE: I have add-on map images, so your display may look different. I believe AerobrakedMFD uses the same map images that MapMFD does.

You can see the green line showing the projected flightpath. The upper right corner contains a crosshair representing the base, and is used for higher accuracy.

Once you are under 200k alt, you can use the <MOD> button to change the info displayed, at the bottom in the screenshot is the "" Dist line that displays the distance from base the flight ends.

attachment.php


Hope this helps,

Tommy
 
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