Reentry Error DGIV

[tekinozbek]

Hi guys,

As I've read in Atmospheric Reentry tutorial, I do my base alignments when 90 degrees to the base, on orbit. I reduce the distance of the first, or second orbit to around 1km.

Then, I move on to Aerobrake MFD for deorbiting burn.
Now here is the problem:

• When I burn retro, on opposite side of the base, the grey line lays on yellow line. However the reentry angle is either greater than 1.5 degrees or the speed is higher than 8, and the autopilot says "NO! I wont re-enter at this speed!"
• When I set 1.5 reentry degrees to below 1.4 degrees, the grey --+ indicator on AerobrakeMFD starts moving away from yellow.
• Sometimes, the yellow --+ indicator doesn't even extend as long as the grey line, therefore I can't event put them together, on top (--+ of yellow line doesn't come on top of the --+ of grey line).

Can someone help me through this? I undock from ISS then burn +NML or -NML for BaseSync, then use Aerobrake to check de-orbiting burn, but I never set the reentry parameters correct.

By the way, quoting:

-- "As the near pass is in the first orbit, and you are in the opposite side of the planet, you may start the deorbit burn." (Atmospheric Reentry)

and

-- "Entry angle should not exceed 1.5 degrees for Earth. Ideally it should not exceed 1 degree. It means you have your deorbit burn at the lowest possible altitude." (Atmospheric Reentry)

These two quotes don't match each other because sometimes, the opposite side of base isn't the lowest possible altitude.

So when do we burn?

 

[garyw]

Lower your orbit so that the low point is on the side of the planet opposite to where the base is that you wish to land at.

For example, landing as KSC means you need to lower your orbit over western Australia about 200km.

 

[tekinozbek]

I did that, but this time, as I came closer to earth, the --+ grey indicator started to move away from the base --+ yellow indicator.


-----Post Added-----


By the way, I would like to add this: my periapsis is never exactly on the opposite side of the base, does that really make a difference?

 

[RisingFury]

The AeroBreakMFD will only work correctly once inside the atmosphere - for Earth, that's 200 km.

Before you do your deorbit burn, make sure your orbit is below 200 km so you don't get massive speeds and so that your AeroBreakMFD will work correctly.

 

[tekinozbek]

I will try that, thanks.


-----Post Added-----


Nope, didn't exactly work. I ended up over Houston instead of Cape Can.

 

[Tommy]

I find that using AerobrakeMFD's Map view more useful, the one that show the planet map. You can safely re-enter directly from the ISS's altitude. Set your target in MapMFD, and when you are on the opposit side of the planet (when "dist" stops increasing (around 19M) and begins to decrease de-orbit. Watch the green line showing your path on AerobrakeMFD and stop burning when the green line ends close to your target. You should have a PeA of around 50k. This will give you a re-entry angle quite close to one degree, which is very survivable in all the glider type ships.

You'll have to wait until you are below 200k alt before you can set your AoA, Aerobrake doesn't consider lift and drag before then. Somply adjust your pitch until the green line ends at your base (banking if needed to correct heading) and engage the atitude hold AP. You'll usually need about 35 degrees AoA.

For ships without atitude hold AP's, like the sock DG, AerobrakeMFD has a pitch hold and bank hold function that are better than nothing (but not nearly as good as the DGIV's or the XR's)

 

[RisingFury]

I will try that, thanks.


-----Post Added-----


Nope, didn't exactly work. I ended up over Houston instead of Cape Can.

Do your deorbit burn around 16 000 km from the target, when deorbiting from lower then 200 km altitude. That way you'll go in steep enough to have enough energy.

 

[Tommy]

From 200k alt, you can de-orbit a DGIV anywhere from 14M to 20M away from target. You just have to manage your re-entry to get the range needed. That's where AerobrakeMFD comes in. Although, for shorter, faster re-entries Aerobrake isn't as good. It assumes a constant AoA, when a quicker re-entry requires frequent changes to the AoA to manage heating.

Using the longer (de-orbit opposite of the target) re-entry gives you a more predictable, lower temperature reentry. Using AerobrakeMFD makes pin-point re-entry and unpowered landings reasonably easy.

 

[tekinozbek]

I have to de-orbit on the other side because I use the re-entry autopilot on DGIV. When I use manual re-entry, I burn the thing

 

[RisingFury]

I have to de-orbit on the other side because I use the re-entry autopilot on DGIV. When I use manual re-entry, I burn the thing

Turn on the RCS and the control surfaces and you'll have enough torque on the vessel to keep it at 40° AOA at all times. That allows you to adjust your nose higher or lower to vary your reentry trajectory.

Lowering your nose will produce more lift and will slow your vertical speed (or as a better measure - your slope angle), but will temporarily increase strain on the heatshield. Raising your nose will increase your vertical speed (and your slope angle), but will temporarily decrease the strain on the heat shield - at least until your vertical speed increases... and you're trapped.

So keep you have to act way before you get into trouble. Keep your nose down when you see your slope go over 0.5°.

 

[Tommy]

I use the re-entry autopilot on DGIV.

Which reentry AP do you mean, PRO 104 or PRO 105?

If you are using the full-auto reentry AP (PRO 105) you have to de-orbit at the correct distance from the base. This distance varies depending on the altitude you are currently atltitude. From the ISS's altitude, you'll de-orbit 17,20 M from the base, and burn until your re-entry angle is 1.2 degrees.

If you are using the "Manual Reentry" AP (PRO 104) you can de-orbit opposite the base, and use AerobrakeMFD to help determine your AoA. Since the AP only adjusts in 5 degree increments, that's a wee bit tricky. Usually, on setting will leave you short while the next causes an overshoot. Use the setting that comes up short, and watch your hull temps. If needed you can lower your AoA a bit to help avoid overheat, but try to keep the AoA at whatever will get you closest to (but not past) your target. As you get slower, AoA adjustmets will have a smaller effect on your range, and you can slowly lower the AoA to get a bit more range and get closer to the target without going past it.

Learning how to manually manage a reentry is hard at first, but once you get the hang of it it's a snap. If you can't do it, you're pretty much stuck flying just the DGIV.

 

[HiPotOk1978]

Lots of good advice here, but one thing that hasn't been said. Reentry is a merge of science and art. Even still when I reenter with anything but a dry XR5 (which for me is the most easy craft to reenter), I still wonder if I am gonna make it... Rarely do I ever do a reentry that is what is called 'Dead Stick' (you glide all the way down to the runway w/o using your engines after deorbit burn). I just skimmed the article so I may have missed it. S-Turns are a wonderful thing to do to bleed off speed if you have too much. The idea is that the shorted distance between where you are and where you wan to land is for arguments sake, a straight line. Turning left and right causing you trajectory to not be a straight line but a series of S's and by doing so making your trajectory to be longer will give you more atmosphere to work with to keep from overheating

Hope my little post helps

 

[Tommy]

Reentry is just a matter of managing the descent rate according to the deceleration rate. I can reenter a fully fueled, fully loaded DGIV or XR series craft safely every time.

If you are using a high AoA (30 -50 degrees) a descent rate under 80 m/s will be safe. For a low AoA (under 20 degrees, used with extremely high bank for extreme crossrange reentries) you'll have less drag (and therefore less decelleration) so a descent rate under 30 m/s is required.

The main thing is to watch the vertical speed and VACC, and to think ahead. Keep the VACC very low so you don't end up not being able to "pull out" without burning up, and keep the hull temps at less than 80% max. AoA adjustments need to made before you are in trouble, small adjustments made early work much better than large adjustments made just-in-time.

With Aerobrake, and a de-orbit burn 180 degrees from base, I've never been in any danger of overheating and can make a deadstick landing consistantly. It's almost boring for me now, it's so routine. At least until touchdown - a fully loaded and fueled XR-5 is almost impossible to land "deadstick" without braking the landing gear. At least, I haven't managed it yet.

Also - have your descent rate below 80 m/s by the time you get down to 65k altitude. Above that alt you won't burn up, so start reducing the descent rate at about 75k alt.

 

[Messierhunter]

It's almost boring for me now, it's so routine. At least until touchdown - a fully loaded and fueled XR-5 is almost impossible to land "deadstick" without braking the landing gear. At least, I haven't managed it yet.

Dear lord man, why in the 'verse are you trying to land fully fueled? Are you trying to kill your poor little ummus? The virtual humanity!

I'm guessing you're just trying to come up with a new way to challenge yourself. For myself, the biggest rush so far has been to attempt a direct reentry/deadstick landing in an xr2 or DGIV directly from the moon. Agentgonzo taught me the trick, which is to go inverted with a small AOA for the early part of the re-entry, and carefully manage your vertical velocity so that you stay at about 75-80km altitude until you reach a safe horizontal velocity to assume a normal reentry. It's a very sensitive manuever in the early going; if you dip too low or your perapsis isn't high enough you burn up very quickly, but if you don't keep your aoa high enough you bounce off into space or you expose the cockpit to too much heating.

 

[Tommy]

I put together a tutorial on a direct re-entry from the moon a few months ago. Requires the XR-2. Tutorial uses IMFD, BasesnyncMFD, and AerobrakeMFD, but they aren't required to run the flight recording. It covers the whole flight from Brighton Beach to Cape Canaveral with annotations and a PDF Manual (manual by Mark316).

http://www.orbithangar.com/acc_viewaddon.php?ID=3660

And yes, re-entering a fully loaded and fueled XR-5 was just an attempt to challenge myself, and help me get landings down better. Still a bit weak on those, I break a lot of landing gear!

 

[Sky Captain]

Dear lord man, why in the 'verse are you trying to land fully fueled? Are you trying to kill your poor little ummus? The virtual humanity!

I'm guessing you're just trying to come up with a new way to challenge yourself. For myself, the biggest rush so far has been to attempt a direct reentry/deadstick landing in an xr2 or DGIV directly from the moon. Agentgonzo taught me the trick, which is to go inverted with a small AOA for the early part of the re-entry, and carefully manage your vertical velocity so that you stay at about 75-80km altitude until you reach a safe horizontal velocity to assume a normal reentry. It's a very sensitive manuever in the early going; if you dip too low or your perapsis isn't high enough you burn up very quickly, but if you don't keep your aoa high enough you bounce off into space or you expose the cockpit to too much heating.

This is one of my favorite things to do in Orbiter to make direct reentries in XR2 from the Moon or Mars, I have even managed to successfully reenter from Mars with a velocity of 15 km/s

 

[Tommy]

For inverted direct entries, a high AoA works just as well, and has the advantage of higher drag. You'll slow down to suborbital velocity quicker, meaning less time flying inverted manually, and reducing the chance of pilot error. It also means less distance traveled during entry and makes for a more predictable glidepath, increasing your chances for a deadstick landing.