AeroBrakeMFD Trying a high PeV reentry and failing

[Scared Bunny]

Hi,

I'm trying to do a reentry where I reenter Earth's atmosphere at velocities between 10k and 11k m/s. I'm flying the DGIV and I'm not trying to land somewhere. At the moment I'm only trying to prevent myself from bouncing back into space.

I already found out that 10k+ m/s is too high a velocity for the DGIV to bleed off inside the atmosphere towards a sub-orbital trajectory. At least when using the DGIV reentry autopilot.

After some thinking and experimenting using AeroBrakeMFD I came to conclusion that the only way for a DGIV to reach sub-orbital velocity is when it reenters "upside down". That is, with a bank angle of 180 degrees and an AOA of about 20-30 degrees. In this orientation all the lift produced by the DGIV fuselage is directed towards the earth's surface. The negative lift thereby negates the upward acceleration caused by the orbital speed the DGIV still has at that point. Once the DGIV reaches speeds in the order of 8k m/s, I can then rotate the craft into an upright orientation and do a normal reentry.

That's the theory. Practice is that the DGIV Pro104SpecNN reentry autopilot does not allow a bank angle beyond 90 degrees. Why I don't know. It seems trivial. AeroBrakeMFD's autopilot does allow bank angles beyond 90 degrees, but the autopilot does not correct for side slip, and at bank angles near 180 degrees the autopilot often becomes confused. If the bank angle is 180 degrees and because of some side slip the bank angle changes to -180 degrees, the autopilot wants to perform a complete 360 degree spin around the craft's ventral axis. Even though a minute correction of some degrees in the opposite direction would have done the same thing. In other words, AeroBrakeMFD's autopilot is too unstable, and a bit obtuse.

Questions:
- Is the upside down orientation the only way for a DGIV to do a high velocity direct reentry?
- Is there some other MFD that allows better control of AOA and bank angles during reentry?
- Is there perhaps some way to adjust the DGIV's reentry autopilot to accept bank angles greater that 90 degrees?

Thanks

 

[Tommy]

The answers are Yes, No, and No, although AttitudeMFD may work a bit better than AerobrakeMFD's AP.

The XR vessels are easy to do an inverted re-entry in becuase you can shift the COG a bit to the rear and hold the higher AoA, but for the DGIV (or stock DG) is not too hard to do it with a low AoA. Aim for a PeA of 70k (DGIV) or 65k (stock DG), then use elevator trim to maintain altitude. Keep in mind the the vessel is "unbalanced" when inverted so you'll be adjusting the trim fairly continuously - keep the VACC low or you'll end up in trouble. You can safely use the airbrake on a stock DG, and usually on the DGIV as well if you keep the altitude high enough. The low AoA means low drag, so plan to make up to a full orbit during re-entry.

 

[dgatsoulis]

Questions:
- Is the upside down orientation the only way for a DGIV to do a high velocity direct reentry?
- Is there some other MFD that allows better control of AOA and bank angles during reentry?
- Is there perhaps some way to adjust the DGIV's reentry autopilot to accept bank angles greater that 90 degrees?

Quick answers:
1: No, but it's the most fuel-efficient one.
2: I can't think of a better one than Aerobrake MFD, it controls your AoA and bank, and you can see where you'll land.
3: No, there isn't a way, unless DanSteph decides to implement one. (Well... there is an Attitude MFD somewhere... but i haven't tried it. Not sure if it would work.)

Now, let me explain:

1: (For speeds 10-11.5 km/s) You could try entering the atmosphere 'upright' with a VERY high AoA (something like 65-75 degrees, or even more).
Ofcourse, the DGIV would "sink" very fast and you would be burnt into crisps in a few seconds... but you DO have powerful main thrusters. If you have enough fuel, you can burn (even with the turbopumps, if neccesary), untill
your vertical speed is close to zero, at a preselected altitude. (72-73 km works just fine for those speeds).
Here is a tip: Watch your temp. When it's about 4/6 away from you meeting Santa (up-close), start applying thrust untill your vertical speed is close to zero. Give it a few tries. It does work for those speeds. (A bit more difficult for 13+ km/s though).

2: Don't use the autopilot and the Aerobrake MFD at the same time. Let one of them be "in charge".

3: There's no need for DanSteph to implement an "inverted" Autopilot. The DGIV-2 can handle "upright" re-entries with speeds up to 12.5-13.5 km/s. (At least, those are the ones i've tried).

Have fun, happy orbiting!


Edit: Tommy beat me to it, but when it comes to advice, ALWAYS listen to what he has to say!