Flight Question Post-Reentry: Transitioning XR5 from stalled to flying.

[Scav]

Hi guys.

Setup: Past peak heating; later stages of re-entry. I'm at between 200,000' and 150,000 feet of altitude, at a range of mach 10 and mach 9.

Symptoms: At this altitude and velocity range, vertical velocity will begin to decline rapidly at an AOA of 30-35 degrees, from an initial value of ~-300 fps. The rate of vertical velocity decay is approximately 20-60 degrees per second squared, rising exponentially over several seconds' flight time; there is little time to recognize the shift from stalled-wing flight to natural flight as my only cue to this is the multifunction HUD on the right.

Aggravating circumstances:

1. Altering attitude control from AOA-centered to pitch-centered, and forcing the nose down when the vertical velocity reaches critical decay will dramatically increase the rate of decay and 'pop' the ship into positive vertical acceleration, resulting in a 'phugoid'-like pattern in the air. This will, naturally, blast me well over the vertical path which at multi-mach flight less than two hundred miles from my destination is rather undesirable.

2. Doing nothing and maintaining a nose-high attitude will precipitate an incipient stall into an advanced stall. Do not want, especially over a patch of fake pink flamingos.

Question:

Is there a better way to mitigate the violent 'porpoising' effect I've outlined here? I've noticed the DGIV autopilot (which probably has a MUCH smaller airframe to work with) seems to have an algorithm to slowly lower the nose after a given AOA, velocity, and altitude variable. Must I mimic the same to get the desired result (a smooth transition)?

Thanks in advance.

 

[ADSWNJ]

Simple ... use Aerobrake MFD alongside the XR-5's built-in attitude autopilot. You can see the entire projected reentry corridor, with G's, heating and vertical acceleration. You'll notice that the reentry glideslope and angle creates very different effects. Try 0.7 degrees slope and 38-40 degrees AoA for a start, but vary the nose up and down all the way down the slope to keep things smooth and to track to the target.

 

[Tommy]

First of all, PLEASE use the metric measurements as given by orbiter - alt in km, velocity in m/s or km/s. Those are the numbers most of us use - and I'm not going to be bothered converting english system figures to metric.

As such, there's not much I can say about your current profile since you used numbers that are fairly meaningless to me as they are.

I can say this much - you should be transitioning from High-AoA to conventional flight at an altitude of 20km - 25 km, and at a velocity of around 1km/s (plus or minus a couple hundred to adjust for distance), and about 100km from the runway.

To make the transition, just hit the spacebar to disable the AP. You will have an increase in VS - but often this just brings a high negative rate closer to zero. The XR-5 is BIG, so it has a lot of rotational inertia. It will take longer for the vessel to rotate than a DG-IV or XR-2, not much you can do about that.

If you are bouncing too high during the transition it most likely means you still have too much velocity.

 

[Scav]

Okay.

First off, apologies for the conversion confusion. I've spent WAY too much time in my life sitting in front of the flight sims. :blink:

Secondly . . .

I can say this much - you should be transitioning from High-AoA to conventional flight at an altitude of 20km - 25 km, and at a velocity of around 1km/s (plus or minus a couple hundred to adjust for distance), and about 100km from the runway.

This actually helps me out a lot, and I'm glad you said that. I've been shooting my reentries pretty much by feel, aided by the MFD's when it comes to the actual flying of the vessel; this gives me the opportunity to be more proactive rather than reactive.

I'm going to take a look again at how I've been doing things, but seeing as how I've been transitioning at that fast a speed (around 1.8-2 k/sec) definitely seems to answer why the transition is so abrupt.

If you are bouncing too high during the transition it most likely means you still have too much velocity.

Yes . . . at that airspeed the air is still much like skidding on a glass wall, to use an analogy.

I guess the biggest thing that was stumping me was the L/D relationship at that speed -- the higher the AOA, the greater my sink rate, but only up to a point. Once that point is reached, the wings are producing more lift than drag, resulting in a nice launch back upward.

So, to restate the principle, I need to lower the AOA at the right time to stave off the XR5's natural tendancy to produce gobs of lift at the wrong time, and in a VERY circular way, come back to what you were talking about: 20-25 kms high, ~100 km from the runway at around 1 km/sec of velocity as a target for when this should happen.

I'll work this out again when I have some time, with those targets in mind. Thanks again!

 

[Tommy]

The method most commonly used with AerobrakeMFD is a lengthened re-entry at a constant AoA, and results in a rollercoaster ride. If I'm performing an "optimal" re-entry (fairly consistent descent rate) I'll have an AoA of anywhere between 35 and 45 degrees (depending of vessel mass - higher mass = lower AoA) for most of the re-entry. I usually find I have to lower the AoA by 5 to 10 degrees toward the end to maintain a steady descent rate - so I'm at 30 - 35 degrees AoA by the time I transition.

Keep in mind that an XR-5 has a HUGE amount of lift available - so if you are re-entering light (no cargo, low fuel) you may actually want to hold off on the transition until you are down to 600m/s - 700m/s.