Flight Question Banking Entry

[aftercolumbia]

I've seen a lot of "How do I enter and land?" and I'm a little surprised at the paucity of information on bank-controlled lifting entries. Almost all entry tutorials I've come across are alpha modulated and nearly ballistic, which is not normal for real spacecraft, and IMHO, quite difficult to fly, especially if you're trying to do it manually. I discovered this after I uploaded this 101 minute monster:

I used CamStudio to capture the video (excellent software, did not affect Orbiter performance at all - http://camstudio.org - the quality issues are my fault and somewhat on purpose to keep the upload time measured in hours rather than days.) I captured the window area of my screen, and had behind the Orbiter window, a text file which I typed into. I did not capture audio (I have no mic right now, and the audio stream would have added significantly to the size of this file.) There were a few places where I wanted to type during the flight, but was too busy with the craft, so I added annotations in Youtube.

This flight starts by showing how to set up the Delta Glider to passively maintain a 10.7deg angle of attack (alpha), which is high enough to require yaw input and is somewhat more realistic than what is normally possible.

I have found entry and descent nearly impossible without the AerobrakeMFD located here:

[ame="http://www.orbithangar.com/searchid.php?ID=2139"]AeroBrakeMFD[/ame]

This is from a guy who made a tutorial before it existed:

http://aftercolumbia.tripod.com/entrytutorial (pop-up/malware advisory)

The deorbit for the current tutorial had an apoapsis (maneuver point) over the destination of 306.1km and a periapsis of 69.9km.

The first step in entry is to stabilize the glide so that you're not skipping out of the atmosphere. This requires keeping an eye on your vertical speed and bringing it to a low value which smooths out the curves on the AerobrakeMFD's energy and heating/loads displays. (Most entry tutorials I've seen don't even use them.)

Roll reversals are tricky because they bring the lift vector through vertical and lead to a reduction in sink rate and slower deceleration. If not compensated for, an overshoot will result. (In real life, the Shuttle DAP did this automatically, but apparently Shuttle engineers didn't study it much until after the STS-107 accident, despite the fact that the PASS had been in operational service for twenty-two years by that point! Roll reversal effects are likely part of the reason no Shuttle pilot has ever successfully controlled an entry manually (meaning the Control Stick Steering (CSS) fly-by-wire mode) in the Shuttle Mission Simulator, despite hundreds of attempts.) This tutorial shows several roll reversals.

The Delta Glider never displayed entry flames during this flight because I kept it cool enough. Part of the reason is because the long track from west of Australia to Cape Canaveral gave plenty of room and time to dissipate the orbital speed, and the flight never pulled even three gees. I also did a visual runway alignment (I can use the HSI, but IMHO that is the subject of another tutorial.)

As the craft slows, you can see how the DG's awesome L/D of 4.5 actually begins to turn the craft at speeds as high as Mach 10. I turned the craft from flying north on a "downwind" leg over Miami at about Mach 3, to come down on Runway 15. I can hardly wait to do this using the Delta Glider IV.

Terry

 

[agentgonzo]

Orbiter's aerodynamics modelling is far from perfect. Especially at high (>mach10) speeds. My understanding (which may be wrong - please correct me if I am) is that the orbiter aerodynamics model generates far more lift at high mach numbers than you'll get in real life, meaning that you can do your 10° AoA reentry and get sufficient lift to slowly bleed off the speed. In reality, you'll get very little lift at high speeds and reentry will be largely ballistic (subject to atmospheric deceleration) at high mach-numbers.

Also, in reality, the shuttle requires high AoA to push the shockwave further in front of the craft to prevent overheating. At lower AoAs, the profile of the shockwave is a 'sharper cone' and is located closer to the nose of the craft which increases the heat flux onto the skin of the craft and would be too much for the TPS (thermal protection system) of the craft.

But , in orbiter you can have very shallow reentries with very low temperatures. I did one a few years ago in the XR2 and kept the skin temperature below 150°C the entire way.

 

[aftercolumbia]

I don't think anyone models skin temperatures in Orbiter with much accuracy, and also, the real issue with the Shuttle's necessarily high alpha is wing and drag loading. The shockwave standoff distance is mostly a function of how round the object is, and this is part of the reason why wing panels RCC 8L/R and 9L/R are hotter than the nose (the other part of the reason is because this is where the sweep kink is and most significantly, where the shock wave from the nose interacts with that of the wing.)

The Space Shuttle's L/D at 40deg AoA is about 1.5. Real-life banking entries have been accomplished with as little as 0.3 L/D (i.e. Gemini), Apollo had 0.5 L/D, and Curiosity is about to set a new record of 0.18 L/D at Mars. (Viking did not use a banking entry profile, btw.) I seriously doubt that "conventional" wings could provide more than about 2.0 L/D in the hypersonic realm, but if one could control charged plasmas using electrostatic or electromagnetic fields (Avro was researching along these lines in the 1950s), than one could use them to protect sharp wings and also provide much higher L/D at hypersonic speeds. This, combined with Delta Glider's fusion powerplants, forms a plausible explanation for her high L/D at all speeds. I recall from the old forum days that the Delta Glider's subsonic aerodynamics were proven implausible in X-Plane, and that she lacks directional stability. From that, it is obvious that if a craft similar to Delta Glider ever came to reality, it probably wouldn't look much like the one we see in Orbiter.

Terry