From what I have read, these effects are so esoteric, that many more recent designs decided to go without. After all, during most of re-entry, the fin is in the wake of the spacecraft and can not have any effect.
You don't land such a lifting body on a runway without parawing, the landing speeds would have to be very high (even if you reduce the weight of the spacecraft a lot). And for inherently instable aircraft, we have invented FBW, which can do quite some magic on aircraft that just shouldn't be allowed to fly.
The problem they are alluding to is adverse yaw coupling. For a lifting body with little to no horizontal fin area on a subsonic approach, a lower body flap does not provide enough pitch/roll authority. You can see that the third iteration has additional upper body flaps because of this. While this solves the pitch/roll problem, it introduces an interaction with the vertical fins. An extended upper body flap produces a high-pressure region in front of the flap, which also pushes on the fin next to it. This pressure on the fin causes an unwanted yaw moment, which is doubly bad for a delta body because sideslip usually turns into a roll moment. By putting in a central fin, the pressure field acts on both the side fin and central fin, canceling the yaw moment.
Most of the designs lacking a central fin and upper body flaps are only good in the hypersonic regime and require parachutes to land.
Kocmolyf's response rings like a bell, doesn't it? I can actually visualize that area of instability and the problem with yaw movement. I'm guessing this is why the X-37B has it's tail fins positioned diagonally the way they are and at the angle.
I'm thinking your little machine is light enough that you will actually be able to skip across the upper atmosphere on reentry so effectively that if planned right (maybe a half an orbit) you will be able to de-orbit without even flaming out should you choose not to. It's one of the things I do "for fun" occasionally with the X37B - start skipping in and out of the upper atmosphere beginning around India and doing my final full atmospheric entry about half way across northern S. America. It's almost disconcerting. Great fun.
Man I'm looking forward to giving this one some test flights! Looking good Pipcard!
I have a confession to make before it's too late: I have never actually managed to re-enter and land any spaceplanes other than the XR2 and XR5.
Why? Because they are coded with a center-of-gravity (CoG) shift feature, allowing them to hold a high angle of attack during re-entry (e.g. 40 degrees or more). I am usually hesitant of re-entering non-XR spaceplanes due to their lack of attitude hold autopilot, but I tried manually re-entering the [ame="http://www.orbithangar.com/searchid.php?ID=5834"]Dream Chaser[/ame] once and it couldn't even hold itself up.
What kind of AoA are you supposed to have during X-37B/C re-entry (the normal way, not the occasional way)?
You should be able to reenter with a DGIV, The AP is different but accomplishes the same tasking.
After watching AutoFCS, I can bring anything back to a runway. But I 'd hate to be the crew (LoadMFD) they'd be all squishy after some of my recoveries.
Your craft looks pretty good so far Pip. Like Urwumpe mentioned FlyByWire has "fixed" may problems with instability, but what happens in the event of a failure? The opening scenes of "The Six Million Dollar Man" showed a crash of a lifting body craft being tested. Not fake, but actual footage of the M2-F2 lifting body; while your craft shows lessons learned from such a crash.
Funny you should ask this - I think I have a pdf just for you (fellow micro-shuttle fan). I've got "Footprint Problem with AoA Optimization for High Lifting Reentry Vehicle" open in another window.
Quick answer as per the text - lowest boundary for initial descent is 18.656 deg. At 20 deg. it quickly exceeds the heating rate boundary. The quasi-equibrium gliding part is a bit more complicated. In a nutshell, it really changes depending on the Mach you're traveling at - mach 25-18 AoA around 18.6, around mach 12 it's around 10 deg. I'll get back to you on this with some better data as time permits (it's one of those days out here I'm afraid - my wife had emergency surgery this AM and I'm hanging out in a waiting room as I type this - a nice distraction actually). I'll see if I can't find where I found this translated Chinese Journal of Aeronautics article and post a link in the pdf section here.
I'm a heretic here. Your confession actually makes me feel better! :tiphat: I prefer Buck Rogering my way around and while I have done things by the "book" on several occasions to prove to myself I could, I generally fly by the seat of my pants for all re-entries. It almost becomes intuitive after a while, at least in constantly guestimating where to begin skipping and what kind of AoA to attempt. With the X37B and C, it's actually not hard. The real trick is finding that sweet AoA and doing little skips while bleeding off m/s and not getting too much altitude between skips.
Mass vs. lift? The shuttle is huge compared to X37B. I'm guessing it's along the same lines of why it's easier to skip a small stone a dozen or more times across a pond while a stone bigger than your hand is lucky to skip 3 times before sinking. Same principle.
Those 90 degree turns I'd mentioned. Rolling past that gives you "negative lift", or a decreased altitude while still bleeding off the airspeed. If the descent rate is too fast, roll back to 70 or so. Watching the track on MapMFD tells me when I should roll over in the other direction (those pesky "S" turns).
Several craft want to "fight" the up elevator input, you just have to keep at it and deal with it. With Surface MFD I keep track of how my speed is bleeding off. Once I'm down out of hypersonic, I can relax a bit and lower the altitude a bit more. Usually this time I'm pretty close to the landing point anyway. Say an altitude of 25 Km, field (more or less) in sight, and mach 2-3. Dead-stick that sucker right onto the runway.
Takes practice though. And about 30 minutes from deorbit burn to wheel stop with no time acceleration once entry interface begins (that's just asking for trouble).