Different Aero Model Launching from SPH vs. VAB?

[JonathanPerregaux]

Okay, I'm baffled by something. I built a large, jet-propelled hydrofoil in the VAB. During my R&D, I would roll it off the launch pad and guide it into the water to test. It ended up being such an efficient hydrofoil that it actually took off from the water into the air at a certain (insane for water) speed, allowing me to fly it around. I then made the thing a high-altitude flier as well as a water-loving boat. Eventually I got my "boat" to the point where I had it flaming-out in the upper atmosphere, having originally taken off from the ocean.

I eventually decided to launch the same craft from the SPH so I'd have a convenient runway. I opened the craft file from the VAB into the SPH and launched it. All of a sudden, it's flipping around like crazy on take-off. If I get it up in the air at all, it wobbles and twists, then ends up crashing. No matter what I do to modify the craft, I cannot not fly it. I tried using SAS, MechJeb, and Atmosphere Autopilot (all of which worked from the VAB), to no avail.

Finally, I returned to the VAB, re-opened the same craft, rolled it off the rocket launch pad to the runway... and ended up taking it for yet another smooth, fast flight into the wild blue yonder. It was stable as can be.

Do ships launched from the SPH obey different physics than the same ones launched from the VAB? Does opening a VAB craft in the SPH "corrupt" it somehow?

Edited January 5, 2017 by JonathanPerregaux

[sardia]

Do you have a picture of the craft?

[fourfa]

Perhaps there's a change in "control from here" axis between the two buildings? Flipping around crazily sounds like stock behavior for engines and control surfaces pointing an unexpected direction

[Sharpy]

Share the .craft please. Not only would it make troubleshooting easier, but it sounds like a nice craft!

[n.b.z.]

I have occasionally seen struts that were placed using n-fold symmetry getting messed up when transferring a vessel between SPH and VAB.

I am speculating that this could, maybe, also happen to auto-struts. So in case you're using auto-struts, you could try resetting / re-doing these in the SPH.

Other than that, I have no idea, but will be watching this space. I have my own "Different Aero Model" theory about launching from VAB/SPH, versus launching after using "Revert to launch". I have often observed large performance differences after reverting, but when I made a thread about that, nobody could confirm my findings.

[sal_vager]

Sounds like you need some gameplay help, but yeah sharing the craft file would be best.

[JonathanPerregaux]

Okay, here's my hydrofoil:

https://kerbalx.com/Hamhole/Spase-Bote-X1

The way the hydrofoil works is simple. First, get it in the water. Press Action Group 2 to deploy the fore, inboard elevons (and retract the landing gear). The elevons provide the lifting ability in the water, otherwise you'll sink at speed. All other elevons should not be locked in place so they'll provide control both on water and in the air. Only the bottom three elevons need to be locked.

When you get the boat up to about 30 m/sec. it should have risen up out of the water and will be slicing just over the surface with barely anything touching (as pictured). You will need to back off on the speed because it will pick up quickly. Unless, of course, you want to fly it up out of the water. When that happens, press Action Group 3 to retract the locked elevons used for water mode so you'll have full flight control in the air.

I stripped as many mod-based parts as possible from the model, including MechJeb, but I did use TweakScale to make the rudders and front landing gear larger.

The issue with flying it from the SPH may have something to do with the elevons reversing their directions from what I built, but I have not confirmed this. They must be reversed in order to work. Sometimes, even from the VAB, I find the elevons locked in strange directions.

Edited January 6, 2017 by JonathanPerregaux

[bewing]

Hmmm. I DL'ed it, but action groups 2 and 3 are missing. I rebuilt them.

And yeah, the rudders don't make contact with the water in stock.

And the back wheels are not quite on straight. Which makes it crab a lot when you are taxiing.

Getting speed wobbles (and subsequent RUDding) at 70 m/s ...

OK, turning on SAS at around 70 m/s fixed the speed wobbles. So I was able to get the boat into the air from both the SPH and VAB without any big issues. Got a bit of pitch instability which makes the nose bob while it's in flight. If you go too fast too low, it does start to go unstable (with all that negative dihedral) -- but that happened from both the VAB and SPH.

And yes, the elevons are being silly when it comes to their deploy direction -- but for me it was identical for both the SPH and VAB.

 

General quibbling:

You don't need those extra 6 radial air intakes at all. Replacing the outer 2 shock cones with medium circular intakes gets you plenty of air at all speeds.

 

 

Edited January 6, 2017 by bewing

[bewing]

OK, so I did a lot of messing around with your plane -- and I can't see any aerodynamic differences between launches from the SPH and VAB in stock. YMMV when it comes to any mods, of course.

And it's quite a nice little flying boat. I'd make a bunch of little mods to it to suit myself -- but still, a very nice little boat. With just a little less drag, it would be an amazing SSTO.

 

[JonathanPerregaux]

Yeah, I'm never sure just how many, or what kind, of air intakes I need. I was trying to get it to flame-out altitude, which I did, but went overboard (a boat pun?) in the process.

The elevons are my biggest headache. I kept second-guessing myself and readjusting their deploy direction. It's a great flying boat like you said, but rather bouncy in the air. It was never designed to be a plane, per-se, but it would be nice to be able to land it without the thing spinning out of control.

[bewing]

I was able to reduce the bounciness a lot with some tweaks. Getting rid of some sets of elevons & adding an extra canard pair.

Like I said, straighten the rear wheels first, using Absolute rotation mode. Then increase their friction, and decrease the friction on the 2 front wheels. That should help a lot for landing.

[JonathanPerregaux]

Thank you for the air intake advice. I've been highly rocket-focused in KSP, with a few odd aircraft here and there. Any other design advice is appreciated.

This "aqua-aircraft" ("Airquaft." — Elmer Fudd) is grossly anhedral (wings folded downwards) and rolls over like crazy at the slightest nudging. It usually goes into an unrecoverable death spiral after stalling at high altitudes. Would you suppose that adding any dihedral (wings folded upwards) air foils could compensate for this? I didn't want to add too many lifting surfaces, otherwise it would never stay in the water to fulfill its original purpose.

[bewing]

1 hour ago, JonathanPerregaux said:

Thank you for the air intake advice. I've been highly rocket-focused in KSP, with a few odd aircraft here and there. Any other design advice is appreciated.

This "aqua-aircraft" ("Airquaft." — Elmer Fudd) is grossly anhedral (wings folded downwards) and rolls over like crazy at the slightest nudging. It usually goes into an unrecoverable death spiral after stalling at high altitudes. Would you suppose that adding any dihedral (wings folded upwards) air foils could compensate for this? I didn't want to add too many lifting surfaces, otherwise it would never stay in the water to fulfill its original purpose.

Heh. Actually, after I added some dihedral wing and did some other playing around, I got your spase_bote to 324 m/s in hydrofoil mode. But then I couldn't get that design to lift off anymore.

But yeah, the roll instability thing was one of my main concerns when I was messing with your design and seeing if I could turn it into an SSTO spasebote. If I were going to mess with it some more, my basic aim would go like this:

You have those front and rear winglets angled down at 45 degrees (the anhedral that you already are very familiar with) -- and then you have your hydrofoil elevons at 90 degrees to that, so they are angled 45 degrees to vertical in the opposite direction. So, my basic point would be -- both of those angles are way too extreme. The elevons are fighting each other in the yaw direction; giving you large forces trying to rip your wings off sideways. Additionally, I was getting very little benefit out of the rudders that I could see. I couldn't really steer the thing (in bote mode) at over 40 m/s effectively.

So, I tossed all the radial air intakes, and made the rudders into aero tailfins on top of the craft. Changed out the outer pair of shock cones for circular intakes, as I said. Switched out your big landing gear in the back for a pair of medium gears (which made the thing sit level with stock wheels, which was nice). But what all that did was to clear out the entire underside of the spasebote. I also added one more small fuel tank just behind the cockpit to rebalance the CoM, and moved your nacelles down one click to align your thrust vector with the CoM more exactly. And, as I said, got rid of all the "upper surface" elevons -- because they are not pointed in the right direction to help with pitch and roll control.

And then yes, I tried adding a pair of raised dihedral wings to lift your CoL up -- to help with the roll instability thing.

But what I was about to say is that since -- after all that messing around -- there is nothing on the bottom of the spasebote, it seemed like the next logical step would be to make your anhedral winglets be at maybe 10 degrees instead of 45. And then attach the hydrofoil elevons at 20 degrees off the winglets, so they are angled at 10 degrees to the water, instead of 45 degrees. I think doing that may help your roll problem a lot (along with adding more control surfaces and reaction wheel that is devoted to roll and pitch control).

(And if you like, I can post some of my test versions of your craft for you to look at.)

Edited January 6, 2017 by bewing

[bewing]

OK, I tried out my suggestion. And ...

Woohoo! 400 m/s in bote mode (at 415 m/s it started to break apart). Heh! This is fun! It takes off from the water at 150 m/s if you tell it to. Flies really nice.  Lands really nice at 55 m/s with the front elevons deployed and full of fuel! Takes off at 100 m/s from land.

OK, ok, you have to try this. Sorry for modding your craft so heavily, but you just have to try this.

Craft file:

http://pastebin.com/raw/88PnLrte

(actually, I probably should have taken that extra oxidizer out of the two bicouplers -- forgot to do that)

Edited January 7, 2017 by bewing