0.17 spaceplane stability issues

[Elmetian]

So I've been mucking around with 0.17 for the first time today (stupid RL job taking up my spare time :@) and I've found that my 0.16 spaceplanes are now unstable.

As you can see, I've got the CoM and the CoL more or less aligned (which I achieved painstakingly through trial and error in 0.16 I might add), and the CoT aligned along the plane's long axis, yet when the atmosphere thins above 30km the nose tips over to the left or right. I know that the avionics package is really for atmospheric flight and not reaching orbit, but my 0.16 SSTOs didn't have these issues before the update. It almost seems as though the avionics been nerfed (or made more realistic I suppose).

Can anyone else see where I'm going wrong?

Edited September 29, 2012 by Narcosis

[Grizzlol]

If it only tips when you are applying power then it is because the command module's gyro power is not enough to counter the forces exerted on the craft during acceleration. Hardly helps having a million wings and things clipping through other other things, either. Everything seems to be centred and aligned, though, so the non-vectored toroid engine should be fine, but you could try a vectored one to see if it makes a difference.

[sal_vager]

Try moving the center of gravity forwards of the center of lift Narcosis, that works for me, like how an arrow is weighted at the front.

Where you have it now you'll have a very twitchy aircraft, and if it were behind the C of L you'd have a devil of a job keeping her flying straight

[Fuzzy Dunlop]

If the problems are only occuring above 30 km it's probably not a centre of lift issue. And it only occurs on 0.17? Maybe the order the tanks empty has changed and this is shifting the Centre of Mass?

[7rex]

I too have the same issue, I have a plane that is a dream to fly in the thick part of the atmosphere. As soon as the atmosphere thins the plane goes nose up and spins end over end till it crashes.

[Elmetian]

Thanks for the replies guys.

Try moving the center of gravity forwards of the center of lift Narcosis, that works for me, like how an arrow is weighted at the front.

I tried this I think. It makes it more stable on the ascent to orbit, but almost impossible to pitch the nose up in the atmosphere without the nose oscillating from side to side. Might try it again, though it's a pain in the bum to rearrange the tanks.

If the problems are only occuring above 30 km it's probably not a centre of lift issue. And it only occurs on 0.17? Maybe the order the tanks empty has changed and this is shifting the Centre of Mass?

Nope, the ordering hasn't changed either. This is just an issue with 0.17, and I have my suspicions that Grizzlol is sorta right about the gyro's force being insufficient. Thing is, it flew fine in 0.16 regardless of my fancy wing arrangement. The only problem was a slight tendency for the nose to lift when the flight surfaces became useless in the upper atmosphere.

My plane doesn't glide very well now either. Before 0.17, I could bring it down unpowered with all tanks empty after a deorbit burn and it would glide smoothly. Now at about 10km up the nose drops and it goes into a steep dive even with the avionics on. It almost feels as though the efficacy of the gyro and the control surfaces has been reduced by quite a bit.

Also, I know the 0.16 fuel exploit was a bit daft, but it seems that no matter how much fuel I strap on this thing it still doesn't have enough to reach orbit. The few times I've managed to manually overcome the instabilities, I run out of fuel at 50km altitude. The best I've done was a suborbital lob with an apoapsis of 65km with the SSTO pictured, and it carries 2000 units of fuel more than my original Buzzard Nk1 which could reach a 100km circular orbit with relative ease and still have enough fuel for a deorbit burn.

Almost seems as though SSTOs are now impossible in KSP, except maybe an unimaginative rocket with a simple wing strapped on

Edited September 30, 2012 by Narcosis

[vexx32]

I've seen plenty of working SSTOs in the Spacecraft Exchange built for 0.17, so I think you just need to rethink your methodology.

If you're having stability issues where the atmosphere is thin, it's going to be tough to correct without vectoring engines, since control surfaces do practically nothing up there. You could always use RCS, I suppose.

[Fuzzy Dunlop]

You could try adding an SAS unit or two. That might help.

[Elmetian]

I've seen plenty of working SSTOs in the Spacecraft Exchange built for 0.17, so I think you just need to rethink your methodology.

I can't see any since 0.17 was released. There are a few shuttles that use disposable stages but no SSTOs that I can see, and certainly no horizontal take off SSTOs. I know the flying wing design isn't exactly suited, but short of making a VTOL rocket with a simple pair of wings slapped on, I can't see elaborate designs ever being viable now.

If you're having stability issues where the atmosphere is thin, it's going to be tough to correct without vectoring engines, since control surfaces do practically nothing up there. You could always use RCS, I suppose.

Hmm. The reason for the toroidal engine is its improved thrust to weight ratio, and the fact that the vectoring LV-T45 has a nasty habit of being knocked off when pitching up on take off. I've tried models with more RCS thrusters and they felt like a bodge to me. And like I said, the avionics package never had this problem in 0.16. I couldn't get it to orbit with the 0.16 fuel bug fix installed, but my suborbital attempts never ran into this stability issue.

You could try adding an SAS unit or two. That might help.

I've tried this as well. The SAS modules are designed to keep the rocket pointed in one direction, but they're as good as a chocolate fireguard for atmospheric flight.

They also don't fix the other problems I'm having in 0.17. The aircraft masses 30.44 units fully fueled and 15.14 dry, and carries 3100 units of fuel. It only uses 200 units of fuel to get to 13km altitude when the rocket stage is ignited. After that, my most efficient ascent profile only gets it to 65km. I'm using small tanks so they reach their dry weight faster, but without the game being redesigned to calculate the rate of loss of mass, and without jet engines that can take me closer to 20km altitude, I don't see this working any time soon.

Edited September 30, 2012 by Narcosis

[EndlessWaves]

There are a couple of working designs for 0.17 SSTOs in the K-prize thread:

http://kerbalspaceprogram.com/forum/showthread.php/11214-The-K-Prize-100-reusable-spaceplane-to-orbit-and-back

Without the fuel bugs you definitely have to build more efficiently. 15 tons dry weight does seem a little excessive. Just over 1/3 of that is obviously essential (engines 3.4 tons, fuel tanks ~2 tons, cockpit 1.25 tons) but what is the rest being used on?

[Elmetian]

There are a couple of working designs for 0.17 SSTOs in the K-prize thread:

http://kerbalspaceprogram.com/forum/showthread.php/11214-The-K-Prize-100-reusable-spaceplane-to-orbit-and-back

Some interesting designs there, but most seem to be just a few liquid fuel tanks with a cockpit and a pair of wings. Nice to see it's still possible ofc, though I'd like to see how they handle when descending to landing.

Without the fuel bugs you definitely have to build more efficiently. 15 tons dry weight does seem a little excessive. Just over 1/3 of that is obviously essential (engines 3.4 tons, fuel tanks ~2 tons, cockpit 1.25 tons) but what is the rest being used on?

Heh, <i>well</i>.... Canards, wings, control surfaces, landing gear, RCS fuel, RCS thrusters, the avionics (there are two to maintain symmetry), struts and NCS adaptors come to 7.74 tons. The rest is comprised of the cockpit, engines and fuel tanks. I think you might have underestimated the fuel tanks, because 14 small tanks and the Mk2 fuselage and the two Mk2 adaptors comes to 2.95 tons dry. I know there's a lot of dead weight, but I like my fancy wings.

Anyway, after giving up on keeping it stock, I whacked on a Mk2 bicoupler and another two small tanks. It now has enough thrust and fuel to reach orbit

Still encountering aerodynamic problems on reentry though

Edited September 30, 2012 by Narcosis