Another Mk3 lift problem topic

[capi3101]

O.o that's an insane amount of reaction wheels you have there... why? You shouldn't need anywhere near that many to keep it controllable in space, so I can only assume they exist to counteract a serious aerodynamic flaw.

The SAS helps the plane to hold its attitude in atmosphere without user input (i.e. they're there so I don't have to mash down on the "S" key for half-an-hour). Actually, including SAS is one of DocMoriarty's design suggestions, and for that very reason - He suggests "1.5 kN of SAS per tonne of takeoff mass". The big ASAS wheels provide 30, and I have 8 of them - from that you can guesstimate the take-off mass of that plane (somewhere between 140-160 tonnes). As it was I had to add all the canards up front because the plane didn't have sufficient pitch authority during the early test flights.

Wound up with that big Monoprop tank as ballast - at the time I'd forgotten that Vernors work on LF/LOX instead of monoprop...if I redesign that plane sometime in the future, I may swap it out with an X200-8 or X200-16.

Edited January 16, 2015 by capi3101

[Anquietas314]

The SAS helps the plane to hold its attitude in atmosphere without user input (i.e. they're there so I don't have to mash down on the "S" key for half-an-hour). Actually, including SAS is one of DocMoriarty's design suggestions, and for that very reason - He suggests "1.5 kN of SAS per tonne of takeoff mass".

That's completely unnecessary in my experience; between the control surfaces and just plain building the plane (ba-dum-tsh) properly, you should be able to fly in a straight line with SAS enabled using just the cockpit. Heck, you should be able to fly reasonably straight with it off. In space, that's usually still sufficient for acceptable rotation speeds/stability. Seeing as you have a probe core, you'd probably need one reaction wheel on top of that, but otherwise they're not really necessary.

[capi3101]

Necessary, no. But it sure does come in handy.

[Northstar1989]

This is a common problem in the current stock game. The issue is not that the wings don't lift enough, it is that they are designed to lift Mk2 planes.

Actually, part of the problem *IS* that the wings don't produce enough lift, at least in the stock game anyways. To get more realistic (higher) lift values, I strongly suggest you play with FAR...

I suggest using the B9 mod Wings with all your Mk3 planes. You can extract just the wings if you don't want all the rest of the mod.

Until Squad gives us some larger wings, that is going to really be the only option to building Mk3 planes without massive part counts on the wings... unless you want to resort to welding.

:chuckle:

You've never heard of Procedural Wings/Dynamics mod? (it goes by either name) THAT is the only real solution to lifting a Mk3 fuselage. Just add a single wing part to each side, enlarge it to HUGE proportions, and add some control surfaces (and vertical stabilizers). Weld the Mk3 fuselage parts if you really want- but as stated FAR seems to have some issues with welded parts, and you can easily get a <120 part SSTO with a Mk3 fuselage if you just use the Procedural Wings mod...

- - - Updated - - -

What's wrong with 35 degrees? That's typical of a plane. The only way to get a higher AoA is more speed which means more power. Ideally you want an ascent angle that will get you up to maximum speed just as the engine loses it's air. If you do it right you won't have to change that angle the whole way up, if you don't you have to level off to build speed, which is still ok.

35 degrees is NOT a typical AoA for a plane (are you sure you don't mean angle of ascent? AoA is defined relative to your prograde vector, NOT the horizon, and you can easily achieve 35 degrees with a high climb-rate..)- *ESPECIALLY* using FAR. And there are MULTIPLE solutions to bring it down:

(1) Increase thrust to increase velocity. In stock, lift only increases linearly with speed (which is why I and the devs agree the wings produce too little lift), but in real life and FAR lift increases exponentially with speed.

(2) Decrease drag to increase velocity. In FAR, this means designing the wings to have a higher sweep and making sure there are no protruding objects (such as radial batteries) that disrupt the airflow. In stock aerodynamics (which STINK), well tough luck- even cargo bays don't serve any purpose (much like nosecones), as they actually INCREASE drag with the silly linear mass-drag relationship (you're better off mounting a payload on top of the plane with a radial decoupler and struts in stock...)

(3) Reduce wingload by decreasing mass. Any extra mass on the plane you don't need will increase the force of gravity, and require a higher AoA to produce enough lift to stay airborne at a given altitude/speed. Extra mass also decreases your TWR, although with a plane (in FAR) this is less of a problem as you will simply accelerate until drag and thrust balance each other (assuming the extra mass doesn't increase drag- say it's inside a cargo bay). In stock, reducing mass is actually even MORE important, as there is a silly/stupid linear relationship between mass and drag (a full fuel tank experiences more drag than an empty one, for instance...)

(4) Reduce wingload by increasing wing area. Simply adding more wings will allow you to produce the necessary lift with a lower AoA. Although extra wing area will increase your drag at a given Angle of Attack, it will allow you to fly at a lower AoA in the first place, and thus can actually REDUCE drag in level flight when you're already pushing very high AoA values just to stay airborne... Peak Lift/Drag ratio usually occurs as a MUCH lower AoA than peak lift- around 6-10 degrees instead of 16-24... You also need less airspeed to produce the same amount of lift with more wing area, so you can get away with more drag...

I hope all this helps.

Regards,

Northstar

[Alshain]

Actually, part of the problem *IS* that the wings don't produce enough lift, at least in the stock game anyways. To get more realistic (higher) lift values, I strongly suggest you play with FAR...

They don't work in FAR either, not without an absolute ton of them, which is what the OP was trying to avoid. The B9 modular wings are larger and configured to have the same lift as several of the stock ones making it less parts.

:chuckle:

You've never heard of Procedural Wings/Dynamics mod? (it goes by either name) THAT is the only real solution to lifting a Mk3 fuselage. Just add a single wing part to each side, enlarge it to HUGE proportions, and add some control surfaces (and vertical stabilizers). Weld the Mk3 fuselage parts if you really want- but as stated FAR seems to have some issues with welded parts, and you can easily get a <120 part SSTO with a Mk3 fuselage if you just use the Procedural Wings mod...

Regards,

Northstar

Heard of it, I've used it before. You've never heard of reading a thread before? We discussed this already. The "only option" remark you quoted meant MODS are the only option, not B9 specifically.

Besides, PWings sucks.

Yes, I was talking Angle of Ascent, sorry my mistake.

Edited January 16, 2015 by Alshain

[capi3101]

Quick question regarding B9 - is there a list of the part stats anywhere? I'm debating whether or not I want to install it (I've got a pretty hefty mod list as it is).

[Fearless Son]

They'll blow on tri-couplers; that happened to me with my own "lift a full orange tank to orbit" spaceplane design...

http://i.imgur.com/shTk6cq.png

Do not take this the wrong way, capi3101, but the ugliness of that design is admirable. There is just a kind of "brutal elegance" in the way that thing is built, a kind of "I don't care about what it looks like, it works, damnit" attitude that does more than the stuff I have been trying (and failing) to get into orbit.

Good work.

[capi3101]

Do not take this the wrong way, capi3101, but the ugliness of that design is admirable. There is just a kind of "brutal elegance" in the way that thing is built, a kind of "I don't care about what it looks like, it works, damnit" attitude that does more than the stuff I have been trying (and failing) to get into orbit.

Good work.

Er, thanks......I guess...........

I do have a tendency to favor function over form. Not just in KSP either.

[Merandix]

Ok... I've made some tweaks... she flies pretty well now. Not much performance by a long shot, but she will take the designated mass up to a 100+ km orbit quite easily in about 20 minutes time... But now I've suddenly run into the weirdest problem... It used to be fine, but after some rudder tweaks, my shiny plane suddenly developed quite a severe yaw problem at very high altitude where none was before. Normally, I'd say this would be caused by the rudders... but since there's hardly any air left by the time it occurs that wouldn't be entirely logical.

This occurs with the four wing mounted RAPIERS in rocket-mode, some time after firing them. it happened for the first time at about 53 km up... Since the engines are placed completely symmetrical, and did NOT have this problem before when they were placed exactly the same, it can't be the intakes or the engines, since they're closed when it happens.

Checked the intakes: closed

Engine Thrust is symmetrical

Engines are mounted symmetrically

RCS is off (turning it on doesn't help much other than burning fuel).

Torque-wheels are torquing

Everything looks like it should be al right, yet the plane starts wildly deviating off it's course a little while after closing the intakes. First yawing, then roll and pitch also become unstable (which may be in part due to me fighting it).

I did remove a torquewheel earlier... but could that be it?

[Anquietas314]

Merandix: That sounds like asymmetric flame-out. Double check all your intakes are mapped to your action group correctly. Also, what do you mean by "Engines are mounted symmetrically"? That is, did you place them symmetrically by hand, or place them in symmetry mode? The latter causes asymmetric flame-out by default. The intake build aid mod can be used to fix that though. Another (unlikely) possibility is that you've hit one of the quirks of fuel flow and for some reason one of the wing-mounted tanks is draining faster than the other, making the center of mass move to one side of the plane.

It's probably not your issue, but I would also make sure the RAPIERs are configured to use manual switching (automatic causes issues when you have more than one). Note that you need to do that one engine at a time - the tweakable ignores symmetry.

[Merandix]

I would fully agree if they were airbreathing, but this is at over 2000 m/s, in closed cycle mode... It'll go straight for about 30 seconds after hitting the rocketry, and then it'll suddenly start 'drifting' wildly. I'd have to take another testflight and try to see how the tanks in the wings are draining. That sounds the most plausible actually. Though I did (in pure bewilderment) click on the large fueltanks in the wings, and at least those were draining equally.

Also, and again, this happens about 30 seconds AFTER I've (manually) switched from airbreathing to closed cycle. So intakes, intake mapping (they actually are closed, and correctly mapped to each engine) or asymmetric flame-out aren't applicable. Also, it didn't happen before. That's what puzzles me mostly.

Note that the addition of an RCS system bloated her part-count a little, but she's still not lagging... which makes me think going too fast too low is actually a bigger cause for lag than part count.

-edit- though I've since discovered that I can run them at near 100% thrust (just one notch shy of full power) without fear of them going boom, so I AM actually producing far more thrust than earlier... hmmm... maybe that's related. A shame this problem doesn't occur until about 15-20 minutes into the flight (low engine count means she's not much of a performer ).

Edited January 17, 2015 by Merandix

[Anquietas314]

I would fully agree if they were airbreathing, but this is at over 2000 m/s, in closed cycle mode... It'll go straight for about 30 seconds after hitting the rocketry, and then it'll suddenly start 'drifting' wildly.

Make sure all the intakes are mapped to that action group, including those meant for the turbojets at the back. Those can have asymmetric flame-out too. You should probably also have that action group toggle the turbos.

[Merandix]

That it does.

All intakes are tied to that action group. Also, the turbojets are toggled OFF, and the RAPIERS switch mode to closed cycle when I press it. 20-30 or so seconds after I press that button, things are starting to get strange. I've also confirmed that indeed ALL intakes are in that action group. So it's also not the case of one remaining open.

Test flight was um... paused... so it's still in progress...

[capi3101]

If the fuel is draining symmetrically, it may simply be that you don't have sufficient yaw authority in the upper atmosphere. I still think you need more rudder, or to put back that torque wheel that you took out and see what happens.

[Anquietas314]

Test flight was um... paused... so it's still in progress...

Well, I realize it's been about 3 hours, but if you still have it like that, maybe check you haven't accidentally set the trim to yaw in whichever direction? It seems like that would be hard to do accidentally, but I guess it's possible. At 53km your control surfaces and wings should be almost useless so we can probably rule those out if as you say it's a serious problem. The reaction wheels in the cockpit and cargo bay should be more than enough to overcome a minor aerodynamic issue at that altitude. Have you checked all of the wing tanks to check they're symmetrical? Also, do you have a joystick plugged in? If so, check that its dead zone is set properly and something isn't pushing on it. Another long shot: check none of the engines have thrust limiters set.

[Merandix]

Installed that third ASAS again and that seems to have solved the problem. It's still weird because the craft is inherently stable, and the port and starboard tanks are draining symmetrically. Oh well. It flies now. Now to do final tweaking on the oxidizer, and she should work as intended.

Also accidentally setting trim did not occur to me! Thanks for pointing that out. Though it wasn't the cause, it's good troubleshooting advice.

Thanks everyone for your advice! At least now I can say that while hard, it actually IS possible to build a low relatively low ttw, functional large Mk3 space plane!

[Anquietas314]

Oh well. It flies now. Now to do final tweaking on the oxidizer, and she should work as intended.

At least it's fixed .

At least now I can say that while hard, it actually IS possible to build a low relatively low ttw, functional large Mk3 space plane!

Yeah, but it's still more efficient (both in terms of fuel and your time) to build one with good TWR so you can get out of the atmosphere quickly, but rebuilding spaceplanes blah blah blah .

Congrats for getting it to work though .

[Merandix]

I guess you just told me an option for a next project. Though that should be a bit less involved, as designing for more engines from the ground up shouldn't be too hard

Just to be curious, how quick does your craft get up there? Mine takes, depending on how stupid I'm piloting it (Yeah, worthless pilot) 18-25 minutes to get up there.

[Anquietas314]

Just to be curious, how quick does your craft get up there? Mine takes, depending on how stupid I'm piloting it (Yeah, worthless pilot) 18-25 minutes to get up there.

Well, right now I can't play KSP because my gaming rig is broken, but iirc most of my planes get to orbit within about 5-7 minutes - not much more than it takes with a rocket.