Making extremely large stock planes take off.

[Pds314]

I have designed and built a massive spaceplane with 40 RAPIERs and 4 NERVAs. It weighs around 400 tonnes and can currently hold 112 Kerbals in cockpits and Hitchhiker modules. It flies fine and can get to takeoff speed from the runway. The problem is that either the heavily reinforced landing gears break or the plane falls off the runway. Trying to steer it hard can break the gears. Any tips for taking off gargantuan planes like this one?

EDIT: it is in FAR.

From memory, it's primary wing surface is a trapezoid 10.5 metres long, 21 meters at the front, and 42 at the rear. This is 330 m^2, but it has an additional wing so it should have enough of a wing surface considering the speeds it reaches.

In other words. It isn't going off the end of the runway. It is collapsing before or during takeoff. Sometimes it manages to take off and fly a few hundred meters, minus some engines and wing parts and landing gears. It just breaks first.

ANOTHER EDIT: I'm trying to avoid solid fuel because I intend it to be an SSTO and the nose starts raised by a structural fuselage, 1.25 meters. Also, The most significant problem seems to be that the front landing gears are bending over and parts of it are breaking off of them, causing their structure to compromise and the plane to try and steer off the runway and them to break.

Finally! Pictures are available:

So yeah, it is basically going 'splode right as it is taking off or shortly before. I don't think skids work well at those speeds either.

Edited March 4, 2014 by Pds314

[Blue]

Pictures often help diagnosing design problems.

From the sound of your description, it would appear that either its lift is unbalanced, or it doesn't generate enough lift to overcome its weight at all.

[Pds314]

Pictures often help diagnosing design problems.

From the sound of your description, it would appear that either its lift is unbalanced, or it doesn't generate enough lift to overcome its weight at all.

Will post pictures. I'm on a phone. May post in 4.5 hours.

Edited March 3, 2014 by Pds314

[Pds314]

Whoops. Forgot something important. It is in FAR.

[Rakaydos]

You might try launching it from jacks.

Instead of landing gear, use structural girders. For lift off, throttle up, then fire enough sepratrons to get your nose in the air.

[TechnicalK3rbal]

You might consider rocket assisted takeoff.

[capi3101]

Hmmm....general principles I would apply stock are:

One Turbojet per 9 tonnes payload (everything but the wings, engines, and intakes)

One pair of Swept Wings per six tonnes of plane

Two Mk1 Fuselages per Engine

One Ram Intake per tonne of plane.

Now, I know you're using FAR and I'll admit I don't use FAR and am not familiar with the parts. But, you should be able to compare the FAR parts to the stock ones and see how well your plane is measuring up. It sounds pretty massive in any event. Could easily be a case where the solution turns out to be MOAR STRUTS, but try comparing your plane to these principles first and see how well it's measuring up.

[Pds314]

You might try launching it from jacks.

Instead of landing gear, use structural girders. For lift off, throttle up, then fire enough sepratrons to get your nose in the air.

You might consider rocket assisted takeoff.

I would, but it is intended to be an SSTO.

[Pds314]

Hmmm....general principles I would apply stock are:

One Turbojet per 9 tonnes payload (everything but the wings, engines, and intakes)

One pair of Swept Wings per six tonnes of plane

Two Mk1 Fuselages per Engine

One Ram Intake per tonne of plane.

Now, I know you're using FAR and I'll admit I don't use FAR and am not familiar with the parts. But, you should be able to compare the FAR parts to the stock ones and see how well your plane is measuring up. It sounds pretty massive in any event. Could easily be a case where the solution turns out to be MOAR STRUTS, but try comparing your plane to these principles first and see how well it's measuring up.

One turbo per 10+ tonnes of plane: I have one RAPIER per 10 tonnes, but FAR means you need fewer engines.

Swept wings: FAR aerodynamics make a big difference here. I do have roughly the same area as the appropriate number of swept wings.

Mk1 fueslages: NEVER USE THESE! You should just use fuel tweaked without oxidizer. Rest assured, it has enough fuel.

1 intake per tonne? I'm not gonna Airhog THAT much. That means I should have 10+ intakes per engine. I currently have 1.075 intakes per engine.

I think MOAR STRUTS may be the answer, but I do have the wings and body pretty much glued solidly together and I'm not sure how I can reinforce the landing gear any more.

Also, FAR doesn't add parts. All it does is change the aerodynamics and makes some very minor nerfs to the jet engines. Anything built in FAR is guaranteed to be loadable without FAR and is almost always flyable if it was so in FAR. The exception is rockets which need less Delta-V in FAR because the air is realistic and not "made of pea-soup." (which also means Eve ascents are much, much easier).

Edited March 4, 2014 by Pds314

[magnemoe]

I would, but it is intended to be an SSTO.

How about using hotkeyed 48-7S to lift nose and optionally give more trust, you can also run the nervas during takeoff for 240 more trust.

[AlexisBV]

Looks like you only have 5 gears touching the ground. Maybe you need more to share the load? Landing gears tend to fail quite spectacularly when overloaded and moving. Check that when the game loads the plane on the runway, if the gears buckle or bend in any way...

[Claw]

Well, I will caveat my thoughts by saying I haven't built a 400 ton airplane and I don't use FAR.

However, my thoughts from looking at your picture:

1) Your rear landing gear look too far back. I don't know where your CoM is because it isn't in the picture, but your rear gear look to far back.

2) You have a lot of control surfaces at the back of the plane.

3) I'm guessing your wings have a lot of flex in them when you try to rotate/takeoff.

4) Based on how much low wing area you have, your CoL might be low compared to your CoM, which is probably why you have so much upper tail surface.

I have a few other guesses at things, but since you're using FAR I'll save those because they may not apply.

One of the things that may be happening is that because of #1, 2 (and sometimes #3) above, when you try to steer or rotate your plane, you are placing tremendous downforce on your rear landing gear. #1 makes it hard enough to get the nose up off the ground, but when you have a plane that heavy and you're trying to do all the downforce rotation directly above the landing gear, they're going to buckle.

If you are in fact experiencing wing flex (#3), then it's going to cause extra up/down and side forces on all those landing gear that are attached to it. When you try to steer the plane, the wings flex more which causes the gear to buckle, which causes you to have to steer more, which causes the wings to flex, which.... hopefully you get the idea.

Also, if you're having wing flex, even if you do get off the ground your plane probably doesn't want to fly straight.

For your tail section, the angles are probably also causing them to flex around. The way they are attached, the extra bends, and the lack of struts are going to cause them to flex around. Also, if your CoL is low compared to your CoM, it's going to cause your plane to want to roll over on takeoff.

If any of the above is correct, those are the things you'll want to attack first. If not, you could provide a craft file and I'll give it a test flight. Maybe that will help me get a better idea what's causing your problems. I fly stock KSP, but I would imagine the takeoff problems you're having shouldn't be to different than with FAR.

[Pds314]

How about using hotkeyed 48-7S to lift nose and optionally give more trust, you can also run the nervas during takeoff for 240 more trust.

Yeah but running the NERVAs on takeoff is EXTREMELY inefficient and plus, it doesn't need more thrust, it needs to take off without crushing the landing gears.

[Pds314]

Looks like you only have 5 gears touching the ground. Maybe you need more to share the load? Landing gears tend to fail quite spectacularly when overloaded and moving. Check that when the game loads the plane on the runway, if the gears buckle or bend in any way...

The reason only 5 gears touch the ground at the back in that pick is because the wings are lifting off the ground.

Normally, all 9 rear and 6 front gears touch.

They don't buckle when the craft loads, they buckle while rolling and especially while trying to correct yaw. Only the front ones buckle.

[Claw]

Yeah but running the NERVAs on takeoff is EXTREMELY inefficient and plus, it doesn't need more thrust, it needs to take off without crushing the landing gears.

I agree. It looks like you already have around 2.0 TWR. If you stood it on end, it could probably take off straight up.....

[Claw]

They don't buckle when the craft loads, they buckle while rolling and especially while trying to correct yaw. Only the front ones buckle.

Ahh, if it's the front gear that you might still be overloading them when the wings flex. However, you can try and unlock the steering via tweakables. That might give them some freedom to move and give you more ability to steer, instead of forcing them sideways. Even my small planes will sometimes tumble when pushing sideways too hard against the front gear. And small taps on the yaw to straighten the nose.

[Pds314]

Well, I will caveat my thoughts by saying I haven't built a 400 ton airplane and I don't use FAR.

However, my thoughts from looking at your picture:

1) Your rear landing gear look too far back. I don't know where your CoM is because it isn't in the picture, but your rear gear look to far back.

2) You have a lot of control surfaces at the back of the plane.

3) I'm guessing your wings have a lot of flex in them when you try to rotate/takeoff.

4) Based on how much low wing area you have, your CoL might be low compared to your CoM, which is probably why you have so much upper tail surface.

I have a few other guesses at things, but since you're using FAR I'll save those because they may not apply.

One of the things that may be happening is that because of #1, 2 (and sometimes #3) above, when you try to steer or rotate your plane, you are placing tremendous downforce on your rear landing gear. #1 makes it hard enough to get the nose up off the ground, but when you have a plane that heavy and you're trying to do all the downforce rotation directly above the landing gear, they're going to buckle.

If you are in fact experiencing wing flex (#3), then it's going to cause extra up/down and side forces on all those landing gear that are attached to it. When you try to steer the plane, the wings flex more which causes the gear to buckle, which causes you to have to steer more, which causes the wings to flex, which.... hopefully you get the idea.

Also, if you're having wing flex, even if you do get off the ground your plane probably doesn't want to fly straight.

For your tail section, the angles are probably also causing them to flex around. The way they are attached, the extra bends, and the lack of struts are going to cause them to flex around. Also, if your CoL is low compared to your CoM, it's going to cause your plane to want to roll over on takeoff.

If any of the above is correct, those are the things you'll want to attack first. If not, you could provide a craft file and I'll give it a test flight. Maybe that will help me get a better idea what's causing your problems. I fly stock KSP, but I would imagine the takeoff problems you're having shouldn't be to different than with FAR.

1. Good point with the rear gears. Not sure if it will help much seeing as I can't really angle it up too much more than it already is.

2. Yeah. I might add some canards, but again, it isn't an issue of flying off the end of the runway or not being able to take off. It is an issue of of being damaged before takeoff.

3. Actually, they have some flex, but not too much to be flyable. It isn't bending in half or anything and the wings only break bits of the fuselage off if it is re-entering at stupidly aggressive trajectories.

4. The large tail is indeed partially made for that purpose. Seeing as the ladders have to be there for getting off of the plane, I can't see how I would put the wings on top. Again, it has little-to-no trouble flying, even at low altitude, but it has a lot of trouble not breaking on takeoff.

Actually, the tail fins don't at all lack struts. If you look very closely, you can see they have quite a lot of them.

I may upload it later. I'm currently tweaking a few things and seeing if I can fix it based on suggestions.

[capi3101]

One turbo per 10+ tonnes of plane: I have one RAPIER per 10 tonnes, but FAR means you need fewer engines.

Swept wings: FAR aerodynamics make a big difference here. I do have roughly the same area as the appropriate number of swept wings.

Mk1 fueslages: NEVER USE THESE! You should just use fuel tweaked without oxidizer. Rest assured, it has enough fuel.

1 intake per tonne? I'm not gonna Airhog THAT much. That means I should have 10+ intakes per engine. I currently have 1.075 intakes per engine.

Intake-wise, Try 6-7, or possibly eight per engine. Ideally you want to delay the switchover of your RAPIERS as long as you can (less oxidizer needed that way). One per engine ain't gonna cut it; I've had designs like that flame out on me on the stinkin' runway...

Speaking of RAPIERS, are you just experimenting with them, or do you have a specific purpose in mind with that particular engine? I'd give you the standard "Turbojets have lower mass and higher Isp" argument for Turbojet/Rocket vs. RAPIER, but I imagine you've heard it before.

In any case, looking at your screenie, I think you're putting too much mass on the back wheels when you go to lift your nose. Try reinforcing the wheels first, then adding more wheels if that doesn't work.

Something you may consider (and I've done before) is actually having three sets of landing wheels - nose gear, a rear set close to the center of mass, and a tail set that responds to an action group command (this is the set that supports the tail of the plane when it's stopped). On take off, when you get close to takeoff speed, use the action group to raise the tail set. You should be going fast enough at that point that, if all is well, the rear end of the plane will stay off the ground. You then just have to pitch up; the plane pivots on the middle set (the ones close to the center of mass), and you're off to the races.

[Claw]

1. Good point with the rear gears. Not sure if it will help much seeing as I can't really angle it up too much more than it already is.

2. Yeah. I might add some canards, but again, it isn't an issue of flying off the end of the runway or not being able to take off. It is an issue of of being damaged before takeoff.

3. Actually, they have some flex, but not too much to be flyable. It isn't bending in half or anything and the wings only break bits of the fuselage off if it is re-entering at stupidly aggressive trajectories.

4. The large tail is indeed partially made for that purpose. Seeing as the ladders have to be there for getting off of the plane, I can't see how I would put the wings on top. Again, it has little-to-no trouble flying, even at low altitude, but it has a lot of trouble not breaking on takeoff.

Actually, the tail fins don't at all lack struts. If you look very closely, you can see they have quite a lot of them.

I may upload it later. I'm currently tweaking a few things and seeing if I can fix it based on suggestions.

If you're not having problems in the air with the tail, then no need to change it. And sorry I missed the struts. I could only clearly see one in that picture so I wasn't sure.

Talking about the front gear, the other thing I have noticed is that if you attach them to components that are radially attached, the joint sometimes flexes. I guess what I mean by that is that it isn't always the landing gear the problem. In your case, I've seen problems with the connection between that structural fuselage and the aircraft fuselage. Sometimes moving the joint up or down a little bit changes the way the forces are applied to the joint and it flexes a lot less. I don't know if that's happening here, because it's sort of a "maybe/maybe not" thing I've seen with similar landing gear designs.

Forgot to add... Sometimes even if it's just a little wing flex, having your landing gear attached to the wings can cause wobbling and steering problems with the plane (gear attached to the wings). For instance, the plane below was very unstable on the runway, veering to the right every time. And if it did manage to get airborne, the wings would flex so much that it would yaw and roll. I added two key struts, and it flew solid as a rock because I eliminated wing flex to the rear gear (even without a vertical tail).

It's not nearly as heavy as yours (only 90ish tons), but it only has one pair of landing gear at the back.

Edited March 5, 2014 by Claw

[Pds314]

The gears Are only attached to the wings at the back. The front, ones are attached to fuselage and their heavily reinforced mounts are crushing and bending.

[JumpsterG]

Just a shot in the dark, but I remember hearing that landing gears attached directly to wings caused problems for some reason. Maybe try attaching little cube struts (massless, dragless at this time) to the wings first and attaching the gears to those?