what is wrong with this plane?

[Lechu]

Why it backflips just after the take off?

[mk1980]

it flips because the center of lift (blue/black indicator) is in front of the center of mass (yellow/black indicator)

you can try to move the wings further back so the CoL ends up behind the CoM

 

 

[VoidSquid]

Just what @mk1980 said.

Or you could mount the engines inverted and simply fly backwards  

[Lechu]

23 minutes ago, mk1980 said:

it flips because the center of lift (blue/black indicator) is in front of the center of mass (yellow/black indicator)

you can try to move the wings further back so the CoL ends up behind the CoM

 

 

But it is also backfliping:

I have tried numerous configurations, there must be something fundamental wrong with this plane.

I am sitting on this for two days. My first attempt on SSTO, and I cannot even get off.

[VoidSquid]

Put the COL behind COM, as @mk1980 said.
As per your screenshot, COL is too close to COM and too low.

[HvP]

In addition to making sure that the center-of-lift is behind and slightly above your center-of-mass marker, consider the direction of your thrust vector. The purple marker shows you where your thrust vector is, that's the line of force that your engines create.

For stable flight the purple marker should be directly in line behind the yellow center-of-mass marker. This is often difficult to judge because it's normal for the center-of-thrust marker to be to the distant rear of the craft. To make it easier, try lining the camera up directly behind (or in front of) the plane looking directly along the center axis of the plane. If the purple marker is above the yellow center-of-mass then it will cause a leveraging torque that is constantly forcing a downwards rotation - if the thrust marker is below the center-of-mass then it will cause a rotational force resulting in constant upwards pitch.

Think of it like this. Your plane is hanging from a string that is attached to a balance point at the spot the yellow mass marker is. Your engines are trying to push the plane forward, and that force is applied to the spot where the purple thrust marker is. You want them to push from directly behind the point the plane is hanging from or else it will cause the plane to tip over.

Complicating things though is the center-of-lift. This is a force applied by the air you are moving through. It adds another element of force to your plane. Imagine this force as the place where your plane catches the wind. If you are moving forward, and the blue lift marker is behind your yellow mass marker then you can move forward and the air just pulls the rear of the plane further in line behind the front of the plane. If the blue lift marker is slightly above the center of mass of the plane then you get lift pulling the plane into the air - although it also creates a slight tendency to pitch upwards, but as long as you have enough control from your pitch control surfaces this is desirable.

However, if the blue lift marker is a good deal below the center of mass of the plane then you'll find that the plane is constantly trying to turn itself upside down. Try to imagine a lever arm attached to the yellow ball the plane is hanging from, and that this lever extends to the blue ball showing you your lift point. If you thrust forward and this blue center-of-lift catches the wind, what happens? The blue ball will be pushed backwards until it is directly behind your center-of-mass, but doing this will rotate the plane up or down.

Your best options are to:

A) Adjust the placement of your engines so that the purple center-of-thrust marker is in line behind the yellow center-of-mass marker. Then...

B) Adjust the placement of your wings and control surfaces so that your blue center-of-lift marker is slightly behind and either in line with or very slightly above your yellow center-of-mass marker.

C) Realize that moving around the wings will also shift the center-of-mass and center-of-thrust, so...

D) Go back to step (A) and repeat each step as necessary until things line up properly, then...

E) Test your configuration in flight, note the behavior of the plane and...

F) Revert to the space plane hangar and continue adjusting as needed for further testing.

Edited December 19, 2019 by HvP

[Lechu]

3 hours ago, VoidSquid said:

As per your screenshot, COL is too close to COM and too low.

But it is still backfliping.

Edit: I have not seen HVP's post. Thanks for the detailed answer, I must to think about it.

Edited December 19, 2019 by Lechu

[IronMaiden]

3 hours ago, Lechu said:

But it is also backfliping:

By backflipping we're assuming you mean the nose goes up over the top, this would be caused by your center of lift being in front of your center of mass. Take another image with your plane rotated so it looks like it's pitched up over 5°. If you mean front flipping, tail goes over the top, that would be caused by your center of thrust going over the top of your center of mass, pushing your nose down. It's tough to say exactly without having the craft file. Can you share it?
 

There's a whole lot wrong with your plane,  but it should still be able to get to orbit with a few small tweaks. 

[HvP]

1 hour ago, Lechu said:

Edit: I have not seen HVP's post. Thanks for the detailed answer, I must to think about it.

You're welcome, of course.

I hope you don't mind if I add a little something else. Airplanes can be difficult to get right, and space-planes are much harder. Don't get discouraged because everyone has a hard time with them in the beginning.

One problem you will probably notice later is this: Your jet engines and your vacuum engines don't both work at the same time. This means that your thrust vector changes when your jet engines stop working and you have to switch to the rocket engines. If you want to see where the thrust vector is during atmospheric flight, then turn the thrust on your rocket engines down to zero (0) in the space plane hangar. And then, to see where your thrust will be during space flight from the SPH turn the thrust on your jet engines down to 0 and your rocket engines back to 100%. Now you can see where your thrust vector is for each stage of your flight. You might be surprised how much it changes. Just make sure to turn them both back to 100% before you launch it.

There is a section of Tutorials in this part of the forum if you look at the top of the "Gameplay Questions and Tutorials" section. You might find some instructions on building planes that you find helpful.

Above all, it's a game. You're trying new things, experiment, and just try to have fun with it. Good luck!

Edited December 19, 2019 by HvP

[Lechu]

On 12/19/2019 at 9:40 PM, HvP said:

Your jet engines and your vacuum engines don't both work at the same time. This means that your thrust vector changes when your jet engines stop working and you have to switch to the rocket engines. If you want to see where the thrust vector is during atmospheric flight, then turn the thrust on your rocket engines down to zero (0) in the space plane hangar.

Great thanks. That was the problem. The (experimental) Vector engine, that was not supposed to be activated on the runway, has been throwing off the center of thrust in build mode. Discounting this engine, the center of thrust was way in front of center of mass and center of lift.

 

I do realize that I'll need to make many tweaks before this plane reaches orbit, but I was unable even to start, without the ability to take off...

For now I was able to reach the height of about 30 km,  my current aim is to get this plane suborbital, for long enough time to deploy a satellite.

[HvP]

Good to see that you've been able to get it up to a decent height @Lechu.

And just so you know, having the thrust marker forward of the CoM/CoL isn't really a problem. It just means that you're pulling your weight instead of pushing it. What you really don't want is to have the thrust marker above or below (or left or right of) the center line going through your center-of-mass. That causes rotational forces that will pitch and spin your craft.

If you take that horizontal line coming out of the purple thrust marker, imagine extending it horizontally in both directions. Now try to line up your thrust so that this line would pass right through the center of your yellow mass marker. This is actually a bit difficult in KSP because the editor doesn't have any good way of showing you this except either guessing, holding a ruler up to the screen, or turning your camera view so that you are looking straight down the middle of your craft from the front or back.

I don't think this is much of a problem for you right now, but it could be a small part of it. It's also possible to very slightly rotate the entire engine a tiny amount up or down just a degree or two if needed. As long as the thrust is aiming through your CoM.

If you are comfortable using mods then you should look up RCS Build Aid. It has a function that allows you to see how much torque your engines would apply to your craft. Of course, it also makes it much easier to balance RCS placement if you need that in the future.

Edited December 22, 2019 by HvP

[Lechu]

2 hours ago, HvP said:

And just so you know, having the thrust marker forward of the CoM/CoL isn't really a problem. It just means that you're pulling your weight instead of pushing it. What you really don't want is to have the thrust marker above or below (or left or right of) the center line going through your center-of-mass. That causes rotational forces that will pitch and spin your craft.

Yeah, it is logical, but in my case no amount of aligning thrust with the CoM would make any difference. On the other hand, once I put a thrust marker of the air breathing engines (with thrust of rockets disabled) behind the CoM, the flipping problem was gone right away.

[Lechu]

Quote

my current aim is to get this plane suborbital, for long enough time to deploy a satellite.

...I said.

Well, I have send this thing into suborbital orbit with apoapsis 250 km.  It gave me enough time to deploy the satellite and circularise its orbit, then switch to the plane before it hits the atmosphere. Then the wings are ripped off. Literally.  I have thought, that these big engines, working with reversed thrust, will let me slow down. No dice. I have a quicksave just before reentry, and the spaceplane gets ripped apart every time, no matter what I do.

Now, what is wrong with this plane?

 

Edited December 25, 2019 by Lechu

[HvP]

It may not be the plane so much as your reentry angle.

If you're going up to 250 km suborbital then you probably have a very steep reentry. Space planes seem to work best for me reentering the atmosphere with a periapsis no lower than about 40 km, but since you are suborbital your trajectory would of course have to intersect the ground. This causes you to fall through the thin upper atmosphere very fast which doesn't allow you to slow down very much before you slam into the denser atmosphere at high velocity. In an effort to slow yourself down you have to try and pitch up quickly which over-stresses the connection to the wings and they fail.

So here's what I think might help. Once you are leaving the atmosphere on ascent, see if it's possible to burn more horizontally aiming for the horizon. Try not to push your apoapsis higher than about 80-90 km. Instead, focus on making your trajectory wrap around the planet as much as possible. Go long, not high. You should still have time to launch your payload since you are just trading height for distance.

On the way back down, you will now drift for a longer period of time in the high atmosphere where you can pitch upwards or downwards, and use that drag to slow you down a lot more before you fall into the denser air. You'll have more time to bleed off speed and won't always require as much force pulling up.

Also, a tip: Don't set the wings to "Rigid Attachment" in their right-click menu. While this makes them more rigid, it also doesn't allow them to bend before they break. Autostrut to grandparent or root part would work better.

Edited December 25, 2019 by HvP

[TheFlyingKerman]

Fixing one engineering problem after another is what KSP is about

The big airliner wing is known to shear off during high g turns. Even if it doesn't it has extremely poor heat tolerance (1200K) so it is going to melt. The big Goliath engines and circular intakes are designed for low altitude and speed (< mach 2, much less than orbital) and produces no thrust whatsoever while you are reentering.

p.s. the number of intakes on that plane is excessive, it was useful in old versions (in case you base your design on older tutorials) but not now.