SSTO Lifts Gone Wild! Losing control after space plane lift off.

[jessesomething]

Hi there, Kerbalnauts!

My first post here, actually. So many other great threads on this forum but I just can't figure out this space plane design. I've read probably fifty different tutorials and guides on making an SSTO and have used all the methods described in making this plane. But, it just can't give me a stable lift!

I'm using two RAPIER engines with liquid fuel, and two Attila thrusters which aren't used until I reach my orbital altitude. The wing design is a little different but I didn't think there was any problem with it. I monkeyed around with straightening them out with the elevons but didn't seem to help.

The plane is able to take off from the ground at the end of the ramp but isn't able to get any lift until it reaches there. As you can see, I am employing the typical weight/thrust ratio needed for a space plane. When I reach the end of the ramp for takeoff it lifts up REALLY quick when I slightly pull back the controls for a little lift. Then, I'm usually able to equalize it but when it get back towards prograde, it starts to ease over to the left or right without any reason. Once it does this, it just starts spinning slowly around and I'm not able to regain control.

What am I doing wrong here?

Top-view

You can see the wheels are aligned well and in front of the elevons (maybe too far?).

The wingspan is larger but seems balanced, and both tilt a little to the front at the ends. Also lots of struts!

Getting ready to lift off. Pulling up does not lift until the end.

Here we go again!

Bath time!

Lemme know what you think. Thanks!

[AlexinTokyo]

I think the immediate problem id that somehow your lift force is downward instead of upward. You can see this by the direction of the blue arrow coming out of the CoL ball.

I'm not sure how this could have happened, though. Did you invert the wings when attaching them?

[jaredkzr]

It might just be the perspective but picture number 2 appears to show the wings angled down, IE the right side of the wing in the picture is lower than the left side. I think this is what's causing your blue lift vector to be inverted which would cause all sorts of trouble similar to what you've experienced. Double check that the wings are perfectly horizontal.

Also, welcome to the forums and congrats on your first post!

[eddiew]

Unless I'm missing something, you only have one pair of control surfaces, and they're not far behind the CoM. Whatever else may be amiss, you're definitely lacking leverage - lift in this case comes from the rear elevons angling up to push the back end down. While still on the runway, you're trying to pivot the plane around the rear wheels by pushing on down on the shorter of two levers - meaning you need more down force than the plane has weight. I'd consider swapping the two forward wings for some active control surfaces, say a pair of canards, tweaked to only operate for pitch. These will directly lift the nose without fighting leverage, and should help get you the control you need. You'll probably be able to have the rear elevons handle only roll as well, which will help

As for yaw instability - more tail fins!

[cicatrix]

Why the lift vector is pointing down? I think this is the problem.

[capi3101]

I think others have hit on the problem - the lift vector should be pointing upwards, not downwards. The likely cause of this is the way you have the wings attached. Wing position has a lot with how the game calculates lift in general, and it assumes that certain wing parts will be aimed certain ways (for example, a Wing Connector A is designed to generate lift if it's placed with the long end touching the fuselage/adjacent wing; turn in sideways (so it looks like a Wing Connector and it generates no lift.

I suppose it's possible that the game is calculating negative lift for the wings...certainly in a quantity enough to overcome the lift that is being generated by the lifting body parts you're using (such as the Mk2 Cockpit).

[cantab]

Flipping the wing parts 180 degrees won't matter. The important thing is their front-back axis needs to be aligned to your direction of flat. If you turn them 90 degrees you get no lift.

Failure to take off until you reach the end of the runway is simply because your plane cannot lift its nose to angle its wings. For a tricycle gear like yours, move the main gear closer to the CoM to make it easier to lift the nose, and/or add more pitch control authority whether it's more or bigger elevons or moving the ones you have further back. Alternatively for any style of landing gear ensure the plane sits on the runway with the wings angled up.

Yaw instability is because your tail fin is too far forward. At best it's doing nothing and it might even be destabilising the plane. Think of the tail fin like a weather vane, it goes at the back and it tends to make the plane point into the wind, ie towards the prograde marker.

[jessesomething]

That helped immensely. The wings were angled down a bit and didn't notice. Also moved my gear back and added more wing support. Still didn't take off till the end but I'll make some more adjustments and should be ok. Thanks guys!

[jessesomething]

Another question. I'm trying to use my atilla thrusters when I reach an altitude where I can't use air intake for my rapier engines. Where does the Attila or Plasma thrusters start working? They don't seem to work inside the atmosphere.

[eddiew]

...Where does the Attila or Plasma thrusters start working?...

Might want to ask in the dev thread for the relevant mod - definitely not stock parts

You can of course kick the rapiers over to rocket mode. It's a small craft, but if you pull up hard you might be able to clear the atmosphere on a suborbital?

[cicatrix]

Another question. I'm trying to use my atilla thrusters when I reach an altitude where I can't use air intake for my rapier engines. Where does the Attila or Plasma thrusters start working? They don't seem to work inside the atmosphere.

It's from Interstellar. AFAIK you need to place your engines immediately adjacent to the reactor. And all these engines produce virtually no thrust in atmosphere. They're vacuum only.

[capi3101]

That helped immensely. The wings were angled down a bit and didn't notice. Also moved my gear back and added more wing support. Still didn't take off till the end but I'll make some more adjustments and should be ok. Thanks guys!

Still not taking off......what's the mass of your craft? You've got the KER display up in compact mode in your screenie. Two RAPIERs oughta support up to 26 tonnes of takeoff mass, so I'm guessing that you don't have enough lift-generating parts on the design for its mass. As a rule you want the sum of the lift coefficients of all your lift-generating parts to be roughly equal to the mass of the craft in tonnes (a 26 tonne craft would need a total combined lift coefficient of 26, the equivalent of 13 Delta Wings; you could get away with a 1:1.15 ratio, which comes out to 12 Deltas, but the 1:1 is easier to remember). I'm also seeing a grand total of two Elevons on that craft and small ones at that...Elevon 1s, I believe. You might not have sufficient pitch authority to bring the nose up. Any of those problems could be cropping up with your design here.

[MarvinKitFox]

Another question. I'm trying to use my atilla thrusters when I reach an altitude where I can't use air intake for my rapier engines. Where does the Attila or Plasma thrusters start working? They don't seem to work inside the atmosphere.

Atilla thrusters need pure vacuum, and 60+Mw of power.

I'm thinking you are a wee bit shy on power there. sonny. Is that rig capable of more than about 2Mw, in space? Those are puny reactors, and the stacked radiators only really work in air.

[Flixxbeatz]

Welcome to the world of SSTOs, where fixing one problem on your ship grants you another problem, and fixing that another problem gives you one more problem.