SSTO space plane pitching up uncontrollably on ascent to orbit

[davidpsummers]

I am tweaking my big SSTO. I've got a working design and I'm trying to improve performance. In my latest iteration (picture below), I have run into a problem. At some point it will start pitching up and I can't stop it. Any help?

Edited October 6, 2015 by davidpsummers

[capi3101]

Usually when that happens, it's because your CoM has shifted too far aft as your fuel has drained.

To diagnose the problem, empty out all the fuel from your tanks in the SPH and see where it winds up. Or you can install a mod called RCS Build Aid; it has a "dry center of mass" marker that does pretty much the same thing. Check to see where the CoM is when the tanks are dry; if it's aft of your CoL, there's your problem. If not, we can work further through the diagnostic process.

[Padishar]

My initial guess would be that you have too much of your fuel too far forward so as it drains the CoM shifts back until it is much closer to the CoL (or even behind it). Either try tweaking all the fuel out to see where the dry CoM is or install something like RCS build aid which can show you both dry and wet CoM markers without needing to mess with the tanks. Also, at present, you may need to account for the fact that the drain is split equally between the tanks rather than taking a larger proportion from larger tanks. This can make the CoM move in unexpected ways as the fuel drains if you have tanks of different capacities...

Edit: darn Ninjas...

Edit 2: Given you have multiple engines at different heights, you should make sure that you engines' thrust vector is aligned with the CoM. A slight imbalance can probably be handled by SAS etc but as the fuel drains and the CoM shifts this might change. Also, as you get higher the lift generated will decrease and the lift is actually helping to keep the nose down (the lift force is trying to rotate the plane around the CoM).

Edited October 5, 2015 by Padishar

[Van Disaster]

Pitch the plane in the hangar & check whether the CoM/CoL relationship changes, you may find CoL shifts towards CoM as you pitch. Usually it's CoM shift from fuel burn though. Also you need rather more horizontal control surface area - I'd look at using things the size of that tail fin.

[davidpsummers]

I did check the center of thrust and made sure it was through the COM. I did a flight where I checked the fuel in the tanks when this happened. I then went to the SPH and adjusted the fuel distribution to be as close as possible. The COM was still ahead of the COL.

I didn't check it at different pitch. I can see how that would bring them closer, but not how it would reverse them? But I'll check it out and get back to you. I have to run through where the fuel distribution was at the time.

[davidpsummers]

I didn't check it at different pitch. I can see how that would bring them closer, but not how it would reverse them? But I'll check it out and get back to you. I have to run through where the fuel distribution was at the time.

Well, pitching the thing in the SPH say the COM and the COL get close. But don't switch? The wings are mounted low, maybe move them higher?

I was thinking about more control sufaces, but wonder if I wasn't try to solve a fundamental design problem with brute force at he expense of drag. I'll try adding some. (Whats a good step up from the AV-R8? As far as I can tell, nothing else pivots like it does?)

[Van Disaster]

As your CoL & CoM get closer together then the pitch-down moment from CoM being in front will get less... and as it's so far back all that body up in front of it is going to give a nice nose-up amount of body lift, and you'll get a positive feedback loop if you don't control it.

Build static canards & attach control surfaces, or just remove all the ones you have & use a pair of that part you're using for the tail. They don't *have* to be all-moving...

That had the same issues as yours, I just shifted aero balance a bit and made sure I had enough control authority to catch it trying to pitch backwards. that's for FAR, but this is aero basics which are valid for both.

If you want a naturally stable craft then move the wings forward a little & build a big tail. It won't be as efficient as a canard craft because you'll have one set of surfaces pushing down, but it is a naturally more stable configuration than fixed canards. All-moving canards will stall before the main wings, but you're back to not having any big enough.

Edited October 6, 2015 by Van Disaster

[davidpsummers]

I may also try moving the wings higher up on the craft. If its COM/COL balance when pitched up, that should help?

[capi3101]

I may also try moving the wings higher up on the craft. If its COM/COL balance when pitched up, that should help?

A CoL located below the CoM will also cause instability/greater maneuverability. In that particular case, the plane will have a tendency to want to fly inverted. That's probably tolerable in the upper atmosphere, but for landing, well......

[Van Disaster]

A CoL located below the CoM will also cause instability/greater maneuverability. In that particular case, the plane will have a tendency to want to fly inverted. That's probably tolerable in the upper atmosphere, but for landing, well......

A little dihedral will cure that, but that's effectively the same as moving the wings upwards anyway.

CoL below CoM will probably make pitch-up CoL movement issues worse - in the side shot of the plane pitched up, if the wings were lower the CoL ball will move down the axis of lift ( which is the arrow pointing straight up out of it ), so if you move CoL down that axis the distance the wings are from the bottom of the body you can see it'd probably end up in front of CoM at that pitch.