Why do my shuttles keep pitching up and tumbling

[TheKSPBeginner]

Edit: nobody seems to understand that launch is going fine

Answer me @Pak

Edited October 29, 2019 by TheKSPBeginner
E

[Vanamonde]

We'd have to see the shuttles to get some clues. 

[PositronLance001]

also shuttles are a lot more trouble than they're worth, unless you're trying to go for replicas.

Speaking from experience here.

[Geonovast]

1 minute ago, PositronLance001 said:

also shuttles are a lot more trouble than they're worth, unless you're trying to go for replicas.

Speaking from experience here.

Practially?  Definitely.

But as someone who struggled with it for a long time, then finally figured it out, then stopped for awhile and promptly forgot how to build/fly them, once you get it down, they're incredibly satisfying to fly.

 

That said, as stated above, until we can see what you have, there's no a whole lot of advice that can be given.

[Lisias]

Your shuttle has a fat ass. You need to have the nose as the heavier part of the shuttle on reentry, as the aerodynamic forces at that stage are so brutal than easily overcomes any wing lifting and control surfaces authority.

Problem is that a too heavy nose will make the thing hard to glide, so you would want to have fuel tanks on both ends of the craft, and pumping fuel (or anything else, as monopropellant, oxidizer, etc) between them on the different stages of the flight.

Edited October 26, 2019 by Lisias
yeah. moar tyops!!

[TheKSPBeginner]

First they were pitching down but now they’re pitching up please help

[TheKSPBeginner]

It tumbles when I turn on the engines

[Xd the great]

1 hour ago, TheKSPBeginner said:

It tumbles when I turn on the engines

Rotate the engine around, move the fuel tanks around...

Basically, have center of thrust go through center of mass.

And use launch clamps to stabilize the shuttle on the pad.

[TheKSPBeginner]

The OMS engines

[Xd the great]

6 hours ago, TheKSPBeginner said:

The OMS engines

Yep, rotate them so their thrust will also go through center of mass. I usually set SSME thrust (and other engines' thrust) to 0, remove the side boosters and external fuel tanks, then see if Center of Thrust goes through Center of Mass.

Edited October 27, 2019 by Xd the great
Edited for clarity

[TheKSPBeginner]

They are aligned with the COM but my maneuver node keeps moving away, sending it into a spin

Edited October 27, 2019 by TheKSPBeginner
I am using the Cormorant Aerotechnology O-90 engines

[pincushionman]

3 hours ago, TheKSPBeginner said:

They are aligned with the COM but my maneuver node keeps moving away, sending it into a spin

Unless you’ve added a control point that is aligned with your (rotated) thrust vector, your thrust won’t be aligned with “vessel prograde”. You’d need to eyeball an offset from prograde during your burns, which would be kind of awkward.

[TheKSPBeginner]

Thank you, will try

[TheKSPBeginner]

Didn’t work

[Xd the great]

23 hours ago, TheKSPBeginner said:

They are aligned with the COM but my maneuver node keeps moving away, sending it into a spin

Did you try with stock engines?

[TheKSPBeginner]

Probably would’ve provided the same result.

[Xd the great]

4 hours ago, TheKSPBeginner said:

Probably would’ve provided the same result.

If you are using ksp v 1.8, the mod may not have been updated, and bugs will occur. 

[Rocketology]

A balanced space shuttle type architecture in KSP is arguably one of the most difficult things to engineer in game. As others have stated above, ensuring that your Center of Thrust is aligned through the Center of Mass is only the beginning. As the CoM of the shuttle will move toward the orbiter and aft as you burn fuel (both solid and LFO). Thus, the Center of Thrust must also move to compensate; and this is where it gets difficult.
If you are trying to build a NASA shuttle analog, you may need to thrust limit the Vectors if the shuttle is wanting to pitch forward, may also wish to use the rotate tool and rotate them in the pitch axis 5-15 degrees where they mount to better align through CoM. If your design is wanting to pitch backwards, the SRB's may be overpowering the Vectors, depending on your setup and that SRB's TWR climbs rapidly as they burn solid fuel, so what may be balanced at launch won't be shortly after lift-off. If this is the case, and you have the Breaking Ground DLC, you can set a KAL 1000 controller to thrust curve (decreasing thrust limit over time) the SRB's - this will require a lot of trial and error to figure out the ideal curve for your design and may vary by  payload mass.
If you do not have the Breaking Ground DLC, you can still thrust curve stock SRB's - you just have to thrust limit them in pairs, I did this for my shuttle long before the Breaking Ground DLC. This requires extensive trial and error to get right, and for my design, it took 14 Kickbacks per side to accomplish, thrust limiting in pairs from 90-50%. Longer still to tune them so the shuttle could fly both empty and max payload with the same SRB setup. I then used a small LFO tank to fuel Cub engines that I used for engine gimbal. The only reaction wheel on mine is from the Mk3 cockpit and the shuttle is very smooth in flight.

Here's some screenshots of my original setup (NOT a NASA analog), all stock, pre-BG DLC (note the attachment setup using the docking ports no longer works with changes due to Breaking Ground DLC):

[TheKSPBeginner]

This problem is only taking place in orbit

[TheKSPBeginner]

Edit: no it isn’t

It’s because my COM is too low. What do

Edited October 30, 2019 by TheKSPBeginner
No it isn’t

[Xd the great]

12 hours ago, TheKSPBeginner said:

It’s because my COM is too low. What do

Rotate the OMS engines so they point through the new COM.

And by the way, you can edit your original texts.

[TheKSPBeginner]

They are going through the COM, always have been

[Xd the great]

29 minutes ago, TheKSPBeginner said:

They are going through the COM, always have been

Did you switch on SAS to manouver node?

Were any other engines firing?

[TheKSPBeginner]

Just the OMS, and I am using MechJeb 

Edited October 30, 2019 by TheKSPBeginner
E

[Xd the great]

2 minutes ago, TheKSPBeginner said:

Just the OMS, and I am using MechJeb 

Could mechjeb be the problem?

Did you try limiting the throttle of the OMS?