Shuttle OMS Stability

[Wizard Kerbal]

I’m playing on Xbox, so no mods.

I wanted to build a shuttle that had the OMS on the top and back of the vehicle instead of 2 terrier engines inline with CoM on the sides, but when It came time to use it, it started front-flipping out of control, and I see why Matt Lowne designed his shuttle as he did. 

Any help?

Edited February 5, 2021 by The wizard of me
Grammar

[Zhetaan]

That seems to me to be an asymmetric thrust problem.  You'll need to turn on the centre of thrust indicator in the VAB and ensure that the line coming out of it points through the centre of mass.  It's a bit tricky to do since the line doesn't come out of the front of the marker, but you can do it by rotating the view of the vessel while in the VAB.

Before you get that far, be certain to remove any boosters that will be expended before you light the OMS engines; you don't want to include engines that will be staged away by the time that the OMS fires because they and their tanks will throw off your centres of thrust and mass in the VAB, which would lead you to optimise for the wrong configuration.

Once you do that, you'll need to rotate the OMS engines to align the thrust with the centre of mass.  When I do it, I try to get my view behind the centre of thrust and rotate my view so that the centre of mass disappears behind it, and then rotate engines to align the thrust vector, too.  This is much easier to do if you first rotate the whole rocket to be on its side or even switch to the SPH for this portion; thrust and mass don't care about the direction of gravity.

Next, you'll need to drain the propellant tanks and see whether the centre of mass shifts appreciably.  If you're using something like an SSME (that's a Space Shuttle Main Engine, which is the engine that used the propellant from the big orange tank on the real Shuttle) instead of, or in addition to, an OMS system, then you'll probably need to move the orange tank (or equivalently-coloured tank) up on the shuttle's belly so that its centre of mass is on the same line as the thrust.  That way, the centre of mass as the tank drains only moves closer to the engines along that thrust line, and you either eliminate or significantly reduce the asymmetric thrust.

Note that if you have wings or other engines, then the thrust of the shuttle may not point precisely where the OMS engines do.  Also, as you go into vacuum or change engine configurations, the powered flight performance of the shuttle will change as well.  There's no good overall solution for that, but you can still optimise the engines' alignment for where you intend to most use them:  for example, don't take lift into account at all if you intend to use the OMS engines only in orbit or the upper atmosphere.

I hope that helps, and if it doesn't, then please do let me know (and provide more details so we can help you more).

[Wizard Kerbal]

1 hour ago, Zhetaan said:

That seems to me to be an asymmetric thrust problem.  You'll need to turn on the centre of thrust indicator in the VAB and ensure that the line coming out of it points through the centre of mass.

Before you get that far, be certain to remove any boosters that will be expended before you light the OMS engines; you don't want to include engines that will be staged away by the time that the OMS fires because they and their tanks will throw off your centres of thrust and mass in the VAB, which would lead you to optimise for the wrong configuration.

Once you do that, you'll need to rotate the OMS engines to align the thrust with the centre of mass.  When I do it, I try to get my view behind the centre of thrust and rotate my view so that the centre of mass disappears behind it, and then rotate engines to align the thrust vector, too.  

Next, you'll need to drain the propellant tanks and see whether the centre of mass shifts appreciably.  If you're using something like an SSME (that's a Space Shuttle Main Engine, which is the engine that used the propellant from the big orange tank on the real Shuttle) instead of, or in addition to, an OMS system, then you'll probably need to move the orange tank (or equivalently-coloured tank) up on the shuttle's belly so that its centre of mass is on the same line as the thrust.  That way, the centre of mass as the tank drains only moves closer to the engines along that thrust line, and you either eliminate or significantly reduce the asymmetric thrust.

Huh. Ok. From the start that’s what I did with my main engines, but I didn’t do it with OMS thrusters. That should help!

Edited February 5, 2021 by The wizard of me
Like a true weeb I said ODM and not OMS thrusters