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My SSTO stops working at ~23k or 1400m/s


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I am kinda new to the game and can't quite wrap my head around the problem with my design and each time it goes above 23k / 1.4km/s it spins out of control or just doesn't have any oxidizer left (most of the time both)
the whole design probably sucks but I searched every tutorial and still don't get how it's supposed to work

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That is an interesting design with different engines for different altitudes.  It looks like the thrust is imbalanced and is pushing it out of control.  And yeah, it doesn't have enough oxy.  With all those different engines it takes a lot of tuning to get them all balanced all the time.  Also, the COM and COL might be way out too.

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Out of control spins after some time in flight can often be caused by the COM shifting relative to the COL as fuel is drained from the tanks. In the SPH you want to check where your COM is with full fuel load, with the liquid only tanks empty (to represent transition to rocket power) and with all tanks empty (for landing - you might reserve some fuel for a powered landing approach, but checking the balance when empty is a good idea anyway).

If the COM moves significantly and especially if it moves behind the COL, then you might want to try redistributing fuel tanks or changing flow priorities to try and keep it better balanced during different phases of flight.

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You’re trying too hard with all those different engines. Try swapping the Rapiers for Whiplashes and the Vector for a cluster of four NERVs, which will keep your fuel situation much more manageable as they both only need fuel and not oxidiser. If that isn’t enough to climb out of the atmosphere, a couple of SRBs slung under the wings or a pair of Swivels directly under your tailfins might give it the necessary kick. Ditch the big monopropellant tank and the docking port- that thing is too big to try and dock it efficiently- and then open the CoM and CoL markers and gradually drain fuel from the tanks to see if they shift to an unstable position as the tanks empty.

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