I created an SSTO which works fine in Kerbin atmosphere, has plenty of fuel to get into orbit, however, when trying to circularise the orbit and using the nuclear engines in orbit, the SSTO keeps on pitching up, even when all control surfaces are turned off. Any help please?
The SSTO has a mass of 56.3k kg, 4900 units of liquid fuel and 880 units of oxidiser, with 4 rapier engines and 2 nuclear.