How fast did you get in atmosphere with your jet engine? If you didn't get upwards of 2km/s velocity with it, you're losing a massive amount of potential delta V due to inefficient launching. Jet launching is much, much different than rocket launching. You need to stay in the low 30's of km altitude until you get your jet's top speed, then start climbing again, slowly, as long as you can while avoiding flameout and eventually firing the rocket once you are as high and as fast as you can go. Keep in mind that a good jet can actually get in to an orbit (though air braking will cause it to not actually work). My best on a jet is an apoapsis of 250km and a periapsis of 40km with a speed of around 2300 m/s at the periapsis. We're talking only providing like .5kN of thrust with the jet engine in such thin atmosphere, but you can slowly work your way up, and just barely provide thrust that you eventually reach an equilibrium, but it's a hassle to fly like that. (Jet + Ion drives makes for a really easy orbiter with no oxidizer).