X-15AD-4 with solid fuel rocket boosters at full thrust.
Heat energy derived from the combustion of fuel and oxidizer at high pressure and temperature is converted into kinetic energy of motion by expansion of the combustion gases to a lower pressure and temperature through an exhaust nozzle. The fact that the gases are accelerated by their expansion through the nozzle means that force has been exerted on them to speed them up. The reaction to this force, acting on the forward end of the combustion chamber, gives the engine a forward push.
At the time of publication, rocket engines were still not supported in either Microsoft® Flight Simulator X or Lockheed Martin® Prepar3D®. Your simulation platform still sees the X-15A-2's XLR-99 engine, and the solid rocket boosters on the X-15AD-4, as jet engines. Rocket-propelled flight is therefore not 100% accurate in the simulator although we tried our best to emulate it as much as we could in the flight model, considering the present limitations. We sincerely hope that all simulation platforms will offer true support for rocket engines (and ballistic control) in the near future.