How can a modified ramjet produce static thrust without a turbine?
Well the Pratt &amp; Whitney J58 engines which powered the SR-71 worked in a similar fashion, using a bypass system which allows the engine to operate as a ramjet at high speeds and a turbojet engine at lower speeds.

By the very nature of a ramjet, it relies of stagnation pressure at the intake and inlet throat to pressurize the airflow prior to combustion.  It is just not possible to achieve these kinds of stagnation pressures at speeds under Mach 2 or so.  There was an article in Air &amp; Space magazine a while back about two enterprising Canadian teenagers who tried to fashion a homemade ramjet from iron sewer pipes to power a sled.  They couldn’t get the design to work for this reason.

As such, you would unfortunately need another type of engine to accelerate an aircraft to these fast speeds where a ramjet could work.  Turbojets work well for this application.  So do rocket boosters.  Injecting oxidizer would not be very useful here as airflow through the engine is not fast enough to ensure sustained combustion.