First, most turbojet type engines can't handle supersonic airflow into the compressor section. Supersonic jets avoid this problem by using sophisticated inlets to slow the airflow to subsonic speeds before it reaches the compressor section. The most complicated (and fastest) instance of this was the SR-71. It's inlets where so cleverly engineered that a high percentage of the engine's total thrust at the maximum speed of Mach 3.2 (declassified not too long ago) came from those inlets.

A ramjet engine has a similar limitation - the airflow through the combustion chamber can't be supersonic. That limits the top speed to something like Mach 5-6. To go faster, you need a scramjet (supersonic combustion ramjet), an idea that has been around for decades but has proved very difficult to accomplish even in a wind tunnel, much less in actual flight.

Even when the scramjet engine is developed, heat will be the real killer. Go read that link about the final moments of the Columbia. It reentered the upper atmosphere doing about Mach 25. The heat was in the thousands of degrees. You don't want to stay in that kind of heat any longer than you absolutely have too, even if you engine were still able to make usable thrust (questionable since the combustion temperature may well be less than the airstream temperature at those speeds). Trying to go faster, such as the Mach 45 that you suggested, would mean even higher temperatures and bigger engineering challenges.

I still like the idea of using a mothership aircraft to go as high and fast as possible before releasing a small rocket to take the payload into space. DARPA is working on a project (Rascal) along these lines that should reach Mach 3+ at around 100,000 feet. It's designed to launch microsats up to about 75-120 pounds into LEO, but the idea can eventually be scaled up for larger loads.

The faster you go, the more energy it takes. More energy means more propellant, which means more weight at takeoff. That means you need more energy to go fast, a classic snowball effect. That's why the Concorde is now a museum piece - it took too much energy (and carried too small payload as a result) to be profitable. That's why Boeing had to drop their High Speed Commercial Transport idea (designed to carry 300 passengers on transpacific length flights) - the engine technology just wasn't there to make the project work and be profitable. Maybe someday...