A comment to this question suggested, followed by a smiley, that dropping it [a ramjet engine] from orbit might start it. This got me thinking if this really would be possible.
This Skybrary article suggests that the minimum theoretical starting speed for a ramjet engine is as low as about 100 knots, but the same article, and Wikipedia and many other sources state no significant thrust is produced below mach 0.5 (or even higher).
A streamlined object such as a bullet in freefall will reach a terminal velocity of about 90m/s or 175kts, which would be within the suggested theoretical starting speed.
But this is where my math skills fail me: In order to start, the ramjet will need to be able to force air through itself, this will obviously create resistance. Therefore a ramjet might not be described as a streamlined object, and it might not be able to achieve the speed necessary to start and produce enough thrust to further accelerate to an operational speed range.
Further complicating things migh/will be the altitude from which the ramjet migh be dropped. We know that re-entry from orbit will yield vey high velocities, but these are achieved in very thin atmosphere, and I have no idea how this plays with the operation of a ramjet...
So: is it possible to start a ramjet by dropping it from an altitude, and what might the order of magnitude of this altitude be?