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Ramjets are more efficient than turbojets supersonically, not worth bothering with subsonically, and don't work at all under a couple hundred miles per hour.
A "motorjet" is a turbojet with no turbine, that utilizes an external source to drive the compressor.
The Idea is for a ramjet, which spike
houses an electric motor that drives retractable fan blades. (the parts of the idea which are original are "electric motor" & "retractable")
At low speed the fan is run, blades extended, properly feeding the (motor)jet. When the desired speed is reached, the motor is turned off, the blades retracted, and operation as a full ramjet, with no moving parts, commences.
(At an opportune time during the ramjet phase the blades can be partially re-extended to recharge the batteries.)
Note: There's the possibility of using the engine as an electrically-driven "plain old" ducted fan as well, without a running jet, if the internal contour of the ramjet allows for it.
[search-engine skimmers: this isn't the same principle as the J58 found on the SR-71: that's a turbojet which has a variable bypass after the compressor to the afterburner]
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||An electric motor with the power output
needed will be bulky and very heavy,
although using exotic materials may help.
||There would need to be gearing to get the
fan disc up to the required speeds.
||Once at unity output, probably better to
declutch the rotor than try to retract the fan
||//motor...heavy// I question whether the motor+fan assembly would weigh as much as the innards of an equivalent full speed-range turbojet, but regardless the entire thing certainly won't weigh as much as a ramjet and a separate turbojet or piston-engine.
||//gearing...speeds// mmm.... say a 2ft wide inlet that's 6.3ft circumference - at rest the blades go supersonic at about 10,000rpm: doesn't sound too difficult for an electric motor... variable-pitch isn't completely out of the question, just feather them then fold (or let the wind fold them).
On the other hand, if each blade was hinged in the middle and counterweighted, on the end of a fixed rod (quick visual: "drinking-bird toy"), then it could be folded without net centrifugal force rearing its ugly head, after which the motor (regeneratively of course) stops them.