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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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Annotation:
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An electric motor with the power output
needed will be bulky and very heavy,
although using exotic materials may help. |
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There would need to be gearing to get the
fan disc up to the required speeds. |
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Once at unity output, probably better to
declutch the rotor than try to retract the fan
blades. |
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//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. |
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//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). |
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<ponder>
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. |
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