|VFDR is the acronym for Variable Flow Ducted Rocket. It is different from the Ramjet not only because the fuel is solid but also because the sustainer fuel is lit and burned using an oxidizer blended into the solid propellant. This mixture burns very rich and completes the combustion of the fuel material using air from an inlet duct system. This is unlike a Ramjet which burns its fuel using solely air from the atmosphere. The variable flow denotion indicates that the fuel burn rate, or intake air flow, or both may be throttled via some metering mechanism. The advantages of the VFDR is that the fuel itself is ignitable and is solid making the weapon more reliable and simpler. The disadvantage is that VFDRs have an IpSec* in the 600 sec bracket compared to a pure Ramjet which is around 1200 sec. For comparison a solid rocket is around 250 sec and a turbojet is about 3000 sec. A VFDR missile tends to have 2 to 3x the sustainer performance of a solid rocket sustainer. However, because a VFDR incurs additional weight and drag from the intake system, valving interstage, and has to live with a relatively inefficient nozzleless integrated booster, overall performance tends to be only about 1.5~2x better. This nonetheless is still a very tangible improvement. The Meteor, for instance, is a VFDR missile.
In the early 2000s, the USAF/USN awarded various contracts to Aerojet for the development of practical VFDR solutions applicable to tactical missiles as a safeguard against the transformation technology thrust such as a hydrocarbon Scramjet and Dual Combustion Ramjet not panning out in time for next generation US weapons. This resulted in three VFDR solutions. A 13.5" (MARC-282) has since been put into production for use on the GQM-163 Coyote Mach 2.5 sea skimming target for the USN. The 10" VFDR (MARC-290) was recently test fired with HARM front end. Finally, a 7"Aerojet booster has now surfaced with an AMRAAM front end.
GQM-163A Sea Skimming Supersonic Target (SSST) with MARC-282 13.5" VFDR sustainer motor.
MARC-290 10" VFDR integrated with AARGM (AGM-88E) front end for test firing.
7" VFDR (unknown designation) with AIM-120 front end.
* 1 IpSec = 1 lb of thrust for 1 lb of carried expendables (fuel and/or oxidizer) burned. And IpSec of 250 seconds means that 1 lbs of propellant gets you 250 lbs of thrust for 1 second,1 lbs of thrust for 250 secs, or somewhere in between.