Excellent stuff Guy
Your comments on internal volume and wing-loading are sobering. To answer your question about what would need to go inside ... in this specific example - beyond fuel, engine, controls, guidance, etc. - it needs to accommodate a BLU-116 AUP warhead (or non-US equivalent).
Weight can be reduced - dumping the undercarriage; restressing for a one-flight lifespan; simplified controls; etc. But quartering the available volume is the BIG reality check here.
For powerplant, the MQ-28A engine is (AFAIK) a Williams FJ44 ... no idea on the exact sub-type. So, we have a thrust anywhere between 1,900 and 3,000 lbf; a diameter of 20.7 to 23.0 inches; and a dry weight of 460 to 535 lbs. My 'close to 50%' choice would have been the PW610F producing 950 lbf; a diameter of 14 inches; and weighing about 260 lbs. But in length (46 inches), the PW601F isn't much different from the
Ghost Bat's FJ44. Needing to drop down to a quarter of the volume is a whole 'nuther deal!
The Williams WJ38-7 (F415-WR-400 for the
Tomahawk Block IV TLAM-E) might be small enough but I can't find any published dimensions. I suspect that F415 details are still classified ... and, therefore, possibly unavailable for any non-US programmes.
All this has me wondering if turbofans mightn't be the best approach. They give you much better sfc and are quieter but you need so much more space to shoe-horn one in. That probably explains why SCALP/
Storm Shadow are using the punchy-but-thirsty Microturbo TRI-60-30.
Another 'old school' turbojet option would be the Teledyne J402-CA-400 turbojet - diameter 12.5 inches; dry weight 101.5 lbs; length 29 inches. That's gets us down to 'quarter scale' but the Teledyne only produces 660 lbf with twice the sfc
Anyone have any other ideas?