*The abilities of the DG/XR engines are not physically impossible, it's just that concepts that allow that sort of performance come with a whole lot of technical baggage and engineering constraints that these vessels simply don't portray.
I've wondered about this. Take X-Plane as an example. According to their website, the way it works is, "...by reading in the geometric shape of any aircraft and then figuring out how that aircraft will fly. It does this by an engineering process called "blade element theory", which involves breaking the aircraft down into many small elements and then finding the forces on each little element many times per second. These forces are then converted into accelerations, which are then integrated to velocities and positions."
With that in mind, I have wondered what would happen if you built a Delta-glider model that would work in X-Plane. (Just the shape / weight distribution / flight characteristics of the Delta-glider. Minus the fantasy engine technology.) Then mount the Delta-glider under the wing of another plane, carry it up to an altitude of, say, 10 kilometers (about 33,000 feet) in X-Plane, and let it go.
What would happen?
I suspect ... if X-Plane's "blade element theory" works how I imagine (which I envision as like a virtual wind tunnel simulator) ... that the Delta-glider would "fly" like a rock all the way to the ground. (Maybe tumble end over end once, or do a couple of cartwheels before crashing into the ground.)