The hybrid approach is how I saw attempts on Solar System Speed Record occuring. Yes, you could get up to 0.21c (ish, I can't remember the exact numbers without a dig, and the higher speed was assisted by gravity slingshot), but you run into the two main problems. These are Frame Dragging, and Conservation of Momentum.
IIRC, Speed Drives create a distortion field to allow them to move. This is patially a gravitational effect, which is why we have the mass/speed curve we do. I'd say once the field hits a certain threshhold (differing for each vessel) it 'jumps' to the fractional c values. With the field 'complete', it semi-isolates the momentum of the ship from the outside universe. This means that when the speed drive is turned off, all the ship has is the momentum prior to 'jump' plus whatever the acceleration drive managed to impart.
Frame Dragging occurs when the drive field distortion impinges on normal spacetime, thus we get a sort of friction. The higher speed ships of Fenspace reach these speeds by either power to mass ratios (small ships with BIG engines, which is reasonably easy) or manipulating the drive field to reduce Frame Drag (We could call this Frame Interface Dynamics, which leads to really complicated engines like the Midnight's and ridiculous amounts of processing power for simulations).
'Waved Acceleration Drives also have a drive field. It serves to protect the ship's occupents from the stresses of high Gs, but it also affect delta-v.
N.B: The mechanics above are all my interpretation of how things work. Other peoples interpretations will differ.
IIRC, Speed Drives create a distortion field to allow them to move. This is patially a gravitational effect, which is why we have the mass/speed curve we do. I'd say once the field hits a certain threshhold (differing for each vessel) it 'jumps' to the fractional c values. With the field 'complete', it semi-isolates the momentum of the ship from the outside universe. This means that when the speed drive is turned off, all the ship has is the momentum prior to 'jump' plus whatever the acceleration drive managed to impart.
Frame Dragging occurs when the drive field distortion impinges on normal spacetime, thus we get a sort of friction. The higher speed ships of Fenspace reach these speeds by either power to mass ratios (small ships with BIG engines, which is reasonably easy) or manipulating the drive field to reduce Frame Drag (We could call this Frame Interface Dynamics, which leads to really complicated engines like the Midnight's and ridiculous amounts of processing power for simulations).
'Waved Acceleration Drives also have a drive field. It serves to protect the ship's occupents from the stresses of high Gs, but it also affect delta-v.
N.B: The mechanics above are all my interpretation of how things work. Other peoples interpretations will differ.