This seems to not have consideration for hypersonic air flow. If this is just a lifting engine it can only provide a small fraction of delta v.
This implies dropping to presumably start a scramjet? Otherwise, all you're getting is a bit of altitude and softer max Q.
The surprise advantage of air-launched vehicles is you can launch from any airfield. The surprise disadvantage is you need strength two directions; vertically, when it's thrusting, and horizontally, when it's hung underwing.