From what I understand from various air-launch studies, top-mounted launch is about the hardest way to do air-launch possible. While it avoids issues with ground clearance and carrier aircraft modifications somewhat it has major difficulties from an aerodynamics standing.
Once the carrier aircraft is "relieved" of the load it wants to climb and you can't do that with the carried vehicle ABOVE you. So in order to get a clean seperation distance you need to have large wings on the rocket, (increasing drag and limiting your launch altitude) AND you have to perform a "negative-G" toss manuver. This is where you enter a shallow dive to gain speed, pull up into a climb and suddenly shove-over into a dive while simultaniously releasing the rocket, fire-walling the throttles and turning away just in case the rocket DOESN'T fire and comes falling back down!
Transformational Space (T-Space) did a lot of work on studying the various aspects and types of air-launch types and proposals some of which is available in various studies and papers on the AirLaunch LLC website:
http://www.airlaunchllc.com/TechPapers.html
and here for an overview:
http://mae.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pdf
(Which actually details this concept: Boeing AirLaunch concept, page-6. I should note that the authors of THIS paper are also the authors of the majority of the papers at the AirLaunch LLC website so they have evidently substantially changed thier minds on the "best" method of air-launching a rocket vehicle)
The biggest drawback for concepts of air launching booster vehicles has always been the limited ability to "use" off-the-shelf military and civil aircraft without extensive modification almost always leading to the need for custom built carrier aircraft. Both the modifications of existing aircraft or building all new aircraft have historically been major expenses. Some of this has changed with studies for using large cargo aircraft as carrier vehicles but the limitation on space remains significant, and recent studies by AirLaunch exanding on the T-Space launch vehicle concept have qualifed that current 747-class aircraft actually ARE capable of being used as carrier aircraft with less extensive modifications that previously thought.
(By using different landing gear hydralic systems that allow "over-filling" the gear a ground clearance of up to 8 feet is possible while still useing essentially the already qualifed landing gear that comes on the 747. This allows both a cost savings on certification as well as not haveing to extensivly modifiy the airframe with extended landing gear systems)
The majority of the modifications costs now almost equal the conversion costs of a 747 air-freighter into a fire-tanker aircraft with the added benefits that the modification still leave the air-freighter with significant "on-board" cargo capability AND the ability to be used as a fire-tanker as well as a launch vehicle carrier aircraft.
See:
http://www.airlaunchllc.com/AIAA-2008-7835-176.pdf
Randy
