I am not a rocket scientist but I think the horizontal take-off is a huge drawback.<p>* The wings would only be useful at relatively low speeds and then just add lots of mass.<p>* Wings that are efficient at high mach-numbers would be less effective at take-off.  Presumably lots of fuel would be burned at low speed to make up for this.<p>* The structure would need to be strong enough to function whilst horizontal and filled with fuel.<p>* It would need a large heavy undercarriage capable of supporting the entire take-off weight.<p>I am unconvinced that horizontal take-off has much benefit anyway.  A flight within the atmosphere to a launch site would inevitably use lots of fuel and eliminate any payload capacity.  Also, it is hard to believe that anyone is going to allow a massive hydrogen fuel tank to fly over populated areas.<p>A more traditional single stage to orbit would make more sense IMHO.