Hello Archibald,
no need to get formal and address me as "M. Bayer" - just call me Martin, but I'm good either way, just as long as you don't start using "Mr." or "Sir", because then I usually know I'm in trouble 
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To me the Hopper was always a dead end niche concept that might have been a modest stepping stone to introducing partial reusability and associated learning effects and lowering pure launch cost for smaller payloads, but in my view only true and *full* reusability will allow to not just deploy but also routinely retrieve and return payloads and thus open up a whole new set of missions and orbital activities - something FSSC-16 would have supported at least to some extent, so the comparison is really apples vs. oranges. Based on your statements above, I can now see though that you likely looked at it as yet another potential avenue to the concept under discussion here.
You mentioned using the X-33 design, so I assumed you were referring to a VTHL system - apart from the needlessly complex tank shapes that caused the concept's demise, I'm not sure though whether the *fat* lifting body aerodynamics would really be a good fit for an at least somewhat aerodynamic jet powered ascent as compared to a more conventional and slender wing-body configuration like the Teledyne spaceplane or FSSC-16 (or IMHO really for *any* aerodynamic maneuvers, for that matter). Adding jet engines to both vehicles obviously adds complexity and weight for both instead of just the booster in the almost bimese case, and if you used the original X-33 as the point of departure for two *completely* identical but separately launched stages, you would now incur a lot of other similar inefficiencies like a true bimese, e.g. orbital capable TPS and OMS on both stages, having an empty payload bay on the booster, both active and passive refueling equipment on both stages, etc..
Instead of just taking Mitchell Burnside Clapp's breezy assertion that buddy type suborbital refueling is really a lead-pipe cinch at face value, I'd want to see *any* two vehicle rendezvous, linkup, and propellant(s) transfer in coasting ascent per se (irrespective of propellant densities, temperatures, number of propellant components, etc. - for proof of concept you could initially even use water, for all I care) *repeatedly* demonstrated reliably in actual flight and representative environments in principle before I would put *any* practical credence in that concept for routine operational purposes. The risk factors remain, and of course that applies even exponentially more so to the flock concept.
I also note that, at least after cursory perusing of your uploads, none of the papers really provide a detailed, honest to god head to head comparative analysis of a TSTO vs. an aerial hookup system. In fact, for example in the 2006 AIAA paper "Economics of Separated Ascent Stage Launch Vehicles" a footnote on page 8 characterizes an assumed structure mass fraction with admittedly remarkably candid and refreshing honesty explicitly as a "Wild Guess". The closest reference I could find so far to some kind of comparison is on page 4 of the AIAA 2004 paper called "The Flock Booster Architecture – Low Cost Access to LEO via Sustained Fueling", which refers to two winged boosters mated belly to belly for a vertical launch off a standard launch pad. Apart from a basic booster delta v calculation, there are however really no quantitative comparative analyses with two separately vertically launched stages, only the qualitative assertion that "[w]hile vertical launch is possible, the time required to lift and mate the boosters on a launch pad is very costly, and requires both specialized equipment and a dedicated launch pad", whereas apparently (nearly) effortless horizontal launch is automatically assumed for separately launched stages. But even under the assumption of vertical launch for a rocket powered launch vehicle with at least partial reusability, Elon Musk might have a tought or two on the actual time, cost and required equipment to do so .
Martin
