Dunno, the adverse aerodynamics of the two vehicles in “biamese” configuration at max-Q? That is, if the vehicles are mated together, they may be less aerodynamic or less controllable than if they flow by themselves. The Shuttle “stack” is not a true biamese, but the shock waves from the SRB’s and the Orbiter and the ET mixing it all up must have been a serious aerodynamic design challenge.

The other problem could be the “stuff falling of the tank and hitting the tiles.” Imagine if the Shuttle Orbiter could be lofted into suborbit, the External Tank lofted into suborbit, and the tank fueling (or oxidizing) up the Orbiter outside the influence of aerodynamic forces and in free fall. You would not have the problem of, ahem, “stuff” falling from the Tank and damaging the Orbiter.

On the other hand, the suborbital in-flight refueling mission profile — it would probably come about like “synchronized diving” (do they do that in the Olympics?) Two spacecraft on two pads, far enough away that they don’t risk blast damage to each other, but with synchronized engine start and synchronized liftoff. A computer gives the “GO” for both launches if both sets of rocket engines successfully reach operating thrust at the same time for the hold-down arms on each pad to be released at the same time.

We would watch two rocket plumes streak skyward and then lose sight of them. We would “watch” the suborbital refueling represented by the “artists’ animation” on TV.

http://twitter.com/Doug_Comstock: Peter Homer wins $250k 1st prize and Ted Southern wins $100k 2nd prize in Astro Glove Challenge. Congrats to both for a great competition!

http://twitter.com/pkhomer: Re-Pete! My astronaut glove wins first prize at NASA Centennial Challenge!

Worthy opponent Ted Southern teamed with Nik Moisiev, both past competitors, also beat NASA glove to win 2nd prize.

Cryogenic boil off is not such an issue if launching from above most of the atmosphere.

Which is also very good news for ISRU. Kerosene/H2O2 would be nocryogenic like MMH/NTO, which solves boil-off issues and allows propellant transfer with proven technologies. In addition it would not be nearly as nasty. Performance would be slightly less, but not critically so and ISRU potential would be a lot better. All in all, a fine near term propellant combination for landers and other transfer craft based at Lagrange points.

Who said High school algebra puzzles were useless….

With H2O2 the OF ratio is so skewed you only need to transfer one propellant.

Also if the vehicles are VT is there any downside to having them take off already coupled…..

We were focused on a “near SSTO” design using suborbital refueling as an enhancer for payload. You could think of it as a biamese idea except for the fact that you do the stage integration with a hose, and after launch. Mathematically, they’re not so different. Scroll down; it’s below figure ten.

It’s also worth pointing out that the receiver-side propellant transfer hardware arguably could be common for both the atmospheric propellant transfer, needed to load the rocket propellants in the first place, and the exoatmospheric propellant transfer. We imagined a “transfer kit” in the payload bay of the tanker-configured vehicle, with the necessary pump or pressure feed systems to push the propellant out into the receiver.

The precision flying needed to effect the transfer is probably easier outside the atmosphere, given the lack of disturbances. We pictured a back-to-back concept, but didn’t think it through completely, so perhaps another approach would be preferred.

An aircraft designed for the suborbital point-to-point mission would be a good fit for this application. The extra delta-V obtained from a second propellant transfer could deliver net payload to orbit. A first look at the thermal protection system requirements suggests that the more rapid heat rates from the suborbital case is probably the designing requirement.

Finally, if you can refuel exoatmospherically in a suborbital trajectory, you can do so in an orbital trajectory. And that makes a great many things possible, clearly, as advocates of orbital propellant depots have argued for some time.