In the discussion on Jons’ last post it was mentioned that seven cores could be used in the VTVL booster technique with the outer six semi-permanently attached as a single unit. If they are close to gas-n-go on landing, then keeping a much larger LV booster together might make economic sense. A very large Reusable Launch Booster (RLB) could make good sense
The thought is that the center core is mated to the three diameter payload shroud on the ground and lowered into position after the six booster is moved to the launch pad and cleared for flight. This would allow for frequent launch of large volume payloads with the expenditure of a single booster.
At right is the landed cluster with a crane lowering the center core and high volume payload shroud into the center position. At left in color is the nozzle layout. The high expansion ratio center nozzle has the lower expansion ratio booster nozzles in close proximity. The outer six nozzle ring entrains air flow to prevent flow separation of the vacuum nozzle at sea level.
The Falcon9 series may not be appropriate for this concept as 63 engines at lift off may be a bit much to deal with as well as the problem of getting the expansion ratios right for the upper stage in a complex layout. Their next engine series may be a different story. I have wondered on occasion why they didn’t go with an aerospike arrangement with all the gas generator exhausts to the middle.
Blue Origin may have a better shot with the single engine cores. The ULA Vulcan in the works may have potential. The Delta might be modified in this manner with a bit of VL development on their part. Atlas and Antaries have the two nozzle boosters which may make it a bit more challenging, though a bit of clever could go a long way.
It should be built into the design of the RLB that individual cores could be changed out fairly rapidly for maintenance or repairs. Also it would allow for transport of the entire vehicle in sections by ground or air. Seaborne transport might carry it as a unit if desired.
If the demand arises, it seems possible for several companies to have 100+ ton LVs with 10+ meter  shrouds in the next ten years.
johnhare
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Re: Your sketch of the high-expansion exhaust nozzle.
Have a look at the sea-level nozzle on the existing <a href="http://i.kinja-img.com/gawker-media/image/upload/s–wyfASjRR–/1231905701160982855.jpg"New Shepard launcher. A high-expansion nozzle will fit well within the stage diameter.
Re: Payload shroud.
If the first stage boosters are bolted together and fully recoverable, then you might be able to attach the aerodynamic shroud permanently to the ring-stack. Clam-shell opening during staging, then closed for re-entry.
(You might not even need to close the shroud for re-entry. With the right design, it might serve as a drag system.)
“The high expansion ratio center nozzle has the lower expansion ratio booster nozzles in close proximity. The outer six nozzle ring entrains air flow to prevent flow separation of the vacuum nozzle at sea level.”
Unless you have cross-feed, you don’t want to fire the
upper-stagecentre-core until after staging. Otherwise you’ll need much more fuel in the centre-stage, which defeats the purpose of identical length stages. And if you’re bolting the ring-stack together as one piece, you don’t want the added cost of cross-feed. If you have cross-feed, you don’t want to join the ring-stack together as one piece (you’d shed the boosters in pairs.)Oops, try that again: New Shepard
I had assumed cross feed as a given. OTOH, firing the center after staging only costs 1/7 of the thrust at lift off and may not be worth the trouble unless the extra performance was a clear necessity.
Permanent shroud doubling as drag device looks like a clear win. Saves cost on an expendable shroud and weight on a separate drag device.
One benefit of firing all engines at liftoff is assurance that all engines are working. It’s not unknown for engines to not start up properly in flight. Detect a problem engine in the seconds before clamps release and you can abort on the pad to make repairs.
Re: Running the centre vac-engine.
You’ll need to turn off the engine, or throttle it way, way down, during staging when it’s being drawn through the centre of the other stages. Otherwise you need a heat-shield/ablative tube all the way down the inside of the outer ring. I’m not sure you’re really gaining anything.
(Plus you’ve got the issue of trying to tune the “altitude compensating” vac-nozzle to work with that over-pressure from the outer engines at sea-level. If you get lucky, it “just works”, but… how often does the universe reward aerospace engineers with “just works”?)
Re: Cross-feed.
If you have cross-feed, you wouldn’t bolt the outer-cores into a single pseudo-stage. You’d treat them as three opposite-pairs of boosters. IMO it’s one or the other. If you do both you’ve got the added costs of developing cross-feed and the added inefficiencies of all-in-one.
I’m thinking the ground handling and recovery advantage of the all in one would outweigh the performance advantage of dropping in pairs. It also seems possible to me that the 6 ring booster could incorporate the full diameter expansion nozzle I suggest in the next post. Instead of the center burning all the way as I thought, it might be possible to RTLS with the center stage and payload if there is a starting problem. The cost of an in flight abort being the just booster fuel seems like a desirable characteristic.