Paul451 made a suggestion on the last post that triggered a thought on boosting payloads on many current vehicles at a reasonable cost. I claim 50% credit for this concept with Paul getting the rest.
He suggested that the tanks on the last post should be thin walled balloon tanks with a structural truss from the thrust structure of the first stage to the second stage. This would allow modular stacking and swap outs with minimum pain. It would also allow greater flexibility in modifying vehicles for different missions. The thin wall tanks were to compensate for the mass of the structural truss as well as for cost reasons.
My quibble was with the truss mass, though the ability to go with cheaper and lighter tanks appealed greatly. My thought based on his thought started with using an existing LV for the structural truss with the thin wall tanks attached to the sides much like in the previous post. Then SRBs are attached to the outsides of the strap on tanks. My squeamishness about SRBs is not shared by several of the experienced companies that actually launch rockets.
Several companies use strap on SRBs as a matter of course. The Atlas sometimes uses a single SRB, which answers many of the concerns expressed in comments about asymmetrical loads or thrust vectors expressed in the previous post. A variety of existing and well known SRBs are available to launch companies right now, as opposed to the liquid engines that I would prefer. SRBs are frequently mentioned as relatively inexpensive engines. My qualms have to do with the unfortunate failure modes.
If two tanks are strapped onto an LV with direct feed to the core engines, the core will be far too overloaded to leave the ground. Add enough SRBs to the outside of the strap on tanks and the vehicle will have the TW to launch. At SRB burn out, the core vehicle’s engines will be at full vacuum thrust. A lofted trajectory will allow the total vehicle to have a TW of less than one including the strap on tanks. The vehicle first stage mass ratio from Mach 3-4 and 25 miles could be as much as double that of a standard core vehicle doing a ground launch. Payload could triple or so.
An Atlas 5 with this modification could match the theoretical payload of an Atlas 5 heavy with the expenditure of one RD170 instead of three. Other vehicles run by companies not squeamish about SRBs could get similar results.
As pointed out by Peter in the previous post, it is not required to stop at two strap on tanks as the Saturn I for instance had multiple parallel tanks.
In the event of SRB cato, the strap on tanks act as shrapnel catchers. This might make it possible for standard LVs to use SRB boosters with less concern about losing the core vehicle and payload. The payload might make it into LEO instead of GEO after losing the extra boost and propellant. A tug could be designed and tasked to rescue these stranded vehicles and take them to their intended destination.