One of the items that seems to be missing from the current crop of LVs is any hint of compensating Nozzles. So a nozzle normally flies that is over expanded at sea level, optimum for a few seconds at some intermediate altitude, and under expanded through most of the flight. The Isp hit from under expansion at altitude reduces the velocity attained by that booster, which is well known to hurt mass ratios even more. Perhaps worse is the thrust loss from over expansion at lift off. At lift off, every pound of lost thrust is perhaps 0.85 pounds less GLOW possible. I think compensating nozzles are, if obviously not essential, definitely a serious performance and thus profit enhancer.
One concept I believe to be original allows a single expansion nozzle for several chambers that is the full diameter of the launch vehicle. If an aerospike is a reversed bell nozzle in concept, then this one is partially re-reversing the aerospike while retaining the atmospheric wall through the lower altitude portions of the flight.
In this concept, the multiple engines are in a circle as far out as possible under the lower propellant tank. The expansion nozzles all are connected on the outer perimeter as well as completely enclosed in the center. When the ambient air is higher pressure than the Â exhaust in the center volume, the flap valves hang open to let in the air. As air pressure decreases with altitude, the exhaust pressure gradually pushes the flaps closed. At high altitude the flaps are closed and sealed such that the multiple engines have a single expansion nozzle 12 feet or more in diameter giving the maximum possible expansion ratio.