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  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.
johnhare
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I think you’ve reinvented the expansion-deflection nozzle.
I think the air connection makes
It different as well as using multiple nozzles. The flow direction is similar but not identi al. I’m not fanatical about it being new and original though. IMO this might be worth exploring with the multiple engine vehicles.
Why not just hang a flexible curtain between the nozzles of the outer ring of 8 engines in the Falcon 9 FT? I think this accomplishes much of what your flaps accomplish. At high altitudes, when the engines are underexpanded, some of the exhaust already reverses direction and heads back to the vehicle. The curtain causes the pressure under the base of the rocket to increase to approximately the exit pressure of the engine bells. There is 3.4 m^2 more area at the bottom of the F9FT than its nozzles, so if the exit pressure is something like 50 kPa, that’s about 170 kN of extra thrust that could be had, an extra 2.3% of vacuum thrust. If it acts like an aerospike and increases the effective expansion ratio, so much the better.
At liftoff, the outer engines would all be gimballed to reduce the diameter of the curtain. Unfortunately, I think you lose a bit of thrust at takeoff because the local air pressure at the rocket base drops below ambient. If the curtain makes the vehicle bottom act as an aerospike, then maybe it’s possible to reduce the sizes of the engine bells and make the engines exhaust at 100 kPa at sea level, and lose nothing to overexpansion. Right now the rocket probably loses about 350 kN (5%) to overexpansion at takeoff.
As the rocket rises, ambient pressure drops to the exit pressure of the nozzles, after which point the base pressure is effectively above ambient, perhaps significantly so if there is an aerospike effect. The nozzles can gimbal outward to increase the effective size of the combined engine bell.
This scheme reuses the heat shield and gimbal mechanism to get an aerospike. During landing, the center engine is operating at near minimum throttle and so has the worst-case flow separation problem. Reducing the expansion ratio of that center nozzle makes it possible to throttle the center engine down even further during landing, which should improve manoever time and robustness of the hoverslam.
It’s not really that novel I’m afraid. It basically looks to me like the design for a type of artillery shell known as a “Base Bleed.”
The gas generator the shell uses is replaced here by airflow but it’s pretty much the same idea. Gerald Bull, in designing artillery pieces, developed the base bleed concept to a very high level to improve flight times for projects like the Iraqi supergun.
https://en.wikipedia.org/wiki/Base_bleed
I followed your link to see whether I agree or not. I think it is a case of somewhat similar in general. The expander deflector Jim mentioned earlier looks to be far closer in layout and intent. The flexible skirting Iain suggested though is quite different and worth further thought.
Over expansion is strongly linked to chamber pressure. Higher the pressure, higher expansion ratio is optimal for sea level. Higher optimal sea level expansion ratio means less need for compensation at high altitude. And better isp at launch.
Case and point, russian high pressure engines.
Actually, with all thrust upgrades, Merlin 1D FT is probably under expanded even in sea level operation. Or just about optimal.
Also im not sure how well would the plume fill that extra space inside, as flow is supersonic…
The advantage of high pressure engines being well known, what is your point?
Higher the pressure, less difference altitude compensation makes. Basically at what point does extra mass of larger and complex common nozzle negate benefits of extra isp?
Trade studies are always needed when evaluating different approaches. My suggesting an idea doesn’t mean I think it should be implemented without thought. High pressure engines have some difficulties as well.
Interesting concept. Fascinating even. But I would prefer an ejectable nozzle insert. An ablative nozzle insert configured to optimize off-the-pad performance, and then ejected at altitude. The actual engine (regeneratively cooled) nozzle would be optimized for altitude. Of course, would that optimized nozzle still have the issue of being underexpanded for at least some portion of the flight? I don’t know, but, I think, at least the off-the-pad expansion issue would be resolved. AND, you’d be using a conventional engine and nozzle design.