Author Topic: Counter Intuitive Thrust augmentation  (Read 6051 times)

Offline redneck

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Counter Intuitive Thrust augmentation
« on: 03/01/2023 11:18 pm »
There have been a few recent discussions of means to increase rocket thrust during the first minute or so of flight. That's when gravity and back pressure losses are eating most of the thrust. There are three basic means of improving thrust. More mass in the exhaust. More velocity in the exhaust. Reduction of losses from gravity and/or back pressure.   The ideal would be a vacuum nozzle that had no losses in atmosphere.

This is concerning the nozzle back pressure losses in the early stages of flight. A first stage nozzle is some compromise between overexpansion at sea level and under expansion in vacuum. A first stage nozzle that expands such that PE/PA is 0.5 loses 1,000 pounds per square foot due to the 14.7 psi pushing down while the exhaust is at 7.35 for a net down force of 7.35 psi which shows up as less thrust for an over expanded nozzle. A 5 foot diameter nozzle at this time gives 20,000 pounds less thrust than an smaller optimized nozzle. Double the diameter  for better altitude performance and things get rapidly worse. But shrinking the nozzle down to sea level optimum gives away a lot of performance at altitude.

I think that is a reasonable summation of what everybody here already knows. I am not current and am going from memory. I did some playing around with various altitude compensating (actually compensating for variations in PE/PA as the nozzle doesn't know how high it is) nozzles several years back. Aerospikes to me are too complicated and development risky. I found at least two concepts that should compensate allowing large nozzles to fly efficiently from sea level. One was what I call a notchbell nozzle with a notch in the skirt of a large standard nozzle interacting with the atmosphere. I was able to get them to work with compressed air, though they are a bit trickier than I first expected. The other method is what I will attempt to describe here. It involves a secondary flow in an unusual direction.

When playing with the various nozzle concepts, I had an over expanded nozzle set up on a scale. When it was blowing down from the air tank, I used a slightly modified shop air blow gun to see what happened to the strings glued to the nozzle for flow visualization. I was able to kick the flow off of one side and get a strong bounce on the scale.  5 pounds  isn't much, but at the pressures and sizes I was using. it seemed like a lot. It entrained some air back up into the nozzle and focused the flow to the side opposite the blow gun. As far as I can tell, this technique could let a vacuum nozzle fly efficiently from sea level.

My original plan was to work with friends in the suborbital companies. Flying often from sea level to 100,000 feet or more would have been ideal, especially for the lower pressure engines involved. The nature of the business being that I would have to fund the first efforts myself, I never had enough money to make it happen. Those companies are all gone now.

Possibly someone will find it worthwhile to do a bit of simulation or possibly do the shop air experiments with proper instrumentation.

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #1 on: 03/01/2023 11:24 pm »
Trying to add a sketch

Offline InterestedEngineer

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Re: Counter Intuitive Thrust augmentation
« Reply #2 on: 03/01/2023 11:47 pm »
Cool idea.

A problem you didn't face at that scale and power is oscillations that would blow a real vacuum nozzle apart, as the air coming back up the edges would oscillate to destruction.

Some sort of injection of some sort of high pressure molecules around the edges of the nozzle, would, I think, cause a higher pressure zone preventing the atmosphere from coming back up the nozzle.  A 'virtual' nozzle, I think.

Another problem is that the bottom area of almost all modern (esp. reusable) boosters is all used up, so pounds/square foot (or Pascals if you prefer) is at a premium, and a vacuum size nozzle would waste that precious area.  A system design constraint.   This directly affects TWR and thus you'd trade better Isp for gravity losses, probably a wash or worse.

As a general note, improving the booster performance by 10% (i.e. 10% more payload at same altitude and velocity for MECO) improves final payload by 10%, by simple division inside the rocket equation for the second stage (initial mass and final mass increase by 10%).

That 10% improvement is relative to the initial mass of second stage, so whatever the second stage masses (say 1450t), 10% is 145t (most of which is added fuel for the second stage).  So an improvement can weight quite a bit, in the case of SpaceX booster 1-2t per 33 engines for 10% at MECO is a fair tradeoff.

I'm not sure whether one could plumb a high pressure gas feed to a booster engine for < 1-2t per engine.  The other complication for packed booster surface area is plumbing gets harder and harder.




Offline Jim

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Re: Counter Intuitive Thrust augmentation
« Reply #3 on: 03/02/2023 12:01 am »
makes no sense.  Instead of carrying extra gas, just increase the propellants going into the engine

Offline sevenperforce

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Re: Counter Intuitive Thrust augmentation
« Reply #4 on: 03/02/2023 03:28 am »
makes no sense.  Instead of carrying extra gas, just increase the propellants going into the engine
Or you could have a dual-cycle engine that is open cycle at sea level (more thrust and higher chamber pressure) and exhausts into the nozzle extension to prevent flow separation, but closed-cycle at altitude (lower chamber pressure, lower thrust, but higher specific impulse) to altitude-compensate in that way.



Described at length here.
« Last Edit: 03/02/2023 03:28 am by sevenperforce »

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #5 on: 03/02/2023 07:58 am »
makes no sense.  Instead of carrying extra gas, just increase the propellants going into the engine

This was a real observed effect that gained thrust per amount of air involved. Net Isp at sea level would improve. Increasing propellants requires increasing pressure and pump size. This is about getting more performance out of a given engine.

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #6 on: 03/02/2023 08:09 am »
Engineer
""Cool idea.

A problem you didn't face at that scale and power is oscillations that would blow a real vacuum nozzle apart, as the air coming back up the edges would oscillate to destruction.""

By controlling the separation and directing the flow, the oscillations are eliminated.  Your other objection about the real estate under the rocket being restricted is true for the Starship class. I think smaller vehicles with less vertical density could benefit. New Shepard and Electron class vehicles in particular.

@Sevenperforce,
This one is about getting more performance at sea level out of an unmodified rocket that is over expanded at that point. Not about adding cycles and turbo machinery.


Online edzieba

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Re: Counter Intuitive Thrust augmentation
« Reply #7 on: 03/02/2023 10:39 am »
Move the injector to the middle (so the exhaust isn't asymmetric and you don't have a small segment of the nozzle trying to handle all thermal duties, and you do not have a 'kick off' line where the flow detaches from the bell and applies large forces), turn it into a deflector (so you don't have to supply additional working fluid) and you have an ED nozzle.

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #8 on: 03/02/2023 11:08 am »
Part of the usefulness of this would be the ability to use existing nozzles.  Also entrainment of air is a critical part of the compensating flow which wouldn’t work down the middle without some plumbing.   This also could be placed such as it could be discarded at 20,000-40,000 feet altitude when it ceases to be useful.    IMO, a standard bell nozzle is best at higher altitudes.   

Offline Jim

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Re: Counter Intuitive Thrust augmentation
« Reply #9 on: 03/02/2023 01:10 pm »
makes no sense.  Instead of carrying extra gas, just increase the propellants going into the engine

This was a real observed effect that gained thrust per amount of air involved. Net Isp at sea level would improve. Increasing propellants requires increasing pressure and pump size. This is about getting more performance out of a given engine.

Your setup had errors and false readings.

Offline Jim

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Re: Counter Intuitive Thrust augmentation
« Reply #10 on: 03/02/2023 01:11 pm »
Just use an extendable nozzle

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #11 on: 03/02/2023 04:13 pm »
I was using threads in the nozzle in the same manner as yarn tufts on aero surfaces to watch flow patterns.  Never Hearn of yarn tufts giving a false reading. 

Offline Jim

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Re: Counter Intuitive Thrust augmentation
« Reply #12 on: 03/02/2023 04:50 pm »
I was using threads in the nozzle in the same manner as yarn tufts on aero surfaces to watch flow patterns.  Never Hearn of yarn tufts giving a false reading. 

yarn tufts on aero surfaces, key word there.  You don't know what is happening to the rest of the flow from the surface to the centerline.

And what was the pressure going into the nozzle vs the blow gun?

« Last Edit: 03/02/2023 04:55 pm by Jim »

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #13 on: 03/02/2023 09:09 pm »
Jim,
It only takes one person duplicating my results to refute your objections. 

That does not mean ready for Implementation.  That means verification for further investigation.

Offline sevenperforce

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Re: Counter Intuitive Thrust augmentation
« Reply #14 on: 03/03/2023 02:13 pm »
Jim,
It only takes one person duplicating my results to refute your objections. 
It would only take one other person who also set up an improperly-controlled experiment to come up with anomalous results and thus confirm that improperly-controlled experiments produce anomalous results.

What is your argument here, exactly? That adding pressure to one side of the nozzle can prevent flow detachment at the other side of the nozzle?

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #15 on: 03/03/2023 02:40 pm »
Not at all.  The result is that the secondary jet kicks the flow off of one wall and lets the atmosphere interact with the follow in the same manner as an aerospike. Or if you prefer, it shortens the effective nozzle length at lower altitudes.   

Offline Jim

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Re: Counter Intuitive Thrust augmentation
« Reply #16 on: 03/03/2023 04:07 pm »
Not at all.  The result is that the secondary jet kicks the flow off of one wall and lets the atmosphere interact with the follow in the same manner as an aerospike. Or if you prefer, it shortens the effective nozzle length at lower altitudes.   

you don't know that.  Tufts are not enough to base that conclusion on.

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #17 on: 03/03/2023 06:27 pm »
Actually I do know that.   It’s clear to one knowledgeable in the field. 

Offline Jim

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Re: Counter Intuitive Thrust augmentation
« Reply #18 on: 03/03/2023 06:40 pm »
Actually I do know that. 

No, you don't.  You have no idea what the rest of the flow in the nozzle is doing. Like you said "do the shop air experiments with proper instrumentation". 

It’s clear to one knowledgeable in the field. 

Apparently that doesn't include you then.  Like you said "I am not current"
« Last Edit: 03/03/2023 06:42 pm by Jim »

Offline sevenperforce

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Re: Counter Intuitive Thrust augmentation
« Reply #19 on: 03/03/2023 06:44 pm »
What is your argument here, exactly? That adding pressure to one side of the nozzle can prevent flow detachment at the other side of the nozzle?
Not at all.  The result is that the secondary jet kicks the flow off of one wall and lets the atmosphere interact with the follow in the same manner as an aerospike.
It sounds like you're saying the same thing I just said.

There are a number of ways to control flow separation in a bell nozzle. You can go the RS-25 route and use a nozzle lip that doesn't diverge sufficiently, leading to higher pressure at the exit; this has the disadvantage of losing the maximum possible expansion at altitude. You can do an expansion-deflection nozzle which pushes the flow sideways at the throat; this has the disadvantage of lost thrust at launch due to gas entrainment in the center of the exhaust flow. You can do a dual-bell nozzle that provides a specific surface for the flow separation to attach to; this will be slightly less efficient in vacuum than a smooth surface and will have significant gas entrainment at launch.

What you appear to be proposing is the use of an additional gas flow in one region of the nozzle, which you hypothesize would interact with the exhaust flow to "push" it to one side, preventing flow separation on that side while accepting it on the near side, purportedly for increased total thrust.

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #20 on: 03/03/2023 07:13 pm »
It doesn’t add thrust.   It decreases the losses from pressure at exit being lower that the atmospheric pressures at low altitudes. 

Offline Bruce Dunn

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Re: Counter Intuitive Thrust augmentation
« Reply #21 on: 03/04/2023 12:36 am »
This concept appears at least in principle to be related to the technique of Liquid Injection Thrust Vector Control.  A liquid is injected into the exhaust stream partway down the nozzle.  It vaporizes, creating a stream of gas which pushes the main exhaust stream to one side.  For economy, the liquid can be an oxidizer, which reacts with the fuel-rich exhaust gases to generate hot gas.  A ring of injectors can be used to vector the exhaust of what is typically a solid rocket.  The Titan launcher solid rocket boosters used injection of N2O4 to vector thrust from a fixed nozzle.

When thrust is vectored, there is a small additional increment in thrust related to the additional mass flow, and potentially due to changes in the expansion of the main rocket exhaust which is "squeezed" into a smaller area (similar to the proposal being discussed here).  Potentially, a ring of liquid injection ports all working at the same time would symmetrically create extra gas which would squeeze the main exhaust away from the nozzle walls and increase its exit pressure, reducing the overexpansion losses at sea level.  As the rocket rises, the flow of liquid injection would be tapered off to match the reduction in atmospheric pressure.  Whether it is worth the trouble (extra mass, extra equipment) is not clear without detailed calculations.
« Last Edit: 03/04/2023 04:15 am by Bruce Dunn »

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #22 on: 03/04/2023 08:02 am »
Whether it is worth the trouble (extra mass, extra equipment) is not clear without detailed calculations.
« Last Edit: Today at 04:15 am by Bruce Dunn »

That is the key question on any different system added to a standard engine. New systems must necessarily be guilty until proven innocent. I stumbled across this while working on something else right about the time the recession decided I didn't need to have extracurricular activities. Going from the aerospike side, I never looked at LITVC closely. Also, when I ran some numbers, thrust augmentation seemed to gain more from reducing losses than actual thrust.

Offline john smith 19

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Re: Counter Intuitive Thrust augmentation
« Reply #23 on: 03/04/2023 09:16 am »
That is the key question on any different system added to a standard engine. New systems must necessarily be guilty until proven innocent.
A lesson so many advocates of various "cunning plans" fail to realise. novel <> better  :)

I stumbled across this while working on something else right about the time the recession decided I didn't need to have extracurricular activities. Going from the aerospike side, I never looked at LITVC closely. Also, when I ran some numbers, thrust augmentation seemed to gain more from reducing losses than actual thrust.
Yes. the nozzle is literally acting as a suction cup, pulling the vehicle back onto the pad  :)

Decades ago the semiconductor industry developed this effect as a way to pick up wafers without sucking up dust particles that could embed on them at the same time.
MCT ITS BFR SS. The worlds first Methane fueled FFSC engined CFRP SS structure A380 sized aerospaceplane tail sitter capable of Earth & Mars atmospheric flight.First flight to Mars by end of 2022 TBC. T&C apply. Trust nothing. Run your own #s "Extraordinary claims require extraordinary proof" R. Simberg."Competitve" means cheaper ¬cheap SCramjet proposed 1956. First +ve thrust 2004. US R&D spend to date > $10Bn. #deployed designs. Zero.

Offline john smith 19

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Re: Counter Intuitive Thrust augmentation
« Reply #24 on: 03/04/2023 09:39 am »
It doesn’t add thrust.   It decreases the losses from pressure at exit being lower that the atmospheric pressures at low altitudes.
Indeed. Personally I always liked the idea of "louvres" or "flaps" at the right diameter of the nozzle that would open (on the S71 I think they were referred to as "suck in doors" but at a much larger scale) when the pressure was below atmospheric. This way the engine is sucking in air from in front of (or above if vertical) so sucking the engine to the pad become pulling it off the pad.

BTW there are some NASA studies on (roughly) wrapping a rocket engine in a duct (or a duct around the stage skirt) that reckoned you could increase T/O thrust somewhere between 15-50% over a part of the flight. The joker is how heavy does that ducting have to be?

I never really understood why NASA didn't pursue it. Their massive phobia about averting risk? Lack of a current mega-project to use it on? Who knows.

Breaking a flight into segments shows that takeoff is the time you want maximum thrust (you could say maximum momentum transfer between the vehicle and the pad)

Under the "simple, but crazy" notions in this area include (for example) a long umbilical to feed a water spray in the low pressure area of the nozzle in the first say 10m of takeoff. Just pumping extra reaction mass out the back. Then the connector separates and the reel retracts. I'm pretty sure someone (several someones probably) has already had this notion.

Like a lot of these BOTE notions however while the idea is simple the implementation is much more involved. :( As always does the performance boost  you get in this segment justify the hit you take on complexity and mass?
MCT ITS BFR SS. The worlds first Methane fueled FFSC engined CFRP SS structure A380 sized aerospaceplane tail sitter capable of Earth & Mars atmospheric flight.First flight to Mars by end of 2022 TBC. T&C apply. Trust nothing. Run your own #s "Extraordinary claims require extraordinary proof" R. Simberg."Competitve" means cheaper ¬cheap SCramjet proposed 1956. First +ve thrust 2004. US R&D spend to date > $10Bn. #deployed designs. Zero.

Offline Jim

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Re: Counter Intuitive Thrust augmentation
« Reply #25 on: 03/04/2023 11:39 am »

I never really understood why NASA didn't pursue it. Their massive phobia about averting risk? Lack of a current mega-project to use it on? Who knows.


They don't have a massive phobia about averting risk.  Just no reason to use it.

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #26 on: 03/04/2023 09:55 pm »
from John Smith 19,
Under the "simple, but crazy" notions in this area include (for example) a long umbilical to feed a water spray in the low pressure area of the nozzle in the first say 10m of takeoff. Just pumping extra reaction mass out the back. Then the connector separates and the reel retracts. I'm pretty sure someone (several someones probably) has already had this notion.

Like a lot of these BOTE notions however while the idea is simple the implementation is much more involved. :( As always does the performance boost  you get in this segment justify the hit you take on complexity and mass?


Most here probably wouldn't believe all the crazy stuff some of us used to kick around. I have pages of concepts that should work technically.** And notebooks full of things that won't. Not to mention all the ones that went in the circular file early. Tether, nuclear, KERO/LOX hybrid to 650 seconds without radioactive exhaust. Oxygen pellets launched from the moon to provide impact deceleration to incoming craft. Asteroid beanstalk using the rotational energy to throw ore to a processor in Earth orbit while adjusting the orbit of the asteroid in our favor. And many more. Fun while it lasted.

**Should work technically is totally different than guaranteed to work in hardware. And even further from being a good idea from the business side.

Offline john smith 19

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Re: Counter Intuitive Thrust augmentation
« Reply #27 on: 03/04/2023 11:24 pm »

**Should work technically is totally different than guaranteed to work in hardware. And even further from being a good idea from the business side.
Exactly
MCT ITS BFR SS. The worlds first Methane fueled FFSC engined CFRP SS structure A380 sized aerospaceplane tail sitter capable of Earth & Mars atmospheric flight.First flight to Mars by end of 2022 TBC. T&C apply. Trust nothing. Run your own #s "Extraordinary claims require extraordinary proof" R. Simberg."Competitve" means cheaper ¬cheap SCramjet proposed 1956. First +ve thrust 2004. US R&D spend to date > $10Bn. #deployed designs. Zero.

Offline sevenperforce

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Re: Counter Intuitive Thrust augmentation
« Reply #28 on: 03/06/2023 03:40 pm »
This concept appears at least in principle to be related to the technique of Liquid Injection Thrust Vector Control.  A liquid is injected into the exhaust stream partway down the nozzle.  It vaporizes, creating a stream of gas which pushes the main exhaust stream to one side.  For economy, the liquid can be an oxidizer, which reacts with the fuel-rich exhaust gases to generate hot gas.  A ring of injectors can be used to vector the exhaust of what is typically a solid rocket.  The Titan launcher solid rocket boosters used injection of N2O4 to vector thrust from a fixed nozzle.

When thrust is vectored, there is a small additional increment in thrust related to the additional mass flow, and potentially due to changes in the expansion of the main rocket exhaust which is "squeezed" into a smaller area (similar to the proposal being discussed here).  Potentially, a ring of liquid injection ports all working at the same time would symmetrically create extra gas which would squeeze the main exhaust away from the nozzle walls and increase its exit pressure, reducing the overexpansion losses at sea level.  As the rocket rises, the flow of liquid injection would be tapered off to match the reduction in atmospheric pressure.  Whether it is worth the trouble (extra mass, extra equipment) is not clear without detailed calculations.
One question I find interesting here is whether there is a difference in thrust if the ring of "liquid injection ports" is activated all the way around or only in certain portions of the nozzle. Flow in a nozzle is supersonic; nothing you do downstream can impact anything upstream. And thus the expanding exhaust cannot provide thrust to the nozzle unless it is actually in contact with the nozzle.

So if you activate the entire ring of liquid injection ports, you're pushing the exhaust into the center, where any remaining pressure it has will be lost. On the other hand, if you were only to activate two of the "liquid injection ports" opposite each other, then presumably it would "pinch" the exhaust flow and push it into the opposite axis, causing it to continue to push against the nozzle all the way to the nozzle exit at the two points 90 degrees from the injection port locations.

Does that make sense?

Online edzieba

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Re: Counter Intuitive Thrust augmentation
« Reply #29 on: 03/06/2023 08:49 pm »
Flow in a nozzle is supersonic; nothing you do downstream can impact anything upstream. And thus the expanding exhaust cannot provide thrust to the nozzle unless it is actually in contact with the nozzle.
Those are convenient simplifications, not truisms. Otherwise, gas generator exhaust injection into the nozzle bell would cause a drop in both thrust and propulsive efficiency (instead it increases both), and exhaust gas recirculation around a vehicle's aft section would be impossible (when in fact it is both possible, observable on images and video of vehicles in flight above the atmosphere, and a factor that needs to be taken into account for thermal modelling).

Offline sevenperforce

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Re: Counter Intuitive Thrust augmentation
« Reply #30 on: 03/07/2023 03:18 pm »
Flow in a nozzle is supersonic; nothing you do downstream can impact anything upstream. And thus the expanding exhaust cannot provide thrust to the nozzle unless it is actually in contact with the nozzle.
Those are convenient simplifications, not truisms. Otherwise, gas generator exhaust injection into the nozzle bell would cause a drop in both thrust and propulsive efficiency (instead it increases both), and exhaust gas recirculation around a vehicle's aft section would be impossible (when in fact it is both possible, observable on images and video of vehicles in flight above the atmosphere, and a factor that needs to be taken into account for thermal modelling).
I'll push back on this slightly. Adding gas generator exhaust into the nozzle is useful because it increases the mass flow coming out of the nozzle; the expanding combustion chamber exhaust pushes the gas generator exhaust outward laterally and into contact with the nozzle, where it is redirected downward, producing thrust. That doesn't require any upstream propagation in a supersonic flow. Similarly with exhaust gas recirculation: once you're high enough that the engine is under-expanded, the exhaust gas leaves the nozzle in a chaotic flow at the lip rather than a clean supersonic flow, and that chaotic flow can readily be entrained back up around the vehicle by the low-pressure regions causing parasitic drag.

My point was that if the point of the liquid injection system was merely to disturb the exhaust flow along the nozzle surface, then you might prevent flow separation but that's it. If the liquid injection is functioning to add mass flow, like the gas generator exhaust injector, then you have a different scenario. However, the OP's description suggested that the effect was to "push" the supersonic flow over to one side, preventing flow separation on that side; if that's true, then you are directly increasing thrust.

Image to demonstrate. On the left is an over-expanded nozzle with a high flow separation point, causing parasitic drag (shown in red). In the center is this imagined circumferential liquid injector, which prevents flow separation and fills the void to prevent parasitic drag, but results in a loss of thrust because some of those central pressure lines can't fully expand against the nozzle. On the right is what the OP seems to be describing: fluid injection from one side that "pushes" the rest of the flow over against the nozzle, allowing the exhaust to flow properly all the way down that side of the nozzle and provide thrust all the way through.

Offline redneck

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Re: Counter Intuitive Thrust augmentation
« Reply #31 on: 03/07/2023 10:36 pm »
Not fluid injection in what I started with. An energetic counter flow to detach the flow off of one side such that the atmosphere can fill the void. Controlled flow separation can  be a good thing. Uncontrolled and chaotic can destroy a nozzle. Fully attached flow that is over expanded costs performance.

Offline sevenperforce

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Re: Counter Intuitive Thrust augmentation
« Reply #32 on: 03/08/2023 02:19 pm »
Not fluid injection in what I started with. An energetic counter flow to detach the flow off of one side such that the atmosphere can fill the void.
So, precisely what I showed in the image, but with gas being blown up from the nozzle lip on one side rather than fluid being injected down?

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Re: Counter Intuitive Thrust augmentation
« Reply #33 on: 03/08/2023 04:57 pm »
Not fluid injection in what I started with. An energetic counter flow to detach the flow off of one side such that the atmosphere can fill the void.
So, precisely what I showed in the image, but with gas being blown up from the nozzle lip on one side rather than fluid being injected down?

Almost except entrained air providing most of the effect    Plus the center of nozzle flow is at less pressure than the attached at the wall.  A bit of vacuum effect there. 

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