Why Not Use A Cluster Of Smaller Magnetic Nozzles Instead Of One Big One?

[Spacescifi]

Why not?

 

This is in regard to stuff like mini-mag orion pulsed propulsion.

 

What? Would magnets screw each other up or something being in such close proximity?

 

Normal rocket nozzles are clustered together as opposed to using one big one, is there any good reason why you would not or cannot do the same with magnetic nozzles using a mini-mag propulsion system?

 

[kerbiloid]

It will be heavier than a single one, have more possible nodes of failure, irradiate and heat each other from side, and still cause same problem with cooling.

[sevenperforce]

7 hours ago, Spacescifi said:

Normal rocket nozzles are clustered together as opposed to using one big one

Nope, they aren’t.

Multiple rocket nozzles are never clustered in place of a larger nozzle.

Multiple rocket CHAMBERS, on the other hand, are often clustered, either in connection to a single turbopump or as a cluster of completely separate engines. There are a variety of reasons for this, mostly related to combustion stability but also for economies of scale and additional thrust. But of course the multiple rocket chambers will need multiple rocket nuzzles.

There would never be any need for multiple rocket nozzles attached to a single combustion chamber. 

[wumpus]

1 hour ago, sevenperforce said:

Nope, they aren’t.

Multiple rocket nozzles are never clustered in place of a larger nozzle.

Multiple rocket CHAMBERS, on the other hand, are often clustered, either in connection to a single turbopump or as a cluster of completely separate engines. There are a variety of reasons for this, mostly related to combustion stability but also for economies of scale and additional thrust. But of course the multiple rocket chambers will need multiple rocket nuzzles.

There would never be any need for multiple rocket nozzles attached to a single combustion chamber. 

You might want extra nozzles on an expansion-cycle hydrolox engine, but even then I'd expect additional combustion chambers would be better.  Expansion-cycle hydrolox engines are more or less limited to combustion chamber+nozzle sizes similar to RL-10 or higher, but you could get greater performance from more combustion chambers and nozzles.  For some reason I remember hearing the nozzle was the limiting factor, but I suspect that the chamber might be as well.  No idea if they attempt to cool the chamber.

No idea if a mini-mag has scaling issues.  And as mentioned, the reasons for multiple combustion chambers are combustion-specific issues.  And if you went multiple nozzles for a hydrolox expander-cycle it would be for thrust *inefficiency* (you want to scavenge the heat from the thrust to power your turbopumps).

[magnemoe]

19 hours ago, sevenperforce said:

Nope, they aren’t.

Multiple rocket nozzles are never clustered in place of a larger nozzle.

Multiple rocket CHAMBERS, on the other hand, are often clustered, either in connection to a single turbopump or as a cluster of completely separate engines. There are a variety of reasons for this, mostly related to combustion stability but also for economies of scale and additional thrust. But of course the multiple rocket chambers will need multiple rocket nuzzles.

There would never be any need for multiple rocket nozzles attached to a single combustion chamber. 

Lots of rockets has multiple rocket engines. Saturn 5, SLS, the shuttle, Falcon 9, Electron, Proton comes to mind as the most famous. 

Now for something like mini-mag  its an pretty huge engine and unlike chemical engines I guess building it larger makes it easier and more efficient. Making the Saturn 5 with just one larger first stage engine would be much harder and you could not shut off an engine at the later part of burn to reduce g forces. 

[sevenperforce]

19 hours ago, magnemoe said:

Lots of rockets has multiple rocket engines. Saturn 5, SLS, the shuttle, Falcon 9, Electron, Proton comes to mind as the most famous. 

Zero of those have clustered nozzles coming from a single combustion chamber.

Multiple engines or multiple chambers make perfect sense. Multiple nozzles just for the sake of clustering nozzles makes no sense. 

[Gargamel]

There might be some discrepancies in how people are interpreting the question.    @Spacescifi, are you asking about one engine with multiple nozzles, or multiple engines each with one nozzle? 

[magnemoe]

On 12/6/2022 at 5:49 AM, sevenperforce said:

Zero of those have clustered nozzles coming from a single combustion chamber.

Multiple engines or multiple chambers make perfect sense. Multiple nozzles just for the sake of clustering nozzles makes no sense. 

True, it makes little sense so I assume he implied engines not nozzles for this reason. 
Now fusion engines are radically different from chemical engines and they probably scale much better up than down. 

[Spacescifi]

On 12/6/2022 at 5:23 AM, Gargamel said:

There might be some discrepancies in how people are interpreting the question.    @Spacescifi, are you asking about one engine with multiple nozzles, or multiple engines each with one nozzle? 

 

Every nozzle I assume needs an engine behind it... that was ny presumption.

 

It is unnecessary to have s cluster of nozzles fed by a single engine since it would serve no purpose.

 

Every nozzle needs an engine.

 

In fact every rocket I have seen I reckon has an engine behind the nozzles.

[sevenperforce]

43 minutes ago, Spacescifi said:

Every nozzle I assume needs an engine behind it... that was ny presumption.

Well I often see you asking whether rocket nozzles need to be bigger or smaller or whatever because of "heat" or some other reason, so how would I know?

48 minutes ago, Spacescifi said:

It is unnecessary to have s cluster of nozzles fed by a single engine since it would serve no purpose.

Well, that's not necessarily true. You can have an toroidal aerospike which uses a single annular combustion chamber but discrete nozzle throats. Such a configuration could be used for gimbal control or additional altitude compensation or just overall structural stability.

In a similar vein, there were a number of early ballistic missile designs which used one sustainer nozzle inside of a larger annular boost nozzle, both of which (I believe) were fed at least in part from the same chamber:

This is from the Lance missile. I'm not 100% sure that it is a single combustion chamber, but even if it isn't, you could still imagine an engine with such a configuration, especially if it was solid-based. The popular RPG-7 antitank missile has a single solid-fueled combustion chamber that feeds multiple nozzles:

And the same is true of the solid-fueled escape motors for the Apollo and Orion capsules, which have multiple nozzles fed from one central combustion chamber (ignore the bit up top, that's a separate solid-fueled steering motor):

In other theatres you have the R-5 air-to-air missile that used multiple nozzles fed from a single combustion chamber:

And in a rocket-combined-cycle airbreathing engine, you can absolutely route the exhaust gases of a single chamber through multiple nozzles or multiple geometries. That's sort of what an afterburner is, in a way; you're using the nozzle from the primary combustion chamber as a secondary combustion chamber.

But let's not forget the most famous single-chamber multiple-nozzle engine of all, the 1970s Mars Viking lander engine:

Spoiler

Amazing to think that all those nozzles are fed by a single chamber.

This was a monopropellant engine and so they forced the monoprop through a catalyst bed in a circle so that centrifugal force pushed the active combustion region to the outside of the chamber and then through the nozzles.

So when I said that "there would never be any need for multiple rocket nozzles attached to a single combustion chamber" I wasn't entirely right. There would probably never be any need for such a design as the main boost engine of an orbital launch vehicle. But there are clearly plenty of situations where there is some specialized purpose that requires multiple nozzles with a single chamber.

[Spacescifi]

3 hours ago, sevenperforce said:

Well I often see you asking whether rocket nozzles need to be bigger or smaller or whatever because of "heat" or some other reason, so how would I know?

Well, that's not necessarily true. You can have an toroidal aerospike which uses a single annular combustion chamber but discrete nozzle throats. Such a configuration could be used for gimbal control or additional altitude compensation or just overall structural stability.

In a similar vein, there were a number of early ballistic missile designs which used one sustainer nozzle inside of a larger annular boost nozzle, both of which (I believe) were fed at least in part from the same chamber:

This is from the Lance missile. I'm not 100% sure that it is a single combustion chamber, but even if it isn't, you could still imagine an engine with such a configuration, especially if it was solid-based. The popular RPG-7 antitank missile has a single solid-fueled combustion chamber that feeds multiple nozzles:

And the same is true of the solid-fueled escape motors for the Apollo and Orion capsules, which have multiple nozzles fed from one central combustion chamber (ignore the bit up top, that's a separate solid-fueled steering motor):

In other theatres you have the R-5 air-to-air missile that used multiple nozzles fed from a single combustion chamber:

And in a rocket-combined-cycle airbreathing engine, you can absolutely route the exhaust gases of a single chamber through multiple nozzles or multiple geometries. That's sort of what an afterburner is, in a way; you're using the nozzle from the primary combustion chamber as a secondary combustion chamber.

But let's not forget the most famous single-chamber multiple-nozzle engine of all, the 1970s Mars Viking lander engine:

  Hide contents

Amazing to think that all those nozzles are fed by a single chamber.

This was a monopropellant engine and so they forced the monoprop through a catalyst bed in a circle so that centrifugal force pushed the active combustion region to the outside of the chamber and then through the nozzles.

So when I said that "there would never be any need for multiple rocket nozzles attached to a single combustion chamber" I wasn't entirely right. There would probably never be any need for such a design as the main boost engine of an orbital launch vehicle. But there are clearly plenty of situations where there is some specialized purpose that requires multiple nozzles with a single chamber.

 

I had thought for a brief moment before posting my last comment that multiple nozzles routed to one engine could help with throttling... but then I thought not really since that is based more upon the engine itself.

Yet it is true that for probably less expense and complication for vectoring thrusters you may want to just route off the main engine.

 

Reduces waste heat as well.... yeah... I went THERE.

[kerbiloid]

3 hours ago, sevenperforce said:

you could still imagine an engine with such a configuration, especially if it was solid-based

Post-boost vehicles of ballistic rockets love the single-chamber-different-nozzles design.

(Not all of them, but turbo-pumped and solid fuel ones).

***

Also, the anti-tank missiles. Though, their nozzles arre fixed.

Spoiler