Questions About Rocket Nozzles...

[Spacescifi]

Often in scifi the only part of a rocket engine the audience ever sees is the nozzle.

IRL the engine is attached to the beginning of the nozzle and a tank feeds it.

I am going to make an assumption... correct me if wrong please. Every nozzle has a rocket engine behind it typically. You could actually make rocket engine feed multiple nozzles but I think it is more common if you need good thrust to use mutiple engines. Since one engine has a limit on power, which can be increased with more engines of similar or greater power.

Main Question: What happens if your chamber pressure is too high and your nozzle is too small? I have been told that bigger nozzles are needed for higher chamber pressures, but was not told the reason why.

Lemme guess.

Spoiler

Boom?

 

 

Secondary Question: Higher chamber pressure equals more thrust right? But there is a limit on how high the chamber pressure can be too right?

Third Question: Can you have multiple rocket engines on ONE nozzle? Or is that just silly and no good reason for it because you will probably melt the nozzle trying that?

Edited August 31, 2023 by Spacescifi

[StrandedonEarth]

1. Nozzles need to expand the exhaust out to ambient pressure (or as close as practical in vacuum). Otherwise it’s under expanded and inefficient. 

2. However high the chamber pressure goes, the pump pressure has to be higher to push it in there. AFAIK it’s how high the materials can withstand, but it gets heavy…

3. I don’t see why not. If you have developed a very good but small turbopump, it may be worthwhile to use multiples to feed a large chamber rather than develop a larger unit. The A4/V2 engine used 18 injectors, because that’s what they had working at the time. The Soviets went the other way; large chambers are prone to combustion instability, so they used a larger powerhead feeding multiple smaller chambers. 

[sevenperforce]

3 hours ago, Spacescifi said:

Every nozzle has a rocket engine behind it typically.

If it's a working engine, yes.

3 hours ago, Spacescifi said:

You could actually make rocket engine feed multiple nozzles

This is very common in Russian designs and other engines with Soviet legacy, but less common in Western designs (the Rocketdyne XLR-89-5 is a notable exception). However, it's important not to confuse the entire engine assembly with individual combustion chambers. Here's an example, shown by the venerable RD-180:

This is one liquid propellant rocket engine with a single thrust structure, a single gimbal mount, a single turbopump, and a single propellant flow system. However, there are two combustion chambers and two nozzles.

I suspect that when you are calling something an "engine" you are actually talking about the combustion chamber. You would never want to make a single combustion chamber feed multiple nozzles, because the pressure loss along the path from the chamber to the various nozzles would be too inefficient. The only exception would be for a very low-impulse system, like a set of attitude control thrusters or propulsive vents.

Try to think about individual rocket engine combustion chambers like the individual pistons in an automobile engine and the entire rocket engine assembly like the completed automobile engine+fuel pump+crankshaft system.

3 hours ago, Spacescifi said:

but I think it is more common if you need good thrust to use mutiple engines. Since one engine has a limit on power, which can be increased with more engines of similar or greater power.

There is no real limit on the power from a single engine. When you're talking about rocket engines, you need to be conscious of both power input and power output. The power input is the amount of power that is generated by the turbopump in order to push the propellants into the combustion chamber(s). The turbopump is basically a jet turbine engine all on its own and can be almost arbitrarily large; the fuel turbopump for the Aerojet M-1 engine produced a 75,000 horsepower, and the RD-170's single-shaft turbopump produced a whopping 230,000 horsepower. That's up there with the horsepower of the largest diesel engines ever produced, although it is dwarfed by the effective horsepower of airliner engines (like the GE-90's 21 trillion horsepower).

The drive to multiple combustion chambers was not an issue of power, but an issue of combustion stability. Once you end up with a gigantic combustion chamber (the volume of the F-1 engine combustion chamber was almost a full cubic meter), the flow of massive amounts of propellant starts to produce currents that can impede combustion. That's why the Russians used multiple combustion chambers, and it's one of the reasons that a company like SpaceX would rather have dozens of smaller Raptor engines than build a gigantic Super Raptor.

3 hours ago, Spacescifi said:

Main Question: What happens if your chamber pressure is too high and your nozzle is too small? I have been told that bigger nozzles are needed for higher chamber pressures, but was not told the reason why.

Lemme guess {boom}.

Nope, not boom. Who told you that bigger nozzles are needed for higher chamber pressures? Or did you just assume that? That's not how it works.

When exhaust comes yeeting out of the combustion chamber, it has a lot of heat and a lot of pressure. The nozzle allows the exhaust to expand, trading heat for velocity and providing a surface against which the expanding exhaust can push to produce thrust. Complete expansion is impossible without an infinitely long nozzle, so you have to truncate the nozzle somewhere.

Pressure is the force that a fluid exerts on a containing surface (and on itself). Force is proportional to acceleration, so the higher your chamber pressure, the more rapidly it will accelerate as it travels down the nozzle. At very high combustion pressures, you can get away with a relatively short nozzle, because of how quickly the exhaust expands. At low combustion pressures, you need a longer nozzle to give the exhaust time to fully expand. So it's exactly the opposite of what you were told.

If your nozzle is not large/long enough for your design pressure, it won't hurt anything; you're just wasting pressure that could otherwise be converted into thrust (the exhaust will spill out the edges of the nozzle before it has fully expanded).

3 hours ago, Spacescifi said:

Secondary Question: Higher chamber pressure equals more thrust right? But there is a limit on how high the chamber pressure can be too right?

Higher chamber pressure equals more thrust for an equivalent sized nozzle. If you increase the pressure, the forces acting on the nozzle are greater, and so even though you still have spillover at the edge of the nozzle, you will have gotten more out of the exhaust flow by the time it reaches the edge of the nozzle.

If you want to increase thrust directly, you have to increase the amount of propellant going into your engine, usually by spinning the turbopumps faster. There is a limit to how fast you can spin your turbopumps before you just kinda have to build a bigger turbopump.

There is no real limit on how high the chamber pressure can be, except for the fact that a higher chamber pressure requires a heavier chamber (to contain the pressure) and a beefier turbopump (to push the propellant into the chamber at a higher pressure) which means your engine will be much heavier overall. So there's a tradeoff between chamber pressure and engine weight. There's also a tradeoff between chamber pressure and turbopump power consumption. The turbopump typically burns a portion of your propellant to provide the power to push the rest of the propellant into the chamber, but if you ramp up your chamber pressure too high, then you end up using up too much of your propellant inside the turbopump.

3 hours ago, Spacescifi said:

Third Question: Can you have multiple rocket engines on ONE nozzle? Or is that just silly and no good reason for it because you will probably melt the nozzle trying that?

It's perfectly fine to have multiple combustion chambers feeding a single nozzle -- it's common in aerospike designs. Certainly no problem with melting the nozzle, if the nozzle is regeneratively cooled.

I'm not sure why you're always afraid of things melting. Melting is certainly a common challenge in the world of rocket science, but it's a technical/efficiency challenge, not a basic limitation of physics. Melting is usually NOT the limiting factor on making an engine bigger or more powerful.

Edited August 31, 2023 by sevenperforce
Added commentary.

[Shpaget]

On 8/31/2023 at 6:19 PM, sevenperforce said:

I'm not sure why you're always afraid of things melting.

I wonder that too. I would imagine that during the engine design phase, thermal management would be something to consider.

Also, why would the chamber pressure be to high or nozzle too small? Again, those things should be solved during the design phase, unless you are assembling a rocket engine from assorted parts you find at a spaceship scrap yard; parts that were never meant to go together. Do you worry about what would happen if the piston in your car was too small for the cylinder?

[darthgently]

On 8/31/2023 at 12:19 PM, sevenperforce said:

There's also a tradeoff between chamber pressure and turbopump power consumption. The turbopump typically burns a portion of your propellant to provide the power to push the rest of the propellant into the chamber, but if you ramp up your chamber pressure too high, then you end up using up too much of your propellant inside the turbopump.

Good point