When You Need Less But Big Rocket Nozzles

[Spacescifi]

 

Assuming you had a legitimate SSTO using some advanced nuclear engine or super hot propellant you would also want less but huge main engine nozzles.

Another reason is because you are not using a chemical reaction so chemical combustion does not matter. The last reason is thermal.

Air conducts heat, so having one nozzle lose heat to the air on ascent is better than radiating heat off to each nozzle side by side.

 

Point? If we ever do make SSTO's, main engine nozzles will be optimized to just be one big one.

 

If I am wrong or in error feel free to correct me.

 

Otherwise you may add what you know.

 

Than you.

Edited April 19, 2021 by Spacescifi

[Shpaget]

In a good deal of engine designs, before the fuel is burned in the combustion chamber it is passed through the double walls of nozzles to cool down the nozzles so they don't fall apart.

Rockets don't really spend much time in atmosphere so ambient air is not particularly efficient and reliable at cooling the nozzles. In vacuum there is only radiative cooling, which is not as good as convective.

Big nozzles are more difficult to design and build.

They are floppy.

[RealKerbal3x]

4 minutes ago, Spacescifi said:

Assuming you had a legitimate SSTO using some advanced nuclear engine or super hot propellant you would also want less but huge main engine nozzles.

[...]

Air conducts heat, so having one nozzle lose heat to the air on ascent is better than radiating heat off to each nozzle side by side.

Most of the time where engines are clustered and/or arranged in a way that would prevent radiative cooling from being effective, regenerative cooling or another method is used to regulate nozzle temperature.

See the SpaceX Starship engine bay, where multiple engines are located inside a skirt that prevents excess heat from simply radiating away. Here, some of the propellant has to be circulated throughout the engine nozzles for regenerative cooling. Other methods of engine nozzle cooling, such as ablative cooling, would also work, but I suppose we're assuming that this SSTO is reusable, and an ablative coating inside the engine that must be replaced every time isn't great for reusability.

(Also, most of the time rocket engines don't use the air as a heatsink, as it's easier to just release it as infrared radiation, and rockets spend most of their time in vacuum or at least extremely thin air anyway).

[wumpus]

I was under the impression that using fuel to cool the nozzles was relatively common.  The shuttle certainly used it, and any expander cycle (RL10 and later developments) engines have to use it.  So they have been in use for quite some time.  RL10 dates from 1962.

Spacex obviously prefers it as they simply refuse to use ablative systems as it prevents reuse.  Presumably anyone else interested in reuse will try to to avoid ablative systems as well.  No idea how common ablative systems are,  but I suspect that all early rockets used a combination of air-cooling and ablative  systems.

I strongly doubt that radiating away heat will work for anything with significant thrust.  My guess is that anything that looks this way is being held to a stable temperature by the exhaust gasses, and can survive that temperature (typically in model rocketry and other amature levels).  Leave radiant cooling for ions and other electric thrusters.

[kerbiloid]

The true way.
Use hydrogen.

Like this, but hydromethalox
https://en.wikipedia.org/wiki/RD-701

 

[StrandedonEarth]

36 minutes ago, wumpus said:

Spacex obviously prefers it as they simply refuse to use ablative systems as it prevents reuse.  Presumably anyone else interested in reuse will try to to avoid ablative systems as well.  No idea how common ablative systems are,

Have you read Liftoff! by Eric Berger yet? I just finished it; it was a good read. Interesting tidbit from the book: In the early days of Merlin development, they had to make that choice between ablative and regenerative cooling. Despite already wanting to re-use rockets eventually, Mueller and Musk decided regenerative would have too many headaches early on when they needed to get the engine working. So they decided to go ablative at first, which proved to have its own set of headaches, adding weight and robbing performance. So Merlin and Merlin 1A were ablative, and Merlin 1B was abandoned when work on the regeneratively-cooled Merlin 1C proved to be surprisingly straightforward and problem-free. They would have saved a lot of problems and headaches if they had started going regenerative from the beginning.

[KSK]

Some thoughts on this.

Rocket engines of any kind are going to be hot and for efficient operation you want to run them as hot as possible. When it comes to rocket exhaust, the hotter it flows, the faster it goes (and the higher it blows when anything goes wrong, but that's another matter). 

With that said, rocket engine nozzles losing heat isn't a significant problem.  Simplistically, the engine exhaust doesn't spend enough time next to the cold nozzle (or curtain of cold gas next to the nozzle) to lose a significant amount of heat. 

To put some numbers on this, according to Project Rho, a solid core nuclear thermal rocket will run at anywhere from 2300K to 3100K.  Even at 3100K, that's cooler than the Space Shuttle Main Engines which, according to NASA, run at up to  3315 degrees Celsius (3588K).

There are materials which melt above those kinds of temperatures but not many. Even then, it's not much good having a super high melting point alloy if it's  too expensive/too hard to machine/too toxic/too brittle/too reactive with the exhaust gases etc. etc. End result, rocket engine materials are probably not going to be optimized for pure melting point.

So in practice, the problem is to stop your rocket nozzle from melting. Various ways of doing that as folks have already discussed on this thread. If anything, heat loss from multiple nozzles will help very very slightly but not enough to be an issue either way.

 

[Spacescifi]

1 hour ago, KSK said:

Some thoughts on this.

Rocket engines of any kind are going to be hot and for efficient operation you want to run them as hot as possible. When it comes to rocket exhaust, the hotter it flows, the faster it goes (and the higher it blows when anything goes wrong, but that's another matter). 

With that said, rocket engine nozzles losing heat isn't a significant problem.  Simplistically, the engine exhaust doesn't spend enough time next to the cold nozzle (or curtain of cold gas next to the nozzle) to lose a significant amount of heat. 

To put some numbers on this, according to Project Rho, a solid core nuclear thermal rocket will run at anywhere from 2300K to 3100K.  Even at 3100K, that's cooler than the Space Shuttle Main Engines which, according to NASA, run at up to  3315 degrees Celsius (3588K).

There are materials which melt above those kinds of temperatures but not many. Even then, it's not much good having a super high melting point alloy if it's  too expensive/too hard to machine/too toxic/too brittle/too reactive with the exhaust gases etc. etc. End result, rocket engine materials are probably not going to be optimized for pure melting point.

So in practice, the problem is to stop your rocket nozzle from melting. Various ways of doing that as folks have already discussed on this thread. If anything, heat lftoss from multiple nozzles will help very very slightly but not enough to be an issue either way.

 

 

Well...with advanced nuclear designs like LANTR and NSWR (I know NSWR is not safe) having multiple nozzles means multiple reactors does it not?

Is not one big reactor and one big nozzle better than a bunch pf smaller ones or not?

Extra complexity tends to invite Murphy's law right?

Imagine if they were FUSION reactors LOL?

 

Again...if I am in error...let me know.

 

Thanks.

[KSK]

Well who the heck knows with fusion reactors since we’re nowhere near building one that we can fit on a spaceship.

A LANTR though (as I recall) is essentially a solid core nuclear thermal motor which can burn the hydrogen exhaust from the reactor with oxygen, thus trading off ISP for added thrust when required.

You’re absolutely right that simpler tends to be better because it makes for fewer things to go wrong. With NTRs though, I honestly don’t know if one big motor is simpler than multiple smaller motors.

Full disclosure - the closest I’ve gotten to nuclear rocket science is reading up on it as research for a sci-fi story, so this next part is going to be horribly simplified at best and quite probably wrong at worst.  But with that said, let’s give this a go.

A solid core NTR has a number of design challenges. Firstly, you’re trying to get as much propellant as possible through the reactor as quickly as possible. The reason for that is obvious - higher mass flow equals more thrust. That propellant also needs to be as hot as possible, because hot propellant = fast propellant = more efficient rocket.

That, right there, is not an easy problem to optimise. Bear in mind that an NTR is basically heating propellant by passing it through channels in a hot block. Make those channels too narrow and you get good heat transfer at the expense of flow rate. Make the channels too wide and you get good mass flow at the expense of heat transfer.

Adding to that complexity, that propellant that you’re forcing through the reactor is also going to be moderating the fission reactions in that reactor. That can be useful - towards the end of Project Rover they’d got to the point where they could control the power output of the reactor purely by controlling the flow of propellant through it. But at the same time it’s also another complicating factor.

Finally, there are material considerations. An NTR needs a seriously powerful reactor because it needs to impart a lot of heat to a lot of stuff in a short period of time (to use the technical explanation  ). So, unlike a chemical rockets, NTRs absolutely are optimised around high melting materials. Those tend to be ceramics or cermets, neither of which handle thermal shock very well.  Try leaving a brick on a fire for a couple of hours and then dumping cold water on it if you want an example!

This is (yet another) problem when designing your NTR reactor which almost inevitably is going to have searingly hot reactor elements right next to cryogenic propellant.

Taking all of the above into account,  you’ll see why I honestly have no idea whether one big reactor is going to be simpler than multiple small reactors.

[Spacescifi]

3 hours ago, KSK said:

Well who the heck knows with fusion reactors since we’re nowhere near building one that we can fit on a spaceship.

A LANTR though (as I recall) is essentially a solid core nuclear thermal motor which can burn the hydrogen exhaust from the reactor with oxygen, thus trading off ISP for added thrust when required.

You’re absolutely right that simpler tends to be better because it makes for fewer things to go wrong. With NTRs though, I honestly don’t know if one big motor is simpler than multiple smaller motors.

Full disclosure - the closest I’ve gotten to nuclear rocket science is reading up on it as research for a sci-fi story, so this next part is going to be horribly simplified at best and quite probably wrong at worst.  But with that said, let’s give this a go.

A solid core NTR has a number of design challenges. Firstly, you’re trying to get as much propellant as possible through the reactor as quickly as possible. The reason for that is obvious - higher mass flow equals more thrust. That propellant also needs to be as hot as possible, because hot propellant = fast propellant = more efficient rocket.

That, right there, is not an easy problem to optimise. Bear in mind that an NTR is basically heating propellant by passing it through channels in a hot block. Make those channels too narrow and you get good heat transfer at the expense of flow rate. Make the channels too wide and you get good mass flow at the expense of heat transfer.

Adding to that complexity, that propellant that you’re forcing through the reactor is also going to be moderating the fission reactions in that reactor. That can be useful - towards the end of Project Rover they’d got to the point where they could control the power output of the reactor purely by controlling the flow of propellant through it. But at the same time it’s also another complicating factor.

Finally, there are material considerations. An NTR needs a seriously powerful reactor because it needs to impart a lot of heat to a lot of stuff in a short period of time (to use the technical explanation  ). So, unlike a chemical rockets, NTRs absolutely are optimised around high melting materials. Those tend to be ceramics or cermets, neither of which handle thermal shock very well.  Try leaving a brick on a fire for a couple of hours and then dumping cold water on it if you want an example!

This is (yet another) problem when designing your NTR reactor which almost inevitably is going to have searingly hot reactor elements right next to cryogenic propellant.

Taking all of the above into account,  you’ll see why I honestly have no idea whether one big reactor is going to be simpler than multiple small reactors.

 

Good answer!

 

Put of me is laughing as I am picturing you saying all if that in a British accent.

Maybe you know this, maybe you don't, but accross the pond in the USA, there is an assumption, likely influenced from movies and TV, that smart or classy people have British accents.

Because the ones on TV tend to. So when you admitted you don't know but will give it go it made me want chuckle.

Also...saying give it a go is a phrase among others I know people in the UK use more often than folks where I am from.

Virtually most all of my UK media knowledge of how they speak comes from watching Dr Who during the Ecclecton-Tennant era.

 

EDIT: Hope I did not needlessly offend anyone. I actually like British culture, what bit of it I know, from the accents to the fish and chips.

I just found it amusing since on TV among American characters, I can't remember the last time the British guy did not know something 

Probably that all just too much Dr Who though, since he and now she is the hero andTime Lord extraordinaire.

 

Regarding the NTR, perhaps you and I accidentally solved the problem?

 

Want greater heat into propellant? Narrow flow.

Want more flow? Build a BIG REACTOR.

I think the really hardest issue is thermal shock on the reactorbecause of cold propellant.

 

So that is hopefully an engineering issue where mankind can engineer new materials that can take the thermal shock.

One day.

 

Edited April 20, 2021 by Spacescifi

[KSK]

Thanks!

Your imagination is accurate -  and no offense taken. I am British, which probably comes across in my choice of phrases.