Hybrid engine injector plate

[jjrgray]

Quick question, is it necessary in a hybrid engine to have a 'shower-head' injector plate in order to diffuse your 1 big oxidiser stream from your injector into multiple smaller oxidiser streams? Would this help with more uniform combustion of your solid fuel grain? Would this help with the vaporisation of the liquid nitrous oxide (that I will be using) coming from the oxidiser vessel/tank through the injector and into the combustion chamber? Design points to give you context is that I am not using a U-C injector, the oxidiser vessel/tank will be top filled and the fuel grain that will be used is (aluminised) paraffin. Any help would be much appreciated.

 

[rocket_troy]

People have claimed improved performance from ideally oriented and geometric injectors with N2O hybrids. Most of the time, a fair portion of the N2O will vaporise into 2 phase flow as it travels through and leaves the injector. What's more, don't forget N2O is a mono-propellant - its decomposition is *exothermic* - in fact, nearly half the energy from a N2O hybrid comes just from the decomposition of the N2O molecule, so there's a fair amount of (in chamber) turbulence with ample radical KE for combustion.
These attributes are one of N2O's key virtues allowing more scope for simpler injector design than for a pure oxidizer such as LOX.
So the answer again is - it's up to you; if you want to chase that extra 10 -15% (for general HPR type chamber configurations). Well, maybe those percentages are even a bit generous.

TP

 

[heada]

I can only compare to Skyripper since that is what I had and flew. Monotube 54mm in diameter with K class motors and a single orifice injector. So it isn't required up to that size.

I believe other motors had multiple injector designs and I think it was Rattworks that had an x-form on one of theirs.

 

[G_T]

Skip the aluminumized parrafin. One or the other would have to go, due to the nature of how it would burn. You'll probably burn the aluminum after it passes the nozzle. It may look good on paper. In real life, in these size motors, probably not so much so. You don't have the residence time to burn it. You don't have the temperature to boil it as well as you'd like, and you do have to boil it. It is the difference in temperature between the chamber temp and the boiling point which determines how fast you can heat the aluminum droplets past the boiling point. The wax acts as a coating that has to boil off first - while that aluminum is headed for the nozzle. You'll likely reduce the ISP and the density ISP, though there is a certain offsetting benefit not mentioned here. And if it doesn't get well above the boiling point of the aluminum without burning the aluminum, then you definitely won't be burning the aluminum! Aluminum not burned in the chamber takes away from chamber temperature as dead mass absorbing heat with no returned energy benefit once it gets hot. Essentially, you make a supersonic molten aluminum sprayer.

Mods, if this is out of bounds, please remove.

Gerald

 

[dhbarr]

I can only compare to Skyripper since that is what I had and flew. Monotube 54mm in diameter with K class motors and a single orifice injector. So it isn't required up to that size.

I believe other motors had multiple injector designs and I think it was Rattworks that had an x-form on one of theirs.

Propulsion Polymers Pentamax

 

[G_T]

Rattworks had it as an option IIRC, for improved performance.

On my own I used three impinging injectors, in a 75mm motor. Another who surprisingly isn't showing up in these threads yet has had great success with showerhead. Most anything will work, but the more you break up the stream into smaller streams or droplets likely the better it will work.

Gerald

 

[jjrgray]

Skip the aluminumized parrafin. One or the other would have to go, due to the nature of how it would burn. You'll probably burn the aluminum after it passes the nozzle. It may look good on paper. In real life, in these size motors, probably not so much so. You don't have the residence time to burn it. You don't have the temperature to boil it as well as you'd like, and you do have to boil it. It is the difference in temperature between the chamber temp and the boiling point which determines how fast you can heat the aluminum droplets past the boiling point. The wax acts as a coating that has to boil off first - while that aluminum is headed for the nozzle. You'll likely reduce the ISP and the density ISP, though there is a certain offsetting benefit not mentioned here. And if it doesn't get well above the boiling point of the aluminum without burning the aluminum, then you definitely won't be burning the aluminum! Aluminum not burned in the chamber takes away from chamber temperature as dead mass absorbing heat with no returned energy benefit once it gets hot. Essentially, you make a supersonic molten aluminum sprayer.

Mods, if this is out of bounds, please remove.

Gerald

Okay, so general consensus is that unless I am sure that the combustion chamber temp is high enough to ignite the aluminium powder inside so that it contributes to the combustion, I should forgo the aluminium powder altogether and just stick with regular paraffin?

 

[jjrgray]

Rattworks had it as an option IIRC, for improved performance.

On my own I used three impinging injectors, in a 75mm motor. Another who surprisingly isn't showing up in these threads yet has had great success with showerhead. Most anything will work, but the more you break up the stream into smaller streams or droplets likely the better it will work.

Gerald

Sounds like the showerhead injector may be a good thing to test in static fires then.

 

[jjrgray]

People have claimed improved performance from ideally oriented and geometric injectors with N2O hybrids. Most of the time, a fair portion of the N2O will vaporise into 2 phase flow as it travels through and leaves the injector. What's more, don't forget N2O is a mono-propellant - its decomposition is *exothermic* - in fact, nearly half the energy from a N2O hybrid comes just from the decomposition of the N2O molecule, so there's a fair amount of (in chamber) turbulence with ample radical KE for combustion.
These attributes are one of N2O's key virtues allowing more scope for simpler injector design than for a pure oxidizer such as LOX.
So the answer again is - it's up to you; if you want to chase that extra 10 -15% (for general HPR type chamber configurations). Well, maybe those percentages are even a bit generous.

TP

Thanks for the info, a lot of help .

 

[G_T]

Okay, so general consensus is that unless I am sure that the combustion chamber temp is high enough to ignite the aluminium powder inside so that it contributes to the combustion, I should forgo the aluminium powder altogether and just stick with regular paraffin?

Or use something that is easier to burn than aluminum.

Gerald

 

[jjrgray]

Or use something that is easier to burn than aluminum.

Gerald

I am cautious of straying into the research forum. So what you're saying is that I should use another powdered substance which I mix with the paraffin and this other powder should do the same/similar job as the aluminium powder in speeding up the regression rate, but be easier to ignite?

 

[Johnly]

What's more, don't forget N2O is a mono-propellant - its decomposition is *exothermic* - in fact, nearly half the energy from a N2O hybrid comes just from the decomposition of the N2O molecule, so there's a fair amount of (in chamber) turbulence with ample radical KE for combustion.

TP

The decomposition of nitrous oxide contributes well more than half of the ISP according to ProPep.

John

 

[Johnly]

There is a critical L/D ratio for Nitrous oxide injectors. Violate the "rule" and you can get an energy bump from combustion oscillation back into the tank followed by instant disassembly.

 

[G_T]

Ok, let's back up a bit. You might want to research Paraffin combustion in a hybrid. You need to know what is going on. IMHO you are trying to go too far just yet without knowing enough about what is going on. It would be good for you to pick up a book on the subject and get familiar with it. I don't know if it is still available, but something like this https://aeroconsystems.com/cart/literature-software/hybrid-design-factors-by-bill-colburn/ is a good starting point.

The statement that prompted my response is "to speed up the regression rate".

Why do you want to speed up the regression rate with Paraffin? To put it more generally, what are the advantages and issues with using Paraffin as the fuel in a hybrid rocket motor? You need to be able to answer that question offhand.

Secondly, you need to be able to design your hybrid given the expected regression rate of your chosen fuel. The physical design of the motor depends on the planned regression rate, the planned O:F ratio, Expected oxidizer mass, etc. If it isn't obvious to you how these things interrelate, and dictate major factors in your combustion chamber design, and how things like injector area vs nozzle throat area interrelate, then you aren't ready to design a hybrid. Way too high a percentage of what you might just sketch up that looks ok really isn't. It is touchier to design in some ways than solid propellant motors, and you don't have that experience either.

PERHAPS you could design a working hybrid using one of the hybrid design tools, without knowing these things. But you could design a much better one if you knew more about what was going on.

You could copy an existing design - and that includes the fuel type - and make a working hybrid. But then you wouldn't really know why it worked. You'd have a motor, but would have learned much less than you could have learned.

I highly enourage you! But the journey isn't instant. If you want it pretty quick and want it to work, purchase one or copy one. If you want to design your own, take some time out to learn more before making any design decisions. You aren't ready. Then don't set a deadline just yet. When the design is ready, it is ready.

Go Fever blows up hardware. Unless you have the hardware budget of a certain rocket company you probably can't afford to learn your lessons burning through hardware tests! The 500 I think you said you've sunk into it so far is a small part of the total by the time you are done.

Not a great picture, but 75mm static test. THRP-1 (Tiny Hybrid Rocket Project 1).

Gerald

 

[rocket_troy]

The decomposition of nitrous oxide contributes well more than half of the ISP according to ProPep.

Note I said *energy* not specific impulse.

TP

 

[jderimig]

What is your goal for your first motor? if you want to get a reliable motor and a rocket in the air, a plastic pipe for fuel and a tube compression fitting will most likely get you 85% of the performance of an advanced fuel and complicated injector. For a large motor that 15% might be significant. For a small motor you can get the equivalent impulse gain by adding a few centimeters to you tank length without the extra complexity. How big is your motor?

 

[jjrgray]

What is your goal for your first motor? if you want to get a reliable motor and a rocket in the air, a plastic pipe for fuel and a tube compression fitting will most likely get you 85% of the performance of an advanced fuel and complicated injector. For a large motor that 15% might be significant. For a small motor you can get the equivalent impulse gain by adding a few centimeters to you tank length without the extra complexity. How big is your motor?

I have already built a hybrid motor for static firing using GOX and paraffin, but because I want this motor to eventually fly in a rocket I decided to switch to N2O. The engine I built before was a round 54mm in diameter, so I assume this new engine will be designed relatively to the same size.

 

[jjrgray]

Ok, let's back up a bit. You might want to research Paraffin combustion in a hybrid. You need to know what is going on. IMHO you are trying to go too far just yet without knowing enough about what is going on. It would be good for you to pick up a book on the subject and get familiar with it. I don't know if it is still available, but something like this https://aeroconsystems.com/cart/literature-software/hybrid-design-factors-by-bill-colburn/ is a good starting point.

The statement that prompted my response is "to speed up the regression rate".

Why do you want to speed up the regression rate with Paraffin? To put it more generally, what are the advantages and issues with using Paraffin as the fuel in a hybrid rocket motor? You need to be able to answer that question offhand.

Secondly, you need to be able to design your hybrid given the expected regression rate of your chosen fuel. The physical design of the motor depends on the planned regression rate, the planned O:F ratio, Expected oxidizer mass, etc. If it isn't obvious to you how these things interrelate, and dictate major factors in your combustion chamber design, and how things like injector area vs nozzle throat area interrelate, then you aren't ready to design a hybrid. Way too high a percentage of what you might just sketch up that looks ok really isn't. It is touchier to design in some ways than solid propellant motors, and you don't have that experience either.

PERHAPS you could design a working hybrid using one of the hybrid design tools, without knowing these things. But you could design a much better one if you knew more about what was going on.

You could copy an existing design - and that includes the fuel type - and make a working hybrid. But then you wouldn't really know why it worked. You'd have a motor, but would have learned much less than you could have learned.

I highly enourage you! But the journey isn't instant. If you want it pretty quick and want it to work, purchase one or copy one. If you want to design your own, take some time out to learn more before making any design decisions. You aren't ready. Then don't set a deadline just yet. When the design is ready, it is ready.

Go Fever blows up hardware. Unless you have the hardware budget of a certain rocket company you probably can't afford to learn your lessons burning through hardware tests! The 500 I think you said you've sunk into it so far is a small part of the total by the time you are done.

Not a great picture, but 75mm static test. THRP-1 (Tiny Hybrid Rocket Project 1).

Gerald

Okay baring this in mind then I will take significant design influence from Aerotech's RMS hybrid engine and will read more deeply into the subject to better understand how everything works. For the time being I'll change my designs to be more in line with their 54mm motor line. I've attached the schematics below for what I'll be mainly using. To be honest Aerotech explains everything pretty well to my surprise, but can someone explain why there are two different fuel grains (forward and aft fuel grains)? Initially I thought the forward fuel grain was part of the solid propellant preheater that breaks the nitrous into its constituent nitrogen and oxygen, however I looked further and there is a separate labeled preheater grain. So are the aft and forward fuel grains just different sized fuel grains made of the same fuels - whats the deal with it?