Helical port design for a hybrid

[mael]

Hello all, hope your are well,

I'm designing a hybrid rocket (slowly and step by step), and I would like to know this things : How to design the helical port of a Hybrid rocket ?
Once I'll have my regression rate, I would be able to determine the O/F reaction area.
And I guess that both canals have their middle at the center of radius (in order that the flamme touch the wall and the other helical port at the same time).

(Another thing I was thinking about is leaning inlets of GOX in order to improve the "vortex" along the helical port ?)

Those are important elements of helical canals but I need to determine other things, as the number of spire and the length or the diameter .

 

[G_T]

GOX is not ideal due to the high pressure and low density. You don't get much oxidizer for the weight of the hardware. That makes it unsuitable for rocket use.

Why a helical port?

Gerald

 

[mael]

GOX is not ideal due to the high pressure and low density. You don't get much oxidizer for the weight of the hardware. That makes it unsuitable for rocket use.

Why a helical port?

Gerald

Hello thanks for the answer !
this is only for statics fires for now, and helical because it seems that it has some interesting caracteristics, increasing the O/F reaction area for the same volume I think?

 

[G_T]

Reaction_area x regression_rate x fuel_density = fuel_mass_flow_rate

You make your fuel mass flow rate suitable for your oxidizer mass flow rate at your target O:F ratio for the pair.

Your fuel is also your insulation. You want some left at the end of the burn. But the more you have left the more useless weight your motor is carrying. So choose a fuel grain geometry that leaves you with insulation while carrying the least excess weight. There are several options for this.

Fuels burn differently depending on the viscosity and thickness of the melt layer generated, and the oxidizer flow velocity.

Wax and wax-based fuels can have a melt layer that gets frothed up into waves like breakers at a shore, shedding lots of droplets of fuel. Therefore high regression rate, and good turbulent mixing. Disadvantages once scaled up beyond a certain size for pure wax (mechanical) or certain burn duration (too much web required). Wax-based rather than pure wax can address the issues.

Other fuels have a slower regression rate and smoother mixing. So use a longer grain and less web. In other words, geometry needs to be adjusted to be appropriate for fuel burning characteristics.

Smoother mixing is a minus, by the way. Swirl flow, turbulence, these sorts of things are an advantage. They provide faster mixing of the fuel and the oxidizer.

Better atomization of a liquid oxidizer is good. Not using a liquid oxidizer (or solid if you really want to try a reverse hybrid) just doesn't make sense for a flight vehicle. You NEED to have at least some density to your oxidizer.

I'm not sure of any advantages to a helical port. Helical flow, sure. Port in the grain? Not really. Mostly disadvantages.

Gerald

 

[mael]

Thanks a lot for this answer !

I'm not sure of any advantages to a helical port. Helical flow, sure. Port in the grain? Not really. Mostly disadvantages.

Gerald

Helical port does not lead to a helical flow (was palnning to lean the GOX injectors in the direction of the helical port) ? If not I'll try to find a better shape for a "vortex" (if it's a way to call what I'm searching for) in google scholar documentation.

 

[mael]

Reaction_area x regression_rate x fuel_density = fuel_mass_flow_rate

You make your fuel mass flow rate suitable for your oxidizer mass flow rate at your target O:F ratio for the pair.

Gerald

I think it's what I've plane to do

 

[mael]

Your fuel is also your insulation. You want some left at the end of the burn. But the more you have left the more useless weight your motor is carrying. So choose a fuel grain geometry that leaves you with insulation while carrying the least excess weight. There are several options for this.

Gerald

I'll think about not centring in the middle of the radius as I said then.

 

[G_T]

A helical port leads to linear flow through the curved port, like a curved pipe. That's not the same thing at all as helical flow in a more normal port - such as swirl combustion. Don't take "linear" too literally.

I suggest backing up a ways, and reading some books on hybrid rocket design. There are a few out there.

BTW, with GOX you are more likely to make something more like a blowtorch than a rocket motor. Oxidizer mass flux is likely to be too low for it to function well as a rocket motor exhausting to atmospheric pressure.

For some entertainment if you can get on the Research subforum (US citizen, L2 cert) then you can read up on a hybrid motor of mine - THRP-1. I put lots of design details in the thread.

Gerald

 

[mael]

Agains thanks a lot for all those infos !
Even with a small engine + a well designed nozzle I won't be able to access 1atm at the exit ?

 

[rocket_troy]

Even with a small engine + a well designed nozzle I won't be able to access 1atm at the exit ?

Not sure what you mean again there? Anyway, so long as you can achieve 2atm chamber pressure or higher, you can achieve choked flow. About 4 bar and you'd definitely achieve supersonic flow out of a reasonably designed de laval nozzle.

TP

 

[mael]

 

[mael]

 

[mael]

View attachment 518721

This regression rate will be different from the one of the a simple port engine ?
I guess yes from I think having understand of what you said + what I try to learn on the net.

This would come from what you said about improving the regression rate with helical flow for the same m°ox if I got it well.

 

[rocket_troy]

Yes correct. There's been considerable work done on improving regression rates in slower regressing fuels. Helical ports and swirly injection are some of the methods that have shown to make a significant difference.

TP

 

[mael]

Yes correct. There's been considerable work done on improving regression rates in slower regressing fuels. Helical ports and swirly injection are some of the methods that have shown to make a significant difference.

TP

I love swirlers if it's what you are talking about !

 

[FredA]

Aren't swirlers the tassels on pasties?

 

[mael]

Re:
This question will look stupid but here it is :
Does increasing the regression rate change the O/F optimal ?

--> For a gift m°ox : if r° increase --> m°fuel increase too --> O/F changes.
Do we reduce Area of reaction in order to maintain ancient O/F ?

Once again it shows that I have lot of to learn, but I do want to !

 

[mael]

View attachment 518721

Ok so after having think + search on the net I came to this :

measure of the regression rate thanks to comparaison of pre/post-combustion in order to determine precisely the specific hentalpic ratio of the nylon or asa choosed.

edit:
I have to continue searching haha it's not usefull to do so...

 

[mael]

Ok so after having think + search on the net I came to this :

measure of the regression rate thanks to comparaison of pre/post-combustion in order to determine precisely the specific hentalpic ratio of the nylon or asa choosed.

edit:
I have to continue searching haha it's not usefull to do so...

it does not even make sense...

 

[rocket_troy]

The regression rate doesn't affect what the *optimal* O:F is. That is very ingredient dependent. There maybe some instances with fuels like waxes where you might need to lower the O:F (based on mass discharge) because a non trivial fraction of the fuel is expelled out the nozzle before complete combustion. There might also be instances where you choose to run the chamber rich due to other considerations, although that's generally nothing to do with regression rate.

TP

 

[mael]

You was talking about it.

 

[mael]

The regression rate doesn't affect what the *optimal* O:F is. That is very ingredient dependent. There maybe some instances with fuels like waxes where you might need to lower the O:F (based on mass discharge) because a non trivial fraction of the fuel is expelled out the nozzle before complete combustion. There might also be instances where you choose to run the chamber rich due to other considerations, although that's generally nothing to do with regression rate.

TP

Thanks for your help, really !

 

[rocket_troy]

--> For a gift m°ox : if r° increase --> m°fuel increase too --> O/F changes.
Do we reduce Area of reaction in order to maintain ancient O/F ?

What generally happens is this: let's assume our oxidiser flow rate is constant and we're talking classical hybrids. So, our fuel will be regressing at a given rate, so our port will also be expanding at that given rate (x2 for diameter), out port *area* will also be increasing to pi*r^2. Assuming a simple round core port, the fuel surface area will be increasing to pi*d*length, however Go (oxidizer flux rate) will be reducing proportional to the increase in port area and remember that if we use the regression formula r=a*Go^n, then a reduction in Go reduces r. So, in one sense, the area of the fuel surface is increasing with the increasing port, but the regression rate will likely be reducing at the same time. Which of the competing influences outdoes the other is depending on the length of the fuel grain and the flux exponent of the fuel.

TP

 

[RalfB]

Hello,
While I can't contribute to the design of spiral ports, I tried it on a paraffin and epoxy grain and it worked great.

Viele Grüße Ralf

 

[rocket_troy]

While I can't contribute to the design of spiral ports, I tried it on a paraffin and epoxy grain and it worked great.

Nice. Do you have any comparison to non spiral cores with equivalent fuel and setup?

TP

 

[RalfB]

Hello, here are the measurement data.

1. Mittlerer Schub: 282N
2. Spitzenschub: 600N (außer Zündpeak mit 880N)
3. Impuls: 803Ns, was einem J-282 entspricht
4. Brenndauer: 2,85

If you are interested in our hybrid test day and you want more comparison values, check out the trade here:

https://www.raketenmodellbau.org/forum?action=viewthread&threadid=802753&page=1
Greetings Ralph

 

[G_T]

Hi Ralph,

Thank you for the link!

Gerald