Rocket tooling dimension questions

[Maserface]

Hey folks

wanted to ask a few questions regarding different the geometries involved in rocket tooling

1. Nozzle size, using rocket tool sketcher as a base point, the ID:Nozzle ration is 2:1, because I experiment with lots of different fuels, I would like to increase that to 2.5:1 or 3:1. The way I see it, I could increase the spindle length, or tighten up the nozzle, the latter makes more sense to me, because I could easily drill the nozzle out if I get catos, and longer spindles are more fragile. What are the thoughts of the experts on this?

2. Nozzle geometry, while it may be a large factor with supersonic exhaust gases, I see no evidence to suggest that the fuels we work with are greatly effected by parabolic nozzle shapes. Has anyone gathered real data on this? I remember SLD saying somewhere that extravagant nozzle shapes simply take away from potential tube space that would be better spent loading with fuel.

I would love to hear what you guys think about this, and any other tooling developments that I may be overlooking.

Edited October 27, 2014 by Maserface

[Oinikis]

As for the nozzle size and chamber pressure. I'd rather choose longer spindle with wider nozzle, rather than tighter nozzle, because longer spindle and wider nozzle will result in less progressive burn (the difference between initial and final thrust/chamber pressure will be smaller). As for the chamber pressure. You can determine chamber pressure by so called Kn (It is a ratio between burning fuel area and cross section of the nozzle). Depending on fuel and casing, after exceeding certain Kn, you will over pressurize your motor, and it will blow up. You can poke around and find the max Kn, after which motor will either explode, or require unpractical and heavy casing. For basic rammed motors the critical point should be expected above 100, maybe 200, depending on the fuel and casing. At 80-100 Kn, it is easy to get a motor working without issues together with having tolerable efficiency.

As for the shape of the nozzle. Convergent part of the nozzle helps to compress the gas, increasing exhaust velocity, and is very easy to make, just have dowel with a 45 degree dent (That is most widely used convergent angle). The divergent part could be made by having widening spindle at the base, but it's not really worth it. The idea is, that after the gas exit the neck, they expand, but still has some pressure in it, and essentially the exhaust cone presses against those expanding gases, giving them extra velocity and producing more thrust. But in case of rammed rocket, pressure we are working with is way less than 20 bar, and those rockets you see flying into space with fancy expansion bells? way over 200 bar. The point where it is worth considering having expansion bell is way above 30 bar, and rockets with these pressure almost always have metal casings. However, I make 16 bar rcandy motors, and at first theyr nozzles were "shapeless". after making nozzle with exactly same cross section area, but with convergent-divergent shape, I got an increased Isp by almost 10 seconds, but divergent part was really small, and I think this was all thanks to convergent part.

[schroedinger]

All reports i read about the delaval shaped nozzles on our bp rockets allways had the same result, they don't really help us, specially the divergent part can harm.

 

But there is an other reason to use the covergent shape, this is that it helps directing the flame towards the exhaust, specially with endburners you will often find that the tube willl burn through just above the plug if a flat rammer is used. A connical rammer will help with that.

[dagabu]

I can agree with both Oinkis and schroedinger, the nozzle shape doesn't seem to matter much is lower impulse rockets like tame BP. I will have to go dig out all my curves to show you the same fuel, same spindle and different nozzles and how the impulse changed.

 

Yes, there were some very good, the one below did a very nice job increasing the thrust but in the end, the additional clay and tube took most of the gain away.

 

http://pyrobin.com/files/new%20nozzle%20mix_1.jpg

 

The De Laval shape works well with high speeds but can actually reduce thrust wit5h slower burning fuels.

[pyrojarjar]

Thanks for Posting this Mason- I've always wondered these same things. I've used 60° instead of the normal 30° on the divergent side of the nozzel and although I don't have a forcegauge...the eye test suggests that it has helped a little.

Cory