New and having problems

[Crimson0087]

I have been working on a PVC rocket motor. Here are my specs. I use 3/4 inch PVC pipe. For nozzle I use a PVC pipe connector with a small PVC retainer on the end. I use Durham's rock hard water puddy and a washer for the nozzle. On the other end I use Durham's rock hard water puddy with a piece of PVC cut and cemented end as a retainer. The fuel is melted sugar and kno3 35/65. I started with a 7/32 nozzle then a 5/16 nozzle and a 1/4 nozzle. The tutorial I used was using 7/32 but all of mine explode....I was about to try a 10mm and larger nozzle but I feel like these nozzles are huge and won't produce thrust.....I felt the sugar and have 3 segments wrapped in cardboard and cored with 7/32 and placed into the motor. The entire motor is 9 inch long with roughly 7 inch fuel. Do larger higher fuel motors require larger nozzles? The tutorial I was using told me 7/32 but that exploded. The only deviation I made from that tutorial is that I used more fuel. The tutorial uses 65 grams I'm using 100g

[gdeputy]

Yes, you need a larger nozzle if you increase propellant surface area. See more info in your other thread.

[stix]

Crimson. When you say "explode" do you mean the casing fails/splits, or the header or nozzle pops out?

 

When does the motor explode - straight away or part way through the burn?

 

Also, are you using straight KNSU or doped with red iron oxide?

[Crimson0087]

firebreather, it explodes nearly instantly. Or the end cap gets fired out. I am using hydraulic cement reinforced by an internal pvc ring. it explodes within the first second. I am using straight knsu 65/35. It is inhibited in the outside only with double wrapping of aluminum tape used in duct work. I am using 3 2 inch long pieces of fuel with a 9/64 inch hole drilled (roughly through the center). According to the KN ratio the surface area of the burn is 3309.5 mm squared and the nozzle was 12mm for a kn of 275. According to the chart on nakka rocketry that should yield a pressure under 1000 psi. The most recent trial as described blew the plug out of the top. Do i need a larger nozzle or perhaps just a better plug in the top. or is my grain not really being inhibited on the outer surface and yielding a surface area of burn over 12k?

[NeighborJ]

It sounds to me like the core in the grain is too small. The core should be at least the same size as the nozzle, otherwise the grain itself restricts the flow of gasses.

[stix]

. . . According to the KN ratio the surface area of the burn is 3309.5 mm squared and the nozzle was 12mm for a kn of 275.

 

Crimson, your initial surface area of 3309mm2 is similar to what I get. However, your Kn ratio is incorrect. But I think I see your math issue.

You cannot simply divide 3309/12. This does give you approx. 275, so I guess that's where you are going wrong.

 

You have to divide Area by Area. You are dividing Area by Length, which won't work. Therefore (sticking with millimeters), the area of your 12mm nozzle is 113mm2

 

The calculation now becomes 3309/113 = a Kn ratio of approx. 29 which means a pressure of less than 75psi.

 

If you're getting the explosion straight away, then it's probably not an inhibitor failure because the flame wouldn't have had enough to propagate up the sides. I also doubt that it's the Nozzle. I reckon even if you had no nozzle and just straight open grain, you would get the same issue.

 

How are you igniting these? Are you using visco stuffed right down into the end? Maybe you're getting initial ignition which then creeps under the grain and the pressure has nowhere to go because the fuse is in the way.

 

I agree with NJ that the core (3.6mm) is way too small. There's not much point having a small core like that if the nozzle is larger - you're just wasting fuel. Ideally, the core diam. should be the same as Nozzle or larger.

 

Are you able examine the remnants after the explosion - this might give you some clues.

 

btw. my username is stix

Cheers.

[Crimson0087]

I am igniting with a fireworks fuse stuffed down the hole

[gdeputy]

I am igniting with a fireworks fuse stuffed down the hole

 

if its stuffed, you may be cato'ing right off the bat because the fuse is blocking the nozzle. Do you have smaller fuse?

 

As others have noted above, if your core diameter is less than your nozzle diameter, you're essentially starting out 'nozzleless'. In other words, the propellant core is acting as the nozzle, and you should use the grain core diameter as the nozzle diameter to calculate Kn. As a rule of thumb, your grain core needs to be larger than your nozzle throat, otherwise things wont go as expected.

[Crimson0087]

Ok so tomorrow's trial is set up like this....4 grains. 1.25 inch long by .75 diameter. With 1/4 inch core. I am using a 7/32 nozzle. My math gives me initial burn surface area of 7.06 and nozzle area of .0376 for a know of 187. Hopefully it goes well.

[Crimson0087]

Ok so tomorrow's trial is set up like this....4 grains. 1.25 inch long by .75 diameter. With 1/4 inch core. I am using a 7/32 nozzle. My math gives me initial burn surface area of 7.06 and nozzle area of .0376 for a know of 187. Hopefully it goes well.

[stix]

I get the same Kn ratio of approx. 187. Therefore around 560psi.

 

Your new test sounds much more promising. Just to be on the safe side, maybe you could ignite it around half way in, instead of all the way to the end?.

 

Anyway, looking forward to hearing about your results. Be safe, and make sure you protect yourself and others.

 

Cheers.

[Crimson0087]

So I just BARELY put the fuse into the tip of of the fuel and the fuse is substantially smaller than the hole so I know thats not occluding it. I believe that it once again blew the nozzle out of the back end. I am using a nozzle made of durhams rock hard water putty that is reinforced by an internal pvc ring. The internal pvc ring was maintained but the water putty is not blown out. Perhaps I should use hydraulic cement for the nozzle. However, shouldnt the water putty hold? Is my pressure too high or is it a nozzle failure. It looks like it started to have some thrust but then when the nozzle blew out I believe thats what shot it into the dirt. Heres the footage. Any advice welcome

 

 

https://www.youtube.com/watch?v=5Lw0DpVzwbU

[Crimson0087]

I know that the math (KN ratio) doesnt support it but maybe I need a large half inch nozzle?

[gdeputy]

I know that the math (KN ratio) doesnt support it but maybe I need a large half inch nozzle?

Not with the 1/4" core size, no. Nozzle throat > core diameter = bad

[gdeputy]

So I just BARELY put the fuse into the tip of of the fuel and the fuse is substantially smaller than the hole so I know thats not occluding it. I believe that it once again blew the nozzle out of the back end. I am using a nozzle made of durhams rock hard water putty that is reinforced by an internal pvc ring. The internal pvc ring was maintained but the water putty is not blown out. Perhaps I should use hydraulic cement for the nozzle. However, shouldnt the water putty hold? Is my pressure too high or is it a nozzle failure. It looks like it started to have some thrust but then when the nozzle blew out I believe thats what shot it into the dirt. Heres the footage. Any advice welcome

 

 

https://www.youtube.com/watch?v=5Lw0DpVzwbU

Did that blow the thing in half, or just blast it back into the ground?

 

If you're still using just tape to inhibit the outside of the grains, thats most likely not working. Are there any propellant grains left that you can check if there's burning happening on the outside surface?

 

Need to get to something that doesn't blow apart as a starting point. Putty in PVC is pretty crappy for a nozzle solution, but you're limited with PVC. Graphite would be a better bet, and you can usually get that cheap via mail. You can just drill a hole in it for a nozzle and dont bother with entrance/exit cone on the nozzle for initial tests. Then pin it to the PVC or use some other kind of positive retention. Adhesive, etc is not going to work well.

[Crimson0087]

It just blasted into the ground. Which is pretty cool but still not what I'm after lol

[Crimson0087]

People get these things to work by just hammering in kitty litter.... Why won't water putty work lol

People get these things to work by just hammering in kitty litter.... Why won't water putty work lol

[Crimson0087]

Any recommendations for I hibiting outside?

[NeighborJ]

Crimson, the water putty sets overnight but it takes a week or so to fully cure. It will also release moisture back into your fuel grain if used too soon.

 

Most people line their grain forming mold with a turn of paper and hold it in place with vaseline as an inhibitor. I reapplied the vaseline to both the inside of the motor casing and the outside of the inhibitor paper before loading it into the motor. Any glued surface needs to be free of oils or vaseline before gluing to ensure a good bond and the pvc glue also takes time to set properly. The PVC cement sets in seconds but also stays soft enough to force off for up to 24hrs, the solvent needs a chance to completely evaporate.

 

I am sticking with my origional assumption that the core is just too small to make a functioning motor, open it up so the thing has a chance to breath.

Edited October 4, 2018 by NeighborJ

[stix]

This is what I would do:

 

Forget about Kn values, forget about Bates grains and just go back to the simple basics. I know a lot of people suggest to start BIG then back off. My view is contrary, start SMALL then work your way up.

 

I think you are taking on too much without understanding some basics. This is not an attack on you, but it's important to understand that in the pursuit of scientific discovery, if it doesn't work out over many tests (which it hasn't) then it's time to go back to the drawing board (a useful metaphor worth remembering).

 

All because you see someone make a successful rocket motor on Youtube, doesn't mean it translates directly to you or the methods you use. Making rocket motors isn't the same a baking a cake. It's not that easy.

 

Anyway, 3/4" casing ID is a good size for testing and should yield a decent "E" class motor using only 32grams of fuel.

 

Start off with only one grain. Make it around 60-70mm long with a core of 1/4". Make the nozzle the same diameter.

 

Instead of wrapping your motor with an inhibitor, pour the fuel into a sleeve. 2 to 3 winds of glued thin (manila) cardboard is good enough. You can use the 3/4 ID pvc pipe as the retainer. The fuel will absorb into the card and therefore stick to it and become the inhibitor. You'd be looking at a burn time of between 1.5 to 2 seconds, so it should hold out fine.

 

Easier said than done I guess. You'll need a smaller rod/mandrel to make the inhibitor sleeve. It will take some trial and error and a calculator. In the end you should end up with a grain of fuel that can be inserted into your pvc casing very snuggly. Less chance for blow through.

 

You can probably buy some tools on this forum to make it easier, or you make them yourself. It's up to you.

 

Start small, if there is a failure, it's much easier to work out where the problem is, otherwise you end up with too many variables and no idea which way to turn or what to try next.

Edited October 4, 2018 by stix

[Crimson0087]

Sounds like a good idea. Thanks

[Crimson0087]

I finally got a successful test today....well semi-successful. It did not CATO (which I think means explode but im not sure what the acronym means). I had 4 bates grains in there. Some of the changes that I made were as follows.

 

1. Instead of 3 long bates grains I made 4 shorter bates grains because I used a formula I found to help size it appropriately for a neutral burn. Apparently 2.25 inch was a progressive burn.

2. I still used craft paper as the inhibitor but I wound this much thicker this time. I wound it about 5 times stuck it in my pvc pipe mold and packed the fuel into it hammering melted fuel in with a mallet to compress it.

3. I drilled the core out at 1/4 an inch but I did use a 7/16 inch nozzle. My next go around I am going to attempt a .25 inch nozzle. With my setup this should be a kn of around 200.00

 

I think my problem has been that the inhibitor was not working and that I had a nozzle set up for an inhibited fuel while having all sides burning.

 

It did not cato but as you will see in the video it had a good amount of thrust and burn but then the last part of the burn was sluggish. I am not sure why this is? I was thinking maybe when I hammered the fuel into the casing that I perhaps closed or compressed the core on the last one or maybe even 2 fuel grains. It was a pretty tight squeeze. I think next time I will place a metal rod through the core as slide it into the tube to ensure it stays open.

 

https://www.youtube.com/watch?v=dBeGqbkCRec

 

Any thoughts?

[Crimson0087]

Unless my fuel wasnt completely dry. When I mix it I dissolve the kno3 and sugar in water and boil off the water and raise it to melting point of the sugar before packing it. If it continues to be sluggish like that I think I will slowly boil off the water untill it is completely dry and only when its dry will I heat it up to melting point of sugar.

[stix]

I think that sluggish part was probably caused by moist fuel.

[Crimson0087]

Motor Weight 170 grams / 6.0 oz.

Motor Length (Typical) 190 mm / 7.5 in.

Motor OD 27 mm / 1.06 (1-1/16) in.

Casing ID 20.6 mm / 0.813 (13/16) in.

Propellant Type KN/Sorbitol (65/35)

Propellant Weight 62.5 grams / 2.2 oz.

Grain Type BATES (outside surface inhibited)

Total Grain Length 142 mm / 5.6 in.

Number of Grain Segments 4

Segment Length 35 mm / 1.375 (1-3/8) in.

Grain OD (w/o inhibitor) 19 mm / 0.75 in.

Core Diameter 6 mm / 0.25 in.

Maximum Kn 255

Nozzle Throat Diameter 5.6 mm / 0.219 (7/32) in.

Nozzle Throat Insert #10 SAE Flat Steel Washer

Nozzle Exit Diameter 14 mm / 0.563 (9/16) in.

Nozzle Expansion Ratio 6.6:1

Casing Stock Length 215 mm / 8.5 in.

 

 

How does he come up with that kn? Using formula on his own site I come up with a max kn of 219