Static rcandy rocket test

[Oinikis]

After many CATO's I finally developed working class E rocket motor. After first successful test after last modification, I've decided to put this one on a dynamometer, capture data, and figure out the characteristics of the motor, so here it is:

https://www.youtube.com/watch?v=_RNRwZbZmj4

According to the burn profile end disc areas of this BATES grain motor struggled to ignite.

The motor was a bit leaky (I saw some black stains around top plug, but they were tiny, so I guess the effect wasn't very big.), so performance could have been better.

Burn time = 1.4 sec

Total Impulse = 23.6

Specific Impulse = 52.4 sec

Effective exhaust velocity = 514.5 m/s

Average thrust = 17 Newtons

Peak thrust = 34 newtons

Initial Kn = 89

peak Kn = 96

final Kn = 67

Grain weight = 45 grams

 

http://i.imgur.com/B2zWx8b.png

Here is scale drawing of the motor. I designed it as research motor, for bigger rockets, using more of these grains. The bulkheads are cast from heat resistant concrete mix binder. It's very strong and hardens overnight in sealed enviroment. They are cast ~1.5mm wider than ID, then tapered using sandpaper, and glued in using epoxy-concrete mix (It makes epoxy much more heat resistant). Then I drill 4 4mm holes into each bulk, and put some epoxy-concrete mix to lock it in. So, what do do you think about my rocket, and what I can do to help end discs ignite. Also, what do you think should I do, launch several of these and move on developing a longer, more powerfull 2 grainer version of this motor, or launch these some more, and try to lift various payloads?

Edited August 7, 2014 by Oinikis

[Bobosan]

Interesting build. Can't offer any suggestions as I have never made rcandy motors. I can offer a welcome back!

[Oinikis]

Well, thank you then.

[dagabu]

End disks ignite? Not sure what that is. I suggest you move on to multiple gain motors.

[Oinikis]

End disc surface of BATES grain. They struggle to ignite, and the graph was more similar to coreburner rocket, rather than BATES grain rocket. According to BATES grain calculator the burn profile should be kind of flat (progressive by like 5%, and then regressive by like 10%). I think about cutting small groves into end disc areas of the grain. I think sometime soon I might do another test of this engine, but I'm going to make sure these end discs surfaces ignite together with core. Somehow.

I'm going to keep similar combustion chamber pressure on multigrainers, so top bulkhead will be kept the same, however nozzle will need to be bit bigger, to maintain similar combustion chamber pressure, and I also want to make it bit longer, so it has "funnel" in the inner side of the nozzle, to help the gasses get compressed easier, and some sort of expanding hole on the outside, acting as a expansion bell. I wonder what proportions should I use?

Edited August 8, 2014 by Oinikis

[TYRONEEZEKIEL]

I'm interested to see how you cast your individual grains. I could see some obstacles arising there

[Oinikis]

I do not have any footage from manufacturing the grains, but here is the grain, tooling and plastic wrapper I wrap my grains in, so they don't get wet.

http://i.imgur.com/xVz5erC.jpg

The thin paper winding where fuel is going to stay is put in outer mold (the short piece of tube you can see), then put on, and centered on the spindle. When I cook rcandy I take piece of rcandy, make ball in my hands, and poke onto the wooden spindle. Then I take the short dowel, and press the fuel down from all sides. I find this works much better than cored dowel, becouse it doesn't stick, lets air come out much easier, and "massages" rcandy in much better, leaving no air gaps, cracks, or joints between increments. I never had issues with my grains, the only issue is when the grain is put against bulkhead it struggles to ignite, and I need some way to keep a fair gap between, so these end areas could ignite as soon as the core ignites, without making the tube longer. I think slight cross shaped groove in the bulkhead using angle grinder might help ignite it, or maybe when casting the grains, making some grooves in soft fuel, like crosses and stuff to leave some gaps. I think if I can have that cross ignite, whole end disc area would ignite too.

Edited August 8, 2014 by Oinikis

[dagabu]

Interesting that you don't use a dowel that goes all the way through. I found that just a 7/64" rod down the center made for good burn through, blocking the fire at one end of the grain with a two grain system seems to block the fire propagation to the second grain.

 

Use and angle grinder to make a groove? Um, NO! Power tools have no place with fuel grains.

[stix]

I like your thrust meter

 

I'm assuming you are using the std. mix KNSU 65/35? If so, it should deliver a specific impulse of around 100 > 120. Are you sure your calcs are correct?

 

If you did actually get around 52 then you didn't have enough chamber pressure and/or your fuel underperformed - water not driven out enough? Did you use the recrystalised or melted method? Also the fact that it 'leaked' may have caused a drop in pressure.

 

Anyway, I'm into model rocketry myself (and a bit of pyro) and have done a lot of experimentation with sugar motors and have come up with a satisfactory fuel mix and consistent method of casting grains. I'll post an image of the graph and specs so you will be able to extract some info and compare with yours.

 

Looking at your specs I think your nozzle throat area should be larger and the fuel grain longer which would make it safer. But I certainly would not go longer in grain with that nozzle - and yes, creating a 'convergent' section would help with flow.

 

I used to cut a cross in the ends of the grains with a hacksaw to ensure ignition. Then I ended up just inhibiting them, and in effect making a 'core' burner and therefore increasing thrust over time.

 

I wouldn't advise stepping up in size until you have sorted out your construction methods ie. why it leaked and if the fuel is up to scratch. I ended up using KNDX fuel wich is dextrose/glucose powder because it melts at a lower temp and burns slower, which is what you want for a core burner.

 

btw. The jagged look in the graph was electrical artifacts in the recording device.

 

If you have any questions I'll be happy to help.

 

[EDIT] oh, I saw one of your other posts and you obviously know how to make r-candy

 

Cheers.

Edited August 29, 2014 by stix

[Oinikis]

For some reason I did not saw these last 2 posts. I did test later and got 60 seconds Isp, but still core burner profile. I solved the problem by having cross of BM pressed against the ends of the grains, and it sure does the trick. Today I did test of modified version(helping ends ignite with BM, and De Laval nozzel (45,15 degrees), and thrust curve really looked like it should look, however, it wasn't a success. For some reason it got very bad leaks, and even on the curve you can see where the leaks starts, and I lost like 1/3rd - 1/4th of total impulse. However the nozzle performed so well, than even with those very bad leaks I got 61 seconds, which is an improvement.

I need to figure out the leak problems. I think it's linked with bad quality tubes, couse I used tube I originally dismissed as unfit one, but ended up using, couse new tubes looked even worse. I have no idea, but lately I lost my ability to roll. I just can't roll, it's like learning to walk again.

As for the chamber pressure, 6 mm washer I use here works no problem, but preassure is estimated to be according to Kn 1.6 MPa. If I try to use 5 mm washer, pressure is estimated to be 2.2 MPa, and it just blows either plug out (usually nozzle). I think after a successful test 6mm De Laval nozzle, which is yet to happen, I might try using 5 mm one, using beefy retainer pins. Rocket you saw had 4, 4mm holes drilled in each plug through the tube, and later filled in with epoxy-concrete powder mix (Is like clay, is rock hard when set). Later found 3 works just fine. 4, 6mm ones will be 3 times stronger, and I think could handle 5 mm.

Mix is 66.6/33.3, or 2/1 and 0.5 water with KNO3 dissolved before adding sugar, to make sure it all recrystalizes. I make grains with care and love, and don't feed them after midnight.

Dag, i'm not that stupid, my original idea was to use a file, not an angle grinder.

Edited September 16, 2014 by Oinikis

[stix]

Oinikis,

 

Have you had a thorough inspection of the casing to determine where the failure (leak) is occurring? Perhaps the flame is propagating up the walls and igniting the outside of the grain. This would cause thermal runaway (more pressure = increased burnrate = more pressure. . . recurring) leading to ultimate casing failure.

 

It's very interesting that there are many ways of making sugar fuel. I pretty much do similar to yourself - roll balls or long snakes and press them down into the mould with a cold rod. Works for me.

 

. . . 20mins later . . . . . .

 

Ok, so I went back and reviewed your original post and video, I also looked at my own posted results (SKINT-16.PNG) and have come to some observations & conclusions (correct, imagined, assumed or otherwise).

 

In no particular order:

• Your fuel looks very good
• Your Motor Casing is not strong enough
• Your fuel length is not long enough
• Your grain dimensions are too short & fat
• The video looks and sounds not powerful enough
• Complicated naming conventions of your motors

Looking at your diagram (B2zWx8b.png), and if this is the only grain you used, then I'm not surprised at your results.

 

If you compare it to my posted results, there is some revealing info to be considered (oh no not the bullet points again!)

• I used less fuel
• My motor is smaller
• I have more max thrust
• Total Impulse (mid-E, yours high-D)
• My specific impulse was around 40% better

Why?

 

I think your casing is not strong enough to get to the pressures required for the sugar fuel to attain higher Isp - You don't have enough fuel burning for that to happen because the grain is too short.

 

I use aluminium tubing as the casing (I'm not suggesting that you do), I also use 2 grains, resulting in a total fuel length of 80mm long x 18mm wide. Long and thin like the space shuttle boosters. The down side to this is a short burn time (1.2s in my case)

 

I did a few tests on my thrust meter first (SKINT-16.PNG) and then let one go in a model rocket body with a parachute recovery system - It took took off so fast that I couldn't track it! - I lost sight of it and don't even know if the parachute system worked (timer based). Gone forever.

 

Perhaps I went too long - a bit shorter and wider might be better? That's the fun of experimenting.

 

[EDIT] Reason for editing: Deleting nonsensical comments.

Edited September 17, 2014 by stix

[Oinikis]

The leak was due to bad quality tube. I think my glue was too diluted and I used too much of it, so it shrank more than usually, leaving air gaps and stuff. outside of the grain does not burn, becouse it is inhibited with thick paper, and my fuel is well massaged in, leaving no air gaps. If too much pressure were to build up, it would simply blow up.

I designed this grain in such a way, that it has a slightly regressive burn, becouse regressive burn makes sense, so you have quick start, and get velocity quick, and later thrust drops for more of a cruise, and to keep similar G loading throughout. If you want to make chamber pressure,you just can make smaller nozzle, instead of making longer grain, which will make the thrust more progressive. However, short grains only makes sense when used in multiple grain configuration.

The reason this motor is short, is becouse this is my first rocket of this kind, and I wanted to figure stuff out, using single grain, so this is sort of research motor, to prepare for making multigrainers, becouse this motor is terribly inefficient in terms of fuel/empty mass ratio. I want to make a modular family of rockets based on this. This is the pinky of the family. Bigger ones will include 2, 3, maybe 4 grain version, and sustainer-cruise sort of endburner motor, with grain shaped like long cylinder, with all side inhibited, and instead of having flat bottom, having a cone shape, to increase surface area, while making it near neutral burn profile.

 

According to burn graph, and time it started leaked, I estimated I lost about 1/4 of total impulse due to flawed tube. I should hit above 70 seconds specific impulse.

I need to figure out how to make stronger tubes, and using more serious retainer pins I might be able to raise chamber pressure. I should try using less diluted glue, and apply less of it, becouse the tube are too wet when rolling in my opinion.

 

Do not expect me to make 5 segment Space Shuttle SRB, I worked summertime job at cemetary just to get money to make rockets out of stuff which was never intended to be used this way. If I had lathe, more tools and stuff, I would be making high performance reusable, full metal motors, and that would be great.

BTW here's the vid

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

Edited September 17, 2014 by Oinikis

[stix]

No probs oinikis, it's 11.30pm here in oz and i'm done - chat soon.

[kpknd]

Oinkis

 

How did you make your thrust stand, are the markings in pounds, kg ? I like the videos.

[Oinikis]

The meter is made from wood. there is a stator, on top of it is a slider, which is sliding on clothes hanging wire, so it doesn't fly off, and some side keepers are attached to the sides of stator, to keep the slider in line. then there is spring. Markings are in kg, or in 10s of newtons, so calculating is easy, and metric system rocks.

[stix]

... Do not expect me to make 5 segment Space Shuttle SRB, I worked summertime job at cemetary just to get money to make rockets out of stuff which was never intended to be used this way. If I had lathe, more tools and stuff, I would be making high performance reusable, full metal motors, and that would be great...

 

BTW here's the vid

 

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

 

 

Yep, understood - It sounds like you've had to work hard to get things together and I admire your resourcefulness and inventiveness in building your 'thrust meter'.

 

I'm looking forward to seeing more vids.

 

Cheers.