How I got my rcandy grains consistent and fuel performance characteristics

[Oinikis]

So some of you might remember me having some problem with my grains being inconsistent. The test result were very inconsistent and were basically of no use. There are two main problems I fixed to get consistent results:

 

1. Fuel. I now use 65/35 KNO3/Sugar mix, ballmill it, and melt it dry. This makes very fast, powerful and consistent fuel.

 

2. Inhibitor. I used 2 rolls of thick kraft paper. This was the problem. Now, I take a strip of that paper, put aluminium tape on top, and roll it with wood glue. I also aluminium tape the joint on the inside, giving it a smooth surface all around. This way rcandy sticks to the aluminium, sealing it's surfaces.

 

I have been doing tests using this reusable rocket motor right there. The plates with the nozzle are interchangeable, giving me the ability to change it's diameter, thus the Kn.

 

http://i.imgur.com/9ukgChY.jpg

The grain improvements really made the grains consistent and the burn profiles seem to match predicted Kn progression. Here are the graphs of carious configurations (from left to right:6mm nozzle; Kn of 135 and 7mm; Kn of 99):

http://i.imgur.com/tI48Uve.jpgSo, I did numerous tests with varying Kn's, calculated average-peak pressure (with "flat" nozzle thrust=pressure*nozzle area), calculated burn rates, specific impulses and plotted them on graphs:

http://i.imgur.com/biC2Cll.jpgLeft graphs is chamber pressure (BAR)/Kn, right top is burn rate mm/s / pressure, and right bottom is Isp/chamber pressure. Notcie how high chamber pressure and burn rate are, that's because of ballmilling.

 

Another thing to note, is that beyond 44 Bar, Isp does not increase. My theory is that at lower pressures gasses still combust when exiting the rocket, thus loosing energy, while once you reach 44 bar, gasses finish combustion inside and give away all it's energy. This shows us that using this kind of fuel, going past 50 bar is impractical.

 

Using this data I designed a motor with 2 grains of the same type (OD 34mm, ID 10mm, height 50mm, mass 70 grams, so 140g with two grains), with nozzle of 9mm, which gives max Kn of 120. It uses concrete plugs with De Laval nozzle and aluminium casing. Using a steel plate with a notch cut into it, I rolled the ends of the tube inwards to keep the concrete in place. This method is extremally reliable, it held 90 bar no problem, and this design only creates like 40 bar. Here are some photos and the burn graph.

http://i.imgur.com/P2pR7Vo.jpg

 

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

 

http://i.imgur.com/Is9IOue.jpg

 

So as you can see, the results are pretty consistent, one rocket might have a bit longer grains tho. Isp is about 117 seconds, total impulse about 170 Ns, and max thrust is 45-50 kilograms. I determined the chamber pressure via burnrate, and calculated that the nozzle coeficient is about 1.7

 

So here are the results of my research. Now that I have the technology to build these motors, I'll work on parachute and equipment, and hopefully soon I'll launch some rockets. I have already stuff planned and calculated out, I just need to wait until arduino nano will arive, because I will use it for the timing and deployment.

[stix]

Congratulations Oinikis, it's good to see that you've got those initial problems sorted out and in a very impressive way! An lsp of 117 is very good indeed. Tmax at 45-50 kilos must have made one hell of a noise!! . I know that you've put in a lot of research and effort and it's now paying off - you certainly deserve it.

 

It's also good to see someone doing research, ie. making & testing motors, recording the results and then being able to analyze the data and show the final results.

 

How are you going to use the Arduino Nano? Will it be just a simple delay based on altitude calculations for determining apogee - then ignite the ejection charge - or an altimeter - or something else?

 

Cheers.

Edited March 22, 2016 by stix

[Oinikis]

Yes indeed it made one heck of a noise.

 

I like to first test and calculate stuff, and then using data create it. Using this data I have right here I can design and make basically any rocket I want. But there is more to it. Currently, i'm finishing school, and I plan to study aeronautical engineering in aircraft designing. They have labs, and do various projects, inluding some rocketry. This will show my engineering capabilities. Also, I work one aerodynamic tunnels, gliders, and as a sideproject, I made an arduino rf link, and hope to make a RC plane, and ultimately one day, a rocketplane, or rocket lifted glider.

 

Anyway, back to earth. I plan to have the parachute pushed out by a rubber band, which will be held by a servo, and when time comes, the servo will release the rubber band, and it will push out the parachute. As for the timing, I'll set a time, when to deploy after launch. I will have some sort of a pin or a contact attached to the guide (launch) stick, and then when the rocket will take off, the contact will be pulled out, breaking the contact in the circuit (contact between the GND and some digital input), and start the timer.

 

As you might have not known, I also have a limited amount of knowledge about aerodynamics, and a wind tunnel. Before doing this rocket research, I did some research on aerodynamic surfaces. Right now I have folowing graphs:

angle vs. center of force

angle vs. lift coefficient

I have two version of the latter, using surfaces with different aspect ratios. The data is not very serious, but it will help me determine the fins. Actually, in the near future i'm planning on to building a much bigger wind tunnel, and do some more serious research.

 

Now, using my current wind tunnel and some knowledge, I'll determine rocket drag coeficient, and using it and the burn profile, I'll do a simulation of the flight in excel, to get the timing, predicted apogee, etc. Using this data I will set the timer. I plan on having the parachute deploy after apogee, so it deploys mid fall, so it spends less time in the air, so it drifts less.

 

I also plan on having blinking LED's for enhanced visibility on the ground. But that might not be necessary.

 

One more thing, I have already made the parachute, and figured out the folding. I fold it in a similar manner man-rated ones are folded. Due to the nature of parachutes, it's quite easy to calculate approximate drag of the parachute, so I made it to let the rocket fall at around 8 m/s, which is quite fast, so it won't have much time to drift sideways.

Edited March 22, 2016 by Oinikis

[stix]

I do remember your wind tunnel experiments. I'm not sure where you got to but obviously you made some good progress. I can see your point in doing it, otherwise how else can you plug in the Cd (drag coefficient) into your altitude calculations?

 

. . . I will have some sort of a pin or a contact attached to the guide (launch) stick, and then when the rocket will take off, the contact will be pulled out, breaking the contact in the circuit (contact between the GND and some digital input), and start the timer . . . .

 

Sounds like a good idea. I did a similar thing a while back using a "reed switch" with a good magnet inside one of the launch rails. Took me some time to make a timer circuit (and battery) that would fit into a small (20mm diam) tube.

 

Make sure when you do become a Cosmonaut/Astronaut that you get me a front row seat for the launch.

 

Cheers.

[Oinikis]

So now I shall work on the parachute module, Nano is only necessary when fitting it all aboard. I'm still not sure how I should do it. should I have the nosecone come off, and parachute to slide out the end, or some sort of doors, and expel it sideways.

 

As for the front seat, We'll see where life takes us, but one thing is clear, I should never stop and reach for the stars. I made gliders before I could read and write, and since then I wanted to fly.