Experimental rocket.

[gdeputy]

http://www.graphitestore.com/ is one option i've used in the past. Also http://aeroconsystems.com/cart/graphite-rods/medium-grain-graphite-x-12/. And ebay.

 

Longshot, but if you know anyone who works at a foundry where they use electric arc furnaces you can sometimes get the used electrodes. Not super high quality, but cheap/free. I still have a few chunks of that laying around that are 8-10" in diameter and a few feet long.

[douglas423]

Well you guys have been right(i knew you were right about the nozzle) there is slightly a melted part on the nozzle now. This was my 2nd static test tonight and it blew a hole in the motor casing. A hole on the side just like the last one. Should i try a motor casing out of thick fiber glass?

[NeighborJ]

Before giving up on the aluminum casing I'd try a few more things. The core burner motor design counts on being ignited at the top of the motor. This ensures that the core ignites almost instantaneously and burns evenly, which allows the fuel to be completely spent just as the case wall is exposed. A true even burn is almost impossible to achieve so the inhibitor sleeve is used to prevent the case from burning while the last few remnants of fuel are consumed.

By using the slow fuse for ignition the core ends up igniting at the bottom first, which causes the case near the bottom to get exposed prematurely. That fuse can still be used but I'd suggest wrapping several turns of masking tape arround the fuse all the way to the top and leaving an inch exposed to j hook it in the very top of the core.

A lot of motors use aluminum cases and it is a fully accepted and proven casing material so I would not change the casing without exhausting all other options.

[douglas423]

Alright ill re try it using that. I will also use an electric ignition in my launch. I have another problem as well, re drilling the holes in a new casing? Most times they end up being uneven and i have to drill completely new holes in my nozzle. I'll try to be careful and drill them right. How do i ignite it? Do i make it in the very top in the wall of the core? Could you maybe provide some photos? Thanks!

[NeighborJ]

You will find that the graphite nozzle is an unsuitable material to drill and tap into. Instead you will need some sort of retainer ring which you can tap screws into, I've attached some pics from the aerotech catalog for you to find some inspiration.

Umm, I guess it posted the entire catalog, look at the RMS plus reloadable motor. It utilizes threads on the retainer ring but could just as easily been fastened with screws.

07-08_aerotech_catalog.pdf

Edited November 3, 2016 by NeighborJ

[Arthur]

There may need to be a thermal insulator between the fuel and the casing, perhaps just a sheet of paper or two or three turns of aluminium foil. This keeps some of the heat off the pressure body til the burn is over (probably 10 - 30 seconds).

[douglas423]

I'll try it with a fuel with sheets of paper. This last time it had so much force it bent 3 screws that held the bulk cap on the back. Should I use more screws? I was using some 6-32x1/2" stainless steel ones. I included a photo of the before and after 2 use nozzle and the exploded body from the first test. Edited November 3, 2016 by douglas423

[gdeputy]

If you have access to a lathe, retaining the nozzle and forward bulkhead with snap rings is a simple solution. Bolts work fine too, but yeah, sounds like you need to use more.

 

RE insulation / preventing burn though, the most common way its done in both commercial and home made motors is to cast the propellant grains in a cardboard tube that fits in another tube of cardboard or phenolic that serves as the insulator. You want to avoid having the flame front hit any of the case. Aluminum just wont hold up if exposed to the flame directly. The insulator should fit the length of the motor case, and seal against the forward bulkhead and nozzle. A chamfer on the inside edge of the nozzle and bulkhead that fits into the insulator will also help. Use plenty of grease to seal everything up. A few wraps of paper are probably not going to cut it. Foil will conduct heat real well and isnt a good insulator.

 

It would help immensely for you to see some other folks motors hands on and see how they all go together. If you buy a pre-made motor case you can see how they work, as well as get liner/casting tube sets that fit. Sources include

 

Loki (these guys do commercial motors, but the hardware is a very common design for homemade motors, and they also sell casting tube sets that fit their cases, nozzles, etc...)

http://www.lokiresearch.com/secure/store.asp?groupid=5520031443530

 

Gorilla motors (similar to above, same common snap-ring design)

http://www.rocketsrus.com/gorilla-38mm.html

 

The snap ring motor design is so incredibly simple, anyone with a lathe and very basic skills can crank them out.

[Arthur]

While one layer of metal foil is a poor insulator several layers make a remarkably good insulator. To ensure that the centre of the grain burns over it's entire length sometimes it's lit at the closed end so that the whole length burns from the start.

 

If you make the grain in a card or foil tube then you can make the centre burn space whatever shape that you want to have max thrust at the beginning when there is max mass present.

[douglas423]

I only have a wood lathe but I might could still do it. The nozzle holds fine it was just the bulk cap. I'll try using cardboard or very thick paper with glue. Would someone mind posting a photo of how the igniter is set in the fuel? I keep thinking about how to light t from the top in a core burner. Edited November 3, 2016 by douglas423

[NeighborJ]

Wow, Douglas, after viewing the pics of your motor I can see a few more possible issues. I revisited your first post to get the actual dimensions which I originally thought to be a typo. The total motor length is 3"? And the dia is 2.75 od? These proportions immediately raise some red flags. My concerns are that the motor is too short for the diameter and in order to make it work the nozzle would need to be so large that the motor has little thrust at ignition and near the end of the burn it will be over pressurized. This would be caused by the dramatic increase in surface area. Does anyone else have any opinions about this? I'm not trying to tear your design down, I've just never seen such dimensions as your motor so I need to ask.

[stix]

I'd have to agree with NJ, that this geometry isn't ideal. Also, are you making true bates grains?

 

In any case, I think that because of the short fuel grain, the nozzle would have to be smaller to get up to pressure at the beginning, and as a result will be way over pressurised at the end. The combination of the aluminium being heated and the extra pressure may well have caused the Al tube to blow through. For that diameter tube, the wall thickness could/should be thicker, but in this case it's probably safer as it's acted as a safety release.

 

I've made some APCP motors a long time ago, but AP is so expensive here that I've turned to sugar rocket motors. Nevertheless the fundamentals of motor design still have relevance.

 

Douglas, I'm all for experimenting - I love it, it's what I enjoy the most. But there is some scientific method involved.

 

Start small and work your way up. 3/4" would be a lot easier to perfect your manufacturing methods. Test/analize the results. Decide which way could improve it. Read up elsewhere and ask questions here. Once you get the small one working well to good specs, then you'll be in a position to use all your hard learned knowledge and make something bigger.

 

Most people who start experimenting with anything start off small and work their way up. I'd advise that you take a step back or two and re-evaluate where you are and what you want to actually acheive. You'll get there quicker in the long run.

 

If your current tested motor is around 2.5" ID then in my view an ideal grain length of around 12" or even longer and a core and nozzle somewhere around 3/4" diam. A lot of fuel for just experimenting.

 

Smaller is faster, easier and cheaper.

 

Cheers.

Edited November 4, 2016 by stix

[douglas423]

I have in the past made smaller rockets 1" in dia. This ID of this rocket is 2.375" and on both of those test I used a fuel grain about 1 1/4 inch tall. I believe it's failing because of the pressure and heat, from not having a inner sleeve to protect the walls. Should I make the pressure/fuel area be bigger than the 1 1/4 it was? The nozzles ordice is 1/2" it seems to have a lot of pressure. How about I make the core bigger in the grain? Wouldn't that reduce the pressure some? Edited November 4, 2016 by douglas423

[stix]

Ok, so I take it that your 1" motors were successful? If so, apart from size, what other differences are there?

 

So what is it you are testing - fuel types/mixes etc.? I don't really understand why you are making a short fat grain that's wider than its height. Which what sides are inhibited?

 

Interesting concept. A bit more info would help.

[douglas423]

I'm testing to see if the body blows out like it has or if the nozzle melts. Using more fuel would just have more power and would blow it out more I would like. In these past two motor test I put fuel directly into the motor casing with no sleeve. In this next motor test next week I will put a cardboard tube or lots of sheets on cardstock paper around the fuel.

[Mumbles]

Unless you're planning to use these squatty motors, it doesn't make a lot of sense to not test under conditions a real rocket would encounter.

[Arthur]

This is probably a good time to check where you are aiming for and whether what you are working on is in line with that. Study the properties of grains with shaped cores, they are useful for adjusting the time of burn and the thrust profile.

 

A firework rocket has whatever core can be made with a particular nozzle size, and HP rocket has the grain and the nozzle made separately, both can be optimised for thrust and thrust profile.

[douglas423]

I'm going to look into grain geometry more and how big my core needs to be. Along with the size of the fuel and inhibitor sleeves.

[stix]

Making the 'fuel core' wider in diameter isn't really going to achieve anything. It would be the same as a small core, and after the fuel burns it would reach that same point after time. Making the fuel core longer however will make a big difference.

 

This is where you need to understand Kn ratios. The Kn ratio is the exposed surface area of fuel divided by the nozzle throat area. This may change over the burn of the motor depending on the geometry of the grain. If you are serious about motor design then It's an absolute MUST to know what Kn's you're running at. Otherwise you won't understand what has to be done to make a different model work.

 

It's not that hard to work out (for round cores). Simple online calculators to work out surface area, a pen and notebook should suffice. For different geometries like a star shape it may be worthwhile looking at BurnSim, which I think someone already mentioned.

 

As Mumbles mentioned, a short fat 'squatty' motor is an inefficient design in the 'real world'. That is, if you were to fly it, there would be unnecessary aerodynamic drag - whereas you could use the exactly the same amount of fuel in a longer thinner design (think space shuttle booster) and it would slice throught the air easier and acheive a higher final altitude.

 

I'll say this once again, I think you should go back to the smaller motor first and see if you can optimise that design, refine your techniques and get a better understanding of what's going on before moving to a rather large one.

 

You obviously have a passion for it so I'm sure you'll get there. I think everyone who has participated in this thread wants to see you acheive that too.

 

Cheers.

[douglas423]

I'm going to look into grain geometry more and how big my core needs to be. Along with the size of the fuel and inhibitor sleeves.

[Arthur]

I'd suggest that you find your local rocket shooting club and join! Then plan some research and tests round having something ready and tested to fly at their next shoot dates. Let them get some airspace cleared officially and you turn up with a useful rocket -that's more than just a motor! and see whether it flies according to your calculations. Do expect to have to justify every calculation and test of your rocket before they let you fly. All the rocket clubs have to tread very carefully round the explosives, flight and counter terrorist regulations of your county/ state/ country of residence and flight. Expect issues taking a high power rocket across administrative boundaries

 

Hopefully a rocket club will fire from a space where you can stand well back from a cato, currently you should assume that several test motors will explode and make all the metal into shrapnel, and dig a crater. you need blast screens and lots of space. Look at NASA's efforts, when it goes wrong it goes very wrong, occasionally people get killed. Once you have a rocket that flies you need to have a method of collecting the used motor and rocket body minimising the damage to property, may well include timed multi stage parachutes, -you don't want to pick up the remains in the next country if they drift a bit (10,000ft on a parachute and you can fly about 40 miles

 

A typical rule of thumb is to design a rocket to explode then back the power off a bit, they are always close to going bang, commercially and experimentally you need to chose a safety factor, and be able to prove that your design is safe but will still fly!

 

Bring on the pictures! We need pictures

[dagabu]

While one layer of metal foil is a poor insulator several layers make a remarkably good insulator. To ensure that the centre of the grain burns over it's entire length sometimes it's lit at the closed end so that the whole length burns from the start.

 

If you make the grain in a card or foil tube then you can make the centre burn space whatever shape that you want to have max thrust at the beginning when there is max mass present.

20# bond that's been treated with borax makes an excellent insulative layer when combined with layers of aluminum foil but is easily defeated by the gasses.

 

Making a phenolic tube for casting is easy and gives much better insulative properties than aluminum foil, paper, cardboard or aluminum tubing.

[douglas423]

I'll look into phenolic tube for it. The nearest rocket club is over an hour away and the next launch is in over 150 days as I said before. But I'll look for some more forums so I don't have to post on this one about this stuff.

[Arthur]

You need to join that club so that you will be accepted to launch your rocket at their meeting, than you need to have a design and test and development plan so that your design comes up to your expected standard in time to check it passes the club standard to launch. All that in 150 days!

[NeighborJ]

Douglas, don't get discouraged, everyone here is looking out for your best interest. The easiest way to make your first launch date is to call the club president and ask what is needed for launch clearance. He can either give you some literature or put you in touch with another member who can walk you thru the process. Personally I'd rather try to have a respected member guide me and then vouch for the design. It will make things a lot easier to get permission for launch.