Rocket Cores

[Arkangel]

The pyro section is looking a little bare, and a question springs to mind.

A rocket with a core the full length has certain characteristics that you might like or not. Since the engine is ignited and burns along the length of the core (if ignited correctly, from the top) The pressure generated is proportional to the surface area of the exposed fuel. So as the fuel burns, the circumference of the core, and therefore area of exposed fuel, leading to a rise in pressure inside the engine. This leads to a gradual launch, with the rocket accelerating more toward the end of the burn. (looks cool if you have a model Saturn V) It also leads to Cato's in testing along the way, as the motor body or end plug rupture.

A way to make the pressure more consistent might be to change the full length core for a conical one. It would look a bit like a shaped charge cone and this way, you'd still get a decent surface area, but the pressure would be consistent most of the way through the burn. You could have a faster fuel without worrying so much about cato's and chuffing. If it doesn't blow up in the first 2 or 3 seconds, it isn't going to. My rockets often get off the launch pad and explode after a couple of seconds.

I don't suppose this is a new idea, but has anyone tried it?

:)

<small>[ February 18, 2003, 07:55 PM: Message edited by: Arkangel ]</small>

[a_bab]

What kind of fuel do you use ? "Normally" a rocket engine burns 1-2 seconds at best (Estes). As about the conical profile, I saw somewhere on the net a conical spindle plan.

[Anthony]

One way to eliminate the increasing pressure is to use a cored grain that burns from both the inside and the outside.

[Tuatara]

There are many ways to 'core' a propellant grain, each with its own characteristic thrust/time curve. What you describe is effectively a modification of the 'end burner' configuration. The book I have, 'Making Amateur Rockets' by John H. Wickman (get it, its great!! <a href="http://www.space-rockets.com/Cptechno.html" target="_blank">CP technologies</a> ), also has 'moon burner'- an offset round core, 'c-clot burner' - rectangular slot extending ~1/3 in from the outside of the grain, and the segmented cored burner, composed of sections which burn from both ends an from the middle. You can also have multiple cores, and I've seen a 'star' core designed to give very high initial thrust.

a_bab : rocket engines can be made to burn as long as you like - limited only by fuel, and the melting point of your engine casing :D

[Guerilla]

End burner provides flat burn curve, but with the cost of pressure. If you want a decent thrust you need a core to increase burning area.

A rather easy way to obtain a neutral curve with a respectable thrust is to make so called bates grains. They work basically like Anthony said, burning from inside and outside, of course both ends have to be covered with something to prevent them from catching fire. Other option might be to cover only the top of the grain. The latter gives a bit lower thrust.

The grain lenght should be approx. 1.5 times the grains diameter, depends on core diameter which should be 1/2 - 1/3 of the grains diameter.

These grains are then packed into a rocket body and lit from the top. Preparing the grains shouldn't be very difficult, at least with a castable fuel (such as KNO3/sugar candy)...

[gamin guru08]

I'm more into rocketry than anything else right now but I'm learning. So on that note...rocketry is my specialty.

One way to make the thrust curve more flat is to make the core into a star shape. This provides the most surface area at the start of a launch and as the star begins to burn away then the whole is rather large providing around the same amount of surface area as at the beginning. The amount of thrust created(in theory) will still be higher at the end, but not so much as with a normal cylinder as a core. The only problem with this is finding a easy way to cast it. Right now I use a coring rod that I pre-heated to ram it through my KN/Sucrose rockets. A star shape could be done in the same way but I have no idea where to or how make one.

You guys probably already knew this since it is rather general but I'm just putting my .02 in.

[PyroTech]

Hey, Tuatara.
I would like to buy that book, but is it worth the money? Their site looks awesome. Can you tell me, what I can expect from the VHS tape and the software? That would really help me.
Thanks

[frogfot]

Gamin, can you give some info on your coring tool for candy rockets?
Also, do you think drilling core will be safe?

[Tuatara]

PyroTech: The CP Technologies book is definitely worth the money. Its well laid out, is a comlete A-Z on building rockets and rocket engines (over 160 pages). There are four complete engine designs in the back, from 1" dia class G to 4"dia class L ( thats got 2.6lbs of propellant in it <img border="0" title="" alt="[Eek!]" src="eek.gif" /> ). I've only watched the video once, but it does show the process of making the propellant and loading it into the motor, and some footage of engine tests. The software is fairly basic, but does ease all the calculations.

[Arkangel]

Everyone mentions the star shaped cores and other complex methods, without saying what they think about the conical "core" I'm talking about.

There is a reason why I'm persisting, so bear with me. Anything other than NO core or a simple cylindrical core involves a great deal more effort. If you make a rocket using a conical Mandrel, with a 30degree point on it. You'd have a cross between a core and a flat end, which would increase the surface area immediately available 2 or 3 times that you'd get with a flat end to your grain, in the same size tube.

You'd therefore immediately get 2 or 3 times as much thrust, and since it would more or less keep it's shape as it burnt, you'd get CONSISTENT pressure. And you'd get it easily.

Now the advantage I see to this, over the many other methods you've mentioned (star, crescent, C-slot etc) is that you could have a much longer rocket with more fuel in it. The longer it the rocket is with a full length core, the higher peak pressure is going to be. With a conical "semi-core", the only limit to the time you could burn it for would be the walls of the engine not burning through. That would be ideal for a multi-engined rocket, where you have the main "cruise" engines, plus some boosters. The boosters are short and high impulse, but your mains will continue pushing it along, getting lighter all the time for as long as they had fuel.

After my mini-MLRS, I'm thinking something like this:

<img src="http://www.boomspeed.com/arkangel/soyuz.gif" alt=" - " />

Sorry, I'm a bit caned at the moment, am I making sense here?? :)

<small>[ March 19, 2003, 07:40 PM: Message edited by: Arkangel ]</small>

[Tuatara]

I think your reasoning is sound, regarding the conical core.

However ...
Engine melt down is the biggest problem, as the combustion chamber temperature is likely to be around 2000C, and there aren't too many structural materials that can withstand that kind of punishment for long (got a good source of Inconel?)
One other issue to be aware of is nozzle erosion. Most of the commonly available nozzle materials will be eroded by the Mach 1, 2000C gas stream, meaning a prgressive drop in chamber pressure if your burn rate remains constant. Graphite is one of the few materials that doesn't erode appreciably (for amateur scale engines), that should be easily obtainable.

You will also be limited in your maximum lift-off weight.

Despite these *negative* comments, I think the conical core idea would be very effective for smaller rockets, with engine burn times of around 10 to 20 seconds, and the ease to manufacture is a good offset to the disadvantages.

Please post a vid of your next rocket project - the one after the mini-MLRS :D

[aster]

i made my rocket, many times, the nozzle is made from tile grout, somekind of cement, the body is made from aluminium tube, the propellant is black powder cast with the using of water during preparation, the propellant is formed like cylinder, with the hole along it's length, but i experienced fail in launch the rocket, first, my rocket just burn all the propellant on the ground, i think this caused by too wide hole of the nozzle exhaust, the rocket not moved even one inch, the second try i narrowing the nozzle, my rocket go KABOOM ...!! before moving... <img border="0" title="" alt="[Frown]" src="frown.gif" />

[Arkangel]

Well I don't have anything so grand in mind, but <a href="http://www.moonrace2001.org/n1_launch.shtml" target="_blank">this</a> is a 1/16th scale model of the Russian N1 moonshot rocket. These guys have done an amazing job, and some of the pix are very impressive.

I'll do a seperate thread for my multi engine jobbie, when the time comes.

aster, your nozzle should be something like 1/3 (0.33%) the diameter of your rocket tube. Read through some other threads though before you post too much more on this. There are a lot of relevant threads open, with answers to your problems.

[Anthony]

33%, Arkangel? :)

IIRC it was stipulated that it must be area not diamater, but for small engines, 1/3rd diamater seems to work and is easier to work out.

aster, if you cast your BP grain with water, then it likely shrank when it dried inside the rocket body. This will often lead to a CATO.

[Arkangel]

Ahaaaaa

Now I've harnessed the power of Paint shop pro to give you THIS

<img src="http://www.boomspeed.com/arkangel/conicalcore.jpg" alt=" - " />

A diagram of my conical core. If this fuel is quite fast, you'd get a pretty strong initial boost, which would gradually decay as the "v" of the cone became flattened during combustion. But do you see how you could make this motor as long as you wanted?

If you wanted to be really sexy, you could mix in a faster fuel in the top half of the motor, to the point where it was 100% fast fuel by the time the cone had burnt to a flat.

And notice the ablative liner on the lower part of the motor inner. I'm not sure what you could use for that - suggrestions welcome. (on the stinger missile instructions from Skylighter, they suggest soaking the area of the rotational thrust aperture with sodium silicate (I think) Might that help?

<small>[ March 19, 2003, 07:18 PM: Message edited by: Arkangel ]</small>

[Anthony]

The water glass should help prevent erosion of the rocket body. I was going to suggest a piece of thin walled metal tube for the ablative armour, but it'd be easier to just use a complete metal motor body.

If you need a source of water glass, I've got a product for car engines containing it written down in a little book somewhere...

[Arkangel]

I wondered about lining the motor body with a couple of layers of Aluminium foil...........?

Water glass??

[0EZ0]

Water Glass = Sodium Silicate

Arkangel, with the supposed conical core you have illustrated, we can see a greatly increased stress on the rocket engine nozzle. It could just be blasted out or eroded to a sufficient enough degree for the engine to fail. A way to counter this might be by using a turned metal/graphite nozzle. But by using metal, you might have to consider building the body also out of metal. This could lead to greater cost, more labour intensive manufacture and of course increased fragmentation hazard.

If you really are having trouble with rockets exploding on the launch-pad, try experimenting with decresed core lengths. I would agree that any standard experimental fuel(eg. KNO3/sugar formulations) and conical shaped core could have an overall negative effect on performance. But why not try an end-burning design with a standard nozzle? I'm sure you could get a rocket off the launch-pad with any well made Black Powder with that design.

Maybe you could fully negate the use of a nozzle altogether using cored nozzleless Black Powder engines.

Here is a good link for Nozzleless BP rockets:

<a href="http://www.angelfire.com/nv/bpnozzlelessrockets/index.html" target="_blank">http://www.angelfire.com/nv/bpnozzlelessrockets/index.html</a>

<small>[ March 19, 2003, 09:36 PM: Message edited by: 0EZ0 ]</small>

[Arkangel]

Ahaaa, thanks. :)

I'm not having trouble with rockets exploding........well, here and there I have, but not regularly or recently. The idea of the cone rather than a normal core is that you'd get a much steadier pressure curve and could have longer burning engines.

I don't know what the formula is to work out the area of a cone, ( <a href="http://www.mathepower.com/english/kegel.php" target="_blank">this might help</a>) but the surface area of a regular core can be calculated as follows:

2 x pi x radius x length of core. Compare that to the area of the conical core.

The difference here is that the regular cylindrical core gets bigger all the time, and you therefore have an increase in pressure throughout. There's a theoretical limit to the size of motor you can make with this design (with the same construction/fuel etc etc)

The cone should give you a steady pressure curve, with a motor that burns for as long as you want - just have a longer tube. If you tried that with a normal cored rocket you would get to the point where something would fail.

Incidentally, I've been trying <a href="http://www.roguesci.org/cgi-bin/ewforum/ultimatebb.cgi?ubb=get_topic;f=3;t=000418" target="_blank">this</a> stuff, making 1" plugs of it and drilling nozzles and it's superb. 5 minutes ago I dissected the motors I fired last week fitted with these nozzles.

1. The epoxy held the plugs in unbelieveably well, far stronger than either the tube or the nozzle plug on their own.

2. The plug material itself was still very hard and strong. I managed to crack one open from the side, but it was very reluctant and I really can't imagine these nozzles, or the fixing method failing.

(btw, went through that BP nozzle-less site a while ago, thanks)

<small>[ March 19, 2003, 10:20 PM: Message edited by: Arkangel ]</small>

[Tuatara]

For your ablative liner you could use EDPM rubber sheet glued to the casing using RTV Silicone. EDPM ablates at 1000F, and has fairly good insulating properties, which will help to protect the engine casing.

Compared to aluminium, EDPM has half the density, twice the Cp, 1/1000 the thermal conductivity, similar melt/ablation temp, and 10 times the heat of fusion, so it should make a far superior lining material.

[Arkangel]

Any ideas where I might get some? Is there a normal application for the stuff?

[Tuatara]

EDPM rubber is a commonly available synthetic rubber - I guess its used wherever you'd normally use sheet rubber, its not a rocket specific product! My local engineering supply shop carries it , along with Neoprene, natural rubber, butyl rubber, silicone rubber etc. You could try any rubber goods supplier. I suppose any rubber type might be a good candidate for lining a combustion chamber - but I don't have all the data. Natural rubber would probably be the cheapest, EDPM is fairly pricey IIRC.