21 replies to this topic

[dr thrust]

I launched a couple tonight, one with a 1" payload and one with a 2"payload.

The one inch worked perfectly, right height, burst at apogee.

The 2" went about 60 feet then fipped upside down and hit the ground under power then burst.

I am getting there though

good, with a end burner( i presume thats what your using) your not going to create a lot of thrust, but you'll have a longer duration of flight( = greater height under no load).
so to lift a heavier payload , ie a three inch shell atop a 18mm motor, your going to need thrust by burning more fuel per meter/second.

how is this achieved i hear you say, well its down to motor design at the moment with the end burner configuration, only a small surface area of the propellant is able to burn, ie the "end!" which say is 18mm, has a surface area of 254 mm(squared).
so to get more thrust we need to change the motors internal geometry, by creation of a hollow core.
this now allows us to achieve a greater surface area of propellant, well use a cylindrical core for a example , a 9mm dia x 175mm high core will have a surface area of 496mm (squared)and the area will keep expanding as the propellant is used= more fuel burnt per sec.
i think lol

[Vic]

With long cored rockets you do get high initial thrust compared to a end burners with short cores running on hot fuel. The nice thing about long cores you don't have to ballmill your BP for hours screening is fine as long as your chemicals are as well, and you don't need willow charcoal. I don't know if I am right here but even with long cores the fuel grain at the bottom of the core burns away very rapidly to the wall of the tube, shortening the core and thus the burn surface area giving you ever decreasing thrust.

I remember a thread from somewhere I think APC saying shortish but pointed cores on whistle rockets gave more thrust than long cores, does any body know the maths on that?

[Atom Fireworks]

After reading this thread it inspired me to have a crack at rockets seens astho my last attempts just blew up on take off.

I had some plastic pipe at work around 14-18 mm I.D and wall thickness of about 2.5mm. I found some steel rod that was a little to loose fitting in the tube so i put it in the vice and whacked the hell out of it with my old trusty lump hammer. It then fit nice and tight with abit of linishing.

To make the rockets all i did was compress some Bentonite clay in the tube about 15 mm, then compressed some meal powder ontop of this in a teaspoon increments until about 30mm for the top, i then compressed a further approx 10mm of clay on top leaving a void of around 20mm for a passfire to payload.

Then i used a narrow pair of scissors to bore a nozzle, once i hit BP the nozzels size goes from approx 3.5/4mm to about 8mm at the bottom of the nozel.

Ive made 8 now 4 tonight and 4 last night and they work really well, ive lifted a 10gram salute and a 20grm film canister with stars and burst the others were testing for consistancy.

They dont fly all that straight, fairly erratic but i suspect my stick not long enough so off to BnQ tomorrow for some thin dowell.

I never thought rockets would be so much fun

P.S i know my method is rough arse, not exactly proffesional but it seems to work really well i might see if i can record a motor on some scale and see what kind of thrust it gives.

[Vic]

After reading this thread it inspired me to have a crack at rockets seens astho my last attempts just blew up on take off. I had some plastic pipe at work around 14-18 mm I.D and wall thickness of about 2.5mm. I found some steel rod that was a little to loose fitting in the tube so i put it in the vice and whacked the hell out of it with my old trusty lump hammer. It then fit nice and tight with abit of linishing. To make the rockets all i did was compress some Bentonite clay in the tube about 15 mm, then compressed some meal powder ontop of this in a teaspoon increments until about 30mm for the top, i then compressed a further approx 10mm of clay on top leaving a void of around 20mm for a passfire to payload. Then i used a narrow pair of scissors to bore a nozzle, once i hit BP the nozzels size goes from approx 3.5/4mm to about 8mm at the bottom of the nozel. Ive made 8 now 4 tonight and 4 last night and they work really well, ive lifted a 10gram salute and a 20grm film canister with stars and burst the others were testing for consistancy. They dont fly all that straight, fairly erratic but i suspect my stick not long enough so off to BnQ tomorrow for some thin dowell.I never thought rockets would be so much fun P.S i know my method is rough arse, not exactly proffesional but it seems to work really well i might see if i can record a motor on some scale and see what kind of thrust it gives.

You can't beat home made rockets, but please try to use aluminium or any non sparking materials even wood dowelling will do for a rammer and a sharp drill bit turned by hand for the choke/core.
You could try filing down a brass bolt or screw and mount it on a block of wood and save the trouble of drilling. By doing this you will be increasing your safety considerably.

[digger]

You can't beat home made rockets, but please try to use aluminium or any non sparking materials even wood dowelling will do for a rammer and a sharp drill bit turned by hand for the choke/core.
You could try filing down a brass bolt or screw and mount it on a block of wood and save the trouble of drilling. By doing this you will be increasing your safety considerably.

Yep, could be dangerous.

Many many moons ago in the eighties when I was a teenager. I made a few rockets with the steel rammer method. Admittedly the internet was not around and I was ill informed. The composition may have been a little more sensitive than black powder. Anyway I was holding the rammer in one hand and the mallet in the other. So I was battering away and whumphh the rammer went into orbit (thankfully outside) and it backfired into my clenched hand. Needless to say I got a rather severe burn. But I was fortunate that I did not have a steel bar sticking out of my head.

[dr thrust]

I don't know if I am right here but even with long cores the fuel grain at the bottom of the core burns away very rapidly to the wall of the tube, shortening the core and thus the burn surface area giving you ever decreasing thrust.

I remember a thread from somewhere I think APC saying shortish but pointed cores on whistle rockets gave more thrust than long cores, does any body know the maths on that?

interesting,but surly with the erosion of the core wall the extra surface area will make up for the bottom of the core burning up, is the lack of thrust at the end of the motors burn simply down to the lack of propellant?.

as for the pointed whistle spindles, are they referred to as "pusher's" , i found them a real pain to press ( with the ever diminishing core) you need about five rammers !
also the propellant was being extruded into the rammer under pressure, not ideal!.
i did have a brain-wave though, of filling the motor casing with propellant and pressing the pointed core into the propellant by the means of machining a shaped end on the rammer

[robbstech]

So if i decide to make core burners......how do I make the delay to ignite the payload? do I need some delay composition of just a bit of time fuse (or visco)?

Thanks for the input guys....keep it coming!