12 replies to this topic

[MarcCallaghan]

Folks, I need some advice and guidance.

 

First, some context:

I'm a 'techi' teacher and wish to knock up a few small rocket engines with which the kids can simulate a rocket car - essentially the Bloodhound Challenge. So the kids will make small model cars which will roll along the deck and be tethered to a static line in order to keep them in a straight line.

The kids will make the cars but only I will make the rockets, fit them to their cars outside and light them once the kids are away a safe distance.

 

Materials:

I purchased some fuse, Pottasium Nitrate (KNO3) and some icing sugar.

 

Method:

I mixed a ration of approx 70% Pottasium Nitrate to 30% Icing Sugar. I measured quantities by weight and added a pinch of Iron Oxide. I then gently warmed it up over a hot plate until i got a rough syrup consistency and then packed it into a thin plastic waste pipe (approx 1inch x 3.5inches ). Once cooled, i blocked both ends with a filler (like polyfiller) and then punched a hole in one end using a screwdriver.

 

Result :

Pretty poor. I wedged the rocket motor between 2 weights and lit the fuse. It started fine but seemed to create more smoke than thrust. Half way through the burn the pipe also failed and turned into a molten mess.

 

Request:

I need some guidance. I'll soon have 30 kids, each with a model (approx 12inches long and made from covered foam and balsa, etc). All desperate to see their car whiz across the tennis courts and my best efforts resemble wet farts. I'm going to get lynched.

 

Oops, almost forgot. Before someone suggests buying the commercial ones. Their like rocking horse poo in our neck of the woods and even if i could get my hands on them, we couldn't afford them.

 

Thanks

 

 

Marc

[digger]

Simple answer, buy estes motors and you will be certain of being on the correct side of the law (you can buy them mail order for not much money, £10 - £15 a pack for larger ones).

 

More inspiring answer (keeping below 100 grams NEQ)

 

Change from icing sugar to normal sugar (no additives), also ensure that you have a core 75% ish the length of the of the motor. You could use an old biro pushed into the mix before it sets. More surface area = more gas in a shorter time = more thrust. The other obvious one is a smaller nozzle.

 

Hope this helps

 

Gareth

Edited by digger, 13 March 2013 - 06:52 PM.

[Atom Fireworks]

And for your casing issue, wrap a few layers of paper around the casing with pva or some other form of glue to secure, the more layers the better the motor will withstand the duration. Also if doing it the core burner way you will not have too much of an issue on that front as it will be insulated by the propellant for a short time.

[digger]

And for your casing issue, wrap a few layers of paper around the casing with pva or some other form of glue to secure, the more layers the better the motor will withstand the duration. Also if doing it the core burner way you will not have too much of an issue on that front as it will be insulated by the propellant for a short time.

 

Why I suggested a core. With a core the casing issue will most likely solve itself as the grain burns from inside to out, so when the heat hits the PVC there is no propellant left.

 

With a cored BP rocket I have managed to get casings down to 1.5mm wall thickness where I would have needed 5 - 7 mm with end burners.

Edited by digger, 13 March 2013 - 07:13 PM.

[digger]

P.S. With sugar rockets a core is pretty much essential

[Deano 1]

Hi Marc, ideally you could do with some tooling, maybe being a techi you could make some.

Try this for a bit of a guide.

 

http://www.skylighte...n=Speed-Rockets

[Deano 1]

Also Marc, you could use the black electrical round conduit from B&Q, dirt cheap and 17.5mm I.D.

[fruitfulsteve]

I tried something similar a while back with toy cars and boats and found the estes motors had a little to much inistial thrust which resulted in both the cars and boats flying up into the air spinning around a bit and crashing. It could well be that they were a little light. So and i haven't actually tried this yet, but perhaps using the 'drivers' of a large cathering wheel might well give a more gentle inistial thrust and a longer burn time !!

[barnsley-mark]

I also tried something similar - rocket motors (bought or made) have a high initial thrust tapering off towards the end.
To push a car or boat etc you are wanting the reverse.
And any slower burning motor will tend to burn the side of the tube before you have all the useful thrust out of it.
Nozzle size and shape is very important, a rocket tooling spindle is a very good investment and a must for consistency

[pyrotrev]

As well as the above try adding 2...3% finely powdered iron oxide as a catalyst - does wonders for burn rate 

[Sparky]

I've quite a lot of experience with Estes rockets and rocket powered planes / cars. As most have correctly pointed out the motors you are thinking of will invariably be far too powerful and a short impulse. However you are tethering the car but I still think it will still be a problem. Either make sure your car is pretty big (heavy) or you use very small motors / low power drivers. If you get the power to weight ratio right it will actually look a lot better too.

 

I personally think you should try and end burner with a very under oxidised BP green mix or sugar mix. You'll get masses of smoke plus a long slow burn. I'd also use paper tubes for it and you could make your own with craft paper and wallpaper paste, buy kitty litter to make clay nozzles with a not too large aperture. I may even go and video one I have made if I get time tonight/tomorrow. I used 3/4 inch ID tubes about 5 inches long. I think a 3 mm nozzle and I use a screened green mix BP of about ratio 50 KNO3 35 C 15 S. I get a burn of about 4 or 5 seconds and if I use them as a rocket it lifts it's own weight and a small header gently and slowly into the sky but they go surprisingly high.

[pyrotrev]

Thinking about it, this would be an ideal application for the old Jetex reloadable motor!

[Vic]

Thinking about it, this would be an ideal application for the old Jetex reloadable motor!

 

I used Jetex motors on cars made of balsa wood, had them tethered to a fixed line to run in a circle.

Not a lot of thrust from these motors, so light car and nice smooth surface to run on. I chose the school hall to do the test run when nobody was about, started off nice and slow but after about the third lap the speed was so great it pulled over the lab support stand and went flying in to the wall, the place was full of smoke. I was never found out!

Edited by Vic, 17 March 2013 - 05:16 PM.