Common BP engine dimensions - why are they always so long?

[Ferret]

The largest advertised consumer size is 8 oz, and you can buy that size in those 12 pack assortments. But you can buy larger rockets in smaller packs that are 1 lb motors. They don't actually say they are 1 lb motors on the package, but if you measure the inside tube size it is 3/4". And it's not a tube inside of a tube either, I found one of the rockets in my yard after it fired, and I could see through the tube down into the nozzle. I have had 8 oz and these 1lb motors, and the 1 pound rockets are definitely louder when they take off than the 8 oz. So i think the largest size you can buy is actually 1 lb.

[dagabu]

The largest advertised consumer size is 8 oz, and you can buy that size in those 12 pack assortments. But you can buy larger rockets in smaller packs that are 1 lb motors. They don't actually say they are 1 lb motors on the package, but if you measure the inside tube size it is 3/4". And it's not a tube inside of a tube either, I found one of the rockets in my yard after it fired, and I could see through the tube down into the nozzle. I have had 8 oz and these 1lb motors, and the 1 pound rockets are definitely louder when they take off than the 8 oz. So i think the largest size you can buy is actually 1 lb.

 

Did you happen to find the label? I would be interested in finding out what those were.

 

-dag

[nater]

Dag,

 

There are rockets available that appear to be genuine 3/4 motors, they have labels stating UN0336. Find me at PGI and we can discuss this further.

[dagabu]

Dag,

 

There are rockets available that appear to be genuine 3/4 motors, they have labels stating UN0336. Find me at PGI and we can discuss this further.

 

Interesting, 1.4g you say? If you are manufacturing, I will find you there.

 

-dag

[dangerousamateur]

What I meant is, I can do two rockets, one long and one shorter, with the same amount of BP, and also about the same burning surface. The shorter motor can have a bigger diameter and thus can also have a bigger burning surface.

 

And if the nozzle is to wide, one can use hotter BP.

 

 

So the longer motor will not necessaryly have more thrust.

Edited June 29, 2012 by dangerousamateur

[dagabu]

What I meant is, I can do two rockets, one long and one shorter, with the same amount of BP, and also about the same burning surface. The shorter motor can have a bigger diameter and thus can also have a bigger burning surface.

 

And if the nozzle is to wide, one can use hotter BP.

 

 

So the longer motor will not necessaryly have more thrust.

 

Now I get it!

 

Yes, if the same surface area is burning, the thrust will be the same. But only if the nozzle opening is the same and the ability to achieve linear flow is the same as well. In practice, a rocket motor with a larger cross section will require more thrust to overcome the additional drag and the burning gasses inside the motor casing will not flow out of the nozzle as fluidly as gasses with a linear flow (straight line) and will cause more turbulence causing the casing walls burn out faster.

 

You will find that burning chambers on larger motors will have a taper before the nozzle opening, this is the convergence and it allows the gasses to to be funneled down the nozzle walls and out of the nozzle opening. The larger the cross section of the convergence or the flatter the convergence, the more turbulence is created and thus a higher chamber pressure and reduced thrust. There are many books, papers and mathematical formulae that speak to the issue but most will only have a paragraph or two directly related to the convergence while they speak mountains to the divergence as this is where the greatest improvements can be found.

 

In the end, the difference may not be detectable in our rocket motors but would be huge in high impulse rocket motors like APCP.

 

I hope this helps some but by all means, build what you like and take video of your creations, we just love video!!!

 

-dag

[mabuse00]

I also prefer shorter motors, because of faster ramming and tube supply

 

One thing to consider is that when you compare your two motors is the alteration of the burning surface during the burntime.

 

The burning surface and thus the thrust increases until shortly before the cored part is finished(more or less, depending on erosion and spindle taper).

 

So your nozzle setup has to cover a range.

 

Now when you compare the short thick motor to the long slim motor you will see that the difference between the surface at the beginning and the surface at the end is greater with the shorter motor.

 

->Your nozzle setup has to cover a broader range, and will rather be to big at liftoff and to small in the end.

Or in other words, the longer engine can be made with a hotter setup.

 

 

At least in theory. I don't know if this plays a large role in practice.

[dangerousamateur]

So if I have the free choice, the very long motors are better.

Unfortunately mostly do not have the free choice, unless I go for the notorious PVC pipe...

 

Or do endburners, only my BP is to slow.

 

 

I've got a large supply of very high quality parallel wound tubes wich are 100mm long and have 22mm internal diameter - what do you guys think, is it possible to make decent Coreburners from them?

[Blackthumb]

Would like to try some of those.. If I were you, I would make custom tooling if you have an 'endless' supply. Those are almost 4" long with an ID of .87"...think they would make great, fast core burners. They are a bit longer and wider ID than the ones I regularly use.

 

MIne are 5/8 ID x 3" and I get estimated heights of 500' or better....depending on the payload.

 

[californiapyro]

If you are looking for a 1.3 rocket motor bigger than 8 oz, vulcan makes 1 lb and 3 lb rocket motors. They have metal nozzles, and the 3 lb (while the package says it will lift 150 gram) will take 500gr up to display height easily it is of note that the 3 lb is only 6" long

 

http://www.youtube.com/watch?v=iErfXgPnkRo

Edited July 7, 2012 by californiapyro

[dagabu]

Would like to try some of those.. If I were you, I would make custom tooling if you have an 'endless' supply. Those are almost 4" long with an ID of .87"...think they would make great, fast core burners. They are a bit longer and wider ID than the ones I regularly use.

 

MIne are 5/8 ID x 3" and I get estimated heights of 500' or better....depending on the payload.

 

The tubes I use are almost the same as yours, DA and they work very well using UT type tooling and a clay nozzle with medium BP. I flew a dozen of them this weekend with 1" crossettes and the onlookers were really jazzed the combination.

-dag

 

 

If you are looking for a 1.3 rocket motor bigger than 8 oz, vulcan makes 1 lb and 3 lb rocket motors. They have metal nozzles, and the 3 lb (while the package says it will lift 150 gram) will take 500gr up to display height easily it is of note that the 3 lb is only 6" long

 

http://www.youtube.com/watch?v=iErfXgPnkRo

 

Thanks Cali, now I have a name for them. I ran into a reseller this weekend that told me about them as well. They are pretty expensive though and homemade seems to be a lot cheaper. To have commercial motors would be very nice though.

 

-dag