Public Missles AGM 256 Pit Bull

[Bill Gibson]

A friend of mine built one, launched it one time....and gave it to me! he did a beautiful job on it! whats a good 29mm motor to fly it with? It weighs 33 ounces. Ive ogt a one shot G80-4 Aerotech motor....would this be a good choice?? What might be some others? Don't want to lose it as he did such a great job on it! Many Thanks!

 

[Wallace]

https://www.nar.org/SandT/pdf/Estes/G80T.pdf

 

[Wallace]

That's the Estes version of their G80 that was made for them by Aerotech. May be slightly different so do a quick search just to be sure. To get from Newtons to Lbs. If I remember correctly it is 4.45 to one. Do some quick math and see if you'll have safe liftoff ratio and you're good to go..

 

[Wallace]

Give a man a fish and you feed him for a day. Teach a man where to find beer and he'll drink all day...Or something like that. Point being, now you know how to easily calculate any combo. Or you can just go straight to Thrust Curve.org and never learn a thing...Btw. I am mostly lazy enough to use it myself, but find it important to know how to operate without calculators and such.

 

[BSNW]

Hello Bill-
First. I am glad that you received such a nice rocket. If you are new to the hobby, I hope you find enjoyment in it and find folks who are willing to help as they did with me when I first got started. Keep asking questions, go to some launches and see how other folks do it. There is a lot to learn.

So the way I do it is take 33oz and divide by 16 to get pounds (the unit I like to work with). 33/16=2.06 lbs. You want a safe thrust to weight to launch. Usually it is 5:1 (more in windy conditions). So take the weight of your rocket and multiply by 5. 2lbs x 5 = 10 lbs. This means you want to find a motor that gives at least 10lbs of thrust for at least (IMO)...1 second.

Find the thrust curve of your motor. You can find it either on Thrust curve .org or from the motor manufacturer. Basically you look at the thrust over time graph.You can convert Newtons to pounds by taking Newtons and divide by 4.45. For example 275 newtons (average) is 275/4.45=61.79 lbs of thrust.

In your case, the G80 looks like it has (from Thrust curve) around 100 newtons over about 1.25 seconds. 100/4.45=22.47 lbs of thrust! (I got that from Thrust Curve.org).

Your rocket weighs 2.06 lbs so 22.47/2.06 = 10.9. This means your rocket will have a 10.9 to 1 thrust to weight. This is very good....it will really scoot. This is a good motor for more windy conditions.

Most people go with the average thrust...this is okay, but some motors have much higher (or sometimes LESS) thrust spikes at the beginning of the burn. So looking at the Thrust/Time curve/graph is more helpful.

I hope this helps. Have fun flying your rocket. IMO, a G40 is a great choice too. At 1 second...it has 12 lbs of thrust. So 12/2=6 this means you are still at 6:1 Thrust to weight.

Have a great time, welcome to the hobby and keep asking questions. Remember...EVERYBODY was new to this at one time.

Regards,
Andrew

 

[Jim Hinton]

Hi Bill;

I've been flying a Pit Bull for a couple of years now. It's had a few boo-boos, but it soldiers on today. PML rockets are quite durable and most incidents can be readily repaired. It will fly well on most 'G' motors with a 4 second delay. I would lean towards higher average thrust motors. The G80 should work well, I've had excellent results with the G 64 and G 76 reloads.

I usually fly this rocket on 'H' motors with a ten second delay. ( I have rarely been accused of leaving the motor out ) The H 210 R, H 220T, and H 250 G have all worked very nicely, ejection occurs about exactly at apogee. You will need to obtain an L1 certification in order to purchase H motors however.

PML kits use piston ejection and it is frequently necessary to reduce the ejection charge in order to protect the rocket. This isn't a real big deal with G motors, they are usually pretty close to the desired charge anyway. The stock ejection charges on H motors will require reduction however. You can get this information on the PML website. Enjoy the rocket, mine flies very nicely.

Jim