5' PVC Missile: Scratch/Building a High Power Rocket: Part 8? Delay assembly and Motor Test

[1crackit]

if i put a larger amount of r candy like say 250 grams how much will it burn and will the pvc melt

[rocketmanbkk]

@1crackit : The pvc will soften as it is heated up. If it melted you would not have a working motor. If you use more fuel you have to make sure your casing can handle the chamber pressure. The easiest way to do this is to enlarge your nozzle throat. To large a nozzle and you have too little chamber pressure (a big smoke bomb vs a rocket motor)

[2012goingNutz]

great idea.

[456ner]

how do you make those igniters? ive tried everything to get my core burning motor to light.

[rocketmanbkk]

@456ner : Video no. 6 in the series shows how I make the igniter.

[oliwic]

Nice idea for the delay grain/deployment system, but did you know that if you add some bicarbonate of soda to the delay grain mix it slows the combustion of the delay grain, and also try adding some powdered dye (rit dye) to the fuel/delay mix as it colors the smoke and is good for tracking smoke (orange and blue work best).

[rocketmanbkk]

@oliwic : can't get the dye in Thailand but baking soda is no problem. Thanks for the reminder. Great thing with this design is I can literally make the delay grain as long as I want.

[AHGlasher]

Nice idea for the deployment system, and it may be too soon to ask this, but it didn't seem to be powerful enough to deploy unless your foward velosity is near zero and pressure on the nose cone is reduced as well. I'll continue to watch and see.

[rocketmanbkk]

@AHGlasher : I test every system before launching. There is more than enough pressure to push the parachute out. I don't know how to link these videos so sometimes they are hard to find. The last video in this series is:

"How to Build a High Power Rocket: Final Video-The Launch"

[rickrapp1]

we have been trying to make the fuel using Spectracide Stump Remover that is supposed to be KNO3, but the burn rate is very slow at the mixture % that you use. I dont think its pure KNO3. Any suggestions on where to buy KNO3? What do you use?

[rocketmanbkk]

@rickrapp1 : Just curious, what is your burn rate at 1 linear inch? Spectracide Stump Remover is 99% or more PURE KNO3. You should have no problem using that. Try a small batch 50 grams or so with just that and sugar and light it and see how it does. Do a 65/35 mix. Then do one with the corn syrup and see how much it slows down.

[shooternash]

Can you please explain further how to increase your "burn" time? As how to make it last longer, essentially increasing the height of the launch. Thanks.

[rocketmanbkk]

@shooternash : If you want more altitude (thrust) you can add more fuel. But you don't want to over pressurize. You will have to increase nozzle throat size accordingly. But this won't increase burn time. Adding a surfactant will increase burn time but also decrease ISP in some cases. If your motor is an end-burner, burn time will be determined by grain length. If it's a core burner, burn time is determined by web thickness, thus you need a larger diameter motor.

[mabsahmed786]

I have a similar question to one before....hope u can answer.

I notice that u put the ignitor right up towards the delay grain, I would have thought that the propellant burns from the bottom up(ie putting the ignitor at the bottom most grain) so that it reaches the delay grain at the end......

[rocketmanbkk]

@mabsahmed786 : You are confusing an end-burner (used in most pyrotechnics: fireworks, black powder rockets, etc.) with a core burner which most high power rockets use. Ignite the grain near the nozzle end and the motor doesn't power up to full thrust before gasses exit the nozzle. All the grains aren't lit before maximum pressure is built up. Light it from the top and

all grains and surface areas ignite before gasses are expended out the nozzle.

[1000martijns]

the fuel burns not so long kan you do som on that

[rocketmanbkk]

@1000martijns : The outer diameter of the grain is less than 1.5 inches. The current burn time is already around 2 seconds. Standard r-candy burns even quicker. It would be difficult to slow the burn rate more and maintain a high ISP.

[MastaMag]

I like the chute deplyment system.

[asimov13647]

Question: what keeps the ignitor from firing your ejector charge? Suggestion: use PVC fuel cells that screw together instead of gluing them together. For examle, the nozzle section could screw into the fuel assemply allowing you to test different nozzle configurations.

[rocketmanbkk]

The electronic system has worked well. I've yet to actually use the delay system in a launch.

[Damong1321]

Between your vids and Richard's info I've almost completed my 1st project. Looking forward to your chut build video (hope you have one in mind). I'm going with sorbitol instead of sucrose in a solid grain H class. Don't won't to push the burst rate. US 1/2 sch40 is rated 370psi.

Any unfortunate motor explosions?

Kick A videos! Keep'em comming.

[levip1611]

cool design but the fuel test seemed weak did you just have one section of the r-candy or what happened to it shouldn't it take like 5-8sec each amounting in like around 30sec burn time?

all looked cool though man

[rocketmanbkk]

That was a very strong motor. Not max pressure but very good. The internal pressure was around 450-500 psi. PVC can take about 800 psi safely. It would have put our rocket up a couple thousand feet no problem.

[rocketmanbkk]

No, No, burn time is based on web thickness. The distance from the core to the outer diameter of the Inhibited ( meaning the outside walls of the grain sticks to the casting tube) Bates grain. The burn time is determined by the thickness of the web ( the distance from the core to the grain wall) because the propellent burns from the inside out.

[rocketmanbkk]

This propellent has a diameter of about 1.375 inches. With a 10mm core that makes the web about only 1/2 inch. It only takes 2-3 seconds to burn that 1/2 inch under pressure. Compare that with the huge size of the Shuttle solid fuel boosters which are also core burners. Long burn times. The bigger the diameter the grain, the longer the burn time.

[bguffer]

I like these videos. Hope you keep producing them.

[AirrowRocket]

Very good but I'm lost on your deployment system and the grain didn't seem to burn hot enough or long enough.