Igniter Position

[Mohinish]

Hello Everyone!

I am new to model rocketry and completely do everything on my own.So,got struck at a point

Problem is "Does the position of Igniter really matter ?"

I mean where should we keep the igniter is it at the opening end of motor or else some what deep inside the motor?

Please help e and also tell the science behind it.

 

[OverTheTop]

The correct answer is to read the instructions for the motor you are using. The BP motors seem to have the igniter inserted just inside the nozzle. For the larger composite motors the instructions say to put the motor all the way into the motor usually, starting ignition at the head of the motor.

BP is quite easy to ignite so I think it doesn't matter where it begins to burn. With the current crop of Estes igniters they are more reliable if they are actually touching the side of the core.

Composite (ammonium perchlorate composite propellant, APCP, the same fuel the space shuttle boosters used) is more difficult to light, and the burn rate is exponentially proportional to the pressure. By lighting at the head the burn has a bit more back-pressure, due to viscosity effects in the long skinny core, so the burn builds pressure and progresses to the entire core quickly. Igniting at the nozzle end has almost no back pressure and the burn can even fizzle out.

There have been attempts to make solid motors that are throttleable, but working out a practical variable nozzle throat is not a trivial task.

 

[Mushtang]

Shove it all the way up as far as it will go. When the igniter starts the BP burning at the top the flame will ignite the rest of the inside surface of the hollow core. This will cause the BP to burn evenly and with the most pressure.

If you barely insert it and the BP ignites from the bottom it will still burn, but the non-burning powder will have to be ignited by the powder next to it that burns. It will be like a fuse which eventually burns from bottom to top. This will still happen very quickly compared to a fuse but still significantly slower than a jet of flame shooting down the hollow core from the top will.

A slower igniting/burning engine will burn longer, but produce much less thrust. In some cases it won't be enough thrust to get enough velocity for the fins to keep the rocket pointed up, and your rocket will spin after leaving the rod.

 

[Zeus-cat]

Based on his previous posts he is using homemade sugar motors. I have no idea where the igniters go in those motors.

 

[heada]

Based on his previous posts he is using homemade sugar motors. I have no idea where the igniters go in those motors.

In a BATES configuration, at the top just like APCP. You *can* do end-burning sugar based but its rare.

Ideally you'd want a BP pellet at the top of the top grain similar to the CTI process but there is a fine line between instant pressure versus over-pressure. A little too big or too fast and CATO.

 

[Bat-mite]

Question is: is it an end-burning geometry or a core-burning geometry. For core burning, ignite the top grain and allow the flame escaping through the nozzle to ignite the rest of the grains. For an end-burner, ignite the bottom, and then the flame moves upward as it consumes the propellant. An end-burner burns like a cigarette, but upside down.

 

[jqavins]

The answer is the same for all motors: shove it all the way in. For an end burner that's just clear of the nozzle. For a cored grain that's all the way to the top. For a semi-cored grain like the old B14 motors it's half way up. Doesn't matter; shove it up until it won't go up no more.

 

[Bat-mite]

The answer is the same for all motors: shove it all the way in. For an end burner that's just clear of the nozzle. For a cored grain that's all the way to the top. For a semi-cored grain like the old B14 motors it's half way up. Doesn't matter; shove it up until it won't go up no more.

Sometimes the simplest answer is the best.

 

[OverTheTop]

Interesting. Everyone says to "shove the igniter all the way in" for BP motors. Given the shape of the wires on the Estes igniters that places it just inside the nozzle and nowhere near the top. Something doesn't add up .

 

[heada]

Interesting. Everyone says to "shove the igniter all the way in" for BP motors. Given the shape of the wires on the Estes igniters that places it just inside the nozzle and nowhere near the top. Something doesn't add up .

What Estes BP motors are not end-burners? For an end-burning motor, just inside the nozzle *is* the top of the propellant that you have access to for placing an igniter.

 

[Mohinish]

Yeah I have been igniting it from end all these days and then it lifted up went over and landed everything's good but I can see the motor still burning for a few seconds like 10-15 seconds after it dropped down.(just throwing out some gas)

So,I was thinking I should start igniting from the top of Motor like SHOVE IT ALL THE WAY UP.
But,does this really change anything?
Please help me

 

[Mohinish]

By the way again this is a homemade sugar motor with sugar and kno3.
I don't have any commercial model rocket equipment of any company selling in my Country.So, this is the only way.

 

[heada]

Yes, it does make a difference in getting the motor started. It will start faster and easier when ignited from the top. Since you are making your own motors, build 2 and start them each way and compare how easy the start, how fast they start and how good the burn is.

 

[jqavins]

Can you give a more detailed description of the flight? It seems that you're saying you lift off OK, arc over, and come down still burning. You say "everything's good", but coming down while the engine is still burning is definitely NOT GOOD. An engine still burning when the rocket comes down can cause a fire on the ground or an injury. This is not a safe situation. This could be caused by your igniter placement, but I doubt that's the whole story. It could be the igniter placement in combination with something else, or it could be something else entirely.

Perhaps I have misunderstood you, and it may be that the situation is not as dangerous as I think. I hope that's the case. However, if your rockets' and motors' performance are the way it seems then I would like to help you improve until you can fly safely.

We can't discuss motor construction or chemistry in this part of the forum, and unfortunately you are not permitted to access the portion where we can. Therefore, no one here can post any advice for you in these areas. Still, there may be a way to deduce if there are other factors on which we can advise you, if you are able to provide us more information.

Would you please describe the flight, step by step? Does the rocket take off quickly or slowly? Does it begin immediately to arc over, or does it rise close to straight up for a time and then begin to arc over? Does it seem to still be accelerating when it arcs over? I assume you are using a parachute. What are you using for parachute deployment? Does it deploy correctly? If there is no parachute deployed, is rocket still accelerating when it hits the ground? And so on. The more information you can provide, the better.

Do you, by any chance, have access to a motor test stand? If you can generate a thrust vs. time graph then please do these steps: make two motors as nearly identical as you can; place one in the test stand, light it with the igniter as you've been doing until now, and record the data; place the other one in the stand with the igniter all the way up and take that data. If the thrust graphs indicate that there is a potential problem with the motor's construction or chemistry then I'm afraid we can't help you. What I hope is that someone can determine from these graphs combined with the detailed flight description if something else is at the root of the problem.

 

[Titan II]

By the way again this is a homemade sugar motor with sugar and kno3.
I don't have any commercial model rocket equipment of any company selling in my Country.So, this is the only way.

You may have already consulted these sites. If not, they are highly informative regarding your type of motors.

https://www.nakka-rocketry.net
https://www.jacobsrocketry.com

 

[Bat-mite]

Take a look at these two diagrams. The first is an Estes-style, end-burning, black powder motor.

Notice that "all the way up" is to the bottom of the yellow propellant area. The igniter head must be all the way up to the yellow. Many LPR flights end with igniter burnout because the igniter is not all the way up to the yellow.

Now, here is a composite, core burning motor.

Here, the igniter needs to go all the way up the core to the bottom of the delay charge.

It is accurate to say that for all motors, the igniter must go "all the way up;" however, based on whether the motor is a core-burner or end-burner, it looks very different.

 

[Mohinish]

Thanks for the reply to everyone.
First of all ,I am absolute beginner.In my country I dont even have access to Commercial Model Rocket Motor.So,you can imagine what else I have.I don't have launch stand,igniter system anything.I barely learned everything from internet and built it using PVC pipe and 3D printed rocket body.

So,I got succeeded to make it fly and my next target is to make it go further higher and deploy parachute.

Speaking of ignition after even rocket landed please watch this video and tell me your views.

https://drive.google.com/file/d/1AHKHURs1biR9EIqT6gcqRhqv8YpsQkEz/view?usp=sharing

Please do copy above link and watch the video

 

[Mohinish]

Also,by looking at the video please tell me about the second rocket launch(the one with body) how it is and how much height it would have reached and comments /suggestions are most welcomed!

This might seem very small for everyone who are in this form but this took a lot of time for me and I am really passionate to build more so please help me.

Thank you.

 

[heada]

It appears your thrust to weight ratio is too low. Normally we design for 5:1 ratio meaning if your rocket weighs 1kg, you should have 5kg of thrust. This is to provide enough velocity so that the fins and give stability and you have a good flight. Also, the second launch was of a rocket but the rocket fell apart when it landed. How are you attaching the fins?

I understand you don't have a test stand with load cells to get good data. To get an idea of the performance of your motor, you can use a bathroom or kitchen scale. Place the motor on the scale with the nozzle pointed up. Set up a camera to record the display of the scale and ignite the motor. Do not hold a camera by hand, use some other device so you can be away from the motor. You should be using an electrical process to start your motors, not fuse or flame as that is dangerous and you could be hurt. When you watch the video after, record the thrust that shows on the scale as often as you can and then you can create a graph of thrust over time.

 

[jqavins]

I don't have launch stand,igniter system anything.

That is what I expected. I asked because it would be helpful if, for instance, another hobbiest you know might have one. I didn't expect so, but it was worth a try.

The method that Aaron suggested would be very helpful. If you have a way of taking the video at high speed, such as a setting on you phone for it, that would help as well, especially for the first part of the burn. I understand you may not be able to do this. Whatever you can provide will be useful.

This might seem very small for everyone who are in this form...

Not at all! To have come as far as you have without any help is really quite impressive.

For many of us, helping others to succeed is a big part of the enjoyment we get from this hobby. Helping you to continue safely and to reach your goals is our pleasure.

 

[rmorales2169]

Hello Everyone!

I am new to model rocketry and completely do everything on my own.So,got struck at a point

Problem is "Does the position of Igniter really matter ?"

I mean where should we keep the igniter is it at the opening end of motor or else some what deep inside the motor?

Please help e and also tell the science behind it.

Hello there Mohinish,

Hello Everyone!

I am new to model rocketry and completely do everything on my own.So,got struck at a point

Problem is "Does the position of Igniter really matter ?"

I mean where should we keep the igniter is it at the opening end of motor or else some what deep inside the motor?

Please help e and also tell the science behind it.

Well hello there Mohinish,

Actually I’m also curious about where to place the igniter. I’ve made tests over tests regarding this and have seen al sorts of behaviors using the exact size motor with the exact APCP based propellants. Have placed the igniter just a bit over the nozzle, in the middle and way up.

I would have to actually have pressure sensors in all 3 positions to have a clearer idea.

Hello Everyone!

I am new to model rocketry and completely do everything on my own.So,got struck at a point

Problem is "Does the position of Igniter really matter ?"

I mean where should we keep the igniter is it at the opening end of motor or else some what deep inside the motor?

Please help e and also tell the science behind it.

Hello there,



Well I see most if not all recommend to place the igniter deep into the motor until it won’t go further no more. Yet I’ve made tests using the same size motor with the same size propellant grains (APCP) with the same core diameter, same nozzle and have just modified the igniters position in 3 different locations. Right before the exit, at the middle of the motors length and all the way above till it won’t go up no more.



My thoughts for right above the nozzle is that too much pressure is at the exit, since there is pressure and APCP BR increases with pressure the upper pressure can’t exit and you end up with a series of (big explosion like) bursts. Which is definitely not useful and is dangerous. In the middle it is just the same case.



By the way the propellant ignition towards the top will never advance as if it where a fuse or as if you where lighting a strip of propellant in open air. That’s way to slow. It travels upward much much faster because of the pressure.



Igniting it from the top will further increase the burn rate and a lot more pressure will be created and the more chances there will be for the motor to explode.



This of course depending on the specific mixture of the propellant, the type and size of the core, the motors length and the nozzle throat.



So from my point of view this is actually a very good question but has not got a definite answer because there are so many things depending on it.



But it certainly makes a difference where you place the igniter.



If you are using only commercial motors, then stick to the motors specific guidelines, but if you’re making your own propellant, be it KNO3 based AP based or KP based or whatever, we’ll then your in to a lot of math that doesn’t always result as predicted.



Good luck and yes I know I didn’t quite answer you’re main concern.