Bundling motors together

[Sulphurstan]

HI everyone.

 

I want to bundle 3 similar motor together in order to get on of my heavier rocket take off, and fly higher.

 

I have 2 questions:

1) If total acceptable weight for ONE motor is 175 grams, will the total acceptable weight of the THREE be 175x3=525 grams?

2) I fairly well know the thrust curve, global impulse, average thrust etc of ONE Motor, How can I apply this to the THREE motor bundle?

 

-> Goal of all this is to compute the gloabl acceptable weight and the apogea (for setting my delay after motors burnout)

 

Thank you in advance if there are some answers about it!

[mabuse00]

 

1) If total acceptable weight for ONE motor is 175 grams, will the total acceptable weight of the THREE be 175x3=525 grams?

Since you can never ensure they all work perfectly simultaneously I would rather think of some factor like 0,8 or something, just to be sure. 400g or something.

 

 

I fairly well know the thrust curve, global impulse, average thrust etc of ONE Motor, How can I apply this to the THREE motor bundle?

Assuming you manage to ignite them perfectly simultaneously, just multiply. 3x the thrust, and thus 3x the impulse.

In practice there will be an overlap of course, depending on the quality of your motors, the ignition ect.

 

That's why you should never count on 3x the thrust.

Imagine the extreme worst case, one motor is allready burnt out before the other start, the thrust of the single motor might not even move the rocket from its pad, in this case it would be a total loss of 33% impulse.

Edited August 18, 2017 by mabuse00

[Sulphurstan]

Mabuse

 

Thank you for this reply. I was already planning to have a 0,75 or so factor on the end result (safe side).

I ll put all this in my excel simulation sheet, see if my plan of lifting a 200g shell in a reasonable height for long burning stars with 3 or 4 C6 motors is achievable or not.

[Arthur]

Most of your answers are given by the time accuracy of the ignition fusing. Once you get all the rockets lit at the same time within hundredths of a second, things should approximate to n times the thrust. Getting a long launch guide tube or rail should start it flying in the intended direction until it gets to speed. Clever use of one igniter and several quick match lengths comes to my mind.

[Sulphurstan]

Arthur. Yes QM firing is the plan :-) !

I also thought about quickmatching my visco fuses (they are 60 mm length, and very reliable in burning speed), but I prefer the QM entering into the nozzle, sounds safer to me in terms of having accurate timing...

 

Tom Rebenklau's videos show some rocket of double / triple motors.

I have to watch closely, if he uses QM entering inside the nozzle for the firing..?

[dagabu]

For the sake of timed ignition, I would use e-matches in direct contact with the fuel grain. You can also use quick match all tied together and ignited with 1 e-match or section of visco. I have done a few clusters and they are fun but the build time for three 1 pound rockets and the clustering, timing and creative sticking is eclipsed by the ability to use a single 3 pound whistle motor with a greater impulse.

 

They are fun to make and I think any serious rocketeer need to do it at least once.

[Sulphurstan]

Thanks Dagabu.

I wish I could build my own motors, but i have such an impossible neighborhood that I cannot test my stuff as I want...

Just imagine the catos and the neighbors asking and so on... nimby people..

 

So making motors, jee!! I would die in time before getting it right!

So I admit, I use commercial c6 and d9 motors to lift up my shells, so at least, when I test it (not at home, but somewhere else), there is something to see for the audience

 

But yes, it is kind of frustrating not to build my own motors..

Anyway, there is already so much job to get a reliable shell building "procedure", that I take this challenge first.

 

I'll probably launch my first 2 or 3 c6-bundle rocket in October, with a ring pattern in the shell, and will make it with qm tied to one visco, as you suggested

[stix]

Apart from clustering, the other method for multi-staging is one on top of the other. ie. the Saturn V.

 

As Dags suggested, for most pyrotechnic applications just make a bigger one. But it's indeed always good fun experimenting

 

You may want to test those C6 motors for consistency, I've noticed some considerable differences in the past with both the C6 and the D12. Perhaps there are some 'fake' Estes motors on the market. Similar total impulse but significantly different thrust profile.

[Baldor]

Multistaging is only good if you have enough power in the first stage to lift the payload and the rest of the stages decently, then you can achieve more height. If your bigger motor have difficulties taking off with your payload, only clustering will help.

[OldMarine]

Just for the Hell of it I'm going to bundle 8- ½" motors around a ¾" motor just to see what it will do. The ½" rockets will be BP+20% saly whistle and the ¾" will be 2 increments of saly with the same mix as the smaller ones with a Win39J delay before a ball heading with BP and Ti since I expect it to be high enough the salute would be a waste.

Edited August 24, 2017 by OldMarine

[Sulphurstan]

OM.

That's nice experimenting. If you can tell us the result of this flight once fired!

[stix]

Well OM, you certainly don't do things by halves.

 

If you can, a video would be great.

[OldMarine]

I'll video it. I have the ½" rockets pressed with 2 having passfires (redundancy) to light the ¾" center rocket. I decided against straight whistle in the center rocket and will instead use a whistle/BP mix of 60/40 since my ACME test shows the straight whistle will put it in the next county. I plan to put a completed rocket on the stand to see if it will do what I hope but I'm feeling pretty good about it.

[mikeee]

Patrick,

 

Bundling high power whistle fuel motors together can create issues to the adjoining motors.

Some of the other Pyro's I have talked to over the years have had similar problems when attempting this.

The frequency and sound emanating from each motor can cause the fuel grain in the adjacent motor

to fracture and cato. The only time I was successful doing this was when tempering the fuel in the adjacent

motors to limit the impact on the fuel grains.

[OldMarine]

Patrick,

 

Bundling high power whistle fuel motors together can create issues to the adjoining motors.

Some of the other Pyro's I have talked to over the years have had similar problems when attempting this.

The frequency and sound emanating from each motor can cause the fuel grain in the adjacent motor

to fracture and cato. The only time I was successful doing this was when tempering the fuel in the adjacent

motors to limit the impact on the fuel grains.

I'm using whistle tamed with BP. Put one on the stand tonight and it didn't go well. There was too much lag between the ½" rockets burning out and ignition of the ¾ one. It was just more than 1 second but I think it would be enough to allow the rocket to lay over a bit. I think I'll drill the delay back a bit on my passfire rockets so the ¾' fires while 6 of the ½" are still burning.

Nothing exploded so it was sort of a success!

Edited August 26, 2017 by OldMarine

[OldMarine]

Getting ready to try another test model with a different fusing method and a slightly altered setup after some advice from here and FW. I'm going to run QM to each of the ½" rockets from a common bucket containing a bit of BP and ignited with visco. I'm going to make sure the nozzle ends of the smaller outer motors are an inch or so below the nozzle of the ¾" motor to help prevent premature erockulation of the "main motor". I'll also cover my passfire fusing with foil tape for the same reason. Here's a mock-up of my latest iteration with my proposed stick attachments. Had to fatten my NEPT thick wall tube up with 7 turns of 70lb kraft to get a nice tight ring but it shouldn't affect anything:

[mikeee]

Patrick,

 

I have stacked 1 pound whistle motors on top of each other and used a little bit of BP on the

the top of the whistle fuel grain to help ignite the upper motor and eject the spent motor from

the 2nd stage motor. The top motor is secured to the rocket stick and the lower motors are

glued on with hot glue. As the 1st motor burns it heats up the hot glue and when the 2nd motor

ignites it ejects the spent motor from the rocket stick. These are a lot of fun to make and a short

piece of inner or outer cardboard tubes hold the motors together. I have gotten (3) motors to work

on one rocket stick on several occasions, two motors work 99% of the time.

[NeighborJ]

As a kid, it was commonplace to both cluster and stage estes rocket motors. They didn't always work properly however a club member showed us that if the staged motors were attached with a friction fit sleve and a small vent hole was drilled in that sleve, no additional help was needed to ignite the next stage, it served to prevent the burnt motor from ejecting from the rocket before the next stage lit. In this mannor we were able to successfully stage even clustered motors.

Occasionally I would see a few members j hook a piece of jetx fuse into subsequent stages as well for extra insurance. These rockets were an awesome sight, leaving a dense white column of smoke behind

[OldMarine]

Well, I learned something valuable today. ½" rockets get hot enough while burning to release hot glue! Put one together to put on the stand and didn't put the gum tape around the perimeter as I did before and upon ignition I had chasers flying all over. OOPS!

[Sulphurstan]

OM.

Regarding hot glue, I could only confirm :excl: .

That's why I now fix the stick to the motor tube:

- with hot glue

+ with 2 slings of clove hitch

+ the 2 clove hitches glued to the motor with cyanocrylate.

[OldMarine]

Yeah, I think I'll put several turns of hemp twine around the next one with a light paste wrap over that.

I wish I'd gotten video but I was too busy ducking and dodging.

[Sulphurstan]

First c6 bundle rocket, launch on sep 16, will try to make a video that day

[OldMarine]

I've ordered some pulpy ¾" ID tubes for my central rocket. The larger OD of these should allow me to better anchor the ½" rockets to it and each other. I'll have to hand ram these tubes because they won't handle pressing so I'll have to use just hot BP in them. I'm sticking with the BP+20% whistle in the smaller ones. I have enough of those pressed to make 5 more test models.