Single D or Double C's?

[Bruiser]

I follow Chris Michielssen's blog and he recently built a Semroc Centurion that has swappable engine mounts. One is for a single D engine and the other is uses double C engines.

I thought that is rather cool and started thinking about doing the same thing to a Big Bertha. I've never clustered before and want to give it a try.

I started wondering which version would go higher so I opened up the Estes catalog. Here is what I found:

C6-5 N-sec 10 Thrust 15.3newtons Duration 1.6 sec weight 25.8 grams

D12-5 N-sec 20 Thrust 32.9 newtons Duration 1.8 sec weight 43.1 grams

From this data it seems the D12-5 is more powerful and weighs less than two C6-5 engines. Am I correct in thinking the rocket will fly higher with the D12-5?

If so, considering better performance at a lower cost with the D12-5 why cluster with the C6-5 engines? Is there a "cluster" factor" to figure in?

BTW, I've decided on the Citation Patriot kit. Not only can I complete the requirements for my cluster badge but with all the decals, I think it will also qualify for that badge as well. Plus, they had one at Hobby Lobby

-Bob

 

[kuririn]

For a Big Bertha size rocket, three engine clusters are far more common than two.
The Ranger was designed by Vern Estes in 1962, and was a precursor to the Big Bertha. It was the first commercially available clustered rocket model. Semroc has a reproduction:https://www.erockets.biz/semroc-flying-model-rocket-kit-ranger-kv-83/
Here are the parts list and instructions:https://www.semroc.com/Documents/pdf/Ranger.pdf
When the original Ranger was introduced, 24mm C and D engines were non existent. So to power larger, heavier rockets clustering was the only answer.
Now days, with the multitude of larger engines available, clustering is done mainly for the "coolness" factor.
A misfire in one engine is less likely to result in a U turn power prang with three engines as opposed to two. Don't ask me how I know.
Special ignition techniques and hardware may be needed for clusters. Lots of info on that here and in technical reports on the net. I would NOT use stock Estes igniters. Third party or enhanced igniters would be my choice. But that's just me.

 

[Bruiser]

Well the Ranger and the instructions were interesting. It looks basically like a Big Bertha with a payload section and has the clustered engines. I saw some things I haven't seen before too:

1) Based on the photos in Chris's blog I didn't think 3 BT 20 tubes would fit in a BT60 tube. It looked like two barely fit.
2) It does not look like there is much "real estate" left on the centering rings to make the notch and have room to turn the engine mount and "lock it in" as in Chris's blog
3) The ejection baffle only has one baffle disk. I've only ever seen two disks with holes offset from each other before in that style.
4) Dual parachute where two sections come down separate from each other. Haven't run across that before.

So no-go on standard ignitors... I am going to do some more research on clustering. I have been wondering how you connect the igniters together. A search is in order

Thanks,
-Bob

 

[jadebox]

I follow Chris Michielssen's blog and he recently built a Semroc Centurion that has swappable engine mounts. One is for a single D engine and the other is uses double C engines.

I thought that is rather cool and started thinking about doing the same thing to a Big Bertha. I've never clustered before and want to give it a try.

I started wondering which version would go higher so I opened up the Estes catalog. Here is what I found:

C6-5 N-sec 10 Thrust 15.3newtons Duration 1.6 sec weight 25.8 grams

D12-5 N-sec 20 Thrust 32.9 newtons Duration 1.8 sec weight 43.1 grams

From this data it seems the D12-5 is more powerful and weighs less than two C6-5 engines. Am I correct in thinking the rocket will fly higher with the D12-5?

If so, considering better performance at a lower cost with the D12-5 why cluster with the C6-5 engines? Is there a "cluster" factor" to figure in?

BTW, I've decided on the Citation Patriot kit. Not only can I complete the requirements for my cluster badge but with all the decals, I think it will also qualify for that badge as well. Plus, they had one at Hobby Lobby

-Bob

You swapped the durations for the two motors. The C6 has the longer duration.

The two C6s actually have a little greater total impulse and longer duration than one D12:

https://www.rocketreviews.com/combined-motors-3082.html

Anyway, we don't always do things like clustering for performance reasons. Sometimes it's just fun to challenge ourselves and try something different.

 

[kuririn]

Cluster clip whips, while not absolutely necessary, may make hook up easier:
https://www.rocketarium.com/Launch-Equipment/Cluster
This technical report, while dated, still provides useful information:
https://www.spacemodeling.org/jimz/manuals/tr-6.pdf
One method of enhancing stock Estes ignitors for faster, i.e. simultaneous, ignition:
https://modelrocketbuilding.blogspot.com/2018/08/upgrading-estes-starter-igniters-part-1.html
https://modelrocketbuilding.blogspot.com/2018/08/estes-starter-igniter-upgrade-update.html
And of course, a 12 volt power source (I prefer an auto battery) with enough amperage to reliably ignite all motors.

 

[Scott_650]

Well the Ranger and the instructions were interesting. It looks basically like a Big Bertha with a payload section and has the clustered engines. I saw some things I haven't seen before too:

1) Based on the photos in Chris's blog I didn't think 3 BT 20 tubes would fit in a BT60 tube. It looked like two barely fit.
2) It does not look like there is much "real estate" left on the centering rings to make the notch and have room to turn the engine mount and "lock it in" as in Chris's blog
3) The ejection baffle only has one baffle disk. I've only ever seen two disks with holes offset from each other before in that style.
4) Dual parachute where two sections come down separate from each other. Haven't run across that before.

So no-go on standard ignitors... I am going to do some more research on clustering. I have been wondering how you connect the igniters together. A search is in order

Thanks,
-Bob

The method used on the original Ranger was filling the gaps around the motor tubes with glue soaked tissue paper. I think it works but since my first cluster rocket I built that way in the early 70s was never recovered (that happened a lot back then to 9-10 yo me) I can’t give you firsthand info

Two motor clusters are, oddly seeming at first glance, since on a three motor setup one failure to ignite doesn’t necessarily crash your rocket. Don’t let clustered rockets intimidate you - multiple flame/smoke streams are awesome!

 

[Flyfalcons]

Personally, I find C's to be just about perfect.

 

[Zeus-cat]

From the NAR data a C6 has 8.82 Newtons and a D12 has 16.84. So two C6 would have 17.64, less than 1 Newton more than the D12.

 

[SDramstad]

But the 2 Cs would weigh more then 1 D. Along with more weight from 2 motor mounts.

 

[Bruiser]

Ok, I think I have found a way to make the interchangeable mount a possibility with a cluster of three. Thank goodness this is a 3 fin rocket or it wouldn't work.

Three is better than two, right?

Thanks for all the help, I'll start a build thread sometime this week

Need pictures...

 

[kuririn]

Bob,
That's outstanding! On your own you came up with an interchangeable system very similar to the one that's used in some Semroc SLS kits. TTW and twist lock. Pure genius.
https://www.semroc.com/Documents/pdf/SLS Aero-Dart.pdf

 

[Bruiser]

Not really. I saw the system on the centurion then I saw the system on the hustler and worked from them. It is definately semroc inspired.

Bob

 

[BABAR]

For your cluster recommend either you get some black powder to dip your Estes starters in, find some old Quest igniters, or try the first fire micro igniters for a more reliable cluster ignition.

 

[Bruiser]

I will get some black powder to enhance the ignitors for the cluster. I know there are different sizes so I have to do some research to determine which one..

I am also going to get or make a harness that has hook ups for three ignitors.

I will also verify that the launch system used at the local filed has enough power for a cluster. I bet it will because they shoot off many different rockets but I'll ask just to cover the bases.

Thanks,
-Bob

 

[dpower]

Clusters are always cooler! Check out MJG technologies for BP starters that work great for clusters. I’ve used up to 7 in parallel.