Engine Mount Length

the model rocket show
eggtimer rocketry
Onebadhawk
The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
J

JKeeper

Active Member
Joined
Dec 28, 2009
Messages
29
Reaction score
0
Hey All

I plan to attempt my first clone, a Big Bertha, and have noticed that the Bertha comes with what appears to be a 9 inch motor mount. I plan to launch on 18m motors and was just wondering if there was a reason for the extended mtm, and if it is necessary.

Oh, and i was also wondering, what is the best material to make centering rings for a rocket such as a Big Bertha? I was planning on using 1/8 Inch Balsa wood.

Thanks for all of your help!!!:)
 
An extended motor mount reduced the volume of the cavity in the larger body tube, making ejection more reliable. There is less area to pressurize for the ejection charge that way and ensure ejection.

Also, a longer engine mount will help reinforce the main Body tube, not that the Big Bertha needs it.
 
JKeeper said:
Hey All

I plan to attempt my first clone, a Big Bertha, and have noticed that the Bertha comes with what appears to be a 9 inch motor mount. I plan to launch on 18m motors and was just wondering if there was a reason for the extended mtm, and if it is necessary.

Oh, and i was also wondering, what is the best material to make centering rings for a rocket such as a Big Bertha? I was planning on using 1/8 Inch Balsa wood.

Thanks for all of your help!!!:)
Click to expand...


Laminate two or three layers of cerial box type cardboard together with white glue, scuff up the glossy side of the cardboard with rough sandpaper so the white (or yellow) glue can soak into the cardboard. It will be more then strong enough. You could use three ply lite plywood, available at www.balsamachining.com. They also sell premade centering rings.
A single layer of balsa will probly crack along the grain.
 
Last edited:
While I have not made any cardstock centering rings, I have made and used plenty of balsa ones.

I used to use a compass and an xacto knife, but have had good luck using and held 'circle-cutters' I got this one at Walmart.

Just remember to try some practice pieces, and cut the larger part of the CR first. Ask me how I know...:eek:
 
When your on www.balsamachining.com 's home page, select the title "Kits & Building Supplies". Then scroll down to Centering Rings. Fin stock (balsa, ply and basswood) is near the bottom of the same page. Most things you'll need are on that page.
Balsa Machining has a flat shipping rate of $6.00. I would get as many scratch building items on one order as possible to save on shipping fees.
 
Last edited:
JKeeper said:
...and have noticed that the Bertha comes with what appears to be a 9 inch motor mount. I plan to launch on 18m motors and was just wondering if there was a reason for the extended mtm, and if it is necessary.
Click to expand...
As STRMan indicated, the motor tube forms a stuffer tube to reduce the volume. You need this on a Big Bertha. I might even make it longer than 9", maybe ~12". I've seen too many times where the ejection charge wasn't strong enough on a stock Bertha. Reducing the volume will make the single ejection charge more effective.

Doug

.
 
Standard centering rings aren't worth making yourself; just buy a bunch from the supplier of your choice. I like the fiber ones I get from BRS hobbies but any will do just fine. Otherwise the advice you got above is excellent.

I have made such rings out of the thinnest light aircraft ply I could find(couple bucks for a big sheet at hobby lobby or micheals). I cut the outer circles with a pair of shears, then drilled the inner circles and used a dremel sander tool to expand the inner hole to the right size and polish the outer circle. After doing this, I then placed an order for lots of rings so I would never need to do it again...

Marc
 
Thanks for all of your replys! I was planning to just get by with a standard engine mount lenght but i think its worth it to keep it at about 10 inchs. As for Centering Rings, Im not sure exactly what i will do, I think i may just order some!

One more question, I know the Big Bertha is 2 feet from tip to nozzle, But i havn't been able to find anything about the actual body tube lenght, where could i find this info??? And would it be benificial for me to lengthen or shorten the body tube in anyway, as some people on Der Red Maxs??
 
Marc_G said:
... After doing this, I then placed an order for lots of rings so I would never need to do it again...
Click to expand...

I laughed when I read this! :roll: I feel the exact same way. There are lots of folks that enjoy making their own rings. I'm not one of them. :(

Since I use standard size tubes for airframes and motor mounts, I just purchased a quantity of rings to fit the tubes I use. For about $20 I have enough rings to last a long, loooong time. :clap:

*** The Modern Big Bertha uses one full piece of standard Estes BT-60 18" long ***
 
Last edited:
JKeeper said:
Thanks for all of your replys! I was planning to just get by with a standard engine mount lenght but i think its worth it to keep it at about 10 inchs. (snip) ...i havn't been able to find anything about the actual body tube lenght, where could i find this info???
Click to expand...
The stock length is 18" - ie, an uncut Estes BT-60 tube.

With a 10" stuffer tube, you'll still have about 7" of BT-60 to pressurize at ejection. That's way plenty for your chute, wadding, shock cord, etc. So if you wanted to make the stuffer a bit longer, you'll still have ample space for the recovery system.

Have you downloaded some plans for this kit? They are available on the web.
https://plans.rocketshoppe.com/estes/estK-23/estK-23.htm

Doug

.
 
scsager said:
I laughed when I read this! :roll: I feel the exact same way. There are lots of folks that enjoy making their own rings. I'm not one of them. :(

Since I use standard size tubes for airframes and motor mounts, I just purchased a quantity of rings to fit the tubes I use. For about $20 I have enough rings to last a long, loooong time. :clap:
Click to expand...
Amen!

I have a very good stash. But it mainly supports Estes tube sizes. With Semroc's vast offering of Centuri sizes, it gets much more complicated, in which case I sometimes cadge something up. Similarly, for the occasional Quest mod, or a complex cluster pattern, I'll do likewise. And if it's 2am and I'm against the deadline, I'll get out the fly cutter.

Otherwise, I buy them from one of the many affordable vendors.

I have also bought custom cut rings from Bill @ BMS. I draw them up in Autocad, and, if I get more than a couple rings, the setup charges are quickly amortized.

Doug

.
 
JKeeper said:
One more question, I know the Big Bertha is 2 feet from tip to nozzle, But i havn't been able to find anything about the actual body tube lenght, where could i find this info??? And would it be benificial for me to lengthen or shorten the body tube in anyway, as some people on Der Red Maxs??
Click to expand...


www.rocketshoppe.com

Estes Body Tube/Kit Reference List

K-23/1223 Big Bertha BT-60 18.00" Motor tube BT-20B 8.65"



And would it be benificial for me to lengthen or shorten the body tube in anyway, as some people on Der Red Maxs??

I can't see how it would benifit you in any way. :rolleyes:
 
Last edited:
plano-doug said:
Otherwise, I buy them from one of the many affordable vendors.
Click to expand...

This usually involves Doug seeing me at lunch once a week, giving me a sly sideways glance and saying "hey Don, if you have any scrap (insert material type) and I send you a file, could you bring me some rings next week?"
:neener:
 
BMS has bulk packs of MM rings cheap. I like the black fiber ones. They are strong and Elmers wood glue works good with them.
 
JKeeper said:
And would it be benificial for me to lengthen or shorten the body tube in anyway, as some people on Der Red Maxs??
Click to expand...

If you want to, go right ahead. That is the reason I buy my tube from BMS, they are 34" long!

It is difficult for me to leave any kit 'stock'. I either use longer BT's, or larger motor mounts (13 to 18, 18 to 24, 24 to 29mm...), different fins, etc. Who wants a rocket that looks like everyone elses?

A Big Bertha would be cool stretched, with basswood fins and a 29mm motor mount.

You could use an adapter to still use 24mm, and when you really want it to rip, pop in the 29mm. Wheeee!

Do what you like, and dare to be different. ;)

BTW, my stretched Mini Max really rips on 18mm power! :D
 
At one point, BMS offered a BB clone kit that featured a 34" long body tube in addition to the 24mm motor mount.

Unfortunately I didn't get in on that deal.
 
Thanks Again! I plan to make a big order of body tubes and other miscellaneous parts from BMS, but WOW noes cones are expensive!:jaw:
 
JKeeper said:
Thanks Again! I plan to make a big order of body tubes and other miscellaneous parts from BMS, but WOW noes cones are expensive!:jaw:
Click to expand...


Go to www.semroc.com and select 'All Categories' on the home page. Give their nose cones, body tubes and other scratch building parts a good going over. Orders over $10.00 ship free.
 
SecretSquirrel said:
This usually involves Doug seeing me at lunch once a week, giving me a sly sideways glance and saying "hey Don, if you have any scrap (insert material type) and I send you a file, could you bring me some rings next week?"
:neener:
Click to expand...

Hey Don, if you have any scrap (insert type of material) and I send you a file.....
;)
 
JKeeper said:
Hey All

I plan to attempt my first clone, a Big Bertha...

I'm curious why do you want to clone it? Now upscaled I'm right there with you. Just curious.

You've gotten some good advice I would add lose the plastic chute and get a light nylon chute; you'll pay less too! Also, if you are serious about being a "future space scientist", then you need RockSim, dude! Hint: Xmas is coming!

One small correction, Bertha is 24" from nc to end of fins.

I have great affection for the Baby Bertha. I had one for 5 yrs and then the nc didn't kick and she came in hot. The cone was shattered, but all I had to do to get her back in the air was cut off an inch of bt and add a small piece of bt. Well a few more flights later, it did it again, so I did again too. And, later, it land sharked and I rebuilt her again. On the test flight she seemed sluggish on a C6 (she had gotten longer with each repair), so I added 4-13 mm mmts to the base of the fins and added another stick of pipe (bt) and now she is 41" with a 5 motor cluster! Unfortunately, the season ended before I could take her for a test flight.

Good luck with your project!

Tsolo L2
NAR 82124
TRA 10359
Click to expand...
 
Cluster King said:
JKeeper said:
Hey All

I plan to attempt my first clone, a Big Bertha...

I'm curious why do you want to clone it? Now upscaled I'm right there with you. Just curious.

You've gotten some good advice I would add lose the plastic chute and get a light nylon chute; you'll pay less too! Also, if you are serious about being a "future space scientist", then you need RockSim, dude! Hint: Xmas is coming!

One small correction, Bertha is 24" from nc to end of fins.

I have great affection for the Baby Bertha. I had one for 5 yrs and then the nc didn't kick and she came in hot. The cone was shattered, but all I had to do to get her back in the air was cut off an inch of bt and add a small piece of bt. Well a few more flights later, it did it again, so I did again too. And, later, it land sharked and I rebuilt her again. On the test flight she seemed sluggish on a C6 (she had gotten longer with each repair), so I added 4-13 mm mmts to the base of the fins and added another stick of pipe (bt) and now she is 41" with a 5 motor cluster! Unfortunately, the season ended before I could take her for a test flight.

Good luck with your project!

Tsolo L2
NAR 82124
TRA 10359
Click to expand...

Thanks for the encouragement! I am really just cloning the big bertha for the fun of it! And Cloning is a lot cheaper then buying the $25 kit plus i get to practice more building skills!
Click to expand...
 
Ok Guys! My Plans have changed a little! Im still going for a Big Bertha Clone, but i think im going to install a 24mm Mount instead! But BMS seems to be the only supplyer that sells a "big Bertha" Noes cone. I was wondering, what type of noes cone is that of a big bertha? Measurements would be helpful:) Thanks!!!
 
JKeeper said:
I was wondering, what type of noes cone is that of a big bertha? Measurements would be helpful:) Thanks!!!
Click to expand...
From BMS, the BNC60MS, @ 2.5" long, is equivalent to the current, stock PNC60 that Estes uses on the BB.

The BMS BNC60L, @ 3.125" long, is the same length as the classic cone, from the 1960's, used on the Big Bertha, Ranger, Farside-X, et al.

So, you have a choice. But either one will work just fine.

https://www.balsamachining.com/CONES.htm

Doug

.
 

Similar threads

B
stuffer tube / motor mount construction ?
Replies
4
Views
465
BABAR
B
J
Questions after an ‘almost’ perfect Boosted Bertha launch. W/video
Replies
12
Views
726
bjphoenix
B
Azamiryou
Motor mount design
Replies
13
Views
727
bjphoenix
B
ShakyShane99
Estes Murcury Redstone help.
Replies
11
Views
679
cwbullet
cwbullet
Wrightme43
Estes Corvette Class Build Questions
2
Replies
35
Views
2K
Wrightme43
Wrightme43

Latest posts

Back
Top