Cluster/Large Rockets

[Engineer Kelly]

Im building a 3 motor rocket. The diamter is around 1.5 inches, the entire model is around 16 inches long. Im wondering if the three motors ejection charges will be strong enough to pop the nose and parachute. I thought about using stuffer tubes. Is there a mathematical way of determining this?

Also, how do the large models i see eject their very large recovery devices and noses?

[barone]

Yep.....you've got plenty there for recovery system deployment.......and welcome to the forum.....

[Engineer Kelly]

I was looking at your profile and noticed your location. I'm in northeast MS. Corinth to be precise.

[CPMcGraw]

...Im building a 3 motor rocket. The diamter is around 1.5 inches, the entire model is around 16 inches long. Im wondering if the three engines ejection charge will be strong enough to pop the nose and parachute...

Welcome to the Asylum! You sound like you're well on your way to belonging here... :D

I'm going to make a number of suggestions, some of which have become part of our FAQs (or should be, if they're not). From a personal POV, these have helped me get more out of the hobby.

1. Go get "The Book". Which book?

The Handbook Of Model Rocketry, Seventh Edition, by G. Harry Stine and Bill Stine. (http://www.amazon.com/Handbook-Model-Rocketry-NAR-Official/dp/0471472425/ref=sr_1_1?ie=UTF8&s=books&qid=1205637627&sr=1-1)


2. Read "The Book", cover to cover.

3. Put these URLs in your browser "Favorites" section:

A. Ye Olde Rocket Shoppe (http://www.rocketshoppe.com/)
B. Ye Olde Rocket Plans (http://www.oldrocketplans.com/)
C. JimZ (http://www.dars.org/jimz/rp00.htm)
D. The Ninfinger Files (http://www.ninfinger.org/~sven/rockets/rockets.html)
E. SEMROC (http://www.semroc.com/)
F. Balsa Machining Service (http://www.balsamachining.com/)
G. BARCLONE (http://barclone.rocketshoppe.com/)
H. Apogee Rockets (http://www.apogeerockets.com/)

4. Go to JimZ, look for the Estes Cobra (K-10) and Estes Ranger (K-6), and download the plans.

5. Go to YORP, look for the "Custom Rockets" section, and download the Landviper (#10001).

6. Study these three plan sets and use them to get an operational idea of what you can do with your design.

7. Go to Apogee Rockets and download the DEMO of RockSim. Play with it. Learn how to combine parts to create a flyable design. Get comfortable with it. Then order the full version ($100). Best C-Note you'll ever spend in rocketry, short of a few Semrocs that is... :rolleyes:

8. Get to know the technicals of rocketry while having fun with the flying and building of models. If you don't have fun doing this, why bother?

9. Build models as often as you can. Get familiar with the way the better models are assembled. Read through our various building threads here on YORF and try out some of our methods for construction and finishing.

10. Don't be afraid to lose a model. Someone who has never lost a rocket has never flown a rocket.

11. Go drool over at Ninfinger. Get inspired. Imagine a world where nearly everything in those catalogs is available again. Look through nearly 50 years of collected history through the pages of catalogs and other printed documents from the Founding Fathers of this hobby. Now realize that you don't have to imagine! Through the fine offerings from several YORF members, nearly every design you see is either offered in kit form today, or the parts are available separately. You can build nearly every kit found in those pages today!

Enjoy!

[tbzep]

A tweaked version of the above post should be stickied. :)

[Engineer Kelly]

Very nice! Thanks a bunch! I actually purchased the handbook a while back and have been studying it for about a month. I have only flown a few rockets but have designed several along with calculating CP with Barrowmans method on a few. I have used ALTPRED and SpaceCAD a little but haven't tried RockSIM yet. I will definitely give it a try.

Thank you again!

[barone]

I was looking at your profile and noticed your location. I'm in northeast MS. Corinth to be precise.
Well, you are kinda close. Maybe you can join DaveR, tbzep and me for a launch sometime. We've just been talking so far but I think we all want to get together at least once and put some in the air. Maybe one of those second Saturdays at Shelby Farms ;)

[ghrocketman]

A single motor has more than enough "ejection charge" to eject the 'chute from a 1.5" diameter 16" long rocket without a stuffer tube.

[Doug Sams]

A single motor has more than enough "ejection charge" to eject the 'chute from a 1.5" diameter 16" long rocket without a stuffer tube.I gotta disagree (with respect :) )

There's a reason the Big Bertha comes with a stuffer tube. And even with a stuffer, I've had too many core samples with my old Bertha because the charge was only enough to pop the nosecone without getting the laundry out.

In four separate cases, the nosecone came off but the chute seemed to just move out of the way while the ejection gasses went by :( After the fourth coupler was used to knit it back together, it started looking a little like one of Jim Flis' A.C.M.E. Spitfires.

While many current motors - especially those with nuke charges - will indeed do as you state, there are still plenty of motors out there which won't. Many of the motors in my stash (working stash, not collection) predate the nuke era, and are thus marginal in large tubes. Furthermore, not every recent vintage motor can be counted on to have the oomph.

In short, to improve the odds of success, it's better to plan for less than adequate ejection charge and thus either use a stuffer or a cluster. In the case of a 1.5" dia tube 16" long, I'd make the stuffer long enough to reduce the chute space to ~6" aft of the nosecone. (That is, make the stuffer = 16 - 6 - NC-base-len )

As they say in HPR, blow it out or blow it up.


Doug




.

[ghrocketman]

In response, I will say that a stuffer tube can almost never hurt (unless someone is foolish enough to use a BT20 or BT5 instead of the normal BT-50 in LPR as the stuffer tube in a BT60 when using 24mm power :rolleyes: )

If a body tube of BT60 or larger is longer than one 18" tube-length, I always use a stuffer as well, especially if using single 18mm power.
Most all of my BT60 and larger rockets are 24mm + engine sized (or 18mm clusters) so
I never really worry about stuffer tubes.
I don't think I have ever had a 24mm or larger engine (BP, APCP, SU & RMS) fail to eject the laundry since I started flying in 1978, but I may be incredibly lucky.

[barone]

Im building a 3 motor rocket. The diamter is around 1.5 inches, the entire model is around 16 inches long. Im wondering if the three motors ejection charges will be strong enough to pop the nose and parachute. I thought about using stuffer tubes. Is there a mathematical way of determining this?

Also, how do the large models i see eject their very large recovery devices and noses?
You gitting enough info here Kelly?

Bottom line, your three motor cluster should provide adequate deployment of your recovery system.

A stuffer tube (maybe three in your application) might be nice to keep your recovery system as far foward as possible. Three motors will provide a lot of thrust (assuming 18mm here) and your recovery system may tend to fall rearward. Keeping it forward does two things for you....

1. Recovery system has less distance to travel to deploy, thus increasing the chances of a successful deployment (what if something happens and only one motor ignites?)

2. Keeps your CG from shifting rearward with the recovery system. With a three motor cluster, you may be just stable (depending on your fin size and nose weight) and the recovery system shifting rearward might be enough to make you unstable.

As for a mathematical way of determining the amount of pressure needed....sounds like a great research project. :D No one (that I've heard about at least) has done an analysis of ejection charge to volume of pressurization study. It would be interesting to see how much volume can be pressurized by an ejection charge and result in successful deployment of a recovery system.

As for large rockets and their recovery system deployment.....Are you talking like high power rockets......over 2.5" diameter and really long? These usually have an altimeter to fire a secondary ejection charge. The motor may eject a drouge at apogee (just to slow the descend so it doesn't come in ballistic). The altimeter then triggers an ejection charge at something like 400' to deploy it's main chute. With the altitudes these rockets reach, deployment of the main at apogee usually results in a long walk.