altitude prediction?

[shoprobav]

What kind of altitude should i get when i decide to fly my cloned version of the Astron Ranger?
I plan on using 3 C6-5 engines. This rocket will not have the payload section. Just a straight body tube. So i imagine it will be a little lighter then normal. I have a pretty big area to fly in so i'm not to concerned about losing it...although the rocket god's have been know to grab a rocket or two from me. It should be done in a couple of days. I will take pics on that magical day for everyone to see.

 

[Micromeister]

Shoprobva:
That will depend on what kind of a brick you built. How well you filled and finished the model and how well you match the motors. If your model is well finished (no spiral wrap line and 0 grain on the fins, baby's butt smooth finish, waxed before launch. You've weighed all your C6 motors and picked three with indentical or very near the same weight. Your ranger isn't much heavier then 80g empty (heavy for a bertha) i'd expect somewhere between 1000 and 1200 ft.

 

[Fore Check]

I'm intrigued by your comment about weighing the engines in the cluster to check for a more "perfect match."

I've never heard of that before. I just grab 2 or 3, preferably from the same pack.

 

[Micromeister]

Fore check:
I'm sure you are aware of the 10% tolerance built into all model rocket motors. If not let me tell you that is quite a difference. Most of the time much of this margin is in the delay mixture, but also in the propellant grain as well. If you weigh your motors you'll usually find a good bit of range between like motors. In some motors this number can be several grams. When getting ready for a cluster altitude meet I will buy as many as 8 or 10 packs of the motors to be used. In a 6xC6 clu/alt event I went thru 18 3 packs of C6-0's matching as many of the heaviest booster motors as possibles to come up with 3 matched sets of 5 boosters and the heaviest C6-7 I could find. You never know how many flights it's going to take to get a closed track, or tracked at all for that matter. This one worked out very well. Tracked and closed on the first flight at 666m (2,185ft) a national record at the time
If using say 3 C6-5 motors in a model, you will always hear Pop, Pop, Pop at ejection. Most of this is the difference in the delay time of the motors, a tiny amount is due to difference in ignition time and the rest in extra propellant burn time. does this help, I think is does especially with booster motors as the heavier the motor means more propellant in those motors. "the racers Edge", possibly enough to make the difference between a nice flight and a national record
I'm not competing anymore because of a back and knee problem but I'm happy to pass along some of the BTC tricks of the trade.
You will need a good gram scale or triple beam balance to take full advantage of this process. If using a digital scale make sure it will tare out to 0g and read accurately to at least .01grams
this process works for all competitions regardless of power class, but is most helpful in PD, SD, HD, Payload, all altitude and of coarse cluster/atlitude events.
Hope this helps

 

[powderburner]

You can also buy a bunch of motors, like Micromister said, and (after sorting by weight) check for production batch variations. (You are going to have to burn a few of your stockpile on a thrust stand.)

There can be small differences in thrust and total impulse, even among motors that fall closely into the same weight range. This gets into minor variations in propellant chemistry, freshness, compaction, shipping and storage effects since manufacture, and other influences on motor performance. You can even get a little thrust variation due to batch-to-batch nozzle quality/accuracy of dimensions, although ALL of these effects are getting down into the last one or two percent of performance variation.

You would do better to spend your time getting a good surface finish on the rocket itself, or doing a little research into better ways to pack 'chutes and streamers to open rapidly after ejection.

 

[Fore Check]

I'm with you guys now. For competition, I can see this makes sense.

For general sport flying of clusters, it seems like overkill. I've had fine luck just using motors from the same pack for sport flying.

 

[powderburner]

Yes, I completely forgot to explain that you only go through this pain and agony for competition.

For sport flying, you can grab any old combo. You can even fly a cluster with an A-B-C collection----and have a lot of fun watching the RSO's face.

 

[Peter Alway]

Unless your Ranger is espcially heavy--epoxy fillets, three coats of automotive paint, and the like, I would expect that C6-7's mght be a better choice. A one-engine Bertha gets plenty of altitude for a 5-second delay, and I think a 7-second delay would be plenty here.