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EchoVictor
05-07-2010, 01:07 PM
Hey all,

I've been working on a project (http://www.oldrocketforum.com/showthread.php?t=6641), and as I'm doing the swing-testing and adding nose weight, I'm getting really close to 113g but not yet stable. According to the NAR website (http://www.nar.org/SandT/pdf/Estes/C6.pdf), that 113g is the mass that Estes has recommended as the Max Liftoff Weight for B and C motors.

My question is, how critical is that Max Weight? If I go over by a few grams (115~120g total weight), will I be OK? I would have to imagine that there's some sort of safety margin built into that 113g spec.

Thanks,
EV

nvrocketeer
05-07-2010, 01:09 PM
Hey all,

My question is, how critical is that Max Weight? If I go over by a few grams (115~120g total weight), will I be OK? I would have to imagine that there's some sort of safety margin built into that 113g spec.

Thanks,
EV

The max recommended weight is related to speed as it leaves the launch rod. If you use a longer rod, the extra quarter-ounce or so should pose no threat.

(Bob waits to be shot down by his betters for speaking out of turn. :-) )

tbzep
05-07-2010, 01:28 PM
It has about as much to do with the coast phase as it does liftoff velocity. Check the C6-3 vs C6-7. Their thrust curves are the same and they have the capability to lift the same amount, yet the C6-7 is rated for lighter rockets due to the delay. The extra weight of the longer delay grain is negligible compared to the 2.5 oz vs 4.0 oz liftoff weight between the two motors. You have to consider both when choosing your motors. :)

tbzep
05-07-2010, 01:37 PM
BTW, I can't see the pics in your thread here at work, but I recall it's going to be pretty draggy. That would make me worry a little more about the max weight of the rocket because you might end up with a really late deployment. I'll try to remember to look at the pics again this weekend.

If you have a bunch of guys in your club that fly oddrocs, they can probably look at it and hold it in their hand and give you a pretty accurate prediction of the flight profile on a C6-3. They will probably be a lot more accurate than you could calculate with RocSim.

jamjammer53150
05-09-2010, 06:38 AM
C's and B's seem to have the same lifting ability ( off the line) and c's more or less just have a 2x longer burn time , that being said . B's are lighter ... and since they have to lift themselves , technically b's can out lift c's in the same rocket.

Or just do what i would do , punch a few 1/2 inch holes and add a few a10's :) they will coax it off of the ground

dlazarus6660
05-09-2010, 06:52 AM
EV,

You have those extra tubes in the wings and the extra body support tube you can put motors into. :confused:
Would but something to see launch! :eek:
Just thinkin' out loud again. :rolleyes: :D

tbzep
05-09-2010, 09:33 AM
C's and B's seem to have the same lifting ability ( off the line) and c's more or less just have a 2x longer burn time , that being said . B's are lighter ... and since they have to lift themselves , technically b's can out lift c's in the same rocket.


Yes, but try launching a heavy rocket that does ok on a C6-3 with a B6-2 and you may end up with a core sample.

jamjammer53150
05-09-2010, 09:57 AM
I was simply speaking of lift off , ....... I actually did that miscalc a few weeks ago loaded up bt80 red max clone with 3 b-6's only 2 light and it archrd at around 30 ft

It did clear the rod nicley however

GregGleason
05-09-2010, 08:28 PM
I don't know if you have thought of this, but you can explore the use of the 18mm AeroTech SU or RMS motors.

Greg

ghrocketman
05-10-2010, 02:35 PM
3 D21T SU 18mm Aerotech motors in a 3x18mm cluster mount BT80 rocket should fly really well.

Rocketflyer
05-10-2010, 02:53 PM
Actually, the D10's may be well suited for this bird. The D21's are good, but hard hitting, kinda like the ol' B14's

jamjammer53150
05-10-2010, 06:01 PM
I saw some old estes manuals , and the showed a cored grain , with a star shaped core , was that the b14's?

Doug Sams
05-11-2010, 02:33 AM
I saw some old estes manuals , and the showed a cored grain , with a star shaped core , was that the b14's?Cored grain, yes. Star shape? Haven't seen that before.

I know the shuttle SRBs have one starred grain at the top, but haven't seen any star-cored BP motors.

Doug

.

ghrocketman
05-11-2010, 10:38 AM
The B14 originally had a drilled core-burning grain.
Later it was changed to where the BP was pressed around a pintle that formed the core.
NONE of them had a star grain, and no BP engines I know of were produced with a star grain.
Some APCP motors have used this grain geometry in the past, but am not aware of any that do now.

I LIKE the hard-hit of the D21 and MUCH prefer it over the lame-o thrust curve of the D10.
If one was going for overall altitiude in a min-diameter 18mm rocket, the D10 is the way to go. Otherwise I prefer the D21, even in my Mars Lander.

Shreadvector
05-11-2010, 10:59 AM
The Aerotech D10 is absolutely perfect in the stock built Venus Probe.

EchoVictor
05-11-2010, 01:30 PM
Well,

I kinda answered my own question in how I'm going to go forward on this one;

"Dang the torpedos. Full speed ahead!"

For comparison, I decided to grab a few of my earlier builds that I fly exclusively on 18mm power. The heaviest one I found is my Sunward Galactic Wave (on the left);

http://i73.photobucket.com/albums/i204/EchoVictor/Rocket%20Stuff/100_0460.jpg

Total mass of this bird WITHOUT the motor is 118g. Adding a C6-3 takes it up to 132g, well over the recommended 113g limit. I've flown this bird at least a dozen times, and never had any issue with it getting off the pad or achieving altitude. I'm pretty sure my Whitestar at 115~120g total mass with motor will be just fine.

Later,
EV

Bill
05-11-2010, 07:54 PM
Total mass of this bird WITHOUT the motor is 118g. Adding a C6-3 takes it up to 132g, well over the recommended 113g limit. I've flown this bird at least a dozen times, and never had any issue with it getting off the pad or achieving altitude. I'm pretty sure my Whitestar at 115~120g total mass with motor will be just fine.



Two main factors go into the maximum weight recommendation which makes it sometimes confusing.

The first and most important is to guarantee sufficient airspeed at the end of the launch guide. This depends mostly on model mass and only a little on drag or wind speed.

The second is to prevent a late deployment. This one depends heavily upon a number of factors other than model mass, and arguably should be left out of the calculation.

As it is now, the max weight should be treated as a rule of thumb for newbies. Experts should verify that peak thrust is at least five times the weight at liftoff, higher for a shorter guide or a windy day and less is allowable for a longer guide on a calm one. Choice of a delay is an equally important but separate decision.

Some examples:

Motors of the same family have a lower maximum weight the longer the delay. This is a direct reflection of the second factor. They all have the same capability to accelerate the model safely off the pad.

Looking in the printed 2010 Estes catalog (yes, it does exist), the B6-2 has a max weight of 4.5 ounces while a C6-3 or C6-5 has 4.0. The C6 has a higher peak thrust. This is a direct result of the short delay of the B6-2.


Bill