# Max liftoff weight (2)E.M. G35

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#### Bowhunter

##### Well-Known Member
What Is the Max Liftoff wieght for a cluster of 2 Elis Mountian G35-6's

#### lalligood

##### Well-Known Member
I don't think I would try that...

EM motors are often enough hard to light singularly. I like them though! ('specially in any Art Applewhite product with a 29mm MMT )

#### Chilly

##### Well-Known Member
Couldn't put a weight limit on it, but it's gotta be light. REAL light. One EM G-35 couldn't even make my LOC Weasel fly straight in a very light breeze.

#### r1dermon

##### Well-Known Member
use a GOOD igniters. magnelites seem to work extremely well, i'd try those.

#### bobkrech

##### Well-Known Member
A quick rule of thumb to conservatively calculate the max lift-off weight in pounds is to take the average thrust in Newtons and divide by 25 for ~5:1 thrust to weight ratio. (Actually 5.6:1 since the actual divisor should be 5 * 4.45 = 22.25 but 25 is easier to do in your head.)

On a windy day, you should be using a 10:1 ratio so you should divide by 50.

Clusters are trickier. For a cluster, to be conservative, it is best to be able to lift-off safely if only 1 engine lights, so for a 2 engine cluster of the same engine, divide the total average thrust by 50, 3 engines divide by, 75, n engines divide by n * 25. For large clusters this may be too conservative, so you may want to divide by (n-m) *25 where m is the probable number of misfires.

So for a dual cluster of G35's, I would recommend (35+35)/50 = 1.4 pounds.

If the clusters are made up of a combination of engines, it's even trickier. Smaller engines light quicker, so it is possible that the smaller engines will light before a bigger main engine. An example would be G or H engine clusters around a J. The smaller engines will light first, and the bigger engine might not light before the rocket lifts off. If it lights in the air, the rocket accelerates non-vertically the air. I've seen this happen and it's scary.

Each case is different, but a suggestion is to choose a main engine that is capable of lifting the rocket safely by itself, and to choose the smaller motors so that they won't launch the rocket without the main. Another solution is to have on-board pryo circuitry to light all the motors. It's very conservative, but it's safe.

Bob Krech

#### r1dermon

##### Well-Known Member
indeed, using good igniters is also a key to success. the more motors, the higher the probability that it will fail. dip some magnelites and make sure you have enough juice to flow through 2 igniters. you dont want one motor lighting and the other not lighting. thats usually not a very good thing.

#### n3tjm

##### Papa Elf
Originally posted by r1dermon
use a GOOD igniters. magnelites seem to work extremely well, i'd try those.
Even with Magnelites, I would not cluster them. They vary to much in ignition times, from 1 to 5 seconds. I have seen the magnelite ignite the delay train, and after a few seconds of the delay burning, the motor decided to start...

#### mike_bar

##### Well-Known Member
Originally posted by EMRR
The Maximum Liftoff Weight for the above is: 3.15 lbs (50.34 ozs) (1427.28 gms)
Is what I came up with with AVG thrust of 70 (by doubling the 35 in the G35)
Nick
You have not factored any margin of error for a cluster misfire.
Do you really think a thrust to weight ratio of 4.99:1 is safe for this cluster?
I suspect the focus on strong ignitors relates to preventing misfires. Is this enough margin?
Your calculators work properly for a single engine, say a 3.15 pound rocket on an Aerotech G80-T.
I would agree with your calculation if the rocket contained a single motor with an average thrust of 70 newtons, and flew on a calm day.
Otherwise, out comes the hardhat, and Heads Up

#### mike_bar

##### Well-Known Member
Originally posted by daveyfire
'nuff said.
Perfect example! Calculators and computer simulations are useful, they have their place, but common sense trumps all in the end.
PS- those Berts look too cool... I have a idea for an odd-roc, but I'm not letting the cat out of the bag until the season opener at CMASS.

#### artu

##### Well-Known Member
Originally posted by daveyfire
https://www.vatsaas.org/rtv/misc/ghs2004/bertrace2004.aspx

'nuff said.

I don't like Ellis motors. I've seen very few ever work 100% right... light properly, burn well, and eject on time.
Sorry to hear that Davey. I've personally flew at least 40 EM motors this year alone. I've seen at least 100 or more fly.

The QuickBurst ignitor seems to work for me the best.

Out of my 40, I had one nozzle blow out, trying a friend of mines home made super burst ignitor. It super burst all right, and burst my nozzle.

He gave one of those to a friend of mine to use on an AT G35, and it blew that nozzle as well.

I think the Ellis propellent is nice and slow burning. Love that I-134, I-69 , and J228.

an I-69 in a LOC Weasel, with tracker, was a great flight.
the ellis mountain E & F motors in Blue Ninjas are a hoot as well.

You just need a proper ignitor. If Bob would provide the QB in every motor, then I don't think many would have such a problem like they are having.

I don't think I would cluster slow burning motors of any brand.

Instead, airstart them while using a central motor with kick.

humm. my LOC ultimate, with 6 G35 outboards , humm....