Max lift off weight for G64-7

[bcook7135]

I had read in a very old post that a G64 has a certain lift off weight that my rocket is very close to. I evidently broke a forum etiquette procedure in reviving an old thread. And the forum police rightly disciplined me. So here is a new post.
I have 2.5lb rocket I was planning to use a G64-7... Rocksim says it will be fine. But, now I am concerned.

 

[jmasterj]

Nah, no etiquette broken. It was just a really old post.

If you have the correct weight and CG in Rocksim, your rail length reflects reality, your rail exit speed is close to 50fps, and the optimal delay is within a couple seconds of 7, send it!

 

[jmasterj]

For what is worth, the weight sounds fine to me. Just make sure your sim matches reality.

 

[waltr]

I have launched my 3 pound LOC goblin of G64's with 4 second delay. Good flights.

Just ensure the delay is set.

 

[SolarYellow]

I had read in a very old post that a G64 has a certain lift off weight that my rocket is very close to. I evidently broke a forum etiquette procedure in reviving an old thread. And the forum police rightly disciplined me. So here is a new post.
I have 2.5lb rocket I was planning to use a G64-7... Rocksim says it will be fine. But, now I am concerned.

This is actually one of those places where ancient threads come back to life productively, and it is generally appreciated to keep on-topic discussion in them rather than having seventeen different threads all discussing the same thing over and over. And over. And over...

On another forum I participate in, there's a technical thread that was started in 2014 and has remained reasonably active all the way to the present time.

 

[Handeman]

I had read in a very old post that a G64 has a certain lift off weight that my rocket is very close to. I evidently broke a forum etiquette procedure in reviving an old thread. And the forum police rightly disciplined me. So here is a new post.
I have 2.5lb rocket I was planning to use a G64-7... Rocksim says it will be fine. But, now I am concerned.

That weight rocket on a G64-7 depends on the diameter. If it's less than 3" dia., send it. If it's over 3", I would drill the delay closer to 4 or 5 seconds.

In either case, the weight isn't the problem.

Those old max lift off weights were scaled to the delay time. For a 10 sec. delay, you needed a light rocket that would coast high enough to need the long delay time. As the weights increase, the altitude goes down and the delay has to get shorter so you still get deployment near apogee.

I have a 20 year old Callisto at 2.2 lbs. that I will fly all day on G64-7 motors or H128W drilled to about 10 sec, a H180W will use the full 14 sec delay.

 

[SolarYellow]

It depends on both weight and drag.

Lighter rockets can accelerate to high speed, but the smaller mass also slows down more quickly due to drag.

Heavier rockets don't accelerate as quickly, but the carry their speed longer against the forces of drag.

More drag reduces the peak velocity as well as increasing the loss of velocity, thereby shortening the optimum delay.

Less drag increases the peak velocity and reduces the rate of loss of velocity, thereby lengthening the optimum delay.

Build accurate models in your sim software and use it. Try a whole bunch of different launch parameters. Look at the effects of wind, etc.

OpenRocket is free. The only reason to not use it is laziness.

 

[OzHybrid]

G64 is a white lightning composition. If the motor is old, they can be a pain to light and may sit on the pad chuffing. During that time the delay may/will be burning the delay element. With a short delay, this will significantly affect your chance success if some of the delay has already been burned before it takes off.
For something with a short delay you may be better using a composition that's easier to light. Blue Thunder or Black Max.
For G64 reloads, I add a piece of BP cut from the solid BP core of a C-6 pushed in to the top of the top slot piece. If you have access to pyrodex, a bit of that would be work too. This gets the G64 lit quickly and prevents any on pad chuffing.

 

[bcook7135]

That weight rocket on a G64-7 depends on the diameter. If it's less than 3" dia., send it. If it's over 3", I would drill the delay closer to 4 or 5 seconds.

In either case, the weight isn't the problem.

Those old max lift off weights were scaled to the delay time. For a 10 sec. delay, you needed a light rocket that would coast high enough to need the long delay time. As the weights increase, the altitude goes down and the delay has to get shorter so you still get deployment near apogee.

I have a 20 year old Callisto at 2.2 lbs. that I will fly all day on G64-7 motors or H128W drilled to about 10 sec, a H180W will use the full 14 sec delay.

Diameter is 3.1”.

 

[bcook7135]

It depends on both weight and drag.

Lighter rockets can accelerate to high speed, but the smaller mass also slows down more quickly due to drag.

Heavier rockets don't accelerate as quickly, but the carry their speed longer against the forces of drag.

More drag reduces the peak velocity as well as increasing the loss of velocity, thereby shortening the optimum delay.

Less drag increases the peak velocity and reduces the rate of loss of velocity, thereby lengthening the optimum delay.

Build accurate models in your sim software and use it. Try a whole bunch of different launch parameters. Look at the effects of wind, etc.

OpenRocket is free. The only reason to not use it is laziness.

I use Rocksim and OpenRocket

 

[OzHybrid]

Going back to your original question "What's max lift off weight for G64" You need to look at the thrust curve.


You have a thrust of at least 90N for the first 0.5 seconds. This SHOULD be long enough to get your rocket to aerodynamic flying speed. Generally considered as 60 km/h at the point it leaves the rail. Thrust/weight ratio is generally accepted as 4:1 but preferred at 5:1 So removing the thrust required to counter the weight gives your rocket an acceleration of 3 to 4 G. This gives the difference in the thrust to weight ranges is 25% more for 4:1 to 5:1 but this gives an increase in the AVAILABLE acceleration of an extra 33% Acceleration goes from 3G to 4G. (The minimum in the Tripoli rules is T/W of 3:1. However at that level, you would need to demonstrate to the RSO that you had done your due diligence a LOT harder. And rightly so)
So 90/5=1.8Kg max take-off weight or 90/4=2.2Kg Both of those weights are into the HPR range. So a G64 will be fine for all MPR rockets(1.5Kg) Provided you've simmed it and can get to a safe altitude for parachute deployment, bearing in mind that motor ejection delay times are not written in stone. There's always some variation.
To see the thrust curves for the motors in OpenRocket, go to select a motor configuration, when that opens the default tab on top right is "filter motors" once you have selected the filters for the motors you want(if any) click the "Show details" tab and you'll get to see thrust curves for any motor you highlight.
Good luck.