Max weight on a Pro38 G69...?

according to the Pro38-XX Motor Reference, the 133G69-12A 62.5g "Classic" G69-12A ,the Max launch weight is 2.97 lbs.... so you are close..... shockie B)

that is actually a significant safety margin. The max launch weight I provided is calculated using the 5:1 rule, which is very conservative, especially since it is a regressive burn providing a bit more power off the rail.

see thrust curve at

I have heard of acceptable ratios as low as 3:1, but under ideal conditions (length of launch rod, zero wind, good stability).

IMO a 5:1 will be fine even in moderate wind ( < 15-20 mph is my guess at max weight )

what say ye merry gentlemen?

- iz

Matt, Rule of thumb Take number from motor designation and divide by 4.5. This gives you pounds. Then divide by 4 again. This gives the max safe liftoff weight.

69 / 4.5 = 15.333 15.333/4=3.8 You should be fine.

MLOW is typically 3:1 (a legal principal not a practical principal)

My recommended "range" is 3:1 to 5:1. 5:1 is FAST.

Anything higher is just rediculous!

I know. Been there, done that!

Jerry

snip

30:1 is really cool

(as long as it's not pointed at people, buildings or vehicles)

-- Eric Benner TRA # 8975 L2 NAR # 79398

Please cite a reference to 3:! MROW as a legal princple (or even a princpal).

But that hadly matters as you are not manufacturing certified motors. ;)

It will be fine on a Pro38, use a sim to get the delay set to the right time.

I've seen 5.5" Loc Mini-Mags flown on Pro38s all the time to great flights.

Art

I have been used as a "consultant" to generate MLOW tables for several manufacturers (AT, USR, IBCo, CD, RDS, others). It is a judgement call, but here are my GENERAL criteria:

Thrust times 2 seconds or less 3:1 Thrust times 2.01-9 sec 4:1 Thrust times 9+ sec 5:1

The problem arises in estimating the DIAMETER of the MODAL rocket to be flown with a particular motor. So I cheat a bit. I assume a smaller modal diameter for long delays and a larger modal diameter for shorter delays.

Jerry

PLEASE pardon the tech post.

NFPA 1127, 2002 Ed. (excerpted for editorial review)

4.9.1 The maximum liftoff weight of a high power rocket shall not exceed one-third (1?3) of the certified average thrust of the high power rocket motor(s) intended to be ignited at launch.

- iz

this man has a contribution to make ...

and makes it!

except to those who refuse it, which is their loss, IMO

tech post all you like, Jerry :)

- iz

Oh, thanks.

assuming : thrust times =< 2 s = hi thrust thrust times >2 9 s = low thrust what actual average thrust values correspond to the above ? most HPR are med to hi at least.... I would suspect your values would be different for LowMR.......

shockie B)

and by that I mean Pro38 G69s.

/ArtU

Since all MR is in catagory 1, it was an "art" to add catagories 2 and 3.

I did that.

Both the subject matter in the form of products, a magazine to promote it, and a dealer network to offer them.

The post facto analysis was from observations of the artwork and subject matter in use in actual practice. Field experience.

Jerry

Let's simplify: divide by 4.5 then divide by 4 reduces to divide by 18. And that gives you a number in pounds. Multiply by 16 to get back to ounces, and you're damn near the number you started with. So why not just call it even, and say ballpark a Gxx can lift about xx ounces.

I've also gone the other way since I usually work in metric: Multiply xx by

25 (20 if being a bit more conservative) and get an approximation to the GLM in grams.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Because most people do not think in ounces but KG, LB and STONES.

Jerry

Is anyone trying to stop him? Jerry is just as free as anyone else to make technical contributions to the hobby.

I agree.

Here is how you figure it out. A G69 creates an average thrust of

69newtons. Divide that by 4.45 to get the average thrust in pounds. So 69/4.45=15.5 So the G69 has an average thrust of 15.5 pounds. Divide the thrust in pounds by the thrust to weight ratio you plan to use. A 5 to 1 ratio is common. You can do less but a 5 to 1 is nice because it is fast enough to boost a rocket in wind. So 15.5/5=3.1 pounds.

Teach a man to fish......