Motor Lift Capacity Questions

I got the below from the Estes site for 18mm or 24mm. Are these numbers fairly accurate? Would these numbers include the motor weight?

I'm just trying to figure the max rocket weight without motor, so I can be a little more comfortable building. I really don't want to go out of site, 300ft to 600ft maybe a little lower or higher, but I do like to see the whole flight. I like clusters and staging and want to eventually do clustered staging.

Are there any particular websites that teach more about this?

Thanks.

A8 85g
B4 113g
B6 127g
C6 113g
C11 113g
D12 226g
E9 283g
E12 340g
E30 600g

Yes the figures are the rocket ready to fly, Rocket, Recovery and Motor total weight. I am not sure about websites that may explain it better.

I can share what I know. In commercial motors we measure thrust in Newtons. Newtons are equivalent to pounds-force, with a conversion constant.

To lift anything, you need to apply enough upward force to overcome gravity. So, if the rocket weighs 85g, then you need to apply an upward force of 85g to hold the object static. Applying more than 85g of upward force lifts the rocket.

The important thing about a rocket, however, is not just life, but stability. I want the rocket going upward at a speed that allows the shape of the rocket to interact with the aerodynamic forces in such a way that the rocket goes straight up, rather than spin around.

I'm sure you're familiar with the CG:CP relationship. If the CG is at least one BT diameter forward of the CP, then the rocket should be stable. But the rocket is ONLY stable when the aerodynamic forces are active upon the rocket, and that doesn't happen until the rocket reaches an optimal speed.

With me so far?

Tests have shown that in optimal conditions, a ratio of 3:1 thrust-to-weight will achieve the optimal speed while the rocket is still on the rail. This is important, because when the rocket leaves the rod, it must be at or above the optimal speed. The rod creates stability while it is getting up to speed.

While 3:1 is good in optimal conditions, wind plays a major factor in stability, and the rocketry community has settled on more like a 5:1 ratio. So, for your 85g rocket, you need to apply 5 * 85 or 425g of thrust.

But, we measure our motors in Newtons, not pounds or grams. The equation for converting grams to Newtons. 1g force = 0.0098 N. So, to get 425g force, you need 425 * 0.0098 = 4.165 Newtons.

Here's the good news. There is indeed a web site that will calculate all of that for you. Go to https://thrustcurve.org . Click "Motor Guide". Enter your rocket's dimensions and click "Run". It will produce a list of acceptable motors for you. By rerunning the test with various dry weights, you can see how the motor selection changes for different weights.

The "dry weight" is the weight of everything EXCEPT the motor. The sim will add in the weight of each motor that it tests.

Go try it, and come back with questions.

scadaman29325 said:

I got the below from the Estes site for 18mm or 24mm. Are these numbers fairly accurate? Would these numbers include the motor weight?

I'm just trying to figure the max rocket weight without motor, so I can be a little more comfortable building. I really don't want to go out of site, 300ft to 600ft maybe a little lower or higher, but I do like to see the whole flight. I like clusters and staging and want to eventually do clustered staging.

Are there any particular websites that teach more about this?

Thanks.

A8 85g
B4 113g
B6 127g
C6 113g
C11 113g
D12 226g
E9 283g
E12 340g
E30 600g

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wouldnt max rocket weight change with the delay used?

Bat-mite said:

I can share what I know.

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Wow, Great! I think I knew a little, very little, but I really did li earn a lot from that post!

Thank you very much!

My idea is to see how big I can make a rocket that is within the motor selections that I want to use, 24mm. Which the biggest 24mm is an F, maybe even a G, depending on what I can find. Been looking at Cesaroni lately and AT. RMS seems a little over my budget but maybe I can swing that.

Time to branch out into more fun stuff and learn a lot.

tomsteve said:

wouldnt max rocket weight change with the delay used?

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Yes, max liftoff weight is dependent upon the delay used.

You might get some useful information from the Apogee Rockets website at https://www.apogeerockets.com/
They have an informative news letter, how-to guides and many instructional videos.

scadaman29325 said:

RMS seems a little over my budget but maybe I can swing that.

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CTI makes a 24mm case for G motor reloads and its not terribly expensive. $25 for the case and $13 for the closure; the same closure will work with all their 24mm cases. AT has the 24/60 case but only up to F motors for that. the 24/40 case does not have the same closures as the 24/60 case

tomsteve said:

wouldnt max rocket weight change with the delay used?

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usually I do it the other way around; change the delay to correspond with the weight of the rocket

The best solution is to use a sim program. Open Rocket is free.

This good stuff. I've got to do a lot of studying.

scadaman29325 said:

My idea is to see how big I can make a rocket that is within the motor selections that I want to use, 24mm

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I would recommend against flying right up to the limits of the motor capabilities. Any one little hitch has potential to make the flight less than ideal. And any combination of hitches could result in a dangerous flight.

rcktnut said:

The best solution is to use a sim program. Open Rocket is free.

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This is the best (and safest) way for you to evaluate different motor and rocket design combinations without opening your wallet.

rcktnut said:

Yes, max liftoff weight is dependent upon the delay used. View attachment 400257

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I am not convinced that this chart is reasonable.
The C12-4 will barely lift a competition egglofter, while a D16-4, with about 25% more total impulse and thrust, will lift as much as three motor cluster of C12-4s???

Alan15578 said:

I am not convinced that this chart is reasonable.
The C12-4 will barely lift a competition egglofter, while a D16-4, with about 25% more total impulse and thrust, will lift as much as three motor cluster of C12-4s???

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That is from a pack of motors, I just scanned it. I don't use any info regarding liftoff weight for any flight (use sims) but after your post it did look "off". I checked into it, Quest site, and the liftoff weight for a D16-4 is listed there at 128g. Information on packaging is wrong I guess. Somethings wrong somewhere!!

thrustcurve.org is a great resource for this. It has a basic simulation that will list motors that can safely be used to fly the rocket description (diameter, weight, Cd).

For example, I have a Super Neon XL, which the package says will fly on a D12 or E9, but thrustcurve.org says an E9 fails their safe flight criteria. It only gets up to 41fps on a 5' rod, but a D12 or E12 gets over 50fps on a 5' rod.

Oh, and IMHO, some of the Estes numbers are much too high for max liftoff weight. In ideal conditions, maybe, but we almost never fly in ideal conditions (zero wind, ideal rod with no friction and perfectly stiff w/o rod whip, full use of the rod length, etc).

dpower said:

thrustcurve.org is a great resource for this. It has a basic simulation that will list motors that can safely be used to fly the rocket description (diameter, weight, Cd).

For example, I have a Super Neon XL, which the package says will fly on a D12 or E9, but thrustcurve.org says an E9 fails their safe flight criteria. It only gets up to 41fps on a 5' rod, but a D12 or E12 gets over 50fps on a 5' rod.

Oh, and IMHO, some of the Estes numbers are much too high for max liftoff weight. In ideal conditions, maybe, but we almost never fly in ideal conditions (zero wind, ideal rod with no friction and perfectly stiff w/o rod whip, full use of the rod length, etc).

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Didn't read my post at all, huh?

Bat-mite said:

Didn't read my post at all, huh?

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Or at least didn't read your entire post. Guilty as charged!

But sometimes having a chart is handy.

Some of Scadaman's numbers are from long delay upper stage motors.

58pan said:

Some of Scadaman's numbers are from long delay upper stage motors.

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Hmmm, didn't notice that. Long delay = less lift?

scadaman29325 said:

Hmmm, didn't notice that. Long delay = less lift?

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No, just that the heavier the rocket, the shorter the delay usually needs to be for deployment near apogee.