Maximum Liftoff Weight

[ghost]

Where do I find the max. liftoff weight for motors. I've been searching (on Google, Aerotech's website and hobby stores) but can't find anything!
I need the max. liftoff weight of the AT F52-5 and Estes D12-0.

Thanks!

[barone]

Try this site...

http://wwwtest.nar.org/SandT/NARenglist.shtml

Then find the motor you want the info on and select it. Gives you all the data for the motor. MAX Lift off weight is provided in grams so if you need ounces you'll have to convert. I usually use this site for metric conversions.......yeah, my memory is REALLY short term.

http://www.worldwidemetric.com/metcal.htm

[Doug Sams]

Where do I find the max. liftoff weight for motors. I've been searching (on Google, Aerotech's website and hobby stores) but can't find anything!
I need the max. liftoff weight of the AT F52-5 and Estes D12-0.There are two kinds of max liftoff weight.

The first is the raw lifting power of the motor. Can the thrust of the motor get the rocket accelerated to sufficient flying speed before reaching the end of the rod? Normally, we would need to the know the length of the rod and what defines sufficient flying speed.

The shortcut version, neglecting these two items, is: The motor's average thrust (12N for the D12) should be 5 times the weight of the rocket: 12/5=2.4N or about 8.6 ounces. This method is a rough rule of thumb and not a rigorous analysis. Also, the delay number is not used in the raw lifting analysis. But, as long as you use something longer than a chop stick, this method should keep the rocket from falling over as it clears the rod on liftoff :)

The other kind of max liftoff calculation uses the delay. But is assumes a lot, and is likewise a rough rule of thumb. Once a rocket motor burns out, it has imparted so much speed on the rocket. At that point, drag and weight will determine the coasting time. The tables that list different MRLOWs for the different delays of a given motor assume a certain weight and drag for a rocket. That's a whole lot of assuming! So, while they try to give additional info beyond what's in the 5:1 rule, they also must do even more hand waving and are thus even less rigorous. Plus, some of the tables I've seen from Estes aren't consistent in their numbers. I've seen them show the same LOWs for lower impulse motors (at equal coast delays).

So, instead of using a table, I prefer to use a simulation tool. These will take into account weight, drag, rod length, delay, etc.

Rocksim is on my PC, but there are also a couple of free, more rudimentary tools out there. Sorry I don't have any links, but you can google wRASP and VCP. Either of these should help you come up with some estimates for these two motors. You could enter a generic rocket and play with the mass and drag numbers, then tabulate the results. You can also enter a rod length and see what the rod speeds are. The acceptable minimum should be in the 30-45 fps range.

HTH.

Doug

[DaveR]

When you see the max lift weight for a motor, would you include the weight of the motor toward the max lift weight or not?

Example: (using the KISS method)
Motor "z" has a max lift weight of 10 ozs.
Motor "z" weighs 2 ozs.
So, would the max rocket weight be 8 ozs. or 10 ozs.? :confused:

[ScaleNut]

the max liftoff weight would be the rocket in "ready to launch" mode (including motor )

[DaveR]

If I understand correctly, the letter on a motor refers to the total impulse of the motor and each letter has twice the impulse of the previous letter. So, if you cluster 2 "A" motors you have the total impulse of a "B". How does this affect the max lift wieght? It seems to me that it would double the lift weight but I don't have any experience with clustering. However, I do intend on building a cluster this Spring so any and all feedback would be most welcome.

Maybe off topic.....
If you were to cluster 2 or 3 "F" motors does that push you into HPR? Meaning, would you need to be certified to fly said rocket? Not that I intend to jump into clustering with 2 or 3 "F"s but was just wondering where you draw the proverbial line.

Rocketry Terminology 101
What is the correct terminololgy, rocket motor or rocket engine? :confused:

[barone]

If I understand correctly, the letter on a motor refers to the total impulse of the motor and each letter has twice the impulse of the previous letter. So, if you cluster 2 "A" motors you have the total impulse of a "B". How does this affect the max lift wieght? It seems to me that it would double the lift weight but I don't have any experience with clustering. However, I do intend on building a cluster this Spring so any and all feedback would be most welcome.

Maybe off topic.....
If you were to cluster 2 or 3 "F" motors does that push you into HPR? Meaning, would you need to be certified to fly said rocket? Not that I intend to jump into clustering with 2 or 3 "F"s but was just wondering where you draw the proverbial line.

Rocketry Terminology 101
What is the correct terminololgy, rocket motor or rocket engine? :confused:Hey Dave,

For clustering, a general rule of thumb is to have the max weight 1/3rd of the AVERAGE thrust. As for MR vs HPR, the general consensus is that up to 160 N-sec of total impulse is considered MR. Above that is considered HPR. Someone else would need to address the certification portion. My understanding is that the certification is required to purchase and use motors of impulses H and higher. Since you are using clustering to achieve the H impulse, I wouldn't think you need to be certified. But, then you get into the BATF realm of operation. If you have so many grams of propellant (I believe the magic number is 62.5 grams), a license is required (explosives license). :confused:

As for terminology......seems anything goes since we all know what we're talking about. However, the NAR motor listing is called "NAR Certified Motors".

[dwmzmm]

In my recent launch attempt of the PDRocketry Gemini Titan - 3, I was concerned about
the maximum liftoff weight of two B6-4's I planned to use for the first flight. When fully
loaded, with the chute, wadding, engines and clear plastic slipon fins, it came to exactly
6.25 oz. According to Estes engine data chart, one B6-4's maximum liftoff weight is 4 oz.
With two B6-4 it's 8 oz. More than enough to lift the fully loaded model.

In reality, only one B6-4 ignited and the model still made a "successful," albeit scarey
ballistic flight with the ejection charge going off just in time to allow for successful
recovery... :eek:

[Doug Sams]

If I understand correctly, the letter on a motor refers to the total impulse of the motor and each letter has twice the impulse of the previous letter. It's the impulse range. An A can be 1.26 to 2.5Ns - ie, baby A to full A.

So, if you cluster 2 "A" motors you have the total impulse of a "B". How does this affect the max lift wieght? It seems to me that it would double the lift weight but I don't have any experience with clustering. Yes, two A's is a B. Two baby A's could be as low as 2.51Ns - a baby B - while two full A's would be a full B, 5.0Ns.

But the key here is that not only does the impulse double, but so does the average thrust. Two A8's make a B16, two B6's a C12, etc. You get twice the lifting capability.

If you were to cluster 2 or 3 "F" motors does that push you into HPR? There are six ways to be high power. The three likely criteria here are rocket weight, propellant weight and combined total impulse.

A rocket with a 3-motor cluster of F's should weigh less than 3.3 pounds, but may easily be over a pound. If it's over a pound, you need FAA notification. But as long as it's under 3.3, it's not high power and no FAA waiver is required.

The propellant weight of three F's could indeed exceed 125 grams total. If so, you need a waiver, and it's a high power flight. Over 113 grams and you need FAA notification. OTOH, three baby F's such as the F39 would have a total propellant mass of 68.1g, well under the limits.

Lastly, three F's could be up to 240Ns combined impulse. You can go up to 320 (combined - ie, in a cluster) before it's considered high power.

(The other three criteria are a max impulse of 160Ns in any single motor, a max propellant mass of 62.5g in any single motor, and a max average thrust of 80N in any single motor (ie, F101's and G125's are HPR). )

Doug