How reliable/accurate are Estes max weight numbers?

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byoungblood

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I've built an upscale (BT-70/60) of an Estes Echostar that has come out to about 10.7 oz with motor, sans final paint.

According to Estes motor charts, it is a tad too heavy for a D12-5, but should be fine on a D12-3, understandable given the weight of the rocket.

I plug it into ThrustCurve, and the smallest engine it recommends is a E20. I'll have to run RockSim again to make sure I have the final assembly weight correct, but it was showing about 40-41 ft/s off of my 4' rod with a D12.

So given the Estes chart and Rocksim I should be OK with a D12-3, but ThrustCurve says I need more motor. Should the Estes charts really be relied on as more than a general guideline, or are they a bit too optimistic?
 
Everything is a general guideline..... but in my experience, 11oz with low wind makes for a nice D12-3 flight.... just don't try an E9

5891731748_748de07e96.jpg
 
A 40ft/sec off of a 4' rod on an LPR is perfectly fine. I would definitely go with a D12-3 over a -5. You want your recovery package to deploy at apogee, not at a full speed descent. I think thrustcurve.org likes to see a faster speed off the guide, more in tune with HPR flying. I however caution you to do your first launch in calm wind conditions.


Jerome :)
 
Thanks guys. My LHS don't carry anything but Estes BP motors so I was going to have to place an order for anything bigger than an E9. I'm going to my first club launch next weekend and I was wanting to launch this rocket for the first time there.
 
The Estes recommendations can be used as a rough guideline, but often can be too optimistic. You need to take into account the rod length, and wind speed, and model stability. For example, I've launched a 12oz Estes Interceptor-E on an E9-4, which is within the recommended max liftoff weight, but only on a 5 foot, 1/4" rod, and only when there is absolutely no wind.

Lower speed off the rod, to a point, can be OK in low winds. The D12 has higher initial (and average) thrust, so really has a higher max liftoff weight than an E9 (the Estes Catalog numbers for the E9 are too high, IMHO). All D12 motors have the same ability to get a rocket to a safe speed off the rod, but Estes lists a D12-3 with a higher liftoff weight than a D12-5, due to the (rough) estimate that a heavier rocket will stop coasting sooner. The catalog numbers also ignore drag and wind speed. Higher winds and/or higher drag (a BT80 model has much higher drag than a BT50 model) require a lower delay number, due to reduced altitude.
 
As long as you keep it at 11 oz or under you should be fine, in light winds. In not so light winds or greater than 11 oz, you will need more punch than the D12 can provide or gets risky.

Just make sure that your chute is packed for quick opening (do not do the "wrap the suspension lines around the chute" thing). You will want the chute loose in the tube and close to the nose cone.

Greg
 
Thanks guys. My LHS don't carry anything but Estes BP motors so I was going to have to place an order for anything bigger than an E9. I'm going to my first club launch next weekend and I was wanting to launch this rocket for the first time there.


If you are going to fly at a club launch, be aware that the Range Safety Officer (RSO) might not let you launch your rocket if you exceed the max lift-off weight of the motor you present during check-in. This is for your safety, the safety of spectators, to give your rocket the best chance of survival and to prevent fire danger (an ejection charge on the ground is not good). Just be prepared to follow the club's safety rules.

Clubs are a great place to see lots of cool launches and meet great people!


Jerome :)
 
Is this the Echostar, byoungblood ?

echostar.jpg

If so then not only do you have a potential total mass issue but you have some additional draggy bits that tend to catch air and can effect the entire flight profile. So flying on a D12 at the upper limit of that motors performance envelope might not be the best idea, especially for a first flight.

Do you have access to an E12-4 ?
 
approximate delay for apogee height.png

This simple chart is a correlation of the optimum motor delay time for a given apogee.

delay time in seconds = sqrt (apogee in feet / 16) within the range of the table.

It uses the gravity drop time of a given altitude to predict the optimum motor delay.

This simple approximation is good for most rockets within a second or so.

Don't believe it? Check it out with a sim.

Bob
 
This simple approximation is good for most rockets within a second or so.

Don't believe it? Check it out with a sim.

Bob

I don't believe it. I checked it out with many sims.

I don't understand how or why you would use this approximation. Expected max altitude is needed to look up the proper delay. To get the expected altitude, you need to run a sim. If you have a sim, then the sim would immediately tell you the optimal delay. So, why the need for the table?

See my typical rocket simulations below. Your table is in error more like 3 sec on average, not 1 sec . At higher altitudes, the error is 5 seconds, which will lead to dangerously late deployments and damaging zippers.

Capture.JPG
 
I don't believe it. I checked it out with many sims.

I don't understand how or why you would use this approximation. Expected max altitude is needed to look up the proper delay. To get the expected altitude, you need to run a sim. If you have a sim, then the sim would immediately tell you the optimal delay. So, why the need for the table?

See my typical rocket simulations below. Your table is in error more like 3 sec on average, not 1 sec . At higher altitudes, the error is 5 seconds, which will lead to dangerously late deployments and damaging zippers.

View attachment 188282
The table was addressed to the original poster for Estes type model rockets, not for high power rocket and high power motors which most of your cases are, and it was based on a number of sims from thrustcurve.

You also should be aware that for any rocket motor, the minimum allowable delay error is +/- 1.5 seconds or +/-20% which ever is greater to a maximum of +/-3 seconds, If your off by 2 seconds on your sims expectation, it's possible to be off by 5 seconds so you should plan on that possibility in any case when designing a recovery system for a high power rocket using motor ejection.

Bob
 
The table was addressed to the original poster for Estes type model rockets, not for high power rocket and high power motors which most of your cases are, and it was based on a number of sims from thrustcurve.

Bob

Umm, no. The table was addressed as "this simple approximation is good for most rockets." No mention of LPR vs. HPR. Whatever.

Your answer still doesn't explain why you take perfectly good simulations and make them less accurate by forcing them into a regression look up table.

I don't understand why rocketeers go to great lengths to make up convoluted, 1950s era approximations when a simple sim on a smartphone tells you everything you need and more.
 
Either way, he sounds like he's good to go unless his Paint weighs more than a couple of Ounces.
 
Is this the Echostar, byoungblood ?

View attachment 188196

If so then not only do you have a potential total mass issue but you have some additional draggy bits that tend to catch air and can effect the entire flight profile. So flying on a D12 at the upper limit of that motors performance envelope might not be the best idea, especially for a first flight.

Do you have access to an E12-4 ?

Yes, but I knocked it down to 3 fins on the upper and lower sections specifically for that reason. I slightly increased the wingspan of the upper and lower fins to help make up for only running 3 fins. I just finished the paint on it today and it came in at 9.6 oz dry, so it should be right at 11 oz. with the motor. Plus I'm using 1/4" lugs so I'm going to plan on using a 5-6' rod with it too.

No, I wish I did. I was talking to one of the hobby shops day before last about trying to diversify their motor selection somewhat. They only carry the E9, if you want anything bigger from them you've got to move up to 29mm motors. I built it so I could run E and F motors in it for lofting a camera and altimeter in the future in the upper body tube.
 
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Happy to say I had a successful flight on the D12-3 today. It weather cocked very slightly off the rod (club had a 4.5' 1/4" rod), but continued straight from there, and deployed almost right at apogee. It landed about 50' downwind from the pad right on the roof of someone's car! :D I estimated the altitude at about 250'.

I'll post some pictures in the LPR section later today.
 
Congrats on the successful Flight. I love the D12s when they stay low and slow. Plenty of Smoke and Fire.
 
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