Weak Estes motors?

[Thermo]

Hi All,

I did some searching on this forum a little while but didn't see anything recent on this topic, so apologies in advance if it's a "dead horse" issue.

Short question: Does Estes produce under-performing motors?

I saw an Estes thrust curve for a C6-5 from 1971, that showed an average thrust of 6 N. The overall thrust time was 1.5 s, as this is a 9 N*s motor.

I saw a more recent report showing an average of ~4.6 N for about 1.9 s. That seems about right...same total impulse but longer thrust duration.

My last 3 flights with a C6-5 were with the same 51.2 g rocket with a very small accelerometer onboard. These 3 flights had significantly different performance, with the first one having an average acceleration of 4.7 g's for 1.8 s, while the other two were at 3.6 g's for about 1.9 s.

But I can't blame the motors yet, as thrust is not the same thing as acceleration. In the latter two flights I had a fair component of wind, while the first one was straight up with little wind.

So the second flight was est. 10 mph of wind, and the third had to deal with about 15 mph of wind. I'm assuming the breeze was enough to lower the acceleration performance.

But...the worst acceleration of the bunch was flight number 2 where the wind was lighter than flight #3. This makes me wonder if one or two of these C6-5's were underperforming.

Otherwise I have to assume that all the variation was due to the wind.

 

[bobkrech]

When the T/W ratio is less than the windspeed in mph for mormal 3' rod, wiind will influence the rocket performance. I[n not that the motor is underperforming, the rocket is underpowered for the wind speed/rod length.

Bob

 

[Peter Olivola]

There is variation in motor performance. NAR certification defines what's acceptable. Since no rockets built to fly on a C motor are at optimal weight (the motors themselves put you over optimal) even small variations in motor performance can produce significant variations in flight.

 

[plano-doug]

So the second flight was est. 10 mph of wind, and the third had to deal with about 15 mph of wind. I'm assuming the breeze was enough to lower the acceleration performance.

Regarding the wind, two things occur due to wind which can consume lots of the impulse.

While the rocket is on the rod, the cross wind imparts some lateral force on the rocket which ultimately increases the drag of the lug on the rod. So, for that ride up the rod, extra fuel is consumed.

The other is the effective angle of attack for the rocket. Once it clears the rod, the cross wind causes the rocket to fly slightly off axis, crabbing its way skyward. This appreciably increases the drag.

So this added drag could be as big a factor as motor variation.

Doug

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[Thermo]

Thanks Bob, Peter and Doug!

Yep, the 2nd flight (worst of the 3) was with a pretty sticky rod...many launches that day from other club members. I was swabbing the rod best I could to reduce the palpable friction between the lug and the rod.

The third flight...better than the second, but still windy...was my own launch rod, nice and clean.

YET...my accelerometer recorded higher peak accelerations in both the high wind cases, which simply weren't seen at that level in flight #1.

And I take your point about engine variation having a big effect on a lightweight rocket (which this one is).

I'll keep an eye on this phenomenon and see how it goes. I have one more batch of C6-5's, probably 5-10 years old, that I got from a pal...let's see how these do.

Cheers-

 

[luke strawwalker]

Also anybody in the motor business will honestly tell you today's black powder they get from their suppliers for making motors isn't as good as in days gone by... like everything else, the quality has decreased (and I heard this DIRECTLY from an Estes employee at a national event).

There's always a percentage of variability that effects things... some motors burn a little 'hotter' and others a little 'weaker', some burn a fraction of a second longer or shorter, etc... and little things, especially when compounded together, can mean a lot...

I think you need a much larger data set than three flights/motors worth of acceleration data to draw any relevant conclusions... A three-axis accelerometer would tell you more about any 'lateral' acceleration that might be 'stealing impulse' brought on by wind or any other factors...

Later! OL JR