Cluster impulse

I'm curious about how one would rate the combined impulse of a cluster of engines. Over the weekend, I launched a total of 5 cluster engine rockets, all with complete success (at least regarding ignition). So, would my Der Green Max - on three A8-5 motors - be roughly equivalent to having a B24-5 motor, or is there a different way of thinking about it? I don't really care one way or the other, I'm just curious. The thing was very quick off the pad with a short burn, which would be consistent with that sort of thinking. By the same token, would the two D12-7's launched in my Binary Star essentially be an E24-7? Again, the performance was similar to what that would indicate.

I would think that's how it works. here's some photos






more here:
https://www.flickr.com/photos/31757945@N05/sets/72157656406444451

Well, do the math!

Estes D12
Total Impulse: 16.84 N-s
Burn Time: 1.65s
Average Impulse: 10.21 N (making the motor a D10.21, not a D12)

Now, a cluster of 2:

Total Impulse: 33.68 N-s
Burn Time: 1.65s
Average Impulse: 20.41 N (making the cluster an E20.41)



edit: idk why I based the letter designation on average...idiot.

I guess that was the nature of the question - was it actually additive, or was there another factor involved? Looks like the additive assumption is valid.

Nice shots, Dave, BTW. You got all my cluster flights from yesterday. Hopefully, I've got a decent shot of mine from Saturday...

thanks, really cool flights. Gotta start the adventures of AP clustering.... much more pucker factor

Full size are all linked in the regional forum thread.

Here is a chart with the motor impulse classification.



An Estes D12 motor is really more like a D10.66 and is a 71.6% "D" motor (calc. total impulse of 17.16 Ns). The dash-dotted line represents the average thrust, but as you can see there is a spike at the beginning of the burn that tails off a half-second after ignition.



So two D12's ignited at the same time are an E21.32 and is a 71.6% "E" motor (calc. total impulse of 34.31 Ns)...



... whereas three D12's ignited at the same time are an F31.99 and is a 28.7% "F" motor (calc. total impulse of 51.47 Ns).



Here are the thrust curves all together:



HTH.

Greg

EXPjawa said:

I'm curious about how one would rate the combined impulse of a cluster of engines. Over the weekend, I launched a total of 5 cluster engine rockets, all with complete success (at least regarding ignition). So, would my Der Green Max - on three A8-5 motors - be roughly equivalent to having a B24-5 motor, or is there a different way of thinking about it?

Click to expand...

Ideally, yes, B24-5 is the correct nomenclature. But that assumes all three motors light simultaneously (and that all three conform reasonably well to the thrust curve). OTOH, if they don't light simultaneously, the curves don't line up perfectly, so the thrust peaks won't line up, so then you get something less than 3x8 for peak thrust.

That said, I pretty much add them up the way you did.

Doug

[Edit] Fred's right. Three A8's would be a C24, not a B24. I should have caught that.

.

A motors = 1 B motor if the total impulse of the A motors is at or near the maximum range for an A motor.

3 A8-5 motors would put you above the B motor total impulse range and into the middle of the C motor total impulse range.

And you need to check the actual measured values for average thrust since the A8 is really an A3.18

https://www.nar.org/SandT/pdf/Estes/A8.pdf

3 x Estes A8-5 would yield a real-world scientifically measured C9.54 with a total impulse of 6.96 newton-seconds. More than the 5 N-s upper B limit but well below the 10 N-s upper C limit.

You can use the "Combine Motors" tool at Rocketreviews.com to display a time-thrust curve for the cluster:

https://www.rocketreviews.com/combined-motors-5588.html

The three A8s become a C motor with an average thrust of about 9.5 Newtons. So, the cluster would be equivalent to a C10-5 motor.

The reason it isn't a C24 is that the Estes A8 is not really an A8. It is closer to an A3.

-- Roger

Thanks for posting that - I didn't know that tool existed. I knew before that the Estes ratings were, um, optimistic. I didn't realize that they were (to a degree) flat out inaccurate, which is kind of disappointing...

EXPjawa said:

Thanks for posting that - I didn't know that tool existed. I knew before that the Estes ratings were, um, optimistic. I didn't realize that they were (to a degree) flat out inaccurate, which is kind of disappointing...

Click to expand...

There's some historical reason for calling the motors A8s instead of A3s. I think the old designation for the motor was A.8 back when the average thrust was expressed in pounds.

-- Roger

Close. The original A8 (metric) had close to 8 Newtons average thrust. The motor changed a bit over time from geometry (centerbore diameter and depth - it is LARGER throat than in the early 1970's) and slower burning black powder.

The result is a motor that has a peak followed by a short tail.

Compare above modern A8 to the thrust curve from 1970:
https://www.ninfinger.org/rockets/nostalgia/70est084.html

jadebox said:

There's some historical reason for calling the motors A8s instead of A3s. I think the old designation for the motor was A.8 back when the average thrust was expressed in pounds.

-- Roger

Click to expand...

D13 was awesome (when they did not 'splode......) That's why it became a D12.

Just like the E15 became the E9.