Maximum safe lift off weight/velocity using E9's

I am building a custom designed 2 engine clustered rocket. My intention is to use Estes E9's for a low slow effect. The max lift off weight for an E9-6 is 12 ounces. My rocket ways 24.5 ounces fully loaded. Rocksim predicts an altitude of 630 feet and a successful flight. Rocksim also predicts leaving the 6 foot 1/4 inch launch rod at less than 30mph. Assuming calm conditions am I ok?

thanks
Jim

With two E9s, that rocket should be OK with a 6 foot rod. Use the E9-4 though, not the 6, and make sure you are fairly confident in your cluster ignition methods. I would recommend a 12 volt system, and I would also recommend Quest Q2G2 igniters.

jhein said:

I am building a custom designed 2 engine clustered rocket. My intention is to use Estes E9's for a low slow effect. The max lift off weight for an E9-6 is 12 ounces. My rocket ways 24.5 ounces fully loaded. Rocksim predicts an altitude of 630 feet and a successful flight. Rocksim also predicts leaving the 6 foot 1/4 inch launch rod at less than 30mph. Assuming calm conditions am I ok?

thanks
Jim

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With the information you provided (MUCH more complete than most, so thank you!) you should be fine with the calm conditions you indicated. As already stated, you should use the E9-4 and only try the E9-6 if you see much better performance and ejection well before apogee.

Any relation to Jon? https://en.wikipedia.org/wiki/Jon_Hein

Rocksim predicts an altitude of 630 feet ..

make sure nothing is within 500' and your out of its range if it goes wonky..

A lot of people may not agree with my assesment or the reasoning there.

But, low thrust to weight, and a little weathercocking could turn into a landshark. IMO. just give yourself some distance....

tra rso guidelines said:

A printout from a flight prediction software package can be presented. In this case the prediction output should indicate the thrust-to-weight of > 5, the initial acceleration of > 5 g’s, or the velocity of the rocket at the end of the rod/rail/tower > 45 f/s.

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your at 44ft/sec, so i would say its prediction is within 3 percent.

given the likelyness of rodwhip, and weathercocking, give yourself some room.

Another question to be asked is whether the 24.5 oz mass spec you're using is from Rocksim directly, or if this is the measured weight of your finished rocket. The adhesives, paint, and occasional misc. hardware that goes into the actual rocket tends to make it a bit heavier than Rocksim's strictly material-based prediction. If you're sparing in your construction methods, though, any gain (and its associated effect on flight performance) would be minimal.

@Microspeed: 24.5oz is the actual weight. Since the E9 was rated at 12oz max lift off weight/motor, I am 0.5 oz overweight. Hence my concern.

jhein said:

@Microspeed: 24.5oz is the actual weight. Since the E9 was rated at 12oz max lift off weight/motor, I am 0.5 oz overweight. Hence my concern.

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Max recommended weights are for average sized models for that type of motor. That means average air drag for a larger model that would use an E9 motor. Since you are clustering and as long as the diameter is not ridiculous, then your 'drag per motor' will actually be less than assumed by the recommended max liftoff weight as published.

Maybe I'm missing something, but by my calcs the max liftoff weight for a brace of E9's is 15.3 oz (single is 7.7 oz). Also, 3 x E9's will have a max liftoff weight of 23 oz.

Anyway, those are my calcs.

Greg

I think as long as you have some nice large fins than you are fine,with a perfect calm day I would try...As long as there is no wind within the first 100 feet or so.:wink:

Estes's values for max. lift-off weight with their E engines seem suspect to me. They are somewhere around 1/5 of peak thrust, not of average thrust. Greg's calculations make more sense to me, though I'm not sure how his average initial thrust was derived - averaged over some initial period of the burn, I presume. Note that in this table https://www.estesrockets.com/rockets/engines/e-engines/1673-e9-4 Estes shows a higher max. lift-off weight for their E engines than for their D engines; I would say the informed consensus here on TRF is that a D12 will get a heavier rocket off the pad more reliably than an E9. 12 ounces per E9 seems like really pushing the limit to me. The 6 foot launch rod helps a lot. I'd suggest trying it with D12s before the E9s.

I agree with RimfireJim that you should use D12's first. The D12 has a much higher thrust than the E9.

You can see that 2 x D12's have about 50% more initial thrust than 2 x E9's. As a bonus, the D12's have about 30% less weight at launch (~39 grams or ~1.37 oz less).

Greg

Dont anybody shoot me here, but arent Estes max liftoff weights a bit on the conservative side? Im sure you'll be fine with E9-4s and then possibly a six second delay depending on where the ejection is fired in relation to apogy. I think its better to be a little early than late in that regard. I also couldnt agree more about sending it up on D12s. Go for a 5sec delay.

El Cheapo said:

Dont anybody shoot me here, but arent Estes max liftoff weights a bit on the conservative side? Im sure you'll be fine with E9-4s and then possibly a six second delay depending on where the ejection is fired in relation to apogy. I think its better to be a little early than late in that regard. I also couldnt agree more about sending it up on D12s. Go for a 5sec delay.

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Estes actually has their max liftoff weights a bit on the optimistic side, if you ask me. D12's would definitely make a good first test flight, but I would definitely go for a -3, not a -5 (unless you really like hair-raising recoveries).

cjl said:

Estes actually has their max liftoff weights a bit on the optimistic side, if you ask me. D12's would definitely make a good first test flight, but I would definitely go for a -3, not a -5 (unless you really like hair-raising recoveries).

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+1

Greg

I built my girl a 3" Merc Invader that flies on interchangeable mounts. Most of the time it flies on 3xD12-5 since HL doesnt carry 7s but is always a bit early. On 2xD12-5 it's after apogee but not much. A D12-3 is perfect for single motor small field flights. It weighs in at 12oz w/out motors. Hope that helps as a reference.

What are the dimensions of this bird?