My quest for an O motor launch.

Flyfalcons said:

18 grams?!?!.........

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Nah, relax.

With the extended MMT and extra CR mentioned above, I'm looking at starting at 6g for the drogue. The main will start at 6.9g. These numbers are based off of a desired 7.2 PSI of pressure, which is sufficient to separate my other 8" rocket with three #8 shear pins. I'm a long way from ground tests, though.

BTW, without adding the extra CR and lengthening the MMT, I am getting 9.1g for 7.2 PSI.

7psi is what I am aiming for as well on my 7.5" build. My 40" main compartment is going to start testing at 5.7G. I believe that "correct" formula given earlier is way, way off.

Flyfalcons said:

7psi is what I am aiming for as well on my 7.5" build. My 40" main compartment is going to start testing at 5.7G. I believe that "correct" formula given earlier is way, way off.

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Most calculators that I have seen are really tailored toward typical level 1, level 2 rockets, 2.6" to 4". They need a lot higher PSI, and so the calculators use 15 PSI. If you take my original MMT dimensions and double the PSI from 7.5 to 15, then you get a charge size of 18g.

Flyfalcons said:

18 grams?!?!.........

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It is a big compartment! Scary charge size. My L3 was around 8-12g, but the compartment was shorter.

The larger tubes require more powder but less PSI to eject the chute. My Nike required 6-9 grams of BP but only 9-10 psi to eject.

In a 16" rocket I had the charge was 32 grams.

MClark said:

In a 16" rocket I had the charge was 32 grams.

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Were you expelling the NC and free-bagging it?

Most BP I EVER used in an 8inch airframe payload [48in long] was 6 grams ...do me a favor and start with that.[or 8 at most]

If it doesn't work you can gladly award me the "dumb a$$'' award & I will accept. lol

blackjack2564 said:

Most BP I EVER used in an 8inch airframe payload [48in long] was 6 grams ...do me a favor and start with that.[or 8 at most]

If it doesn't work you can gladly award me the "dumb a$$'' award & I will accept. lol

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Yeah, that's what I have -- 6g for the drogue and 6.9g for the main. If the drogue charge is overzealous, I will certainly back down from the 6.9g for the main. Going to be a long time before I am ready to test, though.

Bat-mite said:

Were you expelling the NC and free-bagging it?

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Yes.
28 foot chute in a D-Bag with a drogue. Friend's son had a nice 5 mile walk to retrieve bag.

M

Okay, I have done some work -- mostly sanding.

I got the forward Unistrut button mount epoxied in (1/4-20 weld nut).



And I got much of the motor mount built. I still need to sand more around the edges of the CRs to get a good fit into the booster tube.

Impressive.

Pardon my HPR ignorance: what specifically is the all-thread for? I don't recall seeing it in other builds, although maybe I just wasn't paying attention.

neil_w said:

Impressive.

Pardon my HPR ignorance: what specifically is the all-thread for? I don't recall seeing it in other builds, although maybe I just wasn't paying attention.

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It's a choice I like, and by no means "the right way" or "the only way." With the threaded rods connecting the top CR to the aluminum thrust plate, I need less epoxy on the CRs. Only the top of the top CR and the bottom of the bottom CR will get epoxied to the booster. And it makes it very strong with less weight.

Others will argue. The rods are aluminum and very, very lightweight.

Interesting, thanks.

The hardest part is lining up the third CR so that it butts up against the tops of the fins. Basic steps:

1. Fit the aft end of the MMT into the thrust plate (I had to grind it down a bit).
2. Hold the MMT alongside the booster, near a fin slot, with the rim of the thrust plate against the end of the booster at the depth it will be when it is inserted.
3. Slide on a CR and carefully mark with a Sharpie where it needs to go on the MMT so that it is right at the top of the fin slot.
4. Slide on the top CR and mark where it will go (I chose 1/2" down so there was room for a fillet).
5. Remove CRs.
6. I have hex nuts at the top and bottom of each CR, with a washer between the nut and the CR, so they have to be screwed on and in place before the CR is slid onto the rods.
7. Top CR is anchored with nylon-insert nuts.
8. I didn't care where the second CR was (probably not needed at all), so it just ended up where it is.

Oh, and the fourth (bottom) CR will be slid on after the fins are on, and epoxied and get nylon-insert nuts. So there will be a CR butting tops and bottoms of the fins. Thrust plate goes on last, and will be removable as I share it between this and my Formula 200.

*shudder* that much aluminum thread makes me jumpy

Is there a need for the aft centering ring since the thrust plate is already filling the gap and transferring the motor load to the airframe?

Nytrunner said:

*shudder* that much aluminum thread makes me jumpy

Is there a need for the aft centering ring since the thrust plate is already filling the gap and transferring the motor load to the airframe?

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Maybe not. But this method has worked well for me, and, well, if it ain't broke ....

Really nice work John!

Looks like it can take the load.

Chuck C.

Rail Dawg said:

Really nice work John!

Looks like it can take the load.

Chuck C.

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Thanks. Some time after BALLS, I'll need to get back with Scott to see about getting this O motor hardware turned.

This is the structural engineer in me speaking, not from any L3 rocketry experience. I'm not 100% convinced the all-thread carries much load. Those section are fairly long, and aluminum is flexible enough that the Euler buckling loads are going to be pretty small. Plus (unless it's camera distortion) there's already some flex in the all-thread which reduces buckling loads even more.

I don't think it hurts anything and I don't expect this to fail, especially with solidly epoxied top and bottom CRs plus TTW fins. The all-thread might be useful in keeping everything lined up during construction as well.

boatgeek said:

The all-thread might be useful in keeping everything lined up during construction as well.

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Yes, that's a big help.

The all thread is more to spread the recovery load over more that one CR.

boatgeek said:

This is the structural engineer in me speaking, not from any L3 rocketry experience. I'm not 100% convinced the all-thread carries much load. Those section are fairly long, and aluminum is flexible enough that the Euler buckling loads are going to be pretty small. Plus (unless it's camera distortion) there's already some flex in the all-thread which reduces buckling loads even more.

I don't think it hurts anything and I don't expect this to fail, especially with solidly epoxied top and bottom CRs plus TTW fins. The all-thread might be useful in keeping everything lined up during construction as well.

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I catch flack about the R thread for using this forum for input.

Structural Engineer? I'll listen to every word you say.

Chuck C.

Rail Dawg said:

I catch flack about the R thread for using this forum for input.

Structural Engineer? I'll listen to every word you say.

Chuck C.

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[light thread hijack] I absolutely don't mean that Bat-mite shouldn't use the all-thread. Everyone has their own ways of building, and it's all good as long as parts don't rain down. I tend to overbuild, but I like it that way. As my old materials professor used to say, engineering is the science of good enough. I hadn't thought about the tension loads from recovery that djkingsley mentioned, and that's a good point. That's a load case where the all-thread might help.

I just design medium-sized commercial boats (50'-400'), though that's a pretty good model for an unpredictable dynamic environment! I'm also a lowly L2, never flown more than a baby L. There are lots and lots of people with more experience than me that you should listen to more. [/hijack]

boatgeek said:

[light thread hijack] I absolutely don't mean that Bat-mite shouldn't use the all-thread. Everyone has their own ways of building, and it's all good as long as parts don't rain down. I tend to overbuild, but I like it that way. As my old materials professor used to say, engineering is the science of good enough. I hadn't thought about the tension loads from recovery that djkingsley mentioned, and that's a good point. That's a load case where the all-thread might help.

I just design medium-sized commercial boats (50'-400'), though that's a pretty good model for an unpredictable dynamic environment! I'm also a lowly L2, never flown more than a baby L. There are lots and lots of people with more experience than me that you should listen to more. [/hijack]

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I'll be the first to admit that I am no engineer. Never took an engineering class in my life. Everything I know is from taking what worked on smaller rockets and being slightly creative where I think I need to be.

Fortunately, I have about 15 flights on 7.5" - 8" rockets, and all have been perfect with the exception of a motor cato. So I am confident that this beast is going to be able to do what it should. Really, the main thing is the thrust plate. All the motor's upward force is going against that thrust plate; the rest is to just make sure that the apogee charge doesn't cave in the top CR, and the recovery harness doesn't jerk it out.

I was just looking at your OpenRocket file from one of your first posts and comparing it to my 8" Frenzy XXL. It shows your Performer 200 weighing 16 lb less than my Frenzy XXL even though your rocket is 8" longer. I don't see enough differences to account for my rocket being that much heavier. My weigh is correct so I'm wondering if maybe your weight in OpenRocket may not be accurate.

Nathan said:

I was just looking at your OpenRocket file from one of your first posts and comparing it to my 8" Frenzy XXL. It shows your Performer 200 weighing 16 lb less than my Frenzy XXL even though your rocket is 8" longer. I don't see enough differences to account for my rocket being that much heavier. My weigh is correct so I'm wondering if maybe your weight in OpenRocket may not be accurate.

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Parts were weighed and entered, but that is not the final weight. No accounting for epoxy, actual laundry weight, etc. After it is built, I'll override weight and CG.

MMT is in and epoxied at the top joint.



Got one fin tacked on at the root.





Sorry for the blur. It was moving really fast when I took the picture! Er ... um ... not.