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  #1  
Old 10-19-2010, 05:36 PM
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Doug Sams Doug Sams is offline
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Default Over-rated and useless rocketry accessories

At the risk of starting trouble, for which I'm in the mood , I'm listing some of the doo-dads and habits common in this hobby for which I have great skepticism of their net benefits.

Baffles: I've never used one. I've always thought they were a solution in search of a problem. Saving a few pieces of treated toilet paper...well, it ain't much of a savings. And, I'm pretty sure they don't really keep all the burn off the chutes all the time. Right?

Fancy motor retaining hardware (eg, Aero Pack): How hard is it to cook up a couple of Kaplow clips? If you're losing motors due to retention issues, maybe you need to take some remedial rocketry classes

Tapered fins: Yeah, I've done a few. Guess what? Unless you're flying competition, they're more trouble than they're worth. They're more prone to breaking, and, as a sport flier, the added altitude only makes them harder to track. Sure, the occasional indulgence is perfectly acceptable, but for those who have a life, square cut fins work danged fine

Long shock cords: Whoever came up with the foul notion that making the shock cord longer solves the problem of high speed deployment needs to take a high school course in physics. If your rocket is plunging toward the earth at 400mph, I don't care if the shock cord is 300 feet long, when the rocket gets to the end of the cord, there's gonna be one heckuva a jerk on the airframe, the suspension lines and the recovery harness. You can quote me on that.

Clear coats: Over decals? Sure. Over anything else? Use a paint that doesn't need to be clear coated. It's called enamel. Check some out. Save that extra effort for building another rocket.

Q2G2 ignitors: They're awesome. Best commercial ignitor ever for air-starts. But they're not an absolute necessity for ground-started clusters. I've lit many clusters using plain old Solar ignitors, and had excellent results getting all motors lit most of the time, and never had an ignitor related failure. Us old far...timers remember that before Solars, we had nichrome and crap - those were the two choices. Solars were a light years-ahead improvement. Now, 30-some years later, they're still a danged fine ignitor, and all that's needed most of the time.

Alright, that's all I can think of for now, but I'm sure I'll remember three more as soon as I hit the send button...

Doug...feeling rather curmudgeonly...

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  #2  
Old 10-19-2010, 05:55 PM
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Hi Doug. I agree with most of your points--except for one: long shock cords.

I make shock cords WAY too long for 2 reasons: 1) a shock cord that's too long can be repaired and retied several times before it becomes a too short shock cord, and 2) from my experiences with rockets with heavily-wieghted nose cones, a short shock cord can and HAS broken on laundry deployment, or the NC has sprung back and damaged the BT end.

To summarize, I prefer to use an elastic fabric shock cord that's too long. But hey, everyone's different.

Good thread BTW.
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  #3  
Old 10-19-2010, 06:28 PM
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Shock cords--the nose cone and body are zipping apart at a fast, but decreasing velocity. The energy that the shock cord has to absorb is proportional to the square of the velocity. If you want things to stay together, you need to do one of three things:

1--slow down the flying objects more quickly
2--reduce the mass of the objects
3--wait for friction to do its thing and slow them down on its own (i.e. longer shock cord)

Elastic has the nice effect of slowing things down quickly as it stretches, but has the unpleasant effect of snap-back. I strongly prefer chain-stitched (or monkey braided) Kevlar, as it greatly reduces the force on the system while the knots are unfurling, and doesn't snap back. My R & D report at last NARAM measured this and found that the chain-stitched Kevlar took something like 2/3 of the total energy out of the system before actually yanking on the nose cone tether/Kevlar attachment points.

But what do I know? I usually taper my fins
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Old 10-19-2010, 07:12 PM
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The Estes tube marking guide. I like the two discs for the different size tubes, but the provided straightedge is somewhat flexible and gives me inconsistent results. I use aluminum angle instead for marking lines on a tube.


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Old 10-19-2010, 07:20 PM
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Quote:
Originally Posted by Doug Sams
Baffles: I've never used one. I've always thought they were a solution in search of a problem. Saving a few pieces of treated toilet paper...well, it ain't much of a savings. And, I'm pretty sure they don't really keep all the burn off the chutes all the time. Right?


Dog barf is cheap. One almost cannot give the stuff away fast enough.


Quote:
Originally Posted by Doug Sams
Fancy motor retaining hardware (eg, Aero Pack): How hard is it to cook up a couple of Kaplow clips? If you're losing motors due to retention issues, maybe you need to take some remedial rocketry classes


Pretty if you can afford it, but you have to remember to use JB Weld and thoroughly clean the motor tube.


Quote:
Originally Posted by Doug Sams
Long shock cords: Whoever came up with the foul notion that making the shock cord longer solves the problem of high speed deployment needs to take a high school course in physics. If your rocket is plunging toward the earth at 400mph, I don't care if the shock cord is 300 feet long, when the rocket gets to the end of the cord, there's gonna be one heckuva a jerk on the airframe, the suspension lines and the recovery harness. You can quote me on that.


Longer shock cords give more room to install masking tape shock absorbers.


Quote:
Originally Posted by Doug Sams
Q2G2 ignitors: They're awesome. Best commercial ignitor ever for air-starts. But they're not an absolute necessity for ground-started clusters. I've lit many clusters using plain old Solar ignitors, and had excellent results getting all motors lit most of the time, and never had an ignitor related failure. Us old far...timers remember that before Solars, we had nichrome and crap - those were the two choices. Solars were a light years-ahead improvement. Now, 30-some years later, they're still a danged fine ignitor, and all that's needed most of the time.


I have come to the same conclusion and will have to try extending Solar igniter leads with wire-wrap wire. The Q2G2s will be saved for electronic staging.



Bill
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Old 10-19-2010, 07:27 PM
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With small rockets, baffles are a waste of time, IMHO. I like baffles on larger rockets because I hate dog barf (dusty/messy) and commercial wadding can be expensive considering what you are paying for.

I can whip up baffles using scrap cardboard for zero added cost. I don't put them in all of my rockets. It's one of those "in the mood" type decisions I make when I'm building them. Chances of adding a baffle are greater if the rocket is already going to need a coupler to join two body tubes. There's a considerably lower chance if it's just a regular length of tube.
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Old 10-19-2010, 08:13 PM
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I wish the motor clip "finger tab" had never been invented. As far as I know, they were first used on the thin MPC (and early Quest) motor clips. Even on the thicker Estes (and current Quest) motor clips, the finger tabs get bent when used on streamer-recovery rockets whose configurations (no downward-protruding fins) ensure that the finger tab strikes the ground first at landing. After being bent and straightened a few times the tab breaks off, leaving a sharp edge that has to be sanded or ground down to prevent cuts.

Even in rockets where the finger tab doesn't hit the ground, it's one more metal object that the igniter leads have to be kept away from to prevent short circuits. (This is a particularly annoying problem when connecting the firing leads to rockets that use clustered motors.) Also, in models powered by 13 mm mini motors, the finger tab often protrudes inward close enough to the motor nozzle to act as a jet vane, which makes the rocket veer off course or even tumble end-over-end.
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Old 10-19-2010, 08:27 PM
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Quote:
Originally Posted by blackshire
I wish the motor clip "finger tab" had never been invented. As far as I know, they were first used on the thin MPC (and early Quest) motor clips. Even on the thicker Estes (and current Quest) motor clips, the finger tabs get bent when used on streamer-recovery rockets whose configurations (no downward-protruding fins) ensure that the finger tab strikes the ground first at landing. After being bent and straightened a few times the tab breaks off, leaving a sharp edge that has to be sanded or ground down to prevent cuts.

Even in rockets where the finger tab doesn't hit the ground, it's one more metal object that the igniter leads have to be kept away from to prevent short circuits. (This is a particularly annoying problem when connecting the firing leads to rockets that use clustered motors.) Also, in models powered by 13 mm mini motors, the finger tab often protrudes inward close enough to the motor nozzle to act as a jet vane, which makes the rocket veer off course or even tumble end-over-end.


I cut the finger tab off and use that end at the thrust ring end for all those reasons, plus because the finger tab looks ugly to me.
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Old 10-19-2010, 08:33 PM
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Interesting thread!

Quote:
Originally Posted by Doug Sams
Baffles: I've never used one. I've always thought they were a solution in search of a problem. Saving a few pieces of treated toilet paper...well, it ain't much of a savings. And, I'm pretty sure they don't really keep all the burn off the chutes all the time. Right?

I agree for smaller rockets, but I disagree for larger rockets. Using a baffle cuts way down on the use of wadding and saves your chute and shock cord from potential burns.

Quote:
Originally Posted by Doug Sams

Tapered fins: Yeah, I've done a few. Guess what? Unless you're flying competition, they're more trouble than they're worth. They're more prone to breaking, and, as a sport flier, the added altitude only makes them harder to track. Sure, the occasional indulgence is perfectly acceptable, but for those who have a life, square cut fins work danged fine

If the build calls for tapered fins I usually comply, but I do hate the extra sanding.

Quote:
Originally Posted by Doug Sams
Long shock cords: Whoever came up with the foul notion that making the shock cord longer solves the problem of high speed deployment needs to take a high school course in physics. If your rocket is plunging toward the earth at 400mph, I don't care if the shock cord is 300 feet long, when the rocket gets to the end of the cord, there's gonna be one heckuva a jerk on the airframe, the suspension lines and the recovery harness. You can quote me on that.

I find that longer shock cords help to prevent damage to the rocket and cone (if it's balsa) from the nose cone whipping back and hitting the rocket. It also seems to prevent zippering. Although I can't back that up with anything more than my own personal experiences.


The only thing that I can think of as being on my list right now would be excessive sanding and sealing to obtain that perfect finish. I'm just not all that fond of repeating the process enought to obtain the "perfect" finish. Maybe because in the past when I've done that something else always knocks my fnish down a bit. It could be anything to a run in the paint to a bug landing in the final finish coat.
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Old 10-19-2010, 08:38 PM
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I lawn-darted my Mean Machine last time I flew it, and figured as long as I was repairing the front end, I'd try my first baffle (particularly since I wasn't sure the lawn dart might not have been caused by improper use (e.g. too much) wadding-- the ejection charge sounded like a "pfffft!") I have not yet re-launched it to see how well it works. Personally I don't think I'd ever use one on a tube smaller than a BT-60, but that size and above use an awful lot of wadding.

I'm not against longer shock cords, but I've never had issues with the estes-sized ones. Same goes for the tri-fold mount - I have rockets from when I was a kid where the tri-fold mount is just fine, and the elastic cord was disintegrated. I simply cut it out, and glued in some new elastic with a new tri-fold

I don't really get the finger-tab either. The old-fashioned style worked just fine. On the other hand, the plastic fin-cans with the motor retention cap you turn and pull off have never bothered me whereas others here seem to abhor them.

I'm not nearly as religious about fin filling/finishing/yadda yadda as the rest of you seem to be. I mainly build to fly. I do try to make them look decent but if you back up 3 feet you'll never see those tube styles anyway That said I've been doing that on my last few rockets just to see what all the fuss is about

Basically a rocket flying on a 'C' engine doesn't need to be built like it's flying on an 'H' or bigger. Some of the old techniques work just fine.
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