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  #1  
Old 04-16-2009, 01:29 PM
shockwaveriderz shockwaveriderz is offline
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Default timewarp rocketry

There's a thread over on TRF George Gassaway was talking about an A8-3 with a off center thrustline.

George posted:

"It is ironic though that they blew a great opportunity. Ever see the Estes video “Model Rocketry the Final Frontier”, with William Shatner around 1976 or so? There was some great slo-mo footage of model rockets in that. One of the most impressive was ultra-slo-motion and extreme close-up of an ignition of an 18mm engine. The exhaust flame started out crooked, not straight. Then it got straight a bit, and went past straight to angle the other way a little bit, before finally getting stronger and coming out dead straight. Never thought about it before I saw that footage, but the exhaust flame moving may have been from the ignitor igniting one side of the short core of the propellant, with the exhaust gases at first coming from one side of that short core then the other side of the core igniting to cause the flame to swing, to the other way, and finally enough of the propellant igniting and burning outwards (dome-like) to cause enough thrust to start to move. It was fascinating"

I can't wait to see/record this episode, but earlier I had found some 1200 fps slo mo shots of A8-3 engines, which exhibit that off center line thrust. see attached URL's.

This seems to be more common than we may have suspected because we never have seen closeup of exhaust plumes before. Notice how the exhaust plume changes directions as George describes? This may be more common than we think it is.

I support George's posit that perhps its uneven burning of the igniter.. check out this URL at 1200fps of an Estes igniter firing up. If its not the igniter cauisng the uneven burning perhaps its nozzle erosion or propellant grain erosion ?

Another contributing factor maybe the chamber pressures: the engine chamber pressure of a typical Estes engine is greatest during its "cored" portion of burning; then it subsides; perhaps there is an interrelationship between initial ignition pressures and this off center thrust.

I have suspected since seeing these that the propellant grain may be weakest around the nozzle grain interface because this is where the first increment of Bp is pressed. And this 1st increment or so, defines the small combustion chamber (core) that is molded into the base of the propellant grain... ie above this arae you have a simple cigarette burning or end burinng propellant.

The above is all based upon pure speculations on my part
Trip Barber did report back in the mid to late 70's that dealt with the effect of igniters on the burning characteristics of B14 and I think A8 motors. He found that differing igniter designs would ignite B14 at differing points along their cores and therefore the Isp of the rocket engine itself would be affected as would the thrust time curve and hence internal chamber pressures would also differ. There didn't seem to be any igntion dependence on
A8-x motors due to their small intial core area.

EDIT

Down through the years I have literally taken apart numerous EStes and Quest engines in my trying to understand BP engines better. One thing that I noticed is that their is a slight concave surface to the top of the nozzle, like a bowl. The propellant grain is pressed up again this concavity, if you separate the nozle from the propellant grain you will notice that the concavity is pimpled with BP granules that weren't completely pressed. This indicates to me that there might be an inadequate pressure or a limiting pressure that you can press that 1st increment of BP against the concave top of the clay nozzle.

Its my belief that the BP layer just adjacent to this concavity and the pressure being used to press the 1st increment against this nozzle is probably causing the "spit- splatter" effect we see with model rocket engines during their intial burns.
You can see all the above and even d12's that show the same off center thrust:

http://www.payloadbay.com/videos-7.html


terry dean
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  #2  
Old 04-16-2009, 06:37 PM
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Quote:
Originally Posted by shockwaveriderz
I support George's posit that perhaps its uneven burning of the igniter.. check out this URL at 1200fps of an Estes igniter firing up. If its not the igniter cauisng the uneven burning perhaps its nozzle erosion or propellant grain erosion ?

Well even a perfect ignitor will tend to be placed non-perfectly. Like touching the "left side" of the short core, so the left side gets burning first, then the right side.

Or it may be something else. One of the few who'd be very likely to know of this phenomenon would be Mike Dorffler (and probably best of all, Ed Brown), But I wonder if mike is looking at all the other threads like this one?

Thanks for the links to those slow-motion clips.

The Estes Last Frontier” video I mentioned was posted by someone on YouTube, in 2 parts:

http://www.youtube.com/watch?v=fqupfFjSJXw

http://www.youtube.com/watch?v=ISZKIf_2Plw

- George Gassaway

Last edited by georgegassaway : 04-16-2009 at 07:01 PM.
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Old 04-16-2009, 08:58 PM
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It's not just at ignition, there is off center thrust on at least some motors some of the time after the rocket leaves the pad. I had some thrust vectoring contribute to the demise of my Cineroc Omega clone a while back. Both of these images show vectored thrust.

As you can see from the image, It has off center thrust, causing it to veer to image right.



A later image in the sequence that isn't blown up as much, shows the path of the Cineroc Omega and you can still see the vectored thrust.

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Old 04-17-2009, 04:27 AM
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Yeah, that second picture looks like the "Astronaut Farmer" is about to do some plowing...
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Old 04-17-2009, 07:29 AM
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Quote:
Originally Posted by mycrofte
Yeah, that second picture looks like the "Astronaut Farmer" is about to do some plowing...


And it did, with similar carnage that the Astronaut Farmer had. I had a really nice horizontal flight until impact right about the time of 2nd stage burnout. It was way downrange at that point.

My new Ceneroc Omega has been built with 1/4" lugs so I can use a longer rod. Hopefully any future vectored thrust will be somewhat offset by those big fins getting some fast moving air over them upon clearing the rod.
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Old 04-17-2009, 02:06 PM
snaquin snaquin is offline
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Quote:
Originally Posted by tbzep
And it did, with similar carnage that the Astronaut Farmer had. I had a really nice horizontal flight until impact right about the time of 2nd stage burnout. It was way downrange at that point.

My new Ceneroc Omega has been built with 1/4" lugs so I can use a longer rod. Hopefully any future vectored thrust will be somewhat offset by those big fins getting some fast moving air over them upon clearing the rod.


I admired your build of the Cineroc Omega and remember the original thread. I don't think anyone makes a full decal sheet that includes the Cineroc decals that you made for yours. I'm looking forward to the Semroc release of their version of the Omega with Boostervision set up since I'm not really up to designing my own. I'll most likely use 1/4" lugs or rail guides with stand offs when I build mine since that's most popular at the pads where I fly but would have anyway to reduce rod whip.

The off center thrust is so obvious in those pictures and it's really a shame you wrecked such a nice project. Glad to hear you are working on your replacement

.
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  #7  
Old 04-17-2009, 03:17 PM
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That flight of the Omega/Cineroc clone must have been a real sight.
Did it POWR-Prang or Prang just after burnout ?
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Old 04-17-2009, 04:37 PM
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Quote:
Originally Posted by ghrocketman
That flight of the Omega/Cineroc clone must have been a real sight.
Did it POWR-Prang or Prang just after burnout ?


GH,
After flying horizontal on two D12's, it was so far down range I'm not 100% sure. I think I remember hearing the crunch right after hearing the sustainer shut down. It was very close, one way or another, because the altitude you see in the second shot is not far from the max altitude before it went truly horizontal.

Snaquin,

Thanks. I've had the replacement Omega rebuilt painted, and ready for action for a while. The new Cineroc housing has been ready for paint for several months. The camera gear is sitting right next to my Little Joe II, but I just haven't gotten around to painting the thing and installing the gear. I'll get it done one of these days.
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Old 04-17-2009, 07:24 PM
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Inflight Omega

No Cineroc, but still...
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  #10  
Old 04-18-2009, 05:32 PM
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I was a little disappointed by the Timewarp TV show's episode on rockets. I think some of the videos I've taken with my $1000 camera are more impressive than the ones they produced with a camera that costs more than $3000 a day to rent.

The shot looking up at a rocket with clods of dirt blowing past the camera was really impressive, though.

Anyway ... back on topic ... I didn't notice the off-centered thrust in the D12-3 video I made until it was pointed out to me. At first, I was more impressed by the ejection charge. But, it is an interesting phenomenon and I plan to investigate it further with more static tests filmed from different angles.

-- Roger
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