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  #1  
Old 03-19-2010, 05:26 PM
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Tau Zero Tau Zero is offline
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Angry "Uberfast" deployment velocities

I ran into difficulties with my "Prometheus" design having excessively high deployment velocities on "B" motors. The attached "Retrograde" design suffers similarly.

Do "all y'all" think these are just occupational hazards of this particular size (ST-10 with 18mm mount), or *what?*

In other words, if you have any suggestions for fine-tuning this design, I'm all ears.


Thanks,
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  #2  
Old 03-19-2010, 06:20 PM
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I added one ounce of mass located in the ST-730 body tube just aft of the bulkhead and it dropped the rocket's optimal delay to 3.9 sec and it's velocity at deployment to about 11 ft/sec. I also changed the rocket's finish from 'polished' to 'gloss' to reflect the typical 'high end' finish that about 90% of rocket builders can achieve at best. Actually most will build to about a matt finish (filled and spray can painted) for sport launches.
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  #3  
Old 03-19-2010, 06:34 PM
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Have you tried it on a B6-6?

The "optimal delay" in RockSim is a moving target. Don't take the figure that it gives you for Bx-4 motors at face value. To zero in the true optimal delay for a given motor, you need to sim the launch repeatedly with that motor while working through a whole range of delays, including delay times that aren't actually available. At some point the delay that RockSim recommends will reach a plateau, and simming the launch with ever longer delays won't cause it to budge from that number.

Mark K.
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Last edited by Mark II : 03-19-2010 at 06:52 PM.
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  #4  
Old 03-19-2010, 07:38 PM
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Jay,

First thing I notice is that the rocket is still travelling upward at deployment, so that lets you know the delay values are short, and as Mark said, the optimal value is a moving target. However, it does give you a target range to work toward.

Notice that the A8-3 and C6-5 values are close to perfect. They are still moving upward, but they're closer to the right Dv than the B4-4 and B6-4. What you want to do is shift the A8-3 and C6-5 flights until the model is heading downward before they deploy. Since you cannot adjust the delay times, you have to adjust the Dv.

How? Shifting the Dv is also a product of how much drag the model has. Increase the drag slightly, and the model will begin to arc over sooner. What you want to achieve is to have the A and C engines deploy just after apogee, and the two B engines just before.

Scale up the fins a couple of percent and try running the simulation again. See my attached image for the simulation runs I got. I tried to bump the scale up slowly until I came into range, and the resulting scale factor was 25%. YMMV...


A8-3......220'......Dv 3 FPS
B4-4......517'......Dv 14 FPS
B6-4......525'......Dv 19 FPS
C6-5......816'......Dv 10 FPS
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  #5  
Old 03-19-2010, 11:45 PM
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Quote:
Originally Posted by RandyT0001
I added one ounce of mass located in the ST-730 body tube just aft of the bulkhead and it dropped the rocket's optimal delay to 3.9 sec and it's velocity at deployment to about 11 ft/sec.
Quote:
Originally Posted by CPMcGraw
Notice that the A8-3 and C6-5 values are close to perfect. [SNIP] Increase the drag slightly, and the model will begin to arc over sooner. What you want to achieve is to have the A and C engines deploy just after apogee, and the two B engines just before.

Scale up the fins a couple of percent and try running the simulation again. YMMV...
Randy and Craig (and Mark ),

Thanks for your input! I basically ended up adding a bunch of nose weight by swapping out the SE-10 screw eye with two SE-14's (in appropriate places) and a couple of WL-7 washers. That leveled out the deployment times, but obviously cut back on the altitudes.

I'm thinking if I do the same thing to my Prometheus, I might have to eliminate my tag line about "High Performance."

Craig, I was using Carl's standard "Recruiter" fins, so I decided that (British accent) "my talents lie in other directions." (A Bit of Fry and Laurie)


Herewith, "Retrograde" version 2.


Thanks so much, guys! I appreciate the help!

.
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  #6  
Old 03-20-2010, 07:05 AM
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I thought I recognized those fins from somewhere...

But you understand what I was working toward, right? Slowing the rocket down just a tad by way of drag instead of by mass. Adding mass can bite you -- it makes the model fall faster, even under the parachute, which means higher impact V with the ground. By using drag, the increase in mass is minimal, so the descent rate remains lower.
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  #7  
Old 03-20-2010, 09:41 AM
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Quote:
Originally Posted by CPMcGraw
But you understand what I was working toward, right? Slowing the rocket down just a tad by way of drag instead of by mass. Adding mass can bite you -- it makes the model fall faster, even under the parachute, which means higher impact V with the ground. By using drag, the increase in mass is minimal, so the descent rate remains lower.
I prefer adding drag instead of weight, too. An ounce of noseweight nearly doubles the weight. According to the Rocksim file, it was only 33 grams to start with.

BTW, not only does adding mass increase descent weight, it also increases separation forces imparted to the recovery harness. So, while that 30mph Dv is hard on the recovery system, so too is the extra ounce of mass at the screweye.

Doug

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  #8  
Old 03-20-2010, 11:39 AM
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Talking Craig's right... AGAIN!

Quote:
Originally Posted by CPMcGraw
Slowing the rocket down just a tad by way of drag instead of by mass. Adding mass can bite you -- it makes the model fall faster, even under the parachute, which means higher impact V with the ground. By using drag, the increase in mass is minimal, so the descent rate remains lower.
Quote:
Originally Posted by Doug Sams
BTW, not only does adding mass increase descent weight, it also increases separation forces imparted to the recovery harness. So, while that 30mph Dv is hard on the recovery system, so too is the extra ounce of mass at the screweye.
Craig and Doug,

Thanks for your input! After sleeping on it, I think Craig's idea of increasing the fin size by 25% is probably the best way to go with this design. As for the Prometheus... The 175% SLS version works just fine, but the 100% version still has relatively high deployment speeds.

Maybe the solution is a Really Big Streamer? (See third picture.) But that means finding one big enough to slow this model down... (thinking really hard)

.
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  #9  
Old 03-20-2010, 02:02 PM
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I think the solution would be twin very small chutes with very long elastic shock cords. Pack one chute a lot tighter than the other, so they don't deploy at the exact same time. I always like the look of a rocket coming down on two chutes anyway.
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  #10  
Old 03-20-2010, 05:17 PM
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Yeah, definitely use bigger fins rather than adding big hunks of weight just to reduce the deployment velocity.

It's a nice looking design either way, Jay.
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