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Tweener
06-05-2005, 06:23 PM
No this isn't about that '80's group that did an English language cover of Falcos's "Der Commissar". I'm finally settled into my new abode and organized well enough to build. One of my first projects is another Mean Machine bash of the Centuri Thunder Roc. I have decided to break from my previous opinion that a clone should be constructed as close to the original design as possible and take advantage of some of the other bits of tech available. One design mod will be to add an ejection baffle to eliminate the need for wadding. But I'm in need of some advice for the other. I'd like to put a 3 3/4" 24mm mount in the rocket to enable E power launches with Estes engines, and I have four questions.

1) If I choose to launch with a D, what do I use for a 1" spacer? (I'm thinking spent engine casing cut to size, cellophane taped to front of engine for ease of extraction.)

2) What E9 delay would be best for ejection at apogee?

3) Since an E9 doesn't have as high a thrust spike as the D12, will I need a longer rod than 36" for the rocket to become stable?

4) Will the 3/16" rod still be sufficient rather than 1/4"? ( I use 1 piece steel and not 2 piece aluminum, actually cheaper at the hardware store. )

I'm not rocksim literate, and when this is built it will be my first E power capable rocket. I don't want anyone to get hurt! :D

A Fish Named Wallyum
06-05-2005, 07:40 PM
I built and flew a D-powered Thunder Roc a couple of years back. The only thing I used as a guideline was how the Mean Machines performed that I had seen fly. I'd seen several of them at club launches and none of them seemed to have even slight problems clearing the rod. As for the E flight, everything I've ever built to fly on an E has left the rod with plenty to spare. None have been as long as the Thunder Roc, but I've flown an upscaled Satellite Interceptor that was decidedly bulkier. I don't think the lack of initial punch is going to be much of an issue.
As for flying it on D motors after flying it on E's, do you really think you're going to come down the ladder? This isn't one for the local soccer field. :cool:

CPMcGraw
06-05-2005, 08:19 PM
I'm in need of some advice... I'd like to put a 3 3/4" 24mm mount in the rocket to enable E power launches with Estes engines, and I have four questions.

1) If I choose to launch with a D, what do I use for a 1" spacer? (I'm thinking spent engine casing cut to size, cellophane taped to front of engine for ease of extraction.)

Two possibilities:

1 -- Get a 1" length of hard, black fishpaper "JT-50" coupler material.
2 -- Take two CR-2050 rings and a 1" length of BT-20. Glue the rings at each end.

Having said this, read on...

2) What E9 delay would be best for ejection at apogee?

None. See question #3.

3) Since an E9 doesn't have as high a thrust spike as the D12, will I need a longer rod than 36" for the rocket to become stable?

Much longer. Try 80". It doesn't become stable until it reaches 73". See the simulation file.

4) Will the 3/16" rod still be sufficient rather than 1/4"? ( I use 1 piece steel and not 2 piece aluminum, actually cheaper at the hardware store. )

Should be, but you need to see the simulation file (attached) for the engine selection.

I'm not rocksim literate, and when this is built it will be my first E power capable rocket. I don't want anyone to get hurt! :D

Don't sweat it, it comes with time. Just study the sim results carefully, and you'll get all the data you need.

This bird needs composite Aerotech motors (or their equivalents) to fly safely. The Estes BP motors just don't have the needed horsepower to kick this pig. The same applies to the Mean Machine, if you wanted to do the same thing with it.

Craig McGraw

CPMcGraw
06-05-2005, 08:30 PM
Tweener,

You really don't need the "E" motor to get this rocket to fly well. On a D12, the simulator gives me 488', and it reaches flight velocity at 41", meaning you need a 48" rod.

The "E" motor gave me 934', but it needed an 8' rod to do it. That suggests more of a structural tower to keep it straight. Too much support tower for a one-bird project.

Craig McGraw

A Fish Named Wallyum
06-05-2005, 08:34 PM
Sounds like a challenge to me. I've spent the weekend halfheartedly gathering tubes for an E capable Thunder Roc rebuild. Now I've got some wind in my sails.
BTW, when I exhumed the corpse of my original Thunder Roc clone I found my missing 24" nylon chute. :D

CPMcGraw
06-05-2005, 08:54 PM
Sounds like a challenge to me. I've spent the weekend halfheartedly gathering tubes for an E capable Thunder Roc rebuild. Now I've got some wind in my sails.
BTW, when I exhumed the corpse of my original Thunder Roc clone I found my missing 24" nylon chute. :D

And to think, I just bought another Screamin' Mimi at the last HL sale... :D

I may have to do one of these myself...

Craig...

Tweener
06-05-2005, 09:14 PM
This bird needs composite Aerotech motors (or their equivalents) to fly safely. The Estes BP motors just don't have the needed horsepower to kick this pig.

:( (sigh) I had hoped to be able to fly on cheap (er) E power. How about the Aerotech E15-7W? Then I wouldn't even have to change the mount.

CPMcGraw
06-05-2005, 09:55 PM
:( (sigh) I had hoped to be able to fly on cheap (er) E power. How about the Aerotech E15-7W? Then I wouldn't even have to change the mount.

In the simulation drawing, I used a 6" motor mount, but installed the motor block at 3.5" from the rear of the tube.

The E15-7 is one of the motors I used in the list. The simulator hit 1518', and reached speed at 36.25". Still needs the 48" rod.

Best altitude was with the F24-7, with 1719'. Speed was reached at 27.2", meaning you can squeak by with a 36" rod here. I'd still use the 48" rod for the additional speed before free-flight.

Craig McGraw

Tau Zero
06-05-2005, 10:51 PM
This bird needs composite Aerotech motors (or their equivalents) to fly safely. The Estes BP motors just don't have the needed horsepower to kick this pig."Kick a Pig." Now *there's* a name I'd like to see you use for one of your custom designs! :eek:

Cheers,

--Jay
(Who has both ATF and Falco versions of the following...)

"Don't turn around, woh-oh! (Ja, ja!) Der Komissar's in town..."

CPMcGraw
06-05-2005, 11:16 PM
"Kick a Pig." Now *there's* a name I'd like to see you use for one of your custom designs! :eek:

OK, I've put the name into my list of "To Do Models". I've got to think about that one a bit first... :D

Craig

A Fish Named Wallyum
06-05-2005, 11:39 PM
Sounds like a Goony waiting to happen.

CPMcGraw
06-05-2005, 11:40 PM
Blame Jay for this one! :D

Fat, short, odd-looking, belongs in the water and not in the air...

It's yellow, too... :D

Custom nose cone and cored transition (BMS), and the airframe is from PML (PT-3.9).

Estes BT-50 motor tube. Composite F39 with a 3-second delay is the only recommended motor.

Enjoy!

A Fish Named Wallyum
06-05-2005, 11:44 PM
Better idea. Call it "Squeal Like A Pig" and just put a picture of Ned Beatty on a Fat Boy. :eek: :D

CPMcGraw
06-05-2005, 11:49 PM
Kick A Pig should have been posted in the other thread, but it's here for Jay... ;)

My other first thoughts were to make it look like a football, with laces and everything. I might come up with a Flying Pigskin a little later...

...I've got to get that one to spin off the rod, though...

...Maybe a two- or three-motor cluster, with tilted mount tubes, and have the rod going right through the center...

See what you've made me do, Jay!!??? :D

This "thinking" stuff really is dangerous!

Craig

Tweener
06-06-2005, 12:27 AM
In the simulation drawing, I used a 6" motor mount, but installed the motor block at 3.5" from the rear of the tube.

The E15-7 is one of the motors I used in the list. The simulator hit 1518', and reached speed at 36.25". Still needs the 48" rod.

Best altitude was with the F24-7, with 1719'. Speed was reached at 27.2", meaning you can squeak by with a 36" rod here. I'd still use the 48" rod for the additional speed before free-flight.

Craig McGraw
Hmm.. I noticed on a couple of my D12 flights that my previous 'Roc seemed to "hang" momentarily just off the rod. I wonder if rockets like this and the Mean Machine may actually have such long moments on their CG that they have a bit of "inertial stability" that resists changing direction even if they haven't quite reached aerodynamic stablility off the rod. Not that I wouldn't use a 48" rod just to be safe, but wonder what your thoughts are on this.

The reason I started considering this idea in the first place is because the Thunder Roc would be easy to see at higher altitudes, with a better chance of recovery. Still, a calm day is a must if it's going to 1500'. I went with dual 18" chutes on the last one and never broke a fin, though I DID walk a quarter mile to recover one day with 10 MPH winds. (Where's the emoticon for "exhausted"?)

A Fish Named Wallyum
06-06-2005, 05:25 AM
Grip It And Rip It!!! :D
http://www.rocketreviews.com/reviews/oop/cen_thunder_roc.html
Imagine my surprise. Here I was looking forward to my first Thunder Roc flight with an Estes E9 and it turns out that I ALREADY DID IT!
Then again, it did do that hangin' thingy just as it cleared the rod. Now I just want to try one that's built without basswood fins and epoxy fillets. :eek: :cool:

Tweener
06-06-2005, 10:06 AM
Grip It And Rip It!!! :D
http://www.rocketreviews.com/reviews/oop/cen_thunder_roc.html
Imagine my surprise. Here I was looking forward to my first Thunder Roc flight with an Estes E9 and it turns out that I ALREADY DID IT!
Then again, it did do that hangin' thingy just as it cleared the rod. Now I just want to try one that's built without basswood fins and epoxy fillets. :eek: :cool:
I thought I saw an E launch in that review's launch reports. (They say the memory is the first thing to go.)

To Craig,
I've been playing with the rocksim demo, and modified the design for the standard configuration, just like I built before (sans baffle). It seems to do just fine on the aerotech E15 now.

CPMcGraw
06-06-2005, 11:21 AM
Hmm.. I noticed on a couple of my D12 flights that my previous 'Roc seemed to "hang" momentarily just off the rod. I wonder if rockets like this and the Mean Machine may actually have such long moments on their CG that they have a bit of "inertial stability" that resists changing direction even if they haven't quite reached aerodynamic stablility off the rod.

Nah, it's just that there so much distance between the two launch lugs that when the upper lug clears the rod, any slight change in direction will cause the second one to bind. Rod whip can cause it. That's why you need such a large kick to get these long birds started. It reduces the lag time between the lugs exiting the rod.

Craig McGraw

Tweener
06-06-2005, 11:55 AM
Nah, it's just that there so much distance between the two launch lugs that when the upper lug clears the rod, any slight change in direction will cause the second one to bind. Rod whip can cause it. That's why you need such a large kick to get these long birds started. It reduces the lag time between the lugs exiting the rod.
I noticed that, too. I always check my 2 lug birds by putting a finger on the nozzle and lifting up rapidly to check for bind. I used steel wool on the rod then wiped it down with a silicone lubricant to lessen the effect.

Tweener
06-07-2005, 12:10 PM
Craig,

I noticed that the "E" ThunderRoc Clone rocksim launch guide data says "User specified minimum velocity for stable flight: 43.9993 ft/sec". Is this something you input or does the program somehow arrive at that figure? If it was input, how was it arrived at? Just wondering because the sim says my 'D' launches shouldn't have been stable for a little over 4" past the 36" rod that I was using. :confused:

Bill,

What length rod did you use on your 'E' flight?

Excuse me if I seem a little slow, but after all this IS rocket science! :D :D

CPMcGraw
06-07-2005, 02:20 PM
Craig,

I noticed that the "E" ThunderRoc Clone rocksim launch guide data says "User specified minimum velocity for stable flight: 43.9993 ft/sec". Is this something you input or does the program somehow arrive at that figure? If it was input, how was it arrived at? Just wondering because the sim says my 'D' launches shouldn't have been stable for a little over 4" past the 36" rod that I was using. :confused:

Bill,

What length rod did you use on your 'E' flight?

Excuse me if I seem a little slow, but after all this IS rocket science! :D :D

Tweener,

RockSim either calculates a value here, or there is a "pre-set" value in the program. I believe I can change that value, but I did not select that value. I've just run the simulations with mostly default values. What I did change was the temperature at launch time, the launch guide length (note, this doesn't necessarily mean a rod), and the Latitude of the launch site.

Craig...

Tau Zero
06-08-2005, 12:08 AM
Kick A Pig should have been posted in the other thread, but it's here for Jay... ;)"Heh heh, heh heh... Hey, Beavis. Let's go kick a pig!" :D :p


My other first thoughts were to make it look like a football, with laces and everything. I might come up with a Flying Pigskin a little later...

...I've got to get that one to spin off the rod, though...

...Maybe a two- or three-motor cluster, with tilted mount tubes, and have the rod going right through the center...Your football comments reminded me of the following Pigskin-inspired rocket designs from EMRR:

http://www.rocketreviews.com/virtual_contest5.shtml

http://www.rocketreviews.com/reviews/descon8/ncaa.html


See what you've made me do, Jay!!??? :D

This "thinking" stuff really is dangerous!(modestly) Yeah, well, it's the least I can do. As a TV reporter/cameraman, I seem to have an incurable Analytical condition. --No, it has nothing to do with things gastro-intestinal. :eek:


Cheers,

--Jay

A Fish Named Wallyum
06-08-2005, 12:45 AM
Bill,

What length rod did you use on your 'E' flight? :D :D

I think ours is about a four footer.

Carl@Semroc
06-08-2005, 01:33 AM
Craig,

... data says "User specified minimum velocity for stable flight: 43.9993 ft/sec".
That converts to 30MPH. At some point in the past, someone determined that 30MPH was the minimum velocity for stable flight. In 1961, Vern Estes designed a wind tunnel (TR-5) that produced 22 fps air flow and said it was "ideal for stability tests." G. Harry Stine in his first edition used the figures 30-40 MPH not as a minimum, but as typical.

I think RockSim is very conservative. Minimum velocity varies with the geometry of the rocket. Maybe RockSim needs a plug-in to CALCULATE the minimum velocity and launch rod length.

Another consideration for launch rod length. It should be measured from the bottom of the lowest launch lug to the top of the launch rod. Many taller rockets with launch lugs high up the side and engines raised off the blast shield only have an effective usable length of 2 feet from a 3 foot rod.

Tweener
06-08-2005, 03:09 PM
Thanks Carl,

My gut-feeling was that an over-stable design like the Thunder Roc wouldn't need much velocity at all to keep headed in the right direction - as long as it was still accelerating. I can't wait to try that Aerotech E15!

BTW, my order for a set of laser-cut fins will be in as soon as I decide what else I need/want to make at least a minimum order. :D

A Fish Named Wallyum
06-13-2005, 05:02 AM
I built the motor mount and fin can for the Thunder Roc this weekend, but decided to order a BT-60 ejection baffle from Flis Kits to replace the tube coupler in the middle of the two uppermost BT-60 sections. This T-Roc should be quite a bit lighter than my first clone because I cut out the epoxy clay, basswood fins and huge nylon chute right off the bat. Next launch is in two weeks. (I was "under the weather" this weekend. In a completely different way than Craig was. :( )
I also ordered several other parts, including a "complete" set of the canted cluster mounts. They now stock them in 13mm, 18mm and 24mm for BT-50(?), BT-60 and BT-70 respectively. Not sure what project they'll become a part of at this point, but that's part of the fun. (Although that dual 24mm BT-70 mount sounds like it would make an a$$-kicking SPEV for starters.) :cool:

Tweener
06-13-2005, 08:43 AM
I built the motor mount and fin can for the Thunder Roc this weekend, but decided to order a BT-60 ejection baffle from Flis Kits to replace the tube coupler in the middle of the two uppermost BT-60 sections.
That was my thinking exactly. Kill two birds with one stone, so to speak. The added mass would only be the perforated bulkheads, since the coupler's needed anyway. The only concern I'd have is that only half the baffle is effectively anchored in the tube against the ejection pressurre. Make sure it's glued 'real good'. :D