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-   -   2X Goblin Redo (http://www.oldrocketforum.com/showthread.php?t=2280)

Thor 10-06-2007 11:13 AM

2X Goblin Redo
 
This time I am going with standard weight (wall thickness) BT80 tubing and 1/8" balsa fins. I plan to try a double E9 cluster on this one. SemRoc now has the proper nose cone.
SemRoc Nose Cone Can someone sim the double E9 with this rocket for altitude and tell me what delay I should use?
I am trying to stay with BP motors on this one. If it works well, I may do a simlar size rocket and motors with the Der Red Max, my daughters favorite rocket.
Thanks

Thor 10-15-2007 11:36 AM

Opps! Looks like I forgot to say "Please help me!"

tbzep 10-15-2007 02:55 PM

I don't have RockSim, but I can guess for you. :D

I have a BT-80 based Goblin upscale that I fly on single D motors. It has 1/4" thick balsa so it will be more draggy than yours. Your twin E's will give it a bit more weight, so the inertia will help it coast farther than my single D. With that combination, you should be considerably faster at burnout than me.

My rocket flies on D12-3's on windy days and D12-5's on calm days to give it perfect ejections every time. It just kinda hangs in the vertical position on calm days and barely starts a tail slide before kicking out the chute.

With all that info, I'd say your twin E rocket could go with the E9-8 on calm days and drop back to the E9-6 if you have a decent breeze. That's 3 seconds longer than my single D12-5 model, but your rocket should be somewhat more aerodynamic and move a little faster than mine. Six seconds seems a little short, but if it's a breezy day, you might want to go with that anyway because it will start arcing over much sooner. All of this is just a guess without knowing the final weight of your model, so YMMV. ;)

I'm looking foward to seeing how my seat of the pants prediction compares with RocSim and the real flight. :)

Edit: BTW, I just recently finished a BT-80 based Red Max with 1/4" balsa fins built to fly on a single D. I've been flying it on C11-3's in my small pasture. It's still going up a little at ejection, but I've only flown on completely calm days. I'm guessing it will do great on a D12-5 also.

Thor 10-16-2007 10:39 AM

Thanks so much, that helps a bunch! I have been looking at your recent launch pictures and I must say they are AWESOME! Very nice. Do you have a wild guess what altitude you are getting with BT 80s DRM and the Goblin you are flying? THANKS AGAIN tbzep!!

tbzep 10-16-2007 02:29 PM

Quote:
Originally Posted by Thor
Thanks so much, that helps a bunch! I have been looking at your recent launch pictures and I must say they are AWESOME! Very nice. Do you have a wild guess what altitude you are getting with BT 80s DRM and the Goblin you are flying? THANKS AGAIN tbzep!!


Thanks! The Goblin upscale probably hits about 175-200 ft, but I'm just going by the way it looks from underneath compared to the trees. It's definitely a wild guess because I'm standing more or less right under them to take pictures. The DRM with C11-3 is probably ejecting at about 125 ft. I imagine it will get about the same as the Goblin with a D12-5. I should build myself a theodolite and get some more accurate altitudes sometime.

ghrocketman 10-16-2007 03:34 PM

Why the double E9 configuration ?
I would just use a single 29mm mount which would allow the use of 29mm RMS and SU, and 24mm with an adapter which would allow 24mm AP and BP SU as well as 24mm RMS reloads.
A single 24mm RMS F24W or F39T is roughly the equivalent n-sec of 2 E9 BP SU engines with greater kick off the pad. The cost of a single RMS24 F would be less than 2 E9's unless you are buying the E9's only with 40% off coupons at Hobby Lobby.
Normal price for a 3 pak of E9's in my area is around $15...this equates to roughly $10/flight with 2 E9's. RMS24 F24W's are around $20-$22 per 3 pak, which would make the flight cost about $7/flight.
The flights would probably be higher too as the weight savings for the RMS24 would more than outweigh the extra 5n-sec the 2 E9's would provide over a single F24W.

Unless you are dead set on a cluster, I would go single 29mm with a 24mm adapter for the above reasons.
This also would give you the option of flying it on several 29mm RMS and SU engines that have WAY more punch and total impulse than any pair of E9's could even dream of.
The new SU econojet 29mm F27R would be a good one along with the G71R reload in the RMS29 case.

tbzep 10-16-2007 05:05 PM

Quote:
Originally Posted by ghrocketman
Why the double E9 configuration ?


I can't speak for Thor, but there are several reasons that I now stick with BP motors. The two biggies for me are:

1. BP is more "classic", since AP motors weren't widespread until the latter 80's. Yes, I know they were available way before then, but they weren't common to every "rocketeer". I went through my HPR phase and now enjoy limiting myself to smaller rockets that will do fine on single BP motors or BP clusters. :)

2. I can get BP "E" motors at the nearest Hobby Lobby, but I'd have to order AP motors.

With the 40% off coupon, it makes them considerably cheaper to fly than ordering 24mm E15's at $12-$15 each, and about $25 and up for F sized Econojets. RMS has a considerable up front cost on the hardware, and the extra cost repeats if you lose the rocket. Shipping charges by far override any savings on sales tax from online purchases.

When I stopped doing EX/HPR, I completely abandoned AP and went back to BP low power motors exclusively. I found it refreshing to go the opposite direction of the vast majority of the grown males in the hobby that want to go bigger/faster/longer/higher. I want to keep them down where I can watch the whole flight. ;)

ghrocketman 10-17-2007 11:38 AM

I'm not really an altitude or speed freak when it comes to rockets either, but there are several kits that have been marketed as "model" rockets that will NOT turn in any sort of decent flight on BP motors. Two that come to mind are the Estes and Centuri Saturn V's.
They fly to maybe 100' on a D12 or 150' on a 3xC5-3 cluster. Forget about using the dismally LOW thrust E9 in them as they will not get up to safe rod speed.
The Original Estes Maxi brute Alpha, Alpha3, Honest John, V2, Pershing 1A, and S1B are just as bad. Nothing short of an AP E15 produces a decent flight.
The old FSI BP E60 and F100's were good in these when and if they did not CATO, but in this day and age we most likely will never again see a high-thrust high impulse SU BP engine.

The Estes Big Daddy needs bigger (or at least higher thrust) than an E9 for a decent flight as well....if they made a full 40n-sec BP say E30 that would be great.

The Semroc Hustler won't turn in any sort of a decent flight on anything BP that currently exists either, but at least that one has NO BP motors on the recc. list from Semroc....In fact even a AP D15 reload is too little....mine might fly to 100' on a D15 that I used for the first flight. I have not used anything under a full 40n-sec AP E since then.

I fly my RMS24 almost as much as BP A-D engines.

Fortunately my yard borders a 600+ acre farm that I am allowed to recover my rockets into, which allows me to fly basically anything non-waivered up to and including 2xG power.

tbzep 10-17-2007 02:15 PM

I somewhat agree. I don't mind the Saturn V's low flight because it's up close and personal so you can really see the flames and the ejection. It's really cool to watch, IMHO. I have an old Colossus which is just a stretched Maxi Alpha 3 with different decals. It flies quite well on a D12 as does my Maxi Honest John. Maybe I just build them light, or maybe your idea of a good flight is different from mine. :)

The Maxi Pershing is the dog of the whole Estes bunch (not counting the new Outlander). It was marginally stable as slow as it was, and it probably didn't fly as high as the Saturn. It was so marginally stable that I retired it immediately after its maiden flight and it hasn't been in the air since. :eek:

ghrocketman 10-17-2007 02:26 PM

The Maxi Pershing 1A flies REALLY well on an Aerotech RMS24 F39-6T !

I think if you saw one on that engine you might reconsider it's non-flight status.


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