Little Joe II QTV 1:32 Scratch Build

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Texasgrape

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Hello; new to this forum so hope this is posted correctly; I have been in rocketry off/on for decades as a hobbyist with a few scale models under my belt but this is my first scale-scratch build; I am needing some input if anyone has the time:

I am scratch building a 1:32 Little Joe II QTV--I started on it 15+ years ago (back when Balsa machining still made custom nose cones and transitions; TG!) I have recently completed the no frills 'boilerplate' edition; I was wondering if someone could take a quick look-see to see if anything major wrong stands out; I am using OpenRocket--RockSim doesn't like LJ; it won't fly good in that program no matter what I do; so I went to OpenRocket which seems more accurate to me;

Although the stability margin is small (.64) the swing test was 100% successful loaded up and ready to fly; FYI: RockSim has the CP located 1 1/2" further aft than OpenRocket--have no idea which is correct;

Will be using Aerotech F44-4 motor; I simulated many but that motor just seems to bring in the best numbers for its first flight (to me anyway) I welcome input on anything; but other concerns are:

1 Will there be enough pressure to deploy the chutes? There is 6 3/4" of open parachute bay--even with the 10" stuffer tube;
2 After I glued in the stuffer tube/motor mount I realized the motor tube is not heavy walled but I reached in with a brush and beefed it up with 2 coats of slightly diluted wood glue
3 Engine choice?

I am using a 30" main nylon chute and a 12" nylon for the nose cone (command module and above); I am using the 12" as a pilot (semi-drogue) that I read George Gassaway used on his scale models: IDK seems like a good concept--those threads are old; perhaps there are new methods

Add'l info: The 1/4" dowel rod is a place holder for the tower and the siding will go on in the lower recessed portion of the main BT; all balsa has been sanded/filled/primed but I will leave the lower BT portion raw for the test flight; lastly adding 2 oz nose cone weight only changes CG about 1/8" forward so wasn't going to add any

Sorry for punctuation; period and comma bit the dust on my macbook! I've attached some pics of the stats and simulation as well as a couple pics of the build--can link to the swing test if anyone interested--

Thank you to those who keep this forum alive--so many helpful and knowledgeable people!

Dana
 

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mikec

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Post your OR file, I'm a little skeptical that it's properly modeling the effects of the escape tower, which I would expect to have little impact aerodynamically.

I assume your weight and CG are measured values?

The F44 should be OK, but with a 24mm MMT you don't have too many other options without going to Cesaroni; E30 might work but will be a little lower flight.

I think the ejection charge should be OK.
 

Texasgrape

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Thank you mikec for your input; Attached is the OpenRocket file--

Yes the CG is a measured distance and the weight is at 15.95 as weighed; (OpenRocket had the CG within an eight of inch of actual)

I used a dowel because later on I will need to round off to spec the top of the command module and didn't want to screw it up or the transition which will both be used in the final model; hope that works out OK--

Thanks again; Dana
 

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mikec

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That all seems reasonable. I deleted the tower and it didn't make that much difference. This counts as a short wide rocket and probably benefits from base drag, so the 0.5-ish stability margin doesn't bother me too much. The E30-4 also works. Note that what you can buy as an E15 is really the same as the AT E20-4, which is marginal for this rocket off the rail -- it would probably work in low wind.

Be warned that the F44 is pretty punchy and I've had problems with rod whip with it, so the E30 with about 10 g's off the pad may be preferable. Probably the AT F35 24/60 reload would work well, possibly with a bit more nose weight.

In general this is very similar to the 1/45th scale Estes kit, only a little bigger and somewhat heavier. You might want to consider a 29mm MMT to give you more motor options, but this should be OK as is.

Looking forward to flight reports!
 

Texasgrape

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Thank you for your help!

I plan on launching off a mantis using a 4' long-- 1/4" rod in a calm wind-- We get many calm mornings here in the Texas Hill Country but windy afternoons! If rod whip off my rod set up is of major concern I can always order the E30-4--

FYI: I had this LJ II designed with 29mm but changed at the last minute trying to shave weight anywhere I could; still a rookie at designing--

Will send flight reports for sure; will be launching soon--all I have left to do is install the launch lugs and wait for that calm morning!

Dana
 

mbeels

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Very nice, good luck with the flight!
 

mikec

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The Mantis isn't exactly known for its rock-solid stability (I'll admit I've never used one). On the other hand, I've had good flights and not-so-good flights with the F44 using our sometimes-wobbly club equipment; the worst was an Estes PSII Nike-Smoke that took off at about a 30-degree angle from vertical and landed about a thousand feet downrange. But your mileage may vary and I would try both motors as an experiment.
 

Juerg

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Replace the aluminum rod with a steel rod helps a lot with getting the Mantis good to handle bigger rockets.
 

Juerg

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Although the stability margin is small (.64) the swing test was 100% successful loaded up and ready to fly; FYI: RockSim has the CP located 1 1/2" further aft than OpenRocket--have no idea which is correct;
As it is a short rocket, a margin of 0.6 should be fine. But yes, such big variations make one wonder.
I am building a little Joe 2 and was just coming here to check what CPs others had figured...
Looking forward to your results.
 

Texasgrape

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Thanks on advice on rod; I have a 4' two-piece screwed aluminum now so will go tomorrow and get a one-piece 4' steel one; lots going but will be soon looking for a launch window when winds are calm;

All I have left is to glue the launch lugs on; figured I'd center one between the fins and one above CG; any advice? Length? Placement? I see many varying suggestions on both; attached is the approx location I was going to put them:
 

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Juerg

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Putting them closer together makes the rocket less prone to whip lash!

I'd put one around the middle of the rocket (~cg) and one as low as possible. Looks nicer as well imho
 

Texasgrape

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I launched this morning in calm winds and the flight was unbelievably perfect! So a big thanks for everyone's help!

1:32 scale
16 ozs loaded weight
F44-4 Aerotech engine
1/4" x 56" SS rod (48" from bottom lug to top of rod)
Two chutes (connected; see first post on configuration)

Now comes the detail part! Went to Clear Lake NASA to shoot some pics two weeks ago of the LJ II but because of COVID they wouldn't let me in to look; I think I have all I need to finish except for decals; hopefully someone has decal files and would be nice enough to share me so I can resize to 1:32 scale; I will post the decal inquiry in another topic area;

Thanks again! Dana

Link:
 

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Texasgrape

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Thank you; I have contacted him;

I am excited to start detailing and thank you for the links: I live on georgesrockets; he has done so much for the LJ community; I used to live near NASA Clear Lake and would go by and see it as well as Saturn V when I could; Years ago Saturn V used to be out in the open weather; glad they put it inside--it was starting to weather badly in the humid salt air--and the display is awesome now!
 

MaxQ

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As it is a short rocket, a margin of 0.6 should be fine. But yes, such big variations make one wonder.
I am building a little Joe 2 and was just coming here to check what CPs others had figured...
Looking forward to your results.
JUERG....long time...no see! How have you been ? Please post your Little Joe build.
 
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