Discussion in 'Scratch Built' started by lakeroadster, Dec 27, 2018.
Agreed - looking good!
Still layin' down glue fillets in the cluster.... so how about some more FEA ...
The lower fin can.. loading based on component drag @ max speed from the (4) D-12 motors. Nothing apparent that would cause any issues.
Neat how the tie rods bow out in the exaggerated model.. in reality they only deflect .00005 of an inch.
And the Recovery Bay... pretty minimal loading. The lawn dart look is kinda cool. I hope to never see it in reality.
The mercury nose cone gave the analysis fits for some reason... that's why it is suppressed in the screen grabs shown below
Pic 3 would make a neat color scheme!
Maybe on your NEXT version
I found a Peak of Flight newsletter that describes how to modify engine thrust curves to simulate a canted engine(s):
Since it involves modifying only the motor file it should work in OR as well as RockSim.
A bit tedious to crank out the new numbers on the thrust curve, but should give you more accurate sims. Cheers.
Neat idea... a paint scheme that mimics the FEA results.
Thanks for the link. Really interesting.
The canted engine angle on my Lifting Rocket is 12 degrees. That means a thrust reduction of 2.1% for each motor which nets a total reduction for the cluster of 6.5%.
Is that significant, let's find out.
Some research on the ThrustCurve's site shows that NFPA 1125, Code for the Manufacture of Model Rocket and High Power Rocket Motors, allows an alarming amount of motor deviation from stated values....
The total impulse must not have a standard deviation greater than 6.7%.
The ejection delay must not vary more than 1.5 second or 20% (whichever is greater, up to 3s) from average.
The average thrust must not vary more than 20% (or 1N for model rocket motors, 10N for high-power motors, whichever is greater) from average.
So since the standard for the motors is so "loose"... that motor canting 2.1% reduction is negligible.
The 20% delay is pretty crazy too! Plug that into the simulators and your likely to get a warning about the recovery device being ejected at excessive speeds.on
Another interesting fact... the motor case is allowed to reach 392°F during or after firing!
Thanks again kuririn for the link... I get by with a little help from my friends.
Made pretty good progress on the upper section of the Recovery Bay and the Lug Bulkheads.
I went ahead and used the cereal box cardboard for the transition.
Almost finished the attachment bulkheads today. 1/4" studs attach to these that stick out of the rocket to anchor the parachute chords. And there's also a bulkhead at the bottom of the rocket for the cargo bay.
It's made from 1/8" light plywood Centering Rings that are modified and glued together. The studs are retained by ny-lock nuts that are set into the bulkheads.
Pretty fun project.
Finished the fabrication on the attachment bulkheads today. Machined the studs and started gluing them into the bulkheads.... and made a drawing showing how I made the studs.
Today's progress "The Lower Recovery Bay" ...
Bulkhead for external parachute attachment
BT-60 with BT-80 and C-80
Frosted Mini Wheats cereal box cardboard transition.. "Hi I'm Mini. Lets Hang" The lugs, some rubber bands and spare centering rings were used to apply compression on the transition while the glue was drying.
Recovery Bay & Drawings
Next up, Fin Can Section
Wish there was a forum function that allowed me to automatically like all the posts in this thread...
The drawings for this model look incredible!
Thanks for the comments fella's.
I've been searching for a material to use for the cargo lifting inserts in the fins. Originally I was going to use 1/8" thick aluminum, but that just seemed like extreme overkill and the weight was considerable also.
I spent about an hour at the local Tractor Supply and True Value Hardware trying to find something that I could use, but didn't have any success.
So I went hunting in my barn and found some body filler applicators... Perfect! They are flexible and are 1/8" thick, same as the balsa for the fins.
I redesigned the size and shape in my CAD model, printed a template and cut the parts, as well as the fin openings on my scroll saw. The design now has the insert trapped inside the balsa, with dowels glued in to hold it all together.
Wow, this is one of the coolest, most creative, and well-executed rocket projects I've ever seen. Great work with the documentation and presentation as well. Can't wait to see the result, but I'm enjoying the process.
Yeah what he said!
This is pretty incredible. From the engineering, the drawing, the design and the craftsmanship!
Also, regarding the canted engines, I remember reading that it is optimal to have the thrust line of each engine go through a point midway between the CP and CG. This supposedly results in the straightest flight if one of the clustered, canted engines doesn't ignite. I don't recall where I read that, it might have been in an Apogee Components Peak of Flight newsletter.
Wow, brother! You are all in here! I’m really excited to see how this all shakes out!
PS I’m trying to design one too, but I’m low on originality right now.
I believe the rule is to have the line of thrust go through the CG. That way any unbalanced thrust does not create a turning moment about the center of gravity and only creates a side-wards movement. Otherwise, the rocket can start doing cartwheels in the sky.
Tractor motors, on the other hand, don’t point to the CG because they’re typically forward of it (by definition?)
Hey @Daddyisabar, what happens when all the tractor motors don’t light? Guessing you’ve seen that once or twice...
The principle still stands because you can always draw the imaginary line of action for each force vector that has a perpendicular distance from the CG and thus creates a turning moment. Since the tractor motors are near the center-line or axis of the model, moving the CG as close to the where the centerlines of all the tractor motors meet would be a good design feature.
Thanks for the comments and discussion guys..
The Lifting Rocket is a true tractor motor design... the CG is well below the cluster. And that's ok, we talked about that back on post #35.
I've been updating the Open Rocket simulations as I build the rocket.. here's the current simulations.
With the Cargo Bay: If only one of the D motors fires... The Lifting Rocket will be a lawn dart before the chutes blossom.
Without the Cargo Bay: If only one of the D motors fires... The Lifting Rocket will likely be a lawn dart before the chutes blossom.
That's a big part of the challenge / fun of this design.
That depends on how asymmetrical your design is and other factors like hanging up on the rod. How high does it get on two of three motors and what is the delay. Some have arced over and recovered . Some have arced over and crashed. One hung up on the rod, worked it's way off and land snaked. Tractor motors won't save you. I have seen the same results from properly canted kits from trusted kit manufactures . Following all the good rules is not going to save you either because there are too many random factors in each launch. When two of three canted motors light all that is left is prayer.
I remember where I heard that the optimal thrust line ran half way between the CG and CP, this video from Tim VanMilligan, see minute 1:50 or so. He cites Mark Paige (Page?) . Just a curious observation that seemed like an interesting design/engineering question (not intended to distract focus from the rocket that is the subject of this thread).
There is more discussion here and links:
.... that the God's of Thrust, Apogee & Recovery defeat the God of Gravity.
Thank you SO much for all the info on CAD and CFD! I've learned so much, and it is SO useful!!! I posted some results to this thread:
Started work on the fincan. I leveraged my years of hard bound acquired knowledge to assist in ensuring a good flat finished project.
Here's the body tube, sliced for the through wall motor mount
Sometimes it's more productive to make your own centering rings
And here's the 24 mm motor mount. It's set-up for E motors, but with a 1 inch long insert, can also be used with D motors. No, the lower centering ring is not glued on yet.
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