Anima Mundi

Oh my, that is big :eyepop:

*subscribed*



Braden

TRF,

Four tubes done. Not sure you can see form the attached photo but tube quality has improved with practice. By order first tube is on the right and last tube is on the left.

Some final thoughts on tube construction:

1. Unrolling and carefully rerolling cloth bolts is worth the effort. I didn't have much luck making alignment adjustments on the fly. Maximum "jitter" on cloth roll should be two millimeter.

2. Wear three layers of glove. Just peel off the top layer when things get sticky.

3. Don't paint the entire mandrel with epoxy. Instead use a starter strip to tack and align the cloth's leading edge.

4. Work in quarter turn increments.

5. Put on two wraps using a brush while chasing bubbles with the resin roller discussed previously.

6. Brush from the mandrel's center to outside edges. Use long, 30-degree angle and downward strokes. That pulls the cloth tight. Don't go too fast otherwise the epoxy will froth.

7. After a couple of turns squeegee is the tool. Use it to lay down cloth, to smooth wrinkles, to press out air and to work resin up from lower layers. Then lightly top coat with the brush and keep rolling. Resin roller is still useful for chasing bubbles. But the squeegee minimizes that work making the resin roller a secondary tool.

8. Ideal batch size appeared to be 12-ounces of medium-cure epoxy in a 2 quart bucket. Room temperature was about 70 degrees. Any more resin and the pot got too thick to saturate dense cloth. Laminating requires THIN resin.

9. This job requires four people and about three hours per-tube. We had two on each side of the mandrel. Two with brush / squeegee and two with resin rollers.

10. Total cost of four tubes including materials, tooling and disposables is about $600.

11. Everyone who participated agreed this phase of construction was excruciating and horrible.

Feckless Counsel

It's Miller time! Celebrate the milestone--before moving on to the next (dozen) steps...

Feckless Counsel said:

Everyone who participated agreed this phase of construction was excruciating and horrible.

Click to expand...

That will luckily be only a short term memory. In the long run, they will remember being included an awesome project and being part of history in the making. Rocketry, like fine dining, is a communal activity, meant to be shared.

Congrats on the tubes, they look great.

TRF,

Time to cut centering rings. Some of those rings serve no function other than stiffening the airframe radially. Question is what web thickness is required? By web thickness I mean outside radius minus inside radius.

Material is 5 ply Baltic birch plywood in 1/4-inch (6.5mm) thickness. A 3-inch web makes each ring weigh 0.98 pounds. A 2-inch web makes each ring weigh 0.69 pounds or 30% less than 3 inch.

Feckless Counsel

Stringers?

Anyway, I would make one of each and try flexing each to see if one or the other puts you in your comfort zone.

Regardless, stringers will add a bit more to the structural integrity. Of course, there will be a weight penalty.

Greg

Greg,

Stringers, I believe, are a necessity. Carbon tubing is light and relatively cheap enough. So that would allow a 1-inch web? Or is that feckless?

If you use 1/4" carbon fiber stringers, I think you could run a web of 1.1/2" (1 inch seems a little too thin). You might think about running some CF or FG tape layup, to connect the stringer to the airframe for even greater rigidity.

Be advised, this is TLAR engineering. As in "I am not a stress/mechanical engineer, nor do I play one on TV". :blush:

Greg

TRF,

As my good friend said, "appetite for risk is diminished by the time and expense already invested."

I believe these fiberglass skins will support monocoque design. But, like an empty soda can stacked with books, a small "dent" to the side might buckle the mass. Given the time and expense already invested I'm now considering semi-monocoque design.

Semi-monocoque design employs longerons (stringers) to stiffen the skin. Where the skin might buckle longerons shed load in their tension and compression. The penalty, as Greg notes, is additional mass. Distasteful given the light weight goal of this project.

As light weight is the objective question becomes how many longerons are required? Three points determine a plane so I believe only three longerons are required. Those could be 1/8 x 3/8 carbon strip mounted on edge for the "I-beam" effect. The longerons would slot into centering rings spaced at 18-inch intervals.

Any experience or comment?

Feckless Counsel

It sounds good, the stringers will make it a LOT stronger!

TRF,

Cutting large plates and centering rings has been discussed on several occasions. My solution is pictured below. That is a router, junk router base and some steel dowel stock. The junk base is fitted with a polycarbonate base plate and hole to rotate about an 1/8-inch pin. The router, a DeWalt DW621, comes with highest recommendations as a freehand or small table router. Micrometer depth stop is excellent.

Feckless Counsel

TRF,

I've lost my sense of proportion and would appreciate your comments on the size of fin selected to this project.

Attached is a picture of the fin template used as a guide in actual fin cutting. Having made the template I'm concerned the fins are smallish. That "feeling" is for both control and aesthetics.

CONTROL: RockSim indicates three such fins are sufficient for stable flight. They are small, stout and contribute to the low mass goal of this project. But those with general simulation experience would be cautious. Any comment on RockSim's simulation performance to the question of control?

AESTHETICS: Attached is a RockSim rendering of the project. I like the proportions but am unsure of RockSim's tendency to distort. Could anyone say if their rocket failed to match aesthetic appeal first accepted in a RockSim rendering?

I've lost my sense of proportion and want to let RockSim guide my way. What is your experience?

Feckless Again

In the rendering they do look a bit small, aesthetically. And seeing the NC in the flesh this past weekend, truly gives the sense of the monstrosity that you are building. I can't wait to see this thing fly.

Fins look within reason. Which method (Rocksim or Barrowman) are you using for static and dynamic CP? What is your stability margin?

Here's something that MAY bring a little more comfort:
Lower total mass = Lower corrective force needed = smaller fin area needed.
FWIW

Fe Dude,

I really appreciate the opportunity to share those stock materials with the club. Everyone's feedback was instructive, constructive and well received.

Feckless

Tim,

In general margin is about 1.50 using RockSim's method. Reason I can't answer definitively is that RockSim fails to match some hand calculated and measured masses. So I'll have to weigh and then mass-override everything for an "exact" margin simulation.

Feckless

Feckless Counsel said:

Tim,

In general margin is about 1.50 using RockSim's method. Reason I can't answer definitively is that RockSim fails to match some hand calculated and measured masses. So I'll have to weigh and then mass-override everything for an "exact" margin simulation.

Feckless

Click to expand...

You should be able to do mass override and CG entry for each component. That could be done right now versus doing it on the completed rocket (if that is what you mean mass-override "everything").

Troy,

I'm very sorry we missed one another this weekend past. You have made many contributions to the airframe's construction and I wanted to share the "raw" results with you more than anyone.

Your comment does give some comfort and I agree that less torque is required to correct a smaller mass. Torque, as Tim notes, is the product of force and distance. So I'll need to dial in my understanding of "margin" to be sure.

Feckless

Tim,

We follow each other exactly. I intend to weigh and estimate CG for each part as they are fabricated. Those will be inputs for an "exact" simulation. Nothing will be assembled prior to that design validation. Meanwhile I'm sitting at 1.50 margin based on a combination of measurements and estimates.

Feckless

Well ... since you asked about the fins.

If they look small, they are small.

I read a story about the ol' NACA engineering team from Langley VA that came into NASA had an approach that went like, "If it doesn't look right, it isn't right". So, if it were me, I would increase the span, a bit but not much, and add more sweep. Or, you can scale them up 10 to 15% and see how they look.

Look at it this way, ALL eyes are going to be on this baby when the button is pushed, so it might as well look good when the countdown starts.

Greg

TRF,

The attached drawing compares the existing fin design to one having a lengthened root. Existing design is on the right. Lengthened root is on the left. Any comments?

Feckless Counsel

It's subtle, but the one on the left looks better.

It seems more in proportion.

Greg

Do you think 4 fins might help any thing?:confused2::horse:

TRF,

Working on the fincan and associated centering rings. Below is a rough fit of the ring and retainers. Central motor is 98mm surrounded by three 75mm auxiliaries. Should produce a respectable column of fire?

Feckless

:y::jaw::headbang::wave:
That Is HUGE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

TRF,

Adding 4-inches to fin's root calculates an additional 1-pound per fin. Over three fins that's 2.8% of this project's weight goal added in exactly the wrong location. Seems a "hefty" price to pay for fashion.

Still trying to gain perspective I dry fit that fin template into the motor assembly. The picture below almost makes those 2-foot diameter rings look small?

Feckless

Feckless Counsel said:

Adding 4-inches to fin's root calculates an additional 1-pound per fin. Over three fins that's 2.8% of this project's weight goal added in exactly the wrong location. Seems a "hefty" price to pay for fashion.

Click to expand...

I agree. I say go ahead with your plan. Incrementally fabricate, measure, adjust as needed. I don't think you are that far off. The overall profile on the rocket is not that much different than the LOC I-ROC if you ask me.

TRF,

The auxiliaries centerlines are spaced 6-1/2 inches from the entire rocket's centerline. I wanted to space them closer but am afraid to weaken the plywood rings. I wouldn't want that central N to shoot through the nose. Likewise I don't want thrust imbalance to torque the rocket downrange or worse.

The arrangement shown leaves some web between the tubes.

Let's consider a 50% M as an auxiliary. Thrust is something like 100 pounds. Tolerance among motors could be something like 10% neglecting ignition delays. So we have a torque of 65 inch-pounds or 7.3 Newton-meter acting at CG. How much is that? A little more force than is required to open a stiff jar. I believe that fins cutting air at 100 miles per-hour will provide sufficient resistant to that torque.

What do you think?

Feckless

Feckless Counsel said:

Adding 4-inches to fin's root calculates an additional 1-pound per fin.

Click to expand...

Making the root longer will move the CP forward a little as well as possibly moving the CG back (the latter depending on how the fin shape is changing). If you want better stability I would increase the span, not the root. (Though I understand you may want the same fin shape for aesthetics.)

I didn't think your small fins looked bad, but it's first and foremost about the stability margin.