Another Big Dumb Rocket

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Chris_H

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Here is a project I have been working on for a year or so, it is nearing assembly time.

This is for an L3 cert flight to be flown at Balls this year.

This is a through-the-wall design, thus the 'BDR' designation.

The motor for this flight is a CTI M2245.

A 'shakedown' flight is planned for early June.

 
The motor mount.

An Aeropack tailcone retainer. 1" thick ply thrust and centering rings. The fins will be 'let in' to the lower two ply rings. Internal injected epoxy with filler, and external fillets of ES6209 with filler will secure the fins.

The 1" kevlar strap is wrapped around the motor tube. The strap will be the anchor for the recovery harness, rather than an eye bolt in the forward closure.

 
This is most of the recovery system.

The main harnesses are in 9/16" tubular nylon. I like the stretch. The ends of the nylon harness are terminated in kevlar loops, sewn in.

The drogue harness has a third loop sewn in near the AV bay, inside the sewn in blanket.

All external stitches are in heavy kevlar thread.

The drogue harness is 30', the main harness is 25'.

The deployment bag is sewn to fit a 60" Fruity Chutes Iris Elliptical, which fits perfectly. A 15" pilot chute will pull the bag from the main chute.

The drogue and the pilot chute are my own work. All sewing is my own.

The nosecone anchor is an M6 316 stainless stud. I drilled and tapped the original hole in the aluminum tip, and made a more precise G10 disc to secure the tip. The space between the G10 disc and the aluminum tip will be filled with tungsten filled epoxy. It is not much space, and will add about 170 grams. This will prevent the aluminum tip from shifting, minimizing a paint failure point, and slightly improving aerodynamics.

Recovery harnesses are sewn through the nomex blankets.


 
Here is the AV bay so far.

The sled is made from 2 layers of 1/16" G10, laminating a core of basswood.

The studs are M6 316 stainless.

2 eye nuts are at each end of the av bay. A 3/8" loop of kevlar is passed through both eye nuts, with a twist to achieve an 'equalized' arrangement which gives redundancy in case of a singular eye nut failure. Forces are distributed equally to both nuts, with an initial shock absorption at the time of things pulling tight.

Deployment will be controlled by a Marsa 54HD, and a Raven 3. Switches are the Fingertech robotics screw switches, because why would you use anything else.

2 x Turnigy Graphene 2s batteries are fixed to a battery holding mount in the top of the AV bay with 4 zip ties.

Deployment devices will be based on 5/16" i.d. vinyl tubes, and connected directly to the altimeters.

I may install a GoPro video camera into the AV bay.

 
What motor are you flying in that? Is that the big one that took a walkabout in the outback of the playa for a few years?
 
Chris_H said:
What motor are you flying in that? Is that the big one that took a walkabout in the outback of the playa for a few years?
Click to expand...

That 'motor' is reloaded and back in the rocket. I had it all ready to go at Balls last year, but had GPS issues. I scrubbed the flight. I did not want to 'misplace it again!
This is one is nothing fancy..just...hopefully... a fun, easy to prep and reliable 98mm min dia..if there is such a thing.

Tony
 
There might have been a little 'tongue and cheek' in that..


I was making light of the fact that there are such different 'camps' of what makes a good rocket for a cert flight. Some folks are super adamant about building huge expensive rockets that take up a lot of storage space, and are not really performance based at all. And, at the other extreme, someone set an N class record for their level 3 cert flight, ruffling a few feathers on the way. I think there is a lot to building for the field where the flight will take place, and also, more importantly, I want to build in line with my passions, and in the direction of where my ambitions lie. In building larger rockets, this may be the last through the wall design that I build.

Still, this is a through the wall design, so IMO, as long as everything is put together well, which it will be, there should be no problems.

Part of my reasoning for making this thread, is that I had a question about the design. I was hoping that someone might speak up with some critical words about the design. Since I posted it, I made a pretty significant change to the design. No one has publicly spoken up, yet, in design critique. Tony's post offers a big clue. He more or less directly answered my question, though I am not sure exactly where the point of a clear answer lies. It all becomes trade-offs and compromises.

Over the Summer, this rocket will be completed. I have 2 nearly identical AV bays, and recovery systems. I see a shakedown flight or 2, one on a J1299, and one on a J1799. Those should allow a good visual on how all the pieces fit, and how it falls, and it will stay close to the ground. The J1799 will give a little kick to it, so in case it falls apart, it will fall closer than it will with the M2245. :) The fin construction will be rock solid, and everything smooth and precise, I do not expect a structural failure. This seems like a good basic rocket for flying at the playa.


I was very tempted to build minimum diameter, but I am starting with a 54mm rocket for that. Something less expensive to go through the process of getting the fins and tip to tip all nice and pretty, and learning to work with the exact materials for a higher test design.
 
Ah yes. The overbuilding thread!

L3: Big heavy and expensive, small fast and expensive, or somewhere in between!

Good luck!
 
Oh, and I realized that I said that the core of the sled is Basswood, it is not. It is Sitka Spruce from an instrument top that I did not use. The strongest light wood, or the lightest strong wood, whichever you like. Makes little difference. I would have used balsa, if the slab of it I have was not bried so deep in a wood pile. From a technical standpoint, Basswood is good for carving, and might not have any particular strengths for use in a rocket. Sitka Spruce is used in aircraft.
 
Yep, I am still wondering where the high, fast, and cheap is! ;)

Thank you.
 
Nice rocket. Nothing rings any bells for me. 1 inch thick centering rings are thicker than necessary but there’s nothing wrong with that. Nothing big or dumb about this. I prefer nylon for the stretch as well. The only strap I’ve ever had fail without a clear reason was Kevlar.
 
Steve Shannon said:
Nice rocket. Nothing rings any bells for me. 1 inch thick centering rings are thicker than necessary but there’s nothing wrong with that. Nothing big or dumb about this. I prefer nylon for the stretch as well. The only strap I’ve ever had fail without a clear reason was Kevlar.
Click to expand...




Thank you, Steve.



Yes, the thick rings are plenty strong. I work with BB ply all the time, and have scrap of 1" on down. They are from what was basically, junk wood. The difference in time to make rings from 1" instead of 3/4 is not noticeable. They are very quick to make. Sometimes it is difficult to see where a weakness lies in the factory laminations in this ply. With 1" ply, at the cost of a little extra weight, I think a little more stiffness is achieved in the airframe than with fiberglass rings, especially in having one at both ends of the long motor tube. I have no math to back that up, however.


What I was having issue with, was the fins. I have now increased the height from 3.8" to 4.5", to increase the span a little. The stability went up to around 2. It probably would have flown safely the way it was, but I feel better about it now. A little altitude was lost, but I think the whole project is improved.
 
You didn’t say much about the construction of the fins or their thickness. Increasing the span makes them more susceptible to flutter. I assume you’re on top of that though.
 
Ah. It was in the attached file. They are 3/16" High temp G11.

However, the airframe is slotted to .125", as my 3/16" bit cut a slot that was just a little wider than I liked for the 3/16" material that I had. I was counting on that I could open the slots up to 3/16" from 1/8", which I can easily do. I was also counting on checking the alignment of the slots with measuring equipment to see that they are all precisely in line with the airframe, and with no twist. I was planning on checking the same edge on all 4 slots, and then opening up the other edge of the slot for a 'barely slip' fit. The slots cut perfectly in line and need no adjustment. So, I am considering using uni/twill .125" Dragon plate. It is either my time in opening up the slots, or a little extra cost. I can sand precisely to within a few thousandths, or a lot less when it counts. Time is precious, though. Someone today suggested that the Uni/twill .125" Dragon plate is stiffer than 3/16" G11.


I have not gained access to the fin flutter program, I am not sure why. A while back I ask for access to use it, and never heard even a whisper back. In case someone wanted to run this for me, I would be your fan. Perhaps I should send a follow up email.


The alternative 'engineering' that was used, was conversation with 2 people who know from experience, and a similar fin design as a direct example, shared by one of these folk. That came along with a video of a flight of a rocket with fins with a height of just under 1 caliber, like my original fins, that went a little wonky before straightening out. It was suggested that a fin height of at least 1.1 caliber or even a little more, might be safer than what I had planned. As this flight will barely pass mach 2, and this being the largest motor that will fit in this rocket, and basically the same fin and configuration that I am now using is on a minimum diameter rocket that will be flying a much bigger motor, I fudged the math for this engineering. The fins on the design posted above by tfish are 4.5" tall, and pretty similar to mine.


Fins:

4 fins
Root Chord: 11.5"
Tip Chord: 1.5"
Height: 4.5"
Sweep Length: 11"
Sweep Angle: 67.8

There will be a 15 degree bevel on each side of the leading and trailing edge, not razor sharp.
 
For checking fin slots, I first check for a square cut on the end by putting it on a granite measuring plate and checking with a big machinist's square. After the tube is established as plumb, the slots can be checked with the same square.

Using calipers, the spacing of the slots can be checked.

I have seen a commercially cut tube that had slots that were out by a about 0.012" from the top to the bottom of the slot, and the same on all 4 slots. Enough to cause a spin?
 
Cool seeing "4 inch rocket flown on M1850 to 18,881'"

This file sims to 23,961' on that motor. I look forward to seeing how far off the sims are, with only vague expectations, and a hope of a successful flight.


A couple of concepts that I have been thinking deeply about lately, are exactly what is happening in terms of fluid mechanics up in the nosecone during main deployment. My first introduction to fluid mechanics was in the head room of a facility that built racing motors for a class of land speed record vehicle, among other things. The lead mechanic lead me through some questions, which I answered all incorrectly, by a long shot. The current questions about that, are, when the charge causes the gasses to expand, there may be some moment of compressing the deployment bag more up into the nosecone, but maybe not much, and the gasses may build up more pressure between the packed bag and the AV bulkhead faster than the remaining open space in the tip of the nosecone, how quickly does the entire space equalize in pressure? The fit of the bag is excellent, it is round, and sewn to the size of the cone. It almost falls out, even when compressed in. The shock cord burrito on the stiff kevlar lead will not allow the AV bay to travel at all without pulling the bag out. It feels right, and I have had success with a nearly identical setup that did not fit as well. The mechanics of what happens during the charge expansion are fascinating and intriguing. I wish I understood it more fully, like a physicist might explain it.

And, also, I had some tungsten powder laying around for years collecting dust, about 8 oz. of it. With the the way I solved the main anchor in the nosecone, I wanted the best chance to be able to finish the nosecone and not see a crack in the finish where the aluminum tip meets the glass, so I was planning on filling the void inside with epoxy. I added about 7ox of tungsten into the epoxy I used to fill the tip, between the G10 anchor plate, and the base of the aluminum tip, it seemed like a good use for it. The small void is completely filled. In this rocket, and another one, it barely changed the projected altitude, but definitely increased the stability. I wonder if that is from reducing any 'stability harmonics' (I made that up) or vibrations or similar, due to the added weight? The nosecone is heavier in the tip, now. This changes deployment dynamics a little, as when the drogue is deployed, the inertia of the nosecone and AV bay will be quite a bit greater, meaning that I need to give a little more attention to not causing the main to blow at apogee. A blown main at apogee is a failed flight, and a potential to never see the rocket again. I have a similar 4" rocket that will be used to get this 'wired'. Hopefully. 3 x 2-56 screws for the drogue break, and 4 x 4-40 screws for the main will be a start. Ground testing will give more hints.
 
Chris_H said:
Cool seeing "4 inch rocket flown on M1850 to 18,881'"

This file sims to 23,961' on that motor. I look forward to seeing how far off the sims are, with only vague expectations, and a hope of a successful flight.


A couple of concepts that I have been thinking deeply about lately, are exactly what is happening in terms of fluid mechanics up in the nosecone during main deployment. My first introduction to fluid mechanics was in the head room of a facility that built racing motors for a class of land speed record vehicle, among other things. The lead mechanic lead me through some questions, which I answered all incorrectly, by a long shot. The current questions about that, are, when the charge causes the gasses to expand, there may be some moment of compressing the deployment bag more up into the nosecone, but maybe not much, and the gasses may build up more pressure between the packed bag and the AV bulkhead faster than the remaining open space in the tip of the nosecone, how quickly does the entire space equalize in pressure? The fit of the bag is excellent, it is round, and sewn to the size of the cone. It almost falls out, even when compressed in. The shock cord burrito on the stiff kevlar lead will not allow the AV bay to travel at all without pulling the bag out. It feels right, and I have had success with a nearly identical setup that did not fit as well. The mechanics of what happens during the charge expansion are fascinating and intriguing. I wish I understood it more fully, like a physicist might explain it.

And, also, I had some tungsten powder laying around for years collecting dust, about 8 oz. of it. With the the way I solved the main anchor in the nosecone, I wanted the best chance to be able to finish the nosecone and not see a crack in the finish where the aluminum tip meets the glass, so I was planning on filling the void inside with epoxy. I added about 7ox of tungsten into the epoxy I used to fill the tip, between the G10 anchor plate, and the base of the aluminum tip, it seemed like a good use for it. The small void is completely filled. In this rocket, and another one, it barely changed the projected altitude, but definitely increased the stability. I wonder if that is from reducing any 'stability harmonics' (I made that up) or vibrations or similar, due to the added weight? The nosecone is heavier in the tip, now. This changes deployment dynamics a little, as when the drogue is deployed, the inertia of the nosecone and AV bay will be quite a bit greater, meaning that I need to give a little more attention to not causing the main to blow at apogee. A blown main at apogee is a failed flight, and a potential to never see the rocket again. I have a similar 4" rocket that will be used to get this 'wired'. Hopefully. 3 x 2-56 screws for the drogue break, and 4 x 4-40 screws for the main will be a start. Ground testing will give more hints.
Click to expand...

I really think you’re going to be just fine. You’re right; more mass in the nosecone makes it easier to shear the shear pins and shake the main at apogee. Calculate the “weight” of the nosecone at 30 gees of acceleration. Is that enough to shear the main pins? That’s a force I’ve seen in friends deployments. (I don’t have as fancy of recording altimeters with accelerometers.) If not, increase the size and add powder.
 
You are a gem, Steve. Thank you.

I think I can grasp that math, with a little help. I had read about drag separation, but did not know how to calculate for it.

If the nosecone weighs 840 grams, how do I calculate what it's weight is at 30 gees?

I should know this. It is time to learn.
 
People use gram measurements as if they are weight, but that’s simply for convenience. Grams are really units of mass and mass times gravity’s acceleration is weight, which is a force. Force = mass * acceleration.
So, if the mass of the nosecone is 840 grams (0.840 kilograms), the weight (at 1 gee) is 9.8 meters/second^2, which is the accelerationdue to gravity, times 0.840 kilograms and the answer is in Newtons, which is a unit of force.
So, the actual weight of the nosecone would be 8.24 Newtons. At 30 gees, the nosecone weights 30 times that, or 247.2 Newtons.
 
Thank you, Steve.

That was perfect. Like chewing a bite of good jerky, then beginning to see the simplicity in it.
 
This is the easiest way I have seen so far for beveling fins. This is a Foredom 'Typhoon' 15 degree 'fine' carbide bur in a router table with a fence. Setup involves deciding how deep to make the cut so that you are not cutting to, or past halfway. I have used a router bit in the past, with an angled jig to hold each fin, and have considered some other methods. Foredom also makes a 7 degree bit. If it was desired to have a slightly shallower angle, an angled fence could be made. I am ok with the 15 degree angle for this rocket.

The fin is held against the fence, and pushed through the gap AGAINST the direction of the rotation of the bit. This is an important detail, as if you accidentally put the fin in the gap in the direction of rotation, there is a high likelihood of the bit taking the fin and inserting it into the nearest wall, or person, and maybe pulling your fingers into the bit as well. Of all the tools in the shop, the router actually scares me a little, every time I use it.

It takes little effort to do this, and the results are very good. This bur will hold up to G10 and CF much better than a carbide router bit.



Thanks to CJ for this idea!

I increased the fin span to 1.15 caliber, or, 4.6". These fins are quasi-isotropic uni-twill CF, 0.125" thick.

This rocket will most likely be finished in a catalyzed polyester and fully polished. Not sure what color scheme, yet. These black fins will probably not stay black, though.


 
The setup in the above picture seems unsafe to me. I agree that you need to push the workpiece counter to the direction of the cutting or grinding bit. That way your hands are ‘downstream’ of the router bit.

But how do you hold the workpiece (fin) once it is half way through? Seems to me you now have to grab the opposite side of the fin and pull. your hand is now ‘upstream’. If the bit grabs the fin, it will pull your hand into the burr bit.

A better way is to clamp the fin to a LONG piece of wood and offset the router fence by the thickness of the wood. That way hands can be kept ‘downstream’ for the entire cut.

Another way is to cut a small hole in the fence and have just a little bit of the burr bit exposed beyond the fence surface. I’d still use an offset wood block or jointer safety holders for this set up.
 

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