L3 Project - 8" AGM-33 Pike

[mtnmanak]

The last major section to complete was the nosecone. For the Nosecone electronics sled, I JB Welded a piece of .08” FG sheet to ½” aluminum tubes spaced to the all-threads in the nosecone ebay. I also JB Welded the all-thread and U-bolt to the upper bulkhead.





The nosecone ebay is going to be epoxied into the nosecone, so before I adhered anything, I attached a piece of 2000 lbs Kevlar to the aluminum nosecone tip and to the upper bulkhead U-bolt. This is just a safety lanyard in case the epoxied coupler in the nosecone fails. If it should come loose, the Kevlar will keep the nosecone parts attached to the shock cord. Ideally, this cord will never see the light of day again!



I then JB Welded the aluminum nosecone tip in place and epoxied the nosecone coupler in place.



Finally, I attached the bracket for the Marco Polo transmitter to the sled. I chose to orient the transmitter sideways because the antenna works best in a vertical position, but if the nosecone is laying on the ground after recovery, this orientation will ensure the antenna is vertical.

There is plenty of room to add a GPS tracker later, of needed.

 

[mtnmanak]

I installed the nosecone shear pins, so this rocket is functionally complete! Still a lot of work to do - ground testing, sanding, painting, etc, but it could fly now. Still hoping to get it in the air by the end of the month on a shakeout flight.

 

[Troy3003]

That is a beast. Very nice write-up on the build.

 

[BDB]

Great thread. Lots to love here, but I think my favorite is the CNC'd switch mount. I'm probably going to steal that idea but 3D print it.

 

[mtnmanak]

Great thread. Lots to love here, but I think my favorite is the CNC'd switch mount. I'm probably going to steal that idea but 3D print it.

Great idea to 3D print this. I don't have a 3D printer (yet), but am definitely thinking of adding one. The basswood version of the switch bracket is super lightweight. Even as big as this one is, it is only 3 ounces. I am sure you could 3D print it much thinner, though.

The design/use of it was just born out of trying to figure out a way to get the switches closer to the edge of the coupler without attaching them to the coupler.

Would love to see what you come up with if you decide to print a better version!

 

[Tim51]

Excellent build thread. Nicely thought out, innovative build. Have you decided on a colour scheme yet?

 

[mtnmanak]

Excellent build thread. Nicely thought out, innovative build. Have you decided on a colour scheme yet?

I am planning to make all the body parts a deep red and the nosecone and fins bright chrome to match the chrome decals I got from Mark at StickerShock. It should be bright and bold!

I am considering painting the switch bands bright yellow, but haven't made up my mind yet on that one.

 

[mtnmanak]

These are the two color schemes I was playing with. Either red nosecone or chrome nosecone. Chrome nosecone seems to look better in the RockSim rendering, but RS does not do "chrome" justice, so still on the fence. One good thing about more chrome is it should really reflect sunlight, so it would be easier to see it at higher altitudes.

 

[mtnmanak]

Ground testing complete. Main para charge is 4.5g (5.5g backup) and the drogue charge is 3g (4g backup)

Some cool photos of the fireball created by those large charges:

 

[Homer_S]

These are the two color schemes I was playing with. Either red nosecone or chrome nosecone. Chrome nosecone seems to look better in the RockSim rendering, but RS does not do "chrome" justice, so still on the fence. One good thing about more chrome is it should really reflect sunlight, so it would be easier to see it at higher altitudes.

View attachment 477225

View attachment 477226

Will the chrome paint attenuate the GPS signal?

Homer

 

[Tim51]

Will the chrome paint attenuate the GPS signal?

Homer

That would depend on whether the paint in question had actual metallic ingredients.

 

[mtnmanak]

Will the chrome paint attenuate the GPS signal?

Homer

It won't for the Marco Polo transmitter because it uses RF signals and metal does not seem to affect it much (I have literally used the MP transmitter attached to an all-thread rod).

If I replace it later with a Eggtimer or Missile Works transmitter, that is a good question and I will have to check the paint composition, as Tim51 notes.

 

[mtnmanak]

Was able to launch this rocket in a shakeout flight this weekend at URRG in Penn Yan, NY. Great flight. Flew perfectly on an L2200g to about 2000 feet.

Video:



Photos:

 

[rfjustin]

Digging the comfortably safe main deployment altitude!

 

[crossfire]

What did you use for main chute? Great flight

 

[ThePlmbr]

Nice shake down flight! I like the nice green flame with your primed gray rocket!

 

[rfjustin]

What did you use for main chute? Great flight

 

[mtnmanak]

Digging the comfortably safe main deployment altitude!

Amen, Brother. That huge chute in a deployment bag took a good long while to fully inflate.

Had the main charges fire at 1000 feet. Flight data shows it took a good 200-300 feet for the main chute to fully open. With a 65' drogue shock cord, a 45' main shock cord, a 12' rocket and a 20' long parachute, the whole thing stretched out almost 150 feet in the air, so if the main was fully inflated at 700-800 feet, the booster was already somewhere south of 600 feet, which is about as close as I would want to cut it on a rocket this heavy!

What did you use for main chute? Great flight

Justin has the right of it in the post above. SkyAngle Cert 3 XXL. Descent rate was very slow for this 85 pound rocket. Will use it again for the cert flight in November, but after that, I may switch it out for the Cert 3 XL for future flights.

 

[mtnmanak]

Nice shake down flight! I like the nice green flame with your primed gray rocket!

Yeah, those Mojave Green motors have a super cool flame! They are b!#ch to start, but I used a pyrodex pellet taped to the igniter. As you can see in the video, the motor ignited almost immediately.

 

[crossfire]

Amen, Brother. That huge chute in a deployment bag took a good long while to fully inflate.

Had the main charges fire at 1000 feet. Flight data shows it took a good 200-300 feet for the main chute to fully open. With a 65' drogue shock cord, a 45' main shock cord, a 12' rocket and a 20' long parachute, the whole thing stretched out almost 150 feet in the air, so if the main was fully inflated at 700-800 feet, the booster was already somewhere south of 600 feet, which is about as close as I would want to cut it on a rocket this heavy!



Justin has the right of it in the post above. SkyAngle Cert 3 XXL. Descent rate was very slow for this 85 pound rocket. Will use it again for the cert flight in November, but after that, I may switch it out for the Cert 3 XL for future flights.

Yes it was a very nice descent.

 

[stonc024]

Great post thanks! I have the same kit and was trying to figure out how to get thrust plate set-up to accommodate the fact they only give you 40" of motor tube, should have been like 42", or a 50" and let us cut it back.

 

[mtnmanak]

Great post thanks! I have the same kit and was trying to figure out how to get thrust plate set-up to accommodate the fact they only give you 40" of motor tube, should have been like 42", or a 50" and let us cut it back.

Agreed, If I was going to build this again, I would get a longer 98mm tube for the MMT to make it easier to assemble all those CRs and hardware. I would also (and may still do in the future), extend out the booster with a coupler and about 12 inches of body tube. The current configuration gives you barely enough room for a 45" motor, which rules out a couple large N's and any commercial O's I am aware of. As it is, getting the harness, shock cord and drogue into the small booster space is a challenge. I highly recommend a narrow, high-strength kevlar cord in order to reduce the space requirement.

 

[mikekebert]

I am using 20ml syringes with 5” long 8 gauge dispensing needles to inject the epoxy through the fins slots and fill the glue wells.

View attachment 474562

Pardon me for digging up something a month old, but I'm pretty new to HPR. Are the needles meant to be disposable or do you clean them off with something like alcohol or acetone?

 

[mtnmanak]

Pardon me for digging up something a month old, but I'm pretty new to HPR. Are the needles meant to be disposable or do you clean them off with something like alcohol or acetone?

You definitely could clean these large needles out and use them again. At a $1.50 each, they aren't exactly cheap. I chose to throw them out after using them. Even with trying to be somewhat frugal and make a lot of the parts myself, this project is going to end up costing somewhere around $3000 from start to cert flight, so I figured $10 bucks worth of dispensing needles wasn't going to break the bank.

However, I am not sure how many times you will need dispensing needles this large. I inject a lot of epoxy on a lot of projects and normally, these 14 gauge needles are more than adequate:

https://www.amazon.com/gp/product/B07H81211S/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
At 10 cents apiece, those needles are much better for day-to-day use in most builds.

If you buy the syringes in bulk, you can get the 10ml size for about 18 cents apiece:

https://www.amazon.com/10ml-Syringe...r+Lock+Tip&qid=1630501140&s=industrial&sr=1-3
At 38 cents a pop, you can justify them being disposable.

 

[mikekebert]

Thank you very much for your response and the links. My cursory Google search turned up expensive needles, but the ones you linked to are at a price point where they can be disposable. I also agree that for a $3k rocket, $10 of needles is a rounding error!

 

[mtnmanak]

Long post.

TLDR – I chose to use a single recovery system to decrease complexity and I chose a 36” pilot using equations from the military’s Parachute Recovery Systems Design Manual.



I realized that I had mentioned earlier in this thread that I would discuss the recovery system and I had not yet done that. For the most part, I am confident most people reading this thread have long since developed their own methods and procedures for choosing and constructing their recovery systems, but I do want to share some of the conversations I had with my TAPs, as they may provide some insight that is helpful.

First, I would point out that the parachutes I listed in this thread are just planned. I make game time decisions about parachutes when I get to the field based on weather and field size. I think the options I listed in this thread will work generally at the Higgs Field (where I plan to fly the cert flight) on a nominal day, but I would definitely change them up at another field or if it was windy, too hot/cold, etc.

So, I won’t talk about which parachute is the right parachute for the drogue/main – you have to adjust those.

However, two discussions, I think, are relevant. One is how to choose the attach points/recovery method. The second is how to choose a pilot chute to pull the main out of the deployment bag.

For the first discussion, a lot boils down to whether you plan to bring the nosecone down on its own parachute or whether you plan to bring the entire system down on one large main. It certainly seems like the “traditional” method is to separate the nose when the main charge goes off and bring the nose down on one main parachute and the booster/payload bay down on another main parachute. This method has worked for countless flights and would certainly be a good choice.

I felt like two separate parachute recovery systems added complexity and a lot of moving parts. I discussed the issue with Teddy Chernok from Onebadhawk (Teddy built the bridle and shock cords for me) and he recommended a single recovery system and parachute. In order to do this, you really need to follow the system from the apex of the parachute to the anchors on the MMT and ensure there are no weak points in the system. That includes any hardware, such as swivels and quick links.

It seems that conventional wisdom is to use a 50 G event as a planning factor. That seems to be based on high velocity minimum diameter flights. Even on an O Motor, this rocket will not reach speeds that can generate more than a 13 G event, but in the pursuit of overkill, I chose to target a 40 G event as the goal. Assuming the max recovery weight of the rocket will be about 80 pounds (on an Aerotech N motor), that means the whole system needs to be able to handle around 3200 pounds of "shock" force. It should also be noted that the force will not be the same across all components. For example, the only parts of the system that will be subject to the full weight of the rocket are the main parachute and the attach point for the main parachute. By the time we get down to the booster, the only weight on that part of the system will be the booster itself, so it will be significantly less force. For simplicity, I targeted 3000 pounds for all components of the system.

Working from the booster up:

• Calculating the amount of force the centering ring/MMT assembly can handle is a bit difficult. You would have to take into account the tensile and flexural strengths of the fiberglass used, the epoxy strength, the hardware “grip” points, etc. Suffice to say, I am comfortable that the procedures I used would be sufficient to handle a high G event.
• The U-bolts I used on the upper CR are 3/8”-16 316 Stainless Steel U-Bolts from McMaster-Carr. The weak point on these U-Bolts is the threads. The strength rating is derived from a straight pull on the U-Bolt until the threads strip out. This weak point can be mitigated using steel plates and multiple nuts. I also epoxied the plates/bolts to both sided of the CR. The threads are rated to 1,200 pounds of straight pull force. Using the mitigations discussed, I estimated that the bolts would be good for at least 1600 pounds each and, since the force is spread over two U-Bolts, I am comfortable the U-Bolts will handle 3000 pounds of force.
• Next up is the bridle, shock cords and the swivels. Teddy used 7/16” tubular Kevlar and a 3/8” swivel. Both these are rated for shock loads well over 5000 pounds, so we are good there.
• The quick links are a common point of failure. I have seen a lot of people compromise their recovery systems with weak quick links. For this build, I am using McMaster_Carr 3/8” thick quick links rated for 3,900 pounds each.
• The main shock cord is attached to the Ubolt on the nosecone bulk plate and then the main parachute is also directly attached to that U-Bolt. This attach point did generate some discussion with my TAPs. It is difficult to calculate the strength of the U-Bolt here because the strength rating is based on the threads, but, in this case, the force applied will be sideways on the U-bolt, so the force on the threads will be minimal. So, we need the tensile strength of the steel bar itself, which I could not find. However, if we look at other products using the same steel and same 3/8” thick diameter (such as the quick links) we see strength ratings for many thousands of pounds. So, in this case, while I did not have a specific strength rating, I felt confident that the 3/8” thick steel bar would be more than sufficient to handle the 3000+ shock load of a high G event.
• Finally, the main parachute. I could not find any manufacturer that gave an indication of the maximum shock load their parachutes could handle, but I had to make an assumption that if the parachute was rated to bring down a 100 pound rocket safely, they would manufacture it to handle a high G stress load (or, we would probably see a lot of discussion on this forum about parachute failures). For the Cert 3 XXL, the one concern I had was the swivel that came attached. It was rated for a 1500 pound shock load. That seemed very low to me and dangerously close to the possible shock load this rocket could encounter, considering that swivel would have to be able to handle the weight of the entire rocket. So, I had Teddy cut it off and replace it with a 5000 pound rated swivel and a sewn Kevlar strap.

At the end of the day, I was comfortable that I had thought through the possible shock loads and capabilities of the components of the recovery system.

 

[mtnmanak]

The next point of discussion with my TAPs was the pilot chute for the deployment bag. They expressed concern that a 36” Fruity Chute Iris may not have enough drag force to successfully pull the main from the deployment bag.

This discussion may prove helpful for many parachute calculation questions you may have.

For these calculations, I pulled out my old and dog-eared copy of the military’s Parachute Recovery Systems Manual - https://apps.dtic.mil/sti/citations/ADA247666. I spent many years as a Special Forces Officer in the Army and this manual proved invaluable more times than I can count. This book is about as close to a “parachuting bible” as I know of. It handles all calculations for everything from (literally) the Apollo recovery parachutes to human “cargo” parachutes to small pilots and drogues. You can find and calculate just about any configuration of parachuting you can imagine.

For this specific discussion, we need to look at the reference of pilot chute selection beginning on page 267 of the PDF. This discussion assumes you have read much of the preceding 266 pages, but, in short, it recommends a pilot chute have an extraction force of 4 times the weight of the main parachute.

The manual goes into extremely detailed math to calculate some of these figures, but we can assume much of what we need to worry about is going to be simple, near ground level (as opposed to calculating sub-space and high altitude forces) and nominal weather.

We need to know the dynamic pressure for the day in question, but let's assume a normal temperature (60-70 degrees) and a generally normal weather front in the area. For this we use Bernoulli's equation. If we are using MPH as our unit of measure, then dynamic pressure (q) for a normal day in Maryland would be:

q = v^2 / 391.2

This formula can be found on page 62 of the manual.

If we assume the rocket will be falling at about 90 MPH under drogue, the dynamic pressure would be:

q = 8100/391.2 = 20.7 lb/ft^2

We can then then use this to figure out the Drag in pounds for the parachute using the formula:

D = q x S x Cd

Where S is the surface area of the chute (in square feet) and Cd is the drag coefficient.

Area for a 36" chute is 7 sq ft and the Cd for these Fruity Chutes is 2.2

So total drag for a 36" Iris Ultra Compact Fruity Chute is:

D = 20.7 x 7 x 2.2 = 318.78 pounds

The Cert 3 XXL parachute weighs 4 pounds, so the rule of thumb the military would use is that it needs a pilot chute with a total extraction force of 16 pounds. This led me to believe the 36" Fruity Chute parachute with 318 pounds of drag capability (on a nominal weather day at sea level at ~90 MPH) would be sufficient.

 

[mikekebert]

So a 36" pilot chute would provide almost 20x more force than the minimum recommended by the military. If you needed to optimize to save weight or space on the parachute, what kind of margin would be adequate? 2x? 5x? 10x?

If you went with roughly 10x, you could switch the chute from 36" to 24" (Drag = 10 x 16 lbs recommended force = 160 lbs. If D is 160 lbs., chute area would be 3.51. Area is Pi times radius squared, so the radius would be 1.05 ft. and the diameter would be 2.10 ft. or 25")

If you went with roughly 5x, you could switch from 36" to 18" (Drag = 5 x 16 lbs recommended force = 80 lbs. If D is 80 lbs., chute area would be 1.75. Area is Pi times radius squared, the radius would be .75 ft. and thus diameter would be 1.50 ft. or 18").

Does that check out?

 

[Tobor]

The next point of discussion with my TAPs was the pilot chute for the deployment bag. They expressed concern that a 36” Fruity Chute Iris may not have enough drag force to successfully pull the main from the deployment bag.

This discussion may prove helpful for many parachute calculation questions you may have.

For these calculations, I pulled out my old and dog-eared copy of the military’s Parachute Recovery Systems Manual - https://apps.dtic.mil/sti/citations/ADA247666. I spent many years as a Special Forces Officer in the Army and this manual proved invaluable more times than I can count. This book is about as close to a “parachuting bible” as I know of. It handles all calculations for everything from (literally) the Apollo recovery parachutes to human “cargo” parachutes to small pilots and drogues. You can find and calculate just about any configuration of parachuting you can imagine.

For this specific discussion, we need to look at the reference of pilot chute selection beginning on page 267 of the PDF. This discussion assumes you have read much of the preceding 265 pages, but, in short, it recommends a pilot chute have an extraction force of 4 times the weight of the main parachute.

The manual goes into extremely detailed math to calculate some of these figures, but we can assume much of what we need to worry about is going to be simple, near ground level (as opposed to calculating sub-space and high altitude forces) and nominal weather.

We need to know the dynamic pressure for the day in question, but let's assume a normal temperature (60-70 degrees) and a generally normal weather front in the area. For this we use Bernoulli's equation. If we are using MPH as our unit of measure, then dynamic pressure (q) for a normal day in Maryland would be:

q = v^2 / 391.2

This formula can be found on page 62 of the manual.

If we assume the rocket will be falling at about 90 MPH under drogue, the dynamic pressure would be:

q = 8100/391.2 = 20.7 lb/ft^2

We can then then use this to figure out the Drag in pounds for the parachute using the formula:

D = q x S x Cd

Where S is the surface area of the chute (in square feet) and Cd is the drag coefficient.

Area for a 36" chute is 7 sq ft and the Cd for these Fruity Chutes is 2.2

So total drag for a 36" Iris Ultra Compact Fruity Chute is:

D = 20.7 x 7 x 2.2 = 318.78 pounds

The Cert 3 XXL parachute weighs 4 pounds, so the rule of thumb the military would use is that it needs a pilot chute with a total extraction force of 16 pounds. This led me to believe the 36" Fruity Chute parachute with 318 pounds of drag capability (on a nominal weather day at sea level at ~90 MPH) would be sufficient.

Great info! Thank you for sharing, and thank you for the link.

 

[mtnmanak]

So a 36" pilot chute would provide almost 20x more force than the minimum recommended by the military. If you needed to optimize to save weight or space on the parachute, what kind of margin would be adequate? 2x? 5x? 10x?

If you went with roughly 10x, you could switch the chute from 36" to 24" (Drag = 10 x 16 lbs recommended force = 160 lbs. If D is 160 lbs., chute area would be 3.51. Area is Pi times radius squared, so the radius would be 1.05 ft. and the diameter would be 2.10 ft. or 25")

If you went with roughly 5x, you could switch from 36" to 18" (Drag = 5 x 16 lbs recommended force = 80 lbs. If D is 80 lbs., chute area would be 1.75. Area is Pi times radius squared, the radius would be .75 ft. and thus diameter would be 1.50 ft. or 18").

Does that check out?

Don't forget a possible change in drag coefficient. I used a very high drag (2.2 Cd) Fruity Chute for the 36" pilot. I can't find anyone that makes an 18" chute with that high of a Cd. Fruity Chutes 18" Elliptical comes in at a Cd of 1.6.

So for an 18" chute at that Cd:

D = 20.7 x 1.76 x 1.6 = 58.5 pounds of drag. Still more than enough to pull out the chute at that theoretical speed.

A more interesting discussion is to look at what happens when the rocket is moving at different speeds. I used a velocity of 90 MPH based on parachute descent rate calculators. In the test flight I made, you can see in the video that the booster was far "draggier" than the drogue and, looking at the flight data, it looks like the descent rate for the rocket under drogue was actually around 60 MPH. At this speed the drag for the 36" chute and the 18" Chute are (you can do the math):

36" Chute: D = 141 pounds
18" Chute: D = 25 pounds

As we can see, the 18" inch is getting a little small for comfort at the speed. If we compensated for the drag of the booster with a larger drogue, we can imagine the descent rate dropping even further and getting to a point where the 18" chute is not going to work.

So, theoretical design at the beginning of this exercise would have shown the 18" to be a perfectly good choice (in fact, it was my first choice), but when I started playing with the speed variations, I realized the 36" with a high Cd was the better option.