8 inch AGM-33 Pike Build

This project is an AGM-33 Pike Level 3 certification project consisting of an 8-inch diameter G10 fiberglass airframe and a 98mm motor mount. It is a combination 3-split-fin design, with 3 trapezoid 8 inch long straight fins in the rear and 3 swept trapezoid 27 inch long straight fins forward. It includes a payload section for main chute recovery gear and dual deploy recovery. Topped with a 5:1 ogive aluminum tipped nose cone, the rocket has an overall length of 145 inches. Dry weight, including recovery gear and electronics, is about 75 pounds.

This rocket employs standard dual deploy, with a 36” drogue chute deployed at apogee and a 30” pilot chute at 1000 feet that will deploy the main parachute, a SkyAngle Cert 3 XXL, from a deployment bag.

Certification date: 23 October 2021
Location: Upstate Research Rocketry Group, Potter, NY

Certification motor: Aerotech M2050X
Drogue deployment: Apogee
Main deployment: 1000 ft. AGL

Pad weight: 88 pounds (39,916 g)
T:W: 5.2/1
Expected Altitude: 2370 feet (722 m) (Rocksim 10.3)
Expected Velocity: 251 MPH (368 f/s) (Rocksim 10.3)
Actual Altitude: 2310 ft (704 m)
Actual Velocity: 249 MPH (365 f/s)
Descent rate under Drogue: 37 MPH (54 ft/s)
Descent Rate under Main: 18 MPH (27 ft/s)

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Documentation & Rocksim File

L3 Document Submission:  AGM-33 Pike Level 3 Documentation

Rocksim File:

Parts List and components

Excel File of parts and components:  

Airframe

Nosecone Ebay

Main Ebay

Motor Mount Assembly

Rail Buttons

Recovery

Electronics

Fins

Initial Build Notes

Although the airframe components (body tubes, nosecone and fins) are from Madcow, I replaced, or added, everything else. I used my Shapoko 3 CNC router to fabricate 56 parts for this rocket. This took me quite a few months since I was only able to dedicate time on the weekends. I spent a lot of time designing the rocket and parts in Fusion 360 and then fabricating test parts in cheaper materials such as plywood before committing the designs to aluminum and fiberglass. 

While the kit Madcow sells is a great kit and has most of the parts you would need to complete a build like this, I tend to buy the kits as a convenient way to get the bulk parts. A lot of discussion in rocketry forums revolves around using a kit or “scratch” building. I have found that, even if you have a scratch built design or aspirations, you can find many of the parts you want much cheaper in a kit. In this case, the normal price Madcow sells this kit for is a modest cost savings over buying the component parts individually. I got the kit on one of their Black Friday sales for an additional 30% off. So, even though I had planned from the beginning to ditch many of the parts and make my own, it was worth it to plan ahead and buy the kit on sale just to get the bigger airframe parts. As a bonus, you don’t have to slot your own airframe! Another thing to consider is that the individual components are often not available. Madcow often shows this kit as available for purchase, but the 8″ airframes, the 8″ nosecone, the 4″ motor mount and the 8″ couplers are usually out of stock. So, if you wanted to build an 8″ scratch rocket, buying this kit may be one of your only ways of getting the parts from Madcow.

The centering rings (CRs) I made have slots for both upper fin guides and lower epoxy wells, (the spanners for which were also CNCed) and also have holes for all-thread, which will be used primarily for adjusting the CRs precisely, but also add some structural value.

I CNCed all the bulkheads, mostly out of aluminum, but one BH in the nosecone bay is fiberglass.

The ebay parts include a simple fiberglass (FG) sled, a switch bracket CNCed from 3/4″ basswood and a number of fiberglass parts and brackets.

I will go over the recovery system later in this build thread, but I spent considerable time talking with my TAPs about the various strengths and forces that each component would be subject to in a worst case scenario and bought or built the components with those discussions in mind. To that end, the u-bolts used are 3/8″ thick from McMaster-Carr and are rated for thousands of pounds. Likewise, all the quick-links that will be subject to the weight of the rocket are also 3/8″ thick from McMaster-Carr and are rated for 3900 pounds each. The attach point, bridle and shock cords are all custom made by Teddy from Onebadhawk and are all rated for over 5000 pounds. The Cert 3 XXL parachute is more than adequate for this rocket, but only came with a 1500 pound rated swivel. While the cert flight is designed to be low and slow, so will unlikely generate velocities that will exceed the capability of that swivel, I would like this rocket to fly on bigger motors later, so I had Teddy also replace that swivel with a length of kevlar and a 5,000 pound rated swivel.

The all-thread in the ebay has to hold up the weight of the entire booster section as well as absorb the shock of the various events, so I used 4 lengths of high strength steel 5/16″-18 all-thread from McMaster-Carr each rated for 150,000 PSI.

The thrust plate is 1/4″ aluminum and was CNCed to attach the Aeropack 98mm retainer. The holes were threaded for the screws so the retainer housing can be bolted directly to the thrust plate. Normally, I do this so I can remove the housing and use it on multiple rockets, but since this is my cert bird, I am going to be permanently affixing those screws and the housing to the thrust plate.

All the components were washed and sanded thoroughly inside and out with 220 grit sand paper and washed again numerous times. For anyone that has not built one of these bigger rockets, I would invest in some decent equipment for sanding and finishing. Basically, look for the same tools auto body workers use to work on cars. A good random orbital sander and a good size package of sanding discs seems like a must. There has been a lot of sanding. Also, I am not sure how anyone builds anything without a Dremel tool handy, but I would have been lost 100 times on this build without my Dremel.

I used a variety of epoxies in this build – West Systems 105/205 is the workhorse, sometimes mixed with their 405 Colloidal Silica when I need to thicken it up. Proline 4500, JB Weld and RocketPoxy also made appearances. For 5-minute epoxy needs, I am used both JB Weld Clear Weld and West Systems G5 – literally, whichever of the two is close at hand when I need it. 

Here are some photos of the CNC process and some of the components. Note, in the photos below, the wood parts shown were all test pieces I used to make sure my designs were correct. After everything was fabricated, sanded, washed and tested, I dry fit everything countless times to make sure I had things right.

MMT Dry Fit

Motor Mount Build

The toughest part of this build was getting the entire MMT assembly correct, so that is where I began.

The MMT assembly all stems from the placement of the forward centering ring. This CR is also the point that will take the brunt of the recovery shock, so I chose to fabricate it out of 1/4” thick FR4 Fiberglass. Incidentally, all the CNCed fiberglass parts on this rocket were made using FR4 sheets. FR4 is the fire retardant version of G10 FG. The main reason I went with FR4 over G10 is that the place I source my FG sheets from (www.eplastics.com) only sells the FR4. Since it can be substituted for G10, but G10 can not be substituted for FR4, they decided it was easier to only carry FR4. Also, I figure being fire proof can’t hurt!

My TAPs and I had a long discussion about whether the forward CR should be made out of aluminum. It was easy for me to go either way, I had both materials on hand and it is actually a lot easier to CNC aluminum than the thick FR4 – the FG eats CNC router bits for breakfast. The arguments that led to going with FR4 were that it has almost the same tensile strength as the 1/4″ Aluminum (40,000 PSI for the FR4 and 45,000 PSI for the Aluminum), but the flexural strength of the FR4 is much higher than the Aluminum (60,000 PSI for the FR4 and 35,000 PSI for the Aluminum). So, in this application, both materials will most likely not break in any amount of force this rocket can produce, but the Aluminum has almost twice the chance of deforming under a high impact load. If the CR deforms, it is far more likely that the joint between the CR and the tubes will fail. Further, the FR4 bonds to epoxy much, much better than aluminum. Since this CR will be holding up the heaviest piece of the rocket, we decided FR4 was the way to go.

The entire motor mount tube is barely long enough to handle the motor mount assembly. With so little length to spare, I needed to make sure I didn’t squander it. Looking back on it now, I would recommend to anyone building this rocket to use a bit longer MMT to give yourself some more working room. I needed to have some room on the motor mount tube on the fore end to put in an epoxy dam down the road, so I measured about ¾” back from the front of the MMT and marked off the position of the front CR. I then tacked the front CR in place with some CA. This provided the anchor for the rest of the CR placements.

Then I marked off the approximate positions of the fin tabs and extended the lines around the body tube. For tube marking, the traditional method is to wrap a piece of paper around the tube. That worked for me up until the tube circumferences started getting longer than the edge of legal sized piece of paper. That is when I invested in a pipe fitters marking guide. This one I got on Amazon (https://www.amazon.com/gp/product/B004XNZCEO) was more than long enough that I was able to cut off a couple small sections for tubes 4” and below and still have plenty left over for tubes up to about 12” in diameter.

I inserted the 8-32 all-thread in the front CR and locked them in with dual hex nuts on either side of the CR.

For each successive CR, I tested the fit of the fin tabs and then locked in the CR with hex nuts. Using the hex nuts, I could adjust the spacing of each CR precisely as I moved down the MMT. I inserted the lower spanners (which form the glue wells later on) between each CR as I moved along.

Before I could dry fit the assembly in the booster tube, I had to ensure the fins fit into the slots in the booster. The slots were a little rough and needed some shaping up. The ends of the slots were uneven (it looks like the slot cutter they used may not have been aligned properly) and the fins were a hair wider than the slots. So I shaped up the ends of the slots and drew lines around the tube to ensure the slots were all the same length. I then used a Dremel to clean up / slightly widen each slot. Once I sanded down the fin tabs, they all fit nicely and were all aligned.

I also took the opportunity now to use a long piece of angle aluminum to draw lines longitudinally down the body tube along the fin lines and exactly between each fin.

Once I had the fin slots fixed, I dry fit the MMT assembly in the booster tube and ensured all the fins fit perfectly.

After that, I tacked in each CR with CA and locked in all of the nuts on the all-thread with some 5-min epoxy.

I did one more dry fit of the entire booster assembly before I epoxied the CRs in. This is a big booster section!

With everything appearing to fit well, I epoxied the CRs in with West Systems 105/205 mixed with West Systems 406 Colloidal Silica to make a thin paste.

I proceeded to epoxy in each side of the CRs and let the whole assembly cure overnight.

The next day, I tacked in the lower spanners with 5-minute epoxy. The main goal here was not structural, it was to seal the spanners in so they are water tight. These will be filled with liquid epoxy later on, so they can’t leak.

Once the epoxy was cured, I checked the glue wells for leaks by filling them with water.

There were a few leaks and I patched them up with epoxy once they were dry.

I also got the U-Bolts installed in the forward CR and secured them with epoxy.

Next, I marked off the upper and lower sections of the glue wells on the fin tabs to give me an idea of where I can drill some holes. The intent of the holes is to let the epoxy in the glue wells flow through them and provide more of a mechanical “grab” on the fin tabs. The lower section of the glue well is about 3/8” wide, so I made the holes in the tabs 1/8” in diameter. I measured the holes out about 2 inches apart and drilled them out on my drill press.

I then did another water test with the fins in place to test the seals again and to also judge how much liquid each well could hold with the fin displacement. I wrote the volume of each well on the side of the spanner so I could record it later.

I then tacked in the upper spanners (which act as fin guides) into the CRs with CA. I decided to just use a bit of CA for this application in case something goes wrong later with the alignment and I need to knock out a spanner or adjust one. These spanners are just guides and are not structural.

At this point, the motor mount assembly was functionally complete. The rear CR was not epoxied in place yet, although I did tack in the ¼” pieces of all-thread in the rear CR for the thrust ring placement. The MMT assembly is ready to be installed in the booster tube.

Booster Section Build

Rail Button Nut Fitting

Before I installed the MMT assembly, I drilled the hole for the rear rail button.

The rear button needs to be about 6 inches from the end of the booster tube to clear the rear CR and ensure it does not interfere with the CR installation later.

I drilled a 1/2” hole for the Rotaloc epoxy nut and fitted it in place. The steel disc around the nut is large (about 1.5” in diameter) and flat, so it does not sit inside the tube very well. I gently curved the steel disc in a vise and then it conformed nicely to the inside of the body tube. I am not adhering the nut right now, just fitting it for after I install the MMT assembly.

The last thing I did before installing the MMT assembly is mark out the locations of the CRs and the drill hole locations for the upper rail button, the #10 screws I am going to install to give the upper epoxy dam better “grip” to the airframe and the location of the booster shear pins.

MMT Installation

I inserted the MMT assembly into the booster and inserted the fins to ensure everything stayed aligned and then tacked the forward CR in place with some 5-minute epoxy. At this point, the intent of the epoxy is to hold the MMT in place and seal off any gaps in the upper CR. One of the final steps of the booster construction will be to pour an epoxy dam onto the top of the upper CR, which will provide the main structural holding power.

Forward Fins

With the MMT assembly tacked in place, it was time to start epoxying the forward fins. I ensured the body tube was level and plumb. To check for plumb, I used the lines I drew on the body tube earlier. I then clamped the body tube to the stand. It is important for the tube to be level since there will be a significant pool of epoxy in the glue wells and I want the epoxy to be evenly spread across the fins tabs. By making the tube slot plumb, I can later check the alignment of the fin by making sure it is also plumb. In theory, that alone should ensure the fin is exactly perpendicular to the tube. However, the fin guides on the MMT assembly will provide additional fin alignment and I also CNCed several external fin guides to make triple sure the fins are straight.

I used West 105/205 for the epoxy pools for each fin. I am using 20ml syringes with 5” long 8 gauge dispensing needles to inject the epoxy through the fin slots and fill the glue wells.

I injected the proper amount of epoxy for each tab into the wells, inserted the front fin and slid the fin guides in place. After allowing the epoxy to cure, I proceeded to do the same with the other two fins. I allowed the Epoxy wells cure up for a couple days before proceeding.

Rail Button Nut Install

Once the front fins were cured, I removed the thrust plate and rear CR and epoxied the rear Rotaloc nut in place. I tacked it in with some 5-minute epoxy, then placed a piece of tape over the center hole/threads and used West 105/205 with colloidal silica to securely bond the nut to the body tube.

Rear CR

With the rail button nut in place, I used Proline 4500 to epoxy the rear CR back in place. Once it was in position, I used 8-32 wing nuts to secure the CR to the all-thread and then used the left over Proline to coat all the hardware. Before I installed the thrust plate, I poured some epoxy over the rear CR, so right now the intent is to seal all the gaps in the rear CR.

Rear Fins

With the rear CR back in place, I could now install the rear fins. The only difference in the installation of the rear fins from the front fins is that I used a black dye in the epoxy to be able to see if there are any leaks. Also, I did not use the fin alignment guides, I just used the tried and true method of clamping angle aluminum to the front and rear fins to ensure alignment.

Thrust Plate

I allowed the fin epoxy to cure overnight and then poured an 80ml epoxy dam behind the rear CR.

I used JB Weld to epoxy the thrust plate in place. I mixed up a big batch of it and spread it on all the surfaces the plate will touch and used some of the JB Weld to secure the retainer screws on the back of the plate.

Once the plate was installed in the airframe, I spread more JB Weld on the all-thread and secured the plate in place with hex nuts. I put a healthy JB Weld fillet around the edge to seal it in place.

Once the JB weld was cured, I added a 100ml epoxy dam. I died the epoxy red because I plan to paint the rocket red and may as well make that process a bit easier down the road.

Anchor Screws

Before adding the epoxy dam in front of the forward CR, I drilled 4 evenly spaced holes around the airframe just above the forward CR using a #25 bit. I used a countersink bit to create a slight depression for the screws about half way through the frame. The screws will still be proud of the airframe on the outside, but should be seated so only the very top of the screw heads protrude. I then threaded the holes with 1 inch long #10-24 screws. I put a little epoxy in each hole and secured the #10 screws in place. They protrude inside the airframe just above the CR to provide a little more mechanical hold for the epoxy dam.

I poured a 100ml epoxy dam above the forward CR. I dyed the epoxy black to help me see any leaks and drips.

External Fillets

To mark out the external fillets, I used some charcoal on paper to mark and measure the fillet boundaries, then used angle aluminum to mark and tape the fins and tubes. I used Rocketpoxy with some red dye (same reason as above). Maybe the only really interesting point is the shear volume of Rocketpoxy you need on a build this big. All told, I used the better part of a 2-pint kit for the fillets. That somewhere around a pound of epoxy just for the fillets.

With the booster functionally complete, I reweighed the assembly and figured that the epoxy and hardware added 1,461 grams (3.2 pounds) to the booster section. The new measured CG for the booster is at 36.75” from the top of the booster tube. I entered the new information into Rocksim and re-ran all the sims.

The fillets weren’t too bad, but they were so big, it was inevitable that they would have some pits, holes, uneven sides, etc. I used Bondo to smooth out the fillets and sprayed a coat of primer/filler on the joints to make any defects show up. With a bit of sanding the fillets are smooth and complete.

As a recommendation, this is another time a good random orbital sander comes in handy. I have two models. I have a Dewalt 5 inch palm sander that does great work for general purposes and relatively fine sanding. For larger jobs like this one where the main purpose is to get a lot of material off quickly, I also have a Makita 5 inch with a two hand grip. That thing really makes short work of bigger jobs. With that sander and 60 grit discs, I got most of the bondo smoothed out in about 15 minutes.

At that point, the booster was functionally complete.

EBay Build

Switch Bracket

The next thing I worked on was the ebay and electronics. I am using a switch bracket made of ¾” Basswood. It was CNC’ed to hold any combination of ½” diameter switches, lights, buzzers, etc. For this build, the bracket housees a switch for the main computer, a switch for the main battery, an indicator light for the main computer and a switch for the backup computer.

After drilling pilot holes in the basswood, I coated it with laminating epoxy to strengthen the bridge, then drilled the ½” holes needed for the switches and light.

The switches and lights have protrusions, so I used a Dremel to elongate the holes where necessary

Brackets and sled

With the switch bracket ready, I lined up the sled and measured the end brackets for installing the metal supports.

The supports will be held on with 4-40 screws and nylon locking nuts and the sled will likewise be bolted to the metal supports.

With the supports installed, I fixed the switch bracket to the sled with some 4-40 sheet metal screws. This Dewalt angle drill chuck really comes in handy in these circumstances:

Switch Band

I used the switch holes on the bracket to mark out the switch positions on the coupler to drill the external holes.

At this point, the switch band was just held on with tape. With everything lined up, I epoxied the switch band to the coupler:

Battery Holders

Next, I worked on installing the electronics on the sled. I used closed 9V battery cases for this build. I ripped out the case switches and soldered wires directly to the battery terminals, then secured the battery cases to the sleds with 4-40 screws and nylon lock nuts.

Assembly and Testing

The RRC3 and Stratologger computers were secured to the sled with metal 4-40 mounting hardware and everything was wired up with 24 gauge wire. The wire was secured to the sled with electrical tape. The computers were vacuum tested prior to installation.

Once the sled was complete, I drilled the holes in the switch band, ensured all the switches and light lined up and then installed the charge wells, terminal blocks and U-Bolts to the bulkheads.

I did a full system check, continuity checks and test fire of all the pyro channels. Everything worked perfectly.

Nosecone

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, so 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.

Odds and Ends

I drilled the holes for the shear pins in the booster. The holes were sized to be tapped for 4-40 shear pins. To keep my tap aligned, I use a guide I made from a piece of 1 inch thick acrylic. I got the idea from a video posted on Adam Savage’s site: https://www.youtube.com/watch?v=XVEww6Ylw5c.

I also installed 6-32 Lumadyne PEM nuts in the payload bay to hold the ebay into the upper section.

Once I installed the nosecone shear pins, the rocket was functionally complete! 

Ground testing

The charges for the ground testing were calculated using the calculator at Insane Rocketry’s site: https://www.insanerocketry.com/blackpowder.html

The calulation for the drogue charge was 3g of FFFFg black powder. That charge turned out to be good and was used for the primary charge in the cert flight. The backup charge was 4g of FFFFg.

The charge for the main charge was calculated to be 4g of FFFFg. This turned out to be a bit weak, so the primary charge used for the cert flight was 4.5g of FFFFg and a 5.5g backup charge.

Drogue Test

Main test

Recovery

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 fields I normally fly at 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.

Attach Points and Recovery Method

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 rocket forums 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.

Choosing a Pilot Chute

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.

Shakeout Flight Video

This was a shakeout flight for an 8 inch AGM-33 Pike rocket at Upstate Research Rocketry Group (URRG) in New York on 21 AUG 2021. Flight was conducted on an 75mm Aerotech L2200G.

Altitude was about 2000 feet.

Many thanks to Jeff Manning and URRG for their instrumental help in launching and recovering the rocket on this flight.

Certification Flight Video

This was a Tripoloi Level 3 certification flight on 23 October 2021 at Upstate Research Rocketry Group in Penn Yann, NY.

Certification date: 23 October 2021
Location: Upstate Research Rocketry Group, Potter, NY

Certification motor: Aerotech M2050X
Drogue deployment: Apogee
Main deployment: 1000 ft. AGL

Pad weight: 88 pounds (39,916 g)
T:W: 5.2/1
Expected Altitude: 2370 feet (722 m) (Rocksim 10.3)
Expected Velocity: 251 MPH (368 f/s) (Rocksim 10.3)
Actual Altitude: 2310 ft (704 m)
Actual Velocity: 249 MPH (365 f/s)
Descent rate under Drogue: 37 MPH (54 f/s)
Descent Rate under Main: 18 MPH (27 f/s)

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