Madcow FG 4" Frenzy XL build thread

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I think you're wise to go the DMS route given you circumstances. I justified the RMS purchase due to the fact that I'll be driving about 6 hours to a premier place to fly high altitude rockets, the Blackrock Dessert where's there's an incredible amount of flat open lakebed to launch and recover from. Here's a clip from the AeroPac TRA prefecture that flys here:
"The place where we fly is a dry lakebed referred to as the Playa. It is a spectacular natural wonder, the largest flat piece of land on earth. Black Rock has been the site for both world land speed records and for most TRA rocket altitude records. It is both beautiful and spiritual, but can also be hostile. Black Rock is about 100 miles north of Reno Nevada with an altitude just under 4,000ft MSL. It's very empty country"

The launch sites within a few hours of me generally have waivers to 16K, however the Blackrock site generally has waivers to 30K, with a 2 hour window to 146K!
I'd have to be very confident in my tracking setup and skills before I flew an M impulse motor at the sites with the 16K waiver. While I think I could keep the altitude within limits, it's the tracking and unknown landing zone potential that might keep me at bay.
Makes sense and I'm jealous! Yeah I have no idea if launching this on an M will be possible with a 10k waiver. Thrustcurve and OpenRocket think so for the CTI M1101 or the Aerotech M1350 (both go to about 9k), but I won't really know until I get a couple of launches in and can dial in a more accurate Cd. I also have aspirations to take a long drive at some point to a site with a larger waiver, but it'd be pretty awesome if I could try it before then. It definitely will require waiting for an incredibly calm day though.

I'm also debating how draggy I want my camera attachment to be - more drag means less altitude which is good! But I don't want to add asymmetry since a less straight flight will cause way bigger problems than going a little less high.
 
I've made a lot of progress on the AV bay internals but it's a mess still, so that'll be a later post. I've drilled 3x 3/16" vent holes though for the AV bay, a couple of holes for switches (along with tapped #2 holes for screws to hold the switches), and drilled+tapped 3x #4 holes for metal screws to hold the payload bay to the AV bay. I also drilled 1/8" holes in the seams between the booster and switch ring, the switch ring and payload bay, and payload bay and nose cone. I then inserted 1/8" brass rod into the holes and epoxied to one side. This provides both a visual guide and mechanical lock to keep the pieces aligned the way I want. The pictures might make this clearer:

IMG_20170327_214253.jpgIMG_20170327_210416.jpgIMG_20170327_210335.jpg

As you can see, I used one rod on one side of the switch band and two on the other side, making it impossible to insert the AV bay backwards. This is definitely overkill but I'm pretty pleased with it, other than the part where I mis-drilled a hole and now have to fill it in with epoxy (you can see the partial hole between the pair of rods). I obviously still need to file the rods down to be flush, but that shouldn't be too bad. This will be good though - having the forced alignment means I can exactly control the location of the various screws that are on the rocket surface to keep turbulence away from the avbay vent holes and make sure the screws won't interfere with the rail. I'm currently so pleased with this that I'm going to leave the metal rods unpainted since I think the little metal circles will look neat.
 
Ground testing done! I went with 4 shear pins for the main deployment, requiring 84lbs of force to separate, so it can withstand ~40Gs of apogee event shock without separating. I'm using friction fit for the drogue.

The main required 2g of BP (1.0 and 1.5 didn't separate it). My largest (pratt) canisters are 2g, so both my primary and secondary (i.e. from the primary and backup altimeters) will be that size, however as the video shows the separation was plenty vigorous, and the use of shear pins means I don't have to worry much about variance in terms of how much force is required.
[VIDEO=youtube;EqNk2yIaI64]https://www.youtube.com/watch?v=EqNk2yIaI64[/VIDEO]

The drogue required 1.5g of BP. The backup charge will be 2g. The nice thing about 1.5g working is it means that if somehow both of my altimeters fail, even a 38mm J motor (which has 1.3g of BP) has a shot at separating this thing, and a 54mm motor (with 2g) definitely will. This won't help during a L3 flight, but is nice for smaller motors where I won't even bother drilling the delay but will leave it in as a hail mary backup to the pair of altimeters. No lawn darts for me!
[video=youtube;t1awPc-0FkE]https://www.youtube.com/watch?v=t1awPc-0FkE[/video]
 
Thanks for posting video and your shear pin/charge configuration. Those charges look about right. It's good that you're increasing the charge of the backup altimeter.

I wrote up the charge section of my L3 project just Friday. I decided to go with three 4-40 nylon shear pins for the main and three 2-56 nylon shear pins for the drogue. I then used conservatively high shear breaking force values for the pins along with the Ideal Gas law to find the charge required.. and on top of that I added 25% for safety margin. From that, my starting point charges are very similar to what you've arrived at:
Drogue = 1.7g 4F Black Powder
Main = 1.9g 4F Black Powder
 
I would use at least 2 in both sections. Its one thing for a ground test to work but you for sure need it to work in flight. Kind of a safety thing.
 
Thanks for posting video and your shear pin/charge configuration. Those charges look about right. It's good that you're increasing the charge of the backup altimeter.

I wrote up the charge section of my L3 project just Friday. I decided to go with three 4-40 nylon shear pins for the main and three 2-56 nylon shear pins for the drogue. I then used conservatively high shear breaking force values for the pins along with the Idea Gas law to find the charge required.. and on top of that I added 25% for safety margin. From that, my starting point charges are very similar to what you've arrived at:
Drogue = 1.7g 4F Black Powder
Main = 1.9g 4F Black Powder
Lol, your points were what originally drove me to consider and then go with 4x 2-56 shear pins for the nose rather than 3x 4-40. It seemed that 40Gs of holding strength (which is what 4x 2-56 gets me) was at the upper end of what anyone was doing. I did the math (PM me if you want a link to my google doc writeup, I'd rather not share it broadly while still a work in progress) and conservatively, it would take a drogue deployment at >335fps to shear my nose cone shear pins during the drogue event with this setup. Additionally, I'm using tubular nylon for the drogue cord, since it stretches more than kevlar and therefore is less likely to cause a bad yank if the charge is oversized or the cord tangles. The advantage of not going with 4-40 is a smaller main ejection charge, which means I'm less likely to break my tracker or put high load on the rocket during the main ejection event.

In terms of amount of powder, I was hopeful that I would only need 1.5g for the main, but it wasn't enough. On the other hand, I'm pleased that 1.5g was enough for the drogue. I'm guessing you'll need more than that but we'll see.
 
I would use at least 2 in both sections. Its one thing for a ground test to work but you for sure need it to work in flight. Kind of a safety thing.
You think so? The drogue ejection looks plenty energetic to me at 1.5g, and when it's in the air it won't have to lift the ~6lb payload section against gravity (since the entire rocket is in free fall) so it should separate even more energetically in flight. There are downsides to too much powder, including risk of shearing the nose shear pins and causing an early main deployment. FWIW my L3CC and some other folks were there when I did this testing, and we all agreed that the drogue ground test with 1.5g looked great. 2g in the backup is just for insurance - I don't intend on it being needed.
 
Lol, your points were what originally drove me to consider and then go with 4x 2-56 shear pins for the nose rather than 3x 4-40. It seemed that 40Gs of holding strength (which is what 4x 2-56 gets me) was at the upper end of what anyone was doing. I did the math (PM me if you want a link to my google doc writeup, I'd rather not share it broadly while still a work in progress) and conservatively, it would take a drogue deployment at >335fps to shear my nose cone shear pins during the drogue event with this setup. Additionally, I'm using tubular nylon for the drogue cord, since it stretches more than kevlar and therefore is less likely to cause a bad yank if the charge is oversized or the cord tangles. The advantage of not going with 4-40 is a smaller main ejection charge, which means I'm less likely to break my tracker or put high load on the rocket during the main ejection event.

In terms of amount of powder, I was hopeful that I would only need 1.5g for the main, but it wasn't enough. On the other hand, I'm pleased that 1.5g was enough for the drogue. I'm guessing you'll need more than that but we'll see.

The discussion here and a discussion with a local chapter member had me go with the three 4-40s. I wanted to stick with 3 pins to keep the symmetry with pins inline with the fins. I decided that the main shear pins needed a little more than the three 2-56 pins.. so I went with 4-40 pins. My level 2 rocket is a 4" fiberglass with a payload/main compartment that is 1" longer that the 4" FG Frenzy XL will have, given it's long avionics bay. I use four 2-56 shear pins in that and my main altimeter charge is 1.7g with a backup of 1.9g. As far as I know, the main 1.7g charge has always done the job.. up to 7,300 feet so far. The nosecone on this one, however, is the standard 4:1 fiberglass madcow unit.. and is much lighter than the FW nosecone that the Frenzy uses.... thus your slightly larger charge of 2.0g seems reasonable given the additional mass that needs to be accelerated away after the pins shear.
Hopefully I don't have to go too much larger with my main charge.

I think you're right that nylon is probably the best to ensure soft deployment and would reduce the risk of main deployment as you point out.
I'm going with Kevlar shock cords.. but as you point out, there will be a sharper/larger shock load when the cord reaches full extension after deployment. I've already purchased a number of 5,300 lb 7/16" tubular Kevlar 25' shock cords from One Bad Hawk. I may have to double the drogue cord from 25' to 50' to reduce the shock load risk, even though the shear pins have been sized for considerable G load specifically for this event.
 
The discussion here and a discussion with a local chapter member had me go with the three 4-40s. I wanted to stick with 3 pins to keep the symmetry with pins inline with the fins. I decided that the main shear pins needed a little more than the three 2-56 pins.. so I went with 4-40 pins. My level 2 rocket is a 4" fiberglass with a payload/main compartment that is 1" longer that the 4" FG Frenzy XL will have given it's long avionics bay. I use four 2-56 shear pins in that and my main altimeter charge is 1.7g with a backup of 1.9g. As far as I know, the main 1.7g charge has always done the job.. up to 7,300 feet so far. The nosecone on this one, however, is the standard 4:1 fiberglass madcow unit.. and is much lighter than the FW nosecone that the Frenzy uses.... thus your slightly larger charge of 2.0g seems reasonable given the additional mass that needs to be accelerated away after the pins shear.
Hopefully I don't have to go too much larger with my main charge.

I think you're right that nylon is probably the best to ensure soft deployment and would reduce the risk of main deployment as you point out.
I'm going with Kevlar shock cords.. but as you point out, there will be a sharper/larger shock load when the cord reaches full extension after deployment. I've already purchased a number of 5,300 lb 7/16" tubular Kevlar 25' shock cords from One Bad Hawk. I may have to double the drogue cord from 25' to 50' to reduce the shock load risk, even though the shear pins have been sized for considerable G load specifically for this event.
Holy cow why 5300lb? If the rocket weighs 20lbs after burnout that's 265Gs it can handle!! And is every other component in your stack rated that high? For example, I'm using fruity chutes parachutes which are very well made, but the fruity chute main parachute I'm using (60" iris) comes with a 1500lb (failure load not WLL) swivel. To generate 265Gs with the parachute opening, if you had an instantaneous opening of a parachute that would eventually slow your rocket to 15fps, the worst case initial G forces would only be that high if the deployment happened at 244fps. Anyway I guess it means you can be sure it won't break :)
 
Holy cow why 5300lb? If the rocket weighs 20lbs after burnout that's 265Gs it can handle!! And is every other component in your stack rated that high? For example, I'm using fruity chutes parachutes which are very well made, but the fruity chute main parachute I'm using (60" iris) comes with a 1500lb (failure load not WLL) swivel. To generate 265Gs with the parachute opening, if you had an instantaneous opening of a parachute that would eventually slow your rocket to 15fps, the worst case initial G forces would only be that high if the deployment happened at 244fps. Anyway I guess it means you can be sure it won't break :)
For comparison, I put in an order a few days ago for a 9/16" tubular nylon harness from onebadhawk to use for my drogue section harness. It is rated to 1500lbs - I figured there was no sense in getting a higher rating than my main parachute, especially since (a) I'm not using any knots in it so the full rating applies (b) it is being used between my booster and av bay so it will have maybe 3lbs less load (so about 20% less) than the main parachute and (c) the thinner cord would stretch more than thicker higher rated stuff, putting less load on the rest of the stack as long as the 1500lbs aren't exceeded.

Conservatively, with the size of my drogue, the 9/16" cord (and the rest of the stack) should survive a drogue deployment at nearly 700fps! That would of course pop the main chute, and honestly I can't be confident the drogue parachute would survive, but neither of those are affected by the drogue cord strength.

All that said, there is no harm in going with higher rated cord, I'm just curious about your reasoning in case I'm missing something with my analysis.
 
Holy cow why 5300lb? If the rocket weighs 20lbs after burnout that's 265Gs it can handle!! And is every other component in your stack rated that high? For example, I'm using fruity chutes parachutes which are very well made, but the fruity chute main parachute I'm using (60" iris) comes with a 1500lb (failure load not WLL) swivel. To generate 265Gs with the parachute opening, if you had an instantaneous opening of a parachute that would eventually slow your rocket to 15fps, the worst case initial G forces would only be that high if the deployment happened at 244fps. Anyway I guess it means you can be sure it won't break :)

I certainly don't think that 5,300 lbs of tensile strength is required, unless perhaps there's a deployment or other failure at very high speed. Rather, having a reasonable cross section of material such that minor abrasions don't bring down the strength to unsafe levels. Imagine after 20 flights, and your shock cord has had some wear at the point where it exits the booster tube. At that location, the breaking strength would be reduced some percentage. Another thing to consider is that you really want to have a bit of girth to the shock cord to distribute load on the lip of the booster tube should a higher speed deployment occur. This would reduce the chance of zippering the fiberglass. The 7/16" 5300lb tubular Kevlar is slightly thicker, but 1/8" narrower than the stock 9/16" nylon cord. Kevlar is very strong stuff.. and you can get very thin Kevlar cord that is stronger than the stock 9/16" nylon shock cord, but for the reasons I've listed, I don't think it's a good idea.
Here's what Teddy says on his website (https://www.onebadhawk.com/716-tubular-kevlar--2-loop.html):
"These are made from 5300 lb.
7/16" Tubular Kevlar Webbing.
This is incredibly strong material.
These harnesses are perfect
for 4", 5", 6" and up Rockets.
"


I am familiar with fruity chutes. They are very nice quality and function with high drag semi-torus shape on the Iris. Last year, I witnessed a supersonic deployment of an 18" fruity chutes elliptical (used as a drogue) parachute. The shock cord failed above the drogue, and the thinner Kevlar zippered the booster tube. The chute had the same 1,500 lb swivel. The chute's riptstop nylon was torn out of the center of the chute.. like a big hole in the middle as if it "popped". However, the perimeter of the chute was intact. The swivel had yielded under the load and both swivel eyes were stretched out to long oval shapes.. but the damaged chute and deformed swivel managed to remain attached to the booster.. and it fell fast, but landed without much damage.. I think only the nozzle on the M1297W cracked on impact into hard clay dirt clawds. I was amazed that it held together as well as it did.
 
For comparison, I put in an order a few days ago for a 9/16" tubular nylon harness from onebadhawk to use for my drogue section harness. It is rated to 1500lbs - I figured there was no sense in getting a higher rating than my main parachute, especially since (a) I'm not using any knots in it so the full rating applies (b) it is being used between my booster and av bay so it will have maybe 3lbs less load (so about 20% less) than the main parachute and (c) the thinner cord would stretch more than thicker higher rated stuff, putting less load on the rest of the stack as long as the 1500lbs aren't exceeded.

Conservatively, with the size of my drogue, the 9/16" cord (and the rest of the stack) should survive a drogue deployment at nearly 700fps! That would of course pop the main chute, and honestly I can't be confident the drogue parachute would survive, but neither of those are affected by the drogue cord strength.

All that said, there is no harm in going with higher rated cord, I'm just curious about your reasoning in case I'm missing something with my analysis.


I suppose it's a personal preference based on "gut feel" based on one's experiences. I went with Kevlar since I've just reached the point where I retired my first nylon shock cord. It was crusty from all the ejection charges melting a small amount of exposed nylon with every flight or test charge. It had been used in the main parachute section of my rocket. In that same rocket, I had a 5300 lb 7/16" tubular Kevlar Onebadhawk harness for the drogue section.. and, except for some soot discoloration, is in excellent condition. I'd say these cords have endured roughly 20 ejections in a 4" fiberglass airframe using 1.7 to 1.8g 4F.
 
I suppose it's a personal preference based on "gut feel" based on one's experiences. I went with Kevlar since I've just reached the point where I retired my first nylon shock cord. It was crusty from all the ejection charges melting a small amount of exposed nylon with every flight or test charge. It had been used in the main parachute section of my rocket. In that same rocket, I had a 5300 lb 7/16" tubular Kevlar Onebadhawk harness for the drogue section.. and, except for some soot discoloration, is in excellent condition. I'd say these cords have endured roughly 20 ejections in a 4" fiberglass airframe using 1.7 to 1.8g 4F.

Makes sense. In hindsight maybe I should've gone with 3/4" nylon. The good news is (1) I'm using a bent over kevlar strap as my connection to the MMT that extends above the top of the booster so there shouldn't be substantial chafing of my nylon cord and (2) I suspect the booster section will end up weighing enough less than the overall rocket that 1500lbs is still 100Gs or more, making the parachute swivel still the weakest point.
 
A few changes to my recovery stack:

1) I'm upgrading to 25' of 3/4" tubular nylon for the drogue cord (with 2300lb rating). kevinkal@ - your points are good about the nylon being much more likely to degrade over time than the other weak link in the stack (the 1500lb parachute swivel), so it makes sense to have it be a higher rating than my target of 1500lbs.

2) Rough calculations suggest that on my L3 motor, there will be 30lbs or so of drag force trying to separate the rocket at burnout!!! This shouldn't be surprising, since this rocket has a fairly heavy payload+nose, and a very draggy rear end. In addition, even with 3x 1/8" vent holes in the booster, there will be ~5-10lbs of force from pressure differential around that time too (and with a single vent hole it could be ~20lbs). I am not at all comfortable with relying on friction-fit to hold against 40lbs of force, so I'll be switching to shear pins for my drogue separation as well. I'd love to go with 2x 2-56 shear pins (~50lbs of shear strength), but a lot of folks warn about that risking a jam, so I'll probably go with 3x 2-56 (~65lbs of strength). I'll stick with 3x 2-56 for the nose cone as well.

In other news my av bay is basically done, but I don't have pics and right now it is all wrapped in tape in prep for painting, so I'll post about it later.
 
A few changes to my recovery stack:

1) I'm upgrading to 25' of 3/4" tubular nylon for the drogue cord (with 2300lb rating). kevinkal@ - your points are good about the nylon being much more likely to degrade over time than the other weak link in the stack (the 1500lb parachute swivel), so it makes sense to have it be a higher rating than my target of 1500lbs.

2) Rough calculations suggest that on my L3 motor, there will be 30lbs or so of drag force trying to separate the rocket at burnout!!! This shouldn't be surprising, since this rocket has a fairly heavy payload+nose, and a very draggy rear end. In addition, even with 3x 1/8" vent holes in the booster, there will be ~5-10lbs of force from pressure differential around that time too (and with a single vent hole it could be ~20lbs). I am not at all comfortable with relying on friction-fit to hold against 40lbs of force, so I'll be switching to shear pins for my drogue separation as well. I'd love to go with 2x 2-56 shear pins (~50lbs of shear strength), but a lot of folks warn about that risking a jam, so I'll probably go with 3x 2-56 (~65lbs of strength). I'll stick with 3x 2-56 for the nose cone as well.

In other news my av bay is basically done, but I don't have pics and right now it is all wrapped in tape in prep for painting, so I'll post about it later.

Just the other night I was looking at component Cd vs. mach number and thinking about the drogue shear pins again. My Open Rocket model has the Cd at 0.45 for Mach 0.3, and 0.67 for Mach 1.2 or so. The Cd on the booster/fin section rises sharply with mach number until about Mach 1.2 or so. I plotted drag force vs. time and found that the drag force was around 95 lbs at motor burnout. The rocket will weigh about 18.5 lbs at burnout.. so it will be decelerating at 95/18.5 = 5.14G. I also saw that maximum deceleration at burnout was approximately 195 ft/s^2. I think this includes normal gravity of 32.2 ft/s^2. So the deceleration would be 195 - 32.2 = 162.8 ft/s^2. So in G's that would be 162.8/32.2 = 5.05G.. which agrees with the G calculation from total drag force. I think our rockets will have forward section(Nosecone/Payload/Altimeter Bay) weight of about 6.5 lbs. So, 5.14 * 6.5 = 33.4 lbs of separation force at burnout, not including any internal pressure differential force. That's in good agreement with your number.
I agree, friction fit is not a good idea. I also plan to use three 2-56 shear pin screws for the booster. A friend of mine is building the 4" FG Screech for his L3 project, and was planning to use just two 2-56 shear pin screws.. but his booster will have less drag due to single fins and an Aeropack tailcone. So, just 2 pins might make sense in his case.

I also agree with you regarding the vent holes. We think alike. I wrote in my L3 proposal that I would use between 1 to 3 vent holes that are 1/8" or greater diameter for both the booster and payload compartment vents. Three holes would provide faster pressure equalization and reduce the risk that a single vent hole becomes blocked during boost and/or coast. (Blocked by the parachute or other recovery item.)

I'd like to see any pictures you have when you do find time. I'm looking forward to seeing the Av Bay and your paint process and final product!
 
Just the other night I was looking at component Cd vs. mach number and thinking about the drogue shear pins again. My Open Rocket model has the Cd at 0.45 for Mach 0.3, and 0.67 for Mach 1.2 or so. The Cd on the booster/fin section rises sharply with mach number until about Mach 1.2 or so. I plotted drag force vs. time and found that the drag force was around 95 lbs at motor burnout. The rocket will weigh about 18.5 lbs at burnout.. so it will be decelerating at 95/18.5 = 5.14G. I also saw that maximum deceleration at burnout was approximately 195 ft/s^2. I think this includes normal gravity of 32.2 ft/s^2. So the deceleration would be 195 - 32.2 = 162.8 ft/s^2. So in G's that would be 162.8/32.2 = 5.05G.. which agrees with the G calculation from total drag force. I think our rockets will have forward section(Nosecone/Payload/Altimeter Bay) weight of about 6.5 lbs. So, 5.14 * 6.5 = 33.4 lbs of separation force at burnout, not including any internal pressure differential force. That's in good agreement with your number.
I agree, friction fit is not a good idea. I also plan to use three 2-56 shear pin screws for the booster. A friend of mine is building the 4" FG Screech for his L3 project, and was planning to use just two 2-56 shear pin screws.. but his booster will have less drag due to single fins and an Aeropack tailcone. So, just 2 pins might make sense in his case.

Cool that our numbers agree, thanks for supplying them! Though I'm not sure I follow your math? The nose cone section decelerates for two reasons: (1) drag on the nose cone and (2) shear pins/friction holding it to the booster. Your math seems to be assuming that only (2) applies. To see why this matters, imagine the nose cone had massive massive drag (like a forward facing sphere or something). The rocket would decelerate quickly, but would obviously not try and separate. For this absurd rocket though, your math would take that high G decceleration, multiply by the weight of the nose cone, and assume that there was a strong separation force.

Instead, I manually worked out an equation that I've seen online as well (math is too long to post here) by calculating the force of drag on the nose cone, force of drag on the booster, and then finding what force the shear pins much exert such that the booster and nose cone decelerate at the same rate. If M is the mass of the rocket, mb is the mass of the booster section, a is the (open-rocket sourced) deceleration of the rocket at burnout, and R is the ratio of the Coefficient of Drag for the nose cone divided by Cd for the booster, then the force exerted on the shear pins (positive means the rocket tries to separate) is:

Fsep = a * (M/(1+R) - mb)


One other thing - your Cd numbers are far far lower than what openrocket has for me. If I set to "smooth paint", I'm at 1.1 at Mach 1, and have Cd of 0.88 according to component analysis. This is concerning.... I get Cd numbers like mine if I use the .rkt file from madcow.... Did you build your openrocket model from scratch or start from something you found online?
 
Cool that our numbers agree, thanks for supplying them! Though I'm not sure I follow your math? The nose cone section decelerates for two reasons: (1) drag on the nose cone and (2) shear pins/friction holding it to the booster. Your math seems to be assuming that only (2) applies. To see why this matters, imagine the nose cone had massive massive drag (like a forward facing sphere or something). The rocket would decelerate quickly, but would obviously not try and separate. For this absurd rocket though, your math would take that high G decceleration, multiply by the weight of the nose cone, and assume that there was a strong separation force.

Instead, I manually worked out an equation that I've seen online as well (math is too long to post here) by calculating the force of drag on the nose cone, force of drag on the booster, and then finding what force the shear pins much exert such that the booster and nose cone decelerate at the same rate. If M is the mass of the rocket, mb is the mass of the booster section, a is the (open-rocket sourced) deceleration of the rocket at burnout, and R is the ratio of the Coefficient of Drag for the nose cone divided by Cd for the booster, then the force exerted on the shear pins (positive means the rocket tries to separate) is:

Fsep = a * (M/(1+R) - mb)


One other thing - your Cd numbers are far far lower than what openrocket has for me. If I set to "smooth paint", I'm at 1.1 at Mach 1, and have Cd of 0.88 according to component analysis. This is concerning.... I get Cd numbers like mine if I use the .rkt file from madcow.... Did you build your openrocket model from scratch or start from something you found online?

I'm enjoying this in-depth discussion.

I agree with what you say for (1) plus (2). I was being conservative.. but failed to state that. I made the conservative assumption that the drag force from the booster, booster base drag and fins >> drag force from the nosecone and payload tube. (I'm at work and feel a bit compressed on the rare day I reply to a forum post) This assumption seems to jive with your equation by setting the value of R to 0. I did look at the component drags in Open Rocket and at around Mach 1.14 or so, the Cd was at it's maximum value around 0.67. Let me post the actual numbers I have (I wish I were at home as I could just upload a screen capture instead of typing all this):

Burnout Mach Number = 1.14
Drag Components at Mach = 1.14:
Nosecone_____: Pressure Cd = 0.03, Base Cd = 0.00, Friction Cd = 0.03, Total Cd = 0.06
Payload Tube_: Pressure Cd = 0.00, Base Cd = 0.00, Friction Cd = 0.04, Total Cd = 0.04
Alt Vent Ring: Pressure Cd = 0.00, Base Cd = 0.00, Friction Cd = 0.00, Total Cd = 0.00
Booster Tube_: Pressure Cd = 0.00, Base Cd = 0.22, Friction Cd = 0.09, Total Cd = 0.31
Forward Fins_: Pressure Cd = 0.06, Base Cd = 0.00, Friction Cd = 0.03, Total Cd = 0.09
Aft Fins_____: Pressure Cd = 0.14, Base Cd = 0.00, Friction Cd = 0.02, Total Cd = 0.16
TOTALS_______: Pressure Cd = 0.24, Base Cd = 0.22, Friction Cd = 0.21, Total Cd = 0.67


Given that drag forces are a function of the Surface Area of each section, I'm not sure how to calculate the R term.. just by the coefficients?

R = "ratio of the Coefficient of Drag for the nose cone divided by Cd for the booster" = 0.06/0.31 = 0.1935
from your equation, assuming "a" is acceleration in G, a = (195-32.2)/32.2 = 5.05 G
Fsep (R=0.1935) = 5.05 * (18.5/(1+0.1935) - 12) = 17.5 lb (seems too low)

For comparison with R=0:
Fsep (R=0.00) = 5.05 * (18.5/(1+0.00) - 12) = 32.8 lb (seems to be a reasonable conservative value)

I'll take a stab at just summing the Cd values as a rough look...
or the nosecone/payload/altimeter bay "payload section" that would be:
Pressure Cd = 0.03, Base Cd = 0.00, Friction Cd = 0.07, Total Cd = 0.10

For the booster/fwd fins/aft fins "booster section" that would be:
Pressure Cd = 0.20, Base Cd = 0.22, Friction Cd = 0.14, Total Cd = 0.56

R = "ratio of the Coefficient of Drag for the "payload section" divided by Cd for the "booster section" = 0.10/0.56 = 0.1786
Fsep (R=0.1786) = 5.05 * (18.5/(1+0.1786) - 12) = 18.67 lb (seems too low)

I'm interested to hear what you think of the above. Hopefully I understood what you were saying and used the equation properly. I don't feel quite right about the R term still.


I set my finish to polished for everything. I've found that this gives the best match to my results with my Madcow 4" Fiberglass Level-2 rocket. My Open Rocket model should be from scratch.. I've measured and input all dimensions and weights etc. I may have started with my Level-2 rocket model and modified things from there. I can upload my model when I get home if you'd like to see it.
 
Can't reply in depth, but units should be m/s^2 for accel and kg for mass, giving Newton's as the result unit.

Your R reasoning sounds right, openrocket will sum them for you in component analysis.

I'm going to be traveling so limited computer access, but if you get a chance download the rkt file from madcow to compare? Your CD is much lower than what their model yields
 
Wow, first thread I check after a long absence to the forums and I find out I'm not the only one with a 4" Frenzy XL at MMMSC anymore.

Looking good, maybe we'll have to drag race them sometime.
 
if you get a chance download the rkt file from madcow to compare? Your CD is much lower than what their model yields

I took a look at the frenzyxl.rkt model off the Madcow site. I found a discrepancy:
I noticed they have the fin models incorrect. They have the thickness set to 0.375". The fins should be 0.1875" thick. They also have their fin cross section set to square. I chose the rounded cross section given that our leading edge is beveled on the forward fin, and the trailing edge is beveled on the aft fin.

Their model had a Total Cd of 0.83 at Mach 0.30
After I corrected the fin thickness and cross section the Total Cd dropped to 0.45 at Mach 0.30. This matches my model.
 
Can't reply in depth, but units should be m/s^2 for accel and kg for mass, giving Newton's as the result unit.

Ok converting:
M = 18.5 lbs = 8.39 kg
mb = 12 lbs = 5.44 kg
a = 195 ft/s^2 = 59.25 m/s^2

assuming R = 0; Fsep = (59.25-9.81)*(8.39/(1-0)-5.44) = 44.91*(8.39/(1-0.0) - 5.44) = 145.8 N
145.8 N = 32.78 lb

It would appear that the Fsep equation works for units of acceleration in G's, when M and mb are in units of lb or N.
 
Ok converting:
M = 18.5 lbs = 8.39 kg
mb = 12 lbs = 5.44 kg
a = 195 ft/s^2 = 59.25 m/s^2

assuming R = 0; Fsep = (59.25-9.81)*(8.39/(1-0)-5.44) = 44.91*(8.39/(1-0.0) - 5.44) = 145.8 N
145.8 N = 32.78 lb

It would appear that the Fsep equation works for units of acceleration in G's, when M and mb are in units of lb or N.
yup which makes sense, sorry I'm not sure why I objected...

Great catch though on the fins on the .rkt file! I fixed my model and emailed Madcow to let them know. I'd customized most of the part sizes but it didn't occur to me to double check the fin thicknesses... I get numbers like yours now if I set to polished. I have a bunch of screw heads sticking out and stuff and a camera too so I'm leaving at regular paint which gets me 0.75. More drag is good for me anyway.... With this new Cd if the rocket flies as predicted I'll have to travel to a different field to go for the L3 :-( Or add a bunch of cameras for drag!!!
 
You won't keep that rocket under Maine's waiver on Level 3 power. I've simmed almost every motor I could think of, and nothing stays within the 10k
 
You won't keep that rocket under Maine's waiver on Level 3 power. I've simmed almost every motor I could think of, and nothing stays within the 10k

Yeah... Coming to that realization. The incorrect rocksim file had me excited. I've had some thougbts about trying to mount a bunch of cameras around it, maybe to make a 3d video. That might add enough drag :)
 
That would be cool. But I doubt that would do enough to stay under 10. Probably need the smallest M Vmax to do that. Or white thunder. So the M3700 may be your best bet
 
That would be cool. But I doubt that would do enough to stay under 10. Probably need the smallest M Vmax to do that. Or white thunder. So the M3700 may be your best bet

Great to know, thanks! Even when it simmed under 10k the plan was to not plan to do it on a specific day, but just to keep flying and if the winds aloft were completely dead some day to buy the motor from amw and go for it. I'll play with different cameras and see how it flies on Ks and small LS. Maybe I can find something pretty draggy :). Worst case scenario I'll just have to make a road trip at some point to a place I can go for it.
 
The L910 is a great motor for Berwick. Gets about 6500' iirc.

But remember, to get your L3 very Robert and Gloria will need to be present. Unless your L3CC is someone else.
 
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The L910 is a great motor for Berwick. Gets about 6500' iirc.

But remember, to get your L3 very Robert and Gloria will need to be present. Unless your L3CC is someone else.

Yeah Robert is my l3cc. If I decided to road trip somewhere it'd be after finding out if it was a launch they were attending. At mmmsc, even if I manage to add enough drag, it'd still take a super calm day so it's just not something im ever going to be able to line up much in advance. Super calm and no Robert? No big deal it isn't like I'll be able to fly lots of Ms after I cert anyway :)

Looking forward to trying out the L910 though! First flight will be a low one on a j600. Next up probably k445
 
Well best of luck to you in your L3 adventures. I'm planning on starting a project soon. Hope to see you next time I'm up in Berwick:

ImageUploadedByRocketry Forum1492373693.764558.jpg
 
Thanks! I just got back from traveling but a couple of updates:

1) it looks like I can add 0.1 or more to the CD of the rocket with an 808 camera in a largish shroud on each side. This was something I'd been thinking of doing anyway to take a stab at 3d video, and openrocket thinks it would keep my rocket in the low to mid 9k range with an m1101. There's no way to know for sure until I fly on some smaller engines and dial in the altitude predictions, but there's still (slim) hope for Maine!

2) my shear pins take a lot more force to shear than the rocket materials site predicts (https://web.archive.org/web/2011081...rials.org/datastore/cord/Shear_Pins/index.php). I'm using 4 2-56 pins for the main. I have a load scale that will go up to 110lbs, and I found that 1 pin takes 33lbs or so (plus or minus several pounds), 2 takes 57 to 65 lbs, 3 takes 97 to 105lbs and 4 is at least 130lbs. This compared to the predictions on that site of 50, 64 and 84 for 2, 3, and 4 respectively. I get the same results using the shear pins from apogee as I do for those from mcaster-carr. So, on the one hand this is reassuring that my shear pin holes are done well enough that i get a consistent high-force shear. On the other hand I don't have a great theory for why it is so much more than what that site predicts, and this is more force than I need..... At this point I'll leave the main with 4x 2-56 pins as planned, but these test results make me highly inclined to go with 2x 2-56 for th drogue separation. I know that 2 pins isnt as consistent as 3, but I'm not excited about needing to generate 100lbs of separation force (so 150 to be safe) for the drogue event. It would be a pretty violent separation and risk the main coming out and tax the harness in a tangle, and also would mean that the cti 54mm 2g ejection charge wouldn't be enough to rely on for a tertiary (extra backup) apogee charge on non-l3 flights
 
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