L2 rocket scratch build

[Ben Martin]

Behold the stick that I consider my L2 rocket. Joking of course.

In reality it is my scratch built L2 rocket that I have been working on. It is a 3in diameter that will be 62in long when completed. It has tons of 3D printed components such as the centering rings, baffle system, nosecone, nosecone base plate, fins, and motor retainer. The tubes are ones that I got for free from my father, with the motor tube and coupler (with stiffy) coming from LOC. It's a very budget friendly build.

Overall I have mostly assembled the main body, with the centering rings and motor tube epoxied into the airframe. The rear centering ring is off until I epoxy and fillet the fins soon. In terms of the recovery system, it is complete. I have assembled the baffle system and attached the coupler to both airframe sections with 4 screws on each (CA reinforced holes). Only thing that is left to do is finish installing the carbon fiber rods into the fins, epoxying the fins in, putting interior and exterior fillets, and epoxying the motor retainer on.

Looking forward to seeing this thing launch, hopefully for a successful L2 certification. I do have a few questions though:

- Should I use a shear pin on the nosecone or just friction fit? It's a single deployment at apogee and just the nosecone comes off. My concern is there not being enough pressure from the baffle to shear it.

- Does the length have any significant effects on the stability of the rocket? After some research I saw that it could reduce the actual stability when compared to simulations, but I was wondering if you guys had any experience with this.

 

[Nytrunner]

Good project, many learning opportunities here.
I see all the carbon rods have been embedded in the fins (or fin reprints) successfully.

What motor are you planning to use? Couple things on that:
-Peak thrust. Can your mailing tubes withstand it? (have you tried crushing a spare section of tube to get an idea?)
-Ejection charge of the motor. You can look up how much powder is included with the motor and determine how much pressure it'll generate in a given volume using any of the online BP calculators (I like rockethead.com and the Chuck Pierce spreadsheet)

My recommendation is to attempt the shear pin because it's a good thing to learn how to implement.
The baffle will impede the flow a bit, but that pressure ultimately has to be released somewhere. Highly recommend doing a ground test with the motor tube plugged up and a copy of the motor's eject charge.


I want to say I mentioned in your previous thread that longer rocket's requiring greater static margin between CG-CP, plus more careful attention to the dynamic CP change during flight.

Keep building!

 

[Ben Martin]

Good project, many learning opportunities here.
I see all the carbon rods have been embedded in the fins (or fin reprints) successfully.

What motor are you planning to use? Couple things on that:
-Peak thrust. Can your mailing tubes withstand it? (have you tried crushing a spare section of tube to get an idea?)
-Ejection charge of the motor. You can look up how much powder is included with the motor and determine how much pressure it'll generate in a given volume using any of the online BP calculators (I like rockethead.com and the Chuck Pierce spreadsheet)

My recommendation is to attempt the shear pin because it's a good thing to learn how to implement.
The baffle will impede the flow a bit, but that pressure ultimately has to be released somewhere. Highly recommend doing a ground test with the motor tube plugged up and a copy of the motor's eject charge.


I want to say I mentioned in your previous thread that longer rocket's requiring greater static margin between CG-CP, plus more careful attention to the dynamic CP change during flight.

Keep building!

I plan on using an J250W or J450DM once I get it all sim'd out. Regarding the tests, I don't really have the capability to do it on campus as college limits the fun things I can do. Also, what's your recommendation for the static margin during flight?

 

[Bat-mite]

Long, skinny rockets tend to corkscrew going up unless a very long coupler is used. Nature of the beast. It is not so much the CP/CG relationship as it is the wiggle-waggling of the upper tube against the lower. That being said, although it doesn't affect the stability of the flight dramatically, it looks a little oofbeat going up.

I got my L2 on a Vertical Assault.

 

[Ben Martin]

Long, skinny rockets tend to corkscrew going up unless a very long coupler is used. Nature of the beast. It is not so much the CP/CG relationship as it is the wiggle-waggling of the upper tube against the lower. That being said, although it doesn't affect the stability of the flight dramatically, it looks a little oofbeat going up.

I got my L2 on a Vertical Assault.

View attachment 366014

I have a long coupler and extremely friction into both tubes with screws tightening it down.

 

[Bat-mite]

I have a long coupler and extremely friction into both tubes with screws tightening it down.

Should be good to go, then. For your other question, re: shear pins or friction fit, there are benefits to both.

With friction fit, you don't have to drill holes in your airframe, add shims to assure breakage, etc. No risk of a shear pin induced zipper if the pin doesn't shear.

With shear pins, you have consistency. Friction fit changes with the weather. You are often adding/removing tape on the fly, and tape residue can build up leaving a sticky NC shoulder. With shear pins, once you know the amount of pressure needed to free the NC, and use the same size/amount of pins each time, there is no variable.

But, honestly, both methods work.

 

[boatgeek]

3" diameter by 62" long doesn't seem unreasonably long, and it looks like you have good coupler overlap (~4" on each side?). If it was my rocket, I wouldn't put in a shear pin on the nose cone. There's not much weight up there, so it's not going to want to drag separate. I'd use my standard friction fit test (hold rocket by the nose cone, shake vigorously, and the body should slip ~1/8" down the nose cone coupler). For long, skinny rockets, I've seen a recommendation for having the CG 10% of the length forward of the CP. That would be 6" or 2 calibers on this rocket. I'd be happy with that number, and wouldn't have heartburn going to 2.5-3 calibers. It'll weathercock a little more, but otherwise it'll be OK.

What's the ballpark all-up weight?

[edit]For ejection, it's probably worth doing a modified JLCR puff test. Assemble everything together and you should be able to blow the nose cone off by blowing into the motor mount.

 

[Nytrunner]

I can't add more than Boatgeek already did. Keep up the good work, and strap a camera to the side when you fly!

 

[Ben Martin]

I am having issues with the motor ejection charges being too short for the flights, even on the small J motors. This is even with 300grams of noseweight.

 

[Bat-mite]

Can you clarify what you mean by "too short for the flights?"

 

[Ben Martin]

Can you clarify what you mean by "too short for the flights?"

The max delay for the DMS is 14 seconds, the simulations recommend 14.5.

 

[Bat-mite]

Ah, got you. If that is the case, you will have to use electronic deployment. Really, on a cert flight, better to have an actual apogee deployment than an apogee plus or minus three seconds deployment. Motor ejection has its drawbacks, and you have discovered the main one.

 

[Ben Martin]

Ah, got you. If that is the case, you will have to use electronic deployment. Really, on a cert flight, better to have an actual apogee deployment than an apogee plus or minus three seconds deployment. Motor ejection has its drawbacks, and you have discovered the main one.

Issue is that I don't have an electronics and that would completely change the design. Suppose I'll have to figure something out in order to get it down to the delay time. I always overbuild so it is very likely that it will all fall into line.

 

[hartlch]

Do you have the actual as-built and finished mass for the rocket in your simulation? And what kind of finish did you specify? Looks like it should be "matte" or "unfinished". But motor eject is not that precise as Bat-mite said, so 0.5 sec is in the noise.

 

[boatgeek]

14.5s delay vs. 14 is in the noise. [edit: really, I didn't see hartlch's reply when I was typing mine!] You want to hit it dead on, but plus or minus two seconds usually isn't a big deal. This one may be a little more fragile being non-rocketry tube, but it should still be fine. At half a second before apogee, it would separate with the rocket going 16 ft/s or so. You really start worrying at above 75-100 ft/s. You also won't really know how good the sim is until after first flight. Sometimes rockets go higher, sometimes the go lower. Just keep eyes on it and see how you think you should adjust for the future.

Based on max acceleration there, I don't have any special concerns about the flight. The extra-long motor mount reduces the volume the ejection charge has to pressurize, so I'm not worried about the motor ejection being plenty. Looks like a great build!

 

[Ben Martin]

14.5s delay vs. 14 is in the noise. [edit: really, I didn't see hartlch's reply when I was typing mine!] You want to hit it dead on, but plus or minus two seconds usually isn't a big deal. This one may be a little more fragile being non-rocketry tube, but it should still be fine. At half a second before apogee, it would separate with the rocket going 16 ft/s or so. You really start worrying at above 75-100 ft/s. You also won't really know how good the sim is until after first flight. Sometimes rockets go higher, sometimes the go lower. Just keep eyes on it and see how you think you should adjust for the future.

Based on max acceleration there, I don't have any special concerns about the flight. The extra-long motor mount reduces the volume the ejection charge has to pressurize, so I'm not worried about the motor ejection being plenty. Looks like a great build!

So you're saying just leave the delay at max and let it fly? The simulations in open rocket shows around 1.5cal with only 100grams in the nosecone. The stability over time shows it quickly reaching 2 cal and continuing up to like 4cal. I know that openrocket is very conservative so it might even be higher than that.

 

[jjwb22101]

A couple recommendations -
1. Don't go so quickly through the cert levels. They're designed to each take a little while, and in general get more difficult and costly as you go up. Unless you've got a lot of experience with model and HPR from school competitions (TARC, NASA Student Launch, Battle of the Rockets, IREC, or similar), you really shouldn't try to go for your L2 so quickly after getting your L1. I'd suggest spending some time exploring what you can learn with L1 rockets. Fly something with dual-deploy, add some interesting electronics payloads, etc.

2. If you're going to 4,000' you want dual-deploy. Plain and simple. Assuming a reasonable (15ft/sec) descent rate, and moderate winds (15mph), if you have your main parachute deploy at apogee, you'll drift for over a mile (yes, I just used a rudimentary assumption of drift at the wind speed, but it works relatively well). Have fun trying to find that if you aren't flying on a dry lakebed (if you are I'm jealous).

3. As a general rule, you want to avoid controlling your rocket's altitude with ballast. The simple reason for this is that it lowers your TWR, lowers your launch rail clear velocity, and negatively impacts the safety of your rocket (both in terms of the odds that something will go wrong, and in terms of potential dangers if something does go wrong). Unsecured ballast falling from the sky is one of the most dangerous things that can happen with a launch.

What I'd suggest-
Keep building that rocket! The design looks solid, and I like the CF rods in the fins (that may be a technique I steal at some point - it looks like it would be lighter than a pure composite, without the drawbacks of wood). Ground test it with a sealed motor mount tube and an ejection charge of the same mass as is used in most HPR reloads (I think it's about 1.25g but I could be wrong - CHECK!). HOWEVER - I'd suggest not flying this rocket for your L2 cert. Fly it on some H and I motors, get a dual-deploy altimeter, set that up and use this rocket to learn about how that works. If you're feeling really ambitious, throw an arduino or raspberry pi in with some sensors (and maybe a radio!), or mount a camera externally. If everything goes well there, you've got a good understanding of how the rocket flies, and you're confident you can recover it successfully (remember - the vast majority of certification failures are recovery failures - either something falls off, or you can't find the rocket), it might be time to stick a baby J motor in it. Another option (and what I'd personally recommend - it'll give you a chance to learn what works and what doesn't on this rocket, and fix them for the next one) is to build another rocket for your L2. If you go that route, I'd suggest looking at either 4" or 5.5" diameter - that way you're limiting your altitude with higher base drag, which doesn't have the drawbacks of adding ballast.

 

[Ben Martin]

A couple recommendations -
1. Don't go so quickly through the cert levels. They're designed to each take a little while, and in general get more difficult and costly as you go up. Unless you've got a lot of experience with model and HPR from school competitions (TARC, NASA Student Launch, Battle of the Rockets, IREC, or similar), you really shouldn't try to go for your L2 so quickly after getting your L1. I'd suggest spending some time exploring what you can learn with L1 rockets. Fly something with dual-deploy, add some interesting electronics payloads, etc.

2. If you're going to 4,000' you want dual-deploy. Plain and simple. Assuming a reasonable (15ft/sec) descent rate, and moderate winds (15mph), if you have your main parachute deploy at apogee, you'll drift for over a mile (yes, I just used a rudimentary assumption of drift at the wind speed, but it works relatively well). Have fun trying to find that if you aren't flying on a dry lakebed (if you are I'm jealous).

3. As a general rule, you want to avoid controlling your rocket's altitude with ballast. The simple reason for this is that it lowers your TWR, lowers your launch rail clear velocity, and negatively impacts the safety of your rocket (both in terms of the odds that something will go wrong, and in terms of potential dangers if something does go wrong). Unsecured ballast falling from the sky is one of the most dangerous things that can happen with a launch.

What I'd suggest-
Keep building that rocket! The design looks solid, and I like the CF rods in the fins (that may be a technique I steal at some point - it looks like it would be lighter than a pure composite, without the drawbacks of wood). Ground test it with a sealed motor mount tube and an ejection charge of the same mass as is used in most HPR reloads (I think it's about 1.25g but I could be wrong - CHECK!). HOWEVER - I'd suggest not flying this rocket for your L2 cert. Fly it on some H and I motors, get a dual-deploy altimeter, set that up and use this rocket to learn about how that works. If you're feeling really ambitious, throw an arduino or raspberry pi in with some sensors (and maybe a radio!), or mount a camera externally. If everything goes well there, you've got a good understanding of how the rocket flies, and you're confident you can recover it successfully (remember - the vast majority of certification failures are recovery failures - either something falls off, or you can't find the rocket), it might be time to stick a baby J motor in it. Another option (and what I'd personally recommend - it'll give you a chance to learn what works and what doesn't on this rocket, and fix them for the next one) is to build another rocket for your L2. If you go that route, I'd suggest looking at either 4" or 5.5" diameter - that way you're limiting your altitude with higher base drag, which doesn't have the drawbacks of adding ballast.

I respect your opinion, but I have had a lot of experience with rockets throughout high school and now in college. My L1 rocket was completed quite a few months back and I have learned a tremendous amount from it, so much that I am now working on this new rocket with the intent of L2. My view on getting my L2 is that it opens up some freedoms for experimentation that I can't have with an L1, such as 54mm motors. This rocket is not using a ballast to control the rocket's altitude, I will have at max 200 grams of noseweight in it for stability, with 100grams being much more likely. For recovery from 5000ft, I will be using a jolly logic chute release and motor deployment. I would either be using a J250 or a J270 with a 38mm to 54mm adaptor, with the J250 DMS motor being much more likely. The only issue I have right now is that the expected apogee is 16.1seconds and the motor delay is 14 seconds, however, the simulations show that the optimum is 13.3seconds so that falls within the motor.

 

[markkoelsch]

The max delay for the DMS is 14 seconds, the simulations recommend 14.5.

There is a significant variance in delay times. I strongly recommend using an altimeter for deployment. The Missileworks Rrc2+ is very affordable as is the Perfect Flight stratologger.

 

[Ben Martin]

There is a significant variance in delay times. I strongly recommend using an altimeter for deployment. The Missileworks Rrc2+ is very affordable as is the Perfect Flight stratologger.

The current build uses a baffle in place of the dual deployment system, with an option to change it out in the future. In its current built configuration, it is meant for motor deployment with jolly logic.

 

[Ben Martin]

There is a significant variance in delay times. I strongly recommend using an altimeter for deployment. The Missileworks Rrc2+ is very affordable as is the Perfect Flight stratologger.

You suggest that I change out the baffle system with dual deployment?

 

[Nytrunner]

Worth noting, flight time includes motor burn time, delays time does not.

Delays are measured After Burnout, so if you have a 3 second burn time (makes sense for a J270) and a 16s flight , that means you want your delay time ~13s

 

[Ben Martin]

Worth noting, flight time includes motor burn time, delays time does not.

Delays are measured After Burnout, so if you have a 3 second burn time (makes sense for a J270) and a 16s flight , that means you want your delay time ~13s

Do you mind looking over the sims I posted? Bit confused by the delay time in relation to stage separation. Right now it shows apogee a second ahead of the advertised delay time of the motor. It would not be deploying the chute at separation due to the JLCR.

 

[boatgeek]

So you're saying just leave the delay at max and let it fly? The simulations in open rocket shows around 1.5cal with only 100grams in the nosecone. The stability over time shows it quickly reaching 2 cal and continuing up to like 4cal. I know that openrocket is very conservative so it might even be higher than that.

Eh, it's hard to say. I would like 2 calibers if it were my flight. If I were RSO, I'd probably let 1.5 cal fly. If it were my design, I would have had a little larger span on the fins, but that is water well under the bridge by now.

Do you mind looking over the sims I posted? Bit confused by the delay time in relation to stage separation. Right now it shows apogee a second ahead of the advertised delay time of the motor. It would not be deploying the chute at separation due to the JLCR.

I'm usually pretty happy with a delay within a second of apogee. HOWEVER, that assumes that you have a good handle on what the delay time should be. In your place, I'd drill a tiny hair of delay off (1-2 seconds). That will cover you if your apogee is a little lower than you expect. Also, make sure you do a mass and CG override with the rocket complete and ready to fly (without motor). That will get you a better sense of actual altitudes. Until you have all of your glue and fillets is, you're just guessing at weight. The JLCR is a double-edged sword. It's less stress on an off-nominal ejection, but it's also more likely to slide off the chute if the rocket is going fast at separation. That won't fail your cert, as long as you can get the rocket back.

 

[markkoelsch]

You suggest that I change out the baffle system with dual deployment?

Yes sir, that is what I am suggesting.

 

[Ben Martin]

Yes sir, that is what I am suggesting.

I am probably going to do the first few (lower) flights with the baffle then experiment with dual deployment. The ones that you recommended seem pretty easy to set up and use.

 

[rcktnut]

I am probably going to do the first few (lower) flights with the baffle then experiment with dual deployment. The ones that you recommended seem pretty easy to set up and use.

Bingo! Play around with that JLCR make sure you get your chute packing down pat. You do not really want to deploy the main at over 4,000 ft. The JLCR's are nice,but your still depending on motor ejection. A second or so one way or another doesn't matter much as far as the delay goes. Dual deployment is a PITA compared to using the JLCR but is a definite way to assure deployment at the proper time, if everything in the chain is working properly!!

 

[jlabrasca]

What motor are you planning to use? Couple things on that:
-Peak thrust. Can your mailing tubes withstand it? (have you tried crushing a spare section of tube to get an idea?)

Unless the rocket is being driven into a rigid and immoveable wall, the compressive force on the body tube will not be as large as the peak thrust developed by the motor.

I am sure that this was only one of many threads on this topic (Nytrunner, you contributed to this one).

https://www.rocketryforum.com/threa...-the-point-at-which-body-tubes-buckle.146254/

As Over The Top explained in that thread, bending under off-axial loading is a concern. This is what mailing tubes are designed for; to be rigid under bending stress. If you are worried that the airframe might have been compromised -- a crease or other defect introduced in the previous life of the component as a shipping tube -- low profile plywood strakes would get you style points while adding rigidity to the airframe.

The max delay for the DMS is 14 seconds, the simulations recommend 14.5.

As (I think) others haven mentioned delay timing has substantial uncertainty. Open Rocket puts "optimal" ejection just before apogee. Neglecting drag, the speed of the rocket at apogee ±0.5s will be about 5 m/s (about 16 ft/s). It will be moving faster than this when the JLCR deploys.

The JLCR is a double-edged sword. It's less stress on an off-nominal ejection, but it's also more likely to slide off the chute if the rocket is going fast at separation. That won't fail your cert, as long as you can get the rocket back.

I can think of a few thing to try if you decide to worry about the JLCR slipping off (which is a failure mode I have not seen mentioned before) but the worst that will happen is that the parachute will open too soon -- at a speed slower than the speed at which you intended for it to open. It seems likelier that you'll have a long walk for recovery than that the rocket will come apart. As others have advised, maybe get the finished mass of the rocket before you stress over this.

 

[Ben Martin]

What is the least powerful L2 54mm motor? All of the DMS motors seem pretty dang powerful. Looking for a 54mm motor that has a good kick but not much impulse.

Looks like the J250W is the least powerful DMS and is only a 10% J. Other ones would require a casing which I don't have but could probably borrow.

 

[Ben Martin]

Unless the rocket is being driven into a rigid and immoveable wall, the compressive force on the body tube will not be as large as the peak thrust developed by the motor.

I am sure that this was only one of many threads on this topic (Nytrunner, you contributed to this one).

https://www.rocketryforum.com/threa...-the-point-at-which-body-tubes-buckle.146254/

As Over The Top explained in that thread, bending under off-axial loading is a concern. This is what mailing tubes are designed for; to be rigid under bending stress. If you are worried that the airframe might have been compromised -- a crease or other defect introduced in the previous life of the component as a shipping tube -- low profile plywood strakes would get you style points while adding rigidity to the airframe.



As (I think) others haven mentioned delay timing has substantial uncertainty. Open Rocket puts "optimal" ejection just before apogee. Neglecting drag, the speed of the rocket at apogee ±0.5s will be about 5 m/s (about 16 ft/s). It will be moving faster than this when the JLCR deploys.



I can think of a few thing to try if you decide to worry about the JLCR slipping off (which is a failure mode I have not seen mentioned before) but the worst that will happen is that the parachute will open too soon -- at a speed slower than the speed at which you intended for it to open. It seems likelier that you'll have a long walk for recovery than that the rocket will come apart. As others have advised, maybe get the finished mass of the rocket before you stress over this.

Good thing is that only the nosecone comes off during separation, meaning that there shouldn't be too much danger in terms of early or late deployment. I'm really overbuilding it and I'll be coating it in enamel so it will be very heavy. Only concern I have is that the 4ft parachute won't be able to slow it down enough, even the light simulations have it coming in pretty hard.