Makers Corner

I 100% agree with Frank. You should use swivels on all parachutes (drogue and main). They just help the parachute work better to keep any asymmetrical drag from causing the rocket to spiral around during descent.

Check these out; high strength stainless steel with ball bearings and tensile strength ratings....

Ball Bearing Fishing Swivels

This is the URL that explains how to post photos to the Gallery. You can drag and drop photos to the Forum posts.

https://cmass.org/content.aspx?page_id=22&club_id=693092&module_id=722231

I recommend a swivel on all chutes.

We have a Zoom meeting on Tuesday at 7:00 PM. You should attend. We enjoy it when people ask us rocketry questions.

You can contact me through the CMASS website in the membership section and I will send you the meeting information.

Frank D

Thank you again for your advice.

I settled on the Eggtimer Mini and an Astrocam.

I sanded the fins without a cant, but mistakenly sanded the body too (with a power sander) which I've filled in with epoxy.

The max thrust/weight ratio is >7:1 for my smallest motor (G67). I'm going with the Apogee stated CP of 44.9" from the nose (which makes it stable and ignoring the weird Open Rocket #). 

I've decided to skip the drogue chute and assume it's not going to fall as fast as the simulations say.

I’ve also modeled and plan to build in a nose cone Electronics Bay for an Eggtimer Mini Tracker.

Are swivels a must for the main chute?

Where do I get instructions on how to post photo/video here?

I've found that you need to restart Rocksim each time you change the event parameters for the parachutes. If you don't, Rocksim seams to combine the two events into one (I'm guessing here). Just one of many Rocksim quirks. In addition, Rocksim does not include the drag of the separated rocket body. For example: In my model, Rocksim predicts a Drogue decent rate of 80 ft/s. Measured flight data shows a rate of 45 ft/s. The main chute decent rate was pretty close. Make sure you uniquely name the chutes to avoid confusion. The Drogue is typically about 1/4 the size of the Main.

Well, your RockSim is reporting the same CP location as the manufacturer at 114cm (44.9in). Sounds like something is off with OpenRock? I would trust your RockSim and the manufacturer. With a stability caliber of 2.1 the CG and CP are about 8.4 inches apart (Zephyr being a 4" rocket).

I am thinking that you have a units issue affecting the math somewhere. Assuming RockSim is correct, and applying that CP location of 114cm and that stability of 2.1, that puts the CG at about 92.7cm (from the nose of course). Taking OpenRocks CP location of 108cm, yields a stability of (108 - 92.7) / 10.16 which is about 1.5 (10.16 being the rockets diameter in cm). Which is still a decent stability rating. So the big takeaway is that both RockSim AND OpenRock say your rocket will be stable.

Make sure everything is in either in metric or imperial units. Don't forget, NASA has had some pretty spectacular (and expensive) mission fails due to simple math unit errors.... The moral of that story is double and triple check everything!!

Thank you again. I have sanded the fins without a cant and a postal scale is to arrive today to check the max thrust/weight ratio. 

Rocksim sets the CP as114cm“ from the nose yielding a 2.1 stability, but Open Rocket (reading the same file as Rocksim) puts the CP at 108cm, yielding 0 stability! Your thoughts please.

I’ve modeled an added Electronics Bay and Drogue Chute (both models say the main chute opening speed is too high), but the Drogue Chute has no effect (does not slow the descent) in the models. I see no event in the simulations at either the apogee (set) or motor ejection delay time.

You can certainly round fins and fill body seams, it will help with drag and looks, but it won't help stability. Stability is affected by CG/CP, velocity, etc, ... Reducing fin drag will move the CP forward a little, which will actually hurt stability, but probably not very much. I recommend you use Rocksim to get your CP, but measure CG by balancing the rocket on your hand or some other object with the motor inserted. You can then decide if you have enough nose weight. And don't worry about measuring the CP to millimeters. This is a 4" rocket, so you just need to get the CG to be 6-8 inches forward of the CP. Its better to be a little overstable than understable to allow for heavier motors. Extra weight won't matter as long as you still have >5-1 thrust to weight at liftoff, and if you want to prevent weathercocking go with >10-1 at liftoff.

Thank you again for the great advice!

Realize I’ve yet to conduct my first Zephyr launch. I’m just getting ready and getting back into the sport at a higher level. I can’t even make my first launch event until 8/31 with the G motors (pray for good weather) and I’ve yet to paint it. 

My concern is stability. So I will revisit the Rocksim numbers with electronics added in, verify the Open Rocket numbers with a scale, see if anything can move inside, then re-do the string test at a higher speed after adding the new mass. I also might add a drogue chute (just in case: Rocksim says deployment speed is too high), and a 4' parachute to bring the landing speed down (Open Rocket says this is too high & I was recommended to experiment with chutes). If I still need to add nose weight then, I will. I’ll sand the fins rounded without a cant, maybe fill the body seams & wooden fins before painting (I’m not going for altitude), skip the shear pins & ejection testing of supplemental charges (I’ll trust the motor ejection charges), and I do have a 29/38mm motor adapter already (for an Aerotech G78-4G motor I have).

I’ll assume in your experience the Estes Astrocam and Featherweight mini both work OK? Comments?

The Apogee website directly gives the CP as 44.89". If this is confirmed in RockSim/OpenRock, and your measured CG produces a stability caliber of 1.85, I would trust in that and have confidence with it. I'm wondering if your string test for stability simply isn't providing enough airspeed to torque the model's CP around that CG point. The website lists the model as almost 5' long and just under 3lbs (w/o that rocket motor and casing). That's a lotta rocket to be swinging around your head. The "string test" is typically shown/demonstrated with much smaller and lighter weight Estes type rockets. A whole lot easier (and faster) to swing around your head!

I'd be a lot less trusting of the weight measurements in RockSim (I haven't used OpenRock) as I've found that I've often had to play around with the numbers (of all the components) to get everything to add up. I would recommend getting a postal scale so you can make accurate weight measurements of the rocket (and components). Also, nothing wrong with going "old school" for that thrust to weight ratio. Weigh the rocket on the scale, then use the maximum thrust of whichever rocket motor you are looking at to get the ratio (your liftoff TWR is the most important one here)

Good places to get rocket motor specifications:

thrustcurve.org

www.aerotech-rocketry.com

I wouldn't worry about trying to add spin to your rocket by sanding the fins or whatever. There's usually enough (very small) aerodynamic imbalances to cause a rocket to spin a little bit anyway (assuming good fin alignment). The important things here are liftoff stability and TWR. If you have those locked in, you are good to go.

If you are planning on using electronics for your certification flight, I suggest using shear pins when you ground test. That way you know if you have a large enough charge to break them. If you are using motor ejection and the nose cone is attached to a payload tube, make sure it’s secure. My own personal experience, I have a rocket that is single deploy, but the nose cone is on a payload tube. It’s secured using shear pins.

As far as shear pins, you’d be good using 2-56 nylon screws.

Im at the Amesbury launches all the time. It’s plenty of room for recovery of your L1 certification flight.

"but it still flies sideways & backwards with a string taped at the CG"

that's probably not a good sign - and your observation that the rocket flys sideways/backwards is not consistent with the behavior you would expect based on the numbers from Open Rocket. Consider double checking the simulation numbers - Is the mass in open rocket consistent with your measured as-built mass? Is the CG in open rocket consistent with your measured CG?

"Are a couple low & slow launches enough..."

If you enjoy flying g's then do what you enjoy and buy some more. You'll definitely have far more opportunities to fly g's in Massachusetts than high power motors. If you do choose to fly a few more mid(or low) power rockets it may help answer other questions you may have. Also consider many of the other activities that are necessary for high power such as dual deploy, gps tracking, telemetry, etc... are also accessible with mid power rockets. The Zephyr is somewhat limiting for mid power rocketry though - your engine choice is very limited if you stick with the 38mm mount. If you have a 29mm adapter you have a couple other choices but the selection is still pretty limited due to the size of the Zephyr.

"No need for shear pins or testing the ejection charge?"

Consider avoiding the shear pins. Regarding the ejection charge there's quite a bit of discussion https://www.rocketryforum.com/threads/zephyr-on-a-aerotech-29mm-extra-bp-needed.179187/ - if you have questions consider reaching out to Apogee to see what they recommend. The g67 reload will come with 1.4 gram ejection charge - it should be more than enough to ensure deployment of the parachute.

Great advice all. Thank you. I did build the Zephyr stock. I plan to launch it to ~600' in Acton 8/17 with a couple G67-14A motor kits I bought, then for the certification attempt with an H100 in Amesbury 9/20. I’ll round the fins before painting. With the JLCR and chute loaded the CG moved forward in my simulation to a CG/CP of 1.85 body diameters, but it still flies sideways & backwards with a string taped at the CG!

OPEN QUESTIONS:

• Regarding the stability test with a string described above. Is the advice “that’s probably OK” true?
• Are a couple “Low & Slow” launches enough to test the rocket before the certification attempt? Or should I buy more motors (‘to have fun with’ too)?
• Has anyone used the Apogee Simple GPS Tracker? Does it work?
• Do I Trust Rocksim/Open Rocket results for the thrust/weight ratio? (I haven’t actually weighed the rocket)
• No need for shear pins or testing the ejection charge?
• Is a drogue chute needed? Simulations say JLCR will open at 89mph after falling to 300' - I doubt it.

Thank you Todd for your fiberglass insulation comment. I did get a UDDT tool to adjust the ejection delays to 5 seconds for the G67s, and 10 secs for the H100. I won’t even think about CO2 until higher altitudes. When I sand the fins - do it symmetrically and don’t try to cant them to spin the rocket?

Under no circumstances should fiberglass ever be used as recovery wadding. It can smolder from the ejection charge and start a fire when it lands on the ground.

See Sam's comment about cellulose insulation.

I would say sanding the wooden fins is always a good idea. Round the leading and trailing edges for aerodynamics, and then perhaps a sanding sealer to seal the wood grains prior to painting. It will just plain look better.

I bought a Weighmax W-2809 digital postal scale from Amazon for taking weight measurements. It's good to 90lbs and super easy to use; and accurate. I use a small square of aircraft plywood sheet placed over the top of the scale to distribute the weight loading (using the Tare function to zero out the added weight of course) for heavier items.

I'd like to put emphasis on a couple of points I've seen from other posts. You should keep your L1 flight as simple as possible. Focus on the basics. Thrust to weight ratio, make sure the CG/CP is 1.5 - 2 calibers or so to ensure stability etc.; adding nose weight to bring the CG forward if necessary. Make sure your recovery system is strong and solid/robust and the parachute protector will do the job (I myself wouldn't trust anything that will or would be expected to burn though). Things like that. One other thing someone else touched on that I would also highly recommend is to fly that Zephyr "low and slow" where it has a safe liftoff but yet doesn't go so high that you lose sight of it. Cameras (GPS tracking and such) are certainly nice, just be aware of the extra workload required and all that will be going on while you are right there at the launch pad getting the rocket ready to fly....

As far as the black powder for the ejection charge, the instructions that come with that H100 rocket motor should say how much etc. Looks like you will also need that special tool (UDDT) to adjust the rocket motor for the actual ejection delay you will want.

Fly that Zephyr on a few G impulse flights first to try out that video camera first. Play around with different sizes and types of parachutes. See what you can do with that JLCR when using larger parachutes etc.

The most important thing here (other than safety of course) is to have fun.

Has anyone used the Apogee Simple GPS Tracker? Does it work?

Is the Astrocam a good enough camera?

It's starting to look like sanding the fins is a good idea. Should I cant them to spin the rocket?

Fiberglass is never ok. If dogbarf isn't allowed its news to me. Cellulose insulation is non-toxic and breaks down quickly so it doesn't pollute.

You don't need black powder until you do electronic deployment. Get certified first, talk to some people at launches so you don't hurt yourself.

CO2 is never really required, and its a lot easier to mess up than BP. People have successfully used BP at over 100K ft, although it does get trickier at those altitudes. Its not something that matters at east coast launch sites, Personally I used BP at 28K ft with no issues. If you plan to go higher than that, there are people on TRF with more experience :-)

That's probably ok, but if you are keeping it close then you need to make sure you measure it in real flight conditions. Motors vary in weight, chute protectors and dog barf have some weight, electronics, etc, Also remember things shift in flight and the motor weight changes during flight. Personally, I would add a small amount of nose weight so I wouldn't need to worry about all of that, so it is between 1.5 and 2.

My CG is only 1.1 diameters in front of my CP and the rocket flies sideways & backwards when suspended from a string at the CG.

Do I need to add weight to the nosecone? ...to move the CG forward