Back After 50+ Years

I plan on buying the rocket kit with the 38mm motor setup, and also purchasing the adapter insert so I can use 29mm motors. I'm looking for versatility also. I think Apogee also has a video(s) on reloadable type motors. But I won't purchase anything right yet for the Level 1 Cert airframe or airframe. I want to have one day of success with the "Stretch Blobbo" so I know I still have interest in the hobby. I would hate to invest, say $500, and find out I just don't have the enthusiasm I thought I have. "Little steps Grasshopper". Caley Ann

Caley said:

Thanks again. There's a lot to digest here.

... That's why I mentioned that this will probably take me half a year to get done. Caley Ann

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I told you you'd get good advice here. ALOT of good advice. A bigger bunch of enablers you will not find anywhere ! :wink: Having my local rocket buddies help me digest and explain and demonstrate in person helped me. Really helped me. Really helped me alot. I say first things first, fly your BLOBBO as often as you can with as many motors as you can. Get a feel for how your rocket actually performs on black powder and composite motors. Starting out, I see no reason not to use the kit manufacturers recommended motors as a starting point. The key thing you will learn about selecting motors for a rocket is thrust-to-weight ratio. One low tech tool I've used for the last 10 years are a couple of graphs I saved from an old rocketry site. Average thrust of the motor on the X axis and weight of the rocket along the Y.


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https://web.archive.org/web/20120902192900/https://www.info-central.org/?article=121

Ok, one more video link, John Cokers excellent 'Choosing motors' : https://www.jcrocket.com/choosing-motors.shtml

Relax and enjoy your new hobby !

NateLowrie said:

There was more than one sequel? I absolutely loved it too. Really rich and riveting. On another note, the Audible version of Rocket Boys is awesome if you like audiobooks.

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Rocket Boys
The Coalwood Way (takes place during Rocket Boys; he called is "not a sequel, but an equal")
Sky of Stone (Homer returns after a year at college and goes to work in the coal mine)
Carrying Albert Home (not really a sequel, but a humorous take on the "legend" of his mom's pet alligator)

The Trust-to-Weight ratio charts are very useful. Thanks. If I am correct, the manufacturer provides the output of the motor in Newtons, and I just need to choose one that keeps me in the safe range.

Caley said:

The Trust-to-Weight ratio charts are very useful. Thanks. If I am correct, the manufacturer provides the output of the motor in Newtons, and I just need to choose one that keeps me in the safe range.

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Correct. The average thrust is part of the motor name. It's the number between letter and the dash.

Caley said:

The Trust-to-Weight ratio charts are very useful. Thanks. If I am correct, the manufacturer provides the output of the motor in Newtons, and I just need to choose one that keeps me in the safe range.

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Right!

Thrust = how heavy of a rocket can I lift?

Impulse = how high do I want to go?

Bat-mite said:

Rocket Boys
The Coalwood Way (takes place during Rocket Boys; he called is "not a sequel, but an equal")
Sky of Stone (Homer returns after a year at college and goes to work in the coal mine)
Carrying Albert Home (not really a sequel, but a humorous take on the "legend" of his mom's pet alligator)

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I read Sky of Stone and it was great. I will definitely check out the other 2.

Welcome back to the hobby and we hope to see more of you on here. You picked the right place. This forum has plenty of individuals with a wealth of knowledge. Enjoy your time here and and the hobby

I downloaded the trial version of RocSim, but unfortutnately, every time I tried to extract the execution files from the zipped file, my Norton AV said it was dangerous, and deleted it. So much for running a simulator. Each and every simulator I have tried to download has gotten the same treatment from my Norton AV. Drats! Foiled like most times I am trying to get something done. :-(

Caley said:

I downloaded the trial version of RocSim, but unfortutnately, every time I tried to extract the execution files from the zipped file, my Norton AV said it was dangerous, and deleted it. So much for running a simulator. Each and every simulator I have tried to download has gotten the same treatment from my Norton AV. Drats! Foiled like most times I am trying to get something done. :-(

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Right click on the Nortons Shield and turn it off or do it through services.

I downloaded the trial version of RocSim, but unfortutnately, every time I tried to extract the execution files from the zipped file, my Norton AV said it was dangerous, and deleted it. So much for running a simulator. Each and every simulator I have tried to download has gotten the same treatment from my Norton AV. Drats! Foiled like most times I am trying to get something done. :-(

Bill, I'm a bit late replying to you about motors. I did a check on both motors I purchased from the onsite vendor, E9-4, and D12-5, and apparently they fail due to a drop off in thrust. I am assuming that this means that the model is not going to achieve the desired altitude, and possibly cause a problem with recovery because it didn't achieve enough velocity and height. I'll keep looking around for motors that will work. I'm beginning to think that ESTES motors might not be the best choice. I also need to rethink the stock motor hook that is installed, and see if I might be able to use one of those plastic motor retainers. I've about 1/3 of an inch of motor mount tube in the rear to attach such a retainer. Hopefully that will be enough.

I guess I goofed when I depended on the onsite vendor to provide me with the correct motors and other necessities. Shows how little I knew going into this. Caley Ann

Afterburners, Thanks a ton. I am not much when it comes to computers, so this really helped. I finally got the program installed, and hopefully I will understand enough to get it working, and providing me with information needed to choose the correct motors. Caley Ann

Well, That was kind of a flop with the RocSim. I had downloaded a Stretch Blobbo data file for it to use, but unfortunately the weight, CP and CG parameters do not match what I have, and I cannot figure out how to change that data.

I looked around for 24mm rocket motors, and it seems that anything that might possibly work in my model is sold out. The motors I have will not work because my model (Blobbo) is 4 ounces heavier than the supposed stock rocket weight when finished. I guess this sized model is very popular, and it is very difficult for vendors to keep the engines in stock. Anyone out there that might help me locate some engines for my Stretch Blobbo. It is 2.6 inches in diameter, using a 24mm motor up to 7.5cm long. The weight is a robust 10.2 ounces dry. It would be nice to obtain two or three engines for the next gathering in mid April, so I can at least see if the model will work, or perform the famous lawn dart. Caley Ann

Got my Tripoli membership package today. It contained a welcome letter, as well as papers for certifications, and a booklet with rules and also questions that apparently you must answer for certifications above level 1. Since I have a terrible memory, that might take me a while to do. Its almost as bad as my Pharmacy Techicians certification, or my big rig cert tests. UGH!!!! I hate tests. Caley Ann

So if you cannot find them locally try buyrocketmotors.com. they've been great to work with.

I will have to order online because I live in the boonies. The closest town is Palm Springs, which doesn't have much in the way of hobbies. The closest large city is over 100 miles away, and I don't know if any hobby shops carry anything but ESTES products. I order just about everything online because the local stores have very little of anything that I am looking for, other than basic staples of life..

Caley said:

I will have to order online because I live in the boonies. The closest town is Palm Springs, which doesn't have much in the way of hobbies. The closest large city is over 100 miles away, and I don't know if any hobby shops carry anything but ESTES products. I order just about everything online because the local stores have very little of anything that I am looking for, other than basic staples of life..

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Welcome to our hobby. We are fortunate that a few vendors show up to our launches so we can call ahead and they bring them to us here. Gone are the days of brick and mortar for the fun stuff.

I'm still learning about things online. I have been reading the rules that came with my Tripoli membership package. One stated that I must have three times the thrust of what my model weighs. Being that I primarily concentrating on my "Stretch Blobbo" rocket right now, to get it ready for launch, I roughly converted my 10.2 ounces airframe weight to 3/4 of a pound.

Since these engines give thrust in newtons, I found a converter by Googling, so I could input various engine outputs to match my 3x.75=2.25 pounds. This means I need at least an engine that puts out 10 newtons, and preferably 11 newtons.

What I wish to know is just how much over 11 newtons is safe for a mid powered model like mine. I can imagine that if you launched it with 50 newtons of thrust, you would probably disintegrate the airframe due to excessive forces.

Also, I am trying to figure out the delay for the ejection of the parachute. It seems that most of the time, the ejection is happening very near the peak altitude, or just a bit down from apogee. How does this affect the recovery system if it is still coasting to peak altitude, or has descended slightly, say for a hundred feet, when the ejection charge finally goes off.

I am guessing that the Jolly Logic Chute Release could resolve this problem, if it is one. Or am I being too worried about little things? Caley Ann

Easiest rule of thumb is lb75.

If you have a motor number, divide by 75 to get safe-weight in pounds. Going the other way, multiply weight-in-pounds by 75 to get safe-motor-number.

It's not exact, but this yields a bit more than a 5:1 thrust-to-weight ratio which is just about plenty for most flying conditions.

If I understand you correctly, my 3/4 pound model multiplied by 75 would come out to 56.25 newtons. That sure sounds like a lot of power.

Caley said:

If I understand you correctly, my 3/4 pound model multiplied by 75 would come out to 56.25 newtons. That sure sounds like a lot of power.

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Yup. It's a rough rule of thumb, meant to be easy to remember; cover the majority of flying conditions; and ensure you don't need any special length of rod or rail.

Certainly with no wind, a long launch rod, and a well-balanced bird one could go a lot lower. Going further, it's really the peak thrust on most motors which greatly influences the speed off the rail.

I finally gave up on RocSim, and deleted it. It provided absolutely nothing useful in its trial version,.... I just don't know how to work with it. Couldn't even get the help part of the program to work. I'll just have to rely on other people to provide me ideas about what will work in my Stretch Blobbo. If I am ever going to progress on to building a Level 1 airframe, I definitely need to get this little critter flying to tell me if I can go farther in the hobby. Needless to say I am frustrated that I cannot figure out programs that to most people are very simple.

Rocksim, especially the trial version is less than intuitive. I wouldn't worry too much about it.

DavidMcCann said:

Rocksim, especially the trial version is less than intuitive. I wouldn't worry too much about it.

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+1, I computer for a living and there's some pretty counterintuitive stuff in there. But now that I rocket for a hobby, a bunch of it starts to make more sense.

DavidMcCann said:

Rocksim, especially the trial version is less than intuitive. I wouldn't worry too much about it.

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Open Rocket is free and has most of the same features as RocSim. OR is a Java applet so it does require the installation of Java to run, its pretty easy to learn.

ran a few simulations with a weight of 10.2oz
D12-3 rod departure speed 36.8f/s alt 287'
E20-7 rds 45.2 f/s alt 960'
E18-7 rds 41.8 f/s alt 1060'
usable rod length in the sim 39.4", the 1st 2 motors are single use the third is reloadable (AT 24/40 case), I would consider all 3 motors suitable to launch at wind speeds below 10 mph. if the rocket has a motor hook I would suggest investing in 5" zip ties and a side cutter to secure the hook/ remove the tie after flight. hope this helps, have fun
Rex

Rex, thanks for the list of engines. Based on my search of the internet vendors, there just aren't any motors available right now. It seems all the ones that might work in my Blobbo airframe are very popular, and sold out. The remainder of the 24mm motors won't do the job, or are L1 cert required, and probably too powerful for this little beast anyway. I'm going to sign up for notification service on the motors that will work. Hopefully I can jump on and get a small quantity to allow me to fly next month.

give it a month, it is Tarc season. we go through this every march and April, the TARC teams buy up all the single use Es and Fs(and some reloads). D12s should be in stock they don't usually buy them , by May the number of the teams drops and supply slowly catches up. I will note that Wildman rocketry says he has 12 packages of E18-7s in stock(but little else in the E & f range. on the other hand it does clean out all the old stock .
Rex

Caley said:

I finally gave up on RocSim, and deleted it. It provided absolutely nothing useful in its trial version,.... I just don't know how to work with it. Couldn't even get the help part of the program to work. I'll just have to rely on other people to provide me ideas about what will work in my Stretch Blobbo. If I am ever going to progress on to building a Level 1 airframe, I definitely need to get this little critter flying to tell me if I can go farther in the hobby. Needless to say I am frustrated that I cannot figure out programs that to most people are very simple.

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I have the same problems with simulation programs, what seems easy for everyone else baffles me. I've never tried RocSim, but for the life of me I can't get Open Rocket to work. I like to use Thrustcurve.org to help with motor selection.

Caley said:

Got my Tripoli membership package today. It contained a welcome letter, as well as papers for certifications, and a booklet with rules and also questions that apparently you must answer for certifications above level 1. Since I have a terrible memory, that might take me a while to do. Its almost as bad as my Pharmacy Techicians certification, or my big rig cert tests. UGH!!!! I hate tests. Caley Ann

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I did not see an answer to this, but thought I would clarify. That questions packet and the test are only for Level 2 certification. For level 1, it's basically show up with your rocket, fly it and safely recover, and the certifying person inspects the rocket to make sure there is no damage that would hinder you from immediately flying again.