New guy trying to impress the kids needs help with weight calculations

[Archphoto]

Hello all! I've spent some time already searching through the threads, but decided to join the forum and engage in conversation to make sure I get this right!

Our cub scout pack is launching rockets next weekend, the standard A engine kits. I went to the local hobby shop to find something "big and loud" so we could have something with a little wow factor. Stock was limited as is my timetable, and the shop keep offered their window display if I was willing to finish installing the needed bits. It is just over 5' tall and 2 5/8" in diameter, with substantial fins. He said if it survived the launch then I could bring it back, but if not that's ok. I bought the needed parts and a pair of parachutes, some elastic, and a pack of F15-8 engines (the biggest they had) that fit like a glove.

There is room in the engine tube for a much longer engine, but since this is what i could get my hands on I figured I would just 3d print a stop and glue it in. But that got me thinking... Is this engine going to be enough? So our came the scale...

With the engine and all accouterments this big boi weighs in at 1.5 lbs. The max liftoff for this engine is stated to be just under 1lb.

Now I am custom printing the launch rod guides and will be purchasing a metal rod just for this launch, so I have the ability to add length and increase the diameter for increased launch stability if needed, my original plan was to use a 3/16 rod at 4' long.

All I care about is that it gets off the ground and goes "up". If it doesn't hit the ideal altitude for the chutes to land it without damage that is fine, but if it were to leave the rod and turn toward my pack of scouts that would be my worst case scenario.

Most of the weight is in the wood fins at the bottom, if that makes a difference. I looked at some calculators and I'm not sure what to plug in for some of the values, I am freshly back to this hobby and my math skills in this department are very rusty...

I am here seeking your collective guidance and advice! If I do proceed with this launch I will absolutely share the video here!

 

[tsmith1315]

Welcome to the forum!

There is room in the engine tube for a much longer engine, but since this is what i could get my hands on I figured I would just 3d print a stop and glue it in.

That will limit your motor length in the future. I know I'd want something punchier than an F15 in it. Instead, you could make a tape thrust ring on the back end of the motor. Just wrap masking tape around the circumference of the motor, several layers thick (thicker than the motor tube). It only needs to be about 1/4" wide. That will prevent the motor from moving forward. I do it with much larger motors.

Is there a motor retainer in place that keeps it from being ejected instead of the nosecone? A picture of the aft end would be helpful.

What does your pack have for a launch system? If it can ignite composite motors, you may have many more choices.

Look here for nearby rocketry clubs and flyers: https://www.nar.org/find-a-local-club/nar-map-locator/
Find a club nearby, also ask for help here with Seattle in the subject line of the post. I'll bet you will discover more options than what the hobby store had to offer.

As for calculations, download OpenRocket and you can simulate it. You don't need to get detailed, only the external dimensions and mass are necessary for a quick simulation.

Even quicker, you can go to Thrustcurve.org and enter basic dimensions and motor to get an estimate.

-Tim

 

[Archphoto]

Welcome to the forum!



That will limit your motor length in the future. I know I'd want something punchier than an F15 in it. Instead, you could make a tape thrust ring on the back end of the motor. Just wrap masking tape around the circumference of the motor, several layers thick (thicker than the motor tube). It only needs to be about 1/4" wide. That will prevent the motor from moving forward. I do it with much larger motors.

Is there a motor retainer in place that keeps it from being ejected instead of the nosecone? A picture of the aft end would be helpful.

What does your pack have for a launch system? If it can ignite composite motors, you may have many more choices.

Look here for nearby rocketry clubs and flyers: https://www.nar.org/find-a-local-club/nar-map-locator/
Find a club nearby, also ask for help here with Seattle in the subject line of the post. I'll bet you will discover more options than what the hobby store had to offer.

As for calculations, download OpenRocket and you can simulate it. You don't need to get detailed, only the external dimensions and mass are necessary for a quick simulation.

Even quicker, you can go to Thrustcurve.org and enter basic dimensions and motor to get an estimate.

-Tim

Thanks for the reply! There is no stop on the top of the engine tube, and the bottom is threaded. I like the idea of restricting the engine instead of gluing in a stop so future launches can use a longer engine. I am 3d printing a threaded cap currently, and can actually just glue it to the engine since it is minimal material. That way it will not rlejext itself either. I'm intrigued about the composit engines, I have much to learn, but only 7 days until our event.

As for launch capabilities, we have the standard starter kit hardware, and I personally own a 12 channel wireless fireworks launch system. Basically the same voltage and igniter type, just able to fire from a farther distance.

I'll try the calculators again this evening.

 

[Archphoto]

Adding a pic of the aft end and the cap I printed to heep the motor in. I can modify this so it could be glued to the motors as a one time use solution.

Where can a novice obtain bigger motors than a F-15 of the same diameter in less than a week? Specifically in the Seattle area if there are any locals on here!

 

[Zeus-cat]

I also recommend OpenRocket or thrustcurve. Thrustcurve is by far the easiest to use. I'm certain you will find that the F15 will not work in this rocket.

As an alternative to big and loud would be a spool rocket or a "Dare to Be Square" type rocket. These are high drag rockets that are very stable. Since they are high drag the rocket stays low for the entire flight and therefore makes a lot of noise and smoke close to the ground. They always get people's attention.

 

[tsmith1315]

I personally own a 12 channel wireless fireworks launch system. Basically the same voltage and igniter type,

Is it 12V?

 

[tsmith1315]

Where can a novice obtain bigger motors than a F-15 of the same diameter in less than a week? Specifically in the Seattle area if there are any locals on here!

Here’s a regular local launch:

https://www.rocketryforum.com/threads/60-acres-park-redmond-wa-seattle-area.173828/post-2300176


@BEC, @bandman444, Any advice on obtaining something more peppy in the area?

Another link to an area launch:
https://www.rocketryforum.com/threa...eekend-2023-sept-1-4-2023.181134/post-2463417

 

[neil_w]

1) don't use the F15 for a 1.5 lb rocket.
2) A composite Aerotech F50 or F67 would be good. Use OpenRocket or thrustcurve.org to determine delay. Chris at csrocketry.com was super-helpful the last time I needed to get a motor fast for a TARC launch, you can try him.
3) Don't glue anything to the motor. The Aerotech motors have a built-in retaining ring. Just screw on the retainer and you're good.
4) A 3/16" rod is too flimsy for that rocket. A rail would be best, but at minimum use a 1/4" rod. I think I'd want 6' long, but thrustcurve.org will help you figure that out.

 

[lakeroadster]

Home Depot has 5/16" dia zinc covered steel rod x 72" long

 

[rcktnut]

Agree with bigger motor, bigger launch rod as mentioned. Also need a much bigger field than one allowed to launch A-D impulse motors. 60 Acres Park looks good.

 

[Archphoto]

Is it 12V?

No, just a 4AA 6 volt.

I also recommend OpenRocket or thrustcurve. Thrustcurve is by far the easiest to use. I'm certain you will find that the F15 will not work in this rocket.

As an alternative to big and loud would be a spool rocket or a "Dare to Be Square" type rocket. These are high drag rockets that are very stable. Since they are high drag the rocket stays low for the entire flight and therefore makes a lot of noise and smoke close to the ground. They always get people's attention.

View attachment 634699

I'm super intrigued about these, but a week isn't a lot of time to research and build. I do have a ton of old 3d printing filament spools though, so maybe a good longer term project.

 

[Archphoto]

Agree with bigger motor, bigger launch rod as mentioned. Also need a much bigger field than one allowed to launch A-D impulse motors. 60 Acres Park looks good.

View attachment 634747

60 acres is where we are launching. I'll try to locate a larger motor in time and go with a more stout rod.

In the mean time, I'll print some parts and construct a rocket that can use these F-15 motors! Waste not... ya know?!

 

[BEC]

@Archphoto Where/when are you launching locally, and where are you in the area? There are a number of us relative locals on here and I'm sure we can scare up something more usable than an F15-8 (not enough thrust and too long a delay) for that model.

 

[Archphoto]

@Archphoto Where/when are you launching locally, and where are you in the area? There are a number of us relative locals on here and I'm sure we can scare up something more usable than an F15-8 (not enough thrust and too long a delay) for that model.

Launching Saturday morning at 10 am on March 16th at 60 acres! We have 15-20 kids all launching their little Estes Wizards, so we should be there before a solid hour or two before ready for the bigger rockets. I have a C rocket with detachable gliders that we will also launch, and will hopefully have constructed something to use these F engines by then. If we don't get to fire this big daddy it's not the end of the world, but it would be cool to see it go!

 

[BEC]

Ah....I will be setting up for the BEMRC launch at that same time about half an hour (when traffic is light) to the south of you.

The best model for F15-8s I can think of is the Estes Star Orbiter....but getting one and getting it built in a week is sporty at best. Even so, at Sixty Acres it's dicey flying one as it will go to 1600 feet or more on that motor. Depending on where you live....the one place i can think to look for the kit is the HobbyTown USA in Redmond. The one in Tukwila has scaled back its rocketry section some and I don't recall seeing a Star Orbiter in there when I was there a few days ago.

And there really aren't any local sources I know of for a punchier E, F or G for the model you started this thread with. I have a couple that might work (Aerotech/Enerjet F52C-5 for example), but as I say, I'm going to be elsewhere next Saturday running another launch. Hmmmmmm......

 

[tsmith1315]

No, just a 4AA 6 volt.

And there really aren't any local sources I know of for a punchier E, F or G for the model you started this thread with. I have a couple that might work (Aerotech/Enerjet F52C-5 for example), but as I say, I'm going to be elsewhere next Saturday running another launch.

@Archphoto - If you deviate from black powder motors, be aware of the igniter requirements. Starters for composite motors often need 12V. Not a big deal, but it can be frustrating when you have little eyes watching and you can't figure out why you're not getting any fire.

 

[neil_w]

I missed that detail. You’re not going to get a composite motor in the air with a 6 volt controller. And you’ll want at least 30’ of wire with an F motor.

Honestly it sounds like you’re trying to jam an awful lot into a week… a 1.5 lb rocket is not something to trifle with, especially with a bunch of kids present. Be careful.

 

[smstachwick]

I also recommend OpenRocket or thrustcurve. Thrustcurve is by far the easiest to use. I'm certain you will find that the F15 will not work in this rocket.

As an alternative to big and loud would be a spool rocket or a "Dare to Be Square" type rocket. These are high drag rockets that are very stable. Since they are high drag the rocket stays low for the entire flight and therefore makes a lot of noise and smoke close to the ground. They always get people's attention.

View attachment 634699

I’m going to support this recommendation for a high-drag, low-altitude flight with something that decisively not rocket-like.

One of the guys at my local launch yesterday flew a green Irish hat with a D11-P (plugged, no ejection charge) motor installed, that was neat.

Two cardboard disks attached to either end of a motor tube will give you what we call a “spool”, those also spark a few laughs. Drill holes in the disks for rod guidance, figure out your motor retention so it doesn’t go flying out of the tube, and you’re golden.

If you’re still looking to fly that big one some other time, maybe work on making an OpenRocket file for it. That’s your best bet to ensure safe motor selection.

 

[rcktnut]

I made up a rocket like yours, guestimate on fins close enough, 2.6 dia, 60in. long, 24oz. on RS.
An F20-4, F67-6, F52-5 (delays all good) will work fine. Enough speed off a 72in. rod. Altitudes between 700-900 ft. I used some single use G's also, altitudes from 1100-1400 ft.
For me white lightning composite= 10x more awesome than a BP motor.

 

[Aeva]

I’m going to support this recommendation for a high-drag, low-altitude flight with something that decisively not rocket-like.

One of the guys at my local launch yesterday flew a green Irish hat with a D11-P (plugged, no ejection charge) motor installed, that was neat.

Two cardboard disks attached to either end of a motor tube will give you what we call a “spool”, those also spark a few laughs. Drill holes in the disks for rod guidance, figure out your motor retention so it doesn’t go flying out of the tube, and you’re golden.

If you’re still looking to fly that big one some other time, maybe work on making an OpenRocket file for it. That’s your best bet to ensure safe motor selection.

Priority mail stealth? lol I feel like that'd be the best option to put the E in that is a draggy, easy to build rocket. Plus it's probably the coolest Oddroc.

All you need is a box and some decent glue: http://www.artapplewhite.com/priority.html

 

[neil_w]

Before I got back into rocketry, I took my daughter to a little demo launch at a 4H fair. On a small field they launched some LPR stuff and it was cool. They had a couple of larger rockets just for display (include an MDRM) and it was *extra* cool.

What I remember most, though, was that they launched a Cool Whip container. I don't remember what motor it was but it didn't matter, I was blown away by the whole idea that such a thing could fly.

The great thing about launch a saucer-ish thing is that you can put big loud motors in them and they'll still stay in sight and land right near the pad. That's why I have flown my Quinstar so many times, and really look forward to an opportunity to launch my 29mm Quinstar as soon as I can score some F15-0 motors for it. They're fun.

One possibility is just to bring the big one with you for display and then actually launch a saucer of some kind, if there's still time to put one together and acquire the necessary motor(s).

 

[tsmith1315]

One possibility is just to bring the big one with you for display and then actually launch a saucer of some kind, if there's still time to put one together and acquire the necessary motor(s).

That’s great suggestion, Neil. It would build interest and suspense.

It could make a second launch more successful, while not one-upping the kids’ rocket flights this time around.

 

[Archphoto]

Priority mail stealth? lol I feel like that'd be the best option to put the E in that is a draggy, easy to build rocket. Plus it's probably the coolest Oddroc.

All you need is a box and some decent glue: http://www.artapplewhite.com/priority.html

I like this idea, but I can't seem to find this size engine locally. What would the downside of using these F15-8 engines that I have? The ejection charge would still go off, just wouldn't have any function?

 

[neil_w]

The ejection charge could go off after the rocket lands. You don't want that.

 

[Archphoto]

Ok, I have a andoned (FOR THIS WEEKEND) the notion of flying the 5 footer.

Since I have a pair of these F15-8's, and an ample supply of PLA filament... I repurposed 2" x 2 foot shipping tube, whipped up some super basic component designs based on tried and true concepts and am running them on my 3d printer as well speak. I'll have a .75 lb, 3 foot tall rocket that should hit about 950 feet. Yes, the 8 second delay will probably deploy the chute when it is 3/4 of the way down and probably end catastrophicly for the rocket, but I'm only into the build for about $5 and am willing to risk it. I'll post more pics of the build once the fins are attached, and share video of the flight!

Thank you all for the feedback, ideas, and support. I can tell I'll be getting into some crazy shenanigans with your collective help, and look forward to being to contribute a little knowledge of my own along the way!

 

[Aeva]

I like this idea, but I can't seem to find this size engine locally. What would the downside of using these F15-8 engines that I have? The ejection charge would still go off, just wouldn't have any function?

I've seen 3d printed scale downs to 18mm motors, and 38mm scale ups (with plywood) so yes. There's a template pdf file in that link for a 29mm motor mount. It should print to standard printer paper, and then you can tape it to the box as a guide.

I used a leftover amazon shipping box; it can be done with other materials like poster board too. I also recommend papering seams where you join the cardboard to give it a nicer finish.

Yes the ejection charge will go off. You may want to talk with the folks in recovery. I've heard of people taping over the ejection charge with recovery wadding on motor casings (basically a plug, but without the delay and charge), but I'm not sure if you can do that SAFELY with an 8 second delay on such a draggy rocket. It will be past apogee when that goes off, which won't harm the rocket, but might be concerning for the audience.

Also, the motor they listed on the page (G40) was pretty overkill. If you can find them, you can put an E or F with no ejection charge in that and still get some "WOW" factor.

 

[BEC]

Finding a saucer type for an F15-8 is a problem. Granted right now there's not much chance of starting a field fire with an ejection charge firing on the ground at Sixty Acres. Even though it's not going to rain between now and Saturday, the ground will be damp and soft. But it's really bad practice and a bad example to do that.

I don't know/remember if the HobbyTown in Redmond stocks the 29mm Estes motors — it's been many months since I've been in there. But if they have E16-0s or F15-0s (ideal) or -4s of either motor those would be a much better option for that Applewhite model. It's a cool one, by the way.

I really don't think there's any good local choices for flying the model you started the thread with. An Estes F15-8 is NOT it for that model.

One other thought that popped into my head as I was typing this — if that Redmond HobbyTown USA has a Super Big Bertha, it might work on your motors. The delay is really too long (that model does better on F15-6) but it's still a better choice than the -8s in a saucer or in your original model.

I wish I could come up to help, but as I say, I'm running a launch myself on Saturday, some distance south of Sixty Acres.

 

[Archphoto]

I wish I could come up to help, but as I say, I'm running a launch myself on Saturday, some distance south of Sixty Acres.

No worries at all, this was a very last minute thread!

The Redmond Hobbytown didn't have anything in the 29mm size at all, but the Everett shop did have a limited stock of the F engines, including the 15-0's. I just won't be able to make it up there in time for this launch, let alone build another scratch flyer. I will be dabbling in these all in the near future though, and I look forward to meeting you and others like you in the fields along the way!

 

[Aeva]

No worries at all, this was a very last minute thread!

The Redmond Hobbytown didn't have anything in the 29mm size at all, but the Everett shop did have a limited stock of the F engines, including the 15-0's. I just won't be able to make it up there in time for this launch, let alone build another scratch flyer. I will be dabbling in these all in the near future though, and I look forward to meeting you and others like you in the fields along the way!

You can probably make a 24mm motor work too but you'll have to either figure out differences to the design dimensions when you make your cuts, or add some 24 to 29mm adapter that centers the motor in the 29mm mount

 

[rcktnut]

I'll have a .75 lb, 3 foot tall rocket that should hit about 950 feet. Yes, the 8 second delay will probably deploy the chute when it is 3/4 of the way down and probably end catastrophicly for the rocket, but I'm only into the build for about $5 and am willing to risk it.

By obeying the safety codes, should not be purposely launching this with the expected results. It isn't exactly setting a good example either. That being said, did you sim it? I would expect a rocket that size and weight to go somewhat higher than that on an F15.