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

[neil_w]

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.

Eh, doesn't look too bad to me. A -6 would be better, but I think this is safe to fly.

 

[Archphoto]

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.

That is a fair point, thank you. I ran it through thrust curve to get the altitude estimate, and did some math for the free fall time. Really a guestimate on the deployment altitude, but 8 seconds is a long delay. I'm certain it will deploy before it hits the ground, but I'm not knowledgeable enough to determine exactly how high it will occurr or the speed at which it will go which would certainly affect the effectiveness and integrity of the chute. It will be launched after about 24 smaller rockets, so I'll have a good idea of its trajectory path. I'll see if I can gone in my math a little more before deciding on a go-no go.

 

[neil_w]

OpenRocket is your friend. See my post above.

 

[rcktnut]

That is a fair point, thank you. I ran it through thrust curve to get the altitude estimate, and did some math for the free fall time. Really a guestimate on the deployment altitude, but 8 seconds is a long delay. I'm certain it will deploy before it hits the ground, but I'm not knowledgeable enough to determine exactly how high it will occurr or the speed at which it will go which would certainly affect the effectiveness and integrity of the chute. It will be launched after about 24 smaller rockets, so I'll have a good idea of its trajectory path. I'll see if I can gone in my math a little more before deciding on a go-no go.

I must say the sim looks like a much better outcome than your description of the flight. Having a "flexible" shock cord will help, but a deployment of a plastic chute at 74 fps will most likely lead to a shredded chute. A nylon chute would hold together. If you still have a shroud line or 2 left holding the chute chances are your rocket will fall horizontal rather than coming in at a high speed, straight down. Have a fun and safe launch.

 

[smstachwick]

I must say the sim looks like a much better outcome than your description of the flight. Having a "flexible" shock cord will help, but a deployment of a plastic chute at 74 fps will most likely lead to a shredded chute. A nylon chute would hold together. If you still have a shroud line or 2 left holding the chute chances are your rocket will fall horizontal rather than coming in at a high speed, straight down. Have a fun and safe launch.

That sounds basically like streamer recovery, or nose-blow at worst. I know some rockets are designed to do this but I’ve never seen a nose-blow done deliberately at this scale.

I do know some HPR guys recover on 30ft streamers but that’s a lot more drag, and they do it at bigger sites too.

I’d call this flight questionable but not openly reckless. A good candidate for a heads-up flight and lots of distance from the launcher, or for a rethink and equipment upgrade.

 

[Archphoto]

Ok that simulation data is way better than my own questionable back of the napkin math, thank you Neil_w! I actually have a pair of 18" plastic chutes I could go with to distribute the load and am using an elastic band for sure. I'll make a judgement call on the day, but I do feel better about sending it if conditions are favorable!

 

[Archphoto]

Rocket is painted, launch pad assembled, and ready to go for tomorrow!

 

[Archphoto]

Launch day could not have gone better!

We launched over 20 A, B, and C rockets they the kids built, probably more than all other launches at the field combined! Only lost one to a tree and one fly away, but recovered all the rest!

At the end it was finally time for "Ursa Minor" in honor of my kid being in the Bear den. After building a little suspense with a failed ignighter, 30+ spectators counted down to a perfect liftoff! It followed the simulation in the thread above nearly perfectly, with chute deployment just after it began to descend. it only took damage because one of the kids picket it up and dropped it sideways, snapping a fin. Some super glue with accelerator fixed it right up for a second launch.

I did learn the importance of a cardboard sleeve for the engine mount, as the 100% 3D printed tube did deform from the heat of the first launch. I was able to pressure fit the second engine snuggly about 50% in, and decided to send it with nearly identical results. The video link below is of the second launch because I was annoyingly yelling during the first launch lol.

THANK YOU ALL for the advice, support, and encouragement. This was only successful because of all of your help, and I can't wait to get started on my next build!


YouTube Video Link:

 

[neil_w]

Glad everything worked out!

 

[rcktnut]

Looks like you had a perfect day for the launch. Great to see all went well with the big one.
Nice Job!!