Critique my first scratch high power rocket (29mm)

[DarthMuffin]

So I'm working my first scratch HP rocket and am looking for feedback. Goal here is to make something light and zoomy, yet sturdy and reliable. 29mm motors for cost. I should be able to just take it out and launch it, get good altitude, and be able to turn around and do it again all day if I wanted. I don't have any specific altitude goals, just as high as practical without sacrificing reliability and reusability.

So, current plan is a 38mm CF tube (which I have found a source for), slimline retainer, thin FG nosecone (picked that particular profile because it's the only one I can find for sale and in stock [madcow], if you know of another please post a link). Dual deploy with an Eggtimer TRS (antenna probably poking out either up or down, and yes I know the CF will block the signal before it's deployed). I think it's light enough that drogue can just be a streamer for visibility? Size of kevlar shock cord recommended? Will shear pins be needed for something this small? How about the coupler for the avionics bay--just cardboard, quantum tube, fiberglass? Any reason to do anything besides cardboard for the motor mount tube? How about just some rings and omit the mm tube?

Went with 38mm body tube because I think I want the strength of thru-wall fins. Opinion on that? Haven't decided what to make the fins out of, thoughts? Fins are that shape to hopefully protect from landings a little more, plus it looks cool. I can get a little more stability if I sweep them back though.

I have a 3d printer and would make the av-bay mount myself with polypropylene filament. Can 3d print other parts if it would help?

 

[Zeta]

Darth,
Fun project...
My first thought when you mentioned possible multiple launches per day without an altitude requirement was to seriously consider a larger diameter airframe. This will help keep the altitude manageable. Meaning that the 38mm airframe is going to get really high on H motors and the limiting factor for fun could be as simple as wind conditions. If the rocket hitting 4000 + feet consistently winds aloft will control your day far more than if you are hitting apogee at 2000 ft AGL. Additionally part of the enjoyment of the sport is being able to see the whole flight. I am willing to bet that a 38mm airframe is out of sight at 3000 + (consider any amount of hazy sky). So a larger airframe is also much easier to see as well. I have a 2.6" diameter airframe that did about 2600 feet on an H195 and that was just at the limit of my vision. So I think 3" is a great diameter, at that size the room to work inside the rocket for internal fin filets, electrics, and deployment charges etc is much better. Packing a chute in a 3" rocket is a piece of cake. .... Your drawing shows a flight at Mach 1.45 and 10K feet, unless conditions are pristine, recovery may present challenges.... do you fly on a dry lake bed in Nevada ?

 

[DarthMuffin]

Your drawing shows a flight at Mach 1.45 and 10K feet, unless conditions are pristine, recovery may present challenges.... do you fly on a dry lake bed in Nevada ?

Well, I might. I'm in the desert at least. Recovery shouldn't be an issue, but you're right with the trek to get it I probably won't want to fly it multiple times a day. Anyhow, I have fatter rockets if I want to do lower flights and that sim is with the biggest engine I could possibly use.

 

[tsmith1315]

thin FG nosecone (picked that particular profile because it's the only one I can find for sale and in stock [madcow], if you know of another please post a link)

Composite Warehouse lists 3/4/5:1 Ogive, 5:1 VanKarman, and 5:1 Conical. Thin and standard wall, in various colors.:

https://www.compositewarehouse.com/index.php?route=product/product&path=34_43&product_id=100

I've never bought from them, so I can't give a thumbs up or down for their service.

 

[David Schwantz]

Antenna does not need to be outside airframe if you put it in the NC because it is glass.. If you have a CF tube, by all means fins should be also. I would use 1500 lb kevlar cord. I get mine from Emma Kites. Length is usually 3 times length of rocket. but I like a bit more. Shear pins depend on how well you fit the NC in the tube. And how much it weighs. the more the weight, the more it will want to drag separate. I use streamers all the time. Black and orange are good colors for me. I use cardboard MMTs a lot. I also wick thin CA into the end for strength. I would use at least glass for the coupler, even though cardboard would work. But try to find one the right size.If you want TTF then you must have a MMT. Good luck.

 

[teepot]

I like the fin shape. Bigger would be my suggestion. It would move the CP back. TTW fin tabs is a good idea. Your going to need several calibers of stability as the CP moves forward as you near Mach 1. Forgive me if you already knew that. I would use the motor tube. It's something to epoxy to. If you have a fiberglass coupler I would use that.

 

[DarthMuffin]

I like the fin shape. Bigger would be my suggestion. It would move the CP back. TTW fin tabs is a good idea. Your going to need several calibers of stability as the CP moves forward as you near Mach 1. Forgive me if you already knew that. I would use the motor tube. It's something to epoxy to. If you have a fiberglass coupler I would use that.

I just extended the fins a whole tenth of an inch and got 1.9 to 2 cal stability depending on motor. Worth it, even though for the sims I've seen the CP doesn't start moving forward until approaching Mach 3. Looks like cardboard motor tube and fiberglass coupler is the consensus.

 

[thzero]

I kinda like the idea of custom rings to fit the diameter of the motor casing without a motor tube.

 

[DarthMuffin]

I kinda like the idea of custom rings to fit the diameter of the motor casing without a motor tube.

Me too but I was too dumb to realize I'd have nothing to glue the thru-wall fins to. Derp. Maybe a just-long-enough tube and a custom ring or two?

 

[Zeta]

I just extended the fins a whole tenth of an inch and got 1.9 to 2 cal stability depending on motor. Worth it, even though for the sims I've seen the CP doesn't start moving forward until approaching Mach 3. Looks like cardboard motor tube and fiberglass coupler is the consensus.

If you are running sims in the super sonic range consider comparing your O.R. sims with http://rasaero.com/ software .... rasaero is good for supersonic speeds. Of course the buttons will melt off ...hehehehe

 

[David Schwantz]

Have to have a MMT to glue retainer to. Also, the MMT transfers thrust through the tube, to the CRs and then into the airframe.

 

[thzero]

Have to have a MMT to glue retainer to. Also, the MMT transfers thrust through the tube, to the CRs and then into the airframe.

Depends on the retainer design, although I suppose most of the ones for 38mm probably do require a motor mount. Use a thrust plate to transfer the thrust to the airframe. Its an interesting thought.

 

[DarthMuffin]

Depends on the retainer design, although I suppose most of the ones for 38mm probably do require a motor mount. Use a thrust plate to transfer the thrust to the airframe. Its an interesting thought.

I believe the ogive slimline retainer I intend to use will transfer the thrust to the airframe.

 

[waltr]

Looked at OR sim of your rocket.
Even on an F52 altitude is >2200 feet. If you do go with larger motors then a GPS system in the Nose cone would be a good idea.

Increase length of top tube, about 10cm, to give room for main chute plus GPS. Adding GPS into nose adds weight and increases the stability margin.

 

[DarthMuffin]

Looked at OR sim of your rocket.
Even on an F52 altitude is >2200 feet. If you do go with larger motors then a GPS system in the Nose cone would be a good idea.

Increase length of top tube, about 10cm, to give room for main chute plus GPS. Adding GPS into nose adds weight and increases the stability margin.

Yeah, I'm a big believer in a GPS. The eggtimer altimeter I think I will use has the GPS tracker on it. I may go with an Altus something also with GPS if I get the HAM cert before then.