Need advice

View attachment final.ork Attached is the open rocket file. I need good advice on two things and I'll also use this thread for the build progress. You can see the launcher/controller here...
https://www.rocketryforum.com/showthread.php?59171-Easy-scratch-launcher-with-controller

1) How many calibers of stability would be good for this design to be launched straight, weathercock into 15 mph winds and land back near the pad after canopy drift at 15 fps?

2) I'm torn between a drogue & main with no View attachment final.orkdbag, or a main with piston and no dbag. What do you experienced guys think?

I looked at your file - I think you may be overthinking this. For starters - the motor (G64) has the case, closures etc. already built in. You don't need to add these items in your simulation. As a general rule of thumb - at least 1 caliber of stability. If you are actually able to design and build a rocket that consistently lands back near the pad - please share your knowledge

the motor weight as given by OR is for a motor ready to launch(complete with case mass figured in)...adding mass components will throw off your cg figures. I also looked at your file...when I 'took out' the extra mass the rocket was at 1.28 cal. for a rocket where space is at a premium, I'd probably go with a piston (or as an alternative, dog barf).
rex

View attachment final.orkThank you so much guys! That makes things way less complicated. Here's the revised file. I still need to figure out the final recovery method/size and put the rail buttons on.

First piece done. Not much, but its a start. It's just a fiberglass tube laid up around a wax coated .125in rod with JB Weld simply because I had some and wasn't going to buy epoxy resin for the one and only fiberglass part I had to make myself. This will be sandwiched between the baffle closures and hold the eye bolt.

well after adding 15' 1/4" flat braided Kevlar (22g) the stability margin was better...obviously you'll need to check your cg location after it is built, but if it matches the design file it is going to be marginal at liftoff, you should aim for 1.3 - 1.5 cal.
Rex

Second part done. Before and after of eye bolt. I had these in the garage, but the open design bothered me. I closed the eye with solder and polished to remove abrasive edges that might damage the shock cord.

Assuming I don't care about max apogee, why would something like 3-5 caliber not be better since in theory it would fly more into the wind and then land closer to the pad? Sorry if the answer is obvious to you rocket guys.

Rex R said:

well after adding 15' 1/4" flat braided Kevlar (22g) the stability margin was better...obviously you'll need to check your cg location after it is built, but if it matches the design file it is going to be marginal at liftoff, you should aim for 1.3 - 1.5 cal.
Rex

Click to expand...

you mean aside from weight? nothing, the figures I gave should be considered min values. long skinny rockets may require 3 - 5 calibers simply to be stable(the mean machine would be an extreme case). I am of course assuming that you already have the fins cut, if not and you have sufficient material, make them about a 1/4" wider (span), that should help(one could try 'playing' with this in OR just to see the effects ).
rex

Gotcha. The two parts posted above are all I've done so far so everything else can be changed. I was thinking about getting the proper parachute, then setting the airframe length just long enough to accommodate the chute and baffle with no pilot, piston or dbag. Then I'll increase fin span to reach the proper stability. Does this seem like like the correct course?

a few things come to mind, (remember engineering is all about trade offs) your finished rocket will most likely be heavier than your design, with an FG rocket you can usually get away with a higher descent speed(20 fps is common for FG birds), also keep in mind that the size of your fin stock will be a limiting factor. playing around in OR with sport rocket designs can be kind of fun .
Rex

Rex R said:

a few things come to mind, (remember engineering is all about trade offs) your finished rocket will most likely be heavier than your design, with an FG rocket you can usually get away with a higher descent speed(20 fps is common for FG birds), also keep in mind that the size of your fin stock will be a limiting factor. playing around in OR with sport rocket designs can be kind of fun .
Rex

Click to expand...

I just ordered fin stock which will be 3/32 inch G10. Because of the great advice I've been getting here, I'm shooting for 15fps under canopy with the idea that after it's all said and done with epoxy, paint, etc, I will end up somewhere around 20fps or less. I designed the fin shape to be unlikely to hit the ground before the boattail. I'm still undecided about the chute shape, size and whether I'll need a piston.
I'm thinking of just netting the bottom of an elliptical with a spill hole to reduce the chance of malfunction from line overs, twists, et al.

View attachment final.orkOther than updating weights, cg, etc, this should be a finished design. Did I miss anything or am I good to start assembly?

I see a couple of things that you might want to check...you might want to set your lugs to 30 degrees, can't quite figure how you're going to get the retainer on/off with the fin tabs getting in the way . otherwise as it sits now, I'd go for an F52-8 for a 1st flight.
Rex

Rex R said:

I see a couple of things that you might want to check...you might want to set your lugs to 30 degrees, can't quite figure how you're going to get the retainer on/off with the fin tabs getting in the way . otherwise as it sits now, I'd go for an F52-8 for a 1st flight.
Rex

Click to expand...

Thanks for the review. Retainer is the slimline 29-38 boattail with a snap ring, so the retainer will be glued to the airframe and fins. It doesn't show it well in the OR file, but I plan to make the fins taper with and follow the curve of the boattail all the way to the end.

ah, so you don't need to remove it like some other retainers, cool.
Rex

Canopy riser/harness is 5mm tubular nylon made by removing the guts from a 10ft piece of 550 cord.

Attached to the eye bolt, singed the end to prevent unraveling and covered with shrink tube to prevent the hard singed nylon from scoring the softer nylon next to it...



Shrink tube over entire knot and eye bolt to reduce snag points...

Aerotech store doesn't list any F reloads for the 29/40-120 and at Apogee, they charge $28 hazmat for all the Fs. What are the chances I could get it up with an E? Doesn't look very good on sim with 300 something apogee.

Rex R said:

I see a couple of things that you might want to check...you might want to set your lugs to 30 degrees, can't quite figure how you're going to get the retainer on/off with the fin tabs getting in the way . otherwise as it sits now, I'd go for an F52-8 for a 1st flight.
Rex

Click to expand...

try 'Wildman rocketry', Tim lists 8 in stock....
Rex

Checked wild man and it's the same with the $28 hazmat fee. Looks like the only option in e, g or REALLY expensive f. I wonder what makes the f warrant hazmat fee but not the g.

Rex R said:

try 'Wildman rocketry', Tim lists 8 in stock....
Rex

Click to expand...

apogee has stated that the hazmat fee is per box, and you can get about 6 reloads per box... G reloads are 2 grain loads with each grain being less than (iirc) 62.5g while the Fs are single grain loads that may be over 62.5g, 62.5g being the amount of propellant per grain that invokes the hazmat fee. seems silly to me as it encourages people to order larger amounts of propellant at a time. usually folks buy from an onsite vendor, get other things along with your motors, get a group order(and spread the fee out). I try to get my reloads onsite, which means I'll have to wait till January to restock. one could get an F52 and an F22, you'ld still get dinged a hazmat fee of $28 but that would be spread out over 2 reloads ($14 each) which would bring the unit price down.
Rex

Baffle assembly completed. Ejection charge enters the bottom, travels up and through the first plate, down and through the second plate, and then up & out the top into the parachute bay.

Baffle installed

Airframe with fin slots cut and bottom rail guide installed.

Motor tube installed in boattail retainer

Bridle...

All done! I changed the fin design. I used my favorite spray paint, Rustoleum. Sandable primer, then orange enamel base coat, blue enamel top coat, and finally crystal clear enamel. Sanded to 3000 grit and finished off with McGuire's ultimate compound. Now it's sitting in my 90 degree hot box to cure for a few weeks.

Need pictures of your build please