3" PVC Rocket scratch build

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14bis

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Hello everyone! Long time lurker here.


I finally have some time on my hands to make my first post here.
This is a scratch build for a homemade rocket that has flown with great success in the past.


As a bit of back story, I'm from Brazil, and I started my journey into rocketry in 2014, when I joined my uni's rocketry club. I was a part of projects from low, mid and high power rockets. In 2016 I was able to fly at FAR and IREC as an exchange student, and in 2017 I was the team lead for my uni's IREC team.


I know I have a lot to learn and my objective with this post is to shed some lights on the design techniques one could use when short on resources (rocketry is growing here in Brazil, but we don't have easy access to rocketry stores).


A couple of observations:
1) This rocket uses a sugar motor (KNSU), however it's not on the scope of this post to explain it's design. I'll only use it's specs. This motor was throughly tested and performs within it's designed specs. I will not discuss how to make this motor as it belongs in the [Restricted[Research section of this forum.

2) The parachute deployment signal will come from a stratolloger, this is the only part that is not homemade. I used arduino based altimeters in the past, but I don't think they are as reliable as commercial ones.

3) The rocket will have roll control. Once again, this is not in the scope of this post. I can comment on this, but there is a lot to be learned from this post: https://www.rocketryforum.com/showthread.php?35679-Active-roll-control-system-Success

Objectives:
This is my first time doing all of the rocket by myself. I will definitely have some problems along this build that I'd like to discuss with everyone here. Especially with regards to the recovery, sewing the parachute, recovery harness, etc.
I will also like to discuss some manufacturing techniques, as nose cone construction.


Meet the rocket:


NAME: TBD
Motor: J class, 1000 Ns (224.8 lbf-s)
Predicted apogee: 1000 m (3280 ft)
LO Mass: 5.4 kg (11.9 lb)
Length: 1.80 m (5.9 ft)
Diameter: 75 mm (3")

Below are pictures of the Open Rocket simulation. The .ork simulation may be run with a similar total impulse/mass motor.

Let's go!

03yAnnF.jpg

sims.jpg

View attachment PVC.ork
 
As far as project development goes, I'll try to update this post weekly, probably on Sundays, as I can only use my dad's workshop during the weekends.
Anyway, here is the motor section of the rocket:


The motor will be aligned with a cardboard tube, and attached to it will be centering rings and a guide for the fins.


The centering rings and fins were laser cutted from 4 mm plywood.


A 15 mm plywood disk will function as motor thrust retention.

Here's an imgur album with pictures: https://imgur.com/a/A64lE
*Should I upload pictures here or is it ok to use imgur?

Cheers, and see you next week with more pictures :D
 
Hey Gustavo.
Good to see other rockets with sugar motors and PVC. One of my first scratch rockets was 3" PVC with my own erythritol motor.

I see that you have very large aft fins to counter the center of pressure issues brought on by the large forward fins - which I assume you are using to achieve roll control. May I suggest that you can get by with just a tiny set of fwd fins to achieve roll control? Roll stabilizing forces need only be tiny forces, and significantly smaller forward fins would allow smaller aft fins.

The aft fin mounting structure is my main concern. There is a very short couple between the soft PVC shell and the soft (looking) cardboard motor mount. The 3 centering rings help support the couple, but the lever arm of the large fin will easily break a fragile couple. I suggest reinforcing the couple with any of a variety of materials that will spread the loads far out into the cardboard motor mount and PVC body tube. The materials become internal fillets and spread the couple in the direction of the circumference.

Thin epoxy-soaked balsa sticks sized to the gap
Carbon arrow shafts and thick epoxy (I used this on my PVC rocket)
Injected epoxy with chopped fiberglass and carbon


Essentially what you will need to aim to do is create a fin can of strong composite materials that just happens to be skinned on the outside and inside by rather soft PVC and cardboard.

Good luck. PVC is overlooked as a building material.
 
Hi Kyndel! Thanks for the response.


I feel that PVC in the US desert temperatures may be a concern and why it is often overlooked. But here in Brazil we don't have such problem (the humidity helps a lot), it is relatively light weight and a lot of people here in Brazil use it as an airframe. It is not possible to buy HPR commercial motors here (I know some have, but not to "legal" means) and it's propellant handling (AP) is controlled by the military. In a university environment you can have access, but from a hobby stand point it is impracticable, with this being said, the solid motors are all designed and built (mostly following Nakka's and Jim's guidelines) by students, so the weight factor PVC might have over cardboard is "easily" overcome by motor design. Of course, there are many hand made composite and cardboard tubes being used, also aluminum, but PVC is a major candidate for airframe here. It's really interesting the different approaches to design other countries make when taken in account resources availability (IREC is great for this).


As for the roll control: I get what you're saying, this project is like a senior design/project so I can graduate from my uni. The fin size decision was made in the past and it takes into account some problems as roll control effectiveness, roll reversal, servo performance and literature. I thought about going with smaller fins as you suggested, and I have to say that it is a decision I think about a lot, however, since I will not have the opportunity to conduct CFD or wind tunnel tests, nor I will have enough test flights, I decided to go with the larger ones, and while they may present the same problems, I found some literature that can support this decision, while I didn't find any about smaller fins.


As for the fins attachment, I forgot to say that I will put one layer of fiberglass over the PVC tube, and the fins are going to be reinforced as you said. Another major problem we face here in Brazil is that there is kinda of hard to get black powder or at least fine BP. So I had problems in the past with the ejection charge blowing up the tube. Recovery went well and all, and PVC is cheap, but I don't want any explosions :p. I'm still in the process of trying to get BP, but I think one layer of FG is enough and light enough that it won't affect my overall apogee.
 
Welcome Aboard!!! Please add more pics! We all love rocket porn here...

Pointy Side Up!
Jim
 
For the recovery of the rocket I'll use dual deployment, with the drogue coming
at apogee and slowing the rocket to around 55.8 ft/s (17 m/s), and at around 330 ft (100 m) the main chute deploys, reducing the velocity to 24.6 ft/s (7.5 m/s). My initial plan was for a faster drogue descent, but open rocket sim warned me about high descent velocity for V > 21 m/s, Idk how much a problem this is, since it's hard coded (i guess).


Anyway, my first approach was to follow Nakka's guidelines for parachute construction (https://www.nakka-rocketry.net/paracon.html) specifically the semi-ellipsoid shape. This website has resources for sizing the parachute and obtaining gore patterns.
A sample calculation for base diameter is made, that is the input for the parapat spreadsheet.


As for sewing, I'm using the French flat seam (https://www.madehow.com/Volume-5/Parachute.html).


Anyway, here are some images of the sewn canopy (https://imgur.com/a/d5x5Q7E). It was my first time using a sewing machine in a long time,
and it shows, I got a lot of missed seams and loose fits. I'm thinking about forgetting this semi-ellipsoid design and going for flat triangular sheets.


What do you think? Any tips, tricks, recommendations I can get to help with my homemade chute adventures?


Thanks as always!
 
I wouldn’t worry about a descent rate of 100 FPS under rogue. For the altitude of the main I would be more comfortable with 150 meters. Depending on your chute and the way you pack it, it may take a couple hundred feet (2 seconds) or even more to fully unfurl.
 
I wouldn’t worry about a descent rate of 100 FPS under rogue. For the altitude of the main I would be more comfortable with 150 meters. Depending on your chute and the way you pack it, it may take a couple hundred feet (2 seconds) or even more to fully unfurl.

Sorry for the late response, but I also thought the drogue was a bit too large, I think I'll allow a higher descent rate too, the rocket will be sturdy enough to handle the shock. You also raise a good point about the main deploy altitude, I'll revisit it after testing, it is easy to set this on site with stratolloger.
 
Hello once again!

This was a busy week for me, but for the update I'd like to talk about how I made the coupler tubes (Av-Bay included) and how I'm making the nose cone.

For the couplers and Av-Bay, it is simple, I cut the tube longitudinally and, with the help of a guide, I put a layer of fiberglass on it, so now it's outer diameter is the inner diameter of the body tube. This can also be done with just epoxi, but since I had FG already, I decided to use it. Here are some images of the Av-Bay and also the avionics mount, where two stratollogers and batteries will be placed https://imgur.com/a/vA80Zib.

As for the nose cone, this is where it's kinda of tricky. I have to use a custom made nose cone, what I used to do is to cut the PVC in 6 petals and then I'd use a soldering iron, with a flat metal tip attached to it, to "solder" the PVC into shape, then I'd reinforce it with epoxi and use finish the exterior with epoxi and sanding. This is way a lot of a job to do, so for this rocket I'm making one with foam and a lathe, like this https://speedmotionrockets.com/Foam Lathe.html. Here are some pictures of my old method and new one. Using foam is 100% easier and I don't know why it took me so long to do it, I had a little accident with mine, as you can see, but I fixed it with epoxi and I hope it will be ready this week. https://imgur.com/a/2NLM5Mr

Next week I'm looking forward to fiberglassing the tubes/ or going shopping for recovery harness/ or finishing the control system. So much to do! My plan is to fly this rocket by end on June or July. Is it ok to post photos/videos of the KNSU motor? I know about the forum rules, but this would be just for show.
 
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