Modular Rocket

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Jim-K
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Modular Rocket

Postby Jim-K » Sat Nov 16, 2019 12:08 am

With the summer warming up and no flying until April/May, I have started to tinker with a modular rocket design. My aim is to make different sections that can be stuck together to mix and match different motors, recovery, staging etc.

So far, I have 3D printed some ends and tried our a few prototype sections. It all seems to work, but once I set forth with a particular interface, its going to be hard to change. Happy to listen to opinions on features to add or remove.

The basic structure is based upon BT-70 & 80 tubes. This gives plenty of room to fit stuff into the airframe later. 3D printed ends are bonded in with some balsa stringers running longitudinally and a bit of PU foam.

Here are some illustrations:

4710C058-C921-4B47-BCDA-FD6615A9BCCE.jpeg

A53B007C-C347-4CB4-977B-D64605BDF540.jpeg


I built a short section - about 120mm long - to check the longitudinal strength. About 2kN, which should be sufficient for anything I am likely to put in the back end.

2F856838-D77A-4445-BFAE-5EA7B160CB60.jpeg


The features so far are:
- Each section joined by 6 off M1.6 fasteners.
- Four pairs of balsa stringers to assist with longitudinal load.
- Two longitudinal tubes to assist with running communications.
- Radial hard points. Initially closed, but can be fitted with an M2 ‘helicoil’ as necessary.

First up I will build a tail section, a mid section with parachute and a nose section. Fly it with a 24 mm diameter E or F category, and see how we go.

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Ideally there will be time for bigger, faster, higher etc later.

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Re: Modular Rocket

Postby SpaceManMat » Sat Nov 16, 2019 7:48 am

Neat idea. It will be interesting to see how this comes together.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
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Re: Modular Rocket

Postby strud » Sat Nov 16, 2019 11:54 am

I like modular approaches, reuse saves time and reduces risk in new projects.... theoretically.

M1.6 fasteners, wow didn't know they existed!

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Re: Modular Rocket

Postby Jim-K » Wed Jan 08, 2020 11:34 pm

Starting at the top for no particular reason, I have a nose cone assembly.

This assembly is just going to pop off the top with the ejection charge on the end of the motor. The typical nose cone (this one was from Apogee) will do this without modification, but the inner BT-70 does not suit the BT-80 nose cone. I also have plans to use the same nose cone with more sophisticated rockets later. I also just like tinkering.

The 3D printed rings (from 3D Hubs in FDM PLA) came out slightly 'woolly', but the tolerances seem good and they slip together with the tubes neatly. First off, I have bonded in the washers and nuts. This seemed to work best if the joint is torqued up gently so that the nuts and washers are compressed into the rings, though clearly you need to be careful with the epoxy adhesive.

Nut End Ring.JPG
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Bolt End Ring.JPG
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Next I bonded on the BT-70 and 80 tubes (these are identical lengths) followed by the balsa stringers. I also trimmed out the sides to allow access to the bolts, though I could probably have done this later.

Short Section - Unbonded.JPG


Sticking the two parts together was trickier and time bounded with the PU foam and epoxy curing before my eyes. I will try to get some photos next time. The epoxy went on the end rings. The PU foam in between the two balsa stringers (in the trial parts, the PU foamed up and bonded the inner and outer tubes together).

Short Section - Bonded.JPG
Short Section - Bonded.JPG (47.26 KiB) Viewed 3383 times


The nose cone has a similar structure with a BT-70 running about 50% of its length. It also has 1/8 inch aluminum tubes running to the end of the BT-70 to allow communications for later systems. But I didn't take any photographs.

Nose Cone.JPG


The next post will be about the central section. This will contain the parachute, maybe some other things. I might also think about painting the external surface.

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Re: Modular Rocket

Postby SpaceManMat » Thu Jan 09, 2020 11:50 am

Looks pretty nice. Kind of wish I had added a place to put nuts in the cockpit for my X Wing, but I didn't because I wanted to make sure it never fell out because there will be no access to the underside.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

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Re: Modular Rocket

Postby Jim-K » Tue Jul 28, 2020 12:36 am

Been a while, but I have been tinkering in the background. The mid section turned out trickier than I thought - though I may be over thinking things. I have a nose and a tail done.

Tail

Tricky thing was the transition between 24 mm and 56/66 mm diameter zones. I opted for some strategically placed centering rings, epoxy and crossing my fingers. I don’t see any issue with the load transfer into and out of the 24 mm motor mount and have filled a section between the 24 and 56 mm tubes with rigid PU foam.

066AE2A2-01E6-4B50-A287-03AB60B48ECE.jpeg
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The small aluminium tube will carry signal wires if I fancy trying an air start. Here it is again with the tail cone fitted.

079ED6CF-8B53-4886-B504-A3F1FF77D21F.jpeg
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I should have taken some more photos of the fin fitting. They are laminated balsa fitted into slots in the 66 mm diameter tube, butting against the inner 56mm tube. Cutting the slots was weeks of preparation turning my dremal into a router table and making a tool to hold the tail cone. The actual slot cutting took about 5 minutes.

The aim with the forward swept fins is to protect them on impact (with chute deployed).

The painting was also tricky - a craft that I will need to persevere at. The point of the project, however, is to make interchangeable parts. I can always swap the offending parts out if I needed to.

Hope to get it flying this year, or maybe the next.

51BB4C65-EC3B-4CD9-822C-9D8588DB3788.jpeg
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Re: Modular Rocket

Postby Jim-K » Wed Aug 19, 2020 12:26 am

There’s always something. It’s all starting to look very rocket like, but there’s no way to attach it to the launch rail. The standard buttons come with an M4 fastener, but there’s no obvious way to attach one. A through hole with nut on the other side would simply crush the empty space between the 70 and 80 tubes.

Sandwich panel inserts are typically shaped like cotton reels and are potted into a cavity milled into one side.

E6ECFEA8-458F-4CA3-8E7B-BBF72EF64CFF.jpeg


With curved surfaces above and below, standard inserts will not work. Ideally, a 3D printed shape would do it, but that machine is on the Christmas list. I’ll have to improvise.

First thing to do, create a laminated stack of tube walls with the right thickness, roughly 4.5 mm. Bonded under vacuum onto a 70 mandrel to ensure the correct curvature.

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I had a 16 mm drill that made a neat hole in the outer tube but only a tiny one in the inner tube. I cut a chip from the laminated stack and bonded it in place. Not ideal. My laminated stack tended to delaminate - not sure if it was a lack of adhesive or a weakness in the tube material.

Although the paint job was hardly top class, it looks worse now I’ve had to abrade a big patch. Hopefully nobody will notice at a distance.

0203874B-790E-4B0C-A13E-9E28C9D55A9A.jpeg
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Finally, bond a larger 80 tube wall patch onto the outside, tap out the hole and install an M4 helicoil.

F5C66BAE-E8F0-4625-B992-B8B4E44166D6.jpeg
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The overall method seems sound, though in future I would install the hard point before painting. I’ll also model up some inserts in CAD and put them to one side for my next batch of 3D printed parts - should help bulk up a minimum order quantity.

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Re: Modular Rocket

Postby OverTheTop » Wed Aug 19, 2020 9:08 am

The overall method seems sound, though in future I would install the hard point before painting.


Nobody has ever done that before. :P

Looking good.
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Re: Modular Rocket

Postby Jim-K » Fri Aug 21, 2020 5:37 pm

We might be flying here in WA at the weekend, so I figured I should check ‘as built’ stability. If you find any schoolboy errors, let me know before Sunday...

So far I have the following parts in the library:

Nose cone; 90g; 270mm
Chute deploy; 200g; 250mm
Spacer A; 100g; 300mm
Spacer B; 130g; 450mm
24mm Tail; 220g; 280mm

The baseline with the 300mm spacer seems good.

4D314ADA-C6D3-43F9-90D1-D56D475A4053.jpeg


And a standard 3-grain load looks good too.

82EA89BB-2082-4021-B1DA-FBCB420A8695.jpeg


A 6-grain load probably needs to go with the 450mm spacer.

A83D3B39-4AEF-4D60-837C-9FE8231C2C97.jpeg


And should I ever build a tail to take a 29mm load, it looks like both spacers will be required.

35668B2B-1E4E-44C1-89C9-F1835243E1BF.jpeg


I’m sure I read somewhere that 1 to 2 calibers is the ideal, but, as I said, feel free to point out alternative reasoning (before Sunday).

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Re: Modular Rocket

Postby Jim-K » Mon Aug 24, 2020 1:09 am

So first launch and I have the device back. A bit of unzipping on the trailing edge of some tube, but nothing a bit of glass won’t fix.

Pre launch: easy assembly, but need to work on not dropping those M1.6 cap heads. Tricky to find in the long grass.

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Launch was uneventful. Straight off the rail, but tipped over a little and headed off over the car park. Wind maybe? I may need to go up a notch from the F44-6W.

[youtube] https://youtu.be/F_MD9jWpK_M[/youtube]

Nail biting delay as the craft drifted over the assembled vehicles. I suspect it was well past apogee when the chute deployed. That and the lateral movement probably helped the unzipping.

For next time:
- Higher thrust off the rail;
- Higher overall impulse;
- Install an altimeter to match measurements against simulations.

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Re: Modular Rocket

Postby Jim-K » Fri Sep 18, 2020 2:02 pm

A bit more on the chute deployment section (the grey and white sections, though it’s had a paint upgrade). It worked by the way.

D8A887B3-674A-4CA6-9330-F79FED33F880.jpeg
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And an exploded view (figuratively).

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There are a few moving parts, but the main aim was, as far as possible, to protect the chute and connecting lines from the hot black powder exhaust.
At it’s core is a mandrel to coil the braided aramid line around. Inside is an M3 eyebolt (you can see the thread poking out the end) and on the other end a forked tube to help secure the wound aramid.

48E024FA-EEAB-414F-A26C-AC6B2ED0E724.jpeg
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With the forked end pushed into the piston tube, the blue rubber band is no longer necessary.

DCEA03A8-4522-4CB9-A571-D48D5DC60B01.jpeg
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This is then mounted in a slightly larger tube to both protect the aramid line and act as a guide as the piston moves along the inside of the main tube.

D979CD5C-D9CC-4A09-9049-1A22699DF217.jpeg
D979CD5C-D9CC-4A09-9049-1A22699DF217.jpeg (47.79 KiB) Viewed 1966 times


The assembly is then mounted on a short section of main tube. I have used an M3 eye nut simply because - unlike the wing nut I intended - the thread runs all the way to the base. In theory I could also use it to secure something else one day.

FC6A743A-10F5-40CC-B903-937995EED4CF.jpeg
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The piston is formed from alternate layers of terry flannel and balsa. It is a reasonable seal without unnecessarily tight tolerances. I should probably fire proof it with something, however, as it got a little scorched last time.
The assembly is mounted on the end of another tube (piston inside) and a third bit of tube added as a cap. The chute clips onto the ring between the second and third tubes and fits into a cavity roughly as long as the screwdriver.

07B55420-4D23-438A-904D-D8686FD138AB.jpeg
07B55420-4D23-438A-904D-D8686FD138AB.jpeg (58.76 KiB) Viewed 1966 times


Next job: either avionics or larger motor sections, or maybe both.

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Re: Modular Rocket

Postby Jim-K » Sat Oct 30, 2021 11:42 pm

I have been tinkering in the background for a while during the COVID hiatus. I have a few new rocket sections under construction, but have mostly been working with arduino code for various instruments. Something for another post.

This post is to show off the storage cases. I have opted for the AEG boxes in Bunnings (other stores are available) as there are a few different shapes and sizes and they clip together neatly. I was starting to wonder if I might need to go abroad to fly, so needed something that could potentially survive time in the hold.

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Open the lid and I have a tray with the tools and small parts small parts I might need for launch day. Under that are the rocket sections themselves.

4CB8A1EC-FF84-4AAB-835F-F2DCC763AAEC.jpeg
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The tail sections are a little trickier as the fins stick out a bit. I did consider making them detachable, but luckily AEG do a larger box too.

FCA27911-7A39-43F1-8D81-B03EB6CBF644.jpeg
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I suspect I would still need to fill the cases full of styrofoam chips or newspaper if I ever really did trust the boxes to Qantas airport staff. I also wonder how many alarms the black powder and fuel residue would would set off. Has anyone ever been swabbed at the airport following a flying weekend?

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Re: Modular Rocket

Postby drew » Sun Oct 31, 2021 1:58 pm

I also wonder how many alarms the black powder and fuel residue would would set off. Has anyone ever been swabbed at the airport following a flying weekend?


None. I've had my stuff swabbed post Balls twice and it didn't even register on the unit.
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Re: Modular Rocket

Postby SpaceManMat » Fri Nov 05, 2021 10:59 pm

Neat setup with the cases, perfect for this project.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

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Re: Modular Rocket

Postby Jim-K » Tue Nov 16, 2021 11:38 pm

This one took a little longer than expected. I got distracted by the electronics, arduino and other ideas. All done now, though the software is still a work in progress.

0DE5C273-A506-4E6B-9CFF-A77C75FD43EE.jpeg
Complete part
0DE5C273-A506-4E6B-9CFF-A77C75FD43EE.jpeg (115.72 KiB) Viewed 217 times


It’s a tail section capable of running a 29mm motor, to match my original 24mm tail. I figure the BT70 tubes are probably capable of a 38mm and a 54mm too.

The 29mm motor tube has a Wheatstone bridge attached, with the aim of directly measuring the force applied by the motor. It’s a bit messy, but it works. I’ve done some crush tests on the tube and it should be good to over 1kN. I’m not worried about the 29mm motor overloading the tube, but the loads on landing are still a worry.

8F3E62F3-7294-4CE8-8BD1-571765C4C5C4.jpeg
Bridge
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The bridge is energised and monitored by a SparkFun Qwiic Scale unit. I have also daisy chained a 30g accelerometer and some EEPROM onto the I2C bus (not shown), with the aim of detecting and measuring the launch. Because I can (hopefully).

78F7B44B-96D1-4243-A2DC-0EE62A4617A0.jpeg
Qwiic Scale
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I have 3D printed a hatch detail to bridge between the BT70 and BT80 tubes. The hatch itself is just two layers of BT80 bonded together, held on with a set of 8 off M1.6 screws. I only need access when I want to connect the I2C bus. The tail section works just fine in ‘dumb’ mode, and I would like to check how hot it all gets before I’m relying on the electronics.

E6B83902-CDC4-4C23-AD80-B6B7F8511489.jpeg
Hatch detail
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Tube closed
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I opted for 4 fins rather than three to help with stability. As with the 24mm tail, they are 3-ply balsa. This time, however, I have added a glass veil in between the plies to help with in plane stiffness and act as a reservoir of adhesive to prevent dry patches. The fin laminate was bonded under vacuum and the fins penetrate the BT80 onto the inner BT70.

D8485316-C223-4B8F-8AFA-D6DDF6E1F56F.jpeg
Fins to bond
D8485316-C223-4B8F-8AFA-D6DDF6E1F56F.jpeg (62.23 KiB) Viewed 217 times


Still working out how to fit it into an AEG box, but it’s a nice problem to have.


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