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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SpaceManMat
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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 2608 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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Jim-K
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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
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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
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Next job: either avionics or larger motor sections, or maybe both.


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