How hard is it to make a homemade rocket launcher?

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How hard is it to make a homemade rocket launcher?

Postby majortom » Wed Mar 26, 2014 3:31 pm

Just got a price of $72.50 for a rocket starter kit plus $18 for a 3 pack of motors and don't mind that but also saw a video of a rocket made from a cardboard tube and like to idea of building my own.
http://www.youtube.com/watch?v=yurohmsmY_Y
I know I need to buy the motors but was wondering how the launcher works and if it is posible to build my own.
I have played around with electronics for years so soldering is no problems.
Just saw a wiring diagram of a launcher and from what I can see the power from the battery bank travels down the wires and burns out the igniter and this lights the rocket motor.
Is it that simple?
If so a cardboard tube would be free and a battery holder could be anything with a push switch and safety cover to prevent accidental firing.
I like the idea of a push in plastic plug ( clothes peg ) with metal or foil on the tip which completes the circuit as a safety device.
If this is so, then the only cost will be the motors at $18 per 3 or $6 per launch. Maybe cheaper in bulk.

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Re: How hard is it to make a homemade rocket launcher?

Postby Raaahbin » Wed Mar 26, 2014 3:48 pm

It really is pretty simple to make a launcher for standard Estes igniters. You'll find your life is a lot easier if your launch controller includes a continuity test (i.e. something which passes a small current through the igniter - not enough to fire it, but enough to confirm you have a completed circuit) prior to attempting to launch.

I've home made just such a controller in the past, and it worked well. The circuit was:

-> 6V battery (4 x AA cells) -> safety interlock (guarded switch - your peg goes here) -> 6V incandescent bulb (continuity indicator) -> igniter clip 1 -> igniter -> igniter clip 2 -> [back to battery]

When the safety interlock key is inserted, if there is a complete circuit through the igniter, the bulb illuminates, but the bulb functions as a resistor to drop the current to below the firing threshold.

In parallel with the 6V bulb you have your launch button (normally open pushbutton), so that when you press the launch button, the bulb is bypassed, the full 6V is applied across the igniter, and the igniter fires.

For the igniter clips, you want as small as possible while still being resilient enough to withstand regular blasts from the launch.
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Re: How hard is it to make a homemade rocket launcher?

Postby OverTheTop » Wed Mar 26, 2014 4:17 pm

Launcher is pretty simple to build, but you can make it as fancy as you like.

Battery, two switches (button plus "key") and some alligator clips. I use the word "key" as this can be a plug/socket jumpered out which acts as the safety mechanism. When you are arming the rocket you keep it on a lanyard around your neck.

You can make it fancy with meters, continuity test etc. I would recommend against rechargeable batteries as they are always dead when you leave them in the drawer for a while.
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Re: How hard is it to make a homemade rocket launcher?

Postby OverTheTop » Wed Mar 26, 2014 5:06 pm

Forgot to mention. Sites like www.hobbyking.com have some cheap digital voltmeters (meant for RC and similar stuff) that could add some bling to your project.

CAUTION: www.hobbyking.com will cause you to spend money...
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Re: How hard is it to make a homemade rocket launcher?

Postby matthew » Wed Mar 26, 2014 7:29 pm

OverTheTop wrote: I use the word "key" as this can be a plug/socket jumpered out which acts as the safety mechanism. When you are arming the rocket you keep it on a lanyard around your neck.


This is HUGELY important to do. When you have kids around, they love to hold the controller and press the button. Especially while you are distracted while setting up your rocket on the pad and connecting the clips to the igniter.. :shock: :wink:
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Re: How hard is it to make a homemade rocket launcher?

Postby kopius » Wed Mar 26, 2014 9:18 pm

Majortom, not sure where you are looking, but have you checked out the vast variety of products on www.ausrocketry.com.au ?

For less than $90 (what you mentioned about a kit and motors), you can get a kit, motors, launch controller and launch pad. Specifically for the launch controller, check out http://ausrocketry.com.au/hardware/launch-hardware.html
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Re: How hard is it to make a homemade rocket launcher?

Postby majortom » Thu Mar 27, 2014 2:19 am

Raaahbin wrote:It really is pretty simple to make a launcher for standard Estes igniters. You'll find your life is a lot easier if your launch controller includes a continuity test (i.e. something which passes a small current through the igniter - not enough to fire it, but enough to confirm you have a completed circuit) prior to attempting to launch.

I've home made just such a controller in the past, and it worked well. The circuit was:

-> 6V battery (4 x AA cells) -> safety interlock (guarded switch - your peg goes here) -> 6V incandescent bulb (continuity indicator) -> igniter clip 1 -> igniter -> igniter clip 2 -> [back to battery]

When the safety interlock key is inserted, if there is a complete circuit through the igniter, the bulb illuminates, but the bulb functions as a resistor to drop the current to below the firing threshold.

In parallel with the 6V bulb you have your launch button (normally open pushbutton), so that when you press the launch button, the bulb is bypassed, the full 6V is applied across the igniter, and the igniter fires.

For the igniter clips, you want as small as possible while still being resilient enough to withstand regular blasts from the launch.


Strangely enough I followed all that and it makes perfect sense. I will pick up some motors and make a rocket as per... http://www.youtube.com/watch?v=yurohmsmY_Y
and knock up a control box and definately include a safety plug/key.
My only question is what size motors for what size rocket or is it just a bigger motor for more height.
I also like the idea of refillable motors ( liquid fuel etc ) to save money in the long run. I have watched some vids on making black powder motors but they look a bit explosive for me and I like all my fingers where they are. Must be getting cautious in my old age...

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Re: How hard is it to make a homemade rocket launcher?

Postby Raaahbin » Thu Mar 27, 2014 5:16 am

The important thing to determine how big a motor to use is the size and weight of your rocket. Any rocket weighing less than about 100g should fly pretty well on an A for your first test flight, then move up to a B or a C depending on how high it flew and how high you want it to fly (bearing in mind that the rocket will go more than twice as high for each doubling of motor impulse).

If you want to spoil the surprise (there we go with the caution thing again), download a copy of OpenRocket (a free rocket design simulator) which will allow you to "test fly" your rockets without burning a motor - and importantly, without discovering the hard way that your rocket design has a fatal flaw.
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Re: How hard is it to make a homemade rocket launcher?

Postby OverTheTop » Thu Mar 27, 2014 7:40 am

Without doing simulations you can get a good idea using a "rule-of-thumb". Keep the average thrust from the motor at least a factor of five greater than the mass of the rocket (with motor loaded). That gives enough acceleration to get it off the launch rail with enough speed for the fins to stabilise the flight.

Look at the motor ratings/specification to determine the average thrust:
eg a D12-3 would be a "D" motor with average thrust of 12N, and a delay after burnout before firing the ejection charge of 3 seconds. 12N is roughly 1.2 kg, so the rocket mass (fully loaded) would need to be under 1200/5 = 240 grams.
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Re: How hard is it to make a homemade rocket launcher?

Postby Kryten » Thu Mar 27, 2014 10:21 am

No problem making low power rockets out of cardboard tubes (like most of mine), but I recommend that you make a rocket from a kit first, so you can see how it all works.
A simulation program helps. Originally I just used the cardboard cut-out method, then "VCP", which was quite simple, but I couldn't get it to run when I upgraded to Windows 7. I now use OpenRocket.
You must know the weight of your rocket (and its components)
The launch controller can be very basic. Initially I just had a push-button attached to a 6V battery with alligator clips.
I've since made a controller with a safety switch and continuity light.
You must use a launch rod.
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Re: How hard is it to make a homemade rocket launcher?

Postby majortom » Thu Mar 27, 2014 12:29 pm

Thanks for all the advice and links,
I have downloaded that simulator and will check it out.
Knowing what the numbers mean on the motor makes it a lot easier to work out the weight of my rocket.
While launcing my water bottle rocket, I noticed the weight forward of centre was critical otherwise the rocket would cartwheel after burnout. I assume this is the same for "powered" model rockets but maybe not so critical as the parachute charge deploys just after burnout.
I am also curious about fitting a payload bay later on. If the ejection charge shoots the top off, wouldn't that shoot everything forward of the motor out the top as well and if so do I just join the payload bay to the nosecone portion?
So My rocket may split more to the middle than the top as long as the parachute and ejection piston are in the bottom portion.
I will be initially using a keychain camera
http://www.ebay.com.au/itm/808-Keychain ... 2a0a76a620
and later a keyring size altimeter
https://www.youtube.com/watch?v=4tKMAz3YA3A
I also have one of these
https://www.youtube.com/watch?v=fOVc1aqogsg
and will be trying it out for real time video of the flight. I know they work to a reasonable height as I tried one in a mates model plane years ago.
I was also wondering how small I can build a rocket and still use a motor. Does there need to be a space between the side of the motor and the rocket or can the motor be a neat fit in the tube and how short can the tube be?

Thanks again and hope to see some of you at Thunda in 2015.

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Re: How hard is it to make a homemade rocket launcher?

Postby OverTheTop » Thu Mar 27, 2014 12:43 pm

You can check stability on a small rocket with a swing test. Tie a piece of string around the point where the rocket balances (fully loaded). Swing the rocket around your head and if it flies into the direction of travel it should be stable in flight. If it keeps spinning/flipping/rotating it is unstable.

This gets tricky to do when the rockets get bigger :)

For good discussions on all things (including motors, stability, construction, recovery etc as you ask) model rockets I would recommend this book:
http://www.apogeerockets.com/Rocket_Boo ... nstruction

There are other good books around also, so check out your local library. If they don't have anything you can probably request something be purchased.
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Re: How hard is it to make a homemade rocket launcher?

Postby cryoscum » Thu Mar 27, 2014 5:30 pm

OverTheTop wrote:...You can check stability on a small rocket with a swing test...


Big ones too! Maybe something like this! :)
http://youtu.be/psc3cI68Gy8?t=48s
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Re: How hard is it to make a homemade rocket launcher?

Postby OverTheTop » Thu Mar 27, 2014 6:04 pm

I had thought of attaching the the Velociraptor to the Hills Hoist and giving it a spin. That would give the necessary approximation to a strait free-stream flow at that scale.
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Re: How hard is it to make a homemade rocket launcher?

Postby Raaahbin » Thu Mar 27, 2014 9:41 pm

A suggestion for the 808 video camera - make sure to get the #16 version. The different versions have wildly different performance, and although you might get lucky and get some video from another version (or, you might get unlucky as I frequently did, and discover after flying that the camera had randomly failed to record anything, or powered itself off, or crashed), the #16 is good quality and reliable. Of course, you get what you pay for (and pay for what you get), but the #16 is worth the extra money not to waste huge amounts of time and potentially motors stuffing around with unreliable ones only to give up and buy a #16 later anyway.

Here's the authoritative page on the #16 808 camera.

http://www.rcgroups.com/forums/showthread.php?t=1556994
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Total Impulse (Ns):
2019: 2189 (86% K) :)
2018: 736 (58% J) :cry:
2017: 2826 (55% L) :D
2016: 2006 (78% K) :|
2015: 2809 (55% L) :)
2014: 1565 (61% K) :|


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