Project: Single Servo Multiple Drop Mechanism - 3D Printing


Large Child
Hey Gang, so I totally know what you're thinking... Oh lord, ANOTHER project this guy has going on?!?! He's never going to finish any of them... You know what I say to that?

Yeah, you're probably right. :p;)

However, that doesn't mean I don't ATTEMPT to. Sharing is caring and I have an idea I think can really pan out given the time and effort and the teamwork we have all come to love here on the FT forums.

I also welcome all links and references to existing products/projects that are like this - I have not been able to find anything EXACTLY like this. One guy using 2 servos with 3 rubber bands to drop parachute guys. And of course the $25 E-Flite servoless bomb drop.

I don't have any drawings yet, but what I do have is a set of 7 initial "requirements" (probably better termed "desires"):

The device:

  1. operates using a single unmodified 9g servo (the kind you can usually get for about $2 in a pack of 10).
    • can use a pushrod or the servo snapped into the device, I don't care as long as it's easy to assemble, install and use.
  2. is mechanical. No electronics. Levers, gears, springs, washers, screws, pins, magnets, etc. are all fair game. This keeps cost and usually complexity to a minimum.

  3. can install onto the outside of a craft or be installed internally, flush with an exterior surface.
    • Initially I forsee that surface needing to be a horizontal downward facing surface, but maybe we can create a side-facing version that won't get hung up while dropping.
    • And... the thought just hit me that you could do Space Shuttle style piggy backing if you top mounted it, then you could release the mechanism and dive leaving the towed craft aloft.
  4. uses a standard interface to the item being dropped
    • As much as it sucks, I think having an interface mount that you secure to the object being dropped will be necessary to standardize the entire mechanism. HOWEVER, I look to you all to prove me wrong.

  5. can hold an item of a specified weight securely until released by the pilot. I'm thinking I'll set this initially to 250g (for reference, a can of soda (or pop or soda-pop) weighs just under 400g)
    • In practice I would expect items being dropped of a "bomb" nature would not exceed 100g. Think about that 100g motor falling on your head!!!
  6. is modular. Meaning that you can somehow connect or daisy-chain these "drop modules" to have theoretically unlimited dropping potential.
    • If not modular then design can focus on creating a 3-item drop mechanism that can be scaled up for 4+ versions to be created using the same basic concept.

  7. is configurable. Meaning that if you achieve #3, modular form, that you can "flip a switch on the mechanism" to put it from sequential drop mode to single drop mode, which would just drop everything attached to all connected modules on the first activation.
    • This one I know is a pipe dream, but doesn't hurt to write down the thought.

If you're still reading this, thanks for actually reading the entire list of desires. Subscribe to get updates. This won't likely be a high-speed high-volume thread, I want to be methodical with this one. My Monoprice dual-extruder 3D Printer arrives in a few days, but I want to focus on DESIGN, probably through the end of this year unless we come up with something so perfect it must be prototyped.

Feel free to contribute additional desires and I'll incorporate them to the main list with attribution. The next post will be continually updated with the current status of the project with pictures (so it's easy to get caught up when it's 6 months from now).

C'MON, let's design something awesome together!


Large Child
Current Project Status

14-June-2017: Project not dead, just not much work being done on it these days. Working more on plane design and a new EDF-powered hydroplane boat.
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Junior Member
Hi, it might be unfortunate that i post with this topic again, but i already explained i like new challenges and projects... Id take a look at rubber band revolvers (wood ones ore lego mindstorm rubberband turrets) they are built up on one trigger most of the time and hold several bands. Then u could try to use FTs method of releasing bombs (where david gets slapped :( ) could be a beginning!

Good luck



Active member
We got da solution, Boss!

Cat Food Missile Dispenser file under secret projects 1.jpg

Here kitty, kitty, kitty


Active member
Hey, I think I have an idea to develop the above, addendum

OK you need something to rotate the cylinders and something also to control the rotation.

1: Load all 7 missiles in cylinder, now what you have to rotate the cylinder is a light surgical rubber tube, small n stretchy, not too strong, fixed stationary, you wrap it around a spool on the disc like about 2 to 4" diameter, the larger the dia, the more force of rotation and the faster it will rotate, OK this is on a stationary point on the side and also attached permanently to the spool so that you turn it to tension it

2: A servos' arms could be extended so that when one side of the servo is lifted the other side descends blocking cylinder from rotating any more than one angular segment at a time, or no more than one 45 degree segment "pie slice" segment at a time

so you have this servo on a simple 2 position switch, see? Flip it one time and the cylinder rotates 45 degrees and drops one missile, flip switch the other way, it rotates again in the same direction dropping the next missile in line.... on and on

The servo controller would be at one end of the cylinder; the spool tension mech at the other end

Cat Food Missile Dispenser further developments.jpg

I didn't have any restaurant napkins to draw this on, so I just used a piece of paper.

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Active member
Problem may exist

Yah, thought about it some more, that spring loaded like shock may destroy your servo, it may eat servos, just an idea would probably work but not good for your equipment

You might still experiment with this idea of rotating cyl

I still like the cat food dispenser/missile launcher

cat with gun ... here mousey mousey mousy.png


Junior Member
Something just came into my mind... Why dont u just use a modified servo which turns around 360+ (theres a tutorial from FT on this)


Large Child
Yes, if we have to.

Something just came into my mind... Why dont u just use a modified servo which turns around 360+ (theres a tutorial from FT on this)

Yes, this will be my backup if I can't figure it out otherwise. Using a rotating servo will rely on pilot timing to turn it on long enough to drop 1 and off quick enough to not drop 2.

Leonard, I know I didn't provide any sample drawings, but I was hoping to make something that would allow for SCALE bombs to be hung from wings/fuselages snapped in the bottom and released upon request. I'll try to draft something today and post it later.

Don't get me wrong, once I have the 3D Printer I'll be building all sorts of stuff, and the "Rotating High Altitude Cat Feeder" or RHACF design may still have a life of its own. :D What it would be cool for would be to install it INSIDE of something like the Guinea Pig with a hole in the bottom of the fuselage for the drop. And if you wanted to get fancy you could try to put a pair of bomb doors on it that are triggered by the rotation motion. I know - I just complicated a simple idea - don't ever say I didn't do anything for you, you're welcome. :p


Active member
Hey I know about those, good idea for the cat food dispenser

Super idea

This would make the cat food dispenser work real good for missiles, it really might be the best

Here is something I drew up

worm gear on one side with its meshing round gear turning the cylinder using dc brushed motor

the other side is the control switch, an etched disc of copper clad board with those u-shaped etch outs or non-conductive etched areas

what makes this work? you have a servo push on a switch for half a second that closes pwr circuit to the motor and the scr circuit is on then and will stay on until your control disc rotates into the non conductive area because pwr thru the scr goes thru the bar with the Xs, one contact makes complete contact all the time, the other makes and breaks with the etched control patterns that are about the segment width of your cylinder segment

your servo switch is held on momentarily only long enough to rotate the control disc into the conductive region, release the servo switch but the worm geared motor keeps turning till it hits the non-conductive area and the circuit is broken

Here is another thought without the scr, it could just be a matter of timing, you leave out the scr and just hold your radio switch for .25 seconds while the cylinder and control disc turn a little into the conductive region, it continues to turn till control disc hits the dead spot and stops turning, you just dropped one missile

This is super super easy, the hardest part here could be getting a motor mounted with a worm and mesh gear
I found some of these in toys and played with them when I was a kid
They are in toys and printers and scanners

Cat Food Missile Dispenser further developments 2.jpg

alt solution is a stepper motor and a circuit


Active member
Some bombs are dropped from the central area under the CG on some aircraft

Just about a complete thought in the title, but, this Cat Food Dispenser/Bomb Drop cylinder is about what would work for a stealth bomber, see, a lengthwise compartment fitting this description

Unless you kind of want the F4 phantom/Harrier approach where the bombs are on the wings outside with individual mounts

See now, you have Stealth Jet Bombers,

and you have the other kind of Jets with bombs that just hang outside the under the wing

Either are legitimate and in use
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Active member
Simplest Best Hottest Idea Yet! developed in real time

Now here is a very simple idea that would really work easy easy easy

You have a servo in the middle of the wing with 3 control rods going L, 3 rods R all at different lengths for release
With your radio and two 3 position switches one is for Left bombs, the other for the R bombs

this may require two more of your switches, a momentary on switch and another to choose from which bay you drop, on your xmitter, I have all of these all programmable on my Futaba T14SG

I hope you follow the logic of this, it can work easily

while in flight your Left bay is on 1st in the 1st position, hit the momentary drop switch

now switch to Left #2, hit the momentary on switch ..... you dropped bomb 2

now switch to Left #3, hit the momentary on switch ..... you dropped bomb 3

now switch to Right #1, hit the momentary on switch ..... you dropped bomb 1

now switch to Right #2, hit the momentary on switch ..... you dropped bomb 2

now switch to Right #3, hit the momentary on switch ..... you dropped bomb 3

You just dropped 6 bombs with one servo attached to different length ctrl rods

1L is shortest and drops 1st, 2L is med and drops 2nd, 3L is long and drops last, see these Left and Right bomb switches actually determine the length the servo swings in one direction

If your fully loaded and your on Bombs Left #3 and you hit the drop switch then you just dropped all 3 of the bombs on this Left control

Want to drop 2 at once on the Right side on your enemies' bunker in the practice field on one pass?

goto R2, hit bomb drop sw and bombs away!

You follow, understandable?

And the control rod could slide thru a piece of tie wire tied around your bomb with a loop at the top, hey these are foam bombs with a rock in the nose, fins at the back and tie wire 1/3 back from the bombs nose,

do you follow the idea?

******* THIS IS IT: even simpler, just control this servo with one of your knobs, you know it's a rheostat or potentiometer knob on your radio, glider pilots like to use this for controlling the set of the aileron for lift? Just twist a 1/3 L and drop 1, 2/3L and drop #2, 3/3 L drops #3, 1/3R drops #1, 2/3R drops #2

OK this is the simplest control method, the simple twist of the knob, center setting holds all bombs and of course you have 6 ctrl rods on the one servo

Simplest, Best, Hottest Idea Yet and it drops 6 bombs w/ 1 servo and 1 knob!!!!!!!!!!!!!!!!!!!
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Active member
1Servo/1Knob bomb drop further notes... do this, THIS IS IT!

This could be configured many different ways,

Let's say you don't want CG OR the airplanes Left or Right balance to be affected by what you have or what you dropped

put them in a bomb bay under the wing at the CG like 3 in front and 3 in back and run your control rods, make little burrowed pockets to make loading easy, just set the bomb in the pocket with the loop sticking in the hole, with plane upside down I think you will have to load one at a time or twist knob all the way L, all slots are open, put all bombs, when you twist back to center go slow and see that B1 slides onto ctrl rod correctly, B2 next on rod, B3 OK these were the bombs on the L side and you know how servos work, while one side is pulled short, the other side extends long, so to the R side while loading as explained, the L side bombs are held in place.

***add: To help you save $ on control rod just take one of those long straight rods that you like to run from mid fuse to rudder, right? Now you will just use the end straight, it will slip in and out of the 3rd bomb loop just as a plain slick rod sitting in a guide, so for the other 2 just take some more of this rod stock and solder 2 branches where the other 2 bombs go, that's it and hit the servo arm with the other end of the rod! Badda Bing, Badda Boom!

Now I am really invested and interested in this, it is so simple and easy to set up and do, almost no programming at all except to assign a servo to one of your knobs and plug it in to the right xceiver channel and there you go, hook up 6 control rods to 1 servo, 3L and 3R, that flexy cable rod that goes into a nylon sleeve is what you need for this.

Now with this, it's up to you, hang 'em F4/Harrier underwing or Stealth Jet Bomb Bay style.

"Yew can due eeeeet!"
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Active member
Ya know, when I first got back heavy into RC, I had been messing w/ strykers building them from foam blanks LHS sold.
Now when they quit making the stryker, the last one after C brushless 450, was a super bad version Q Someneizy designed, and they used a park 480 driving a 6x6, I liked that motor and mount setup, so much, I bought 4 of those overpriced $50 motors from Horizon Hobby.

I think now, I have to build a jet with one of those motors and drop missiles with it

Have you seen the F4 DTFB thread on RCGroups, I might build that

Here's the phantom f4 design, the thread is an excellent read

View attachment F-4 des by Matti Huaviala 2200lipo 9x7sf

OK since I said "DTFB" that's the stuff from Dollar Tree, have you seen mention of Ross foam board from Walmart and that it's way better than DTFB?

Ross foam board review:

I like to make it a practice to share what I know is good stuff, and you know if you do this, these references are also there for you to return to, like if you asked one day "Now what what that site RCFoamfighters liked for tape, oh, here it is, it's, you can also get their stuff on Amazon, whichever is easy for you.

If you build firewalls like I do, with Great Planes 6-32 blind nuts, the ones with the larger disc footings, you can find these on Amazon in a 24 pack for just Five Bucks!

All this is great and I want to do it, but first maybe I hafta finish my super new wood pod and the 120% Hawker Hurricane! These are suppose to be my first foam board projects to get something flying

Here's a pic of my new power pod, it weighs 3.25 oz as is without hardware.

Fuse 232 ESC  bottom rear 1.jpg Fuse 232 rear 2.jpg

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Active member
"Captain, there be whales!"

Bomb Bay Dropper 1.jpg

One thing, you have to leave enough clear space fwd of the bomb loop for max travel in the other direction release B3 on the opposite side and just the right amount on the aft side of the bomb loop

After all this, you would also need accurate fixed position slots for the bomb loops to go thru, you know this, right? Here's where you might use some of that 1/8 lite-ply and put 6 slots in it where the loops push thru, and your linkage rod would be guided and run on the top of this board, the bombs on the bottom side with the loops sticking thru.

In conclusion, I see this working easy, right here, right now, this way. Simple cheap, effective, and very reasonably functional.

Hey, Now you know, this idea here works either way, on the fuse length Stealth Bomber one narrow bay or Fighter Jet style with bombs out on the wing or along the span of the wing, too.

Same method, same configuration, except, ONE thing on the wing span, wing flex could affect bomb mech unless you use that nice flexy cable and nylon sheath push rod instead of the straight rod they connect servos to control surfaces with. Might want to see which would work and how installation and wing flex would affect each, may need some investigation/experimentation.
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Master member
So. here's the concept:
new doc 1_1.jpg
A pawl, attached to a servo arm pushes a tooth on a retaining pin when the servo is activated, pulling the retaining pin from the bomb or missile's holder letting it drop. Switching the servo off moves the arm back to its original position. The pawl on the servo arm allows it to 'ratchet' over the next pin/tooth in line resetting itself. In the below example the outer bomb would drop first, the switch on the transmitter would have to be switched off to reset, and when switched on again the next inner missile would drop.

Can be done with one servo and a two-position switch. (My DX6i has no knobs, sliders, or 3-position switches)
No Mixes required.

Number of bombs limited by length of rod and space for it to travel. (Could be circumvented by using a gear/winch with the pawl instead of teeth on the wire)
Can't drop all bombs at once.
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Large Child
Great Input!

Thanks Tench745! I like where you're going with this basic mechanism. I started last night drafting some options for the way the bomb would connect to the release. I figured brainstorming this part would lead to an "ah ha" moment on an elegant way to release it.

My goal is to have 3 candidate designs all which I can prototype and actually test to see which is the winner.

Note: These are not to scale what-so-ever. They are just conceptual shapes that can be refined later.

Adapter 001: Simple slot load with pin hole catch. Looks like what we would need for your mechanism.


Adapter 002: The fingers would be held in place by some plate type piece and would drop when that part was shifted from under the fingers.


Adapter 002a: If you slightly modify this and angle the top edge of the fingers, you might be able to snap the bombs into the mechanism without having to coordinate servo movement putting the bombs on. This will obviously depend on the material I use to print it, but ABS should be flexible enough.


Adpater 003: The would be held in place by a flat plate on a rotating hub (picture a servo upside down with a dual servo arm rotated under the fingers. When the plate rotates the bomb would be released.


I think the key to all of this is devising a working and reliable ratcheting function.

I'm also becoming willing to forego the modularity requirement and perhaps just focus on a 3-bomb version for now. I also have this thought in my head (stuck) that using a belt with some teeth can be pulled around (wratcheting) to trigger them in succession. I don't know - time will tell.


Large Child
OK, I just wanted to throw out an update. So I've come up with a mechanism that relies on magnets and a rotating shaft with nubs on it that sequentially push the bomb (that also has magnets) away from the magnets. So far I think this will satisfy most of the requirements including being able to make it modular. Drawings to come. I think these little high-powered magnets I have will work. I'll have to price out something similar to see if they'll be economical for others to build (or to mass market).



Large Child
Rough Drawings - Comments to come

I'd love to have some feedback so I can continue on the design.

I think that I can get the triggering module to also drop a bomb which would be AWESOME for space savings.

Dimensions of the module right now are: 1 1/8" H x 1 1/4" W x 1 3/4" D (It's likely that the trigger module will add 1/4" to the 1 3/4" side making it 2" deep (front to back)

This first drawing didn't know about the modularity yet... So this is a front view of the bomb hanging from the bottom with the trigger shaft above it with the little "fingers" on it that will rotate forward and eventually push the bomb away from the magnets through the slot.


This shows how they can be modular - and you'll see in the other pictures how they connect. The dark squares are magnets. The rectangles in the first module was to show where shaft supports would go, but I think now the walls of the casing will be enough to support the shaft.


Top View: This shows the slot with the magnets on either side (this is drawn full-size) and I overlaid the shaft and trigger to show how they line up.


This is the side view where you see the shape of the shaft connection. I decided on a 8-pointed star (basically) for the female and a basic flat rectangle for the male. The allows you to rotate the trigger shaft to your desired position and lock them relative to one another. This is how you get configurable drop order and frequency (all-at-once, 1-at-a-time, 2 on first drop and 1 on subsequent, etc.)


There's a typo in the top callout - it's lever, not level - UGH! Anyway, this shows how I plan to make the shaft for the basic mechanism out of a 2-part shaft with the trigger sandwiched in between (with a rod/hole connector). The actuator module is shown below and will have a similar construction.


The actuator module came last to design. I was trying to make it so its profile was the same size as the non-actuator modules. Once I started drawing it seemed that I could get that module to also have a slot and magnets. It got tight so I'm going to make the actuator module about 1/4" deeper than the others. I was thrilled that the actuator module will also drop a bomb.

Its basic structure is a lever with a ratcheting function (Thanks Tench745 - your mechanism helped me envision this) that turns the primary gear in one direction a maximum of about 90-degrees which then turns the motion 90-degrees which then multiplies the rotation with a smaller gear on the main shaft. I think the way this is going to be adjusted will be with dialing throw up & down in the Tx.

I will have to experiment with the gear ratios to make sure they make sense. If I can I'd like to make it so that different holes on the lever result in different amounts of rotation which will be necessary depending on how many modules are connected. I'm thinking that there will be a max of 5-6 modules (actuating module w/ drop + 4-5 expansion drops)


I think I'm going to try to mock this up using Legos before I print out the parts. I'm probably a good 1-2 weeks from being ready for that anyway. The printer just arrived yesterday and I probably won't have time to set it up until later this week or the weekend. :mad: :cool:
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