Rocket Test Rig

[Peret]

I've been working on a rocket force test rig for a few weeks now, and just worked the bugs out of the software and got it to the stage where I can put a motor on and test it. Here are some details of the design. It's built on a 4x2 aluminum box section with 1/4 inch thick walls. Inside is a 20kg (44 pound) load cell, but it will take any load cell that fits inside the 1.5 inch internal height. I have a 60kg (130 pound) cell also, but the 20kg cells were very cheap ($7).

 

The motor mount is a mini lathe chuck from Harbor Freight. It can accept any tube size up to 3 inch OD and it grips really tightly. Mounting it was a bit of a puzzle, because it has a #1 Morse taper made of steel as hard as diamonds, so cutting or turning it was out of the question. I solved it by taking a piece of 3/4 inch aluminum rod and boring it first with 3/8, to grip the narrow end of the taper, and then most of the way down with 7/16 that grips the top. The taper fits tightly and is quite hard to pull off. I turned a groove in the sleeve for an external snap ring as a limit. It slides into a piece of pipe threaded into a pipe flange, the height adjusted by screwing the pipe in and out until the snap ring rests on the pipe at 125% full load. The bottom of the sleeve rests on the load cell.

 

I designed my own electronics for this, not being very impressed with the standard options. Electronics being my specialty, I made it quite sophisticated. It has a load cell amplifier, auto-zero circuit, 16 bit ADC and a processor. There is a setup menu to operate it from a computer, and a couple of buttons to work it when you don't have a computer handy. It has a programmable trigger load, so you can arm it, then light the fuse, and when the rocket fires it starts logging at the trigger load and saves the readings internally. The rate is programmable, but as standard it logs 100 readings a second and stores 1 second before the trigger event and 8 seconds after, which is plenty for the kind of rockets I make. It stores up to 15 sets of readings that can be recalled as required whenever a computer is connected. The output is not an analog voltage that needs another box to convert it, but actual loads in pounds (or grams, it does both) that can be captured straight into an Excel spreadsheet. Communication to the computer is by serial RS232 and/ or Bluetooth wireless, so you can operate this from a safe distance with no trailing wires.

 

The board mounts to the back of a 2x4 plate that screws to the end of the box section, so nothing is exposed. It runs off a single rechargeable lithium polymer cell that is also safely enclosed in the box section. I have no idea how long the cell will last before it needs recharged, but I've been debugging for two nights without having to recharge it yet, so it's more than 12 hours.

 

Finally here's a test I did tonight by placing a 25 pound weight on the chuck and then lifting it off gently. I imported the data directly into Excel (Actually Open Office Calc, but it's the same thing) and graphed it with no alterations at all. Y axis is pounds, X axis is 1/100ths of a second. This weekend, hopefully I'll get to try it with a rocket.

 

I plan to publish this for anyone to make.

 

<edit> One day I'll discover how to embed pictures properly in the text.

Edited April 13, 2012 by Peret

[dagabu]

Peret,

 

Would you be willing to sell one? I have all the "stuff" and have built one myself but it is really complicated, yours is divine!

 

This is easily my favorite of all the load cell setups I have seen so far. Nice job!

 

-dag

[Peret]

Dag, thanks for the compliment of asking if you can buy one! I wasn't really planning to make and sell these as I don't have the time, but I will publish a set of drawings and parts list for anyone to make their own, and it can be done with just a drill, file and hacksaw. Ok, I'll probably make and sell the electronics because that board would be beyond most peoples' ability to assemble. The electronics will work with any rig, of course, doesn't have to be this hardware, and I expect you noticed the complete absence of setup pots, as all the setup and calibration is automatic. I haven't finished developing it - I'm going to add an e-match firing circuit and a remote fire command, to make it completely remote controlled, and I have some other ideas.

[nater]

That is a really slick stand. I have the parts to make an amp per Lloyd Spoenburg's design and need a stand to go with it. I like what you have done, simple and clean looking.

[Peret]

I got the aluminum tube here

[dagabu]

I got the aluminum tube here

 

Or HERE

 

-dag

[dagabu]

Dag, thanks for the compliment of asking if you can buy one! I wasn't really planning to make and sell these as I don't have the time, but I will publish a set of drawings and parts list for anyone to make their own, and it can be done with just a drill, file and hacksaw. Ok, I'll probably make and sell the electronics because that board would be beyond most peoples' ability to assemble. The electronics will work with any rig, of course, doesn't have to be this hardware, and I expect you noticed the complete absence of setup pots, as all the setup and calibration is automatic. I haven't finished developing it - I'm going to add an e-match firing circuit and a remote fire command, to make it completely remote controlled, and I have some other ideas.

 

I have Lloyd's board too but it takes 4-9v batteries, is touchy and takes way too much rigging to run. I like the all-in-one package you have and I can certainly do the hardware part myself but would really like the board and software be done for me by others. The e-match circuit and remote firing command would be an outstanding addition but the idea of the recording starting only when input triggers it is the best part. I have to start the recording, light the motor, remember to stop the recording and edit it.

 

A single rechargeable battery would be the bomb (excuse the pun).

 

What a pain...

 

-dag

Edited April 13, 2012 by dagabu

[nater]

I have not assembled my amp yet, but one weakness is needing a computer right next to the stand. Having software and storage embedded in your electronics is the ideal solution in my opinion.

[dagabu]

I have not assembled my amp yet, but one weakness is needing a computer right next to the stand. Having software and storage embedded in your electronics is the ideal solution in my opinion.

 

Completely agreed!! Imagine, walking out to the shoot site, drop the stand on a patch of dirt, set the motor in the chuck, give the key a twirl, press a button, walk back to safety, e-match ignites, electronics do their thing. Perfect

 

Hell, I'm willing to make 25 of the holders if Peret will make the boards and do the electronics.

 

-dag

 

P.S. Carbide tooling works fine taking down the steel they use on the chuck, I have to shorten mine 2" to fit into the tail stock holder on my 9x20.

Edited April 13, 2012 by dagabu

[Peret]

Hell, I'm willing to make 25 of the holders if Peret will make the boards and do the electronics.

 

I like that idea. Let's do it.

 

P.S. Carbide tooling works fine taking down the steel they use on the chuck, I have to shorten mine 2" to fit into the tail stock holder on my 9x20.

 

Carbide insert tooling barely makes a scratch on the quill on my tailstock chuck, which I also need to shorten 2 inches.

[Algenco]

well put me on the list for one

[dagabu]

Carbide insert tooling barely makes a scratch on the quill on my tailstock chuck, which I also need to shorten 2 inches.

 

Really? Harbor freight right? Sounds like you have a tool steel tail, mine is case hardened but a brush with the bench grinder breaks that easily and a blast of Map Gas takes the hardening off almost any metal when water quenched when cherry red. Never mind though, I like the simplicity of your design and making the stepped holder out of aluminum is no problem.

 

Material cost so far is about $60.00 for the hardware alone. This will not be cheap, we will have to get pre-orders and set a comfortable delivery time. I can do all 25 in a long weekend with my son at the drill but the out of pocket investment ($1500.00) will need to be covered first.

 

Do you want me to buy a holder and shorten it for you? Worse case, I will have it ground on a grinder lathe instead of using tooling. It can even turn carbide

 

-dag

 

 

 

http://www.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/i/m/image_16751.jpg

 

[Peret]

That's the HF small chuck, but that's not the one I'm needing to shorten. I mean the tail stock drill chuck for my lathe, which has a quill so long that I have to wind the tail three inches out before it grips the taper.

[Peret]

It will not be cheap however you make a rocket stand. If you do it the Creagan way, by the time you shop for the metal, a foot of this, a yard of that, ten bolts, etc. you'd easily go through $60 at Home Depot, and then you have to make it. The load cell costs the same any way you do it. For the electronics, the traditional way is Lloyd's board - $15 plus parts, plus you have to make it, so let's say $50. After you've made it you have to test and calibrate it, which is ok if you're an EE with professional tools, but for the average guy with a multimeter, factor in a favor from a knowledgeable friend. Then the digitizer box is another $50, and I believe you have to shell out another $100 for software before your computer can even use it. So we're already looking at not much change out of $300, plus any special parts you need made to secure the motors, plus at least a week's assembly work, not to mention $10 of batteries every time you turn it on and the need to have a computer connected to it at all times.

 

Contrast that with the DAGPER^_tm Acme Rocket Stand. No shopping, no cutting drilling or filing, no calibration, no software, no wiring, no batteries. Just take it out the shipping box, drop it on the ground, switch it on and fire a rocket. Hell, fire a dozen and a half of them. Then take it home and upload all eighteen force curves at your leisure, recharging from the cig lighter socket on the way.

[gregkdc1]

I am interested in the electronic as well especially if you throw in one of the 20 kg load cells. Let us know what the cost would be if you decide to go this route. I haven't seen any test rig electronics that look like they would be as easy to use as yours and think you may really have something here. Thanks Greg

 

 

[dagabu]

It will not be cheap however you make a rocket stand. If you do it the Creagan way, by the time you shop for the metal, a foot of this, a yard of that, ten bolts, etc. you'd easily go through $60 at Home Depot, and then you have to make it. The load cell costs the same any way you do it. For the electronics, the traditional way is Lloyd's board - $15 plus parts, plus you have to make it, so let's say $50. After you've made it you have to test and calibrate it, which is ok if you're an EE with professional tools, but for the average guy with a multimeter, factor in a favor from a knowledgeable friend. Then the digitizer box is another $50, and I believe you have to shell out another $100 for software before your computer can even use it. So we're already looking at not much change out of $300, plus any special parts you need made to secure the motors, plus at least a week's assembly work, not to mention $10 of batteries every time you turn it on and the need to have a computer connected to it at all times.

 

GAAAAHHHHH!

 

You are so right, I am at least $200.00 into mine and I still have to make the stand... YUCK!

 

I would LOVE to do this, really I would.

 

@Greg, these would be 100% complete shipped. Battery and all. (Right Peret?)

 

-dag

[dagabu]

That's the HF small chuck, but that's not the one I'm needing to shorten. I mean the tail stock drill chuck for my lathe, which has a quill so long that I have to wind the tail three inches out before it grips the taper.

 

What size lathe? I got a chuck for my 9x20 that fits all but the first 1/4".

 

-dag

[Peret]

I took the rig out for a test run today.

 

 

http://www.youtube.com/watch?v=I9_4KAryVfw

Edited April 16, 2012 by Peret

[californiapyro]

i'm in the market for one, if you get to selling them

[dagabu]

I took the rig out for a test run today.

http://www.youtube.com/watch?v=I9_4KAryVfw

 

OK, that does it! We have to make some of these bad boys! Very nice, Peret, I could not have even envisioned a better setup. Thank you sop much for sharing this with us.

 

-dag

[Algenco]

setup looks great! but I have one concern with it being self contained.

If the motor blows the unit could be severely damaged.

Two seperate boxes connected by 8-10' of cable could save the electronics in the event of a cato, you know it will happen somewhere along the line.

 

That said, to whom do I send my deposit

[dagabu]

setup looks great! but I have one concern with it being self contained.

If the motor blows the unit could be severely damaged.

 

That may be true but if a hold off is used to limit the distance the plunger is able to travel, the unit cant be damaged from a CATO. Also understand that only the load cell would be damaged in the case of CATO and replacement is about $7.00 plus shipping. Not a deal breaker by any means.

 

In fact, would it not be a good idea to have a few different load cells for different rocket motors anyway?

 

-dag

[Peret]

I'm confident that the 1/4 inch thick channel could survive quite a serious incident, and as Dag says, there's a holdoff to prevent the plunger ramming all the way down to the bottom. I particularly wanted to keep the wiring short because the load cell input has a sensitivity of 75 NANO volts, and is very prone to picking up interference. When I first connected the Bluetooth module the RF interference was enough to drive the input off the scale, and it took some careful work to get rid of it. This sensitivity question is important. I designed the electronics for this specific setup, and it took some black magic to get everything working properly. If someone else takes these electronics and puts them in another enclosure of a different design, there is no guarantee they'll work properly, and I can foresee a number of angry dissatisfied pyros calling me an asshole on Passfire for the rest of my life. So I haven't made my mind up yet whether I'm going to sell the electronics alone. I may do a second design laid out for an off the shelf metal box, which I will specify and test for.

 

The load cell selection is a good point. The cheapo ones can do 20kg, or 44 pounds, but the fact I got 22 pounds out of a BP motor gave me pause for thought. I wouldn't test a whistle motor on a 20kg cell. It's not really convenient to make them interchangeable because I haven't seen two yet with compatible mounting holes. I have a nice 60kg cell (130 pounds) that I may try out.

[dagabu]

If someone else takes these electronics and puts them in another enclosure of a different design, there is no guarantee they'll work properly, and I can foresee a number of angry dissatisfied pyros calling me an asshole on Passfire for the rest of my life.

 

Such is my life...

 

Does this mean that you don't want to go ahead and made an off the shelf rig? That would be a darn shame since you have the best rig to date.

 

-dag

Edited April 16, 2012 by dagabu

[warthog]

Mine too Dag...

That is the slickest test stand I have seen yet! While I am not in the market for one, that would be something I'd be wanting more than those more elaborate models I have seen for sure. Maybe once I get my land and start making girandolas a test stand will be needed so I can get them flying right. Come that day I will be bookmarking this thread so I can, well maybe I can, make one like this myself.

 

Really nice work.

Edited April 16, 2012 by warthog