Water Rocket Update

[DogLover]

Howdy all!! I have been waiting for an opportunity to launch for the past few weeks and haven't had one due to Tropical Storm Debbie. Plus I had to find a larger field to fly in. Anyway, Saturday, June 30th, I finally had the opportunity and went and launched my rocket. It was a 8 litre capacity. I put in about 3/4 litre. Using a new bike pump (the last one exploded!!!) I (I should say my dad) got it up to about 80 psi and then we realised there were some minor leaking issues from the quick connect. I chose to launch it and it launched perfectly!!! I am guessing it got to about 100 feet, maybe a little more than that. Anyway, then the new parachute release system deployed just after apogee, about 2.5 seconds in, and the chute deployed beautifully. The chute unfolding in about 15 - 20 feet, and then the shock cord broke!!! It was probably 60 feet up and fell straight down to earth!! That exact thing happened the only other time that I got a successful deploy! Well, after I recovered the rocket, Dad mentions that the line probably broke because it was around 10 yrs old. Anyway, to make a long story short I promptly headed to Wal-Mart and got some new string. I am using fishing line and decided to get 60 lb test. This should hold better.

So, since my release mechanism is completely destroyed, I decided to rebuild it from the ground up. This time I am going with a side eject tube style. I am using a pepsi bottle for the tube and got a spring out of a nerf gun to push the parachute out. I am planning on holding the parachute in with a rubber band system, which would be attached to a servo. I was using a tomy timer system of my own design previously, but this time, I was loking around in Radio Shack and found this: http://www.vellemanusa.com/products/vie ... &id=350494. I got it for $20 and thought that was a good price. I have yet to put it together and add it to the rocket. I do have a few questions:

1. Does it look like this start/stop timer will work very well?
2. What would be the best (and cheapest) way to have it start after lift off?
3. Is fishing line a good choice for shock cord?
4. Should I clobber my Dad for not telling me that the last fishing line I used was 10 yrs old?

Thanks!

[air.command]

Hi, That timer looks like a simple timer that activates a relay and will not drive an RC servo directly. (the link to the timer seems to be broken at the moment so I can't check the specs - it was working yesterday)
You can, however, drive a DC motor from it directly, so you could potentially remove the electronics from an RC servo and just drive the motor directly. Not as convenient though. It may also need a 12V power supply to operate the relay, though often you can also drive them from lower voltages like 9V, but you would have to test it.

Your main problem is going to be able to set the delay short enough in at least 1 second intervals, but preferably higher resolution say in 0.5 or 0.25s intervals since most rockets will get to apogee in under 10 seconds. I am not sure if you can do that with the timer. You may be able to modify/replace some of the timing components and get it to cover the useful range. It's certainly worth having a go at building a deployment timer with it.

The cheapest way of triggering a timer is either using a 'break' wire or 'make' wire. If the timer only supports a 'make' contact to trigger it, then you can set up a pair of spring loaded contacts and insert a piece of plastic between them. Tie the piece of plastic to the launcher and when you launch, the plastic gets pulled out from between the contacts and the timer will be triggered.

If the fishing line is strong enough then it should be ok. Remember making a knot could reduce the breaking strength by up to 50%. We always use braided nylon cord as the main shock cord as it is very strong and flexible. If you have some of that handy give it a go. We use the 3mm cord on both big and small rockets. It's survived quite a few high speed deploys. It also works well as a release string for a launcher.

- George

[DogLover]

Technically, I don't have a actual servo. What I have is a regular DC motor that is geared down and then somehow is limited to only rotate a certain amount. I got it out of a cheap RC that I had on hand. On my last system I ran it from just a 9V supply that was hooked up to a rather complex switching system. Here is a picture of the motor:

IMG_2549.JPG

About the delay time, I asked how to shorten the time on the support forum, and the told me that replacing C2, which had a default value of 100uF, with a lower value, I chose 10uF, the time can be shortened. Also, it turns out that my rockets hit apogee at just under 3 seconds, so if I set it to 3 seconds, then the chute should deploy just fine.

That is a brilliant way to activate the timer!! I will definitely have to try that. I should have thought of that my self though, because I had a similar system on my last rocket. I had a tomy timer on top that had a piece of bent wire attached to it. On the pad I had a clothes pin attached and a piece of fishing line clamped in that. The other end of the fishing line went up to the wire on the tomy timer and prevented the timer from spinning on the pad. At launch the line would come out of the clothes pin and the timer would spin, hitting a pair of metal contact that where attached to the motor and battery and the "servo" would spin, releasing the parachute door. It sounds more complicated than it is probably

I am not sure how you can attach a shock cord without putting a knot in it?? My brother suggested that I use a soldering iron to barely melt the fishing line at the note. I am not too sure how that would work. As far as braided nylon cord, I don't have any of that lying around. Should the fishing line not work, I will try that.

-DogLover

[DogLover]

A quick update:

I got the K2579 working and it works beautifully!!! I am running it off of a 9V battery and I got it down to a 2.75 second delay! I could get it lower if I wanted to. The 10uf capcitor is woring great! I would highly recommend that anyone who wants a cheap easy to put together timer circuit to use this one!!! It works great, did I already mention that?

-DogLover

[Muzza]

I was working on a release system using a similar motor, rather than use a timer system I was going to just wire in a switch that would allow the motor to run in either direction. My plan was that I'd hook a rubber band onto the output shaft and run the motor backwards for the desired delay period(winding the rubber band around and around the shaft), then reverse the motor and have it start working right on takeoff so the delay comes from the unwinding. Hope you get what I mean
Only ever got the idea to a bench testing stage then ended going in another direction (Servo-chron) but still think it had merit, may drag it out of the unfinished project box someday

Muzza

[DogLover]

I tried a very similar system, but it failed miserably. Must have been that I did something wrong!

[Muzza]

Just read the "only rotate a certain amount" part of your description of the motor, so the above idea won't work for you.

Also, not trying to "rain on your parade" , but have you considered how the mechanical relay on the circuit is going to behave under the changing G-forces of a launch? It may not be a factor, but if you start getting weird deployments it could be the reason.

[DogLover]

I don't know! I didn't think of that!!! Yikes!!

[Muzza]

I was going down the same path until someone asked me the same question. Assuming electronic stores are similar the world over, I wouldn't bother asking questions like "which orientation would give this relay the best resistance to accidental operation due to G forces?" or "Do these have a G force rating?" They just look at you like you're an idiot
I'm guessing it's probably not an issue, but without flight testing it you'll never know

[DogLover]

Yeah I am going to test it tomarrow, if I get it done in time.

[DogLover]

Right now I am trying to get a good make switch but it is giving me some trouble!

[air.command]

Good to hear you have it going and over the time period you need.

I am not sure how you can attach a shock cord without putting a knot in it??

Hee hee, ... I wasn't suggesting you don't tie a knot in it, I was pointing out that if you use a 60lb line, it could potentially fail as low as 30lb with knots in it. If you want it to withstand 60lbs go for the 100lb line. It's always better to have a higher safety margin on your shock cord.

@Muzza: That is a good point about the G-rating of a relay and it would be something you would want to test.

Right now I am trying to get a good make switch but it is giving me some trouble!

The normally closed spring contacts from an old relay work great for this. Depending on the relay, you can usually pull them appart and throw away everything but the contacts. You already have a convenient places to solder to them, and you just insert the plastic between the contacts. Depending on how the contacts are made (often pressed together with an insulator between them) they may also come with a mounting hole.

USWR also has a tutorial for a simple spring G-switch that should be easy to make. Which approach works better for you will depend on how the timer handles the trigger and how long a contact needs to be made. When making g-switches be mindful not to make them too sensitive as pressurising the rocket can cause vibration and potentially trigger the timer. You also want to make sure the timer cannot be re-triggered by multiple triggers.

- George

[DogLover]

Hi,

I want too revoke something I said ealier. The K2579RS is NOT a good timer for parachute release. Trust me. It is hard to make it work. How can I delete that post.

Anyway, I do have a working system now, even if it is less than sightly. I ended up making a make switch from two paper clips with a piece of plastic in between them. This prevents the servo from spinning while it is on the pad. Then, I have another pieces of paper clip glued onto the start button on the timer. This is so that the timer can be activated on launch. I need to figure out whether a string or something on the pad will activate the start switch. These are the pitfalls of the K2579RS. I don't recommend it. However, I am using it because why waste $20?

I was pretty sure that you meant what you said about the knots, but I wanted to be sure I was clear!!! I am pretty certain that the 60lb test will work. Upon further examination if the parachute line that failed Saturday, I realised that the problem was that the knot came undone!!! I might try melting a test note with my solding iron.

I didn't think of using an old relay. Of course then again I don't think I have an old relay. Oh well!! Also, I tries using a G-switch once and it didn;t work very well. Infact I have given up on gravity activated systems. They have caused the crash of too many rockets.

I kinda wish that you sold a assembled timer that worked and all you had to do was hook up your servo and battery!!! But, alas, you don't!

DogLover

[DogLover]

Update:

After some very sorry looking bench tests, I have decided that the K2579RS System with need a LOT more work before it is flight worthy. Therefore I am going to throw together a tomy timer system, the same kind I used last time, to deploy tomorrow. Then in the morning I will go fetch the tomy timer part. Shucks!!! I wasted and entire day on something I am not even going to use!!!

-Nathan

[Muzza]

My latest launch detect switch is a disk magnet that can slide up and down in a slot cut into a piece of corflute (like corrugated cardboard but made of plastic) with PET glued either side of it. I have then put a small steel screw down through the corflute at the top, this I adjust until it is only just keeping the magnet at the top of the track. A magnetic reed switch is then taped onto the outside of it. The theory is that as the rocket accelerates the magnet will drop and trigger the reed switch. seems to be sensitive enough on bench testing, hoping to get out and flight test it soon
M