How can I launch a rocket from 200 feet away?

[Max CR]

I am trying to launch a rocket 200 feet away from the rocket. When the switch is thrown to "ON" the electrical circuit is turned on which sends an electrical current into the motors. I can launch the rocket just fine if I am standing right there to flip the switch, but as i increase the distance of the switch from the rocket, then I need more batteries to the circuit. I have attempted to use this with about 4 nine volt batteries to no success. This is a parallel circuit. How can I launch this rocket from this distance??

I basically have to figure out a way to flip a switch to "on" from 200 feet away. Keep in mind that I do not want to pull on a string which will flip the switch! Thanks!

 

[Phrak]

Use smaller gauge wire. That is, wire, where the conductors are larger in diameter.

 

[turin]

Or use AC and transformers, the trick that the power companies use.

 

[sas3]

You could set up a simple timer with a wind-up alarm clock/Egg timer, or use a radio control device.
You could also use a small relay that turns on a connection to another battery next to the rocket.

 

[Max CR]

I am looking to spend the least amount of money possible so I don’t want to use transformers. I considered using a timer but I decided that after all of my work, I really want to be able to press that ignition button! What do you mean when you say I should use larger gauge wires? Are you saying that the copper inside of the wires should be thicker?

I considered using a radio controlled device but devices that used radio control going up to 200 feet got too expensive.

Now the relays seems like it could definitely be useful. The only problem is I don’t know anything about relays. I tried reading up on it by searching on google but I couldn’t find anything to explain to me exactly how the work. How could I use relays for this? Does anybody have any other ideas?

 

[Max CR]

Ok. How can I use the relay located at the following website for what I am trying to do? http://radioshack.com/product/index.jsp?productId=2062483&parentPage=family

Thanks

 

[negitron]

A relay is an electromechanical switch. You'd wire your button to the relay coil instead of to the igniters; it takes only a small current to energize the coil and pull in the contacts compared to the much larger current required to fire the igniters. You'd wire another battery through the relay contacts to the igniters. Since this battery would be right at the lauch pad, you won't have the voltage drop issues you have when trying to power a relatively large load through a long run of thin wires.

And, yes, as noted previously if you use thicker wires, you also reduce the voltage drop.

 

[turin]

Ah, yes, relay is a superb idea. Cheaper might be a power FET. How much current do you need to fire the rocket, do you know? If its on the order of amps, the power FET would be cheaper.

 

[Max CR]

Negitron,

Thank you. That was really useful advice. Now with the link I mentioned above, where will I attach the wires from the button that will turn on the relay, and where would I attach the circuit to ignite the rocket?

 

[Max CR]

The rocket needs 9.5 volts to launch

 

[negitron]

Refer to the data sheet for the linked relay:

http://rsk.imageg.net/graphics/uc/rsk/Support/ProductManuals/2750249_DS_EN.pdf

The wires running from your remote button and battery will connect to the terminals marked COIL; polarity doesn't matter.

At the launch pad, the battery (-) terminal will connect to one side of your parallel igniters and the (+) side will run to either of the COM terminals. Another wire will run from the NO terminal on the same side to the other side of the igniter array.

I would not use a FET, since a long wire run like that can easily pick up enough stray voltage to trigger it prematurely. In this case, low tech is safer.

 

[Max CR]

Wow Negitron! This has been a big problem that I have been trying to tackle for some time now. The idea to use a resistor definitely sounds like it will work. I never knew how to use a resistor until now. I knew that there was some way to use a switch to turn on a completely separate circuit and this is it! It definitely sounds like this will work. Thanks you! This definitely was something important that I needed to know.

I will try this out tomorrow. Feel free to watch the development of my rocket design at my blog, www.briancleaver.blogspot.com.

I do have one last question, though. Will this relay work if the two circuits are used with 9 volt batteries?

 

[negitron]

According to the data sheet, this relay has a minimum pulldown voltage of 9.6 volts. Combined with the voltage drop across 200 feet of wire, a single 9 V battery will probably not suffice. However, there are relays with other coil voltages available although I don't know what's available at Radio Shack. They probably have a 5-volt coil version, since that's a common TTL (a type of digital logic) voltage. That should pull in fine with a 9 V battery and all that wire.

 

[turin]

negitron, you don't think that a Gate resistor is sufficient? I was thinking of a simple voltage divider, with a large third resistance going to the gate.

 

[negitron]

I'm not sure I'd want to count on it when using motors big enough to burn my face off if they fire unexpectedly while I'm setting up. If the OP needs to be 200 feet back during launch, these are NOT little Estes A, B or even C motors.

 

[Max CR]

Haha. These are actually Estes D size motors. The only catch is there are about 36 of them with many of them igniting simultaneously.

Now, when you say that the drop down voltage is 9 volts, do you mean that in order to turn the relay on, you need to send 9 volts into the resistor? The website says “Pickup/dropout voltage 9.6/0.6VDC”. Doesn’t that mean that the minimum voltage to turn on the relay is 0.6 volts? I am also having trouble finding a relay that works with under 9 volts.

Thanks for all of the help.

 

[negitron]

Close. The pickup voltage is the minimum required to pull the relay in; the dropout voltage is the voltage below which the relay will let go. So, you need at least 9.6 volts to turn the relay on and then if the voltage goes below .6 volts, it will turn off again.

This one will work just fine:

http://www.radioshack.com/product/index.jsp?productId=2062480

 

[Max CR]

I looked at the description of that relay but I could not find any informatino about the pickup or dropout voltages. Where did you find that? It says 5 V does that mean that the relay will switch to on at 5 votls?

 

[negitron]

Yes, that's the nominal voltage; driving it with a 9 V battery will be fine, since it's not on continuously.

But, I just noticed it is limited to 1 A switching current, which is probably a lot less than what's needed to drive all those igniters. It'll work okay, but the contacts might burn out after only a few firings. This one might be better:

http://www.radioshack.com/product/index.jsp?productId=2049716

 

[Phrak]

In the expected enthusiasm, all sorts of higher technology solutions are put forward. For cheap, simple and reliable; get a small spool of bigger diameter wire.

How much bigger? Test it on a small engine to find out.

After more reliability? Put two batteries in series once you substanciate that one battery consistently works.

 

[negitron]

So, how much will it cost him to scrap the 400 feet of wire he has already AND buy 400 more feet of wire 2 or 3 wires sizes larger?

 

[Phrak]

So, how much will it cost him to scrap the 400 feet of wire he has already AND buy 400 more feet of wire 2 or 3 wires sizes larger?

Ha, ha Consider the electrical expertise of the questioner. I really like your idea, really. Not too involved. Reliable. If I were to do it, I would use your relay solution.

But are you going to supply a schematic to the OP?

I thought it was 200 feet. I'll find out how much 400 feet of ... 16 gauge wire will set him back, though. What do you think of 16 gauge stranded?

 

[negitron]

Well, it's 200 feet one way. Don't forget the return path.

Schematic, sure:

 

[Phrak]

http://www.mcmaster.com/#8104k112/=2ogvdz"

18 gauge white lamp cord (18 gauge). Total damages = $35, shipping included, one day delivery.

 

[Phrak]

Well, it's 200 feet one way. Don't forget the return path.

Schematic, sure:

You're fast. What are the contacts rated at?

 

[negitron]

Radio Shack: $4.49.

I win.

 

[Phrak]

Radio Shack: $4.49.

I win.

No you don't. What are the contacts rated at? A little short piece of nichrome is something like a dead short. How reliable is that after you've arced them a few times? Not reliable at all.

Next time you want to fire, you've got a dud.

 

[negitron]

12 a @ 120 vac.

 

[Phrak]

12 a @ 120 vac.

How many amps does a 1 inch length of nichrome #30 draw reduced by battery impedence and a couple feet of speaker wire?

OK, 4 inches of nichrome, as if it makes a difference.

 

[negitron]

Depends on what he's using. The instantaneous current can greatly exceed the stated continuous-duty rating of the relay without too much trouble, particularly since the current draw during the re-opening phase will be zero (because the igniters have fused open), ergo no arcing.

 

[Phrak]

Maybe. You're on the edge in several places. We don't care about continuous current---just the spike.

1) we can't see your schematic unfortunately--it need approval.
2) What are the pin-outs? Without pinouts, what's it worth?
3) It's rated at 7-9V on the inductor side. How much current on the coil side is the OP going to need over weenie wires? Can he supply the 9V with battery impedence.

I'd look for a better relay.

 

[sas3]

36 D's, Wow! You have to post some pictures of the rocket/launch.

If you are going to ignite that many at a time you had better use a 12 volt car battery
because a little 9 volt is not going to do it for you.
Or better yet do what I did if you can.
"use an extension cord and just cut off one end, hook it up to the igniters and plug the other end into an outlet”.

 

[Phrak]

Depends on what he's using. The instantaneous current can greatly exceed the stated continuous-duty rating of the relay without too much trouble, particularly since the current draw during the re-opening phase will be zero (because the igniters have fused open), ergo no arcing.

I understand that. And I've also gone back and read the previous threads. There were a few unknowns. Even a whimpy relay should be good for a one-off if you were confindent in the wiring and all.

If I were doing it I'd use a car battery and 200 ft of #14. There are 36 fuses to ignite.Why risk 12 motors igniting instead of 36. After all, the motors would have already cost me $200 bucks, alone.

 

[Phrak]

You're a fine advocacy and gentleman negitron, unfortunately I hav to get some sleep. See you in the morning perhaps.

 

[turin]

Haha. These are actually Estes D size motors. The only catch is there are about 36 of them ...

!

How confident are you that the engines are balanced? Or, do you have some stabilization mechanism to compensate for unbalanced engines. (These things are called "engines", not "motors", right?) That's a lot of oportunity for some creative steering.

 

[turin]

I'ts hard to find specs for these ignitors. I gave up. But some guy did some testing and found that a single ignitor apparently draws a peak current on the order of 10 amps! If you need to fire 36 of these things ... We're going to need a bigger boat.

 

[Max CR]

Well actually I should have been more specific! There are 36 motors, but starting off, only four of them will ignite. The switch to ignite the system only needs to igntie four motors. THe rest will be ignited through other electrical circuits in the design. Yes, I do mean that I am talking about D size engines. I will look into using a car battery also. That is a really good idea to use an extension cord. I would have never thought of that! Duh! I can't plug the extension cord into an outlet, however, since I will be on a launch field.

I am stabilizing this rocket using fins. I used the program RockSim to ensure this. You all can read about my rocket and its progress at BrianCleaver.Blogspot.com.

I will pick up that relay mentioned earlier, attach it to a car battery, and use an extension cord. I will let you all know if I have any problems. Thanks for all of the help.

 

[turin]

If you use a relay rated at 1 amp to fire 4 ignitors that will presumably draw 10's of amps, then you may have a problem, even if the current is momentary. Even if enough electrical energy to ignite all 4 ignitors passes the relay before it burns up, that energy may not be delivered very evenly. If you insist on going to relay approach, I urge you to invest in a more expensive relay that is rated for at least 20 amps.

One thing that occurred to me: you can almost definitely salvage a relay out of a junked car, probably for free. Are there any salvage yards in your area. I have gotten many useful parts from junked cars (never specifically a relay, though).

 

[Max CR]

If you use a relay rated at 1 amp to fire 4 ignitors that will presumably draw 10's of amps, then you may have a problem, even if the current is momentary. Even if enough electrical energy to ignite all 4 ignitors passes the relay before it burns up, that energy may not be delivered very evenly. If you insist on going to relay approach, I urge you to invest in a more expensive relay that is rated for at least 20 amps.

One thing that occurred to me: you can almost definitely salvage a relay out of a junked car, probably for free. Are there any salvage yards in your area. I have gotten many useful parts from junked cars (never specifically a relay, though).

I am not sure exactly where inside of a car to find a relay. Being that most of them are totaled doesn’t help either. Now at Radio Shack, www.radioshack.com, where can I find a relay rated at 10 amps that can be turned on from only 5 volts?

 

[turin]

Call Radio Shack. I'm sure they will be more than happy to help ($$$). Tell them, "I need a relay that can be actuated with 5 volts, D C, and that can withstand 20 amps." Or (and probably better), describe to them exactly what you are doing. They may either recommend a relay for you, or they may even have a better solution. An alternative to Radio Shack is Fry's Electronics, but you may be overwhelmed when you go in there.

As far as salvaging, it depends on the car. But, usually, there is a box under the hood of the car that contains some heavy fuses and relays. If you go the salvage route, I would try to find the starter relay. To do that, follow the thick red cable out of the battery. If you see a box looking thing connected to this cable, with another much thinner wire coming out of it, then 99.999% chance you have found the starter relay. Unfortunately, it is not uncommon to put it right next to the starter, which is under the car. Some cars simply do not have a starter relay (only a solenoid, which requires more current to actuate, and is almost always on the starter underneath the car, so don't bother in such a case).

A car in a salvage yard is essentially totalled, by definition (otherwise it would be on a used car lot). But that does not imply bad electrical components; it usually implies large-scale body/frame damage that would cost more to repair than the blue-book value of the car (an unfortunate little tid-bit when dealing with the insurance company). I would just try to find the newest car, since electrical components corrode over time. I only mention the salvage yard (yet again) because you voiced a concern over cost.

 

[Max CR]

Ok. My brother actually has a totaled car in our driveway. I keep forgetting its there. I will look in his car and see if I can find the starter relay and the car battery. In the meantime, there is a relay that somebody previously linked me to located at http://www.radioshack.com/product/index.jsp?productId=2049716. Wll this be good enough.

 

[turin]

My brother actually has a totaled car in our driveway.

What kind of car is it (year, make, model)? I may be able to help you find the starter relay (if it has one). However, if your brother works on cars, he can most likely show you the assortment of relays in the car. There are a few other relays in there, but I don't know what are their ratings. (You are going to get permission to scavange the relay, right?)

http://www.radioshack.com/product/index.jsp?productId=2049716. Wll this be good enough.

Possibly. If you're willing to drop the five bucks to test it, then go for it. The current rating is a little bit low for my expectations, but then again, I am going on some obscure test write-up that I found on the internet. And, anyway, as has been stated by others here, it may be OK for the first few uses (and I suppose you are only planning on one).

One thing that I don't remember anyone asking, which is surprising: What is the resistance of your 200-foot wire? Did you measure it? If not, you should.

 

[Max CR]

Of course I will have permission to scavange for the relay. Thanks so muich for your help. I will provide the year, make and model of the car when I am able to find it out. I did not measure the resistance of my 200 foot wire.

Now, to refresh my memory, resistance causes the voltage in a wire to drop over a long length of wire due to heating, correct? How can I measure this resistance? Now the reason for me needing to nknow this is because it will tell me how much the voltage is going to drop, correct? thanks

 

[negitron]

If you know what gauge the wire is, there's no need to measure it. You can look up the resistance factor for that particular gauge (normally given in ohms per 1000 ft. @ 20 C) and do some quick math.

 

[Max CR]

Ok. I have made a ccircuit. I am using very thick wire. It is about 200 feet long. I have connected to 12 volt batteries to the circuit. The voltage of the circuit is 0.50 Volts. In order to use the previously mentioend relay from radio shacki, I need 7-9 Volts. In order to get to 7 volts, I need 28 car batteries in the circuit. This is wayy to expensive. Therefore, I am going to look into using a remote control to launch this rocket. Does naybody know where I can buy such a remote control that has a range of 200 feet? I am thinking about taking apart a toy airplane.

 

[Max CR]

Any ideas?

 

[negitron]

Unless "very thick wire" is something on the order of 40 AWG, there is something seriously wrong if your open-circuit voltage after 200 feet is down to .5 volts.

 

[turin]

Max, if you plan to do projects like this in the future, I strongly recommend investing the $15 or so for a multimeter. How exactly are you measuring voltages? Do you have a strict voltmeter?

I'm with negitron: there is something seriously wrong with your conclusion. You don't want a large voltage drop from one end of the 200-ft wire to the other; you want the large voltage drop to be across the igniter.

 

[Max CR]

I understand that I want the voltage level to be as high as possible but the voltage level just isn't high. It is just at .50 volts. I thought this was due to resistance. Is it not? I have one car battery connected in series with another car battery. Then, the two remaining wires are connected to an ordanary extension cord. Going to the other end of the wire, I can plug my multimeter into the other end of the extension cord and get 0.50 volts.

Am I doing something wrong?

 

[Raap]

Could it simply be the car batteries running out of juice?