How can I launch a rocket from 200 feet away?

[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?

 

[negitron]

Am I doing something wrong?

Probably. What is the voltage at the batteries as measured from the terminals your long wire is connected to? It should be around 26 volts, assuming good, fully-charged batteries. When you say the batteries are in series, do you have the (+) terminal of one battery connected to the (-) terminal of the other, with your wires connected to the remaining (+) and (-) terminals? They should NOT be connected (+) to (+) or (-) to (-).

 

[rcgldr]

What about a two stage relay system? The car battery and the large relay are close to the rocket, and a lower stage relay and lower power source are located some distance away.

You could use a radio control transmitter, a receiver, receiver battery, and strong servo to throw the relay hooked up to the car battery + igniter circuit, or to the equivalent of a starter switch (note some cars use buttons to engage starter circuits these days).

 

[negitron]

I think he needs to solve his fundamental circuit issues before going with anything more complex.

 

[Max CR]

Great news! I found that the voltage of the battery was at a high level without the 200 feet of wire. When I added the 200 feet of wire into the circuit, the voltage dropped to 0.50 volts. There are two reasons for this. Reason one: One of the batteries were dead and not working correctly. Reason two: There was a RIP in the wire!

This is why I was getting such a low voltage reading. Today, I tested the voltage over the 200 feet of wire, and the voltage dropped by 0.01 volts. I tested the circuit with four igniters and they all ignited simultaneously!

I have figured out how to launch the rocket from 200 feet away. It turns out that I do not need any starter relays or anything else. All I am using is a 13 volt car battery and 200 feet of wire.

Thanks to everyone for their help.