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I'm helping a friend with a physics high school project and we are thinking about doing some rockets fueled with chemicals and a chain reaction for launching.

The general idea is: the first rocket will be launched manually, but when it starts to fall (velocity reaches 0 and acceleration becomes negative on the y-axis) an Arduino will release a lock to the second rocket, and so on. In order to do that, we would need some way to measure the acceleration of the rockets on air. Googling it a little, I've found some stuff about XBee devices, but it would make the project quite expensive.

Can you guys suggest me any other techniques?

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  • \$\begingroup\$ robotshop.com/blog/en/… \$\endgroup\$ – filo Aug 29 '17 at 8:05
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    \$\begingroup\$ An Arduino is not a magical device which can do all kinds of stuff. It needs input and output devices so that it can take input from sensors (like an acceleration sensor) and based upon that input activate something. XBee devices are overkill for what you want/need. There are also 315 and 433 MHz modules which are much cheaper. They're for remote on/off control like garage doors. Search for the 315/433 MHz boards on Ebay and use the receiver module with the AGC as these are more sensitive. Not clear if you need wireless comms. Also unclear what the 2nd rocket does. \$\endgroup\$ – Bimpelrekkie Aug 29 '17 at 8:08
  • \$\begingroup\$ What you need is an accelerometer chip Google is that way-> \$\endgroup\$ – RoyC Aug 29 '17 at 8:08
  • \$\begingroup\$ Welcome to EE.SE. Shopping questions are not allowed. (See Help Center.) I've edited your post to fix it. Do a search for multi-stage bottle rockets and see what tricks they use to solve this completely mechanically. You might get some ideas which would feed into an electrical design. \$\endgroup\$ – Transistor Aug 29 '17 at 8:08
  • \$\begingroup\$ @Roy: No, he doesn't need a accelerometer chip. That won't tell him when the rocket hits highest altitude after the engine cuts out. Acceleration is a constant 1 g downwards after the engine quits. Inside the rocket, this feels like 0 g due to it being a inertial frame at that point. \$\endgroup\$ – Olin Lathrop Aug 29 '17 at 11:27
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The general idea is: the first rocket will be launched manually, but when it starts to fall (velocity reaches 0 and acceleration becomes negative on the y-axis)

You can use an accelerometer, it measures acceleration. In freefall the accelerometer measures zero on all axis. That is easy to detect.

accelerometer freefall and impact
(source)

Accelerometers aren't expensive. Sparkfun sells various boards with analog and digital ones for around 10-20 USD. Just make sure you do not exceed the absolute maximum ratings.

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There is a basic physics flaw in what you are trying to do. The problem is that the acceleration of the rocket measured in its own inertial frame is 0 as soon as the engine cuts out. A accelerometer is useless in finding the 0 velocity point where the rocket is at its highest altitude.

What you want to do is measure altitude, not acceleration. Look for things called altimeters. Release the next rocket when the altitude value levels off or starts going down again.

A arduino is also inappropriate here. On a rocket, size and weight matter a lot. You want to strip down this circuit to the smallest size and weight you can, so starting with something large designed for totally different tradeoffs makes no sense.

Use a small microcontroller directly. With the right electronics, maybe even a 8 bit A/D is good enough. Actually, even just a comparator input would be good enough with a analog circuit that gives you the derivative of the altitude. Something like a PIC 10F204 may be all you need. That has a built-in comparator. The rest is simple firmware to sequence, control, and time the process of starting the next rocket engine. A major feature of the 10F204 is that it comes in the tiny and light SOT23 package.

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  • \$\begingroup\$ If he measures maximum altitude the velocity of the rocket will be 0. He will have lost any attitude control shortly before then. He needs to fire the next stage shortly after the first stage burns out. that can be sensed by an accelerometer. This has very little cost as the acceleration from the second stage will add to the residual velocity from the first stage and the path of the rocket will be a little more predictable. \$\endgroup\$ – RoyC Aug 29 '17 at 18:15
  • \$\begingroup\$ @Roy: Some of that may be so, but is not what the OP asked about. The OP said "starts to fall (velocity reaches 0 and acceleration becomes negative on the y-axis)". That's just plain wrong. Acceleration is 0 the whole time after the engine cuts out. As for attitude control, there isn't any at 0 velocity, but then again, the rocket shouldn't suddenly change orientation for no reason then either. \$\endgroup\$ – Olin Lathrop Aug 29 '17 at 19:23

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