Finding small rockets in tall grass

Discussion on ground control/GSE and recovery equipment. This includes launch pads, triggers, chutes, streamers etc. Includes other items such as simulation and other computer software, etc.

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air.command
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Finding small rockets in tall grass

Postby air.command » Mon May 27, 2013 9:30 am

Due to the expense and weight of GPS transmitters they are not always practical for small LPR and MPR rockets. Also due to the rocket's typical small size they can be difficult to find in tall grass if they land some distance away.

At the last NSWRA launch we tested a simple low power laser device for locating these types of rockets. The LaRF (Laser Rocket Finder) proved very successful with 4 out of 4 rockets located one of which was ~850 feet from the launch pad. All were found quickly and within a couple of meters of where they were expected. We even found one within 2 minutes after a lawn dart 2 weeks earlier. The rocket was black and green and half of it was buried. We previously had 5 people looking for it for half an hour without luck.

The design is based around a laser line generator used for aligning tiles or pictures in the home. The one we used is a Stanley Intellisensor Pro that is also a stud finder but we are only using the laser for this application. The light source is a red class II laser of less than 0.16mW. The beam spread is approximately 45 degrees from the source so the further you get from it the laser intensity is greatly reduced. The beam is approximately 4mm wide.

Now it is never a good idea to look directly into a laser and so we did a number of calculations to work out what the beam intensity is at a certain distance, and to make sure it was safe to use. Because of the wide spread of the beam the intensity is quite low at the distance at which we are using it. However to further reduce the laser intensity we used a polarizer rotated relative to the polarised laser beam.

Bracket with polarizer attached:
Image

Laser attached to bracket. The bracket can be mounted on a tripod.
Image

Finding rockets

Finding rockets is simple. You just watch where the rocket lands and then aim the laser in the direction. The laser is far too weak to see in daylight and so you have to sight along the top of the laser at the landing location. The unit we have has sight marks which makes it very easy. You then turn on the laser and start walking towards the rocket. You occasionally look back over your shoulder to see if you are on track. If you can't see the beam you need to walk perpendicular to the beam until you see it again. This keeps you on the correct line of sight. It was amazing to see just how far you drift from the line when just walking in an open field when you think you are heading in the correct direction. As you walk down the line you just look for the rocket. It turns your 2 dimensional search problem into a 1 dimensional search problem.

It's essentially the equivalent of someone standing at the launch site directing you through a walkie-talkie "left a bit... right a bit".

Because the beam is a vertical line you can travel up and down over rough terrain and still see the beam,

Diagram of the setup:
Image

A limitation of the LaRF is that the rocket must land within line-of-sight, and you have to see it land of course. If it lands behind trees then you won't be able to see the laser if the trees block your view of the LaRF. Also at distance while it was easy to see the beam it was difficult to see where to look for the beam. We have now added a black and white stripped sighting screen under the LaRF to make it easier to find the beam source from a distance.

LaRF with a sighting screen to make it easier to see from a distance.
Image

Looking back at the LaRF. Observer shift between the two photos is approximately 20cm with distance to LaRF ~50m (telephoto used)
Image

This is the view from about 150m (telephoto used for this shot). You can see the sighting screen becomes important.
Image

Here is a beam comparison with the normal beam on the left and the polarizer attenuated beam on the right:
In this photo the screen is 1m from the LaRF. and the portion of the beam you can see in this image is approximately 70cm high.
Image

- George
Crop Circles: ... just a bunch of guys looking for their rockets ....

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Re: Finding small rockets in tall grass

Postby Space Mark » Mon May 27, 2013 10:22 am

Genius!
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Re: Finding small rockets in tall grass

Postby rocket_troy » Mon May 27, 2013 10:40 am

Very Nice - we want one! I had a similar idea for our club but using a bright red LED inside a mirror ID tube to create a very narrow beam, but this is much better!

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Re: Finding small rockets in tall grass

Postby air.command » Mon May 27, 2013 11:38 am

I've tried putting a bright white LED in the eye piece of an old telescope to see how well that would work by looking into the big end of the telescope. The horizontal beam divergence was quite wide however at distance so you didn't quite have the same accuracy. I was going to try adding a half red filter and half green filter just before the LED so that you had that center line. If you see red you are too far over to one side and if you see green you are too far over to the other side. Just like a glide slope indicator on ships. The laser line generator though is a lot simpler and smaller. I guess you could use one of those barcode scanners as well.

It would be nice to modulate the laser say at 40kHz and then have a sensor on the back of your cap that looks for the 40Khz signal with a little buzzer to let you know when you are on the beam. That way you can keep looking forward for your rocket without really stopping to look back.

I guess you could take that a step further and put the laser line generator (or at least a mirror) on a rapidly spinning stand and modulate the beam so that it produces a different signal say every 0.25 of a degree. (you could repeat the pattern every say 10 degrees) That way multiple people can take bearings from the LaRF towards their rockets, dial in the signal pattern on their hat and follow their path. That way you'd really just need one for the launch site that runs all the time and people could go look for their rockets anytime they like.

- George
Crop Circles: ... just a bunch of guys looking for their rockets ....

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Re: Finding small rockets in tall grass

Postby rocket_troy » Mon May 27, 2013 11:45 am

Awesome ideas George! But I agree, the simple system you've suggested is an excellent solution in itself.

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Finding small rockets in tall grass

Postby kopius » Mon May 27, 2013 3:58 pm

Very nice work. Have to place with this ourselves.
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Re: Finding small rockets in tall grass

Postby rocket_troy » Mon May 27, 2013 4:11 pm

kopius wrote:Very nice work. Have to place with this ourselves.

It's one of those essential bits of kit that I reckon every rocket club needs. If anyone is interested in doing small run of these, I'm definitely interest in further discussions.

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Finding small rockets in tall grass

Postby kopius » Mon May 27, 2013 6:29 pm

Stupid auto correct. That should have been play.

Anyway, perhaps a green and a red version so busy launches (most QRS launches) you could potentially track two at the same time ;-)
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Re: Finding small rockets in tall grass

Postby air.command » Mon May 27, 2013 7:00 pm

We bought this one from Bunnings for $79:
http://www.bunnings.com.au/products_product_stanley-intellisensor-pro-level-stud-finder_P5660079.aspx?filter=categoryname--Laser+Levels
It actually comes with two laser line generators. The smaller one is powered by 2 AAA batteries, while the bigger one is powered by a 9V but both work the same. I am not sure how long the batteries last but I would expect at least a few hours of continuous operation.

You can use them both simultaneously as long as they are pointing at least a few degrees apart or if you locate them a few meters apart then there shouldn't be any confusion. When we were looking for the rocket at the 850 foot mark the beam would have still been only a few centimeters wide.

I think it would be particularly useful for where small bits fall off rockets like nosecones, fins, motors or the first stage of a small 2 stage rocket.

- George
Crop Circles: ... just a bunch of guys looking for their rockets ....


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