# Another Thesis Bleg: Short Pulse Duration Stroboscope

I figured it might be a good idea to bring this one up on the blog, just in case. Over the weekend, I finally got my flow test rig setup for my thesis. The rig is pretty similar to the fuel half of our igniter cart–basically it’s just a fluid tank with a regulated pressure input, some fill plumbing, and a valve between it and the water jet nozzle. I should have the linear amplifier for driving the piezoelectric crystal fairly soon (Charles Pooley of Microlaunchers is helping me out with the design–he’s very good at analog electronics design).

The experiment I need to run involves using high-speed photography techniques to determine if the pulsed nozzle is creating a pulsed flow. In order to have a clear comparison, I’m also doing some runs with no driving voltage on the piezoelectric crystal, basically as a control experiment. The smoother and more stable and steady I can get the unpulsed jet, the more obvious even the slightest fluctuations are going to be for the pulsed case.

I was able to enlist the help of XCOR’s Mike Massee for help with the photography portion, but we quickly realized that the nice strobes he had were far too slow for the task at hand. Those strobes were mostly for the entertainment business, so they had a pulse duration of about 250 microseconds. The problem is that I’m trying to detect oscillations on an 0.33mm jet that’s traveling at nearly 60-80m/s when it leaves the nozzle, and in 250 microseconds, the jet travels nearly 15mm (or about 45 jet diameters)! I ran the numbers, and in order to prevent the picture of the jet from completely blurring, I need to have the jet move less than 33 microns (about a tenth of the jet diameter) during the flash. The problem is that this implies a pulse duration of around 0.5 microseconds.

That’s really darned fast.

I discovered while doing some googling after our test run that the strobe I had at BYU that I thought was so crappy actually had a minimum pulse duration in the 0.5-0.8 microseconds range, and that that was actually on the high end for stroboscopes.

Here’s my problem. The thesis draft has to be far enough along that I can get my advisor to agree that I’m ready to schedule my defense by the 18th. This means that the thing is basically written and that I’m in the last minute polishing phase. The experimental validation is very important, and to drop that at the last second will make it a lot harder for me to get my thesis accepted. Since it turns out that the strobes we have just aren’t even close to what I need to pull this project off, I need to come up with another option. That means I need to come up with either a high speed camera setup, or a high speed strobe (with pulse durations less than 1 microsecond) within the next week.

The company that makes the strobes that I used at BYU (They’re General Radio Strobotac strobes) wants almost \$6k for the strobes, and doesn’t appear to rent them. There are a couple of places that sell “nano-pulse” systems that use xenon spark flashbulbs to create a flash of light that lasts less than 100 nanoseconds. Those would be even better than a Strobotac (due to the shorter pulse duration), but as far as I couldn’t find any of those for rent, or for sale used on ebay. It is possible to rent high speed cameras, but they tend to run several hundred to a few thousand dollars just for a day or two, which is out of my price range.

So, my question is, does anyone who reads this know a way I could get my hands on something like that within the next week? Anyone have any ideas?

~Jon

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#### Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.

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Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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### 6 Responses to Another Thesis Bleg: Short Pulse Duration Stroboscope

1. Ed says:

Maybe you could get your hands on an old Super-8 camera and modify its drive motor to take pictures at 2000 frames a second, and set up some really bright lights so that the film will expose properly in that short timespan. Then again, I don’t know if anyone even makes Super-8 film anymore.

Or perhaps you can do something with multiple cameras, sort of like they did in that scene from the Matrix with Trinity hanging in mid-air; I’m thinking an array of 10 cameras that are 36 degrees out of phase. With some software tricks you’d have a 4D visualization at high speed.

2. C. Scott Ananian says:

I’d read the datasheets for your favorite LED manufacturer very closely. The bandwidth of an LED is pretty darn high. The reason that strobes are used instead is because LED’s aren’t nearly as bright. But 500ns (2MHz) is pretty darn slow for an LED. You should be able to wire up a number of them (be mildly careful with capacitance) and compensate for the low brightness with a longer exposure. This depends on your ability to manually tune the LED frequency to match your pulse frequency, but it seems like you can control both sides of that pretty closely.

I think that should solve your problem.
–scott

3. Bruno Berger says:

As Scott said… try an array of high power LEDs. We got some good results with such a setting. Keep the capacity of the wiring an the driving electronics as low a possible.

Bruno

4. Paul Breed says:

I’d second the LED suggestion.

5. tankmodeler says:

If you have any contactes in the aero engine industry (and I mean the OEMs not the repair or parts suppliers) you might want to see if one of them will loan you a camera. You could let them have copies of your research. The hook for them is that it may have application to the flow from oil jets to bearings inside turbomachinery, always a prime concern to engine manufacturers.

The manufacturers should have several types of cameras and light set-ups to capture things like blade off tests and bird strikes. You’ll need to get right to the guys in the R&D or design departments to swing such a thing in a week, though.

I know this because I worked at such a place for 15 years and know that they generally have a number of cameras available (failed blade off events at 5000 fps are very cool!)and they always need more info on fine jet fluid flow.

HTH

Paul

6. Anonymous says:

How about a pulse laser (LED)? The pulses are measured in nanoseconds, you can pass it through a magnifying glass to cover more area, and you can get some very high speed film and expose it in total darknes with the shutter wide open (bulb setting).