The "Open" Part

As I said in the previous post, I've been working on things that run on magic smoke.   The board has reached a certain level of maturity where I'm willing to share.   By that I mean I've tested it and it all works - Its not all that pretty and there are some things I'm working on improving.   But the design is at a point where I feel someone else could pick it up and have a reasonable chance of reproducing it.

So, I've uploaded all the relevant information, schematics, layout, gerber files, PDFs to a project page on the www.rocketmoonlighting.com site.

Check It Out

• Parallax Propeller P8X32A - 8 cores, 80Mhz each
• 8 12 bit, single ended Analog input channels, max 44k total samples per second
• 3 high current, low side MOSFET drivers (approximately 20A each)
• 12 servo channels, independently configurable as inputs or outputs
• Direct drive of AC Delco D514 ignition coil
• Onboard MPU-6050 Six-Axis Motion Sensor
• Designed to run on 2 cell Lithium Polymer battery

Electrons

I've been working on some engine controller modules lately - not much rocket plumbing related things going on.

Below is a picture of some of the boards I've had made.  I'm expecting rev 3 to be delivered early next week.  This is my first "serious" circuit board design effort and I've found it to be more challenging than I thought initially.   The 2.5 x 3.0" board replaces a stack of 2 2.5x3.5" boards in addition to adding some functionality.

I'm planning on making the design available when its finished.

Valves By Request

I had a request for additional details on some of the valves I use.   I polished up my drawing and loaded it on the website, www.rocketmoonlighting.com/valves.   It includes a little bit of useful details about o-ring material as well as McMaster-Carr part numbers.

Open Source Hardware is all the rage these days.  I'm not sure that OSH will be quite the revolution that OSS was - replicating hardware from a "source" has a higher barrier to entry than replicating executable software from source.  It also takes more time and effort to develop new hardware - I wonder about the sustainability of OSH because in the end, people have to be compensated for their time somehow.

In any case, part of the point of this post is that I do requests!  So if people have things they want to see, drop me a line or post a comment.

Plastic Rockets

Sort of.   I ordered two of my latest chamber designs in plastic from shapeways.com.    The cost is very inexpensive and even though i can really fire them, they do help to visualize the design.   Its also useful for fitting up the chamber to other components like the test stand.

The picture above is the 50 lbf scaled up design that I did on request.   The material is Shapeways' Frosted Ultra Detail and it came out very well.   The regen passages are very crisp and well defined.   The only drawback is that they are still full of powder.  I actually mailed this part over to the individual who asked about the feasibility of scaling up the thrust.  

As you can see in the picture below, the 50 lbf version is comparable in size to my original chamber, which is only 12 lbf.   The original chamber was fairly conservative in size, and so the results of testing and additional analysis indicate that the chamber volume can be  quite a bit smaller for a given thrust level.   The other plastic model is version two of the 12lbf motor - notice the size difference.   This part was printed in Shapeways' Transparent Detail material and the results were not quite as good.   I'll be using Frosted Ultra Detail in the future.

3D Printed Rockets: The Cost Curve

If you've been around the rocket community, you may have heard of the "square cube law".  Basically, there are some physical parameters of machines that vary as a function of the square of length dimensions.   Thrust in a rocket engine for instance, is a function of the throat area, which is in turn a function of the throat diameter squared.   Volume and therefore mass is a function of the cube of a length parameter.   As a result, when scaling up the thrust of a rocket engine, the mass only increases with the square root of the ratio of the thrust.

So what?  In manufacturing, part of the cost of making a part is the amortization of the capital cost of the machine used to make a part.  In conventional machining techniques, labor and other costs also play heavily. However, in additive techniques like DMLS, the cost to make a part is largely driven by the cost of the DMLS machine:  as a result, the cost of a part is generally proportional to the volume of metal in the part.  


Consider the three chamber designs above.   The chambers progress up in thrust by a factor of 10 from right to left.   So the first chamber is 10 lbf thrust, the second 100 lbf, and the third 1000 lbf.   Because of "the square cube law" the chamber on the right only has about 10 times as much metal material as the chamber on the far left - despite having 100 times the thrust.   Do to the wonder additive manufacturing, we could estimate that if the 10 lbf chamber cost $1, then the cost of the 1000 lbf chamber would only be about $10. In real numbers, that means a 1000 lb chamber could probably be printed using DMLS for less than $10,000 - a pretty significant price point compared to other manufacturing techniques.

I recently got an email from someone asking about the feasibility of a 50lbf version of my DMLS chamber.  I spent a few hours and drew the above model.   Interestingly, this model has nearly the same metal material as Version1 of my DMLS chamber - which means that despite being rated for 5x the thrust, it could be made for roughly 1.5x the cost.   The primary mass reduction comes from the learning curve of the first DMLS chamber and being able to downsize based on the test results from V1.

More Testing

Here are three more videos of DMLS Chamber testing. The first two are long duration tests of the regen cooling:







This second long duration video is at a slightly higher O/F ratio, thus resulting in a lower flame temperature and reduced heating of the chamber walls.







In the last video, I was experimenting with camera settings trying to better capture the plume color and Mach diamonds. It didn't work too well. What did work well was the multiple starts/stops on chamber. Each stop/start is commanded remotely.

Chamber Designs

I'm trying to decided what/if my next chamber will be. Should it be purely self-pressurized like the first one? Or should I pressurize the propane to get Pc and therefore Isp and thrust/weight up a bit? Should it be about the same thrust level as the first at 5 kgf? Or should I go for more like 10 kgf or even 20 kgf?

Here is a picture of my design for essentially the same specs as V1, just refined for improved cooling and manufacturing.



Here is a derivative of the above design for a chamber pressure of 200 psi and thrust of 20 kgf.

DMLS Rocket "Group Buy"?

I'm finishing up design modifications to my DMLS printed chamber: http://www.rocketmoonlighting.com/projects/printed-chamber

I am planning to have the latest version printed sometime in the next few weeks - there are a few minor tweaks and improvements over the original which worked quite well.

There is a significant price reduction per unit when ordering more than one part, so if people are interested, it makes sense to buy more than one. Depending on the number of units, the price could be around half what it costs to make a single part. The price would be in the very low 4 digit range.

If you're interested, per the specs on the website above, drop me an email (i n f o at rocketmoonlighting.com). If I get a few responses, I'll do some homework and send out an email with more details to those who expressed interest.