Rocket Moonlighting

Update - Throttling the DMLS

2011-12-11T12:42:00.003-05:00

Just a short note - I tested the DMLS chamber in the throttling configuration. All the previous tests were with on/off solenoid valves. Thrust level reached approximately 80% before the test was stopped when the stand started rolling.

Testing Update

2011-08-13T11:04:00.013-04:00

The DMLS chamber has accumulated 18 starts and 152 seconds of firing time. The last test was 31 seconds in duration reached something close to steady state according to the regen coolant temperature measurement.

The graph shows the measured temperature of the N2O in the regen coolant jacket near the exit. Notice the temperature actually decreases in the last few seconds of the test, even though the propellant flow rates are constant through out the entire test. Notice also that the N2O is a gas at that temperature.

Some of the early tests ended with glowing red sections of the chamber. I think there are several things going on here. One is that there are some manufacturing flaws in the injector that result in a non-symmetric injection patter, which I think is making locally hotter combustion. Another is that, at low chamber pressure conditions, the regen design is really marginal - I spent most of my time making sure it would work well at the 100% thrust level. I think the main issue is that the channels in the barrel section of the chamber are spaced too far apart.

When I first saw the red-hot walls, my first concern was that the N2O was decomposing in the coolant channels. However, based upon the regen temperature measurements this does not seem to be the case. Also, the pictures and video indicate that the hottest wall sections are between channels which is the opposite of what you would expect if the nitrous was decomposing into N2 and O2 at 3000 degrees Fahrenheit.

The next tests will be focused on step-wise increasing the chamber pressure to the 100% level and decreasing the O/F ratio. Once I have the operating limits mapped out, I'll decide whether to revise the design, or move to the next step of the project.

Testing Update

2011-07-23T12:28:00.002-04:00

I've finally fired the DMLS regen chamber! Two tests so far of 2 and 3 second duration at 50% power level and relatively high O/F ratios

Here's the video of the second test:

What I've demonstrated so far-

- You can actually print a "tube stub" in your motor and use a Swagelock nut/ferule set to attach instrumentation

- My additional measurements (temperature and regen pressure) are working well.

- The direct spark ignition with the D514A coil is working.

- There don't appear to be any blockages or leaks in the regen jacket.

- C* efficiency is reasonably high - approximately 90%.

The big question still remains to be answered - will the motor survive long duration operation as the temperatures reach steady state.

No Testing Yet

2011-06-11T19:41:00.002-04:00

Everything is ready to go on the stand - all the instrumentation is connected and checked out - I just haven't made the time to buy another nitrous bottle and test. I'm measuring the following in this setup:

N2O supply pressure

Propane supply pressure

Regen jacket pressure

Regen jacket temperature

Chamber pressure

Oxidizer flow rate

I have a flow meter installed on the fuel side, but the flow rate will be too small for the meter. I'd love to measure fuel flow as well, if some one has a 0-1 gpm turbine meter that they're not using...

The injector for this chamber is a pintle configuration based upon the design I showed in previous posts. I had a local shop EDM the 8 0.014" injector holes in 1/8" instrument tube and then pinched off the end of the tube.

This is a picture taken through a microscope showing the EDM holes. The price was extremely reasonable - drop me a line if you interested having some small hole EDM work done.

Hopefully I'll be able to do some limited ignition testing soon.

What Its All About

2011-03-04T23:24:00.003-05:00

Engineering is about making things. For me, it is incredibly satisfying to conceptualize something and then make it exist and work as intended. I think that is why 3D printing technologies have captured my imagination - they are a step towards a Star Trek style "replicator". They offer the promise of shortening the loop between concept and reality.

I've been dreaming of different projects for the DMLS process for over three years now. A couple weeks ago, I took the plunge and had a chamber printed in 15-5 stainless steel. The first picture is the CAD, the second obviously the manufactured result. It really is a thrill to send electrons across the country and open a box a few days later to find the electrons made metal.

Now, some of you are saying "but you can do that with conventional machining" and you are right. But having a part machined has a certain activation energy - it requires a non-trivial amount of effort and skill. 3D printing is still not effortless - there are rules about what geometries are printable and of course you have to be able to generate a 3D model of your part. But the barriers are much lower. And there is also the fact that it can make geometries which are expensive or even impossible with conventional manufacturing techniques. It is still expensive - my part cost 4 figures even though it is quite small. In fact, it represents the larges single purchase I've made to date on my rocket hobbies. But as with any other process, buying in quantity is cheaper.

Everything in grey in the CAD picture came as is from the DMLS machine. As you may notice it is a regen chamber and instrumentation ports are integral to the DMLS part. Threads for the spark plug and inlet fitting were printed slightly undersized (extra metal) and then I cleaned them up with a tap by hand. The part came out more or less as I drew it, although there were some imperfections that affect the design. I'm talking with the vendor about ways to modify the design to avoid those problems on future builds.

Testing will follow soon. Had this been my "day job" I could have tested the motor the same day that it was delivered. Installing fittings and tapping out the holes took about an hour of slow work. The mounting ears fit directly on my stand as is. Unfortunately, life is a bit busy these days for me. And I still need to get some thermocouples for coolant temp measurements.

Questions welcome!

Happy Holidays!

2010-12-12T13:16:00.001-05:00

The Poor Man's Pintle

2010-11-20T23:59:00.002-05:00

A bit of a delay since the last post - quite a lot of things going on outside of blog land.

Two posts ago I showed a method for sealing off the end of a stainless tube. But to what end? I was brainstorming on a easy way to make a pintle injector. I took the tube shown in the previous post and drilled 8 holes in the end. The result is less than optimal because my mill's spindle is really too slow for the size hole I need. But the result was promising.

These tests were run with water in the center and air on the annulus.

Data Data Data

2010-09-05T10:35:00.000-04:00

Here is some data from the last igniter/small rocket test I did (video in previous post). In this case, I'm sampling chamber pressure at 5kHz. You can see its a little rough which is not particularly surprising given the injection arrangement.

This next plot is a FFT of the above data. You can see a low frequency mode at ~125Hz with multiple harmonics.

Tubes

2010-08-27T21:19:00.004-04:00

Lenox Laser makes some really nifty orifice fittings. I recently ordered some of the 1/4" tube fittings because I wanted some orifices for my igniter that are smaller than the 0.014" available as flare jets.

The 1/8" tubes got me thinking about another project and so I was looking at buying some of the smaller orifice fittings and modifying them for my own use. The problem was they really only have up to 3" lengths without added cost. And even at $30 they seemed a bit pricey to buy and carve up. A search of the web found no easy source for the blanks they make so I tried my hand at making one myself.

I first tried to make a mandrel using a 1/8" ball end mill. That didn't work particularly well. I had a ball bearing spinning tool for the lathe left over from another project, so I tried it.

It worked very well. The tube in the picture below is my very first attempt with the technique. I was anticipating having to spot weld the end but it is water tight to 60 psi which should be good enough for what I have in mind.

Additional Testing

2010-08-23T07:13:00.001-04:00

I modified the nozzle to a slightly larger throat area and added a diverging section. This test configuration had a 0.014" propane orifice, a 0.027" N2O orifice, and a throat of 0.201". It was running slightly lean at a chamber pressure of 110 psi. I plan to mount it to the thrust stand and get some performance data next.

Nitrous Propane Rocket Igniter

2010-08-13T10:05:00.000-04:00

Second video, slightly smaller orifice size, chamber pressure of 110 psi.

More Igniter Testing

2010-08-13T09:55:00.000-04:00

I am testing various nitrous oxide orifice sizes. This one generated about 135 psi of chamber pressure. And the video is in focus this time.

Loooooooong Time

2010-08-08T08:41:00.003-04:00

I've built a new igniter. Same basic design but a bit bigger and uses a 14mm plug, the Champion L78V. I've also included a port for measuring the pressure inside the igniter combustion chamber.

Here are a couple of videos of it in action.

Engine Testing

2010-03-21T22:24:00.001-04:00

Here is a video from the first day of "full up" engine testing. You can see that the igniter works and the fuel valve in motion. You can also see that when the main valves open, the combustion efficiency is pretty poor.

Sparks!

2010-03-07T16:45:00.001-05:00

I realize that I've probably done this topic to death but I spent a lot of time last fall on ignition systems.

So, here are a couple of photos of sparks. I was trying to capture the visual difference between the CDI and induction style systems.

The first photo is with the LS2 coil and a surface gap plug. This photo is pretty close to what you see with the naked eye, the purple color is very distinctive.

This photo is with the RCEXL system driving a standard small engine plug with the electrode cut off. The resulting gap is enormous and none of the induction systems produce enough voltage to jump that gap even at atmospheric pressure. The CDI system delivers less energy per spark than the induction systems I've tried. The resulting spark is pencil thin and bright white.

February

2010-02-28T20:57:00.003-05:00

The Franken-chamber has been tested with the torch igniter. The igniter has been very reliable and easily lights the main chamber. It is easily 10 times bigger than necessary and makes over 10% of the main engine's rated thrust running by itself. However, the size of the igniter is set by the smallest practical orifice size which is 0.014" in the NOS flare jets I am using. If I switched to vapor propellants, it could be made much smaller while making the plumbing much more complicated.

Testing of the main chamber has shown the combustion efficiency is extremely poor. The easiest approach to improving efficiency is to increase chamber length. With the stainless steel chamber, it was easy to cut the nozzle end off with the band saw and weld in another barrel section. The result is sort of comical looking - yet another testament to the limits of scaling rocket engines

I hope to be able to test again soon. In the meantime, I've already starting working on the design of the next engine as this one has about reached the end of its patchwork life.

A Non-update

2010-02-10T20:12:00.003-05:00

I've been testing the Franken motor as well as another igniter for a different application. Finished upgrading the test stand hardware and software with flow meters for both oxidizer and fuel. That has already been very useful as I was able to accurately determine the O/F ratio the igniter is running. It will light as rich as 7:1 but runs very rough. Smoothest operation seems to be right around stoichiometric at 10:1 or a bit leaner.

I've also been working on an avionics experiment that is coming up this spring. More on that to come soon.

Hybrid Tips

2010-01-16T19:43:00.002-05:00

At 8 bars chamber pressure and 3.5 nozzle area ratio, a nozzle is horrifically over expanded. An over expanded nozzle will contribute significantly to poor ISP performance

c* is a better measure of combustion efficiency, being a function of mass flow rate, chamber pressure, and nozzle throat area. At low chamber pressures, use frozen chemistry rather than equilibrium in PROPEP.

It is common practice in hybrid motor design to include a "pre-chamber" upstream of the fuel grain - it will help improve performance and stability.

Back when I was doing N2O/HTPB hybrids, I measured c* efficiencies from 50% to 84% all with the same hardware - changing mainly fuel grains.

Ozone Part II

2010-01-15T21:09:00.002-05:00

AC Delco Coil Near Plug - aka "The LS2 Coil"

This is where it gets interesting. The OEM application for this coil is the GM LS2 V8, powering Cameros, Corevettes, and various GM trucks and SUVs over the past decade (PN# D514A). It is termed coil near plug because they are mounted on the valve covers and there is one for each cylinder. The complete setup with coil, weatherproof pigtail, and sparkplug lead can be had off ebay for around $45 (from different vendors, of course). This makes it the second most expensive setup but I think well worth it.

This coil is immensely popular with the DIY electronic engine control community. You can read all about it on this page. The most attractive feature of this coil is that it can be triggered with almost any signal that rises above about 3 volts. In automotive lingo it has a "built-in igniter" meaning the power transistor, protection diodes, and noise suppression components are in the coil package itself and so it can be triggered by a low-current logic level signal. It even has a dwell limiting feature which keeps the coil from burning up if the trigger pin is held high too long. I am triggering it directly from a logic pin on my 3.3V micro-controller - not a single component between the micro's pin and the coil connector. The optimum dwell time is about 5 ms at 12V, which is perfect for a 50% duty cycle at 100Hz - I've run it up to 200 sparks/second with good results as well. Spark energy is visibly higher than the other three inductive systems with a fat, purple spark. It pulls a healthy 2.9 amps at 100 sparks/second and should run on power from 6-16 volts with no issues. When you throw in OEM automotive engineering and reliability (even if it is from GM), this system looks really attractive.

RCEXL CDI System

This is a capacitive discharge ignition module for radio controlled small engine applications. Like the LS2 coil, it may be triggered via a 3.3V low current signal. It is designed to operate over a more narrow range of voltage (4-6V). As a hobby system, I would expect that the engineering, quality, and durability is somewhat less than the LS2 coil. However, it is a CDI system and should deliver higher voltage to the plug. It pulls about 0.5 Amps at 5V and the sparks are pencil thin and bright white. The spark plug cap has a unique metal shell and shield high voltage cable that grounds the ignition system directly at the plug and helps to reduce radiated noise. However, as I discussed in a previous post, this ignition seems sensitive to shorts of the spark plug - it is far more likely to cause processor resets with a short than the other inductive systems. It weighs only 150 grams with cable and boot which is about half of the LS2 coil setup.

Wrap Up

I think the LS2 setup is the obvious winner for most applications given the level of engineering, simplicity of interface and integration, and operating voltage range. It might save a pound over a MSD type system and would probably provide better reliability. The RCEXL system might be preferred if you were on a severe weight or power budget or you were trying to ignite mixtures at high pressures (+200 psi). For my igniter, which lights at just over ambient and runs at 100 psi, all four options ignited with no measurable differences.

The Smell of Ozone

2010-01-12T21:27:00.009-05:00

As I mentioned in my previous post, I tried out several different spark ignition systems as potential replacements for the C&H CDI box I have been using.

From left to right are a spare coil for my '76 Ford Pickup, a Tecumseh small engine coil, an AC Delco "coil near plug" module, and the RCEXL CDI System. The first three are all designed to be used in inductive discharge, or Kettering, ignitions systems. Most of the testing I did was at 100 sparks per second. I did not measure spark energy or current as I don't have the equipment necessary.

Ford Can Coil

This is a fairly standard coil used on almost any older car with a distributor and points setup. One can get a coil like this at any automotive parts store by asking for a coil for a random car built before 1975 or so. It is easy to drive with almost any transistor per advice from Thomas McNeill. I actually used a power MOSFET that I have been using to control solenoid valves. I tried this coil while I was waiting for others to ship because I had it already on the shelf. I wouldn't recommend using it otherwise. It generated the weakest spark at the lowest cost of the four options.

Tecumseh Coil

This coil was tested by the folks over at the Rocket Work Bench website (PN# 30560A). It was inexpensive and gave fairly good results so I thought I'd give it a try. As was noted before, it gets fairly warm, especially running at 100 sparks/second. That is not really surprising as it was probably designed for around 3000 RPM or 25 sparks/second. It does seem to produce a fairly energetic spark and is triggered the same as the can coils.

I'm going to make this a multi-parter because its late and I'm being paged.

To be continued ...

Configuration Control

2010-01-09T17:39:00.000-05:00

Those of you who work with software know what SVN and CVS are. And you've probably found yourself in the position of having code that used to work several versions ago, but you've messed it up and you pull your hair out trying to figure out what went wrong. So, what you do is go back to the last working version and start over, making smaller changes.

It sure would be nice to have SVN for hardware.

I posted a video of my torch working well. I then welded it onto the chamber for testing and found that every time the spark triggered, the microcontroller would reset. In the standalone configuration, I had never experienced a reset. Unfortunately, I had made several changes at once that contributed to the resets.

First, I had made a new custom PCB to replace the protoboard I was using previously. Because the board was new, I focused on it as the source of the reset issue. I did eventually find an issue with the ground plane design which was contributing to the resets (It is the first PCB i had ever designed). However, the resets continued but now they only occurred when propellants were flowing in the igniter!

At this point I began to suspect that my ignition box was bad. I have been using a CDI box designed for RC engines for some time now and thought perhaps it had been damaged. After perusing this page: http://rocketworkbench.sourceforge.net/igniter/igniter_blog.html and talking with some of my fellow Arocket-eers, I thought I would try a simple inductive coil setup. I bought via mail several different hardware options including a replacement for the CDI I had been using (I'll post a follow on about all the different ignition systems).

The various setups still suffered from similar problems - at this point I was baffled! Eventually, I made an Edisonian discovery on the bench - if the spark plug electrode was shorted to ground when the coil attempted to fire, it would cause a reset every time. Placing a bead of water over the electrode and touching the ground had the same effect.

As it turns out, when I welded the torch to the chamber, I had welded a new spark plug bung onto the igniter. This was done as an afterthought - I wanted to clean up the multiple bungs I had welded onto the igniter when trying different threads for different plugs. My intention was to maintain the exact spacing, but I must have made the new bung slightly shorter than the original.

I added a spacer to the bung to move the tip back just a little --- and everything worked just as it had before. I inject both propellants as liquids and apparently the spark plug tip was seeing too many liquid droplets, causing the spark to not fire and the resets. All of the ignition systems work just fine in this configuration, including the original CDI box.

So, SVN for hardware would have saved me tons of time, headache, and grey hairs.

Stay tuned for a post about ignition options.

Why TIG?

2009-11-08T19:56:00.003-05:00

A short while ago, someone on a mailing list I frequent asked "Why TIG?". In context, he was asking why TIG is preferred over other forms of welding, such as MIG. The above picture illustrates one of the reasons. Keep in mind, I'm basically a rank amateur when it comes to welding...

Frankenrocket

2009-11-06T21:23:00.003-05:00

I've welded the torch igniter onto my current chamber. As you can see, its rather disproportionate ... a good example of "minimum gauge" issues. The igniter is built around the smallest reasonable orifice sizes (0.014") for the fuel, and the various design constraints lead to what you see here. It was dubbed the "Frankenrocket" by my brother ... and as such it will be known.

I hope to be able to test it this weekend, but in the meantime, I've been working on the next iteration of this engine. I'd like to get away from the igniter for obvious reasons and get back to direct spark. So with some cleverness, a 8mm thread plug, and a step up in my fab skills, I think I've come up with a very compact design that moves the spark plug back onto the injector face and still yields a much smaller engine. I've made a fair start, but I burned out some MOSFETs on the lathe, so I'm stuck waiting for Mr. Postman...

What does 100 bucks look like?

2009-10-27T20:23:00.003-04:00

This:

From left to right, a standard platinum plug for my car, CR8HIX, R0045J, CM-6, R847, and ME-8. All of them except the first are NGK. All of the interesting NGK plugs are racing plugs and run about $25 each.

The ME-8 has a 1/4-32 thread and is TINY. It is basically the same size as an R/C glow plug. In general, it looks way too fragile and still costs twenty bucks. I'll be trying the R847 next.

More Igniter Testing

2009-10-25T14:17:00.005-04:00

As you can see in the photo, the NGK surface gap plug is holding up well. This is the first plug of its type I've used and you can see that it looks basically untouched after 30 firings or so.

I did some more testing of the igniter. As it turns out, it appears to have been running extremely rich before. I leaned it out some to get smoother operation and there is a big difference in the metal temperature. At the previous mixture ratio, the steel never glowed red.

Fire again

2009-10-17T09:28:00.003-04:00

Over the last couple of months, I've been testing direct spark ignition of my NOP engine. It works, but it can be flakey, and usually makes a rather disconcerting "pop" at ignition. I also tried RC glow plugs, but with standard currents and voltages, they don't provide enough power (i.e. heat to the coil). I think that with PWM you could control or set the power level to get the right wire temperature but that's a rabbit hole I didn't want to go down right now.

So, I finally decided to build a torch igniter. I've been testing various plumbing configurations, spark plugs, and orifice sizes and I think I've arrived at something that works fairly well. It accepts liquids for both oxidizer and fuel, which is nice. I'm not sure if other's torch igniters run on on gaseous oxidizer or not.

The next step is to weld it onto the motor.

Progress?

2009-08-10T22:03:00.003-04:00

So, I was out of town for a week on my day job. But right before I left, I was able to try out the Omega amplifier that I bought. The good news is that it works perfectly. The bad news is that the signal it outputs has the same amplitutde as its power source, which in this case is 24V. My TTL counter on the DAQ board didn't mind the 24V one little bit. However, when I went back to use the A/D section of the DAQ, it didn't work. A quick inspection revealed burnt resistors... probably from trying to sync all the current from 24V. So, the board went back to the manufacturer. And I have a non-working test stand.....

I took the downtime as an oportunity to build a Thermocouple amp that I've had partially completed for a while. I now have a four channel amp that outputs 1 mV/degC from Type K thermocouples.

Still no fire...

Sucess is Bittersweet

2009-07-27T14:45:00.003-04:00

Last night after ordering the Omega amplifier, I went to go put away all my electronics gear. Just for kicks, I thought I'd try to get the LM339 based circuit to work one more time. An Eddisonian inspiration led me to try grounding one of the magnetic pickup lines at the amplifier, and wouldn't you know it, the noise problems went away.

So, the circuit below seems to work perfectly in my application. To recap, I have the following

- A Cox AN8-4 turbine flow meter

- A magnetic pickup which outputs a 10 mV minimum peak to peak signal

- A Measurement Computing 1208FS

I'm sure that it never would have worked had I not bought the Omega unit.

Make vs. Buy

2009-07-26T22:06:00.002-04:00

I recently bought a nice turbine flow meter off Ebay. I was planning to use the counter on my DAQ board to measure the frequency of the turbine and convert frequency into flow. Unfortunately, the flow meter magnetic pickup turns out to have a millivolt peak to peak output and the counter expects a TTL (0-5V) signal.

So, says me, "Self, how hard can it be to cobble together some Radio Shack parts to fix this problem ..." After a little bit of work and a trip to the store, I ended up with a circuit that sort of works, based on a LM339 comparator IC. The schematic is below.

Now, this particular setup has a couple of features.

1) When the turbine is not spinning, the DAQ counter reads 60 Hz. This is actually not an issue, because 60 Hz is below the low limit of the turbine. I simply have to remember that if the DAQ board says the flow rate is 0.125 gal/min, its actually zero.

2) If i touch any part of the circuit between the DAQ card and up to and including the body of the meter itself, the DAQ counter reads about 20 kHz. This IS a problem, as the max frequency of the meter is about 1800 Hz. However, its not clear to me if this would be an issue in operation. I seem to remember something about grounding problems causing these kinds of issues....

Now for the point of the post. Omega sells a nifty little box that is designed to do exactly what I need in this case. It costs $70. So, should I have just bought one instead of futzing around with making my own? I mean, $70 is a pretty nice dinner for me and the Mrs. So far, I have about 3 hours into this tiny project. Call it 4 hours by the time i transfer the project from the bread board to a perf board and come up with some sort of enclosure. So, is the value of my time less or more than $17.50/hour?

And here it is ...

2009-06-26T22:38:00.002-04:00

Tonight, I drilled the injector holes and welded up the motor. The injector holes were surprisingly simple to drill at 0.06", 0.022" and 0.013". I did find that the middle sized drill bits were only good for about 6 holes in the stainless.

With the exception of the spark plug bung, I'm very happy with how it turned out. Welding is definately simpler than trying to design, fab, and assemble a bolted joint.

A night's work

2009-06-22T21:45:00.002-04:00

Its like watching paint dry ...

2009-06-21T20:00:00.005-04:00

Well, the good news is my day job has slowed down a little ... bad news is that I still haven't made any fire and smoke :(

After running through multiple repair cycles on the injector head, I finally gave up about two weeks ago and started building another injector. The one I have was designed to be bolted on to the chamber. I used that design because I didn't have a welder and I thought it would be nice to be able to remove the head and swap it out. Since I now have a welder and have been practicing with stainless steel, I've since come to the conclusion that welding is a MUCH simpler way to go (which is the advice that John Carmack gave a while back). If you do end up swapping injectors, its really not a big deal at all to cut the old one off and weld the new one on.

The new design also moves the spark plug off of the injector itself and onto the chamber. This was done to free up the space and make the injector much simpler to machine.

While I don't quite hate stainless as much as Paul Breed, I do find that it is much less forgiving than other materials. The main thing is to simply be patient - use the recommended cutting speeds and feed rates which are an order of magnitude less than aluminum. On the plus side, it is showing me some bad machining habits that I didn't know I had.

Below is a picture of the part I finished today, the cover for the fuel manifold on the injector. Also in my hand are two ruined 1/8" cobalt end mills. One I simply broke from over-feeding (feeding to fast/too much force). The other one I literally cooked from over-heating. It still cuts, but it more closely resembles a ball-end mill :)

Thermocouples

2009-05-02T21:09:00.003-04:00

Still no testing to report but in the interest of keeping the blog alive, I thought I'd share a minor little tidbit.

If you want to measure the temperature of a metal part, say a combustion chamber, it can be hard to get a TC bead in good contact with the metal. A better approach is to actually weld the TC wires to the part. Through the magic of TCs, you can actually weld the two wires independantly. Tonight I thought I'd try it out on a piece of scrap stainless tube with the TIG welder my wife bought me last year.

It turned out to be much easier than I thought it would be. (Please ignore the practice spots :)

Magic Smoke

2009-04-07T22:00:00.004-04:00

Progress is still crawling along at glacial speeds due to my day job. I have flowed propellants through the motor, discovered some leaks, repaired leaks, and reassembled. The motor is on the stand once again and awaiting time for testing.

Tonight while I was testing the ignition system to see if the trigger rate had any effect on spark quality, I was reminded of what I learned in EE lab - semiconductors run on Magic Smoke. If you let the smoke out, they don't work anymore.

I'm using an ignition module designed for small R/C aircraft engines from CH Ignitions.

After being triggered at 200Hz for 10 minutes or so, it suddenly stopped sparking. Because it was warm to the touch, I decided I would disassemble it and confirm that I had let the smoke out.

200 Hz corresponds to about 24,000 RPM in a single cylinder 4 stroke, so its not great surprise that it quit on me. I've also not lost much time because I originally (2 years ago?) bought two modules. And that reminds me of one of the things I've learned working on these projects - Always buy at least one more than you think you'll need.

Where does time go?

2009-02-22T21:05:00.002-05:00

I can't believe that Feburary is almost over. Due to my day job, I haven't had much free time to work on the new N2O-propane motor. However, today I finally finished mounting it on the test stand. I intend to test ignition and thrust levels using very short duration tests. After I've gotten that worked out, I will go through the trouble of adding the regenerative cooling to the chamber.

The Rest of the Story

2009-02-17T20:21:00.012-05:00

Here are the results for the other two cases...

M 1.851

p 29530 Pa

rho 0.398 kg/m^3

T 258.4 K

V 596.4 m/s

Drag 1750 N

Cd 0.764

M 0.936

p 27640 Pa

rho 0.377 kg/m^3

T 255.6 K

V 300.0 m/s

Drag 327 N

Cd 0.596

Unreasonable Request

2009-02-16T05:02:00.007-05:00

I offered to run some CFD for Paul Breed of a simple "rocket like" geometry - a 8" by 8' tube. I picked an arbitrary ogive nose cone and a bluff tail.

Here is the Mach number contour for the 150 m/s case.

The free stream conditions were:

M 0.468

p 27580. Pa

rho 0.376 kg/m^3

T 255.4 K

V 150.0 m/s

Drag 55.1 N

The calculated Cd is 0.40. 300 m/s and 600 m/s results to follow.

Test Fitting

2008-12-11T20:32:00.002-05:00

Another Part

2008-12-05T13:36:00.003-05:00

Shiny New Parts

2008-12-02T17:50:00.004-05:00

This is the injector for the next version of the N2O-Propane engine.

What is Love?

2008-11-26T17:30:00.003-05:00

My wife bought me a Miller TIG welder as an anniversary present. I love her very much.

Happy holidays!

A Sunday Project

2008-11-03T06:35:00.003-05:00

This is what I did yesterday. In total, it was about 6 hours of work. This is my first project in stainless steel and I was a bit nervous after having read of the problems others have had. I'm releived to say that it wasn't as bad as I expected. I did shatter two carbide inserts threading the external threads. However, the trick seems to be to take very light cuts.

The brazed in tubes are new to me also. The braze material is a Silver with Nickel alloy from McMaster. Torch brazing them wasn't much harder than pipe soldering although you do have to be carefull about burning the flux.

Valve Speed

2008-11-01T09:15:00.005-04:00

Some one asked how fast the valves move with these actuators. I did a test this morning with a 6 cell NiMH battery.

Here is a full speed video of an open/close cycle with no pressure:

Here is the same video at quarter speed:

Here is a quarter speed video of a second test at 750 psi line pressure:

With no pressure, the average frame count of the open and close was 8.5 frames which corresponds to 0.283 seconds/90 degrees

With the 750 psi line pressure, the frame count was 9.5 frames, or 0.317 seconds/90 degrees.

Valve for Sale

2008-10-30T22:32:00.003-04:00

This is the latest version of the valves I use. I have one listed for sale on ebay: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270295521784&ssPageName=ADME:L:LCA:US:1123

-RM

Work in Progress...

2008-10-23T07:12:00.003-04:00

New Website

2008-10-12T20:21:00.003-04:00

A blog isn't really a great archival medium so I've created a new website, www.rocketmoonlighting.com. I can still use the blog for, well ... blogging ... but information can be organized by project on the website which may make it more useful.

I've already populated a few project pages. Some of the pictures you've seen before, some not. Among the new stuff, you'll find some drawings of the restartable hybrid motor and the new bipropllant motor.

Requests?

2008-09-21T04:32:00.003-04:00

For personal reasons, I won't be bending metal or testing much over the next couple of months. I will however have time to deskwork like posting to the blog. I was wondering if anyone had any requests. Design? Drawings? Analyses? Data?

The video from this previous post was taken during testing of a bi-propellant engine I've been working on. I've been testing various chamber lengths trying to get good combustion. Here's a picture from a later test. You can see the Swissflow flow meter bottom left.

Flow Meters

2008-09-08T07:15:00.003-04:00

I've been using the SF800 from Swissflow lately to measure propane flow rate. It works well once you realize that the red wire is ground not positive supply. It uses BSP threads which are slightly inconvenient as well. The version I'm using has a fiber reinforced plastic housing and is rated for working pressures up to 230 psi. They have a metal bodied version rated up to 3600 psi that I plan to use with N2O.

Instead of using a steel rotor and a hall effect sensor, this meter has a plastic rotor with an IR emitter/collector pair. It outputs a nice square wave pulse with frequency proportional to flow rate. My DAQ board (from Measurement Computing) has a counter input which can be configured to measure frequency so interfacing to the existing DAQ software is quite simple.

Summer Doldrums

2008-07-23T07:11:00.003-04:00

Things have slowed down a little the past month but I have been working. Not much reportable progress lately however.

I did do a test on a "new" project - it didn't go perfectly. A short video:

Floats as Tanks

2008-06-20T06:54:00.003-04:00

This is a picture of hydrotesting I did on a 6" 316 Stainless steel float from McMaster-Carr (2762K49). It failed at 2150 psi. Interestingly, the failure was a tiny pinhole that opened right at the weld termination. My grease gun was not able to pump enough water fast enough to get the pressure higher higher than 2150 once the hole opened up.

I proof tested a second of these floats to 1900 psi without failure. And 3 more were tested to 1500 psi.

Valves (again!)

2008-06-10T07:19:00.004-04:00

I have developed a direct drive version of the valve I've been using. It was used in my last testing which was described in the previous post.

The packaging comes out quite nicely. They have several advantages to the linkage actuated version - this will be my go forward design.

Along those lines, I have been wondering if there are people out there that might like to buy a pre-made valve like this one. It was actually more of a pain to build than I thought it would be and personally, I prefer to spend my time building rockets rather than various peripheral devices.

Here are some of the relevant details:

CV of 1.6 and is rated for 1.6 gpm of flow at 1 psi delta and 5.1 gpm at 10 psi
rated to 3000 psi
can throttle fairly well and has fast response (as can be seen on this blog)
o-ring sealed and can be compatible with most non-cryogenic propellants, including hydrocarbons and hydrogen peroxide
the servo would be a Hitec digital servo with titanium gears (6.0V or 7.2V input)

If you're interested in buying a valve like this, drop me an email.

Fits and Starts

2008-06-02T07:08:00.003-04:00

Long time, no post. I wish I could say the delay was due to my making lots of progress and not having time. The truth is much closer to not having much to share.

I was able to get out and test some modifications this weekend. The good news is that the urethane o-rings seem to be working. I did a long term pressure test with N2O to see if they would remain sealed for several days. Also, after the testing this weekend they showed no signs of damage.

The main point of this weekend's testing was to test a new configuration with the propane/spark ignition with the goal of eliminating the initial pressure spikes. The new configuration seems to have reduced the tendency towards hard-starts. Unfortunately, the ignition sequence which was quite reliable before is no longer. I spent quite a bit of time just getting it to light at all and stopped testing without arriving at a reliably repeatable sequence.

Valves

2008-05-04T09:07:00.001-04:00

Actuated valves are a tricky thing for rocket motors. Their requirements usually exceed what is available from the McMaster catalog. Of course, if you are willing/able to pay big aerospace money, you can have pretty much what ever you like. Unfortunately, most amateur and "new-space" organizations don't find themselves in that situation.

A common solution is to use ball valves. They have several advantages including reliability, availability, and some throttle-ability. However, the packing around the ball generates considerable amounts of friction, especially when the valve hasn't been moved for some time (sticktion). As a result, ball valves require rather significantly sized actuators which are often heavier and larger than the valve itself.

While browsing through the Swagelok catalog some time ago, I discovered a valve they call a "plug valve". It is similar to a ball valve but with a cylindrical rotating plug that is sealed with o-rings. The result is a valve that has very low friction and thus can be actuated with correspondingly smaller actuators. The primary downside is that the o-rings become a consumable item and prohibit their use with cryogenic fluids such as lox.

Here is a picture of a plug valve actuated with a Hitec servo.

This is a movie of some early testing I did with a garden hose and a standard Futaba servo.

If you look closely in the video in this post, you can see the valve moving in the lower right-hand corner.

As I alluded to in the previous post, I've had several o-rings tear when being closed after use at part throttle with N2O. The valves come from Swagelok with PTFE coated FKM o-rings. I assumed they were not rated for lower temperatures so I ordered some PTFE and urethane o-rings. Below is a disassembled valve with a urethane o-ring.

Last night I did some simple testing with the urethane o-rings and to see how cold the N2O is getting downstream of the valve. The picture of the setup is below showing the valve, thermocouple, and injector orifice.

As you can see, it gets pretty cold. At minimum throttle, the temperature reached a steady state value of ~ -84F after 20 seconds or so. The urethane o-rings are rated to -65F and the PTFE rings are rated to -100F. I'm going to try the urethane rings first because the PTFE rings are very hard.

Last up is a neat picture showing the formation of solid N2O at the orifice exit. Visible are both ice-sickles and snow.

In summary, with the right o-ring material, these valves should work for all storeable propellants such as ethanol, kerosene, and peroxide. They should also work with saturated liquids stored under their own vapor pressure, like ethane, propane, and N2O. The odds of working with LOX are slim to none.

10 Points for Damien

2008-04-23T22:00:00.004-04:00

Recently, I've been working on integrating my test setup onto a trailer so that I'm not constrained by working in the garage. I haven't had any complaints but I felt that doing 60-90 second burns might be pushing my luck in the future.

Last night was the first attempt at using the trailer to test and I was very pleased with how it all turned out. It took almost exactly 1 hour from the time I arrived home from work to the first test. During that time, we loaded the van, drove to the test location (about 10 minutes), and setup the electronics on-site. There were no issues with the test stand and everything worked correctly the first time.

If only the rocket motor were so easy ...

The light sequence is very reliable. I actually had two lights that are not shown in the video - for the day, the motor was 5 for 5 on light attempts. Unfortunately, there is still a fairly high pressure spike at ignition that was triggering a redline on chamber pressure. I had lowered the chamber pressure redline to just a tiny margin above the design maximum - the startup spike is around ~25% higher than that. On the first two tests of the day (not shown in the video), the software redlined at engine start and aborted the test. For the the first two tests in the video, I raised the redline limit and the motor started fine and executed a scripted throttle profile. On the last test, I knew that there was probably a damaged seal in the throttle valve but I decided to initiate a test anyway (the venting between the second and third light is a result of the damaged seal). The end result of that test is obvious - another blown gasket.

I'm not sure yet what the exact cause of the blowout was - the data did not show a pressure spike that was larger than any of the previous tests that day. I have not yet reviewed the data in detail nor have I disassembled the motor to inspect it.

So I have three problems to solve with this motor right now:

1) Pressure spikes at start up - I think I have a plumbing solution to this
2) Blown gaskets at start up - I think solving 1) should help solve this one. Maybe a new gasket material is in order.
3) Repeated failures of valve seals - More on this later

PS - Damien gets 10 points. The propane bottle can be seen in the video at the center of the screen. However, I would not call it a "torch".

Ignition Part 1.5

2008-04-14T06:59:00.003-04:00

So, I've been remiss at posting. Its going to be a few more days before I post "Ignition Part 2". Rather than offer excuses, I'll offer a hint. The picture is a detail shot of the injector side of the motor where all the magic happens. 10 points to anyone who can guess how it all works.

PS - the tank at the bottom of the picture wrapped in electrical tape is a red herring.

Ignition - Part One

2008-03-26T20:53:00.000-04:00

There have been several questions about my ignition system. I thought I would start with what I tried that didn't work.

Some time ago, I began researching the idea of initiating nitrous oxide decomposition with spark energy alone. My literature research turned only one relevant reference, documenting work performed by Pratt and Whitney and Rocketdyne.

"Investigation of Decomposition Characteristics of Gaseous and Liquid Nitrous Oxide", Air Force Weapons Laboratory, Kirtland Air Force Base, NM. 1974.

Pratt and Whitney was able to initiate sustained decompositions reactions in pure nitrous oxide with very low spark energies. However, the reaction was only sustainable in large diameter pipes and at pressures of several atmospheres. In addition, diluting non-reacting species such as helium or nitrogen significantly reduced igniteability.

I decided I would try it anyway.

This is the injector head that I designed for this experiment and am still using. The chamber side of the injector (minus the spark plug) can be seen in this post. It has three ports - two 1/4" NPT and one 14mm. The outboard NPT port is for measuring chamber pressure. 14mm is standard spark plug thread.

The spark plug is a standard Autolite platinum plug I had laying around. I'm using a standard ignition module from CH Ignitions. After trying several different approaches, I ended up triggering it with an old HP function generator I had. I found out from Paul Breed - after I ordered my unit - that CH will sell a custom box that sparks continuously when ever it has power.

So, after designing a chamber and a nozzle, casting fuel grains, shaking down a throttle valve, setting up new DAQ software, and getting the spark box to trigger ....

I found out that it doesn't work.

I was unable to measure any indication that the N2O was sustaining a decomposition. No chamber pressure rise, no clearing of the cloudiness in the exhaust. I tried several different throttle settings and spark rates and finally gave up and moved to plan B.

Which I will talk about in Part Two ...

Aluminum Brazing

2008-03-21T20:43:00.007-04:00

On the 4.5" hybrid motor I'm currently using NPT fittings in the injector to inject propellant and measure chamber pressure. However, there are many reasons to avoid using NPT fittings in injector heads - packaging, leakage, cost just to name a few. Unfortunately, there are not many good options if you don't have a welding machine, especially working in aluminum.

I am considering starting project which is too small for NPT fittings. I would like to use Aluminum brazing materials for attaching the propellant tubes directly to the injector heads. I bought the brazing material from Durafix some time ago for another fabrication test that didn't work out. McMaster-Carr also sells aluminum brazing alloys. Before I machine the injector, I thought I would test out the joining method with a hydro-test - if it doesn't work I'll have to modify my design.

This is a picture of the finished joint on the test piece. The test article is just a 3/8" aluminum tube brazed to a round rod which was left closed on the end. Both the rod and the tube are made of 6061T6.

The test procedure is simple. I fill the apparatus shown below with water and attach the free end to a standard grease gun. I fill the the grease gun with water as well and then pressurize the entire system. I got the grease gun at an Ace Hardware and its really nothing special. The box says its good to 6000 psi, but I've only ever had it up to 3500. It is very important when doing this kind of test to fill the entire apparatus with water - don't use air!

The end result was no leaks, so it looks as though brazing could work as a low cost method for joining aluminum tubing to manifolds.

More Restarts

2008-03-16T21:28:00.006-04:00

Here is a picture of the failed gasket from the hard start.

Other than the failed gasket, there were no problems with the motor. I made a new gasket and reassembled.

Today I tested out some modifications of the start sequencing. The results were good - 4 lights in a row on 5 tries with no hard starts. Unsurprisingly there is some art to getting a sequence which lights reliably without hard-starting. Moving the valve closer to the chamber gave better throttle response as well. I'm working on a post with details of my ignition system (you'll be disappointed). Unfortunately, I'll be out of town this week on business so this movie of today's run will have to do for now.

Random Movie

2008-03-05T21:11:00.000-05:00

This a movie of one of the regression rate tests I did to collect the data I showed in this post. Most of my movies are rather unsteady as this one is because the image stabilization feature on my camera was ON. Newer videos are much more stable.

The only interesting thing about this movie (other than being a rocket motor firing) is that it demonstrates "chugging". The popping at the end of the run is a low frequency oscillation in chamber pressure that is most likely caused by elasticity in the propellant feed system. The issue only ever manifested itself at the very end of a run, when the tank held only N2O vapor and the pressure drop across the injector was relatively small.

Redlines and Blown Gaskets

2008-02-24T19:19:00.000-05:00

I performed two tests today and while I basically failed my objectives, there was some good news. I moved the oxidizer valve closer to the injector hoping to get better throttle response. I also modified the ignition sequencing. The net result was less than desirable - I had a hard start on the second test of the day.

As you can see in the picture, there is fire coming out of the flange that holds the injector plate onto the chamber - the gasket failed when the combustion chamber pressure hit ~1000 psi momentarily. The good news is that there is no other damage to the motor and gaskets are easy to make.

The other bit of good news is that my redline software performed exactly as it was intended to do. I created a module in the DAQ software that triggers when any of the 4 analog channels I'm monitoring (thrust, chamber pressure, injector pressure, tank pressure) exceed predetermined values. When the redline module triggers, it sends a logic signal to the engine controller module and the engine shuts down. In both tests today, the redline was triggered and the oxidizer valve closed. The second test, pictured above, had a duration of about 0.5 s before the valve closed.

Saturation

2008-02-13T21:24:00.000-05:00

I thought I'd share a couple of pictures which highlight the differences between injecting liquid versus gaseous or saturated propellants into a combustion chamber.

This first picture is of a single injection orifice flowing water from my garden hose - the stream is very laminar with very little dispersion. Engines with liquid injection usually use impinging streams to accomplish the mixing that is required for good combustion stability and efficiency.

Robert Watzlavick has several good pictures of an impinging liquid-liquid injector on his website, in addition to this one.

The last picture is of the same injector in picture number one flowing liquid N2O instead of water. In this case, the saturated liquid flashes to vapor immediately due to the much lower atmospheric pressure. The resulting plume is easily 10 times the diameter of the injection orifice. Gaseous injection would probably not result in such a large plume but would still expand much more than the water above.

Restartable Hybrid Movie

2008-02-12T22:08:00.000-05:00

Here is a movie of the motor that I have been working on recently. It is a restartable, long burn N2O/HTPB hybrid. I've included some thrust and chamber pressure data as well - to me the data is more interesting than the movies sometimes.

Of Load Cells and Fourier

2008-01-23T19:03:00.000-05:00

I ran across a post on LLC competitor SpeedUp's website about vibrations in their test stand. The post reminded me of some similar tests I did to measure vibrations in thrust measurements.

As you might be able to tell from the picture in this post , I'm using a beam load cell to measure thrust. It has the advantages of being relatively easy to mount and requiring only one load cell. It has the disadvantage of turning the engine into a vibrating, cantilevered beam supported by the load cell. Fortunately, my data sample rate was 10kHz, so I was able to spot the low frequency ringing by comparing the thrust and chamber pressure data.

In order to determine which frequencies were caused by the test stand and which were "real", I did a hammer test. My test stand is mounted on a steel I-beam, so I turned on the DAQ system and hit the beam with a hammer. The next two charts show the raw thrust data and a FFT of thrust data. The FFT shows vibrations at 31, 33, and 85 Hz, so I can use a digital filter to remove that frequency content from my test data. I use and recommend Sigview to do plots, FFTs, and filtering.

Regression Rate

2008-01-21T07:20:00.000-05:00

When I started this project, I chose nitrous oxide and HTPB for propellants. However, at the time there was no published data on regression rates for that propellant combination. So I started out with a small lab motor that is shown in the previous posts. It was a simple design, similar to motors used in high power rocketry with the exception that the case was 4130 steel instead of aluminum.

That motor turned out to be a good way to do many more things than simply collect regression data. Finding vendors, setting up data acquisition, and learning plumbing best practices are all non-trivial tasks. In keeping with the test a little, build a little philosophy, I find it much easier to incrementally add features as you go. Not worrying about all the features a motor (or any system for that matter) needs to have will often generate more progress than a "single step to solution". Or as my brother says, sometimes you need to just go build something even if you're not sure what it is.

In the intermediate time between when I started and now, a group at Stanford has started publishing data for nitrous oxide hybrid motors. You can read the first page of their papers here and here. Fortunately, their data and mine compares reasonably well as you can see in the chart.

Once I had reasonable regression data and a working test stand it was time to start building a flight worthy motor. However, before you design a motor you have to choose a vehicle architecture. A VTVL vehicle has a much different motor than a sounding rocket. I'll talk a little about motor applications in a future post.

About this blog

2008-01-15T08:09:00.000-05:00

First, "Moonlighting" is a bit of a misnomer.

moon·light [moon-lahyt] noun, adjective, verb, -light·ed, -light·ing.

5. to work at an additional job after one's regular, full-time employment, as at night.

You have to get paid for it to be moonlighting. I'm not, so it's not moonlighting.

I've been working on building hybrid rocket motors in my garage for about two years. Having made some small measure of progress and lacking any eminent prospects for starting a rocket business, I thought I would share some of my lessons with the community. I hope that what I can share may be of some small help to others.

To start, here is a picture of my first motor.

Stay tuned.