Creamy Robot Goodness

Just a quick update about the tool I alluded to in the previous post. A couple of weeks ago I started writing a program, currently referred to as “DragFinder”. It takes the weight of a vehicle (a rocket, usually), a RASP file containing the thrust curve for the motor (such as you might find on ThrustCurve.org), and some flight data (currently altitude vs time). The program then estimates the drag force vs time for that flight.

Next on the list is converting the drag force vs time over to drag coefficient (Cd) vs mach number, which shouldn’t be too bad. I believe that should take both altitude and speed into account, hopefully resulting in a fairly accurate drag estimate.

So how do I verify that my code is working? Here’s where I’d like some help from rocket hobbyists out there, if anybody reads this: I need data. I need information about rockets including their weight, motor used, frontal area or diameter, and flight data (either altitude vs time or acceleration vs time). I’d like data for big rockets, small rockets, high-altitude and low-altitude rockets, everything I can get. I can get data from a few rockets of my own, but the more the better. Real data will allow me to validate that the software works (and find out why, if it doesn’t). I can then be confident that DragFinder will be an effective tool for characterizing the drag on my ramjet project. And, of course, I plan to post DragFinder for free so that anyone else that wants to can benefit from it. Anybody?

So I’ve been working on a project for a couple of weeks now and I guess I haven’t said anything about it on here. Well here it is: I am trying to develop a rocket-boosted ramjet, and I want to be possibly the first amateur to actually fly a ramjet-powered vehicle. (A group in Israel launched a rocket-boosted ramjet a year ago, but it took 5 years of senior design groups and a professor to pull it off, and it only burned for about 2 seconds before burning out prematurely — I’m hoping I can do better.) For the uninitiated, a ramjet is basically the simplest type of jet engine. It has no moving parts, and above about Mach 3 it’s actually the most efficient way to move through the atmosphere. The thing that makes it tricky is that it generates barely any useful thrust until it’s going at least 200-400MPH. That’s a tall order for an RC aircraft engine, but what about a rocket? It’s been done by NACA/NASA and a couple of world militaries, so it’s obviously possible, and I’m hoping to do it myself. I’ve been developing a simple Excel spreadsheet to simulate the performance of a vehicle which is powered by any combination of rockets, ramjets, and cannon power. I’m by no means an expert at this sort of thing, but I think it’s a good start. I pitched my current version over at the pulse-jets.com ramjet forum, but haven’t gotten much feedback yet. I have been exchanging ideas with James Irvine (of Australia), and he may actually make me a simple ramjet engine to experiment with.

Unfortunately with the current version I have been unable to produce a Decker-inspired design that is self-sustaining. I am hoping this is due to an inaccuracy in the drag model. I am considering trying to determine a model of the drag forces empirically, possibly by building a scale model of the vehicle and flying it on a known rocket engine. Perhaps by tweaking frontal area and Cd values in RockSim and in my spreadsheet until they agree with actual flight data I can come up with something usable.

Hmmmmm….. How about a tool that read in flight data in the form of acceleration and/or altitude over time (maybe altitude would be easier for most people to come up with), along with a thrust curve for the motor, and it would produce a graph of estimated drag force? Of course the results would vary from flight to flight, but the more flights, the more accurate the data. Flying the same rocket a number of times, this tool could produce a statistically useful summary of drag forces vs speed. With a direct model of drag forces, one would not even need to mess with something so crude as a Cd. Oooohhhhh….. And one could even fly a version of the rocket without the ramjet engine attached in order to determine how much drag is caused by the ramjet engine itself. Oh man that would be useful….

I plan to make rocket motors for my own personal use here in NJ, and I haven’t found any advice relating to that state yet. Following is a summary of my ongoing attempt to legally make my own rocket motors. So far everyone I have dealt with has been friendly and reasonable. My focus right now is on Ammonium Nitrate Composite Propellant (ANCP), rather than the more traditional Ammonium Perchlorate Composite Propellant (APCP). I provide all of this for informational purposes only, I am not a lawyer and I will not be held responsible for any stupid things other people do. Anyway, here’s what I’ve tried:

• Local fire marshal: I live in the Burrough of Audubon, so this was Bill Schaeffer. He was glad I started by coming to him rather than just going ahead and mixing propellant in my backyard. However, he suggested I talk to the county people.

• County fire marshal: I’m in Camden County, so I spoke with Deputy Fire Marshal Ernest F. Busch. He asked me to send him some documentation so they could start a file and do some research, so I sent him the following:
• MSDS sheets for all chemicals involved in making propellant (AP, AN, aluminum and magnesium powder, HTPB binder, curing agent)
• Links to the Tripoli Rocketry Association (TRA) and the Maryland-Delaware Rocketry Association (MDRA)
• Scans of my TRA paperwork showing the signatures for my certifications
• Scans of all of my TRA ID cards (they happened to document my changes of address)
• Scan of the certificate received from John Wickman’s solid rocket motor design class

He said he would begin researching things, and recommended in the meantime that I try and talk to Lou Kilmer at the Department of Labor (DoL is in charge of explosives). I left a message for Lou but got a call back from the Department of Community Affairs instead.

• Department of Community Affairs, Division of Fire and Safety: they are responsible for the development and enforcement of the State Uniform Fire Code. I spoke with Josh Lazarus, Supervisor of Inspections. His determination was that since I would be working on my own property and making propellant for my own personal (non-commercial) use, the Fire Code does not apply. He then recommended I talk to the Department of Labor.
• Department of Labor:
• I first spoke with Senior Inspector John Jordan. He determined that since none of the ingredients in ANCP are on the ATF explosives list, and neither is the ANCP itself, they would have no jurisdiction over it. However since APCP is on the explosives list, that would probably require a permit from the Department of Labor if I intended to make it. He also said that since black powder (used in ejection charges) is on the explosives list, I would need a permit and a magazine for that, and on top of that, I would need a separate permit and magazine (with yearly renewal fees) for the storage of the electric matches which would initiate the black powder. And if that weren’t enough, if I were to install the black powder and electric matches on my property I would need some sort of installation permit. This all becomes very expensive. John then recommended that I run everything by his boss Howard Black to see what he thought. As for storage of commercial rocket motor reloads, he said that since the NJ Model Rocketry Act says you can store up to 220 pounds of propellant without any sort of permit, he didn’t see a problem there.
• Director of the Division of Public Safety & Occupational Safety & Health, Howard Black: He agreed that storing less than 220 pounds of commercial rocket propellant shouldn’t be a problem. I asked him about the issue of needing a permit for the black powder, and he said that since the NJ Explosives Act allows small arms reloaders up to 5 pounds of black powder (or 36 pounds of smokeless powder) without any permit, he didn’t see a problem with me doing the same for rockets. On the electric matches issue, he deferred to John Jordan.
• ATF: I haven’t spoken with anyone in-depth about this yet, they mainly just pointed me towards the paperwork for filing for a Low Explosives User Permit (LEUP). There are plenty of guides online about getting your LEUP, so I probably won’t duplicate much of that. As near as I can tell no LEUP is required to make AN motors. This (the LEUP) seems like the thing to get though, since it opens up a lot of doors (buying higher grades of powdered metals, etc).
• Local Police: I actually haven’t spoken with them yet, but Howard suggested I talk to everybody I can think of just to get everyone on the same page.

The No-Permit Approach: So what do I do with all of this information? I can’t afford to get permits for every little thing right now, but I want to be legal. So my current approach is to try and do without the things that require permits:

• Black powder: Keeping small amounts on hand should be fine as long as it’s stored in a USDOT-approved storage container (which I haven’t looked into yet).
• Electric matches: Maybe I can just make some substitute (capsules of black powder with a simple bridge wire inside?), or even look into using an alternative like a CO2 cartridge-type ejection system. Motor ejection is always an option for small rockets, though I am opposed to it in most HPR cases.
• Storing commercial propellant: This seems to pose no problem.
• Making custom propellant: Stay away from things that are on the ATF’s explosives list. Right now the best option seems to be using AN-based propellants.

If anyone has any updated information, please post a reply. I will try to keep this updated as I get more information. Thanks!

Last weekend was Red Glare 2, 2007, and my signed certification papers should have reached Tripoli by now.

Pictures of assembling the rocket, prepping it on launch day, and finally… the successful launch.

Video of the launch and recovery

So what’s my next project? The CNC foam nosecone lathe? The bigger-better-all-metal-launcher Jujubee? Something that breaks Mach? A ramjet-augmented rocket, or a tube-launched ramjet? Something with no fins and active stabilization? A rocket that uses GPS to steer itself back to the pad when under chute? A large water rocket staging to a solid propellant rocket? I dunno. What do you think?

Well the launch is done, and it had a measure of success with a flavor of annoyance. The rocket launched and all the new pieces worked perfectly. Unfortunately the cannon had a slow leak that we didn’t discover until the last minute (not enough teflon tape on the joints? loose gauge? not sure yet), and by the time we got the rocket launched the pressure had dropped lower than it should have. It shot up out of the cannon going far slower than it was supposed to. The rocket engine ignited but I think it was already traveling too slow to be stable. The result was about 3 loops in the air, and it actually went almost 100 feet LOWER than the non-rocket-boosted Jujubee launch from 2 months ago. Entertaining in retrospect, certainly, but it was far less impressive than it could have been. The latest predictions (for the rocket stage alone) were almost 9,000 feet up and close to the speed of sound.

The carbon fiber (unidirectional prepreg) was really great. It machined much better than I thought (I was expecting splinters everywhere, and had virtually none), and the rocket came down without a ding on it (which is good considering how fast it comes down — it’s a very dense rocket and parachute space isn’t easy to come by). My custom accelerometer-based ignition circuit worked perfectly, as did my low-current igniters staging the I200 motor. Very nice.

At this point I think it’s too cold to use my PVC pneumatic launcher again this year. I’m thinking of installing some temporary launch lugs and launching the rocket off without the cannon at the next SoJARS launch — the simulations all say the rocket is overstable but I want to know for sure.

Next projects? Not sure. I’ve been thinking a lot about the next generation cannon, big enough for a more powerful 38mm rocket motor, and made entirely out of metal (aluminum for the barrel, steel for the breech/combustion chamber?). I’d power it on either multiple pneumatic butterfly valves (with a burst disc to synchronize them, perhaps?), or just straight up black powder. I’d also like to get my Level 3 at some point, but I’m not sure if that should be sooner or later.

I recently acquired a small amount of round magnesium stock in order to try and experiment with machining it. I’m very impressed with how light it is (something like 2/3 the weight of aluminum) and it machines great, I’d love to start using it all over the place.

Below is a link to a video that Justin took with his digital camera. Unfortunately you can’t see the rocket at all, but over the wind you can make out the countdown, the “poof” of the cannon, me exclaiming “Oh no!”, and Katie laughing hysterically. You can also see the stream of smoke begin mid-air, which is sort of cool in and of itself.

Jujubee video, first launch with high-power motor ignition

If anyone is interested in the small staging timer I built, I’m considering polishing it up and maybe selling a few, so let me know.

So it’s 2:40am and I’m eating a sammich before hitting the bed. Haven’t had lots of free time for food the last couple of days.

So is Jujubee done? Of course not! I still have to lathe up a coupler in the morning. The electronics are all wired up and tested (mostly). Besides the coupler, all that needs to be done is to tie everything together (the chutes, couplers, and airframe sections), pack the chutes, make up some charges for the electric matches, and attach the igniter. Most of that will happen at the field anyway.

I think I just might pull it off. There are some compromises I’ve had to make that, while imperfect, helped me make good progress. For example I cut the fin slots in the airframe but didn’t have time to cut out (and then carbon-fiber over and cure) the fins themselves… so I just cut the fin can from the last Jujubee and that should work ok. I also tried embedding the copper tape underneath the carbon fiber (with a layer of insulation), making it seamless. Unfortunately by the time I’d squared up the tube, the copper situation didn’t look very good. The compromise is to go with outer copper “racing stripes”. They’ll tarnish, but they’ll take about 2 minutes to replace.

With all the extra weight of the new couplers and electronics, the latest (crude) simulation is putting Jujubee at just under 8700 feet up, and 1043 fps peak velocity (about Mach 0.93). That is without the cannon, though.

I hope I have the presence of mind to get some good pictures of this rocket before I send it to its fate. Even with the (medium-grade) RF locator, I still bet it’s a crap shoot whether I ever see this thing again or not.

Well it’s Friday. “Red Glare” (the big launch) started today. I’ll probably end up working on Jujubee all day tomorrow. I wish everything was done, but I do think I’m in ok shape for Sunday. In just a couple of days I designed and built my staging timer. It uses a 250g accelerometer, an AVR, and a mosfet. It lets you, using a serial port, set the acceleration threshold for launch detect, and to set the ignition delay in 100ms increments.

I decided to run copper tape down the side of the rocket to carry the current from the staging timer. I ran a number of tests and it seems very reliable in its current configuration. I can power it up, pull the reset jumper to start the safety lockout timer, and then (after waiting for the timer) a quick smack on the side of the board simulates the high acceleration needed to trigger ignition. After the ignition delay, the igniter burns beautifully. I did, however, end up having to make my own low-current igniters because a 9v alkaline battery wasn’t able to provide enough oomph. Below is a simple explanation of how I made them, for posterity:

Materials:
- 26 gauge shooter wire (aka zip cord, etc), the first 1/4″ or so stripped
- soldering iron, solder, paste flux
- 40 gauge nichrome wire
- Magnelite dip

The concept is that, unlike the pre-soldered igniter wires that often come with Magnelite (see http://www.publicmissiles.com / Webstore / Igniters), we only want a very small loop of nichrome to bridge between the two copper wires (rather than wrapping it multiple times around the insulated section of wire). Using a small piece of nichrome wire (1.5″ is plenty, but most will go to waste), wrap one of the copper conductors several times, very close to the insulation. Then make a bridge, looping halfway around the insulation, to the other copper wire. Apply some paste flux to the two places where the nichrome wraps around the copper. Put a blob of solder on the tip of your iron, and touch it to the flux and copper. This should coat all the nearby copper in solder instantly, and tack the nichrome down adequately. Trim up the wires, carefully bend them into shape if you need very small igniters, and dip in Magnelite. I double-dipped mine after a few minutes to increase the amount of pyrogen per igniter, but I haven’t tested these yet. [Update: I tested most of the double-dipped igniters and one of them had a pretty violent pop. Guess that answers that.]