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!