kyle0100 Posted September 5, 2016 Posted September 5, 2016 For my school chemistry project I am building a APCP rocket. I've decided to use a design similar to the one in this video https://www.youtube.com/watch?v=v5Fch7AemfY. The problem I am having is that he does not list how big the hollow core (don't know the term) that connects to the nozzle is or the height of the grain that he uses. So if I was to follow that recipe except make a kilogram of fuel instead of 650 grams how tall and what diameter should the hole going through the grain be. Ideally I would like an equation supplied so that I can include that in the lab and use it for future reference.
CrossOut Posted September 5, 2016 Posted September 5, 2016 Wow.. id like to know what school you are going to which will allow you to make rocket motors as a school project. Im not against it but you basically are creating a potentially explosive device. Assuming that your intentions is NOT to bring one to class and to keep it as part of professional rocketry, I'm sure your school would still not approve of your project topic either.. Additionally I would NOT recommend starting out with these composite rockets. They utilize extremely powerful fuels which are very dangerous especially for novice builders. I would strongly advise against experimenting with them. Let alone wanting to upscale them. That said, for not im going to refrain from giving additional information about your topic unless i can find some evidence that you are more experienced than you seem. lack of basic mathematics and terminology are kinda obvious indicators for a novice builders. Assuming your school does approve of your project and you have proper safety precautions in place, A much better route for novice builders would be sugar / candy rockets or BP. both can be incredibly powerful and are a good step into the field of rocketry.
kyle0100 Posted September 5, 2016 Author Posted September 5, 2016 First of all, thank you for responding to me. I've already built potassium nitrate rockets and experimented with different catalysts and have had mixed results. When I work on these I only work on designs and in class, I don't actually build it in a class full of students. After school or on weekends I work on it with the supervision of my teacher. I've found certain equations for some of the rocket dimensions but the problem I have with them is what to put in for values that relate to the fuel as they generally aren't listed. If you still have doubts, can you point me in the direction of any books or websites that can help me with some of the terminology or mathematics? Thanks for your time.
NeighborJ Posted September 5, 2016 Posted September 5, 2016 The reason you will not find specs on fuel is because they vary. You will find that your fuel will be different then everyone else due to product purity, mesh size, formulation and a number of other contributing factors. AP motors are not ideal to start out on and extensive testing needs to be done before it can be used as a reliable fuel. And that's not to mention honing of construction techniques will take time, I venture to say that a semester will not be enough time to pick up the skills needed to make all this happen. I do encourage you to check out Richard Nakkas web site to get all the info you will need. 1
kyle0100 Posted September 5, 2016 Author Posted September 5, 2016 I visited that sight and for the core diameter I found the equation mg = Ab x Pp x r. So I was wondering if I can rearrange the equation to find Ab as it is the burning surface area which I can then use to find the radius of the core after I pick a certain height for the grain using the equation Ab = (Pp x r)/mg. From my understanding mg (has a dot over the m) is a prescribed flow rate so is that a value I just pick or measure based on the fuels. If it is a measure that I pick can anyone give me a reasonable number.
NeighborJ Posted September 5, 2016 Posted September 5, 2016 (edited) Unfortunately there is no simple answer to your question. The first thing you should ask yourself is what is the pressure limitation of the planned motor casing? This is important because AP fuels operate at a very high pressure, cardboard and PVC will limit you to around 1000# so they are for the most part out of the question. A few people have made working cardboard tubes and that is when particle size is most important. If you want peak performance you will need a machined aluminum casing with a turned graphite nozzle. You need a burst pressure quantified so you can chose a case pressure, then a burn rate at that pressure of your specific fuel. Then you will be able to calculate a surface area which will be used to pick an appropriate core length and nozzle size. The fuel will be the constant and the core dia and length will need to be tailored to your fuel. Edited September 5, 2016 by NeighborJ
kyle0100 Posted September 5, 2016 Author Posted September 5, 2016 My grandfather is a blacksmith/welder and he has taught me how to do a lot relating to it so I can make a steel case for the motor. So the burst pressure is the pressure at which the casing would rupture so I would use the equation here http://www.engineersedge.com/calculators/pipe_bust_calc.htm . So I guess my question is what equation should I use to find an appropriate case pressure after I've calculated burst pressure and for burn rate can I just make a small batch of fuel and put it in a case similar to the one I'd be using for the finished rocket and use mass/time = burn rate. My final question is what equation would I use after I've calculated burst and case pressure and burn rate as I don't think it will fit into the equation I rearranged.
CrossOut Posted September 5, 2016 Posted September 5, 2016 (edited) I assumed you were smart enough to not build or bring these to school. What are the qualifications and experience your teacher has? I'm Hoping for the best and assuming that he is has a PHD in the subject at hand. That said I have been to many research institutions who will simply take undergraduates and toss them into a research project with little experience on the subject matter. I havn't seen this done for any projects which have such high potential dangers And I think it is a bit unwise to do so for energetic materials.. Seen as how this is your project as he is supervising, I have reasons to believe otherwise. That said i think you should narrow down the focus of your project and eliminate variables to be something you can actually complete rather than trying to build something awesome and possibly kill your self in the process. building rockets in general is already pretty awesome no need to try and be NASA from the get go.. Is there a reason you are getting mixed results with your nitrate based rockets? These rockets are relatively easy to master compared to other options. For the purpose of experimentation and viability to produce publishable document, they are significantly cheaper, reproducible, safer to use, and have known characteristics associated with them. As an added bonus more people on this forum would be more experienced with them and be able to give you advice as opposed to composite fuels. Most of which we shy away from.. You are more or less on the right track.. However, since all fuels are a little different depended on your chemical supplier and minor variations in each batch, you will need to look for better ways to standardize your procedures. once you have a good repeatable method you can begin testing your fuel for its impulse, thrust, fuel consumption, Isp, etc. then you can begin designing your rockets around the fuel you have chosen to use. cores will always vary depended on the type of fuel used. Slower burning fuels can allow for larger cores obliviously. Like i said you would need to analyze your fuel before you can determine an optimal core dimensions. keep in mind that in realistically doing this would take several hours of computer simulation to accomplish. The equation you are asking for would be a differential equation factoring in the fuel characteristics and engine/ core dimensions, etc. I'm more than willing to admit that even having taken several graduate level physics courses i am not capable of supplying equations for your engine. I would even go as far as PHD's in a related field wouldn't be able to supply you with an equation either. As a rough way to do this is simply trial and error based on the fuel weight, volume, and burn charismatics of your fuel. But as i said. In order to do this you would need a standardized method.. What i see in that gentleman's video is definitely NOT a standardized method. All the equations you have supplied us with don't take into account any changing variables. I wouldn't use any of them as they are simply general equations for theoretical use and only hold a slight similar trend for real applications. The calculator you have would be sufficient IF you know the information of your tubes.. And i wouldn't go by what is written on them.. and i realize that the video you watched is using PVC.. I strongly recommend against composite fuel core burners in PVC for safety reasons.. If you have all you that information, your calculator will only calculate the casing's durability under standard conditions and doesn't take into account heating, expansion, impulse of force, etc. Not to mention the data is useless without knowing the characteristics of your fuel. Remember that core burners have an exponential spike in thrust. where as end burners have a initial spike but will eventually flatten out and slow decrease the longer the motor burns. Edited September 5, 2016 by CrossOut 2
NeighborJ Posted September 5, 2016 Posted September 5, 2016 (edited) Thank you crossout, I was starting to feel dumb. I relied heavily on the other guys in the rocketry club for that kind of info. Which is a good idea to join a rocketry club. There you can learn and work hand in hand with people who have done it before. It will take time. Like I said before it isn't something which can be done in one semester. They will also guide you thru all the safety regulations. Edited September 5, 2016 by NeighborJ
CrossOut Posted September 5, 2016 Posted September 5, 2016 (edited) Yep.. Truth be told.. Many composite fuels require Explosive license to produce and use.. I dont know the specifics of where they draw that line but i do know there are regulation associated with them. Found it.. https://www.atf.gov/file/60711/download So there's that. Mind you these are federal Laws. So Local / State laws may be more strict or lax. Edited September 5, 2016 by CrossOut
kyle0100 Posted September 5, 2016 Author Posted September 5, 2016 The only reason that I can think of for my nitrate rockets not being consistent is that it is really humid here this time of year and if I recall correctly potassium nitrate absorbs water from the air (hygroscopic I think). One of the main reasons that I like composite fuel is that I don't have to heat it like I do with sugar rockets as there is always the small possibility of a detonation or more likely caramelizing and ruining the batch. For the standardization I will obviously buy the materials from the same supplies and I would use the same casting molds each time so will that do or are there other methods you recommend? As for trial and error, would I be able to set up a proportion based on my results. For example, if I get the best core size for 50 grams of fuel will I be able to use that to predict the best for 100 grams or will it be a function that I will have to graph in order to find the right core size at the amount of fuel (I will obviously keep the height and and width scaled correctly). Also, do you think steel tubing would work as I have the ability to weld and shape it for the case and nozzle. My final question is what is a safe core size to start with. For example, the diameter of the core should never be less than 1/3 the diameter of the grain just so that even with small amounts I don't make a glorified pipe bomb?
kyle0100 Posted September 5, 2016 Author Posted September 5, 2016 Thanks for your reply Neighbor J. The only problem with joining a rocketry club is that I live in the middle of a cornfield and there aren't any nearby so I can't really join them which is why I was trying to find a method to do this safely here. Also, This project will be at the end of this school year so I have nearly two semesters to do this.
kyle0100 Posted September 5, 2016 Author Posted September 5, 2016 Yep.. Truth be told.. Many composite fuels require Explosive license to produce and use.. I dont know the specifics of where they draw that line but i do know there are regulation associated with them. Found it.. https://www.atf.gov/file/60711/download So there's that. Mind you these are federal Laws. So Local / State laws may be more strict or lax. I was reading about that on the NIRA page and it said those above the age of 14 can get a HPR license. Also I would be testing this at our hunting ground that doesn't have anyone living there in a few miles each direction
CrossOut Posted September 5, 2016 Posted September 5, 2016 (edited) Nitrate fuels are slightly hygroscopic however unless you are storing them for a long period of time this shouldnt be an issue. .I dont see any problems if you are making them and shooting them within a week or two.. Just make sure you are storing them in dry closed container. a good 5 gallon bucket w/ lit would be perfect for this. For rocket casings i recommend something that wont kill you if it blows up.. Paper would be a good option for testing fuels.. from there you can move into PVC as it is a standard for amateur composite fuels. Steel is bad.. as you said its just a pipe bomb should anything go wrong. As for your composite fuels, im going to refrain from any other suggestions unless it pertains to safety. As i said im not a master of the craft and i do believe it is too dangerous for beginners. This is especially so if your mentor is not able to answer the questions you have and you are forced to rely on us for advice.. Choose a topic more of us have experience with so we can all help you. Im sure a rocketry forum would be able to give you more instruction but im pretty confident in that they would want to direct you away from composite fuels as well. I am still going to suggest you step down and revisit sugar or BP rockets in order to increase your understanding of the subject matter and give you something more realistic to achieve. Edited September 5, 2016 by CrossOut
kyle0100 Posted September 5, 2016 Author Posted September 5, 2016 Alright, I'll hold off on composite fuels for awhile. Can I use a binder with nitrate fuels instead of heating them in a pan together?
NeighborJ Posted September 6, 2016 Posted September 6, 2016 (edited) I've never heard of anyone using a no heat method for sugar rockets except for pressing but if I was to try one it would probably be a shellac replacing equal parts of sugar I'd start at mabe 5%. I'd allow it to dry a little until it gets tacky then press. This should greatly reduce the hygroscopic nature of the fuel and should work well as a fuel itself. It would need to be in a cardboard tube so the solvent(acetone) can dry. The solvent is likely to dissolve PVC. You can do density tests to see how well it compares to the normal skillet method. Edited September 6, 2016 by NeighborJ
dagabu Posted September 6, 2016 Posted September 6, 2016 I've never heard of anyone using a no heat method for sugar rockets except for pressing but if I was to try one it would probably be a shellac replacing equal parts of sugar I'd start at mabe 5%. I'd allow it to dry a little until it gets tacky then press. This should greatly reduce the hygroscopic nature of the fuel and should work well as a fuel itself. It would need to be in a cardboard tube so the solvent(acetone) can dry. The solvent is likely to dissolve PVC. You can do density tests to see how well it compares to the normal skillet method. You will have a very weak smoke bomb, I'm guessing. The shellac will probably coat everything and keep it from becoming intimately homogenized making it burn real slowly.
NeighborJ Posted September 6, 2016 Posted September 6, 2016 You are probably correct Dag, it's just the thinnest coating binder I know of and it works awesome on pillbox stars so it looks like it holds the most promise. There is a reason binders are not used.
CrossOut Posted September 6, 2016 Posted September 6, 2016 (edited) Alright, I'll hold off on composite fuels for awhile. Can I use a binder with nitrate fuels instead of heating them in a pan together? Wise decision. I'm glad you understand. There really is no need for a binder. the sugar tends to be sticky enough to hold everything together if you choose to press everything. If you dry and mill all your powders separately and mix them together you can get a pretty homogenized mixture. good enough to make pressed rockets. Pressed sugar rockets can be pretty spicy still and will allow you to play around with core dimensions in a more forgiving manner. I speculate you can probably ball mill the sugar and nitrate together and hand mix in your iron.. if someone can comment to confirm this is safe that would be great. I wouldn't do it until someone does confirm this. Alternatively you can dissolve and melt everything together using corn syrup on a hot plate. That is actually a pretty safe method.. ive never had any accidents with it.. just make sure your hot plate doesn't get too hot.. i recommend an electric pancake griddle for this as it offers pretty good temperature control and no flames obviously. you can get one for liek 20-30$ at walmart or less at a garage sale. there should be enough water while you are mixing it that it wont burst into flames on you. set your griddle on low and add your red iron as you enter the drying process and keep an eye on it. once you reach a good play dough consistency you can pull it off and pack them into your rockets while hot. They dont hold as much power as a composite but they are much cheaper and safer to work with. much more forgiving to mistakes as well. Edited September 6, 2016 by CrossOut
stix Posted September 6, 2016 Posted September 6, 2016 You will have a very weak smoke bomb, I'm guessing. The shellac will probably coat everything and keep it from becoming intimately homogenized making it burn real slowly. Yeah, that's probably about right Dags. In the quest for a "cold set" sugar fuel and binder I've tried shellac - didn't burn very well. I've even tried parlon with acetone - even redgum. I beleive Richard Nakka was sucessful using epoxy resin (West Systems) but also mentions ignition issues. In any case, epoxy resin is expensive so you may as well go for APCP if you can get it and afford it. Simple sugar fuel and/or black powder Rockets should be enough to get you started.
dagabu Posted September 6, 2016 Posted September 6, 2016 Yes, Nakka's system was a great way to utilize KNO3 in an epoxy/cast grain motor but may I point out that sugar is not the fuel in these. Only 2 of the 30 tests had any sucrose in the RNX grains. Also, keep in mind that really any epoxy will work but to a better/lesser degree than West Systems. The AeroMarine 300/21 epoxy resin is very close to the West Systems and MAS epoxy and runs only $200.00 for a 3 gallon kit. The most important thing about this cold pressing system is to make sure the chems are finely ground first. Rich and I had long discussions on the BBS about just how fine and it was agreed that as long as you have a consistent µ size to deal with, you can tweak the ratios to get what you are looking for. The devil is in the air pockets, it's almost mandatory to use a vacuum on all epoxies to cast good grains. If you have the time and money, this is a LOT of fun to work on. For the average person, its a journey to madness!
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