Is This Thing On?

9 Comments

Comment by Jewel Butler on June 29, 2008 at 9:51am

Somewhat like getting your level 1 ,2,3 Sound like you have it basics covered its those little tricks of the trade that you learned with your Tripoli members that they won,t be exposed to .But hands on experience is the best route. Good luck to your class on their next Launch

Comment by John Smolley on June 29, 2008 at 10:56am

Hi David,

Sure sounds like a fun class. Is there a course description with objectives out there?

If so, that might help as I'm (not an aerospace professional) shooting from the hip here.

First, let me say that I think you're on the right track. Education is about immersion--and the more your students are immersed in the project, the better IMHO.

Whats more so that the tendency to borrow ideas from others is minimized, it may be worth considering giving each group a different mission with varying flight profiles and dimensional constraints. As a for instance, one mission might be to get to 10,000' as slowly (yet safely) as possible while another is to achieve a maximal velocity while staying below a certain ceiling. Here I'm trying to keep this from becoming an advanced Boy scout experience given a bunch of Estes kits and a w/e to build and fly them.

The other thought I have is to have every student at least design if not build their own motor. You're likely aware of the canned packages so called EX flier's utilize in their efforts to make motors of given specs. John Wickman up in Montana has an excellent few day class where students are taken thru the basics, and as the final project is to design, build and static fire a motor of given impulse and burn time. This could be a purely academic exercise--having to spend some time playing with whatever sim software you're using, and just turning in a motor design, or real depending on tim and resources.

The two omissions I see in what you have listed--and may be subsumed under one or more categories are instrumentation and payload. I separate the two as here I am thinking of instrumentation as vehicle self monitoring--temperature, vibration, velocity, bearing, altitude etc--the dynamic data that might be telemetered back routinely in a sounding rocket and certainly any orbital shot. Payload is about collecting "external" data--the actual reason for launching the rocket in the first place. While I doubt you have much in the way of monies for doing even primitive science, the mission each rocket team is given might dovetail back to some hypothetical payload. The instrumentation could be real, however. Each team might be assigned to collect certain data with simple transduction and data acquisition systems such as temperature (at nose tip, leading edges, motor airframe (heat soak?), vibration (fin flexing, aeroelasticity of airframe), acoustical issues from the motor, etc. That way after flight each team hopefully gets to analyze and present to the rest of the class their findings and conclusions--always a powerful lesson when the data is real!

Again, just shooting from the hip here. So feel free to ignore some or all.

Comment by David on June 29, 2008 at 12:19pm

Thanks for the thoughts everyone -- it's really appreciated. I'm thinking we'll tie in the small rockets with the usual big group project (we've got some stuff in the works even right now), so there will still be an overall lab goal for the semester. We don't really have a curriculum for the class, per se, as the lab is more along the lines of the "Design, Build, Fly" competition that Aero majors do -- essentially loosely structured with an eventual target goal of actually flying hardware. So it's up to us what we want to do, and how we want to go about it.

Similar to the Aerodesign DBF process, I do agree that designing with an engineering goal in mind is a brilliant idea -- I think I might steal that idea, and can't believe I didn't think of it myself. We usually start an altitude pool before we fly a group project, so we definitely have the competitive mindset already in place (USC football, come on now :)), so maybe a little varied engineering competition (a la MDRA's Mach Madness) is the way to go. While I agree with the idea of making sure everyone doesn't borrow from each other, at the same time seeing the new ideas that come up in an environment where there is a common goal is very inspiring. I'll have to give that one more thought.

I'm definitely having everyone design and build the motors for their rocket. In the past, I've pretty much designed all the solid motors that we've used, so I'd like the next set of people who take over the lab to be able to continue in that vein. We have all sorts of infrastructure for building and testing motors, so I'd hate to see that fall. We can easily do 38mm stuff in our lathe (the cases fit in the chuck), though maybe we'll do something a bit bigger so everyone gets steady rest experience, too. But then vehicle prep time and expense both go up... oh I can't decide...

For my senior capstone project (AME 441), I'm planning on flying a 4" rocket (true 4" airframe so I don't have to machine the entire @#!* OD of the motor case) multiple times on either fast N motors or a fast baby O that I'm working on with pitot tubes, pressure transducers, and an R-DAS in the nose cone to attempt to record Mach wave attachment. (Maybe I'll blog about that when I have it a bit more solidified?) So I can't have RPL stuff upstage my 441 project :D I'm trying to think of other payloads to carry as well, both for my project (5 analog ins, gotta use 'em) and for RPL. Aerodesign usually has a set payload goal, but it's pretty boring -- along the lines of "n number of bricks weighing p grams that must be able to be replaced easily" -- and makes the competition more frustrating. So cool, easy payloads would be great. More ideas on that would be much appreciated.

I remember a "Rockets for Schools" payload several years ago that was called "Peas Under Gees" -- the students put a bunch of frozen peas in a jar of Jello to measure the acceleration of the rocket. At liftoff, the force pushed the peas into the Jello, and using a calibrated experiment on the ground, they could figure out the resultant gee force the rocket experienced on the way up. Granted, this was elementary school, and we have access to things like op-amps and strain gauges, so we could do something much more advanced, but still, the simple concept and the cute name attract me...

Another idea I just got (literally, right now, typing this) is to make sure to have everyone document the entire process correctly. At USC, we have a class called "Mechoptronics" (AME 341, Junior year) that is basically a class about how to properly write reports. We have a large enough group in the rocket lab (me included) who have been through it that proper documentation shouldn't be too much to ask, and we maintain a blog (by "we" I mean "I", mostly, and this idea would fix that disparity) on the RPL site already. Seniors and Juniors could mentor Freshmen and Sophomores in report-writing, and perhaps when the Frosh/Sophs reach Mechop they might be a little more prepared for what's coming at them. Hmm. I'm liking this.

So anyways, that's pretty much my train of thought dump at the moment on this idea. Thanks for the suggestions -- they're definitely helpful. Keep 'em coming!

Comment by John Smolley on June 29, 2008 at 2:01pm

Pees under gees? I like that. But it's just these kind of basic exposures that may lead to some critical insight in a later project design whose ultimate success sepends. Here I am thinking of the recent mission that used Aerogel to capture comet dust.

So you guys machine your own motor hardware--thats cool. You could always sell the used stock on Rocketry planet at semester's end. ;-D I was thinking more of the propellant mixing and casting. Where's that come from? i assume you have the facilities and permits to do that as well.

As I try to think of what I would do with 5 inputs and had a real shop to use, I'm pulled towards what I see as the informational vacuums in the hobby, where theres a multitude of opinions based on hunch and anecdote, but very little, if any, actual data. A couple of those I alluded to in the first response: heat soak from motors and aeroelasticity issues including especially fin flutter. Re the first, there are a number of builders who routinely use cotronics and other high Tg epoxies in minimum diameter high performance rockets under the assumption that heating from below is both quick and severe enough to cause catastrophic disassembly during flight. Yet the more cogent attemps using well known equations of heat transfer tend to cast doubt on it. But no one to my knowledge has taken the simple steps necessary to embed a small thermocouple in the AF and measure the temperature during flight. Certainly we can take the copious data from early NACA studies and put together a pretty good idea where frictional heating is problematic--typically above M2.4.

The other data dearths in my limited experience with HPR have to do with fin flutter issues and airframe flex. Again there are some canned commercial sotware programs out there that may or may not provide enough design guidance to avoid catastrophic shreds, but not having used these while measuring vibrational modes thru different velocity regimes, I just don't know if they are truly that robust. The other issue is all the conflicting opinion about drag coefficients of various shapes. Conical nose cones outnumber all others (I'm talking min diam, let em rip type rockets) at Balls and other venues by maybe 4:1 but in my limited reading, there is real reason to doubt the supposition that such are superior supersonic shapes, at least up until Mach 3. Yet thats what 4/5 rockets get tipped with. Just sort of free association here. But it would be great if the insights available to you guys and other academic associations gets back to HPR in some form--the interplay of ideas/cross-pollination would be a real shot in the arm in the envelope pushing thats been going on for the last couple of decades.

I like the documantation--as future engineers and scientists, such experience is invaluable I would think.

Comment by Mark Koelsch on June 30, 2008 at 1:12pm

David, I think they should all design, build, prep, and fly at least one rocket by themelves. Groups are all nice and good, but most of the time I learned more when I did it myself. I have nver understood the desire of educators to have people work in groups so much. I think it is ok at times, but I think fokls get a lot more out of the learning experience by doing it themselves instead of riding the groups coattails.

As to the idea of airframe heating. I do not know of anyone who has embedded a thermocouple in the airframe. I think it is a great idea. I would put them in the area right by the nozzle, the nosecone tip, and the leading edge of the fins. I would be really interested in that data for varying velocities.

Comment by David on June 30, 2008 at 6:08pm

I agree about the "going solo" thing, Mark... the groups were suggested to me by another student as a perhaps more effective way to spread the workload. But the more I think about it, the more I might think you're right. The goal is just to give everyone a broad overview of the entire process, from start to finish, and working in a group of 3 or 4 might have the same effect as just doing the big projects with the lab... someone will end up with the responsibility while others shrug it off.

And I'm totally loving the thermocouples in the fin fillet idea. One of the meanest motors, thermal-wise, that I've ever fired is a 98-17500 case with Pat G's "Wayside White" formula. It's about an 8 second burn time, high in zinc and other metals, and is absolutely brutal on nozzles and liners. Maybe I'll do up another booster for the 441 rocket with thermocouples and see what happens -- I'm thinking it might be good to put a set in the fillets in addition to the areas you mentioned to see if all the whining about heat soak and extra expense for Cotronics is worth it. (My personal opinion -- I highly doubt it... but that's opening up a whole 'nother can of worms until I get some data back!)

Cheers for the replies!

Comment by Mark Koelsch on June 30, 2008 at 7:55pm

David, I think two thermocouples in the fin might be good. One embedded in the leading edge, and one in the fillet. This would allow you to come up with real numbers on the heating on the leading edge, which will allow for better material choice in the future. The fillet thermocouple will allow you to graph the heat soak of the fillet from various motors. This will also allow you to determine what materials and contruction methods might be best in the future. How thin can you make a thermocouple? I think a surface mount one on the outside of the motor case might be good too...see what the actual temperature is for how long to determine if it will have an effect on the composite through heat soaking by the nozzle.

Lets face it, the individual rockets do not need to be large or particularly complicated. I think a certain mission profile should be determined i.e. max altitude for a given impulse class of motor, or something like that would be good. I would make sure that they design it themselves, design for the use of some form of altimeter based deployment and data acquisition (simple with current altimeters (heck, it could jsut be altitude and/or acceleration)), construct it, prep it, and fly it.

Comment by John Smolley on July 1, 2008 at 2:29pm

Please put an end to the chatter re heating with some conclusive data. I argued this til I was blue in the face on the composite forum, and elsewhere. People want to believe Cotronics is necessary. One argument even suggested (in the face of anecdotal but overwhelming evidence from flyers who used your regular 180F Tg epoxy in some extreme missions) was a single flight was unlikely to cause it, but that repeated flights would weaken the epoxy sufficiently via heat cycling. Nevermind that many rockets sit in the sun all day and can easily reach temps of 150 and above. That needs to be controlled for. Thermocouples can be made or bought in very small sizes, there are even ablatable varieties used for measuring grain temperatures as motors burn.

As to the group vs individual--sure if you got the resources go solo. Having spent more time than most in school and certainly more than I care to admit, I can say pairs work almost as well, in some respects better, if you don't.

Comment by John Smolley on July 1, 2008 at 2:35pm

Oh and lest I forget, the movement of the rocket in most cases cools the rocket from whatever heat soak that occurs, not heats it. There are ample data among the NACA articles that show below M2.4 it's pretty much a non-issue--at least in sounding rockets. The improved heat conductivity/capacity of metal fins/af could skew this result. That would imply maybe M2 overall velocity before local heating exceeded this threshold--just a guess based on the transonic regime starting at 0.8M.

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