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Rocksim for Micromax rockets?

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KevinM

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Greetings all,

I'm new here, so please go easy... I've been tinkering with Rocksim for a Micromaxx design I'm working on but I have a couple of questions that only experience can answer and I'm coming up short. First up - does anyone have much of a feel for how well Rocksim's stability predictions work for Micromaxx sized models? (While I would expect it to be excellent, I'm sure I also read somplace about leaving more margin on really small designs...)

My next question pertains to one of the setup parameters for Rocksim, but I suppose it applies equally to any other larger rocket design. I noticed that Rocksim defaults the "Minimum velocity for sable flight" to a fixed value, but I would imagine this figure to be different from model to model - Is finding this speed not generally worried about, or is there a way to work out (without taking a masters course) what the real number is? Presumably it should be achieved before leaving the guide rail? Any guidance on this (pun intended) is greatly appreciated...

KMc

BTW, if anyone's curious what I'm making, it's a 1/96 scale model of a Russian P-700 Granit cruise missile. (I've removed the wings on the flight model, but left the stab/rudders and oversized the booster fins.) This comes out about the same size as, although a hair lighter than, the Quest mini Tomahawk.

 

MarkII

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Kevin,

I have used RockSim 8 to sim a Micromaxx model or two, and I do think that it gives a pretty accurate estimate of the flight. I think that you are probably correct that the actual velocity needed when the rocket reaches the end of the launch rod to insure a stable flight can vary a little bit from rocket design to rocket design, but the speed that RockSim uses by default provides a margin of safety for practically any rocket design. I'm not versed in the physics of it, but my admittedly uninformed guess is that any variations in that required speed would be slight in any event. The Quest Micromaxx-II motor gives such a hard kick at ignition that few models that are designed for it need all that much in the way of external guidance before the fins can assume the responsibility. They all do need some external guidance, though. A 12" long x 0.049" diameter music wire launch rod is standard.

Your model looks very interesting. It's size and weight should present no problem at all for a Micromaxx-II. Once you have finished it, please post all of the details so that I can build one of those, too. :D

MarkII
 

KevinM

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Your model looks very interesting. It's size and weight should present no problem at all for a Micromaxx-II. Once you have finished it, please post all of the details so that I can build one of those, too. :D

MarkII
Hi MarkII,

Thanks for your response, I'm glad to know that I'm on the right track. What details would you like to see? Here's a photo of a number of test shots I've cast up (but not yet had the opportunity to light off) adjacent to my Tomahawk "reference" model. Notice the decreasing fin span as the models progress from left to the right - My Rocksim model is consistent with the rightmost model, which (if the predictions hold) should fly with sufficient stability. My goal with the short span fins is to be able to safely launch this from a 3/4" tube with internal guide "rails".



Each of these is made of a polyurethane casting resin, cast around a 1/4" mandrel in an RTV mold. Once the resin has cured, a light tap removes the mandrel from the "finned tube" and the resulting piece is ready to accept a nosecone and thrust ring. Although heavier than a paper tube, the nice part about this construction method is that I can mold in all the surface details I want and have them ready for paint when it comes out of the mold.

Here's a couple of shots of one that I finished up as a display piece.




... but I digress from the subject matter of this forum. Does anyone else have any wisdom to share on min launch velocity?
 

MarkII

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Wow, a homemade LPB! :roll: (LPB = Little Plastic Brick. Look it up. With your rocket, I mean that in a nice way, of course.) What would Micromeister think of that?!! :D That IS a marvelous approach to getting scale details onto a micro rocket, and your display model looks terrific. You aren't including the forward wings on your flying model? With proper balancing, it would probably fly fine with them. I didn't realize that you were creating the model out of cast resin. Make sure that you give RockSim the correct weight of the actual built model through a Mass Override if the program does not have this material in its database (and it probably doesn't).

I am no expert on aerodynamics, but from what I have read, the physical dynamics acting on very small flying objects like really small micro rockets (as opposed to larger micro rockets - is that an oxymoron?) work somewhat differently than they do with larger flying objects (see, for example, the difference in flight dynamics of an insect vs. a bird). With the mid-body mounted delta fins, your Granit may indeed be stable in flight, either with or without a little bit of nose weight. After all, regardless of your rocket's shape, the MicromaxxII will boot it into the air pretty good. The Quest Micromaxx Space Shuttle is living proof that even a (small) brick will have a stable flight on this motor. :D I would give it a try with the two delta wings/fins. Here is a review of another micro rocket that has a lot of fin area.

MarkII
 

Micromeister

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Very nifty is what I think.
For some having the opportunity to greater detail is more important then higher flights.
I've done a little casting myself but haven't had the time to really get into it as heavily as rokitflite or others. I did to a micro Patriot some time ago that is still waiting for more work on the molds and surface detailing.
Rokitflite has been using a slightly different way of "slush molding" Polyurethane resin to get a pretty consistant thin shell casting, I'm no good at it at all but I'm sure he'd be willing to help if it gets the mass down a little.
I've been out of the loop the last few months but will be trying to get back into the swing of things shortly. With all the other projects I have in the fire, casting LPB's is pretty far down on my list, Looks like Kevin M has a good handle on at least this one model, prehaps he'll be bringing more on as time premits.

Another Casting Idea is just creating the micro "add-on" Details that can be attached to our paper and cardboard models. I've been carving nozzles, guns, canopies and other little pieces for use in a micro-balloon filled resin set that shouldn't be all the heavy. but it's as I mentioned way down on the list LOL!

Great job there Kevin! Keep up the good work. Super looking add-on wings.
 

KevinM

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No offence taken MarkII, LPB is exactly what they are!

Very nifty is what I think. .
Thanks! I've also done slush-casting before, and although you might get a slightly lighter result I'd be afraid that the inconsistent inside surface would make glueing a motor mount in difficult.

While I'm talking about casting details (in the simulation forum, sorry folks!) I actually went so far as to pressure cast these to be sure I don't wind up with large bubbles where the fins ought to be. After I loaded up the mold with casting resin I dropped the whole thing into a pressure pot and brought it up to 70PSI or so to squish down as many bubbles as possible. It's not perfect, but it seemed to work.

For some having the opportunity to greater detail is more important then higher flights. [snip] Looks like Kevin M has a good handle on at least this one model, prehaps he'll be bringing more on as time premits.
This model is all about the details, but it's not the "whole" model as it were. Once I've worked out all the bugs with launching this P-700 Granit it will actually become part of a much larger model. As for other models from me, that's certainly a possibility... I have also been looking at doing a 1/96 scale RSM-54/R-29RM (SS-N-23), but that one's a ways out and I've not started looking into what the best way to power it would be.

BTW - I'm pleased to report that I got out the weekend before last and launched the lot of these with great success. Successful, stable flights were had by all four although they're so small that finding them again afterwards was a bit of a task. Next up: under water tube launch!
 
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