If I were proficient in that level of math, I might take a stab at it just to see if I could do it. Once.
Bogus sim files posted online - why?
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If I were proficient in that level of math, I might take a stab at it just to see if I could do it. Once.
Well, when it's their software and their kit you'd expect it to be perfect.
ThirstyBarbarian
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Can you calculate speed off the rail, apogee, and optimum delay for a given rocket on a range of motors with different thrust curves? That’s what I use OpenRocket for.
I normally use Rocksim, and learned from someone here to use a mass object for glue/paint weight. I actually use two, one for glue, and one for primer/paint. I weigh and measure all the parts, enter it in, note CG and weight (more for curiousity than anything else), build the rocket, weigh it again and note the new CG. I then enter the weight as a mass object and position it such that the CG matches my measured one. This is usually about halfway between the CG and CP, though it can vary somewhat. Paint weight, while needed to get the final CG, doesn't seem to change the CG much (usually less than a half inch, typically less).
You could ballpark it and just assign a fixed percentage at the start, say 15%. Most of my rockets end up between 15% and 20% heavier after all is said and done. I tend to use more paint, as I like my rockets to look good and that needs typically 3 coats of paint at the least. I'm not going for max altitude after all.
Ez2cDave
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Remember that sims still use formulas, entered by someone else, saving one from have to do it and all of the calculations, manually.
Speed off the rail . . . Thrust to weight ratio, throughout the thrust curve. ( Length of Rail, Acceleration Rate, Time on Rail . . . Velocity, at end of time on Rail ).
Apogee . . . ( one example ) https://www.oldrocketplans.com/pubs/Estes/estTR-10/TR-10.pdf
Optimum Delay . . . Thrust / Weight Ratio, Coasting Charts, etc, etc, etc.
BOTTOM LINE : Since the performance of rocket motors can vary widely and delay times can be erratic, "Sims" or "calculations" are "estimates", at best, particularly when wind on launch day is different than the "simmed wind". The Sim expects everything to happen, exactly as it predicted, based on the info entered ( same thing goes for manual calculations, in truth ).
https://watermark.silverchair.com/2...FT-94EuD7W1ikXZbPvf_nfiwJAj5tO8e2EU6PCfNubzTg
AND
https://watermark.silverchair.com/1...XK7NsRPl7R_8Non_wLfRgatVVsV1dpVQLtF1sAN2IoW4A
Dave F.
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Well yes, that's my kind of point. It's even in their interest, as having good files available helps promote their software.
You probably are. The Barrowman "equations" are just arithmetic (no algebra, let alone anything advanced). There's a lot of arithmetic for anything more than an nFMT, but if you can to basic +-×÷ and follow directions, then you can use them.
Say, here's an idea: how about a spreadsheet where you enter the parts' dimensions and it does the arithmetic for you. Hmm, that'd be a bit tricky to design when you don't know ahead of time how many parts there are; I guess it'd be better as some sort of stand-alone application program where you enter as many parts as you want. It'd be pretty cool to have some sort of visualization of the rocket as you're building it up from the parts. And come to think of it, folks have come up with a few improvements on Barrowman over the decades, so it wouldn't be hard to incorporate those too. You could do all sorts of other things, like rough drag calculations, and some sort of physics calcs when it's launched (but you'd need to import thrust curve data). Someone should write a program like that.
ThirstyBarbarian
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Thanks. I think I’ll use the sims.
Embarrassing confession: sometimes I use a calculator even for calculations that I could do with pencil and paper. I also recently gave up programming my PC by flipping switches on the front panel.
Anti-sim fetish is weird to me. Of course you need to know how to use the tool properly, interpret the results, and blah blah blah. But come on.
Ez2cDave
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Like my high school Math teacher used to say, "calculators are fine, until the batteries die . . . Then what?"
Dave F.
If you lose access to computers for an extended period in the current era, you're facing civilization-threatening problems that will leave hobby rocketry very far to the back of your mind. (Though depending on how organized fights for dwindling resources become before you starve to death or are murdered by an opposing faction, you might want to break out the books to work on designing artillery rockets.)
It's useful to work out the Barrowman equations. It's even more useful to understand them. Same goes for iterative simulations of motion. Calculating stability and performance for every permutation of every component mass, size, shape, and placement throughout design, and simulating every design in various wind, elevation, launch angle, etc. conditions is so time consuming that I can't take seriously the claim that anyone is doing this by hand as thoroughly as a simulator can, nor thoroughly enough not to be a safety issue in high performance flights. I would be surprised for instance if the FAA would take hand calculations as sufficient for approving a Class 3 flight.
For very simple low-power model rockets, sure, I'd even encourage people to work through the math once or twice.
It's useful to work out the Barrowman equations. It's even more useful to understand them. Same goes for iterative simulations of motion. Calculating stability and performance for every permutation of every component mass, size, shape, and placement throughout design, and simulating every design in various wind, elevation, launch angle, etc. conditions is so time consuming that I can't take seriously the claim that anyone is doing this by hand as thoroughly as a simulator can, nor thoroughly enough not to be a safety issue in high performance flights. I would be surprised for instance if the FAA would take hand calculations as sufficient for approving a Class 3 flight.
For very simple low-power model rockets, sure, I'd even encourage people to work through the math once or twice.
Then whip out your slide rule.
ThirstyBarbarian
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New batteries?
I appreciate anyone who can do these calculations, but I’m not sure why anyone would be willing to. A lot of the motors I use have in the range of 30 points in their thrust curves.so for each point you need to calculate the acceleration, speed, and altitude for that segment? And calculate the effect of drag for that segment? Then do the coast to apogee and the delay time calculation? That’s a lot of manual calculation for one motor in one rocket, especially if your batteries are dead! Most of my simulation files have 10-20 motors loaded, so I can pick a motor for the flight profile I want for a given field, given weather conditions, or other factors. There’s no may I’m doing that manually.
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I've been using a solar powered calculator since the early 80s, batteries have never been a problem!
Well, then I guess I'll use one of my other calculators that I have scattered around, where they are handy. 5 different HP RPN models in one drawer, maybe excessive to someone else, pure luxury to me!
Edit: plus 4 slide rules in a box. 3 were my grandfather's, postwar japanese, bamboo, very smooth.
ThirstyBarbarian
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I can’t get OpenRocket to load on my computer.
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Care to explain what you were trying to post with these verbose links to nowhere?
This post is nostalgic Boomer BS. Nobody is using those 60-year-old closed form approximations, let alone solving differential equations by hand so they can say that they are not relying on "somebody else's" equations.
I would love to see your pre-flight analysis with these methods. Then do it a couple more times with different motors and design iterations.
You do realize that TR-10 and the like are rife with assumptions to make convenient hand calculations for the 1960s? Constant thrust, constant Cd, constant air density, constant mass, no wind. Numerical method solutions have none of these limitations , and are thus more safe, accurate, and useful. ...and calculated in a fraction of a second.
I've done the Barrowman calculations for CP on a couple of different rockets, mainly to see how close it came to OR's CP calculation. Both times the two CPs were within .125" of each other, which tells me that I can indeed rely on OR.
My comment to @Ez2cDave —"If I were proficient [...]" was admittedly a little sarcastic (sorry, Dave), but intended to underscore the point that while it may be possible to do that calculation on paper, as well as all the other calcs that OR does on-the-fly, why would I want to spend the time required by manual calculations when OR is completely reliable?
In other words:
That's about as salient as one can get, and hits the nail on the head.
Finally—and quite apart from OR's ability to predict rocket performance based on the data it's given—the ability to design a rocket in OR and see how changes in materials, sizes and weights of components, and motors, all affect performance as the build progresses, is invaluable to me.
That said, I completely understand Dave's desire to 'own' his rocket design by doing the math, however rigorous it might be, and, in the end, be able to say, "I did this!" (Every last bit of it.) Frankly, my hat's off to him for his ability and tenacity.
In the end, if we can't do basic math with a pencil and paper—even in our heads, in many cases—as so many schoolkids these days can't, we're screwed if calculators, computers and software ever go by the wayside. And not just for building rockets.
If calculators, computers, and software ever go by the wayside, it will probably signal the end of civilization. We'll have bigger things to worry about.
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I think you need to update your OS, and perhaps add some more RAM.
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This. I am pretty good at math, but automating long and complex calculations is what computers are best at. They're faster and they aren't going to make silly mistakes the 7th time through a calculation just because their brain is made of meat and inconsistency is inherent. Being able to make a minor change and see the results in a second or two allows me to iterate a design rapidly toward an optimized configuration. To be honest, it's what sucked me into this hobby. I can confidently say I wouldn't be a BAR without OR. If I had to do it by hand, even with a calculator, I'd have much better things to do. Like I'd probably rather go get after the yardwork or make all the toilets in the house sparkle.
Which is kinda the background for the OP. Wanting to know what a kit actually is, going and downloading a file that purports to represent the kit, and immediately seeing that it doesn't actually come even close to accurately documenting the kit I would buy if I went out and bought it today was disappointing, annoying, and frustrating.
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dr wogz
Fly caster
I get a kick out of doing math in Excel: making an a equation or two, and then changing the variable to see the output / results... make a graph even..
it's a tool, like everything else. A fancy tool you nee to plug in, but a quick one, and one less likely to miss a sign or forget to add / multiply a '2'...
it's a tool, like everything else. A fancy tool you nee to plug in, but a quick one, and one less likely to miss a sign or forget to add / multiply a '2'...
You're right, sadly. In fact, it would take us back to the late 19th century, a time when civilization thrived, but without electricity. Today, most of us can't conceive of people ever living that way, let alone actually experience it.
ThirstyBarbarian
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RAM is beads, right? Add more beads?
Maybe I need to add more Java. I hear it wants Java.
Ez2cDave
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Explanation : I was responding to questions asked by "Thirsty Barbarian" . . .
As fast as the various sims are, it concerns me that, in some instances, the program has to be "fooled / tricked" to give accurate results ( Additional, imaginary, "cone" behind the rocket, as one example ).
Dave F.
[never mind, this argument is dumb and pointless]
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NateB
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I can assure you that my brain needs java to write Java and it also needs java for maths. (And most other functions other than sleep)
Ez2cDave
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That is true . . . Of course, with the "tolerances" for actual motor performance in play, the finest sim or equation is, at best, an estimate.
Looking at BOTH Barrowman & the Sims, none of them calculate the "CP shift" when velocity varies, above Mach 1, either.
Basically, a rocket is either stable, or it is not . . .
Personally, I think sims are fine for calculating the CP of a rocket . . . However, I would never trust the CG, as calculated by a Sim. I always manually check the CG, no matter what, and never say "ok, the computer says the CG is right here, so we're good to go" . . .
In closing, my main objection to sims, in general, is that so many rocketeers "blindly" rely on them, without verifying the CG. My advice is to ALWAYS check the CG, manually . . . always !
Dave F.
On this point are are in full agreement. Pre-build CG estimates are good for planning purposes… recommended procedure is to always measure and override mass and CG before making final flight calculations.
My guess is that experienced builders check CG manually. I certainly agree that they should. But checking CG is different than checking CP. I think if you had posted this at the top of the thread, you could have eliminated most of the arguing. That's just me, though.
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Ez2cDave
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