Rocket calc ?'s

[Zippiot]

For chamber pressure (Pc) the formula is
Pc(lb/in^2)=Kn x Isp(sec) x Propellant Density(lb/in^3) x Burn Rate(in/sec)

My question is, for burn rate is that open air or enclosed?

What is a "good" Kn for a regular candy rocket?

Just deciding between fiberglass or aluminum...

[Ray]

Burn rate isn't really an open air/enclosed type of question. Its actually very dependent on several items...one of them being pressure, so your equation ends up being recursive. The math for burn rate is below.

Burn rate = c+ap^n
but can be simplified to burn rate=ap^n, it gives a close approximation.
Where;
c is burn rate in a vacuum (a constant)
a is burn rate coefficient
n is burn rate exponent
p is chamber pressure (psi)

You made one error in the equation by using Isp instead of Isp* SO to restate your equation...after reduction and making some assumptions...like the simplified version of the burn rate calculation and solving for pressure;

Pc=(Kn x Isp* x a x Dwt)^(1/1-n)

Where;
Pc = chamber pressure (psi)
Kn = area ratio (burn area/throat area)
Isp* = characteristic specific impulse (seconds)
Dwt = propellant weight density (lbs/cu in)
a = burn rate coefficient
n= burn rate exponent

a, n and Isp* can only be discovered by experimentation. There are some spreadsheets that will help you to calculate them based on test burns in a ballistic motor.

Isp* is (chamber pressure x throat area x burn time)/propellant weight.

So that's the math...Nakka takes care of that in his spread sheet, and even provides some starting values of a, n and Isp*. He gets to Isp* the long way though, just use the values he has...

In the end you ask two questions, burn rate, and a good KN for a regular candy rocket.

A good Kn is one that delivers the chamber pressure you desire for the motor you have designed. If that motor is aluminum you can run at one pressure, if fiberglass, another, if PVC, another...

The key here is *design* the motor for the casing material and its strengths. You can get there using Nakka's spreadsheet, or you do the math by hand...be sure to use the largest value of Kn that you will see...another set of calculations...

You could also buy BurnSim. It'll do all of this for you.

Did you buy the book Designing rocket motors by Gas Dynamics Lab? All the math you want is in there...

[Zippiot]

OK add another book to the list :)

I am trying to compare 2 different porpellants but each is characteristically different. One is standard 65/35 candy and the other is a KNO3 composite I'm working on. The composite burns at a much higher temp and gives off fewer solids, also is rubbery and flexible. Burn rate inside the engine seems the same but otuside it is near impossible to keep the composite lit, which is understandable...
Thats why I was wondering how to calc the Pc, inside the engine they burn near he same rate but anywhere else its not even close.

Still waiting for the trust stand to accuratly measure Isp*, without a speadsheet anywhere to guesstimate?

[Ray]

There really isn't a good way to guess on the burn rate in a motor. As far as I know the only way to find out burn rate is to burn a motor, time it with reasonable accuracy, then calculate it based on web thickness.

You want to do it in a ballistic motor for the greatest accuracy, for even better accuracy, a strand burner is the way to go.

A ballistic motor is just a very heavy walled motor (to cope with unknown pressures) with bulkheads in either end. One end has a nozzle that you design with different Kn ratios. Do a bunch of test burns, plug the data into a spread sheet and get things like a, n and Isp*.

You would want to do the grains in an end burner configuration that way the Kn ratio remains constant.

Chamber pressure is a very important factor in all of this, so get yourself a good pressure transducer along the way.

Building your own motors is fun, but it isn't cheap. I have a TON of money invested in doing this. I could have flown a lot of commercial motors for the money I have spent in flying a few homemade motors.

The other way to design a motor to work with a given propellant is to just try it. If it blows up, try again with a larger nozzle. Repeat until successful burn. You could also go the other way, start with a large nozzle. Reduce the nozzle size until it blows up, then use the last nozzle previous to the CATO. Please do this in a remote area if that's the route you decide to go...

The down side to the try it and see approach is that its only good for THAT motor...You have to do it for each motor you want to use...change the grain configuration (number, core size, length etc) you have to go through the whole process again. A ballistic motor allows you to predict what you will get in a motor...once you get good at it, static tests are not as critical.

[Zippiot]

It is a mix of interest and physics extra credit for me.

I know it can be one of the more dangerous things to experiment with, but as I am sure you know, the payoff can be great.

How high have you gotten a rocket to go? I'm hopin for over 2k feet, better than the old estes but not quite as good as my aerotech reload.

[Ray]

Sorry, updated my previous post...added some things...

I've managed to push one of my rockets (26 lbs) to 14,631 ft on a motor I've built. It was a 5 grain 75mm APCP motor.

A couple of my friends have gotten about the same altitude with some of the motors I've built.

[Zippiot]

I'm holding a 15 pound weight and am wondering the size of the monstrosity you pushed into the sky?

I like the try and see approach but would like to start somewhere near the final...just to save time and have a lower chance of losing the rest of my hearing...

I definitely see what you meant by endburner, determining the Kn of a bates grain as it burns is nothing short of a physics nightmare...maybe I'll get extra extra credit! Actually it really isn't that bad, I'll make my partner do it shes gotta do more than camera and poster work on this thing!

[Ray]

Another way you can approach it is to burn some motors at pressures you believe will be "safe", be very conservative. Measure the internal pressures, and burn times. Plug the numbers into the spreadsheet from the OTHER book you bought (McCreary) and you'll get an approximation of the a, n and Isp* values you need.

The spreadsheets provided with the book are very good. I prefer to use burnsim for the motor prediction, it works better, but the ones with the book are pretty good too.

Of course, this approach assumes you have a pressure transducer and a way to record the data.

I used a Dataq unit (RS-194), about 25USD, to record the data, and a MSI pressure transducer...its available from Digikey. I used P/N MSP6252P5-1-ND (digikey part number), its about 120 USD. They have cheaper versions. You'll need a computer with a serial port to get the data from the A/D converter. The RS-194 is a starter kit that comes with a software package that will allow you to capture and save the data. You can break into data acquisition for your rocket motors for about 150 USD, not including the computer.

[Zippiot]

Sweet, I'll start saving up!

I'm right on the fence between Sugar rockets and full blown composites...The differeance is being able to buy ingredients around the corner or over the internet. It actually comes out to the same price (the garden store is ripping me off I'm sure) so further down the line I will start the composites again, once I have a better grasp on design aspects and doing calc's. Last set of composites was depressing...Mostly I barely knew waht I was doing and didn't donate the appropriate time. It is rocket science afterall.

[Ray]

Re-reading this I realized that I didn't answer some of the questions you ask...so here goes.

The altitude you get with the sugar motor on your rocket should be better than the estes BP motor, and should be less than your Aerotech reload. The ISP of the BP is lower than the sugar motor, but the sugar motor is lower ISP than the Aerotech. Assuming that you have equal qty of propellant in each.

The rocket was a 4" diameter and approximately 9 ft long. Dual deployment with a 8 ft main set to 700 ft. Dry weight of the rocket (rocket and recovery system) was right around 15 lbs, with the motor, about 26 lbs...Flight was good, very straight with a slight "kick" off the rail. I believe the rail had a small burr on the end that caused the kick. The first photo is of the rocket on the pad (its the orange one to the left). I didn't have time to paint it prior to departure for the launch, so I painted it on-site in the desert...hence the crappy paint job. The second photo is a lift off shot...I think it looks cool, its my back ground now. The event was BALLS 15 at the black rock desert in Nevada. My rocket was just one of the smaller ones launched at the event other photos of the event are in the link. There were some HUGE rockets, several that weighed in over 400 lbs.

If you want to start near the final level, the best way is to use what others have done, in the sugar world, that's Nakka and James Yawn...

Go with composites, you'll be happier, but they are more costly to produce...as you grow in size, you'll find that you'll need things like a mixer, vacuum pump, test stand, cases, scales, etc...the list just keeps going.

[Zippiot]

I HAD everything I needed...it was either ruined or broken..which is sad to me.

I have 3 different coffe grinders I use, 1 for oxidzer 1 for fuel and the last is for the other catagory, I diswasher them (detachable hopper thing) between different fuels/oxidizers to reduce contamination.

Had a triple beam ballance but the dog knocked it off the table, bent it up so its busted, need to find a new one.

I have a weak vacume from an old fishtank pump

Now that I have a welder I can make a test stand

Got a curing box, works well

Just need some nichrome for igniters and to make a few cases/nozzles

Probably am going to play with the 1 time use until I find the correct nozzle and fuel to match what I'm looking for....easy to light burns strong and inexpensive. Overall power should be more than sugar, as long as it is I am satisfied.

[Ray]

Get the biggest scale you can afford. I have a 4000g 0.1g resolution Ohous digital balance. I also have a 7000g 1g resolution generic scale. The Ohous was expensive the generic scale was not. I use the fine balance for the small value measurements, and the generic scale for the bulk items (AP in particular). Discountscales.com is a good place to get them.

A GOOD vacuum pump is required for processing APCP. You want something that will pull to at least 50 microns absolute vacuum. Anything less is just a toy and won't do what you want it to do.

Buy your AP in the particle size you need it at...grinding the stuff could be dangerous. <20 micron AP can detonate spontaneously.

Stop washing your grinders in the dishwasher...remember you eat off the things you clean in there, you don't want residues of chemicals left over.

[Zippiot]

Very good point, I actually wasn't the first to wash it in there...I'll stop. Only used it for kno3 so far.

What is the smallest mesh size you would get of any oxidizer?
I don't want to use metals for starters, any other good fuels (not binder) that wont mess things up?

[Ray]

The mesh size is going to be dependent on the oxidizer...and the effect you are going for.

I work with AP most often, so I'll speak from that perspective, I am most familiar with it.

I think the finest I would go on AP would be around 60 micron. Most of the stuff I've done is with 200 micron, but you can get your solids loading up with a bimodal, or even trimodal mix. As you go finer in particle size you'll see the viscosity increase and burn rates increase. You'll need to account for that in your mix. Remember to stay above the 20 micron limit. AP can and will go high order at that particle size or smaller.

You want to use metals, aluminum primarily, in your mix. It does a couple of things for you. First, it improves ISP dramatically. You get more "bang" for the buck. Second, it tends to make the propellant more stable during the burn. Just a couple percent (or even one) make a much easier propellant to work with. I use 325 mesh aluminum in my mixes...spherical is preferred over flake. Another side benefit is that metals have more density than the AP, so adding them into the mix tends to make the viscosity lower.

Metals can also act as a catalyst...usually elemental (pure) metals don't have this effect, but Oxides do (Iron Oxide, Cupric Oxide, Manganese Dioxide, etc).

[Zippiot]

A friend gave me the recipe
%
67-AP
20-PBAN
2-epoxy (to harden PBAN)
10-Mg
1-Boric Acid (supposedly increases stability)

Does that seem like a reasonable mix or too many solids? He tried to keep it simple so basic measuring tools can be used (still need to find an accurate scale but that is an easy fix)

This time around I wanted to do some calcs before I started burning away stuff but it looks like much testing has to be done to find the Numbers to place in the calcs...Back asswards if you ask me