Another Static Test

[Ray]

Did a static test of a 2 grain 75mm motor last night. The burn went extremely well. The numbers matched Burn sim very, very closely. Only 10 PSI difference between predicted and actual.

Some details on the motor;

2-5" long 75mm grains
0.625" core
0.4531" nozzle throat
Max KN - 332
Max Pressure - 720 psi
Burn Time - 4.3 S
Propellant weight - 1299.3g
Estimated total impulse - 2357 NS
Estimated average impulse - 548 N
Estimated ISP - 185 S

Thrust was estimated because my load cell amplifier (custom designed by ME, so was the load cell for that matter) has an output impedance that is too high, and the A/D converter has an input impedance that is too low, resulting in only about half the voltage I was expecting on the output. A redesign is in process and almost finished. Thrust measurements will follow on the next static tests we do.

To estimate the thrust I multiplied the pressure thrust (area of the nozzle throat times the pressure in the motor) by a nozzle coefficient of 1.25.

You'll notice in the video I posted that the nozzle was over-expanded...Nice solid stable huge mach diamond just after the nozzle.

Comments (good or bad) are welcomed, encouraged even.

[HattoriHanzo]

Looking good! That must be the most homogeneous and stable burn I ever seen in a long burn amatuer rocket motor.

What kind of fuel are you using?

[Ray]

Its an Ammonium Perchlorate/HTPB rubber mix. 1% aluminum, 0.1% Iron Oxide. Sorry, I won't share details unless I've met you face to face and I'm comfortable with the knowledge you have of building a motor. But its not a high performance mix, just an easy and forgiving one.

We try very hard to make a very good mix, with a high density. The batch this came from was nearly 96% of theoretical density, with a standard of deviation of 0.17% across the entire casting of nine grains 2.55" x 5".

Next test of this propellant will be in a 5 grain motor...should be a small "M" class, if we get the nozzle right.

One of the guys I do this with will be flying this motor in a rocket this coming weekend.

[HattoriHanzo]

Interesting fuel composition. I have read a bit about ammonium perchlorate and dont feel comfortable to use it yet. I am sticking with candy fuel for a couple of more motors to gain some experience.

I also noted the absence of smoke when you fired your ammonium perchlorate/htpb fuel.
When I did my first firing of a 800 N average motor with 1s burntime a couple of weeks ago I was stunned about the quantity of smoke it generates. It more looked like a smokebomb :roll: I use kno3/sugar at ratio 60/40.

[Ray]

I have another mix that I am working on that has virtually zero smoke, and its a high performance (good ISP) mix. Medium to medium slow burn. Yellowish, white flame that almost looks like a liquid motor.

Go to 65/35, you'll get less smoke, 60/40 is fuel rich for sugar motors. You'll always have smoke in a sugar motor, its liquids condensing out of the motor...I'm pretty sure there are some solids in the exhaust product too.

[HattoriHanzo]

Go to 65/35, you'll get less smoke, 60/40 is fuel rich for sugar motors. You'll always have smoke in a sugar motor, its liquids condensing out of the motor...I'm pretty sure there are some solids in the exhaust product too.

So this could be a practical example of three phase flow?
I will try 65/35 next time.

[Greg O'Bryant]

Wow Ray that is a nice burn!!

Your video inspired me to go to Home depot to see what I have to work with, really itching to get back into the hobby. I found within a matter of just a few minutes everything that I need to make PVC candy rockets. It is almost too easy and actually kind of scary. I can see why you don't like going into a lot of detail with people when it comes to making something that is as potentially dangerous as making a rocket motor. I still don't have my permits so I had to use my left hand to force down my right hand that was grasping the KNO3 and walk out of the store before I changed my mind!:) I was thinking of making a propellant grain like a bates grain, but it would have a very narrow central core and the sides wouldn't be inhibited. This way there wouldn't be as much stress on the grain. It would have pressure applied to all sides of it. Of course I would still need a liner to protect the engine from the heat. Do you have any experiance with something like this or heard of some one who has tried this? Thanks.

[Mark]

Nice reverberation and echolike.
Mark

[Greg O'Bryant]

Hay I just checked the ATF's Web page From what I can see a person is ok to make rocket engines and store them if they have less than 62.5 grams of propellant each. That is small, I know but is not a bad starting place to get back into making engines! This should make a large E or small F sized motor, and would be perfect for double staged rockets just falling short of the 4 oz cutt off to get an FAA waiver. Oh, I also noticed that they don't have guanidine nitrate on their list of regulated explosives, They have nitroguanidine but not the guanidine salt. This may be a potential loop hole to make larger engines! This is the same oxidizer as in the jet-x engines. right? As for my question reguarding the previos post I was wondering what would happen if you didn't inhibit the outside of the grain, would it still work? I know you would have to redesign your engines to handle the extra surface area, but I think it might shorten the burn time too much. Any input would be apreciated. thanks

[Zippiot]

Beautiful burn Ray, but isn't 185 isp kinda low (not saying I can do any better)?

I have to hold off static tests for a while, I tested the propellant in the book you recomended, size e motor. And 2 candy rocket motors around g sized...Not kidding within 10 minutes 4 squad cars and a BLACK HUMVEE showed up at the park!!! They let me off and told me to call a number (some FAA thing I think) before I do it again...

75mm is huge, do you have a pic of the rocket it will be powering?

[Ray]

So this could be a practical example of three phase flow?
I will try 65/35 next time.

I don't know for sure, but I expect that any rocket motor firing is a practical example of three phase flow. You just need to do the chemistry on the propellant, then figure out if the exhaust constituents are gas, liquid, or solid at that temp.

[Ray]

It would have pressure applied to all sides of it. Of course I would still need a liner to protect the engine from the heat. Do you have any experiance with something like this or heard of some one who has tried this? Thanks.

Thanks for the compliment on the burn.

Everything in the motor will have the same pressure applied to it...including an inhibited bates grain. As long as the grain isn't case bonded.

I haven't done any un-inhibited bates grains, but I have heard of others that have and they've been successful. You'll have to protect the motor casing from the heat. Not sure what it will take.

You could always do a monolithic grain, un-inhibited on the outside and the inside, inhibited on the ends. You should end up with a pretty neutral pressure curve. Do the KN calculations and see.

Its amazingly easy to obtain the materials to make a rocket motor...no surprise there, but amazingly hard to make one work "right".

[Ray]

What are you using for engine casings?

Aluminum tubing.

What's the nozzle material?

Fine grain Graphite

A student group at another university wants to have a rocket competition with M engines. The only way we're going to be able to afford that is if we make them ourselves, so I may find myself making APCP some time soon.

Where are they? There are lots of experienced motor makers out there, you may just be able to contact them and have the motors made for you...for the cost of the materials of course.

[Ray]

This is the same oxidizer as in the jet-x engines. right?

Not a clue...no experience at all with it.

Just get your LEUP (low explosive user's permit), and then don't worry about the size of the grain anymore.

Or wait a while, the NAR and Tripoli have a lawsuit pending that will take APCP off the explosive list. For more info read this.

[Ray]

Beautiful burn Ray, but isn't 185 isp kinda low

Not bad for this mix/pressure. The pressure in the motor is kinda low for safety sake, we'll be increasing the pressure in future burns now that we know the propellent is good. That should improve things a bit. 200-230 is very high ISP for AP, candy is in the 130-140 range. Note that this is estimated ISP based on estimated thrust. Overexpanded nozzle contributes to lower numbers all around...etc.

I have to hold off static tests for a while, I tested the propellant in the book you recomended, size e motor. And 2 candy rocket motors around g sized...Not kidding within 10 minutes 4 squad cars and a BLACK HUMVEE showed up at the park!!! They let me off and told me to call a number (some FAA thing I think) before I do it again...

You static tested at a PARK? are you insane? Sheesh, you're lucky you didn't get arrested. Find someplace remote. You really, really need to start thinking about what you are doing here. Don't screw this up for the rest of us.

75mm is huge, do you have a pic of the rocket it will be powering?

Sadly no...another guy's rocket, I'm just the motor cook. It flew nicely though, to around 8000 ft. I think the ignitor slipped to the bottom of the motor though, it took a long time to ignite.