Merry Christmas to all, I hope that santa was good to you and yours.
On Thursday a couple of my friends static tested some of the 54mm fuel grains I had made previously. The test went nicely. No big booms or anything!
You can watch the video here.
Merry Christmas and static test
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Rocketdork
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bugfreezer
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Rocketdork
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Slow burn vs fast burn...
Assume the same amount of total impulse ie 750 Newton Seconds. Thats the amount of work the motor will do.
A fast burn motor will obviously have a higher average impulse (total impulse / time = average impulse). You'll be able to lift a heavier rocket, accelerate the same rocket to a higher speed and depending on the weight of the rocket, you may or may not get more altitude. Higher speed = higher drag, so getting the thing going really fast can work against you on altitude.
A slow burn motor has lower average impulse, and won't lift as heavy of a rocket. They tend to be slower flights, so you can see them easier, and again depending on the weight of the rocket you'll likely get higher altitude.
Each appeals to rocketeers for lots of reasons. Fast burn motors tend to be "neck snappers" and that's fun. I know one guy that just likes speed, his fastest flight that I am aware of was mach 2.5 (2.5 times the speed of sound). So fast that it burned the paint off the tip of the nose cone, and leading edges of the fins due to aerodynamic heating. Slow burn motors are fun because the usually leave a smoke trail that is fun to follow, and many times the motor is still burning as the rocket goes out of sight. They are used in altitude records almost exclusively. A slow burn motor usually burns for more than 3 seconds...Fast burn can be under a second.
Thrust to weight ratio needs to be about 5-10:1. This gets the rocket moving fast enough of the rail for a stable flight. You want about 45 ft/second by the time the rocket leaves the guide rail. To get there on a 6ft rail you need about 5 g's acceleration (5:1 thrust/weight). The shorter the rail, the higher the accleration and higher thrust/weigh ratio you need, of course, the opposite is true. The motor in this test would be capable of lifting a rocket in the 7-10 pound range...and would have about 2000 ft on a rocket that heavy.
The motor cost me about $3 to make in materials and we cast 4 ft of this size in one 4 hour session. We used 6" in this test. A commercial motor of the same size would have cost about $65...other than the science of doing it you can see some of the appeal making your own motor has. The science is just a damn lot of fun to me too.
In this test the motor burned for longer than the predictions said it should...that means we made a mistake somewhere. The nozzle could have been too big...but the most likely source of the extra burn time was too little Iron Oxide. The mix only has 0.1% iron in it. Iron acts as a burn rate catalyst. A small change in the weight of the iron added can make a big change in the "speed" of the motor. During the mix of this 3 kilo batch, we knocked the cup with the iron in it over, we recovered most of it, but with the total qty only 3 grams, even a small amount lost makes a big differance. I'll know for sure after measuring the nozzle throat, and burning more grains from the same batch.
I wasn't actually present for the test...so I don't have the things I usually look at following a test. There are lots of indicators that you can see in the residue and left overs...
I have video of 5 other ground tests, they can be found here. The server is slow (256kbps) so be patient.
Assume the same amount of total impulse ie 750 Newton Seconds. Thats the amount of work the motor will do.
A fast burn motor will obviously have a higher average impulse (total impulse / time = average impulse). You'll be able to lift a heavier rocket, accelerate the same rocket to a higher speed and depending on the weight of the rocket, you may or may not get more altitude. Higher speed = higher drag, so getting the thing going really fast can work against you on altitude.
A slow burn motor has lower average impulse, and won't lift as heavy of a rocket. They tend to be slower flights, so you can see them easier, and again depending on the weight of the rocket you'll likely get higher altitude.
Each appeals to rocketeers for lots of reasons. Fast burn motors tend to be "neck snappers" and that's fun. I know one guy that just likes speed, his fastest flight that I am aware of was mach 2.5 (2.5 times the speed of sound). So fast that it burned the paint off the tip of the nose cone, and leading edges of the fins due to aerodynamic heating. Slow burn motors are fun because the usually leave a smoke trail that is fun to follow, and many times the motor is still burning as the rocket goes out of sight. They are used in altitude records almost exclusively. A slow burn motor usually burns for more than 3 seconds...Fast burn can be under a second.
Thrust to weight ratio needs to be about 5-10:1. This gets the rocket moving fast enough of the rail for a stable flight. You want about 45 ft/second by the time the rocket leaves the guide rail. To get there on a 6ft rail you need about 5 g's acceleration (5:1 thrust/weight). The shorter the rail, the higher the accleration and higher thrust/weigh ratio you need, of course, the opposite is true. The motor in this test would be capable of lifting a rocket in the 7-10 pound range...and would have about 2000 ft on a rocket that heavy.
The motor cost me about $3 to make in materials and we cast 4 ft of this size in one 4 hour session. We used 6" in this test. A commercial motor of the same size would have cost about $65...other than the science of doing it you can see some of the appeal making your own motor has. The science is just a damn lot of fun to me too.
In this test the motor burned for longer than the predictions said it should...that means we made a mistake somewhere. The nozzle could have been too big...but the most likely source of the extra burn time was too little Iron Oxide. The mix only has 0.1% iron in it. Iron acts as a burn rate catalyst. A small change in the weight of the iron added can make a big change in the "speed" of the motor. During the mix of this 3 kilo batch, we knocked the cup with the iron in it over, we recovered most of it, but with the total qty only 3 grams, even a small amount lost makes a big differance. I'll know for sure after measuring the nozzle throat, and burning more grains from the same batch.
I wasn't actually present for the test...so I don't have the things I usually look at following a test. There are lots of indicators that you can see in the residue and left overs...
I have video of 5 other ground tests, they can be found here. The server is slow (256kbps) so be patient.
"A man without a woman is like a statue without pigeons"
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eddiecanuck
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bugfreezer
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