Propellant Burn Speed versus total height achieved

[Uarbor]

So here is my conundrum. I am trying to figure out how to get the highest overall height with my black powder rockets that are pressed on a universal spindle. I am currently lifting a 4 inch shell. I am using medium to slow fuel and they are currently breaking at about a hundred feet. I'm trying to figure out which is better a longer burn time or just using the hottest fuel possible and relying on the coast at the end to achieve the maximum height. It seems like a longer burn time would end up going higher. My real question is is there a happy medium in between burn time and overall thrust from using hot propellant? Or should I just try the hottest propellant I can get away with with a universal spindle which is pretty darn hot because the nozzle opening is gigantic? I am getting ready to start a new batch of fuel which I currently use the ball billing method and then granulation without dextran method. Edited September 5, 2021 by Uarbor

[justvisiting]

You mention the nozzle opening but I assume you are going nozzleless since you are using milled propellant. By the height, I'm assuming you are using the 1lb size. Using 75-15-10 milled with a hot charcoal like ERC, sumac, or willow, you should get much more height than that. You want the shortest burn time possible in this case. A BT of .35 sec is doable with hot BP, and will get a 4" shell to a reasonable display height.

 

With your medium to slow propellant, you might get away with just adding a nozzle to get more height. In that case, I'd make sure to slightly dampen the propellant to get the most in, and use extra insurance of waxing the tube.

 

Lots of folks use hybrid rockets to get more height with universal tooling. You can mix granulated whistle with your granulated BP, or you could press whistle increments first, and finish with BP. Have you read Steve Laduke's notes on that tooling? He wrote quite a lot about it, since he created the tooling.

 

EDIT: Top-lighting the core on a nozzleless rocket gives more height too.

Edited September 5, 2021 by justvisiting

[Uarbor]

You mention the nozzle opening but I assume you are going nozzleless since you are using milled propellant. By the height, I'm assuming you are using the 1lb size. Using 75-15-10 milled with a hot charcoal like ERC, sumac, or willow, you should get much more height than that. You want the shortest burn time possible in this case. A BT of .35 sec is doable with hot BP, and will get a 4" shell to a reasonable display height.

 

With your medium to slow propellant, you might get away with just adding a nozzle to get more height. In that case, I'd make sure to slightly dampen the propellant to get the most in, and use extra insurance of waxing the tube.

 

Lots of folks use hybrid rockets to get more height with universal tooling. You can mix granulated whistle with your granulated BP, or you could press whistle increments first, and finish with BP. Have you read Steve Laduke's notes on that tooling? He wrote quite a lot about it, since he created the tooling.

 

EDIT: Top-lighting the core on a nozzleless rocket gives more height too.

I am using a clay nozzle. The only reason I have the slower fuel is it is left over from when I was hand ramming. Now that I have a proper press it is my belief that my consolidation issues which were causing bulkhead failures will no longer be a problem. I also purchased a tool that will form a bulkhead with a passfire in it. I might try nozzless sometime soon but I have no experience. My current fuel mixture is 60/30/10 with commercial air float ball milled for 4 hours. I was using slower fuel to avoid my bulkhead failures. But now that I have a press and also a bulkhead former I will be starting at square one with my fuel again. But not immediately because I am spicing up my existing fuel with some recent Black Powder made from Birch charcoal. I made a test batch with two parts of my original fuel and one part black powder. The Rockets definitely flu stronger but they did not seem like they were on the verge of red line or anything. I have actually never had a Bonafide Cato all of my failures are from the bulkhead failing and my header going off early. Anyway I want to get a really good working knowledge with black powder before I move on to whistle. Even though I have the chemicals for whistle and my blast Shield is now complete. I did not know nozzleless Rockets could lift a hefty load. You definitely planted a seed in the back of my mind. Edited September 5, 2021 by Uarbor

[stix]

So here is my conundrum. I am trying to figure out how to get the highest overall height with my black powder rockets that are pressed on a universal spindle. I am currently lifting a 4 inch shell. I am using medium to slow fuel and they are currently breaking at about a hundred feet. I'm trying to figure out which is better a longer burn time or just using the hottest fuel possible and relying on the coast at the end to achieve the maximum height. It seems like a longer burn time would end up going higher. My real question is is there a happy medium in between burn time and overall thrust from using hot propellant? Or should I just try the hottest propellant I can get away with with a universal spindle which is pretty darn hot because the nozzle opening is gigantic? I am getting ready to start a new batch of fuel which I currently use the ball billing method and then granulation without dextran method.

That's a great question Uarbor. It's a conundrum for sure. I think your gut feeling of a "happy medium" is the way to go.

A short sharp and immense thrust at the beginning will subject your rocket to too much air drag. Alternatively, too little thrust over a longer time period will put your rocket motor at the mercy of gravity.

 

There is a happy medium.

 

You can test motors on a "test stand" and then use calculations to determine the final altitude. The hardest thing to determine will be the drag coefficient. That is, the opposing force of the air, as your rocket tries to slice through it.

 

There is probably a really good simulator for that, providing you measure everything.

[justvisiting]

Uarbor, 60-30-10 is not typically used as a ball-milled propellant. Usually, the potassium nitrate and sulfur are fine, and half the charcoal is fine. The other half of the charcoal is coarser, and provides for a sparky, bushy tail. The coarse charcoal that blows out of the rocket to create sparks in the offboard reaction is destroyed by your milling. In effect, at the same time you are milling your propellant, you are weakening it too. The coarse half of the charcoal in standard BP rockets that use 60-30-10 is usually 40-80 mesh. The coarsest charcoal in (unmilled) airfloat might be around 100 mesh.

 

The reason I suggested such a short burn time is because of the huge nozzle hole universal tooling has. I don't think you are in any danger of drag seriously affecting your rocket on that particular tooling, if you are using black powder.

 

Stix, I did quite a few tests of black powders using standard black powder tooling. I was trying to maximize the achievable height of the rockets with a 4" ball shell on top, as predicted by Acme software. I started with poor milling media, and made improvements along the way. Eventually, I made super fast black powder, and actually reached the point of diminishing returns, according to my Acme test stand. It has a preset value for drag, and I don't pretend to understand it. But, the fastest powder in a nozzleless rocket predicted a lower height than slightly slower powder. If I was able to do that, it's easy to see that drag would be a bigger issue with whistle or hybrid propellants.

 

Universal tooling provides about half the lift of standard black powder tooling because of the shorter, fatter spindle. It's a 'one size fits all' tooling that is extremely versatile, but not necessarily ideal for all things. With standard black powder tooling, your goal would be much easier to achieve with much less effort. Not to knock your tooling, just letting you know in case you didn't

[Uarbor]

Uarbor, 60-30-10 is not typically used as a ball-milled propellant. Usually, the potassium nitrate and sulfur are fine, and half the charcoal is fine. The other half of the charcoal is coarser, and provides for a sparky, bushy tail. The coarse charcoal that blows out of the rocket to create sparks in the offboard reaction is destroyed by your milling. In effect, at the same time you are milling your propellant, you are weakening it too. The coarse half of the charcoal in standard BP rockets that use 60-30-10 is usually 40-80 mesh. The coarsest charcoal in (unmilled) airfloat might be around 100 mesh.

 

The reason I suggested such a short burn time is because of the huge nozzle hole universal tooling has. I don't think you are in any danger of drag seriously affecting your rocket on that particular tooling, if you are using black powder.

 

Stix, I did quite a few tests of black powders using standard black powder tooling. I was trying to maximize the achievable height of the rockets with a 4" ball shell on top, as predicted by Acme software. I started with poor milling media, and made improvements along the way. Eventually, I made super fast black powder, and actually reached the point of diminishing returns, according to my Acme test stand. It has a preset value for drag, and I don't pretend to understand it. But, the fastest powder in a nozzleless rocket predicted a lower height than slightly slower powder. If I was able to do that, it's easy to see that drag would be a bigger issue with whistle or hybrid propellants.

 

Universal tooling provides about half the lift of standard black powder tooling because of the shorter, fatter spindle. It's a 'one size fits all' tooling that is extremely versatile, but not necessarily ideal for all things. With standard black powder tooling, your goal would be much easier to achieve with much less effort. Not to knock your tooling, just letting you know in case you didn't

thank you for the info and I realize you are not knocking my tooling LOL. I understand the limitations of it much better than I did before I purchased it. I tried using it with sugar Fuel and could barely get a lift off that's what pushed me into black powder from sugar. The reason for the ball Milling is because most of saltpeter is granular and when I try to mill it alone it clumps up on me. Then whenever I Mill the ingredients together I never have a problem so I figured it would be the most consistent way for me to make the fuel. I did forget to mention that I do ad 80 mesh charcoal for the spark and tail after it comes out of the mill. My goal was to hit on a recipe that works when ball milled because my ball Milling process for black powder works very well and I didn't have very many screens at the time. I now have 40 and 50 a 70 and a 100 mesh screens so I am ready to learn some other methods but I always default to Milling because I'm good at it. Edited September 6, 2021 by Uarbor

[Uarbor]

That's a great question Uarbor. It's a conundrum for sure. I think your gut feeling of a "happy medium" is the way to go.

A short sharp and immense thrust at the beginning will subject your rocket to too much air drag. Alternatively, too little thrust over a longer time period will put your rocket motor at the mercy of gravity.

 

There is a happy medium.

 

You can test motors on a "test stand" and then use calculations to determine the final altitude. The hardest thing to determine will be the drag coefficient. That is, the opposing force of the air, as your rocket tries to slice through it.

 

There is probably a really good simulator for that, providing you measure everything.

thank you for the insight. I guess the answer will be determined the way it always is in Pyro experimentation. And I'm just the man for the job lol

[justvisiting]

You could leave out the 80 mesh charcoal and get a thinner tail but more height.

 

You could go 70-20-10, all milled.

 

You could stay with 60-30-10, but use a hotter charcoal for 15 of the 30.

 

You could use a hotter charcoal, mill it as 75-15-10, and leave the nozzle out. You would get almost no tail though. The delay part above the spindle could be your 60-30-10, and then you'd have nice lift and a pretty good tail.

 

Lots of options to try, and that tooling is very forgiving. When I first looked into making rockets, I was totally lost in the vast sea of information. After making even a few rockets, it all started to become more clear (or less unclear might be more like it

[Uarbor]

You could leave out the 80 mesh charcoal and get a thinner tail but more height.

 

You could go 70-20-10, all milled.

 

You could stay with 60-30-10, but use a hotter charcoal for 15 of the 30.

 

You could use a hotter charcoal, mill it as 75-15-10, and leave the nozzle out. You would get almost no tail though. The delay part above the spindle could be your 60-30-10, and then you'd have nice lift and a pretty good tail.

 

Lots of options to try, and that tooling is very forgiving. When I first looked into making rockets, I was totally lost in the vast sea of information. After making even a few rockets, it all started to become more clear (or less unclear might be more like it

I do believe I will try 70/20 ball milled. But I do want to be able to add some sparkle charcoal. Maybe I will do 75 15 with plus 10% 80 mesh. I guess I will go full hotness and then I can always add charcoal.

[dagabu]

Uabor,

 

Specific Impulse is the term we are relaying back to you by advising you on formulas and such. Because you have a specific nozzle opening, are using BP as your fuel and have a specific shell/weight in mind, you are limited by the theoretical limit of BP with your tooling. In order to increase your impulse, you need hotter BP that will burn faster in order to overcome inertia and propel your shell to a height you find acceptable.

 

The problem is that its a paper tube and a caly nozzle so you experience RUD when the container can no longer contain the poressures. In order to keep the rocket tube and nozzle from failing, you need to de-tune your mix so that your container will not RUD and as you have seen, your impulse falls short of your expectations.

 

Dave has suggested you use the 75:15:10 formula for good reason, The optimum proportions for black powder are: 74.64% Potassium nitrate, 13.51% charcoal, and 11.85% sulfur by weight. This allows for the very best impulse and the greatest height but Dave also suggests making the rocket without a nozzle allowing you to use the most powerful fuel you can make with little risk of RUD.

 

For your desired end results, this is the most likely way to achieve them with this formula.

[Arthur]

With BP rockets the old advice was to make the powder so hot that the rocket exploded than back off the power til it was just reliably stable.

[dagabu]

With BP rockets the old advice was to make the powder so hot that the rocket exploded than back off the power til it was just reliably stable.

 

Often for very good reason if one was working to lift heavy shells to display heights. I think it was Ned whom did that with #12 monsters but backed them off 10% so that there would be no RUD's.

 

In any case, if one wants more height, one needs greater impulse, gotta give to get.