help on my second blackpowder rocket?

[djcruzer]

i have a second tooling that are for 10mm with 3mm nozzle for 100mm cardboard tube the spindle is 70mm long...i used the 60/30/10 mix and i always get CATO...that i have not experienced it on my first tooling that have 10mm with 3mm nozzle for 70 mm cardboard tube with spindle of 40 mm....why i always get a CATO On my second tooling?

[NeighborJ]

Not all charcoals are equal. Some burns too fast for nozzled rockets. The moisture content added to the rocket fuel before use should be around 1% to ensure good compaction and binding. If less is used then the fuel grain turns out crumbly without any cohesion. This results in a faster burn speed or even cato. If the fuel is exposed to humid environments before building the motor it often absorbs enough moisture for use.

[justvisiting]

The second spindle is almost double the length of the first one. I'd suggest:

 

Use the second spindle with hot BP and no nozzle.

 

Use 2-3% water instead of just 1%.

 

Traditional black powder rocket tooling has a nozzle aperture that is half the tube ID. Yours is much smaller. That will lead to CATO.

[NeighborJ]

I tend to disagree. "Traditional" BP motors have a nozzle aperture of 1/3 the tube ID and a nozzle to core length ratio of 1:14. Most modern motors do use the 1/2 tube ID nozzle but still use the 1:14 nozzle to core length ratio.

My point being is that only the 1:14 ratio maters. You could use even smaller nozzles as long as the that ratio is observed.

You are correct in saying the core length of the second motor is too long. 3mm nozzle should never have a core length longer than 42mm.

Edited January 19, 2020 by NeighborJ

[markx]

i have a second tooling that are for 10mm with 3mm nozzle for 100mm cardboard tube the spindle is 70mm long...i used the 60/30/10 mix and i always get CATO...that i have not experienced it on my first tooling that have 10mm with 3mm nozzle for 70 mm cardboard tube with spindle of 40 mm....why i always get a CATO On my second tooling?

 

The longer spindle lenght is most likely to be blamed in the CATOs. It would be very improbable that the casing shall withstand the pressures from almost double the lenght of core using the same nozzle dimensions and same fuel composition. Either tune down the fuel, go nozzleless, or increase nozzle size.....or a combination of all previous.

Scaling rocket motors tends not to follow a linear curve....at least in practice it tends not to. So changing the motor size usually ends up also in bringing about changes to the ratios of dimensions and most likely also changing of fuel composition. It can be quite tricky and challenging to find the "sweet spot".

[djcruzer]

 

The longer spindle lenght is most likely to be blamed in the CATOs. It would be very improbable that the casing shall withstand the pressures from almost double the lenght of core using the same nozzle dimensions and same fuel composition. Either tune down the fuel, go nozzleless, or increase nozzle size.....or a combination of all previous.

Scaling rocket motors tends not to follow a linear curve....at least in practice it tends not to. So changing the motor size usually ends up also in bringing about changes to the ratios of dimensions and most likely also changing of fuel composition. It can be quite tricky and challenging to find the "sweet spot".

do you know any blackpowder ratio except 60/30/10? that can probably work on my second tooling?

[djcruzer]

I tend to disagree. "Traditional" BP motors have a nozzle aperture of 1/3 the tube ID and a nozzle to core length ratio of 1:14. Most modern motors do use the 1/2 tube ID nozzle but still use the 1:14 nozzle to core length ratio.

My point being is that only the 1:14 ratio maters. You could use even smaller nozzles as long as the that ratio is observed.

You are correct in saying the core length of the second motor is too long. 3mm nozzle should never have a core length longer than 42mm.

should i cut my spindle to 42mm to use on 60/30/10 ratio?

[NeighborJ]

As Markx said, scaling changes things because the fuel burn speed does not change, smaller motors use their fuel quicker. On top of that smaller tubes can handle greater pressure than larger ones due to decreased surface area of the tube. Don't worry so much about the physics at first.

 

42 mm would be the optimal (normal) spindle length but if you are looking to dial in your motors to redline then I'd shorten the length to 52mm to start then trim it back little by little until they stop catoing. If you trim it back to 42mm and it still CATOs then I'd look elsewhere as the probable cause.

 

It's a process but once you have things dialed in they are simple and very reliable.

[djcruzer]

As Markx said, scaling changes things because the fuel burn speed does not change, smaller motors use their fuel quicker. On top of that smaller tubes can handle greater pressure than larger ones due to decreased surface area of the tube. Don't worry so much about the physics at first.

 

42 mm would be the optimal (normal) spindle length but if you are looking to dial in your motors to redline then I'd shorten the length to 52mm to start then trim it back little by little until they stop catoing. If you trim it back to 42mm and it still CATOs then I'd look elsewhere as the probable cause.

 

It's a process but once you have things dialed in they are simple and very reliable.

even i cut the spindle to 42 mm there for i will have a thick 58mm blackpowder at top of the core...is this will affect the thrust? or it might only go up and suddenly loose flight due to heavy payload?

i might have planned to have a 25g salute to be attached on top of the rocket...

or i will just shortened the tube as i shortened the spindle?

As Markx said, scaling changes things because the fuel burn speed does not change, smaller motors use their fuel quicker. On top of that smaller tubes can handle greater pressure than larger ones due to decreased surface area of the tube. Don't worry so much about the physics at first.

 

42 mm would be the optimal (normal) spindle length but if you are looking to dial in your motors to redline then I'd shorten the length to 52mm to start then trim it back little by little until they stop catoing. If you trim it back to 42mm and it still CATOs then I'd look elsewhere as the probable cause.

 

It's a process but once you have things dialed in they are simple and very reliable.

even i cut the spindle to 42 mm there for i will have a thick 58mm blackpowder at top of the core...is this will affect the thrust? or it might only go up and suddenly loose flight due to heavy payload?

i might have planned to have a 25g salute to be attached on top of the rocket...

or i will just shortened the tube as i shortened the spindle?

[markx]

do you know any blackpowder ratio except 60/30/10? that can probably work on my second tooling?

Well 60/30/10 is already on the lean side, but you can further decrease the oxydiser content (also worth to play around with decreasing sulfur). Try 50/40/10 or any other combination until you get close.....there are no rules against altering the composition at will, it's trial and error type of work and there really is no universal "fit all" solution. Also you may alter the fuel preparation process by milling for a shorter period of time. This also alters the fuel burning rate towards the slower direction. I also suggest to try wider nozzle sizes, it might just land you in the right spot. You can carefully drill the nozzles to a larger diameter without having to alter the tooling. Wait with the cutting of the spindle, as this step can not be reversed. Try the other options first, then jump to the hacksaw if all else seems to fail.

The solid part of the core on top of the spindle shall usually create a lack of thrust once the channeled part burns out and your rocket tilt off course or even start accelerating towards the ground. Especially if there is payload.

[djcruzer]

Well 60/30/10 is already on the lean side, but you can further decrease the oxydiser content (also worth to play around with decreasing sulfur). Try 50/40/10 or any other combination until you get close.....there are no rules against altering the composition at will, it's trial and error type of work and there really is no universal "fit all" solution. Also you may alter the fuel preparation process by milling for a shorter period of time. This also alters the fuel burning rate towards the slower direction. I also suggest to try wider nozzle sizes, it might just land you in the right spot. You can carefully drill the nozzles to a larger diameter without having to alter the tooling. Wait with the cutting of the spindle, as this step can not be reversed. Try the other options first, then jump to the hacksaw if all else seems to fail.

The solid part of the core on top of the spindle shall usually create a lack of thrust once the channeled part burns out and your rocket tilt off course or even start accelerating towards the ground. Especially if there is payload.

what if i cut the spindle and use 60/30/10 for core and i will use 75/15/10 for thick blackpowder to accomodate the lack of thrust?

[NeighborJ]

The entire length of the tube does not need filled with compacted BP. You want equal or slower mixed fuel above the spindle because it will contain the effects which make the tail display (Ti or added charcoal). I fill my motors to the tip of the spindle with normal rocket fuel then use that same fuel with 5-10% added metal or charcoal to press another 1 to 1-1/2 tube diameters above the spindle. The remaining unused tube can be filled with report comp or cut it short before pressing.

[Mixer]

djcruzer

You could keep it simple by just adding between 2% - 6% Calcium Carbonate to the mix needed for the longer spindle - until the Cato`s stop.

[djcruzer]

The entire length of the tube does not need filled with compacted BP. You want equal or slower mixed fuel above the spindle because it will contain the effects which make the tail display (Ti or added charcoal). I fill my motors to the tip of the spindle with normal rocket fuel then use that same fuel with 5-10% added metal or charcoal to press another 1 to 1-1/2 tube diameters above the spindle. The remaining unused tube can be filled with report comp or cut it short before pressing.

what if i use too much charcoal? will it lose thrust?

[NeighborJ]

Adding too much charcoal will cause chuffing. It will essentially be choking on too much fuel while under pressure. I haven't been able to use much more than 55:35:10 without this happening.

[djcruzer]

Adding too much charcoal will cause chuffing. It will essentially be choking on too much fuel while under pressure. I haven't been able to use much more than 55:35:10 without this happening.

i find a way by not alteringthe tools...so for the 60/30/10 batch i try the toolingand after i drill the 3mm hole with 5mm drill bit this so the nozzle is 5mm while the core is 3mm...when i try it it flew a couple of foot...so i think i will try to use this method but i will alter the ratio to 75/15/10 to hopefully produce much thrust...

[NeighborJ]

5mm nozzle is ideal but make sure you drill at least halfway into the core. If the entire core is 3mm it will become a restriction on its own.

 

75:15:10 will likely be too hot for a nozzled motor. 70:20:10 should be about the hottest mix you can use depending on what charcoal you use.

[SharkWhisperer]

i find a way by not alteringthe tools...so for the 60/30/10 batch i try the toolingand after i drill the 3mm hole with 5mm drill bit this so the nozzle is 5mm while the core is 3mm...when i try it it flew a couple of foot...so i think i will try to use this method but i will alter the ratio to 75/15/10 to hopefully produce much thrust...

Amigo, if you look back over NeighborJ's historical posts, it becomes clear that the dude knows what he's talking about with rockets. Listen to him, and others, before trying to reinvent the wheel on your own. From your posts too, it's also clear that you're a smart guy who's asking the right questions.

 

If your BP isn't complete shit, and I'm guessing it's not, your 75% tests will be time-wasters with a lot of blown tubes/nozzles.

 

"Altering the tools" in this case might simply mean hand-boring out your core to 4-5 mm (instead of 3) after compressing your fuel. NJ suggests at enlarging the core bore at least halfway up the core--ok, try it. Maybe. But I'd personally be boring the whole core to that test width. Me? I'd do a full-core 4 mm then a full-core 5 mm if no take-off energy at 4...

 

Lotta smart folks here. If your BP is satisfactory, and consistanty satisfactory, the dial-in shouldn't be problematic.

 

Our astronauts will be enjoying your handiwork from the Space Station pretty soon!! Then you can work on a rocket to knock off a few of Elon Musk's space junk that he insists on cluttering the vapors with...

Edited January 25, 2020 by SharkWhisperer

[hcb]

i have a second tooling that are for 10mm with 3mm nozzle for 100mm cardboard tube the spindle is 70mm long...i used the 60/30/10 mix and i always get CATO...that i have not experienced it on my first tooling that have 10mm with 3mm nozzle for 70 mm cardboard tube with spindle of 40 mm....why i always get a CATO On my second tooling?

 

I am no expert. I don't know much. But that 70mm core is almost double the core @ 40mm yet the pressure release vent (nozzle) remains the same. You're allowing a significant amount more fuel to burn at one time yet not changing (enlarging) the vent (nozzle). Something's gotta give.

 

--HC