Hi from the French Riviera

The Rocketry Forum

Help Support The Rocketry Forum:

HPman83

Member
Joined
Jan 17, 2022
Messages
5
Reaction score
1
Hi,
I'm starting to build my stick rocket with home made motors. I'm still going back and forth between KNS fuel and BP.
I have a very basic question about stick rocket stability.
I made a launch pad with 2 metal rod to guide the rocket during the first 1.8m of flight.
I successfully launch straight up some BP rocket before.
Yesterday i tried a KNS fuel rocket and I took off straight up for the first 30m then turn 90° and flew horizontally before crashing.
What can cause a sudden turn like that? Is a lost of thrust due to inconsistent fuel can cause this type of behaviour?

Thanks
 

Antares JS

Professional Amateur
Joined
Mar 5, 2020
Messages
2,246
Reaction score
3,631
Location
Eastern Shore, VA
The list of things that can cause that is pretty long and we would need more information to narrow it down. I would suggest starting with some detailed pictures of the rocket, particularly the back end, and more information about the design of the rocket.
 

HPman83

Member
Joined
Jan 17, 2022
Messages
5
Reaction score
1
Rookie, mistake. I didn't take a photo of the rocket and I lost it for good during the horizontal flight. Impossible to find it, maybe landed in a tree or something.

Basically, the one which worked perfectly was the standard stick rocket. A stick glued to the motor, I balanced the CG around 5mm behind the nozzle and added fins at the end of the stick (horizontally and vertically). The stick was made with bambou stick (not exactly bambou, but very similar. It's straight, it's light and available for free in my garden).

The one which took a 90° turn is the same principale, except I put 2 stick instead of one. One stick on each side of the motor. I balanced the rocket to have the CG behind the nozzle and same thing fins horizontally and vertically at the end of the stick. Of course because there are 2 sticks instead of one, they were shorter in lenght (the motor was the same size, made with glued paper, only the fuel was different).
 

Funkworks

Low Earth Orbit, obstructing Earth's view of Venus
Joined
Jul 28, 2018
Messages
3,083
Reaction score
3,063
:welcome:

I can't answer your question but I bet if you post it in a section with more views, you'll get more answers from people who might have seen this before and thought it through.
 

Antares JS

Professional Amateur
Joined
Mar 5, 2020
Messages
2,246
Reaction score
3,631
Location
Eastern Shore, VA
Of course because there are 2 sticks instead of one, they were shorter in lenght (...)

This statement from your description caused me to raise an eyebrow. I would suggest trying again with your one-long-stick design and seeing if that works. When it comes to stick rockets, length of the stick matters quite a bit. Your sticks may not have been long enough to keep the CP of the rocket back far enough.

I would also suggest that when experimenting, change only one thing at a time. This makes it much easier to diagnose problems.

EDIT: Another idea I had, Sugar propellant burns hotter than BP. Could one of the sticks, the bottom of your case, or a chunk of your nozzle have burned away? This would explain why it was straight at first before veering off.
 

HPman83

Member
Joined
Jan 17, 2022
Messages
5
Reaction score
1
EDIT: Another idea I had, Sugar propellant burns hotter than BP. Could one of the sticks, the bottom of your case, or a chunk of your nozzle have burned away? This would explain why it was straight at first before veering off.

It's possible. Because on some other smaller rocket motor I had the casing failed in one spot with some ejection gaz on the side. I successfully launched some KNS motor before but I think my motor had more layers of kraft paper. Too bad I cound find the rocket to validate or invalidate this theory.
 

prfesser

Lifetime Supporter
Joined
May 7, 2017
Messages
2,629
Reaction score
3,323
Location
Murray, KY
The following is speculation:

My understanding is that stick rockets are stable (at least in part) because of the drag created by the stick. Two shorter sticks should have the same drag as one longer stick...but the *effective* center of pressure will be farther forward because the sticks are shorter.

Also, as the propellant burns the motor's mass decreases, which moves the center of mass farther aft. That may have caused the instability in the middle of the flight.
 

HPman83

Member
Joined
Jan 17, 2022
Messages
5
Reaction score
1
Yesterday I tried two stick rocket with BP fuel. I level perfectly the launch ramp and the metal rod guide the rocket for almost 2m. The rocket is guided with 2 ziptight on the motor and the fins are guided by the metal rod too.

The first flight was perfect. Went straight up very high, turned away, went straight down for 6s and crash landed only 6m away from the launch pad.
The second one had a little offset nozzle, then the hot gazes destroy one of the fin and poke a hole in the bambou stick. So it flew a little bit less straight but landed within 18m of the launch pad after around another 6s of freefall.
 

Attachments

  • stick_rocket_launch_pad.jpg
    stick_rocket_launch_pad.jpg
    155 KB · Views: 19

neil_w

OpenRocketeer
Joined
Jul 14, 2015
Messages
13,712
Reaction score
6,952
Location
Northern NJ
The following is speculation:

My understanding is that stick rockets are stable (at least in part) because of the drag created by the stick. Two shorter sticks should have the same drag as one longer stick...but the *effective* center of pressure will be farther forward because the sticks are shorter.

Also, as the propellant burns the motor's mass decreases, which moves the center of mass farther aft. That may have caused the instability in the middle of the flight.
He has said (and latest pic shows) that he has fins at the end of the stick(s). So, stability-wise, this should behave more like a traditional rocket, I would think.
 

Antares JS

Professional Amateur
Joined
Mar 5, 2020
Messages
2,246
Reaction score
3,631
Location
Eastern Shore, VA
He has said (and latest pic shows) that he has fins at the end of the stick(s). So, stability-wise, this should behave more like a traditional rocket, I would think.

Not necessarily. His nose is losing weight instead of his tail as the propellant burns, meaning the CG is moving aft during the burn, so the rocket is becoming less stable until burnout.
 

neil_w

OpenRocketeer
Joined
Jul 14, 2015
Messages
13,712
Reaction score
6,952
Location
Northern NJ
Not necessarily. His nose is losing weight instead of his tail as the propellant burns, meaning the CG is moving aft during the burn, so the rocket is becoming less stable until burnout.
Yes, but the CP is still that of a more of a traditional rocket. It's hard for me to imagine that a long stick with fins at the end wouldn't put CP considerably behind CG, even after the motor loses its propellant mass. But without exact measurements and numbers it's impossible to tell.
 

Antares JS

Professional Amateur
Joined
Mar 5, 2020
Messages
2,246
Reaction score
3,631
Location
Eastern Shore, VA
Yes, but the CP is still that of a more of a traditional rocket. It's hard for me to imagine that a long stick with fins at the end wouldn't put CP considerably behind CG, even after the motor loses its propellant mass. But without exact measurements and numbers it's impossible to tell.

Yes, but the one that went unstable according to the OP was described in post 3 as only half as long. That would be more suspect.
 

HPman83

Member
Joined
Jan 17, 2022
Messages
5
Reaction score
1
Hello,
some numbers about my last super stable rocket (landed only 6m away from the launch pad).
Total Lenght: 52cms
Stick lenght: 42 cms
Weight with fuel: 85 gr
Weight empty: 52 gr
When fueled the CG is about 5mm behind the nozzle.
Empty the CG move 52mm behind the nozzle.
 
Top