PML Striker stability question

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Brad Thompson

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Hi all,

I have just finished building a PML STRIKER and have determined that I need to add some nose weight for a reasonable stability margin. With a CP at 23 inches, the CG without motor is at 20 inches (measured). With most motors I have simulated, the CG moves back very close to the CP.

I am seeking feedback from others that have built this kit and whether they made any adjustments to nose weight.

thanks in advance
 

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crossfire

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Sounds like it doesn't need a lot of weight.
 

Cameron Anderson

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The stability margin is fluid. 1-1 is easy to remember and harp on, but on long skinny rockets, I have found I need 2.5-1 or 3-1; conversely, on short, stubby rockets (like yours), 0.5-1 has worked fine. That said, I wouldn't go too crazy with nose weight.
 

afadeev

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I have just finished building a PML STRIKER and have determined that I need to add some nose weight for a reasonable stability margin. With a CP at 23 inches, the CG without motor is at 20 inches (measured). With most motors I have simulated, the CG moves back very close to the CP.
I am seeking feedback from others that have built this kit and whether they made any adjustments to nose weight.
PML is very good about publishing instructions and .rkt files online:

Have you downloaded .rkt file and simulated your flights in Open Rocket yet?

Depending on what motor you plan to fly, you may, or may not, need to add ballast.
All 38/120 motors are light enough to preserve 1+ calibers of stability.
38/240 - 38/360 motors drop it to 3/4 calibers of stability. As little 100g of ballast in the nose cone will raise it back to 1+ calibers. Or you may choose to fly as-is, with 3/4 calibers of stability. Which may be sufficient, depending on the cross winds at your launch site.
38/480 and longer drop it below 1/2 caliber of stability. You will need 190g of ballast in the nose cone to raise it back to 1+ calibers.

And so on.
 

Tim51

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Nice build. Never seen a Striker before so downloaded the PML rkt file as I was curious. PML's file shows a stock build with a 38mm 3 Grain being a tad under 1 calibre stability:
Screen Shot 2020-05-20 at 18.55.07.png

and 10g ballast bringing stability margin up to 1:
Screen Shot 2020-05-20 at 18.57.17.png

Recommend increasing the NC weight over 10g to give you a little margin of error, in your actual built weight from the file, different motors etc.
 

mikec

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Did you build it stock? If you omitted the piston and the piston tube, as many seem inclined to do, that will move the CG.

The instructions give you the CP location and suggest you balance it for 1 cal of stability with the motor you plan to use. That may be conservative but I wouldn't go below 0.5.
 

Brad Thompson

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Hi everyone, thanks for your replies. To answer your questions:

- I did download a RKT file and simulated using OpenRocket (I recall that the fins had no mass in the sim file). I could not find it on the PML site though and downloaded elsewhere.
- I meticulously weighed each component and the amount of epoxy used at each step to update the sim
- Rocket weight is 1655g complete without motors
- I built it stock as per instructions but did not use expanding foam in the fin can
- I included a HAMR motor retainer
- The simulated stability without a motor is 0.8 cal
- I'm estimating nose weight of 150g to allow for all motor combinations
 

Brad Thompson

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Hi again.

I just checked the PML rkt file and it is the same as the one I downloaded. Note that the fin set has no mass, so the CG in this file in not correct. The fin set weighed in at 229g prior to assembly.

Cheers
Brad
 

mikec

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Note that the fin set has no mass, so the CG in this file in not correct.
In Openrocket at least, it seems like the fin material got imported with zero density. When I changed it to fiberglass things seemed more reasonable.

At any rate, since you've built the rocket, I would weigh and balance it, enter those values as overrides, and see what happens with different motors. I'm guessing you will need at least a little nose weight to maintain a minimum of 0.5 cal stability margin.
 

Tim51

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Hi again.

I just checked the PML rkt file and it is the same as the one I downloaded. Note that the fin set has no mass, so the CG in this file in not correct. The fin set weighed in at 229g prior to assembly.

Cheers
Brad
My apologies - I missed the massless fins on the PML file.
 

mikec

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I am hoping that some Striker owners might offer some comments too.
Are you hoping they'll say that you don't need to add any nose weight even though you have little or no stability margin? Seems unlikely. The instructions tell you to get to 1 caliber.
 

Spree610

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I built mine stock and flew it for my L1 cert yesterday, unsuccessfully. Flew fine on an H123 with 7sec delay. However, it wasn't until I retrieved it that I realized the piston had seized up/frozen and the deployment charge blew out the whole center from the body, though still connected by the shock cords. Good Luck!
 

Brad Thompson

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Coincidentally I flew my Striker yesterday and the recovery failed. The consequence was that the rocket landed on my car and broke my windshield. I feel lucky that it was my car and not someone else’s.

It all happened quickly, but it appeared that the nose cone separated but did not drag the chute out of the stuffer tube. By the time the rocket reached the ground, the piston and chute were out of the stuffer tube.

lessons learned
A) check the piston moves freely before each flight
B) pack the parachute just before the flight (not 4 weeks before)

This was a costly lesson for me.


PS. There were no stability issues
 

crossfire

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Don't use a piston. This has been talked about for years. Temps, humidity, changes can cause piston to swell and stick.
 

mikec

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Don't use a piston. This has been talked about for years.
It is true that there are things that can go wrong, but there are also advantages and if properly used, pistons can work really well. Checking them for smooth motion right before flying is a really good idea.
 

Cameron Anderson

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Don't use a piston. This has been talked about for years. Temps, humidity, changes can cause piston to swell and stick.
I use a piston in every possible situation I can. The only strrwmeres chutes or bad deployments I've had have been in situations where I couldn't incorporate a piston. Pistons lower the ejection size charge size required, protect the recovery components, and get the entire chute into the airstream and don't leave the chute half in half out. Wood and cardboard pistons should be checked for smooth operation as a part of your pre-flight...small price to pay for a much cleaner recovery chain.
And if you don't want to not worry about swelling and humidity, use fiberglass.
 

crossfire

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Well go ahead and use a piston it's your rocket. But I would guess 99% of flyers will not use a piston. They will bite you someday.
 

OZRoc

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Well go ahead and use a piston it's your rocket. But I would guess 99% of flyers will not use a piston. They will bite you someday.
I would hazard a guess that the 99% is a guess only.
I have always used pistons where applicable / appropriate even down to LPR modifications. All successful.
I am now building an L3 craft where I am incorporating pistons into a 7.5" FG airframe. I will be ground testing different CO2 combinations at a LP/MP/HP launch this August. From there I will consider the attributes of CO2 or BP ejection charges. My gut feeling (based on my experience) is that a lower than recommended CO2 charge (40g) will be required for a successful deployment using the pistons. This is new territory to me but I am quietly confident that I will succeed.

I will reply back here good or bad after the trials in August.
Cheers,
Mark
 

Bill Heath

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Pistons work if properly sized for length and kept religiously clean. It is also best for the piston to be made of the same material as the airframe so that thermal expansion or contraction is the same. I have only had one piston failure and it was from not cleaning between flights.
 

crossfire

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I would hazard a guess that the 99% is a guess only.
I have always used pistons where applicable / appropriate even down to LPR modifications. All successful.
I am now building an L3 craft where I am incorporating pistons into a 7.5" FG airframe. I will be ground testing different CO2 combinations at a LP/MP/HP launch this August. From there I will consider the attributes of CO2 or BP ejection charges. My gut feeling (based on my experience) is that a lower than recommended CO2 charge (40g) will be required for a successful deployment using the pistons. This is new territory to me but I am quietly confident that I will succeed.

I will reply back here good or bad after the trials in August.
Cheers,
Mark
God luck. Hope all works out
 
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