Tips for low fuss first-timer groundtesting onsite at club launch?

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rocketsam2016

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I’ll be using electronic single deploy for the first time this weekend (with motor deploy backup) at MMMSC, which I’ve also never been to before. I live in the city in MA, so storing BP legally isn’t possible IIUC given the layout of my home, and even if I was willing to play loose with the storage rules (I’m not), ground testing in my backyard would be a *terrible* idea - 100% chance police would be called. The club has told me I can buy/borrow BP and do ground testing at the site on launch day, but I’d rather not spend all morning on it, so any tips on ground testing at site with a minimum of fuss and busy work? I have a 4” fiberglass airframe with a slightly unconventional interior that works out to about 200 cubic inches of volume to be pressurized with the motor I’ll be flying. My reading online suggests this may need about 1.5g of BP, so I may just start with that and if it looks good go with it.


Alternatively, since this is a kit (Madcow Little John) that is designed for motor deploy only, I could just load the altimeter-controlled charge up with 1g of BP and launch and rely on the motor charge for if the altimeter charge isn’t enough. That would solve the not-spend-entire-launch-day-groundtesting problem :) though I’ve heard that if a charge isn’t enough to separate a rocket you tend to really really cook the laundry…
 
Set the rocket up as much as you can for the ground test before you get there. At our club we treat ground tests almost like a launch. You go past the Range Officer table to the launch area. Set up your rocket for the test. You need to have some way to restrain the rocket for the test. You will want to lay the rocket out so that it is at an angle so the top of the rocket does not go straight up (and then straight back down). We announce the test just like a launch and do a countdown. Ground tests often attract a lot of attention so usually most of the people there are watching you. The top of the rocket will (hopefully) shoot up, but the back end will want to shoot backwards. This is why you need to restrain it. If things don't go as planned you will likely get a ton of advice.

Good luck.

And if you don't separate the rocket, cooking your laundry will not be your biggest problem.
 
Cool thanks for the info. Regarding cooked laundry, I meant that if the motor charge did separate the rocket post-apogee but the electronic charge didn't. My assumption (hope?) is that a kit from a well regarded manufacturer that is designed for motor deploy would successfully deploy on a motor charge.... Nonetheless as you can tell I'm ground testing because I like being sure.
 
Make sure you tell the RSO/Launch director what you're doing first. ;)
 
rocketsam2016 said:
I’ll be using electronic single deploy for the first time this weekend (with motor deploy backup) at MMMSC, which I’ve also never been to before. I live in the city in MA, so storing BP legally isn’t possible IIUC given the layout of my home, and even if I was willing to play loose with the storage rules (I’m not), ground testing in my backyard would be a *terrible* idea - 100% chance police would be called. The club has told me I can buy/borrow BP and do ground testing at the site on launch day, but I’d rather not spend all morning on it, so any tips on ground testing at site with a minimum of fuss and busy work? I have a 4” fiberglass airframe with a slightly unconventional interior that works out to about 200 cubic inches of volume to be pressurized with the motor I’ll be flying. My reading online suggests this may need about 1.5g of BP, so I may just start with that and if it looks good go with it.


Alternatively, since this is a kit (Madcow Little John) that is designed for motor deploy only, I could just load the altimeter-controlled charge up with 1g of BP and launch and rely on the motor charge for if the altimeter charge isn’t enough. That would solve the not-spend-entire-launch-day-groundtesting problem :) though I’ve heard that if a charge isn’t enough to separate a rocket you tend to really really cook the laundry…
Click to expand...


1g or 1.5g probably won't be enough. Start at 2g and go from there. You're use the Pratt charge holders right? As long as you contain the BP that should be enough. If you have your chute wrapped in a nomex blanket, heat damage from the charge shouldn't be an issue. Throw in some dog barf if you need some extra peace of mind. You are looking for the rocket to seperate until the shock cord is almost stretched taut.
 
NateLowrie said:
1g or 1.5g probably won't be enough. Start at 2g and go from there. You're use the Pratt charge holders right? As long as you contain the BP that should be enough. If you have your chute wrapped in a nomex blanket, heat damage from the charge shouldn't be an issue. Throw in some dog barf if you need some extra peace of mind. You are looking for the rocket to seperate until the shock cord is almost stretched taut.
Click to expand...

Thanks for the data point, where does the 2g+ rule of thumb come from? I'll note:

CTI 38mm loads come with 1.3g of BP
CTI 54mm loads come with 2.0g of BP

Madcow sells a non-fiberglass (but otherwise exact same size) 4" Little John that comes with a 38mm motor mount
Apogee markets the fiberglass little john as flyable on 38mm
In both cases this is sold as a motor-deploy-only rocket (I had to get pretty creative to add an altimeter) and nothing in instructions about ground testing or adding additional BP. I am trying to be safe which is why I'm here :)

If 1.3g isn't conservatively good enough though, 38mm motor deploy is bad bad news on this frame and I have a hard time believing Madcow and Apogee both are selling an unsafe configuration of what at this point is a pretty old kit...

Further, the online calculators I found (vernk and one or two others) suggested around 1.5g for the volume calculated (my donut avbay setup reduces the internal volume a bit). I can link to the exact calc I did later

Am I missing something that argues for 2g+?
 
blackbrandt said:
Make sure you tell the RSO/Launch director what you're doing first. ;)
Click to expand...

Lol, I was there that day!

Just like others said , prepare as much ahead of time, pre wire your connection wire to your e-match and run it out a vent hole in your AV Bay and twist together. I stuff dog bark or old Jean material or towel so I don't burn my recovery, make sure motor is plugged / blocked etc.

Here is my test last weekend just sitting next to the low power pads. Bring blanket or something the NC can land on.

The LCO will count down so everyone knows is going on and there are not random BOOMS in the parking area...eh hm.

ImageUploadedByRocketry Forum1476987808.851245.jpg
View attachment 303732
 
Most of the charge calculators I have used estimated less powder than I ended up using after testing. On the other hand, The drogue charge on my 4" X-Celerator is 1.7g for the primary charge, and 2g for the backup. All my larger diameter rockets have redundant altimeters. Since it's a fiberglass rocket, a 2 gram charge won't hurt anything, but a too small charge could ruin your day, and your rocket. It's guaranteed that conditions during the flight will be different than with your ground test, so why not make sure you have a safe, dependable deployment?
Another important factor is how you contain the BP. I used to use disposable nitrile glove tips taped shut, but when I changed over to sealed charge cups with dog barf packed tight and taped over, I got a lot more "bang for the buck" with the same amount of powder.
As far a manufacturers making rockets designed for motor deploy, dream on. Every rocket is different, and it's up to you as the builder to make sure the deployment sequence works correctly, even if it takes extra BP to get your laundry out.
 
Wayco said:
Most of the charge calculators I have used estimated less powder than I ended up using after testing. On the other hand, The drogue charge on my 4" X-Celerator is 1.7g for the primary charge, and 2g for the backup. All my larger diameter rockets have redundant altimeters. Since it's a fiberglass rocket, a 2 gram charge won't hurt anything, but a too small charge could ruin your day, and your rocket. It's guaranteed that conditions during the flight will be different than with your ground test, so why not make sure you have a safe, dependable deployment?
Another important factor is how you contain the BP. I used to use disposable nitrile glove tips taped shut, but when I changed over to sealed charge cups with dog barf packed tight and taped over, I got a lot more "bang for the buck" with the same amount of powder.
As far a manufacturers making rockets designed for motor deploy, dream on. Every rocket is different, and it's up to you as the builder to make sure the deployment sequence works correctly, even if it takes extra BP to get your laundry out.
Click to expand...

Fair enough. Good news/bad news is I'm using pratt cannisters which should contain well. Bad news is size I have caps out at 1.5g so if 1.5 kdoesnt work I'm not launching that day :)

It would still surprise me if a relatively mundane kit that can't be electronic deploy without substantial modification from a manufacturer like madcow was unsafe out of the box when used as instructed...
 
rocketsam2016 said:
Thanks for the data point, where does the 2g+ rule of thumb come from? I'll note:

CTI 38mm loads come with 1.3g of BP
CTI 54mm loads come with 2.0g of BP

Madcow sells a non-fiberglass (but otherwise exact same size) 4" Little John that comes with a 38mm motor mount
Apogee markets the fiberglass little john as flyable on 38mm
In both cases this is sold as a motor-deploy-only rocket (I had to get pretty creative to add an altimeter) and nothing in instructions about ground testing or adding additional BP. I am trying to be safe which is why I'm here :)

If 1.3g isn't conservatively good enough though, 38mm motor deploy is bad bad news on this frame and I have a hard time believing Madcow and Apogee both are selling an unsafe configuration of what at this point is a pretty old kit...

Further, the online calculators I found (vernk and one or two others) suggested around 1.5g for the volume calculated (my donut avbay setup reduces the internal volume a bit). I can link to the exact calc I did later

Am I missing something that argues for 2g+?
Click to expand...

The 2g suggestion comes from past experience. In reality, a 1.3g charge from the motor should be enough. However, the motor charge is tightly contained and the shape has been design to ensure all the powder in burned. So, my arguments are:

  1. In practice, there is some inefficiency in combustion of most rocketeer prepared charges. I have seen 3g surgical tubing charges fail while 2g charges in a long cooper tube have succeeded. It all depends on making sure the BP is properly contained.
  2. There is little downside in using an extra gram of BP. You may have some issue on carboard rockets and you need to pay more attention to your flame protection, but that beats the hell out of a ballistic descent which is a total rocket loss.

In the end, it's a risk mitigation thing for me. I am mitigating far more risk than I and adding by oversizing the charges, so it makes sense.
 
Since you haven't said otherwise, or I missed it if you did, I'm assuming that the nose cone will be friction-fit, as opposed to using shear pins. That being the case, the quality of that fit has everything to do with your ability to deploy with a given amount of BP (aside from the volume, of course). Good preparation would include finessing this fit (just as you've probably done with other rockets). Generally speaking, a "good fit" means that you can lift the rocket with by it's upper section, and not have it separate, but that with little additional work, you can pull the sections apart. I find that this fit can change from day to day with the weather, so get it right before launch day, then check it again when you're at the field. Add tape to the shoulder if it's too loose, remove some (or sand) if it's too tight.

1.5g of BP should produce 15psi or about 180lbs of force, given your 200 cu in volume. I agree that this is a reasonable place to start, and your test will bear that out, or prove otherwise.

Have fun. I hope that a great flight follows.

Mark
 
Last edited:
markjos said:
Since you haven't said otherwise, or I missed it if you did, I'm assuming that the nose cone will be friction-fit, as opposed to using shear pins. That being the case, the quality of that fit has everything to do with your ability to deploy with a given amount of BP (aside from the volume, of course). Good preparation would include finessing this fit (just as you've probably done with other rockets). Generally speaking, a "good fit" means that you can lift the rocket with by it's upper section, and not have it separate, but that with little additional work, you can pull the sections apart. I find that this fit can change from day to day with the weather, so get it right before launch day, then check it again when you're at the field. Add tape to the shoulder if it's too loose, remove some (or sand) if it's too tight.

1.5g of BP should produce 15psi or about 180lbs of force, given your 200 cu in volume. I agree that this is a reasonable place to start, and your test will bear that out, or prove otherwise.

Have fun. I hope that a great flight follows.

Mark
Click to expand...

You are correct, friction fit. I do actually have some shear pins and it'd be trivial to switch to using those. My gut actually really likes the idea of the consistent reprocible shear forces, but it seemed that most folks on TRF think that shear pins for single-break rockets are overkill and the road from KISS to overly complex rockets that create risk of a mistake is paved with individually good ideas :)
 
blackbrandt said:
Make sure you tell the RSO/Launch director what you're doing first. ;)
Click to expand...

rocketsam2016, check with Scott at the launch site. Guy and I will be doing safety and LCO duties, most likely, so it will be easy to coordinate activities. Robert from AMWProX will be on site, too, and will have a wealth of knowledge on which you can draw. There will be lots of people with recorded data about powder requirements for various rocket sizes.
 
KennB said:
rocketsam2016, check with Scott at the launch site. Guy and I will be doing safety and LCO duties, most likely, so it will be easy to coordinate activities. Robert from AMWProX will be on site, too, and will have a wealth of knowledge on which you can draw. There will be lots of people with recorded data about powder requirements for various rocket sizes.
Click to expand...

Yup talked with a few times Scott via the MMMSC facebook account, he's the one who said I could ground test onsite. I figured I'd come to TRF rather than bother him with all my questions. Looking forward to meeting you all this weekend and having a few great safe flights. I'll be the guy with a fluorescent pink Little John trying for my L1 cert after groundtesting in the wings.

And here I was originally just going to skip the altimeter on the cert flight until the Oct 1 launch was scrubbed (giving me time to build my quantum) and my mind was changed by a thread I started (https://www.rocketryforum.com/showt...ronics-for-redundant-single-deploy-on-L1-cert). Now it seems I should be worried the motor charge isn't enough, especially since I'll be certing with a 38mm or 29mm :)y:) load. Ground testing will ease all my concerns.
 
Thanks! And if anyone reading this feels like being the person who signs off this weekend at MMMSC on my rocket and cert if the flight is successful feel free PM me. I was told no problem to just show up with my rocket, the paperwork and ready to talk through the build, CP/CG, flight simulations, etc. and there'd be plenty of people around at the launch happy to do it.
 
Quoting Mistah David here....
Protip #1: don't do it while on the pad.
Protip #2: Don't do it in the parking lot a short hop down from the TRA BOD.
Protip #3: You don't have to use the whole @#%$$%$ can.
Protip #4: Stay clear and block the rocket with something preferably not made of meat.
Click to expand...
 
Just to update the thread, ground testing went great and 1.5g separated the rocket nicely without quite reaching the end of the shock cord (video attached, the force wasn't quite enough to undo some loosely bound z folds in the shock cord, which seems perfect).

During the inaugural flight, I intended to have the motor ejection charge kick off several seconds after apogee as a backup, but it actually triggered before the altimeter charge, and the 1.2g from the motor charge (29mm CTI H399) was enough to separate the rocket completely.

So, I'll definitely be sticking with 1.5g going forward, which conveniently is exactly the size of the pratt cannisters I'm using.

Thanks all for the help!
[video]https://youtu.be/IOS_zrOJKZE[/video]
 

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