Ballast

[cerving]

I'm finishing up a 7.5" Polecat Thumper (5 x 54mm motors) with a 30" extension, to fly it with cluster motors it's going to take about 5# of ballast. I'm thinking about pouring some two-part foam in the nose cone and when it gets a little mushy add some steel shot to it. It would probably take 3 or 4 pours to get it done. Has anyone out there done it this way?

 

[bclark989]

I've done this with regular epoxy (I think I used BSI 30 minute), but only to add about 1 pound. With that much mass, I would worry about retention a bit. Do something to make sure takeoff G's don't cause the whole ballast slug to just dislodge and potentially break stuff.

 

[cerving]

The foam-shot plug would be in front of a bulkhead in the nose cone, so it ain't going nowhere. My experience with foam has been that it doesn't move... and this rocket isn't going to be setting any acceleration records either.

 

[Donnager]

Yeah,

But not 3 or four pours. Usually just one, maybe two. That stuff gets away from me.

I usually just put in tig wire and use epoxy when there's serious weight (>2lb). I have done the epoxy pour with shot and filled the rest of the nose with foam after to keep everything secure.

 

[SolarYellow]

I'm building smaller rockets, but I picked up a jug of this to use as ballast after a recent thread discussed both the risks of solid ballast and the potential for metallic ballast to interfere with radio transmissions from onboard electronics. I figure the glass should be RF transparent and if I only encapsulate the top of it with epoxy, if it comes in ballistic, there's at least a chance of it having reduced potential to damage whatever it hits than it would if it was solid.

https://www.harborfreight.com/5-lb-ground-glass-4070-grit-abrasive-media-63674.html
You can get glass beads at a lower unit price in larger quantities, but this is more than I expect to use in the next decade of building my little rockets.

At some level, one might just use sand, but if you've ever dragged a magnet through sand in the wild, there's a lot of stuff that sticks to it, so I figured the glass might be cleaner on the RF front.

 

[dhbarr]

I would think filter sand probably wouldn't have much extra in it, but that's just a guess.

 

[David Schwantz]

Cris, I have done just what you said minus the bulkhead, it came loose. I now either fill the NC with foam after putting weight in or I make a bulkhead with a tube that goes forward into NC and a plate that screws on to hold everything in place. On my Bullpup I have 25 pounds of Quickcrete in it.

 

[Rschub]

If you use Gorilla Glue and spray it with water after you pour it in, it foams quite a bit. Plus it is glue.
The foam will eventually crush and the weight will rattle around just from normal handling and transportation IMO.

 

[cerving]

There is a 38mm hole in the nose cone... I could sink a tube with some weight in that and attach it to the bulkhead...

 

[cls]

Rocket caviar: good epoxy and lead shot. I've used steel shot, and 1/4" washers.

The trick: push 2 or 3 long finish nails through near the front of the nose cone. Use bails long enough so they enter one side and exit the other. Trim them flush, both ends, on the outside. Yes the tiny holes finish nicely.

Mix enough epoxy to cover the ballast material, and pour it in to the nose cone. It will flow around the nails and cure there and stay for good.

Submerge the end of the cone in a bucket of water, because the epoxy volume gets hot while it cures. (Why we usually mix small amounts on a flat surface.)

 

[StreuB1]

I have used steel washers and 115gn 9mm projectiles for ballast. I used to shoot competitively and I reloaded a lot so I had/have a surplus of copper plated 9mm projectiles for practice ammo.

I drill several 1/8" cross holes through the first few inches of the NC tip, thread 1/8" stainless filler rod (tig welding rod) through the cross holes with about 1/8" to 1/4" protruding on either side. I then dump in the requisite mass in washers/projectiles. Then I mix up a few dixie cups of epoxy and pour them in with a few hours between pours to allow the first to cure up. I roll the NC around every 10min to keep all the bits wetted well. Once it all cures up, I cut off the excess steel cross-pinning rods that are protruding, file/grind them flush, then finish as normal. Never have had a problem.

Sounds complicated but there is no way that ballast plug is coming loose unless you lawn dart.

 

[SolarYellow]

I have used steel washers and 115gn 9mm projectiles for ballast. I used to shoot competitively and I reloaded a lot so I had/have a surplus of copper plated 9mm projectiles for practice ammo.

I drill several 1/8" cross holes through the first few inches of the NC tip, thread 1/8" stainless filler rod (tig welding rod) through the cross holes with about 1/8" to 1/4" protruding on either side. I then dump in the requisite mass in washers/projectiles. Then I mix up a few dixie cups of epoxy and pour them in with a few hours between pours to allow the first to cure up. I roll the NC around every 10min to keep all the bits wetted well. Once it all cures up, I cut off the excess steel cross-pinning rods that are protruding, file/grind them flush, then finish as normal. Never have had a problem.

Sounds complicated but there is no way that ballast plug is coming loose unless you lawn dart.

Smaller rockets, again. I do the same thing, but I use BBs for reduced toxicity compared to lead. Each BB is ~1/3 of a gram, so it gives me adequate resolution even for little stuff. I found the cheapest way to get lead shot, before I switched to BBs, was to just cut open a shot shell from a box of cheap target loads. But as noted above, I'm going to be trying the glass blast media going forward.

On the cross pins, for polystyrene (Estes) nose cones, I use Evergreen polystyrene round rods bought from Hobby Lobby. They glue very effectively to the nose cone. Just drill with a close-fitting bit and push it in, then hit each side with a drop of Tamiya liquid cement. Then the outside surface of the cone is all polystyrene and you don't have to worry about the material difference when blending and finishing.

 

[Back_at_it]

I've done something similar to this on some rockets. I needed to add approx. 1lb to a Phoenix build a couple of years ago and used BB's. The BB's were mixed with BSI 30 min. epoxy. The inside of the cone was sanded with something really rough like 40 or 60 grit then cleaned well. The mix was poured into the tip of the cone and allowed to cure for a couple of days. The mix only filled about 1/4 of the nose cone. I filled the remainder of the cone with great stuff expanding foam and let that cure. That rocket has only been launched four times but so far no issues.

I've used the expanding foam on a ton smaller cones as well. I've had Estes clay come loose on a few models over the years and the fix has always been to shake the cone until the clay is back in place then fill with foam. Works great every time.

 

[jmasterj]

As an alternative to other weights, pennies are cheap/gm and usually available at your local bank.

 

[boatgeek]

Last time I needed to ballast, I put in 3 heavy bamboo skewers horizontally through the nose cone, dropped in some surplus 3/4" bolts and nuts from a fence project, and poured in epoxy lightly thickened with wood flour. It's very solid, not to mention heavy.

 

[DaHabes]

Rocket caviar: good epoxy and lead shot. I've used steel shot, and 1/4" washers.

The trick: push 2 or 3 long finish nails through near the front of the nose cone. Use bails long enough so they enter one side and exit the other. Trim them flush, both ends, on the outside. Yes the tiny holes finish nicely.

Mix enough epoxy to cover the ballast material, and pour it in to the nose cone. It will flow around the nails and cure there and stay for good.

Submerge the end of the cone in a bucket of water, because the epoxy volume gets hot while it cures. (Why we usually mix small amounts on a flat surface.)

This is exactly what I do. If you need to put in a lot of weight, you just use longer and bigger nails placed farther down from the nose cone tip. When you calculate the weight needed, include the weight of the epoxy into the total, it can add up. I use laminating epoxy without any fillers. It flows better. I also mix the lead shot into the epoxy before I put in the nose cone. I want the entire mass to be one solid unit when cured. If you put the shot in first and then the epoxy, it won't fill the voids between the shot. You can always pour a little extra epoxy on top of the mass once it is in place to create a final epoxy dam layer which will be smooth when cured.

 

[SolarYellow]

I also mix the lead shot into the epoxy before I put in the nose cone. I want the entire mass to be one solid unit when cured. If you put the shot in first and then the epoxy, it won't fill the voids between the shot.

There's an argument for having the shot be mostly loose, as long as it will all be positively retained under any loads that could occur in the air. If the NC hits something, there's potentially less damage that will result if it can break open the cone and spill the ballast as loose particles than if the ballast is a solid, monolithic chunk. Also, that's a decent argument for using something less toxic than lead.

 

[cerving]

I think I'm leaning towards BB's embedded in foam... it's a big nose cone, so the volume of foam is going to be much larger than the BB's. Given that there's a bulkhead, the foam isn't likely to go anywhere.

 

[cls]

Foam is terrible. Go ahead, try it, post how it works for you.

 

[DAllen]

Ok...more important question...where the heckin did you find a 7.5" Polecat Thumper? I searched hi and lo for one when I was doing my L3 a few years ago.

 

[droopy]

Cris,

The other question is how have you determined you actually need ballast? I have a 10" Polecat Fatman and the base drag on that thing makes it much more stable than the numbers indicate. I know there are some hacks with OR and Rocksim to try to simulate an approximation of how the base drag affects stability. I had a Goblin or something a long time ago that required a bunch of ballast based on the numbers. After I put it in, it was WAAAYYY overstable which led to its early demise.

Cheers,
John

 

[cerving]

Ok...more important question...where the heckin did you find a 7.5" Polecat Thumper? I searched hi and lo for one when I was doing my L3 a few years ago.

It was gifted to me by a club member that never finished it past the airframe build. Yes, that's about as lucky as one can get... short of winning the lottery. OH, there are probably more lottery winners than people with Thumpers, so I guess I'm more lucky than they are.

 

[cerving]

Cris,

The other question is how have you determined you actually need ballast? I have a 10" Polecat Fatman and the base drag on that thing makes it much more stable than the numbers indicate. I know there are some hacks with OR and Rocksim to try to simulate an approximation of how the base drag affects stability. I had a Goblin or something a long time ago that required a bunch of ballast based on the numbers. After I put it in, it was WAAAYYY overstable which led to its early demise.

Cheers,
John

OpenRocket says I need about 2 kg in the nose to make it stable if I want to use outboards. With such a big and heavy rocket I'm not worried about overstability... and for sure I DON'T want this puppy skywriting.

 

[cls]

I built several 5.5" Thumpers and they were rock stable with stock 38mm MMT. (I have another in the box somewhere... someday!)

But yeah, Andy said upgrade to 54mm, 75mm, airstarts, you are on your own for ballast.

 

[sharkbait]

OpenRocket says I need about 2 kg in the nose to make it stable if I want to use outboards. With such a big and heavy rocket I'm not worried about overstability... and for sure I DON'T want this puppy skywriting.

My 7.5” Thumper has a central 76mm and (2) outboard 38’s, it has 3.5 lbs in the nose

 

[Hobie1dog]

On my Bullpup I have 25 pounds of Quickcrete in it.

Seriously I guess my L3 project has OR saying I need about 4 pounds in the NC. I don't feel bad now.

 

[David Schwantz]

With the canards the weight adds up , up front

Also, she is not a thin wall kit. The glass is heavy, but it will withstand a nuke.

 

[cerving]

My 7.5” Thumper has a central 76mm and (2) outboard 38’s, it has 3.5 lbs in the nose

Is it the standard build or do you have another 30" payload section added on? Mine has the extension... which is probably going to help the stability a lot.

 

[Igotnothing]

With the canards the weight adds up , up front

Also, she is not a thin wall kit. The glass is heavy, but it will withstand a nuke.

Curious to know when and where the testing will be so I can be elsewhere.

 

[David Schwantz]

I'm sorry, but you already missed it. She has flown and did not blow up