Lessons Learned: Expanding Foam Follies

by Burl Finkelstein

I had built a number of different boat tail rockets -- from little ones to some pretty big ones -- and found them all to have the same weakness. If the fins are mounted in the tail cone it is difficult to get a good bond between the fins and the blow molded plastic cone. Even when high-power thru-the-wall construction is used and the fins are glued and filleted to the motor mount and centering rings, you never quite get the same strength of a straight airframe model because of the poorly bonded tail cone to fin joints. A hard landing will frequently create cracks on the outer fin fillets. The exception is if a tail cone is of fiberglass, wood or an other material where the glue can forma decent bond. Unfortunately, most tail cones are made from blow-molded polyethylene nosecones with both ends cut away.

There are several things besides just a good medium grit sanding that are alleged to help bonding to the polyethylene tail cone, but the ressults are inconsistent. Different surface primers, plastic bonders, urethane cement, solvent wash, flaming the surface, even hot melt polyethylene glue all seemed to help, but at other times did not bond any better than plan old cleaning and sanding. As I love the way tail cone models look, I will try anything that may make them stronger.

Foam, I had heard, was being used in large high power models to add support to the fin mounting, by filling all the voids in the fin can of the rocket. The foam also stuck everything together, the motor tube, fin tabs, centering rings, and airframe. Well I just had to try this out!

I first tried it on my 3X scale up 7.5 dia. airframe silver comet, the long swept fins that were completely tail cone mounted, cried out for any thing that could attach them more strongly to the rocket. The large tail cone assembly was easy to fill with foam just like pouring it into a wastebasket. Just let it sit long enough to finish expanding then trim it flat, glue on the top centering ring and finish the build. Being readily available, I used Great Stuff spray can foam from the hardware store. Great Stuff is "moisture cure" -- so as it suggested, I misted the layers of foam during filling the tail with a tiny amount of water. The water kicked off the cure reaction, can you say "catalyzed".

Here is where it got interesting What worked in a big scale should be able to work in a smaller model, or so I thought I saw a PML Little Lunar Express and thought it was a pretty neat retro looking, tail cone model that I knew my wife would like. (If she liked it she would not complain that I had bought another rocket kit). For those of you not familiar with this model it is, half tail cone, and half nose cone, with just enough body tube to couple the two ends together. Long fins mounted to a polyethylene tail cone looked like a good place to improve the strength by my newly tested method of filling the fin can area with foam. I filled the tail about 2/3 the way with Great Stuff to allow for expansion. After it sat for several hours, the foam had stopped expanding and formed a hard biscuit just higher than the open top of the cone.

Since the foam was hard and appeared to be fully cured I trimmed it off flush with the cone and epoxied on the top centering ring. Over the next few hours I filleted all the fins with epoxy thickened with cotton fiber. Being satisfied with a great job, I set it aside and went to bed. So far it looked beautiful.

When I woke up the next morning, the first thing I did, was go and get it to show the cool looking model to my wife.

IT IS ALIVE!!!*

AHHHHH!! THE HORROR OF IT!!!!!!! The tail cone had exploded open at three of the fin fillets! There were chunks of foam the size of hot dog buns at each of the ruptured fillets. The slotted tail cone wall was also now on the outside of epoxy fin fillets. Expansion of the foam (what did not force its way out), had pushed the walls of the cone way out of place. What a mess! Before I could go to work or even get dressed, I had to at least clean up the excess foam. I cut off the big chunks on the outside and forced the tail cone back into shape.

When I came home from work that night it had pushed still more foam out of the slots where the fins entered the cone. This continued for almost a week.

The moral of the story is that moisture cured foam in a deep tight well sealed space does not get enough moisture to properly cure. This probably would have worked if I used a two-part foam like the one PML now sells that uses a catalyst to cure much as epoxy does. The Great Stuff worked in the big open silver comet model because it had the benefit of being misted it with water and a more porous motor mount tube, and plywood fins. The paper motor mount and wood fins can provide minute amounts of moisture to help the cure. The Lunar Express was almost totally sealed from moisture it has a phenolic motor tube, G10 fins, and a very deep cone with a small open area at the top. SO it took forever to cure, unfortunately I sealed it up before it finished.

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* The foam man shown above was created by Rob Cockerham of www.cockeyed.com. View the page [HERE!].