Dedicated to the Promotion of Electric Propulsion in all types of Aeromodeling

An MQ-9 Story pt. 5 – The IR/EO Ball

After getting the rear end or the MQ-9 project sorted out, it was time to work on the front end.
The most prominent feature is the IR/EO ball. This is the acronym for the Infrared/electromagnetic Optical sensor array. There is a whole lot more going on here and you will have to look up the Raytheon site to get more information because there is a LOT of information about this sensor suite! Please note the size of the front “ball” on the full scale aircraft”.
In the next picture you can see that the Chinese model makers did not have the complete engineering drawings at the time of manufacture and as such, the proportions are a little bit off. I am only guessing that they might have been preoccupied with procuring model 35 information during this time period.

I can do better than this.


I was able to get a 3d print of an IR/ER ball.

This 3d printed IR-EO BALL looks different because it is a not obsolete configuration. However it weighs 50grams! OK, I know what you are saying, 50 grams, big deal. BUT, 50 grams up front means 50 grams of tail weight equals 100 grams total weight. This is a 1⁄4 pound folks ! This is already on an overweight scale model that is known to tip stall out of the sky ! So, I have to make a light weight foam copy of this.


Being as cheap as I am, and knowing that on the first flight I could wad this airframe up in seconds, I chose to go cheap. I looked into my piles of leftover supplies and tried a new technique.

You Tube says: pump RTV silicone into water and detergent mixture and you can make a molding putty/dough. As I had all of these supplies laying around from old household projects I thought: Why not try ? The price is right.


RTV spaghetti in a tray of water and dish soap. This gives it enough moisture to cure in thicker sections.

RTV silicone needs moisture to make that vinegar smell to cure properly. Let me tell you, this stuff smells like vinegar ! I did it outside and I am glad I did ! The detergent keeps the sticky RTV under control.

It works pretty well although not as well as expensive commercial mold making silicone. The difference is zero dollars and using up old stock and $50 of first class mold making supplies.

Pretty wild, huh? I can report that it does work OK.


This picture shows the “dough” I have wrapped around the 3D printed master model of the IR/ER ball Plus flange . It is only about 1⁄4” thick and needs 2 days of curing time, OUTSIDE ! Peewhuu!


The silicone mold is a bit flimsy and need a back-up . Plaster is usually used here. I did not have enough plaster so I used some left over drywall compound that I had around for the last decade. It is a bit “chalk-y” but hey, the price is right and I only need to make a part or two.

Cheap plaster and a left over container.


The end result is full functional.


Final result . Does not look like much of a mold.

However, it is “good enough”


Here it is, ready to cast with Home Depot bought Loctite brand Foam sealer. I used Loctite foam as it was a denser, more closed cell foam and would make for a better part. About $6 now invested.

Casting into an open mold did not work well at all.


My first try was a mess. It needed a bit more back-pressure to fill the mold well.

Tape it up and get some back pressure…




A bit of “flash” and excess, but pretty good and only 12 grams not 50 !
I sanded and painted this and it looked pretty good for such a light weight piece.


To be honest, it is only OK. But still, pretty good and definitely flight worthy..

All of the bits and pieces. Note the old sunglasses lens used for the IR window. The Optical part was a cheap flashlight lenses and the electromagnetic laser window was a fancy piece of paper from a greeting card.


Work bench picture.


before and after picture.


That looks about right…At 15 grams it does not affect the weight-and-balance too severely. Time to set up the battery box.
Until next time…