| Manufacturer: | Scratch |
(Contributed - by Drake "Doc" Damerau )
| T' 120 Mortar | ||||||||||||
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| Why A 120mm Mortar? |
| I work at Chamberlain Manufacturing, t' operatin' contractor o' the Scranton Army Ammunition Plant as t' Plant Metallurgist and Laboratory Director. Blimey! Begad! Blimey! We make several products one o' which be t' 120mm mortar family. Begad! I wanted t' make a 120mm mortar as me Descon entry t' show rmr what I do for a livin' and t' show t' guys at work what me hobby is. Arrr! This is an exact scale model! Blimey! I checked t' completed model usin' product gauges and most o' the datum's are out. Arrr! Blimey! OK, so it's nay exact but it closer than most models. Most of the dimensions that are out are only out by a few thousandth's. Aye aye! Begad! Blimey! Sorry Kenny, this ones nay close enough for government work. |
T' Model
| T' mold | |
| I made a mold from an actual mortar. Ahoy! Blimey! Begad! Blimey! (No, it wasn't loaded) T' one you see pictured be t' exact same mortar as t' high explosive mortar but is inertly loaded for trainin' purposes and thus painted blue (FRTR). Well, blow me down! Blimey! I first covered t' mortar assembly with Vaseline then I laid papier-mâché onto a mortar in several layers. Avast, me proud beauty! Blimey! T' finished thickness was about 1/4 inch. T' hardest part was accurately applyin' the papier-mâché t' t' obturatin' grove (OBT groove). Ahoy! Blimey! (T' undercut in t' bourlet, ya bilge rat, just aft o' t' ojive) I laid thin strips in t' grove t' bring it t' t' OD level. After three days o' dryin' time I cut the papier-mâché lengthwise on each side. It be a little difficult t' pry apart but caved in t' me proddin' in short order. | |
 FRTR |
 Covered in papier-mâché |
 T' molds |
 Fin and Boom |
 OBT Groove Mold |
| Glassing | |||
  T' papier-mâché be covered with a
light coatin' o' Vaseline t' facilitate removal o' t' fiberglass. Blimey! Blimey! I coated the
molds with West Systems epoxy and laid 4 layers o' 6 oz glass. Ya scallywag! Blimey! Again, ya bilge rat, me bucko, arrr, the
difficult part be t' OBT grove. Begad! Aye aye! Blimey! Once dry, ya bilge rat, arrr, I cut t' glass so that it was even
with t' sides o' t' molds. Well, blow me down! Avast! Blimey! T' glass popped right out, shiver me timbers, right? OY! Blimey! Nay a
chance. A good portion o' it did but I ended up soakin' them in one o' the
bathtubs for two days and pickin' off t' news paper! Blimey! (My wife is still pissed)
OK, t' paper is off and t' two halves are sanded. Avast, me proud beauty! I expoied them together and
glassed them. Avast, me proud beauty! Blimey! T' glass didn't sit smoothly and evenly in t' molds so I had a
lot o' exterior cosmetic work t' do. Begad! I sanded and glassed for two weeks! |
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| Fin and Boom Assembly | |||
 T' tube and fin section is
called t' "fin and boom assembly" It contains t' explosive
charge t' propel t' mortar. Well, blow me down! Arrr! Blimey! In this case, me bucko, arrr, its a 38mm motor mount tube (mmt).
Can you believe that an LOC 38mm tube is only 0.028" smaller than a real
tube!!! Blimey! Just enough t' take some thin glass and make it perfect. Avast! Blimey! I laid glass
in t' ID on t' forward and aft ends t' size it down t' fit t' mmt. Avast, me proud beauty! Well, blow me down! Blimey! T' 38mm
mmt slid in t' assembly very nicely. Ya scallywag! Avast! Blimey! Once in, arrr, I epoxyied t' mmt in place and
sanded t' forward flat and square. Aye aye! T' fins (all 8 o' them) were made using
1/8" thick aircraft ply. They are slightly thicker than t' real ones. Begad! Well, blow me down! Blimey! No
room for glass here. Aye aye! I took t' real fin and boom assembly and traced t' fins
on t' fin material. Begad! I cut them slightly larger than t' tracin' and sanded
them t' t' proper dimensions. Ya scallywag! Arrr! Blimey! T' placement be easy usin' t' WRASP fin guide
cut out. |
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| Fuse |
 This was t' most difficult part o' t' project. I modeled the
fuse after a real M734 fuse. Avast, me proud beauty! (Yes t' fuse was inert too) I took a block of
balsa wood and drilled a hole t' put a piece o' all-thread about 2" in. Aye aye! I
then commenced t' do t' drill-lathe thing. Ya scallywag! I mounted t' all-thread in my
handy-dandy high speed drill and put t' drill in me vise. I didn't get
it done for t' picture you see below. Ahoy! T' fuse shown be t' real fuse. |
| Paint and other finishin' touches |
  T' mmt be a no brain'er because t' boom is a 38mm
mmt. Avast! For t' first launch I used a 29mm adaptor and secured 3/16" Keelhaul®©™®
rope t' t' forward end o' t' adaptor. Well, blow me down! T' parachute is a 30" Aerotech.
I'm lookin' for a relatively fast decent at t' high school. At a NEPRA launch
I will use a little larger chute and a I161. Begad! Paintin' be a non event. Avast! I
painted t' fin and boom assembly then masked it off after it dried and painted
the body. Aye aye! Again, me hearties, t' fuse on top is a real fuse. No time t' finish t' copy.
Final dry weight after addin' lead t' t' nose area is 41.6 oz, matey, or 1,180 grams.
T' diameter is 120mm and t' overall height is `37.750" |
| OK cool, shiver me timbers, matey, but does it fly? |
 First flight went great. Avast, me proud beauty! I used a G38-4fj. It left
the pad and went up in an arch t' about 800 feet. I wanted t' keep it low
because o' winds and t' small area. |
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