Monday, October 26, 2009

Playing with a Stubby Euphemism

3d by Dick Stafford

Photo KeithAlanK

Photo KeithAlanK

Photo John Lee

I can appreciate a clever euphemism.
The USAF nuclear arsenal is typically modular in nature. A sensible
approach when technology is progressing rapidly. The nuclear
explosive is a seperable unit from it's carrier body, either a
missile or a gravity bomb. In the case of a gravity bomb, the
body is referred to as a drop-shape.   Cute, I like that.

This it the DS-3r; Drop Shape, 3"dia, rear ejection. This is also
the 3rd DS that I've built over the years. Won't be the last either.
I had been sketching and dreaming this particular design for a
couple years already, and when Giant Leap introduced it's 3", 5:1
plastic nosecone, I knew the time had come.  I turned an upscale
Cherokee cone for a rocket buddy in trade for the Giant Leap

I always thought the slo-mo vidclips of gravity bombs with retard
packages looked cool. The small close coupled chute ejecting out
the rear to slow the bombs down. For a low altitude drop, this
gives the aircraft a lead over the slowing bombs so that they
don't explode directly under the aircraft. Nukes often are
configured the same way for the same reason, despite much
higher release altitudes.  The DS-3r has the internal space to do
this well. The motor mount tube extends well into the nose. The
ejection gasses must 1st go forward, then return through the
baffled centering rings to eject the tail cap and the chute which
is packed around the motor. This utilizes the entire internal
volume of cool air to push the chute out before any hot gasses
can even reach it.  Another trick I pulled is that by removing the
nose cone, the entire motor mount/baffle assembly slides right
out for servicing or for chute replacement.

When the DS-3r was nearing completion, I decided that It
needed to be run through RocSim.  I've used it a time or two,
but don't have it.  I still use Barrowman CP calc on paper, and
did it this time, though knowing that it has trouble with rockets
this stubby.  I contacted Dick Stafford of Dick's Rocket Dungeon
fame and he helped me out. After tweaking the mass and balance
in line with the the real model, he found that only one ounce of
nose weight would be required, I added 1 1/2oz.  RocSim
Barrowman CP agreed with paper Barrowman CP, and RocSim CP
was indeed a bit further back.  The wedge airfoil fins [which
RocSim as yet doesn't handle] actually moves the CP still further
back a bit.  The performance sim crosses Estes motors and the
24mm Blackjacks right off the list, too slow off the rail.  E18W is
good for 800ft, the F39T goes to 1200ft.  Just about perfect
upper range, and I may work my way down the motor list over
time.  That new Aerotech E20W looks nice too.


  1. Glad ya like it.
    Flies great too.

  2. That's great looking. How did you build the fins?

  3. Sorry I didn't see your comment sooner.

    The fin sides are 2 pcs of 1/32" linen phenolic, G10 or plywood are ok too.
    Epoxy the leading edges together and tape them, then install a wood trailing edge piece and tape till dry, Add a center spar piece or two to fit, if the skins are too flexible [these got one spar]. Glue on an oversize tip block, and trim and sand to fit. Fillets can be added internally afterwards, and/or foamfilled.
    While I did work from a CAD drawing, I wasn't too concerned about accuracy, just repeatability. For a scale model, I would've used more precision patterns for precut parts, and less trin to fit. I've built at least a 1/2 dozen rockets with hollow builtup fins over the years.

  4. Thanks. I wondered if they were built up or foam core. Really nice job.