I've launched small stuff from underwater (13mm), and I've done underwater
static tests of blackpowder engines up to E9s. They all worked fine except
one of the E9s that was probably sitting in the water too long and didn't
The assumption bing that Bob still has a memory.....
Those "Happy Meals" are arterial concrete... B^P
| In the 70s, someone did a Polaris super scale at a naram underwater.
| Bob should remember who it was, I saw a picture of the rocket in the 'pail'
| in an old model racketeer.
| > Just got to thinking of a Polaris like launch, I was wondering if it had
| > been done already.
| > Some reference pix:
| > TBerk
There was an article about under water launching in an Estes Model Rocket
news back around 1963 +/- a year or so. The rocket was similar to an Astron
Scout, but without the vent porthole.
Back in the 70s John Kalb of Ft. Wayne built a Super Scale Polaris launched
from an underwater submarine.
Many other references in ancient literature.
Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD"
>>> To reply, remove the TRABoD!
That was me. Not doing that anymore due to the high cost of the
rockets/engines, combined with my somewhat altered spending priorities
these days. Any way's, here's the advice I can offer if you ant to go a
little deeper then 4-5' with little estes engines:
1) Do not use paper or wood. They'll soak up water (yes, even if you
paint it), become weak, and immediately turn into pulp the moment you
2) Build your rockets from plastic and fibreglass - lots of it. Water
is about 96 times more dense then air so you'll need much greater
structural integrity to prevent your rocket from dyeing during launch.
On top of that you must not only design your rocket with a lot of
front-rear strength, but a lot of strength around the circumference.
Pressure increases dramatically as you descend into the water, with
pressure increases of 1ATA every 10m/33'. Not enough lateral strength
and your rocket will get crushed before you even launch!
3) Learn a little about hydrodynamics. As it turns out the shapes which
are the most efficient for rockets in the air are not the most efficient
shapes for rockets underwater. Think torpedo - that's the most
efficient shape for underwater travel. Short, squat, hemispherical
nosecone. I've yet to find a modelling program which can determine the
"aerodynamics" for a rocket underwater, but I found that the minimal fin
surface for air flight is more then adequate for underwater. But keep
above this minimum, otherwise the rocket won't be stable once it breaches.
4) Power is everything. As water is ~96x more dense then air it takes a
lot more powerful engine to get things done. I hate to make
generalities, but we found "in general" that for every 660m/2000' an
engine could propel a rocket through the air it could propel a rocket
3m/10' through the water with sufficient velocity to breach and fly a
short distance. That's right, you get about 1/200th the distance you
would in the air. That parasitic drag is a real bitch. Also, longer
burning engines are better. As water is quite dense parasitic drag
becomes a real problem. The added thrust of a quick burning engine can
be countered completely by the increased drag found at higher velocities.
5) Supercavitation can dramatically overcome much of the drag, but
supercavitation is difficult to achieve, even harder to maintain, and
puts tremendous stress on the rocket. Although we had a great deal of
success in achieving cavitation (I don't think we ever hit
supercavitation) it was difficult, expensive, and cavitators rarely
lasted more then one launch.
Any way's I posted quite a bit here, search for my name plus
"supercavitation" in google groups and you should pull up most of what I
wrote. Any way's our deepest "successful" launch was from 36m (110').
When I say successful I mean the rocket breached - in this case about
4m/12' out of the lake. The rocket then fell into the lake and the
recovery charge blew the rockets to pieces. Oh, that brings up point 6:
6) Make sure the recovery charge doesn't go off underwater. For some
reason the amount of charge most of us would use for a recovery system
blows rockets to pieces underwater. Not really too sure why, but we saw
it consistiently with rockets that breached, but fell back to the water
before their recovery charge went off.
I didn't keep much of that kind of stuff - these experiments were done
in co-operation with some friends of mine working on their engineering
grad projects. I was basically there as I had a great deal of
experience building rockets/engines, as well as was a certified SCUBA
diver. My only role with the cavitators was to glue them ontop of the
rockets I built ;-) Unfortunately, all of this was done before I had a
digital camera so the number of photo's I took was rather limited. I
used to have a series of images captured from my old VHS camcorder. The
quality was crap, but you could see the rocket exit the lake and fly
into the air.
Any way's I'll look through my files to see if I can dig up a photo that
shows that kind of detail, and Ill see what my friends have on hand, but
for now I could perhaps give you a physical description:
We tried two types of cavitators, a "recessed cone" cavitator and a
"ball and gear" cavitator. The ball and gear type of cavitator is
probably the easiest to describe, but had the poorest performance
underwater. Basically these cavitators consisted of a series of thick
arms extending radially from a central mounting point (like a gear with
long teeth). At the end of each arm was a round ball. These cavitators
basically worked by forcing a "hole" through the water. The main
problem was that they are extremely difficult to build strongly enough -
most of them either lost an arm or had all of the arms bend back onto
the body during flight. Loosing an arm meant the rocket was destroyed,
as the uneven pressure on the cavitator was sufficient to snap the
rocket like a twig.
The recessed cone cavitator is a little harder to describe and build,
but works a lot better. Basically these cavitators look like the front
of a jet engine - there is a central cone surrounded by a curved
"wall". At the base of the cone, where it met the wall, there was
horizontal slits through the wall. Basically these cavitators work by
accelerating the water and then passing the high-velocity water through
the slits at extremely high speeds. In essence the water would enter
the top of the cavitator, and as it moved through the chamber would be
accelerated due to the cone approaching the wall - basically the same
thing that happens in the throat of a rocket engine. The water would
then exit from the bottom of the cavitator through the slits - the
velocity of the water was sufficient to drop the water's pressure low
enough that the water would vapourize. Vola, rockets surrounded by a
"chamber" of water vapour.
One thing I didn't note in my original post is that the type of thrust
profile you need to make cavitators work is a little different then a
conventional underwater rocket. You still want a relatively
long-burning rocket, but you need to burst of thrust at the beginning to
get the rocket moving fast enough for the cavitators to kick in. We
approached this by using clustered engines - we'd have a large, long
burning engine for the main, and two or three (and once four) small,
quick burning engines to get the whole thing moving. Obviously this
causes a lot of extra engineering problems, especially coming up with a
way to reliably set off all engines at the same time. Lastly, all
attempts we made at staging were essentially unsuccessful. We simply
couldn't come up with a staging device that was strong enough for
underwater use that would reliably stage.
Sorta the same reason you never fire a rifle with an obstruction in
the barrel or with the muzzle submerged. Not sure of the exact
physics and/or hydrodynamics, but a recovery charge that will properly
fire a cone/chute at 1AT WILL, in a Murphyesque fashion, find the next
weakest point of the structure and blow through it due to the
And PLEASE don't test that theory with Pappy's old deer piece...do you
really want to be in the next edition of the Darwin Awards?
missile men' which at one point showed an underwater launch in a clear
container, not more than a couple of feet deep. Not sure of any details, but
if you have/can find the file its a nice video to watch.
Here's a guess at a solution: assuming that the main engine is
powerful enough, might it be possible to use a very small, low-thrust
engine in the nose facing FORWARD, possibly with some suitable
multi-exit venting surrounding the nose, to generate a gas layer to
stream around the body of the rocket while in the water? Sure, it'd
rob you of a little net upward thrust, but you might make that back
and more by reducing underwater drag.