Model Forum / General / Rockets / November 2004

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.

Bryan

Sorry it took so long to reply to this - I was away at a conference and
my news reader only seems to work when I'm plugged into the network at
work. . .

I'm looking for pix.  As I mentioned in a previous post we didn't take
to many, and aside from a few low-resolution video stills I don't have
any of my own.

In order for the engine to ignite we had to keep it dry.  We solved this
issue by plugging the engine (with ignitor installed) with a liquid
plastic.  This bound weakly to the engine, but by ~10' underwater the
differential pressure was enough to firmly seat the plug.  At launch
depth this was more then enough to hold the rocket onto the pad.

Same way you do on land, although our ignitor box was waterproofed
(obviously).  Basically our system consisted of a float with a
garage-door opener receiver on it.  The float was anchored ~30m/100'
away from the launch site.  A wire (later replaced with fiberoptic cable
to deal with shorting issues) ran from the float to the anchor, and from
the anchor to the ignitor box (which was integral to the bottom of the
rail).  You used the garage-door opener to activate the circuitry on the
float, which would then send a signal to the submerged launch-box, which
would then start the rocket.  The advantages of this system were that we
essentially eliminated any current-loss as the batteries were
immediately beside the rocket, and eliminated the need to run cables
100's of feet to shore.  The circutry of the launch box was essetially
the same as what we use (complete with continuity lights and
everything), although the launch button was replaced with a relay
(electric initially, optical later on).  For power we used 3 motorcycle
batteries when launching clusters.

My experiences are identical - my first series of underwater rockets
alpha II's painted with waterproof paint (most of which disintegrated at
launch).  My first successful underwater rocket was actually an alpha
where I replaced the cardboard body tube with a plastic tube.  In fact
~30% of our designs were simply alpha's scaled to the size we needed.  
It's a simple, effective design which is hard to beat.

This may be the case.  We've found that nearly all of our rockets veer
to some extent when they breach.  About the only time this doesn't
happen is if engine burnout occurs before the rocket breaches.  My
theory (no proof, just conjecture) was that exhaust "reflecting"
forwards off of the waters surface lowers the speed of the air around
the fins (maybe even reverses it), causing the rocket to loose
guidance.  Another possibility is that the rockets simply do not breach
with sufficient velocity to be stable, and as a result veer until this
velocity is achieved.  In either case swept fins could counter-act the
veering by using the water for guidance.  That said, we had a long
series of very short, squat rockets with trapezoidal (sp?) fins that
worked very well both in and out of the water.

Underwater CATO's are a real problem - we had one happen while prepping
a rocket for flight.  Knocked me out cold for a second or so, and my
ears were screaming for days.  Resulted in a complete re-design of our
launch & safety systems.

Not even close to a record I'm afraid.  The Russians have developed a
supercavitating torpedo (basically an underwater missile) that has a
range of several kilometres - I assume that it goes up as well as it
goes forward.  It's called the "Shkval".  In fact, it was this invention
that go us started on the supercavitation stuff.  It is impressive though.

This project started off as a little but of fun, but in the end was
actually a graduate project of two of my friends.  We used the rockets
to start the project, but later their lab got enough money to buy the
equipment to study supercavitation in-lab (it costs a surprising amount
of money to get a tank/pump capable of flowing water at >100km/h).  I
believe that they are nearing publication on their studies, so there may
be a real science paper on this soon.  That's the good news; the bad
news is that most of the material I had went with them when they started
grad school.  I used to have a whole series of schematics for our later
rockets and cavitators, I'll put together my old computer and see if I
can find them.  The schematics should be no issue to convert to a format
you can read, the cavitators were all in a format to run a CNC lathe, so
they may be a little more difficult.  As I mentioned before I have
little in terms of photo's; we videotaped most launches but the
resolution of the video is pretty poor.  I posted some stills of them a
few years ago on the binaries group; the may still be stored on google.

I am always willing to talk about this stuff, either here or via direct
e-mail (just remove "nospam" from my addy).  I don't get to check r.m.r
often, so a direct e-mail may be best. . .

Just a word of warning, the launches had a potential to be quite
destructive.  CATO's generate quite powerful shock waves, but we found
that even normal launches could create enough of a shockwave to stir up
silt over a large area, and even crack poorly-designed battery cases.  
Considering the stress reefs are under today you should try to find a
sandy area, far away from reefs, to launch.  I've never launched in the
ocean, but I imaging that currents could be a real issue.

Bryan

Sorry it took so long to reply to this - I was away at a conference and
my news reader only seems to work when I'm plugged into the network at
work. . .

We made a few attempts at artificially inflating the cavitation chamber
by injecting air into the chamber just aft of the nosecone.  The system
never worked right; as you can imagine such a system was quite complex
and difficult to trigger at the same time as launch.  It was about this
time that we stopped experiments, so we never really developed this
system aside form our original (non-working) concept.  Our system
generated gas through azide breakdown (much like an airbag).

Bryan