Подводный ракетный двигатель

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Вот ответы о подробностях конструкции кавитатора:

> What is the ratio of diameters of the cavitator and rocket?

The cavitator should be the same size as a normal cone (i.e. it's OD should be
the same as the rocket).

> Angle of the cone?

You want the water flow to be as close to laminar as possible. A tangent ogive
is best for this. If you cannot make an ogive a straight cone works, but not as
efficiently. Generally speaking I use a cone that gives a 2:1 or 3:1
length:width ratio (i.e. a 2:1 cone would be 2cm long for every 1cm of base

> Number and diameter of the horizontal slots?

Depends on the size of the cone. Generally speaking you want the slots to take
up ~2/3 the diameter of the cavitator. The slots are rectangular in shape and
should be located at the base of the cone, where it joins the outer wall. You
want the slots to be fairly narrow (I generally use slots 8%-10% of the cone
length). I've never tried using a larger number of small cylindrical "slots" -
this may work better.

> Their form: just cylindrical orifice or conical?

Ideally you want these slots to assist expansion. As such an ideal slot would
have a cross section in which it widens from the inside out. However, this is
usually impractical as most materials aren't strong enough to support this. I
usually just grind down the edges on both sides so that the flow doesn't hit any
"sharp" edges.

> How do you test and tune it?

Trial and error. Basically we've designed a standard test rocket that carries
some basic instrumentation that records acceleration. Its designed to allow
easy swapping of cones. We just make a series of cones where one variable is
changed, and launch them through the water (usually horizontally). We then plot
the acceleration of the rocket, which can tell you how well the cone is
functioning. Once you have an effective design it's fairly easy to scale. The
graph for a cavitating cone looks something like this:

| ________________
| ___/ Peak velocity
V| _/
e| /
l| / <- cavitation chamber formation
o| _/
c| _/
i| ___/ <- approx velocity of non-cavitating
t| _____/ rocket
y| /
| / <- initial acceleration

The most important factor in designing a cavitating cone is the compression
ration of the cone. This is described as:

(area available to water flow at the top)/(area available to water flow at the
bottom). The "top" is the opening of the cavitator, the "bottom" is the area
located at the top of the slots. Assuming that the top of the cone is level
with the top of the tube the top" number is equal to the area of a circle with
the same diameter as the inner diameter of the outer tube. Assuming your outer
tube has the same diameter throughout the bottom value will be to top value
minus the area of a circle equal with a diameter equal to the diameter of the
cone at the "bottom" of the compression region. This ratio is vital, as it is
directly related to the increase in water velocity that will occur in the cone.
I've found that you need a ratio of at least 12 to get cavitation chamber
formation, and that the best cavitators run at about 20.

[ слишком длинный топик - автонарезка ]



You want your rocket to be short - its much easier to encase a short,
fat rocket in a cavitation chamber then it is to encase a long rocket.

Extending the cone forward of the outer tube slightly (~10% the total
cone length) seems to improve cavitation. I have no explanation for
why, it just does. This is what this looks like:

| / |
| / |
| / |
| / |
O/ O <- slots

You can artificially inflate the cavity with air. This is common in
military applications. I've just started working with this, and the
technology is very complex. I don't know if it'll be possible to make
this work at the amateur level, but I'm giving it a go.

Finally, fins don't do squat inside of a cavitation chamber. As such I
usually only add fins to the final stage and leave the second stage
finless (my second stages are designed to burn out under water).
Instead I put "bumpers" on the second stage which keep the rocket
centered in the cavitation chamber.

> This sounds very cool. Are you the first or only person doing amature
> experimentation in this area?

As far as I know I am the only one. I shouldn't take all of the credit - I have
two friends also involved who do a large portion of the work.

> What was the cause of the accidental ignition? I guess I was assuming you
> ran a wire down to the launcher so that you wouldn't have to worry about an
> electronics glitch pre-maturely setting the thing off.

We used a relay system in which a fiber optic cable couples the launch
controller to the firing circuitry, which is located under the pad. This way we
can run several hundred feet of cable to the surface without having to worry
about power loss though the cable. What happened was the casing that contained
the battery and electronics leaked. This caused a short that ended up bypassing
the fuse and relays, and thus lead to ignition of the rocket. We are now using
a different system in which the electronic components are sealed inside of
plastic, and the different stages of the ignition system are located in separate
compartments. This should prevent this from happening again.
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