reentry cavitator

The Russian Shkval (Squall) torpedo is built like a
rocket. It flies underwater in a cavity made by a so
called cavitator mounted on its nose. The cavity
drastically reduces viscous drag and enables the
torpedo to fly as fast as 100 meters per second.
http://www.diodon349.com/Kursk-Memor...underwater.htm

A spacecraft reentering the atmosphere moves much faster
than the speed of sound, so its aerodynamics is similar
to the aerodynamics of the cavitating torpedo. It may be
possible to reduce structural stress and temperature of
the reentering spacecraft with the help of the cavitator.
The cavitator would be made of pure copper, cooled, and
mounted on an actuator which controls the attitude and
trajectory of the spacecraft. If this idea does work,
the last stage of rocket launchers can be reusable.

Andrew Nowicki writes:

The Russian Shkval (Squall) torpedo is built like a
rocket. It flies underwater in a cavity made by a so
called cavitator mounted on its nose. The cavity
drastically reduces viscous drag and enables the
torpedo to fly as fast as 100 meters per second.
http://www.diodon349.com/Kursk-Memor...underwater.htm

A spacecraft reentering the atmosphere moves much faster
than the speed of sound, so its aerodynamics is similar
to the aerodynamics of the cavitating torpedo.

No, it is _NOT IN THE LEAST BIT SIMILAR_. Water "cavitates" at velocities
far below the speed of sound in water because it is incompressible, and
because the pressure in the cavity is less than the vapor pressure of water.

Air is =QUITE= compressible, and passing through it at a supersonic
velocity forms _SHOCK WAVES_, which are NOT IN THE LEAST BIT ANALOGOUS
to hydraulic cavities.


It may be possible to reduce structural stress and temperature of the
reentering spacecraft with the help of the cavitator. The cavitator
would be made of pure copper, cooled, and mounted on an actuator which
controls the attitude and trajectory of the spacecraft. If this idea does
work, the last stage of rocket launchers can be reusable.

All your proposal will do is stand the shock-wave off to a somewhat larger
distance. This does little good, because the radiant heat load will still
be the same, as the hot plasma will still subtend the same amount of sky.
The temperature inside a radiantly heated oven does not depend on how large
the oven is.


-- Gordon D. Pusch

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In article ,
Andrew Nowicki wrote:
A spacecraft reentering the atmosphere moves much faster
than the speed of sound, so its aerodynamics is similar
to the aerodynamics of the cavitating torpedo.

Unfortunately, as far as I know, there's no equivalent in air to
cavitation in water. (Air is already a gas, it can't cavitate.)

Besides, the whole point of cavitation in water is to reduce drag.
That is exactly the *wrong* thing to do for a reentering spacecraft,
which wants maximum drag, to decelerate in the thinnest possible air.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

(Henry Spencer) wrote:

In article ,
Andrew Nowicki wrote:
A spacecraft reentering the atmosphere moves much faster
than the speed of sound, so its aerodynamics is similar
to the aerodynamics of the cavitating torpedo.

Unfortunately, as far as I know, there's no equivalent in air to
cavitation in water. (Air is already a gas, it can't cavitate.)

Besides, the whole point of cavitation in water is to reduce drag.
That is exactly the *wrong* thing to do for a reentering spacecraft,
which wants maximum drag, to decelerate in the thinnest possible air.

You can use shock waves to the same effect. The 'spike' on the nose
of a Trident-I/-II does exactly the same thing as it does on the
Skvall, reduce drag on the nose. (IIRC the estimate is that the
aerospike adds 10% to the range of the bird, a pretty good win given
the increase in volumetric efficiency it allows, and it only costs
about 20lbs.)

D.
--
The STS-107 Columbia Loss FAQ can be found
at the following URLs:

Text-Only Version:
http://www.io.com/~o_m/columbia_loss_faq.html

Enhanced HTML Version:
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Corrections, comments, and additions should be
e-mailed to , as well as posted to
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discussion.

"Gordon D. Pusch" wrote:
GDP All your proposal will do is stand the shock-wave
GDP off to a somewhat larger distance. This does little
GDP good, because the radiant heat load will still
GDP be the same, as the hot plasma will still subtend
GDP the same amount of sky...

Most of the radiant heat can be reflected away by
the metal body of the spacecraft. I envision a
double-hull, dewar-like body. If there is little
hot oxygen around the body, the outer shell can
be thin.

The conical, ballistic reentry capsules are very
thick on the bottom facing the hot air, and much
thinner on the conical side. (Russian capsules
have somewhat improved shape of a reflector.)
My proposal is to enlarge the thin, conical side
at the expense of increased risk. If the actuator
fails, the reentry capsule will tumble and burn up.

Andrew Nowicki writes:

"Gordon D. Pusch" wrote:
GDP All your proposal will do is stand the shock-wave
GDP off to a somewhat larger distance. This does little
GDP good, because the radiant heat load will still
GDP be the same, as the hot plasma will still subtend
GDP the same amount of sky...

Most of the radiant heat can be reflected away by
the metal body of the spacecraft.

That simply slows down how fast it heats up. It does not stop it from heating.


I envision a double-hull, dewar-like body.

The vehicle is =ALREADY= in a darned good vacuum !!! Adding another layer
of vacuum will not help !!!


If there is little hot oxygen around the body, the outer shell can be thin.

You seem to have aquired the ridiculous notion that only "hot oxygen"
is a problem. This ridiculous notion is quite wrong. And I again repeat:
The primary heat load on most of the vehicle except for the leadin edges
will be _RADIATIVE_, not aerothermal.


The conical, ballistic reentry capsules are very
thick on the bottom facing the hot air, and much
thinner on the conical side. (Russian capsules
have somewhat improved shape of a reflector.)
My proposal is to enlarge the thin, conical side
at the expense of increased risk.

This sounds like a Very Bad Design Principle.


If the actuator fails, the reentry capsule will tumble and burn up.

This also sounds like a Very Bad Design Principle.


-- Gordon D. Pusch

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(Derek Lyons) writes:

(Henry Spencer) wrote:

In article ,
Andrew Nowicki wrote:
A spacecraft reentering the atmosphere moves much faster
than the speed of sound, so its aerodynamics is similar
to the aerodynamics of the cavitating torpedo.

Unfortunately, as far as I know, there's no equivalent in air to
cavitation in water. (Air is already a gas, it can't cavitate.)

Besides, the whole point of cavitation in water is to reduce drag.
That is exactly the *wrong* thing to do for a reentering spacecraft,
which wants maximum drag, to decelerate in the thinnest possible air.

You can use shock waves to the same effect. The 'spike' on the nose
of a Trident-I/-II does exactly the same thing as it does on the
Skvall, reduce drag on the nose. (IIRC the estimate is that the
aerospike adds 10% to the range of the bird, a pretty good win given
the increase in volumetric efficiency it allows, and it only costs
about 20lbs.)

...At the price of a greatly increased heat load on the "spike."
And again, note that "increasing the range" is the exact _OPPOSITE_
of what one is trying to do in a NON-WEAPON re-entry vehicle.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

(Gordon D. Pusch) wrote:

(Derek Lyons) writes:

You can use shock waves to the same effect. The 'spike' on the nose
of a Trident-I/-II does exactly the same thing as it does on the
Skvall, reduce drag on the nose. (IIRC the estimate is that the
aerospike adds 10% to the range of the bird, a pretty good win given
the increase in volumetric efficiency it allows, and it only costs
about 20lbs.)

..At the price of a greatly increased heat load on the "spike."

Agreed, IIRC that's why it's made of some fairly exotic stuff.

And again, note that "increasing the range" is the exact _OPPOSITE_
of what one is trying to do in a NON-WEAPON re-entry vehicle.

Agreed 100%. I was merely pointing out that Henry's assertion that
such an effect (using shock waves to form a supercavitating effect) is
impossible is completely wrong.

D.
--
The STS-107 Columbia Loss FAQ can be found
at the following URLs:

Text-Only Version:
http://www.io.com/~o_m/columbia_loss_faq.html

Enhanced HTML Version:
http://www.io.com/~o_m/columbia_loss_faq_x.html

Corrections, comments, and additions should be
e-mailed to , as well as posted to
sci.space.history and sci.space.shuttle for
discussion.

(Henry Spencer) wrote in message ...
In article ,
Andrew Nowicki wrote:
A spacecraft reentering the atmosphere moves much faster
than the speed of sound, so its aerodynamics is similar
to the aerodynamics of the cavitating torpedo.

Unfortunately, as far as I know, there's no equivalent in air to
cavitation in water. (Air is already a gas, it can't cavitate.)


While not directly analogous to a cavitation bubble, very strong shock
waves (such as encountered upon re-entry) compress a large amount of
the shocked air into a region near the shock wave itself. In fact, an
analytic technique used for modeling strong shocks is called the
"snowplow" model, where it is assumed that *all* for the shocked gas
is collecting in a thin layer behind the shock. This model shows
surprisingly good agreement with experiment.

This is why, for example, near a very intense point-source explosion,
the pressure can actually drop below the initial ambient pressure
after the blast (shock) wave passes by: the blast has collected most
of the air and kicked it outward, leaving partial vacuum in its wake.

Thus, if you have a strong shock source ahead of the vehicle, there
may be some advantage in riding in the lower density gas behind the
strong shock, in terms of both reduced drag and aerodynamic heating.
How to create the strong shock, however, without resulting in
additional drag and heating on the vehicle?

Derek Tidman has an interest patent on one possible solution:

http://patft.uspto.gov/netahtml/srchnum.htm
Patent number: 4,917,335

Basically, the vehicle fires a forward-facing jet to create a virtual
spike ahead of the vehicle. Tidman proposed a jet of combustible
material, but the effect would work with an inert jet as well. You
can find multiple mentions of similar ideas referenced therein, and
Leik Myrabo has proposed a similar concept using beamed-energy to
create a strong shock stabilized on a virtual spike ahead of a
hypersonic vehicle to reduce drag and heating.


Besides, the whole point of cavitation in water is to reduce drag.
That is exactly the *wrong* thing to do for a reentering spacecraft,
which wants maximum drag, to decelerate in the thinnest possible air.


Exactly. Tidman's patent was for a "transatmospheric vehicle"
(probably a missile or warhead), not for re-entry.
--
Andrew J. Higgins Mechanical Engineering Dept.
Assistant Professor McGill University
Shock Wave Physics Group Montreal, Quebec CANADA
http://www.mcgill.ca/mecheng/staff/academic/higgins/

(Derek Lyons) writes:

(Gordon D. Pusch) wrote:

(Derek Lyons) writes:

You can use shock waves to the same effect. The 'spike' on the nose
of a Trident-I/-II does exactly the same thing as it does on the
Skvall, reduce drag on the nose. (IIRC the estimate is that the
aerospike adds 10% to the range of the bird, a pretty good win given
the increase in volumetric efficiency it allows, and it only costs
about 20lbs.)

...At the price of a greatly increased heat load on the "spike."

Agreed, IIRC that's why it's made of some fairly exotic stuff.

And again, note that "increasing the range" is the exact _OPPOSITE_
of what one is trying to do in a NON-WEAPON re-entry vehicle.

Agreed 100%. I was merely pointing out that Henry's assertion that
such an effect (using shock waves to form a supercavitating effect)
is impossible is completely wrong.

Henry's point (and mine) was that the physics of shock-wave phenomena
have ABSOLUTELY NOTHING IN COMMON with "cavitation." Shock waves are the
result of the non-zero compressibility of gases, whereas cavitation is the
result of the near-_IN_compressibility of water. The two phenomena are at
_OPPOSITE ENDS_ of the relevant parameter spectrum. Moreover, they have
opposite effects on the relevant projectile.


-- Gordon D. Pusch

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