# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/DdaeQ7_gAxx
- **Gondola URL:** https://gondola.cc/posts/70577838-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/f1056cc603.jpg
- **Posted:** 2026-09-18T03:30:49.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

A conventional torpedo hits a wall at roughly 50 knots, and it is not an engine problem. It is the water.

Seawater is about 800 times denser than air, and hydrodynamic drag scales as D = ½ρCdAV². Power demand therefore scales with velocity cubed. Doubling the speed of a 533 mm torpedo demands eight times the shaft power, and the propeller surrenders long before the motor does. Once local pressure on the blade back falls below the vapour pressure of water, near 2.3 kPa at 20 degrees Celsius, vapour pockets form, collapse violently against the blade and destroy thrust and metal alike. Thrust breakdown, not fuel, sets the ceiling.

The Soviet answer was to stop fighting cavitation and weaponise it. Design began at NII-24 in the 1960s, the M-5 prototype was completed in 1972, and the VA-111 Shkval entered service in 1977 with mass production from 1978.

The mechanism:
- A flat disc cavitator at the nose forces the flow to separate cleanly at a sharp rim, dropping pressure below vapour pressure and opening a gas cavity that encloses the whole body
- Rocket exhaust is vented forward to inflate and stabilise that cavity, making it a ventilated supercavity rather than a purely natural one
- Wetted area collapses from the full 8.2 metre hull to the face of the disc, deleting almost all skin friction
- Control surfaces must pierce the cavity wall to bite water, because everything inside the bubble is surrounded by gas

The governing parameter is the cavitation number, σ = 2(p − pc)/ρV². Lower σ means a longer and fatter cavity, so speed becomes self-reinforcing. Cavitator drag follows Cd = 0.82(1 + σ).

Ejected from a 533 mm tube at 50 knots, the rocket then drives all 2,700 kg past 200 knots, roughly 100 metres per second, across 7 km of ocean.

Shkval does not swim. It flies through a hole it digs in the sea.

#stemantics #supercavitation #fluiddynamics #navalengineering #physics

## Stats

- **Views:** 6,280
- **Likes:** 139
- **Shares:** 0
- **Comments:** 2

## Tags

supercavitation, fluiddynamics, physics, navalengineering, stemantics

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