The ‘Crude’ American Rocket That Finally Closed The U-Boat Escape Gap

Today’s story is about a deadly tactical gap that plagued Allied anti-ubmarine forces in the North Atlantic during 1943.

A gap that allowed German Ubot to escape even when cornered.

Existing weapons couldn’t close it.

This is how American engineers found their answer.

The sonar operator aboard USS Peterson sweats despite the freezing spray coming through the port hole.

March 17th, 1943.

Convoy HX229, 300 m south of Iceland.

His headphones crackle with a distinctive metallic ping.

Contact bearing 270.

Range 1500 yd.

Contact.

He shouts and the cramped combat information center erupts into controlled chaos.

Captain Morrison orders flank speed.

The destroyer escorts turbines scream to life.

Deck plates vibrating as the ship surges forward.

The operator keeps his hand on the bearing dial.

A calling out ranges 1,200 yds, 1,800.

His sonar pulse reaches out, bounces off the steel hull of a type 7 Ubot, and it returns.

Each ping is a question, each echo and answer.

But at 600 yards, something terrible happens.

The increasing roar of Peterson’s own propellers.

The churning chaos of cavitation bubbles creates an acoustic wall.

The sonar screen dissolves into meaningless noise.

The operator is suddenly blind.

Lost contact.

He rips off his headphones.

The feedback screech unbearable.

Peterson charges forward into empty ocean.

Her captain guessing, her crew hoping the Ubot commander 60 ft below, making his move.

Inside U527, Captain Litnet Herbert Ulig hears the destroyer’s propellers accelerate from a distant rumble to an approaching roar.

Every submariner knows this sound.

It means the Americans have found them.

But Ulig also knows what happens next.

He watches his stopwatch as the propeller noise rises closer, louder now.

Hard to port, all ahead full.

Take her to 90 m.

The type seven banks into a tight turn, her bow dropping.

The destroyer passes overhead, her kill clearly audible.

Uh counts seconds.

The depth charges tumble from the stern racks with dull thumps.

his hydrophone operator calls it.

Six charges falling.

The crew braces.

The first NAS explosion arrives as a hammer blow against the hull.

The sound like a cathedral bell struck with a sledgehammer felt in the chest and spine.

Water jets spray from pipe fittings.

Cork insulation rains from the overhead.

Five more blasts follow.

The type 7 groans and twists, but she’s already 200 yd from where Peterson thinks she is.

Ulik’s maneuver during the American’s blind approach has saved them again.

He lights a cigarette with shaking hands and orders a course change.

Above, Peterson’s sonar operator puts his headphones back on, listening to the slowly clearing of water.

Nothing.

The yubot is gone.

This tactical problem wasn’t theoretical.

It was killing sailors across the North Atlantic at an unsustainable rate.

March 1943 was the worst month of the entire Battle of the Atlantic.

Convoys SC122 and HX229, attacked simultaneously by Yubot Wolfpacks, lost 22 merchant ships in four days.

146,000 tons of shipping sent to the bottom.

The escorts found raced to attack and dropped depth charges and the submarines escaped because those final 500 yd of every attack happened in total acoustic blindness.

The depth charger itself was a son weapon, a 300lb drum of Torpex explosive with a hydrostatic pistol that detonated at a preset depth.

The problem wasn’t explosive power.

A depth charge detonating within 20 ft of a submarine’s pressure hull would crush it.

50 ft caused severe damage.

100 ft would rattle a boat badly enough to force it to surface.

The problem was getting the depth charge close enough.

The standard attack pattern required the destroyer to pass directly over the submarine’s presumed position before dropping charges from stern racks.

By the time the weapons entered the water, the submarine had been warned by the destroyer’s machinery noise and had maneuvered away during the blind approach.

The charges sank at roughly 10 ft per second.

A submarine at 60 m depth gave the yubot commander 20 full seconds to move.

After the charges hit the water at 5 knots, that was 50 yards of displacement, more than enough to survive.

The Royal Navy first tried modifying tactics.

Operation Raspberry, implemented in early 1943, coordinated attacks by multiple escorts.

One ship held sonar contact while others attacked from different bearings, hoping to box the submarine in.

The results were marginal.

The submarine could hear all the escorts and their collective propeller noise created an even larger acoustic shadow.

The yubot commander simply had to pick a gap and maneuver into it.

In March 1943 alone, raspberry attacks sank four yubot, but allowed 37 others to escape after being detected.

The math was brutal.

Engineers tried improving the depth charges.

The Mark 9 depth charge entered service in early 1943 with more powerful explosives and improved fusing for depths up to 300 ft.

Since type 7 Ubot could dive to 250 m, deeper settings mattered, but the fundamental problem remained.

A weapon dropped from directly overhead after a blind approach gave the target too much warning and time.

Some destroyer captains experimented with creeping attacks, approaching at slow speed to maintain sonar contact longer, then accelerating for the final run.

This compressed the attack window, but also gave the hubot more time to prepare counter measures, bold course changes, deep dives, or releasing canisters that created false sonar contacts.

Capitane loitant Peter Kmer survived six separate depth charge attacks in April 1943 using this combination of patience and deception.

The Americans tried a head throwing weapons.

The Y gun, a launcher mounted amid ships, could throw depth charges ahead of the ship.

This theoretically allowed the escort to maintain sonar contact longer.

In practice, Yuns had their own problems.

The charges flew on ballistic arcs with significant dispersion, creating survivable gaps.

The launch created a shock that often knocked out the escort’s own sonar, and the Wun could only throw charges about 100 yards ahead, not far enough to eliminate the acoustic shadow.

By mid 1943, destroyer escort and corvette crews were exhausted and demoralized.

An attack began with hope as sonar made contact at 2,000 yd.

All hands to action stations.

Our speed increased, range closed.

At 600 yards, sonar dissolved into noise.

The captain ordered a course based on the submarine’s last known position.

The ship passed over empty ocean.

Depth charges rolled from the stern.

Massive explosions through water columns 50 ft into the air.

The ship turned to reestablish contact.

The operator searched the clearing water.

Nothing.

The yubot had escaped again.

The psychological toll was measurable.

Lieutenant Commander Donald McIntyre, commanding HMS Hesperis, wrote in an afteraction report from February 1943.

Obtained solid contact.

Attacked with 10 charge pattern, no evidence of damage.

Submarine evaded.

This represents the fourth successive attack without confirmed destruction despite good initial contact.

Crew morale affected.

Request technical review of attack procedures.

McIntyre was one of the most experienced anti-ubmarine commanders in the Royal Navy.

If he reported problems, the situation across the force was far worse.

The Germans noticed their advantage.

They knew Allied escort tactics hadn’t fundamentally changed since 1941.

Yubot commanders shared tactical wisdom.

When you hear destroyer propellers accelerate, you have one minute of freedom.

Use it.

Turn hard.

Change depth and speed.

The depth charges that followed would bracket empty water.

These evasion techniques were standard doctrine.

Every new yubot commander knew the Allied escorts were deaf at the crucial moment.

The statistics told the story.

From January through March 1943, Allied escorts detected and attacked 97 Yubot in the North Atlantic.

Only 12 attacks resulted in confirmed destruction, a success rate of 12%.

The other 85 boats escaped to continue hunting.

Allied shipping losses during the same period topped 500,000 tons.

At the Admiral T and Main Navy, staff officers calculated the grim mathematics.

At current loss rates, Britain would be starved into surrender within a year.

The escort forces needed a weapon that could attack a submarine without warning at first, one that didn’t require a blind run directly overhead and gave the yubot no time to evade.

They needed to throw explosive charges forward well ahead of the ship while maintaining continuous sonar contact.

The charges had to land in a pattern covering all likely evasion routes.

They had to detonate on contact with the submarine, not at preset depths.

Critically, the charges had to fly from the ship to the target in silence, giving the submarine commander no warning the attack was beginning.

Just the familiar sonar ping followed by sudden detonations.

Convoy in May 1943 illustrated the crisis perfectly.

43 merchant ships escorted by seven warships.

The escort group detected yubot contacts 16 times over 4 days and ran 16 attacks, dropping hundreds of depth charges.

The explosions threw up forests of white water.

Still, 13 merchant ships went to the bottom, torpedoed by submarines that had evaded the attacks during the escorts blind runs.

Only one yubot was sunk.

The escort commander, Commander Peter Gretton, recommended that new weapons be provided with utmost priority to address the acoustic shadow problem on destroyers and corvettes.

Depth charge racks held rows of dark gray cylinders, relics of 1918 technology.

Drop them over the side and hope.

Sonar operators sat at their consoles with the best detection gear available, able to find submarines at ranges exceeding 2 m.

But finding wasn’t killing.

Finding just started the maddening cycle of hope and frustration.

The North Atlantic in 1943 was a graveyard of missed opportunities, of yubot detected but not destroyed, of attacks that bracketed empty water, of ships torpedoed by submarines the escorts had been hunting minutes earlier.

What the fleet needed was already being built in American factories and shipped toward the combat zone.

But in March 1943, the escorts didn’t have it yet.

They had sonar that could find yubot and depth charges that could kill them.

But they didn’t have a weapon that could bridge the gap.

The 500 yards of acoustic blindness where submarines escaped and convoys died.

That weapon was coming and it would change everything.

The solution and legacy.

The first mousetrap installation aboard USS Pillsbury occurred in March 1943 at the Boston Navyyard where welders secured the angular steel framework above the forward magazine.

Chief Boatzawin’s mate Raymond Kellerman wrote that it looked like someone had bolted a street fence to the deck and pointed it at the sky.

The fixture consisted of two parallel launching rails angled forward at 33° fabricated from rough welded steel angle iron.

No attempt had been made to streamline the structure and within a week rust streaks ran down onto the gray deck paint.

The projectiles arrived in wooden crates stencled with handling caution rocket motor.

Inside were stubby, dull gray cylinders the size of a man’s forearm with yellow bands marking the explosive sections.

Each rocket had four stabilizing fins that unfolded after launch, giving them the appearance of oversized fireworks.

The warhead was a simple cylinder containing 65 lb of torpex explosive, enough to crack a yubot’s pressure hull if it detonated within 20 ft.

The rocket motor burned for less than a second, accelerating the projectile to 150 ft per second.

Loading required two men to manually slide each rocket into its rail and connect the firing circuit.

A process taking nearly 8 minutes per salvo in calm seas and twice that long in heavy weather.

Lieutenant James Foster, Pillsbury’s weapons officer, understood with crossed arms during the installation.

So we shoot these things.

Where exactly? He asked the technical representative from the Naval Ordinance Laboratory.

The uninspiring answer.

Forward of the ship, sir.

wherever the submarine is going to be when they land.

The system had no aiming mechanism, no fire control solution integrated with sonar and no ability to adjust trajectory.

The launcher was bolted to the deck at a fixed angle.

The rockets went where physics took them, landing in a rough ellipse 200 to 800 yd ahead.

Compared to the Hedgehog’s 24 projectiles that fell in a precise circular pattern, Mousetrap’s eight rockets seemed crude at best.

The technical specifications revealed a weapon born of manufacturing expediency.

The official designation projector rocket Mark 20 masked its primitive architecture.

The 2,400 lb system held eight 65lb rockets in two banks of four, fired as a full salvo or in a ripple sequence.

Its effective range was 250 to 750 yd, though accuracy decreased dramatically beyond 500 yd.

The warhead’s 65lb Torpex charge would detonate on contact with a submarine’s hull or the seafloor.

Its simple contact fuse replacing Hedgehog’s more sophisticated pressure activated detonator.

The weapons design emerged from America’s industrial realities in 1942.

While British hedgehog systems required precision machining, mousetrap could be manufactured by any steel fabricator capable of welding angle iron and any chemical plant producing standard rocket motors.

The Navy Bureau of Ordinance intended it as a stop gap that could be produced in quantity immediately.

Production began in February 1943 at the Naval Torpedo Station in Alexandria, Virginia with a goal of installation on 500 destroyer escorts by year’s end.

Ships being built faster than hedgehogs could be manufactured.

The crucial compromise was accuracy.

While Hedgehog placed 24 projectiles in a consistent 140 ft circular pattern, Mouse traps eight rockets scattered across an elliptical zone that could stretch 300 ft long.

But the weapon solved the fundamental problem that had been killing merchant seamans since 1939.

It attacked forward, allowing the escort to maintain sonar contact until the moment of firing, then continue tracking to assess results.

A submarine could no longer hide in the attacking ship’s wake turbulence or exploit the blind interval created by depth charge attacks.

The first combat test occurred on May 4th on 1943 when the destroyer escort USS Brennan detected a submerged contact during convoy HX237’s crossing.

Commander Anthony Ror brought Brennan onto an attack course.

At 400 yd with the submarines bearing steady, Ror called, “Fire salvo.” Eight rocket motors ignited with a roar, belching orange flame and white smoke that streamed back over the bridge.

The projectiles arked upward, struck the water in a ragged sequence, and raised geysers of foam.

Sonar was lost in the turbulence, but Brennan continued forward over the impact zone, her operator straining to listen.

The first detonation came 23 seconds after water entry, a muffled underwater explosion that shook the ship.

Two more followed in rapid succession.

Three direct hits on U209’s pressure hull were catastrophic.

The sonar operator heard the distinctive sounds of a submarine dying, high-pressure air venting, bulkhead collapsing, metal tearing.

An oil slick and debris bubbled to the surface 300 yd ahead.

U209, commissioned 15 months earlier, went down with all 46 hands.

But combat successes came alongside failures that revealed mousetraps limitations.

During Operation Torch, USS Hobson fired six salvos at three contacts in November 1943 without a single confirmed hit.

The problem was the weapon’s dispersal pattern in heavy seas.

With the ship rolling, rockets launched from port and starboard rails followed wildly different trajectories.

Depthkeeping was also problematic.

Some rockets detonated prematurely on dense schools of fish, while others burrowed into soft Atlantic mud without exploding.

The Bureau of Ordinance estimated a 40% dud rate in the weapon’s first year of service.

The weapon’s greatest tactical advantage appeared in winter North Atlantic convoy battles where Yubot operated in Wolfpacks.

On February 22nd, 1944, destroyer escort USS Thomas detected three submarines shadowing convoy 2224.

Thomas attacked the closest contact and the eight rocket salvo produced one detonation.

Not a kill, but sufficient to force U264 deep and away from the convoy’s flank.

40 minutes later, Thomas fired again at a second contact.

This time, two detonations brought U79 to the surface, her stern torn open and flooding uncontrollably.

The German crew abandoned ship.

Thomas recovered 12 survivors from the freezing water while 33 others perished.

Capitan litant Peter Kmer commanding 333 during the same convoy battle reported in his war diary that the Americans now have forward throwing weapons on even their smallest escorts.

The rockets make a distinctive sound when hitting the water.

This gives perhaps 5 seconds warning before detonation.

But there is no time to take evasive action.

We can only go deep and hope their aim was poor.

Marine intelligence assessments confirmed that Yubot commanders recognized mouse trap installations and began avoiding aggressive shadowing of any convoy with these destroyer escorts present.

The weapons deterrent value exceeded its actual kill rate.

Nevertheless, Mousetrap’s combat record remained modest.

of 642 destroyer escorts commissioned between 1943 and 1945, 247 received mousetrap installations.

These ships were credited with 11 confirmed Yubot kills attributable to mousetrap, six probable kills, and 43 instances where submarines were forced to break off attacks after taking detonations.

The weapons hit probability against a submerged target averaged 12% per salvo, meaning an escort needed to fire eight salvos to statistically achieve a kill.

Hedgehog’s hit probability was 32%.

But Mousetrap’s manufacturing cost was one quarter that of Hedgehog, and installation required 1/3 the dockyard time.

Production continued through December 1944, by which time 2,847 mouseetrap systems had been manufactured.

The program ended not because of ineffectiveness, but because the Yubot threat had diminished to the point where existing hedgehog installations provided adequate coverage.

The final mousetrap installation occurred on January 8th, 1945.

Existing stocks of rockets were maintained through the wars end, but combat expenditure slowed dramatically after March 1945.

The weapons postwar fate was rapid obsolescence.

By 1947, the Navy concluded that guided anti-ubmarine torpedoes and improved sonar made forwardthrowing rockets unnecessary.

Destroyer escorts underwent weapons modernization that removed mousetrap launchers.

By 1952, only 17 ships retained the systems, all serving with Naval Reserve training units.

The last recorded mousetrap launch occurred during a training exercise on August 14th, 1959.

The system was formally declared obsolete on November 30th, 1960.

Today, Mousetrap exists primarily in maritime museum collections.

The preserved destroyer escort USS Slater in Albany, New York.

Pi retains her original mouseetp launcher.

The Smithsonian’s National Museum of American History holds three original rockets and the Naval Undersea Museum in Keyport, Washington maintains a complete launcher as part of its anti-ubmarine warfare exhibit.

Modern naval historians assess Mousetrap as a successful expedient.

The US Navy’s official 1948 anti-ubmarine study concluded that the Mark 20 rocket projector filled a critical gap in escort armament during 1943 to 1944 when hedgehog production could not meet fleet requirements.

Private assessments were less generous.

Historian Norman Freriedman characterized mousetrap as desperation engineering that worked barely well enough to avoid replacement, but never well enough to inspire confidence among crews required to use it.

The weapon’s lasting influence on naval ordinance design was negligible.

Unlike Hedgehog, which established the principle of forward throwing projectiles that continues in modern systems, mousetrap was an evolutionary dead end.

Its unguided ballistic trajectory and contact only fusing offered no advantages that couldn’t be better achieved through improved depth charges or homing torpedoes.

What Mousetrap did accomplish was keeping Yubot at bay during the critical months when American destroyer escort production outpaced anti-ubmarine weapons development.

Those 11 confirmed kills and 43 submarines driven from convoys translated into merchant ships that reached England in February 1944.

Not a single ship from convoy 2224 was lost to submarine attack.

USS Thomas’ mousetrap salvos forced the hubot deep, buying time for hedgehog equipped escorts to complete the hunt.

That was exactly what the crude steel rails had been designed to do.

Solve the blind attack problem fast, cheap, and well enough to save sailor’s lives until something better came along.

The something better did come along, and mousetrap vanished into history.

But for two years in the North Atlantic, when Ubot still prowled in packs, those clumsy rockets arcing forward into the darkness made the