Ustica le sfere metalliche in flap dc9 ce l'avevano sia Acrid sia Sparrow missili inerti

 Sparrow inerte

Use of Ballast and "Steel Balls"

While standard documentation for the live WDU-27/B warhead describes a blast-fragmentation mechanism, training versions often utilize inert ballast: 

• Weight Replication: To ensure the ATM-7 flies identically to a combat AIM-7, the warhead section is replaced with a telemetry unit (such as the AN/DKT-76) and specialized ballast.

• Ballast Material: In many missile training programs, this ballast can consist of steel or lead shot (small balls) or solid metal plates. This material is used to fine-tune the missile's center of gravity without the risk of an explosion upon impact.

• Purpose: These "steel balls" or shot are strictly inert; they do not function as fragmentation for the training round but rather as a dense material to match the 40 kg (88 lbs) weight of a live warhead. 

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Lethal vs. Training Designations

• WDU-27/B (Live): An annular blast-fragmentation warhead using a steel case designed to break into lethal fragments.

• WTU Series (Training): Training warheads are specifically designated under the WTU (Warhead Training Unit) system (e.g., WTU-12/B or similar variants for other missiles). These sections are completely inert and often contain the ballast mentioned above.

• Telemetry Section: In ATM-7 variants, the space where the warhead usually sits is primarily occupied by a telemetry section that transmits flight data back to the range controllers instead of detonating. 

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Summary of Sparrow Training Variants

Variant

	Purpose	Warhead Status
ATM-7	Live-fire flight training	Replaced by telemetry & inert ballast
CATM-7	Pilot acquisition training	No warhead; captive carry only
DATM-7	Ground crew loading practice	Completely inert dummy round

Acrid inerte

Technical Role of Ballast

Because the R-40 is the largest air-to-air missile ever produced, its weight significantly affects aircraft performance. In inert versions, the internal space where the 35–55 kg HE-Frag warhead would sit is typically filled with steel spheres or lead ballast to ensure the aircraft's flight characteristics remain identical to a combat-ready interceptor.