A Whipple shield (Fred Whipple, 1947 — flown on every crewed spacecraft since, including the ISS) protects a pressure hull from millimetre-sized orbital debris and micrometeoroids not by stopping the impactor, but by shattering it before it arrives. A thin outer "bumper" plate, spaced a gap away from the real structural wall, turns a hypervelocity impact into an expanding cloud of fragments and vapour. The same kinetic energy then lands on the rear wall spread over a much larger area — often the difference between a scratch and a hole.
KE = ½ m v² (impactor kinetic energy)
θ(v) ≈ clamp(10° + 2.2°·(v_km/s − 3), 8°, 32°) (debris-cone half-angle, shield ON)
R_footprint = r_proj + S·tan(θ) (S = bumper–wall standoff)
σ = KE / (π R_footprint²) (areal energy density reaching the wall)
Wall survives if σ ≤ k · t_wall (thicker rear wall ⇒ higher capacity)
This site models the ballistic-limit trade-off used in real shield design (à la the Cour-Palais equation) with a simplified, transparent formula rather than the exact classified/empirical NASA curves: cone spread grows with impact speed once the projectile is going fast enough to shatter and partially vaporise on the bumper (roughly ≥ 3 km/s for aluminium-on-aluminium), the footprint grows with the standoff gap, and a thicker rear wall can absorb more energy per unit area before breaching.
- Diameter / velocity — set the size and speed of the incoming micrometeoroid or debris fragment.
- Standoff — the gap between bumper and hull; more gap gives the debris cloud more room to spread before it arrives.
- Wall thickness — how much structural aluminium the rear wall has to absorb the spread-out impact.
- Whipple bumper ON/OFF — toggle it off to see the same projectile hit a single unshielded wall directly: the energy lands in a crater barely bigger than the projectile itself, and even small debris usually breaches.
Real-world relevance: the ISS's Whipple-type shielding lets a few millimetres of spaced aluminium (plus stuffed Kevlar/Nextel layers in the newer "stuffed Whipple" variant) stop debris that would otherwise need many centimetres of solid armour — armour heavy enough to make the station unlaunchable.