NASA's Glenn Zero Gravity Research Facility gives you 5.18 seconds of free fall down a 132-meter shaft. ESA's ZARM tower in Bremen manages 9.3 seconds with a catapult trick. Both are limited by height. What if we scaled up — a full 1-kilometer evacuated drop tower, taller than the Burj Khalifa, dedicated to microgravity science?

The free-fall time. Using t = √(2h/g), a 1000 m drop gives t = √(2000/9.81) ≈ 14.3 s. Add a catapult launch (fling the capsule up first, let it arc back), and you double it to ~28 seconds. That's triple ZARM and roughly matches what a parabolic-flight aircraft delivers in one arc — but with true 10⁻⁶ g quality, not the 10⁻² g "vomit comet" jitter.

Why the vacuum matters. Terminal velocity of a 500 kg capsule (2 m² frontal area, C_d ≈ 1) in air is v_t = √(2mg/ρAC_d) ≈ 70 m/s. The capsule would hit that in ~7 seconds and stop accelerating — no more free fall. Pumping the tower down to 10 Pa (0.0001 atm) reduces drag by 10,000×, restoring near-perfect g < 10⁻⁵.

The pumping problem is brutal. Volume of a 1 km tower, 10 m diameter: V = π(5)² × 1000 ≈ 78,500 m³. At sea-level density (1.2 kg/m³), that's 94 tonnes of air. Getting to 10 Pa means removing 99.99% of it. Industrial roughing pumps push maybe 500 m³/h effective at low pressure; a bank of 50 pumps still takes ~24 hours per experiment cycle. Turbomolecular pumps get you the last decade.

Structural loads. External atmospheric pressure on the tower walls: 101,325 Pa × (surface area). For our cylinder, that's π × 10 × 1000 = 31,400 m² of lateral surface, giving 3.18 billion newtons of inward crush force. This is why vacuum chambers are round — hoop stress in a thin-walled cylinder is σ = Pr/t. For a 5 m radius at 1 atm and steel yield ~250 MPa (with safety factor 4): t = 101,325 × 5 / (250×10⁶/4) ≈ 8 mm. Surprisingly modest! But buckling, not yield, dominates — you need stiffening rings every few meters, pushing effective wall thickness to ~30 mm. Total steel: ~74,000 tonnes, roughly one Eiffel Tower's worth.

The deceleration. After 14 seconds of drop, the capsule hits ~140 m/s. Stopping it in a 10-meter polystyrene bead pit at the bottom requires a = v²/2d = 19,600/20 = 980 m/s² — a 100 g crunch. Fine for hardware, deadly for humans. Add magnetic braking rails and you can spread deceleration over 100 m at a survivable 10 g.

What does it buy us? Protein crystallization, colloidal self-assembly, and combustion physics all benefit dramatically from longer microgravity windows. At $10,000/second (rough parabolic-flight economics), 28 seconds per drop × 300 drops/year = $84M/year of equivalent research time — versus ISS experiments at ~$100k/kg-day. Payback on a $2B tower: about 25 years, ignoring the science we couldn't do at all before.