Vostok 1 flew an elliptical orbit with perigee 169 km, apogee 327 km and inclination 64.95°. Position on the ellipse comes from solving Kepler's equation for eccentric anomaly E from mean anomaly M = 2πt/T (Newton's method), then the vis-viva equation:
r = a(1 − e·cosE)
v = √( GM·(2/r − 1/a) )
Solving this with the real semi-major axis (a ≈ 6619 km) gives an orbital period of ≈ 89.3 minutes — the historically-recorded value for Vostok 1's single orbit. (Gagarin's full mission, launch to landing, was ≈108 minutes; that figure includes ascent and descent, not just the orbital period, so both numbers are correct for what they describe.)
Retrofire fires the TDU-1 retrorocket, cutting velocity by the Δv you set (default ≈136 m/s, the historical burn) and dropping perigee below the surface. Vostok's descent module was a sphere with no aerodynamic lift, so the trajectory afterward is governed by the ballistic (point-mass) reentry equations:
dv/dt = −D/m − g·sinγ
dγ/dt = cosγ·(v/r − g/v)
dh/dt = v·sinγ
D = ½·ρ(h)·v²·Cd·A, ρ(h) = ρ0·e^(−h/H)
with scale height H = 7.2 km. A lighter capsule or larger drag area decelerates harder, higher up; a heavier, denser one punches deeper into the atmosphere before the deceleration peaks — try the mass and drag-area sliders and watch the strip chart's g-force curve shift. With the stock numbers this reproduces the historically-recorded peak of roughly 8 g, versus the gentler ~3–4 g of a lifting-body reentry like the Space Shuttle.
- Time scale — the real orbit took 89 minutes; speed it up to watch a full pass and reentry in under a minute.
- Retrofire Δv — a bigger burn drops perigee further below the surface, steepening the entry angle and raising peak g.
- Drag on the view — rotate the orbital-plane projection like an orbit camera; scroll/pinch to zoom.
- G-force readout is drag deceleration D/m expressed in multiples of g0 = 9.80665 m/s².