Shenzhou Launch & Return · RocketLab

Meet the Long March 2F

The Long March 2F is China's rocket for launching crewed spacecraft. It is 58.34 m tall — about a 20-storey building — and weighs about 470 t at liftoff, roughly 90% of it propellant.

From bottom to top it works like a relay team: the four boosters and the first stage push the rocket out of the dense atmosphere, the second stage takes over and carries the spacecraft into orbit, and at the top are the fairing holding the Shenzhou spacecraft and the escape tower. Each part is thrown away once it is used up, so the rest of the rocket gets lighter and lighter.

Drag the "Explode" slider to take the rocket apart, and click a part on the right to see what it does.

Ignition and liftoff

The engines blast exhaust gas downward at about 2.5 km/s; the reaction force of the gas pushing on the rocket is the thrust (Newton's third law). The rocket can only leave the pad when the thrust is greater than its weight:

$a = \dfrac{F - mg}{m}$

At liftoff the eight YF-20B engines of the Long March 2F produce about 5,900 kN of thrust. The rocket weighs about 470 t, so the thrust-to-weight ratio is only about 1.28 and the liftoff acceleration is just 0.28g — that is why it seems to rise "slowly". As propellant burns, the mass keeps dropping and the acceleration keeps growing.

Try a heavier payload or fewer engines: when the thrust-to-weight ratio is below 1, the rocket has to burn off some propellant before it can leave the pad.

Through the atmosphere

The rocket gets lighter every second: it burns about 2.3 t of propellant per second, so with the same thrust and less mass the acceleration grows from about 0.3g to more than 4g. The overload the astronauts feel is the seat pushing on them harder than their weight:

$N = m(g + a)$

Air drag $F_D = \tfrac12 \rho v^2 C_D A$ grows with the square of the speed but shrinks as the air thins out. Their product — the dynamic pressure $q = \tfrac12 \rho v^2$ — peaks at about 70 s and 11 km altitude (Max-Q), the moment of greatest structural stress. As the rocket passes the speed of sound, a ring of condensation cloud can form around it.

Dropping dead weight

The velocity change a rocket can achieve depends only on its exhaust velocity and its mass ratio (the Tsiolkovsky rocket equation):

$\Delta v = v_e \ln\dfrac{m_0}{m_1}$

Empty boosters and an empty first stage carried along are just "dead weight" that keeps the mass ratio low. So the escape tower is jettisoned at 120 s, the boosters separate at about 139 s, the first and second stages separate at about 159 s (a "hot separation": the second stage ignites first and its flame blasts out through the open truss interstage), and the fairing is dropped at about 200 s. Every part thrown away makes the rest of the rocket lighter and faster.

Try "No staging": with the same propellant, the rocket cannot reach orbital speed.

Reaching orbit

The second stage pushes the spacecraft up to about 200 km and at the same time tilts its velocity to horizontal. It reaches orbit when its speed equals the circular speed at that height — when gravity provides exactly the centripetal force:

$\dfrac{GMm}{r^2} = \dfrac{mv^2}{r} \;\Rightarrow\; v = \sqrt{\dfrac{GM}{r}}$

At 200 km that is about 7.8 km/s (7.9 km/s just above the ground — the first cosmic velocity). Cut the engine too early and the speed falls short: the spacecraft arcs back down to the ground. Launching eastward borrows the speed of the Earth's rotation, and the lower the latitude, the more you borrow. Once the engines shut down, only gravity acts on the spacecraft and everything inside is weightless.

Orbital flight

The spacecraft moves in uniform circular motion around the Earth, with gravity providing the centripetal force. The higher the orbit, the lower the speed and the longer the period:

$v = \sqrt{\dfrac{GM}{r}},\qquad T = 2\pi\sqrt{\dfrac{r^3}{GM}}$

Shenzhou docks with the space station at about 390 km and circles the Earth every 92 minutes or so — 16 sunrises and sunsets a day. Everyone and everything in the cabin is in "free fall", so they are completely weightless — not because there is no gravity: at this height the gravitational acceleration is still about 8.7 m/s².

Re-entering the atmosphere

The return capsule plunges into the atmosphere at about 7.8 km/s. Over a dozen or so minutes, air drag turns almost all of its huge kinetic energy into internal energy: the shock layer outside reaches thousands of degrees, and layer after layer of ablative material on the heat shield burns away and carries the heat off. Hot ionized air wraps around the capsule and cuts off radio signals — the blackout.

$\tfrac12 m v^2 \approx 1 \times 10^\ \mathrm{J}$

The entry angle must be just right: too steep and the deceleration is so violent that the overload exceeds what the human body can take; too shallow and the capsule skims off the atmosphere like a stone skipping on water. The capsule's trim angle of attack produces a little lift, and rolling changes the direction of that lift to steer the landing point and reduce the overload.

Parachutes and landing

At about 10 km the drogue parachute opens first, then the 1,200 m² main parachute. When drag equals weight the capsule falls at a constant speed, the terminal velocity:

$v = \sqrt{\dfrac{2mg}{\rho C_D A}}$

The main parachute slows the capsule to about 7 m/s. About 1 m above the ground, the retro-rockets fire and cut the speed to about 2 m/s in 0.2 s. By the impulse–momentum theorem $F\Delta t = m\Delta v$, the smaller the change in speed and the longer it takes, the smaller the impact force — the retro-rockets reduce $\Delta v$, and the seat cushions lengthen $\Delta t$.