Propellants, stages and mass

Long March 2F uses nitrogen tetroxide and unsymmetrical dimethylhydrazine. Long March 5 uses hydrogen and oxygen in its core stages, with oxygen/kerosene booster propulsion; the basic Long March 7 uses oxygen/kerosene engines. These combinations imply different tanks, feed systems and ground preparations. A fuel name alone does not specify the whole launch system.

CNSA’s Long March 5 engineering account describes cryogenic conditioning, manufacturing and test infrastructure. When comparing designs, include how propellants are stored and loaded, how tanks and engines are connected, and how operations are rehearsed. A change that improves one performance measure may introduce different structural or operational demands.

The original Long March 5B station-module configuration retained the core first stage and four boosters but omitted Long March 5’s core second stage. It also used a larger payload fairing. The 2020 first-flight account explains these differences as choices for large low-Earth-orbit payloads. Do not apply that original arrangement automatically to every later mission or upper-stage combination.

Dropping spent hardware means later propulsion does not have to accelerate all the earlier empty structure. But another stage also adds hardware, interfaces and separation events. In an original design review, compare the mass saved after separation against the mass and operations added by the new stage; counting stages by itself cannot settle the tradeoff.

For an ideal burn with constant effective exhaust velocity, delta-v equals exhaust velocity multiplied by the natural logarithm of initial mass divided by final mass. The final mass includes the payload and all hardware still attached. This simplified equation excludes gravity and aerodynamic losses; it does not predict a real Long March payload capacity by itself.

Use invented values: initial mass 100 tonnes, final mass 20 tonnes and exhaust velocity 3 kilometers per second. The ratio is 5, giving about 4.83 kilometers per second of ideal delta-v. Add 5 tonnes of nonburned mass while keeping propellant unchanged: the masses become 105 and 25, the ratio becomes 4.2 and ideal delta-v falls to about 4.31 kilometers per second. More liftoff mass did not mean more performance.

A heavier vehicle can have a lower ideal delta-v