Why is the flame of China rocket orange?

As other answers said, there is a great difference between the first and second stage fuels of Chinese and foreign rockets.

The fuel of China is unsymmetrical dimethylhydrazine (CH3)2N-NH2, and the oxidant is nitrogen tetroxide (N2O4). Because N2O4 is decomposed into red nitrogen dioxide (N2O4=2NO2) by heating, China rocket will emit red smoke when it is launched. However, nitrogen, carbon dioxide and water vapor (CH3)2N-NH2+2N2O4=2CO2+4H2O+3N2 are generated by the reaction between dinitrogen tetroxide and unsymmetrical dimethylhydrazine, so the combustion temperature is not high, and a lot of smoke will not be generated.

Foreign fuel is kerosene, oxidant is liquid oxygen, and the combustion temperature is high. In addition, kerosene may burn incompletely, which leads to the mixing and condensation of particles and water vapor, so a lot of smoke will be produced.

Of course, foreign countries will also use liquid hydrogen as fuel, which will produce more water vapor and higher combustion temperature, so it will also produce a lot of smoke.

Other countries use solid fuels, mostly high-molecular polymers, which are similar in chemical composition to kerosene, so they also produce a lot of smoke.

The advantage of unsymmetrical dimethylhydrazine-dinitrogen tetroxide fuel is that, as a liquid rocket fuel, unsymmetrical dimethylhydrazine can be stored and used at room temperature, while dinitrogen tetroxide can directly come from nitric acid plant (nitrogen dioxide obtained by catalytic oxidation is condensed), and the price is cheap. However, unsymmetrical dimethylhydrazine and nitrogen tetroxide are both toxic, and some people died when China launched Austar. Therefore, for safety reasons, UDMH is no longer used in European and American countries.

Although kerosene-liquid oxygen is not dangerous to unsymmetrical dimethylhydrazine, it is necessary to consider how to store liquid oxygen at MINUS 200 degrees, and engine parts need to be precooled to avoid bursting after sudden contact with low-temperature propellant. At the same time, kerosene is expensive and not suitable for commercial launch.

Liquid hydrogen-liquid oxygen is the best fuel, the same fuel weight can provide the maximum thrust, and there is no pollution. However, liquid hydrogen and liquid oxygen are expensive, the engine technology is complex and difficult to master, and both of them are low-temperature fuels, so it is very difficult to save them. Liquid hydrogen also needs anti-static to avoid explosion. The combustion temperature of liquid hydrogen and liquid oxygen propellants is very high, and the nozzle and deflector are a severe test, so liquid hydrogen and liquid oxygen are generally the fuels of the last stage rocket.

Of course, some countries will use solid fuel because their rockets come from ballistic missiles. Up to now, the fuel of most ballistic missiles is solid fuel, which is convenient for normal storage and does not affect the missile shell (ballistic missiles that used to use liquid fuel need to make room for regular maintenance), but this is too extravagant for launch vehicles now.