Take the whole launching process of Falcon-9 carrier rocket, which has just completed the launch mission of manned dragon spacecraft. When the launch button was pressed in the launch command hall at 3: 22 Beijing time, the rocket engine ignited at 3: 23, and the rocket began to lift off, leaving the launch pad and flying into space. After launching for 2 minutes and 36 seconds, the Falcon 9 first-stage rocket was successfully separated from the second-stage rocket. At this time, the separation speed of the first-stage rocket and the second-stage rocket is 3.2km/s, and the height of the first-stage rocket from the ground is about 80km. After the separation of the first-stage rocket and the second-stage rocket, the second-stage rocket engine ignited, pushing the second-stage rocket and launch load to continue to fly to the predetermined launch orbit, while the first-stage launch vehicle did not return to the ground immediately, but continued to glide upward under the inertia, and finally the taxiway was 168 from the ground.

Judging from the above launch process, Falcon 9 is a two-stage rocket. During the whole launch process, the maximum flight speed of the rocket did not reach the minimum launch orbit speed of 7.9 km/s when the two-stage rocket was separated. How did the rocket fulfill the launch orbit requirements?

In fact, this will start with the multi-stage rocket formula of tsiolkovsky, the father of rockets. In this formula, the speed of Falcon-9 carrier rocket during the first and second stage separation is directly related to the thrust M0 when the rocket is launched and the self-weight M 1 when the first stage rocket is turned off, because the greater the ratio of M0 to M 1, the higher the speed of rocket separation, but it is limited by the low specific impulse of existing rocket fuel.

Let's talk about the Falcon 9 carrier rocket. Although the maximum flight speed of the first and second stage rockets is only half that of the first universe when they are separated, the second stage rocket itself is lighter after abandoning the first stage rocket. At this time, the external air is thin and the resistance is small. Therefore, the second-stage rocket only needs a Merlin 1D+ liquid rocket engine with a maximum thrust of 8 1 ton, which can meet the flight requirements of the second-stage rocket. Because for Falcon 9 carrier rocket, the amount of fuel loaded by the first-stage and second-stage rockets is different according to different launch orbits. For example, when launching a spacecraft in low-earth orbit, the self-weight of the second-stage rocket is about 3 tons, the fuel load inside the rocket is 45 tons, and the space launch load is about 10- 13 tons. At this time, the weight of the whole secondary rocket+spacecraft is only 58-666. The vacuum version of Merlin engine used in the second stage rocket has a thrust of 8 1 ton and a rocket thrust-to-weight ratio as high as 1.33- 1.39. If the vacuum specific impulse of Meilin 65,438+0d+liquid rocket engine is calculated as 3 10S, the secondary rocket and launch load can be completely reduced from 80km to 3.2km by multiple ignition. 7.5 km/s launch speed requirement (after the second-stage rocket is separated from the space launch load, the spacecraft can accurately enter the orbit through its own thrust).