Whereas chemical rockets waste too much propellant to reach a net change in spacecraft velocity, electric thrusters can do exactly the same mission using just a small fraction of the propellant.
In the absence of any net external forces - on a system as a whole — - as we discussed last time — the center of mass will always have the same velocity.
在没有外力，作用于整个系统时-，我们上次讨论过-，重心总是，有相同稳定的速度。
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With that the net convergence velocity becomes faster.