I’ve been reading about the conservation of momentum and its applications in deriving the rocket equation. I’m curious whether how can a rocket slow down in space. As far as i can comprehend, a rocket’s change in velocity is proportional to the negative of the exhaust velocity relative to the rocket and the ratio of the natural logarithm of its instantaneous mass over its original mass (before expelling fuel). It can propel itself forward yes (increasing speed), but how can it decrease speed? The only way i can see it is by having the logarithmic ratio of the said masses to be negative (that is increasing instantaneous mass) which sounds really illogical since it implies gaining mass. Another way is by reversing the direction of its exhaust velocity. Is this how a rocket can slow down without the aid of air resistance?
$\begingroup$ Re, "...reversing the direction of its exhaust velocity..." which can be most easily accomplished by simply turning the whole vehicle to face in the opposite direction. $\endgroup$– Solomon SlowMay 27, 2020 at 13:34
You'd need to accelerate in the opposite direction, e.g. turn around and fire the rockets the other way. One could combine this with gravitational maneuvers to minimize the fuel required to do this.
This is indeed as wasteful as it sounds, but it comes from the law that each action has an opposite reaction. This is as you say how rockets work, hot gasses are exhausted at a certain mass/time and with a speed, and this opposite reaction results in the rocket going forward.
The main way is to launch rockets in the opposite direction to cause a delta V in opposite direction. Obviously that's a bit wasteful if you just spent a bunch of fuel speeding yourself. You can also start to use gravitational assists to change velocities with less fuel.
$\begingroup$ Properly speaking, a rocket is only "launched" at the start of its journey. I don't know if there's any special name for the maneuver in which the rocket is turned around for a deceleration burn. $\endgroup$ May 27, 2020 at 15:32