(Technically it’s the “effective exhaust velocity,” but I’ll shorten it to “exhaust velocity” in an attempt at brevity. The force is approximately proportional to the rate at which propellant is consumed.Įach type of chemical rocket has its own chemistry (liquid hydrogen and liquid oxygen kerosene and liquid oxygen solid fuel etc.), and that chemistry combined with the design of the engine results in a characteristic velocity for the ejected molecules, called the exhaust velocity. If the rocket burns fuel at a high rate, the reaction force on the rocket will be high, and so will the acceleration. It means the act of ejecting molecules pushes back on the rocket: every time a water molecule flies from the nozzle, the rocket loses a little mass and in return gains a little velocity.
Now, remember Newton’s third law: for every action there is an equal and opposite reaction. The RL-10 engine, used in the Centaur second stage of the Atlas V launch vehicle, relies on the same chemistry. The RS-25 burns liquid hydrogen and liquid oxygen to release energy, and the resulting water molecules are ejected at high speed from the engine nozzle. It powered the Space Shuttle for three decades and is now being used in the Space Launch System. Take NASA’s RS-25 engine as a specific example. In a chemical rocket the propellant undergoes a chemical reaction that releases energy, and the rocket directs as much of that energy as possible into kinetic energy (energy of motion) of the ejected chemical product. For most of this discussion, I’ll use chemical rockets as the main example, though I’ll briefly mention other types in part 2. In physics there are many ways to convert energy into force, so we need to be specific.
To get started, let’s ask a fundamental physics question: what do rocket engines do? For our purposes the answer is: they consume propellant to produce energy, and they convert the resulting energy into force, which accelerates the rocket’s payload. In Part 1 of this article I’ll develop the basic concepts of the rocket equation, and in part 2 apply the concepts to a worked example: the New Horizons mission to Pluto. This leads to exponential behavior-called the "rocket equation"-which puts tough limits on our ability to deliver large payloads to distant planets. But it turns out we need even more propellant than we think in order to accelerate the added propellant. If we want the spacecraft to go faster, it’s intuitive that we need more propellant. One of the biggest barriers to exploring the solar system is that the rocket we use to launch a spacecraft on its journey must accelerate not only the payload, but all the propellant the rocket is about to use-or will ever use in the future.
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