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The starship catch between the two arms has been all over the internet. And it made me wonder, why do rockets need such powerful engines?

Although I am very comfortable with math, I barely know any physics. My knowledge doesn’t go beyond average high school mechanics, and even that I studied it years ago.

So, my reasoning was this: if we apply a force greater than the force of gravity acting on the rocket (mass x 9.8 N/kg), say 10 newtons more than this force, the rocket should accelerate upwards. Since the force is always there, according to Newton’s second law, the acceleration is there, so it should fly. If we keep on applying this small force and the acceleration is constant, it should reach space eventually, even if it’s slow, meaning without reaching the escape velocity.

This is clearly wrong, but I can't understand why!

Could you please provide an intuitive answer? Don’t hesitate to use math if necessary.

Qmechanic
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Denis
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1 Answers1

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The engines are just powerful enough, but higher power is more efficient.

Let's assume the thrust is only just enough to lift the rocket. In that case, the rocket will burn prodigious amounts of fuel to get nowhere. If the thrust is slightly higher, it will start accelerating slowly, while still burning massive amounts of fuel.

This will get the rocket going. Then, as it burns fuel it gets lighter, so it will accelerate faster. You may have noticed that rockets usually start moving slowly, but rapidly increase their acceleration, even though they have not increased engine power. That's because the rocket gets lighter as the engines burn fuel.

In the case of Saturn V, the most powerful rocket yet, the total mass at take-off was almost 3,000 tons. The 5 S-1C engines delivered a total thrust of around 3,500 tons. In other words, just enough to start going. Fully fuelled, the first stage had a mass of 2,300 tons, of which 2,150 tons was fuel. It burnt through that in 150 seconds, so that every second Saturn V got about 14 tons lighter, and accelerated correspondingly faster.

As slow take-offs waste a lot of fuel, we want rockets to accelerate as fast as possible - within the limits of engine power and what the rocket/payload can endure. Military rockets can take off very quickly because they have been "hardened". Human payloads and spacecraft require gentler take offs.

In other words, it's a trade-off between the desire to use less fuel, and the limitations of the rocket/cargo.

hdhondt
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