Why can’t we go faster in space?

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Okay, I’m an idiot. Just to preface this. But let’s say I’m in space. There’s no wind resistance right? If I accelerate, I’ll maintain that speed. Why can’t I just burn more fuel and go faster?

In: 194

You can, yes.

But you need to bring more fuel to do so, which means you’re heavier. Which means that you need even more fuel to burn to move at all. And that cycle just repeats–more speed = more fuel = more weight = need more fuel.

You can. But you need to carry that fuel, so there’s far more mass to accelerate requiring even more fuel. But in theory, if you had a sufficiently efficient engine or an outside fuel supply that could supply fuel as you accelerate, you can go pretty darn fast.

Now at some point relativistic effects come into play whereby the faster you try to go, the more massive you become, requiring ever more fuel to eke out additional acceleration. But that’s not really ELI5 material.

No you are absolutely right, you can do that.

In fact that is how trips to the moon and Mars works. We use enough fuel to get a good speed going then ride on that speed until it’s time to slow down to land or enter orbit. But since you can’t bring much fuel into space, because it is sooo heavy, we can’t use too much for acceleration or we don’t have any left for slowing down and going back.

If you went in one direction out to deep space eventually you would slow down however. There actually is resistance in space, just very, very spread out. Some particles just hang around in space, even the rare air molecule. Other particles fly close to the speed of light and will hit your spaceship in the opposite direction with a lot of energy. Even starlight will eventually push your ship around.

You can, what you are looking for is specific impulse, ion engines are the ones with the highest impulse and over long periods of time they can get to massive speeds, but that is relative to earth.

When going somewhere in space you have to think not only where is your target but how its moving.

Also how are you going to brake. I f you wanted to go to the moon the fastest way possible you dont only have to think about how you accelerate but how you brake. Braking in space is more limiting than accelerating.

To add,

Regarding the whole rocket fuel thing, that’s called the [Tyranny of the Rocket Equation](https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation). The equation mostly describes the situation of overcoming gravity and lifting a payload off the ground. There are limits to how heavy a rocket can be given our material science today. As a thought experiment, if there were a rocky planet containing life elsewhere, with a civilization of comparable technology to us – if the planet were a bit bigger than Earth, then they could not build a rocket capable of escaping their planet’s gravity. They’d have to invent something more energetic, something stronger and lighter than what we can do today.

Once you’re in space, things are different. We could build a space ship in orbit of any size and load it up with all sorts of fuel and whatever. Technically, we could mine asteroids and other small bodies and potentially make the materials out there rather than down here. You’d have to play a game of time and orbital mechanics, perhaps several lifetimes to get the work done, but hey. It’s not necessarily practical or efficient.

But it would be nifty to accelerate like crazy. Not necessarily press you into the seat crazy, but, constantly crazy. A [1g](https://en.wikipedia.org/wiki/Space_travel_using_constant_acceleration) rocket would accelerate at a constant rate that would put it’s payload under 1g of force. It’s a dandy way to simulate gravity! But it’s also fast. You’re not just getting to your destination, but you’re getting there faster and faster. All you have to do is half way through the trip, turn the ship around, and accelerate in the opposite direction – so you don’t overshoot where you’re trying to go. This could cut down trips across the solar system to just a few days to get as far out as Pluto. A trip across the galaxy would take 12 years – I’m quoting a figure, I don’t know if they factored in slowing down.