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.
we do go faster in space then we used to.
Apollo 11 moved at a snails pace typing out at a 2.7 kilometer per second, voyager 2 is currently on cruise control at 12.4 kmps, New horizons,the Pluto mission hit 16.26 kmps that’s 16,260 meters in a second..
Motion of such large vessels is… difficult… logistically speaking in space.
Lets say, you’re in orbit over Earth, completely writing off the effort already undergone to get there in the first place, and you want to fly to Uranus as you’ve heard it’s particularly phat this time of year.
Well, you get there by burning fuel. Your thrusters overcome the weight of your vessel + the gravity well of Earth and your orbit expands ever outwards until you break free of Earths gravity and you’re sling-shot toward your destination… But, wait. Fuel is a heavy as shit. More fuel means more weight. So Earths gravity is stronger than ever, pulling on your heavy ass fuel tank. So now you need more fuel to overcome the weight of your fuel. *ah…*
That is the major problem. Mass in space is more fundamental than it probably first appears. Anything heavy needs a proportionate amount of energy to propel it. You need to precisely ration fuel, as you’re fighting against the various gravity wells you encounter on your journey, so the weight of said fuel needs to be the perfect ratio for the distance it will actually get you. You need to know when to burn to speed up, and to slow down to take advantage of gravity slingshots. It’s complicated stuff, and not just a “bee-line” to your destination.
So let’s say you figure that out. And you see Uranus on the… erm… “horizon”, and it is indeed as phat as they said. Say you wanted to orbit it. Well, uh oh… How do you do that? As you said, there is no wind resistance, no friction.
Your thrusters have gotten you up to the speed of a few thousand, maybe tens of thousands of meters per second. And it’s gotten you to Uranus… But what now? There are no brakes in space. To slow down, The only option is to burn the opposite direction to cancel out your momentum, which (you guessed it) requires more fuel. Which again increases your mass.
So in short, you need to make sure you have just enough fuel to get you to where you’re going, enough fuel to slow down enough to get captured in the gravity well of Uranus to circularize your orbit. And presumably enough fuel to escape orbit, and return home. Assuming no mistakes of course.
If you really want to learn about rocket science, Kerbal Space Program is a great way to do it. It’s a game, but you will learn the basics and they will answer your question and a good bit more.
Anyone that asks this question should play Kerbal Space Program. If you manage to land on the mun (the programs equivalent of the moon) you’ve answered this question and about 20 other questions you might have about orbital mechanics and accelerating through space.
The short answer is that since there is nothing to brace yourself against, every meter of speed you accelerate is basically your ship going that way because it’s shooting reaction mass out the other direction. The faster and more efficiently you can shoot mass out your rocket the faster you can go before running out of fuel.
Which is basically why an Apollo rocket is the closest humanity has gotten to “not quite a slowly exploding nuclear bomb” (because the fuel used is EXTREMELY explosive). Scientists even talked about using nuclear bombs to go fast into space (Orion Project).
When you accelerate in space you decelerate in time. So asking how you can go faster than the speed of light is like asking which direction you can point a meter stick to make it longer than a meter.
Now here’s the real kicker. As you transfer from time velocity to spatial velocity, the energy required increases exponentially. Eventually you reach a point where it requires more energy than the entire universe to accelerate.
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