Why can't we just keep accelerating in space?

[torr]

I read recently that the space shuttle's top speed is approximately 8km/sec. However, since space is a vacuum and there is no air friction or anything like that to slow it down why wouldn't continually adding thrust lead to continual acceleration. What is it that causes the shuttle to top out at the current top speed?

[Singlebyte]

Space is not a perfect vacuum.  Also, it takes more & more energy to accelerate. Theoretically you could accelerate to nearly the speed of light with enough energy but then time would nearly be at a standstill.  The reason it has hit its limits is because of the amount of energy used (and gravitational slingshot effect) to thrust the vehicle.  

[BitBlitz]

Since NASA doesn't launch Space Shuttles anymore, their top speed is now somewhere around zero.

Regarding things in orbit: that is the orbital speed for their altitude.  Can you go faster?  Sure.  Bring more fuel to space, which requires more power to get off the ground, and even more fuel to get that fuel off the ground--and on, and on.  

Things have been sent out of orbit, which requires an escape velocity faster than 11km/s.

[Vod]

I read recently that the space shuttle's top speed is approximately 8km/sec. However, since space is a vacuum and there is no air friction or anything like that to slow it down why wouldn't continually adding thrust lead to continual acceleration. What is it that causes the shuttle to top out at the current top speed?

One word - fuel.

For each litre of fuel you spend to accelerate, you need to also spend it to decelerate.  The 8km/sec is just an agreed on maximum speed - they trade off time for fuel capacity.

[Foxpup]

The 8km/sec is just an agreed on maximum speed - they trade off time for fuel capacity.

Not true. It's an absolute maximum, you can't go any faster, no way, no how.

Remember, a rocket ejects its exhaust at a fixed velocity relative to the ship. Multiply that by the flow rate (mass per second) and you've got a certain amount of thrust (which, divided by the mass of the ship, is your acceleration). But you've only got a certain amount of propellant on board, and once it runs out, you can't go any faster. You've reached top speed. Changing the flow rate of the rocket does not, in fact, change this speed. If you throttle back to half the maximum flow rate, you can accelerate for twice as long, but you only accelerate half as much, so you end up accelerating to the same speed regardless.

The actual top speed of a rocket can be calculated using the Tsiolkovsky rocket equation:
Δ_v = V_e * ln(R)
where Δ_v is the top speed, V_e is the exhaust velocity, and R is the mass ratio (mass of fully fuelled ship / mass of empty ship).

Note that the absolute mass of the ship isn't a factor: a ship that's twice as massive can carry twice as much propellant, but only gets half as much acceleration for a given thrust, so it balances out. The mass ratio is important, because the ship constantly gets lighter as it consumes propellant. Carrying more propellant enables you to go faster, but only after you've consumed most of it. Also note that increasing your speed requires increasing the mass ratio exponentially; this is why most space rockets look like giant propellant tanks with a tiny ship on top and a tiny rocket motor on the bottom. It's also the reason for multi-stage rockets: dumping spent engines reduces the rocket's empty weight, and so increases the mass ratio even further.

It's not brain surgery.

[pedrog]

Because first law of motion, you need more force so it keeps accelerating.

[Vod]

Not true. It's an absolute maximum, you can't go any faster, no way, no how.

I assume this changes as our fuel becomes more efficient?

[Lucky Cris]

Who the hell are you people?

BitBlitz actually made me LOL, and Foxpup is posting shit in another language!

I need to visit Off-Topic more often 

[mrvicar]

The 8km/sec is just an agreed on maximum speed - they trade off time for fuel capacity.

Not true. It's an absolute maximum, you can't go any faster, no way, no how.

Remember, a rocket ejects its exhaust at a fixed velocity relative to the ship. Multiply that by the flow rate (mass per second) and you've got a certain amount of thrust (which, divided by the mass of the ship, is your acceleration). But you've only got a certain amount of propellant on board, and once it runs out, you can't go any faster. You've reached top speed. Changing the flow rate of the rocket does not, in fact, change this speed. If you throttle back to half the maximum flow rate, you can accelerate for twice as long, but you only accelerate half as much, so you end up accelerating to the same speed regardless.

The actual top speed of a rocket can be calculated using the Tsiolkovsky rocket equation:
Δ_v = V_e * ln(R)
where Δ_v is the top speed, V_e is the exhaust velocity, and R is the mass ratio (mass of fully fuelled ship / mass of empty ship).

Note that the absolute mass of the ship isn't a factor: a ship that's twice as massive can carry twice as much propellant, but only gets half as much acceleration for a given thrust, so it balances out. The mass ratio is important, because the ship constantly gets lighter as it consumes propellant. Carrying more propellant enables you to go faster, but only after you've consumed most of it. Also note that increasing your speed requires increasing the mass ratio exponentially; this is why most space rockets look like giant propellant tanks with a tiny ship on top and a tiny rocket motor on the bottom. It's also the reason for multi-stage rockets: dumping spent engines reduces the rocket's empty weight, and so increases the mass ratio even further.

It's not brain surgery.

I take it you are quoting from leaving the atmosphere of earth and getting in to orbit?
What is the capacity of acceleration and deceleration of a nuclear (or more powered in the future) space shuttle after leaving from orbit?
There is a theoretical  "maximum" acceleration that can be reached and its not 8km/sec .

The theory of relativity does not limit the rate of acceleration only the maximum velocity. However the energy needed is enormous. For instance, in a particle accelerator if you used say an alpha particle and accelerate it to a very high velocity and it collides with another particle, let's say a proton the proton will accelerate very rapidly. - If you can control these factors then you can control velocity, acceleration and deceleration.

[BitBlitz]

Not true. It's an absolute maximum, you can't go any faster, no way, no how.

I assume this changes as our fuel becomes more efficient?

Yep.  You can go faster.  Reduce the vehicle mass, scale up the dimensions to improve the mass ratio, increase the specific impulse of the propellant/motor (efficiency), etc. 

It's not brain surgery.

Anybody who's designed rocket motors and/or put anything in space; put a coffee cup on your avatar's head 

[beetcoin]

i don't know much about physics, but when an object is flying through space.. isn't its motion inert? as in it keeps moving until some form of energy is used to stop it?

[golem]

So where do ion drives (as recentlyish tested fit in) - Slow acceleration, but velocity can carry on increasing until fuel spent?

[Foxpup]

I take it you are quoting from leaving the atmosphere of earth and getting in to orbit?
What is the capacity of acceleration and deceleration of a nuclear (or more powered in the future) space shuttle after leaving from orbit?
There is a theoretical  "maximum" acceleration that can be reached and its not 8km/sec .

I'm talking about a ship in space, free of atmospheric and gravity drag, both of which will of course reduce the top speed. In the worst case, a rocket with a thrust-to-weight ratio of less than one, obviously won't accelerate to any speed at all as long as gravity has a say in the matter. This is why ion drives currently aren't very useful (high exhaust velocity, but low thrust-to-weight ratio).

The theory of relativity does not limit the rate of acceleration only the maximum velocity. However the energy needed is enormous. For instance, in a particle accelerator if you used say an alpha particle and accelerate it to a very high velocity and it collides with another particle, let's say a proton the proton will accelerate very rapidly. - If you can control these factors then you can control velocity, acceleration and deceleration.

For "speed" and "velocity", substitute "rapidity". Happy now?

i don't know much about physics, but when an object is flying through space.. isn't its motion inert? as in it keeps moving until some form of energy is used to stop it?

Yes. It also doesn't go any faster until some form of energy is used to speed it up, hence a maximum speed being reached when you finally run out of propellant.

[counter]

Interesting discussion and I'm in the same boat at the OP.  Also BitBlitz made me lol. 

[notbatman]

Q: Why can't we just keep accelerating in space?

A: Because the "rest mass" of a photon is non-zero and the big bang theory is a gross misrepresentation of the facts. The universe doesn't have a start any-more then it has an end. CMBR is the result of a decaying photon being red-shifted and emitting RF.

Q: What this have to do with the question you ask?

A: "You know your perspective is totally fucked so you let your hands do the driving." -- Heavy Metal 1981

[Lethn]

I don't know as much as the people here but atmosphere and gravity is one thing you need to keep in mind when it comes to space, especially if you plan on actually landing somewhere, you also have to deal with solar radiation as well and that will mean navigating past areas that will likely have more or less radiation depending on where you go. Think of it like navigating through Chernobyl but you have absolutely no friction to stop you so you have to make sure you have enough fuel to get around and then get back home and then there's the fact that there are things that actually move around space on a daily basis like comets and asteroids so you have to account for that too, oh and don't forget those black holes.

Too many people think space is 'just' empty vacuum because they're bloody ignorant, I always laugh when people, particularly news journalists say there's 'nothing' in space as if they know it all and you see planets being discovered and asteroids with tonnes of precious metals in them.

[notbatman]

They're launching bitcoin satellites into space and bitcoin is already the most powerful computing network. This is better than moors law.

[sana8410]

In the simplest terms, we can, but our current methods of propulsion require heavy fuel, which adds to our mass and requires more fuel and so on. There's a point of diminishing returns with current technology.

Beyond that in much faster velocities, the closer you get to the speed of light, the more your mass increases and the more fuel you require to accelerate that mass. As you get closer to the speed of light, it approaches infinity.

[BitBlitz]

So where do ion drives (as recentlyish tested fit in) - Slow acceleration, but velocity can carry on increasing until fuel spent?

They're already being used on deep space probes. An ounce or so of thrust, but they run for years.  Specific impulse is somewhere around 10X the efficiency of chemical rockets, but they also require electrical power.

[noviapriani]

We don't have good enough propulsion technology yet. It all comes down to the weight of the rocket fuel. As you add more and more propellant, it takes more and more of that propellant to just at first build up enough momentum and velocity to leave Earth.