Topic: Why can't we just keep accelerating in space? - page 2. (Read 2700 times)

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.

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.

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.

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.  

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?