# Acceleration in space?

1. Jul 5, 2009

### kashiark

In space if a space craft in the absence of a gravitational field stopped using whatever propulsion system it was using, would the craft instantly stop accelerating and stay at whatever velocity is was at?

2. Jul 5, 2009

### Pengwuino

Yes, with the propulsion being the only acceleration to the ship, it would have 0 acceleration and thus, 0 change in velocity.

3. Jul 5, 2009

### Danger

Yes, but there is no such thing as a total lack of gravity. It permeates the universe. There would be be some, however miniscule, effect upon the craft.

4. Jul 5, 2009

### DaveC426913

Note also that no real drive system will turn off instantly, so acceleration will drop to zero, not go instantly to zero.

But in principle, you are correct. If acceleration is stopped, the vehicle will continue forever with the last velocity it had.

5. Jul 9, 2009

### kevinfr0st

sooner or later it will hit something a moon or plannet or star,time would be erelevent unless it found a orbit :)

6. Jun 19, 2010

### ddd1600

I'm surprised no one has mentioned that space is not a vacuum and that friction:
- caused by the energy exchange between object 1, the spaceship, and objects n -- small particles in space, viz. by the transfer of momentum between the two variables

would cause the ship to slowly decrease in speed, probably never reaching (of course, haha, relativity) any form of inertia...a bottle in the sea, so to speak.

7. Jun 19, 2010

### ddd1600

Now here is MY question, and I'll be coming back to check on it, so use your man-head:

What if one was in deep space (super low density, just a few sparse hydrogen particles mainly), in a ship of course, and wanted to accelerate with very little pre-existing momentum? I guess my question is, following up the discussion of the requirement of a minimum of two variables in order for dA/dT to change at all, how is Gilligan the space sailor supposed to accelerate if there is hardly anything "out there" to "push against"?

See it like this, firing up the liquid oxygen fueled turbo boosters in space is like punching thin air, that is, thin air doesn't PUSH BACK. Am I missing something here or is this simply an extremely inefficient process which one would prefer to avoid altogether by ensuring constant inertia in deep space? Mmmmm.....I would greatly appreciate it if somebody could inform me of a "blind spot" in my logic because I have the feeling I'm not going to get much a better answer than the one I've already provided...I feel like Ender. So alone, in space. :)

8. Jun 19, 2010

### xxChrisxx

Exhaust goes one way, you go the other. The exhaust doesn't 'push on anything'.

9. Jun 19, 2010

### Staff: Mentor

You push on the exhaust gases in order to propel them out the back of the ship. The exhaust gases push back on you, by Newton's Third Law. (That's the "action and reaction" law in case you've forgotten which law is which.)

This works most effectively in a vacuum. If there's any surrounding air or whatever, it just gets in the way and produces drag.

10. Jun 19, 2010

### Cleonis

As xxChrisxx points out, the operating principle of rocket propulsion is recoil.

Imagine a spaceship in outer space, firing a gun. The projectile goes one way, the recoil of the spacecraft is in the other direction. If you fire rounds continuously then you have continuous propulsion. That is how you get propulsion from rocket exhaust.

The disadvantage of rocket propulsion is that you're shedding mass all the time. Eventually you will exhaust your supply of mass to exhaust.

Ion propulsion uses a linear accelerator to accelerate ions to a velocity close to the speed of light. It's the same recoil principle as rocket propulsion, but more efficient because the velocity of the "exhaust" is higher.

Last edited: Jun 19, 2010
11. Jun 19, 2010

### DaveC426913

As other have pointed out, so will I distill:

• Rocket propulsion works better in a vacuum than it does in an atmosphere.
• Rocket exhaust has done its job by the time it clears the lip of the nozzle. By the time it clears the lip, it is just waste, to be disposed of as quickly as possible to make room.
• The atmosphere actually interferes (a little) with the functioning of a rocket by causing (a small amount of) back pressure.