Question(s) related to acceleration

[Urge]

Found this assertion inn another thread, that was realy about something completly different, so I make a new one about it:

You accelerate up to a point where your forward motion is as fast as the exhaust that leaves your engine after that you can exhaust every bit of fuel you have if your engine can't dish out its exhaust at a faster speed it will be a futile exercise.

Is this true? I thought that no matter how weak and silly a engine would be, it would slowly accelerate itself up to it became to heavy to be accelerated further as it approached light-speed (didnt Einstein say that things moving very fast would get more heavy, have more mass?) as long as it had limitless fuel. In my world, a hydrogen-oxygen engine like the one we are using to get up and down from orbit now can potentialy reach 99,99% of lightspeed as long as it had big enough hydrogen and oxygen tanks. But is the assertion above is correct, then a hydrogen-oxygen engine would not be able to go very fast at all. But isnt speed very relative in space annyway?

 

[Jadzia]

Conservation of momentum says the craft will accelerate as long as the engine continues to push mass out of it's exhaust.

And that 'pushing' would be from the point of view of the craft. It doesn't matter how hard or soft the push is. It's all relative.

Terminal velocities are more of an issue in aircraft (ie, in atmospheres) where propulsion occurs from manipulating air pressures. In supersonic aerodynamics, if you're capable of expanding air to zero pressure, your mach number shoots to infinity and you can't accelerate any further. That doesn't mean your going at light speed

The burners on shuttles and rockets also involve supersonic flows of hot gas out of the exhaust, even in the vacuum of space, and while these may suffer from similar mach number related problems, it's not going to prevent you from (theoretically) accelerating infinitesimally close to light speed... You just need a lot of fuel.

 

[Urge]

Thanks:-)

So anny engine - no matter how crappy - can push itself, and any other mass attached to it foreward faster and faster until it runs out of fuel.

 

[SilentP]

Thanks:-)

So anny engine - no matter how crappy - can push itself, and any other mass attached to it foreward faster and faster until it runs out of fuel.

Assuming that you're moving in a vacuum, yes. If not (i.e. in an atmosphere, underwater etc.) you'll carry on accelerating until frictional forces from the surrounding gases, liquids etc. equal your engine's thrust.

 

[Meredith]

The Holy Grail would be a was to convert electrical energy directly to kinetic momentum without loosing reaction mass. This would be a break through that would make colonization of the solarsystem and nearby star systems a reality.

Space ships would have a Nuclear reactor which would generate electricity and convert that directly to momentum. Satellites would last longer because to maneuver they would not need reaction mass for thrusters and thus would last a lot longer and they could be powered by solar cells.