Thrust?

[alexi_drago]

I have an internal combustion engine powered tennis ball launcher in space, will it generate thrust?

The engine has it's own fuel and air supply and the launcher uses spinning wheels to accelerate the balls.

[drkitten]

I have an internal combustion engine powered tennis ball launcher in space, will it generate thrust?

The engine has it's own fuel and air supply and the launcher uses spinning wheels to accelerate the balls.


Yes. The launcher will fire the tennis balls in one direction and be moved in the other direction. Depending upon how the spinning wheels work, the launcher will almost certainly end up spinning around as it fires them, too.

[alexi_drago]

Yes. The launcher will fire the tennis balls in one direction and be moved in the other direction. Depending upon how the spinning wheels work, the launcher will almost certainly end up spinning around as it fires them, too.

To call it thrust, does it matter how the accelerating force between the masses is applied? in this case it's a tractive or frictional force between the balls and the wheels.

[drkitten]

To call it thrust, does it matter how the accelerating force between the masses is applied? in this case it's a tractive or frictional force between the balls and the wheels.

No. Thrust is simply whatever makes something move.

[Dr. Imago]

For every action, there is an equal and opposite reaction.

Newton figured this one out centuries ago.

~Dr. Imago

[Dr. Trintignant]

To call it thrust, does it matter how the accelerating force between the masses is applied?

No. All that matters is that you have a system that starts with the propellant and vehicle having the same velocity, and ending with opposite velocities (from the center of momentum frame). Even the distinction between propellant and vehicle is somewhat dubious; there's no reason you couldn't have two spacecraft pushed apart by a spring or some such--each vehicle would experience thrust from pushing off the other.

There is a wide variety of spacecraft engine types, mostly distinguished by how they accelerate the propellant (chemical, electromagnetic, gas pressure, etc.). And they all generate thrust, of course. The only reason that mechanical systems aren't used is because they don't achieve very high exit velocities, which you need for good efficiency (efficiency defined as low propellant use for a given change in momentum).

- Dr. Trintignant

[alexi_drago]

All that matters is that you have a system that starts with the propellant and vehicle having the same velocity


What about a jet engine where a part of the mass being expelled from the engine was air sucked in through the compressor?

[Dr. Trintignant]

What about a jet engine where a part of the mass being expelled from the engine was air sucked in through the compressor?

Sorry--I was referring specifically to space engines, since your original post mentioned space. Jet engines are different because they collect their propellant from their environment. They achieve very high energy efficiency relative to rockets because they don't have to optimize for propellant mass--they can use low exit velocities, with corresponding low energy requirements, since they can collect as much propellant as they want.

The Bussard ramjet would be the one exception to all this--it collects hydrogen gas from space just as a jet engine collects and accelerates air. It's not clear that it's practical, though.

- Dr. Trintignant

[alexi_drago]

OK, thankyou.

Now, what about a car? can the ground (planet) it is sitting on be considered to be the propellent?
The forces acting on it being the same as the tennis ball launcher but with a much more massive ball and the gravity will stop the car from escaping.

[Toke]

Remember to point the exaust pipe backwards, it will give trust too.

Your trust is determined by mass times velocity of propellant/tennisball
The energy required is determined by mass times velocity squared.

A carbon/grafite engine uses a solar cell to get energy for heating hydrogen and sending it off at high velocity. It saves on reaction mass and will keep a satelite in fuel longer.

Doesn´t work yet.
http://en.wikipedia.org/wiki/Arcjet_rocket

[Dr. Trintignant]

OK, thankyou.

Now, what about a car? can the ground (planet) it is sitting on be considered to be the propellent?
The forces acting on it being the same as the tennis ball launcher but with a much more massive ball and the gravity will stop the car from escaping.

Yes, it's reasonable to think of it that way. In these terms, cars are extremely energy efficient, because they have a huge, "free" propellant mass to push against (plus the propellant is still around to push against when slowing down).

- Dr. Trintignant

[MrQhuest]

The tennis ball launcher as a spacecraft would be an example of an inefficient mass driver type of propulsion. Basically throwing stuff behind you to move forward. Classic Newton.

http://en.wikipedia.org/wiki/Mass_driver

MrQ

[alexi_drago]

Thanks very much again, this is to try to settle an argument but it's being quite hard going trying to get some people to see it this way.

[Soapy Sam]

Throw tennis balls at them till they see sense. :D

[alexi_drago]

Throw tennis balls at them till they see sense. :D

with my V8 launcher. :duck:

[Dr. Trintignant]

Throw tennis balls at them till they see sense. :D

Alternately, you could throw the nearest planet at them. If this proves difficult, relativity tells us that throwing them at the nearest planet is equivalent.

- Dr. Trintignant

[fuelair]

I have an internal combustion engine powered tennis ball launcher in space, will it generate thrust?

The engine has it's own fuel and air supply and the launcher uses spinning wheels to accelerate the balls.

I was going to say yes - everybody beat me to it - but dammit, I will say it anyway!!!! "YES!""

[shadron]

You have thrust (identical in the context to force) at any point where one mass moves with respect to another. This can be measured as a displacement change, a velocity change or an acceleration change; all are affected.

The astronaut who teed off on the golf ball on the ISS applied thrust to the ball, and likewise also applied thrust to the ISS, in an equal and opposite manner. The method for creating the change in displacement/velocity/acceleration is no elevant o the fact that the change happened.