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A NASA ion engine while under test at the Glenn Research Center. It can propel spacecrafts to speeds of up to 320,000 km per hour (i.ibb.co)

submitted 3 months ago by [email protected] to c/[email protected]

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Source: https://en.wikipedia.org/wiki/NEXT_(ion_thruster)

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[–] [email protected] 54 points 3 months ago (2 children)

It seems weird to me that they would measure a thruster in maximum speed and not by the force is generates. Doesn’t the maximum speed depend on the mass it is propelling and a bunch of other factors like friction and gravity?

[–] [email protected] 14 points 3 months ago

This speed is the Specific Impulse which is a measure of how effective the fuel mass is used. It is equivalent to the effective exhaust velocity, which is basically the mean exhaust velocity as the real exhaust velocity depends on the position in the exhaust. The specific impulse is more often given in seconds like in the Wikipedia page for the thruster and this representation is connected to the speed-based representation by dividing the effective exhaust velocity by the standard gravitational acceleration at sea level. Mass is not present in both of these representations because this way they are the impulse per unit mass of propellant.

Electric thrusters reach very high exhaust velocities, but the fuel mass flow is limited which leads to low thrust. Chemical engines reach high thrust, but their exhaust velocities are quite low in comparison.

[–] [email protected] 4 points 3 months ago* (last edited 3 months ago) (2 children)

No, that is the time to maximum speed. Maximum speed depends solely on exhaust speed (how fast the particles get accelerated).

And no friction in space. Well, almost none.

What's weird is that they say km/h. Space and rocket stuff is usually in m/s.

[–] [email protected] 16 points 3 months ago (1 children)

The grandparent comment is correct. There is no such thing as engine-specific “maximum speed”. If you add more fuel, you can always go faster (until the speed of light).

[–] [email protected] 3 points 3 months ago* (last edited 3 months ago)

Even "until the speed of light" has a caveat (hear me out). Would you call getting from A to B in 1 hour on your watch instead of 2 as "faster"? If your answer is yes, then there's no limit to how "fast" you can go. You could get from here to the Andromeda Galaxy in one minute. It's just that for the Andromeda and the Milky Way a LOT more time would have passed.

[–] [email protected] 3 points 3 months ago

You are incorrect. If I am traveling at 90 % speed of light, take a stone and throw it out the back... I am now faster.