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Comment by deepsun | Hacker News Reader

> In order to produce thrust, it has to exit the back of the satellite at a significantly higher speed.

Why? It can exit at 1 m/s and still be thrust. And as long as momentum from engine pushing is larger than momentum of stopping atmosphere -- it will accelerate.

In other words -- chemical engines have exit speeds way lower than 8 km/s, but are able to accelerate spaceships to and above 8 km/s.

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I suppose because they are catching them when they are at ~0m/s in the atmosphere so they're worth -8km/s worth of momentum to begin with. So you would have to fire them out a >8km/s to get a net +ve change in momentum.

> It can exit at 1 m/s and still be thrust

But it won't be enough thrust to counterbalance the force of scooping up air that collides at 8 km/s

In order to produce net thrust, it has to be more than that.

The mass of propellant being expelled is many times higher than the mass of air colliding with the rocket.

If that is the case then yes, agree, but there was no indication that is the case. The air is very thin out there.

In fact, it's linearly proportional, because momentum us m*v, so for 20 km/s propellant exhaust seed and 8 km/s air it needs to spend exactly 8/20=0.4 times the propellant to not slow down.

I understood that they want to used scoop up air to use as propellant. Only a fraction of the air it encounters will be able to be sucked in by the rocket.

You're overlooking the conservation of mass constraint.

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