[grinich]

The ISS needs that much fuel to boost it's altitude due to drag in LEO. But the refueling of satellites by the X-37 is hypothesized to be at much higher altitudes where there is very little drag.

These satellites don't need altitude boosts-- they use propellant for orbital changes (e.g. to "re-task" a spy satellite to a new region). But instead of chemical rockets they often use xenon gas ion engines.

The key thing with ion drives is their "specific impulse." High-thrust chemical rockets are needed to get out of the atmosphere and perform fast burns, but ion engines are often >10x more efficient once already in orbit. And because there isn't a chemical reaction, there is less corrosion and they can last for years.

Refueling xenon for the ion drives of spy satellites would double or triple their mission lifespan. They don't need much -- maybe just a couple hundred kg. And this could literally save billions of dollars since these satellites are so expensive to build.

https://en.wikipedia.org/wiki/Specific_impulse#Examples