Comment by LeifCarrotson | Hacker News Reader

Comment by LeifCarrotson | Hacker News Reader

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It's true that there's no good reason to, say, drop out of geostationary and visit a tanker in LEO. Plane changes are always expensive, altitude changes are what they are, but if your orbit is reasonably compatible it would take a whole lot less delta V to reach a tanker than it would to come back to the planet's surface and re-launch!

This depot is in a sun-synchronous orbit. There are a ton of long-term, extremely expensive, Earth-observing satellites between 600 and 800 km with inclinations of 98 degrees in this area. Yes, it's a large volume of space, and yes, it's not like you can just point yourself at another satellite, fire a thruster, and coast over to it, but if you could extend the lifetime of your billion-dollar meteorological satellite for a few million you might want to have this thing fly over to refuel you.

I doubt that they'll be moving massive satellites with huge telescopes, radars, communication dishes, and solar arrays to the tanker, rather, they'll fly the tanker to the satellites.

Might be better to have a smaller barge (or a bunch of them) which would load up at the tanker and transit over to the satellite with just enough fuel that was ordered? No need to waste energy moving the whole tanker..

If you're in the right inclination and more or less the right altitude, matching argument of ascending node with a target is cheap, just slow; this class of customer orbits is selected precisely because the Earth's oblateness noticeably affects the orbit, so you can get certain kinds of orbit changes and station keeping "for free" from the shape of the Earth's gravitational field.

This tanker is 35kg, so it is the smaller barge.

There are also a proliferating number of sun-sync earth observation small sats. These are often from capital/mass/volume limited new space companies. Lowering their initial launch mass allows them to prove out a business model for a lower cost and then refuel it if it works with future capital. Benchmark's "pay as you go" SaaS-like model is also interesting in the same vein.

Edit: correcting a typo

Depends on the fuel, though. Ion thrusters with a solid propellant are becoming popular in the smallsat business.

> if you could extend the lifetime of your billion-dollar meteorological satellite for a few million you might want to have this thing fly over to refuel you

The pitch for in-orbit refuelling is clear. Less clear is the advantage of a tank in (a close, but wrong) orbit over one on the ground with a launch booked.

Small orbital changes require orders of magnitude less delta-V than a dedicated launch, and launches have to deal with the atmosphere on top of this.

Rockets become more efficient as they get larger, you get lower structure to payload weight ratios and you suffer fewer losses to drag in the atmosphere. The same scaling goes for your fuel tanks. You want to get the biggest fuel tank you can in orbit and then use efficient ion engines to move for whatever delta-V corrections you need.

Doing some back of the envelope calculations, a single 20,000 kg depot could provide fuel at approximately 10% of the cost of individual launches even with an extraordinarily inefficient 300 m/s of delta-V change for every refueling. Realistically you could probably get under 1% with efficient planning.

Certain heavily-populated classes of orbits (like SSO, which this one is using) are very cheap in propellant terms to transfer between. Only as long as you don't care about speed of the transfer, but refueling needs are predictable very far in advance

No idea about the feasibility of this, but it seems like hitching a cheap rideshare to a non-ideal but heavily populated/serviced orbit on a bulk carrier like Falcon 9 or Starship and then contracting with a little ferry to help your sat get to its final orbit could help SpaceX negate one of the big advantages of the companies launching smaller rockets that launch direct to final orbit?

Kind of like remaking the old hub and spoke model, with Starship being the bulk 747, similar to the era when air travel was prohibitively expensive and demand was still too low to support direct point to point everywhere.

Nitpick, but SSO is for imaging satellites that are picky about repeatable lighting conditions (solar angle); but not so much about long revisit times. Weather satellites want continuous observation and (for now) generally go in GEO.

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