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.

Sort of. The promise of on-orbit refueling is three-fold;

First, you can have several tankers in orbit which would allow for an "any day of the week / time of day" launch schedule from Earth for a payload satellite, that then rendezvoused with the tanker in the orbit it found itself in, fueled up, and then did a transition burn to the orbital plane it wanted to be in.

That "wins" because the satellite can go up with empty tanks for its orbital maneuvering thrusters, (so more mass can be allocated to the satellite), and the insertion 'tug' can ride along like an unfueled third stage. That eliminates things like "instantaneous launch windows" which keep satellites grounded if everything doesn't come together at exactly the right time.

Second, station keeping lifetime can be extended (most common value) and that means you can amortize the satellite's costs over a longer lifetime. Many geosynchronous satellites are "retired" not because they don't work, but because they are about to run out of fuel for keeping their place. If you can refuel a $5M satellite and get another 10 years of life out of it, that is a pretty big deal.

And third is the ability to gain fuel post launch as part of a retro-propulsive return profile. The heat shielding of all spacecraft is there because they use "friction" return profiles where they use the atmosphere to slow them down. That is fine but limits the amount of mass you can return because the more mass you displace, the more heat you generate in the return.

If you could fill up on fuel on orbit and use that to cancel your orbital velocity, you could do a return to earth that would stay within the heat limits of ordinary steel which would be safer and easier on the spacecraft.

ULA was circulating design ideas for a cryogenic fuel depot on orbit that were pretty neat.

These are but a few of the limitations that are imposed by having to carry all of the fuel you will ever need on every flight. On orbit refueling would make it much easier to work in space.

Refueling a satellite in orbit presupposes that the satellite actually made it to orbit.

The flip side of amortizing the cost of the satellite over a longer mission is being able to cut your losses a bit in the event that the duration turns out to be 0.

I don't disagree but I also don't understand what you're trying to say here. All satellites are insured and if they don't make it into orbit the insurance claim reimburses the builder for the cost.

Don’t all gas stations require you to precisely match their position and velocity? Like how do you fill up your car without doing this??

I think it might be more reasonable to look at this like air to air refuelling, rather than a petrol station. Either the tanker or the satellite or both must spend a significant amount of fuel to align their orbits and velocities precisely and during that operation they are likely unable to perform anything else.

In air to air refueling, both parties are spending incredible amounts of fuel simply holding themselves aloft at cruising speed. Matching alignment is a literal drop in the bucket by comparison.

The amount of fuel required to align spacecraft in similar orbits is miniscule compared to the cost of getting fuel into orbit to begin with. You're talking 10s of m/s of delta-V.

Probably something like this:

https://www.youtube.com/watch?v=MKF0KXMha2E

There’s a wee bit of difference between matching speed with a gas station and matching speed with a satellite, both in terms of cost and complexity.

Ehh, it's easy enough to eyeball it.

Source: 1000+ hours in KSP :P

Well, if everybody did that, they wouldn't have bollards. Or breakaway hoses.

Spacecraft can change their orbits. You don't fly in a straight line to get there but yes, spacecraft can pull up to it.

The delta-V for such an orbit change is exceedingly small compared to the cost of a launch.

That depends a lot on the change and is not always a true statement.

We're specifically referring to small changes here to service satellites in similar orbits.

Also, for future reference, please consider this site's guideline "Please respond to the strongest plausible interpretation of what someone says, not a weaker one that's easier to criticize. Assume good faith."

Nothing about your post confined the discussion to "similar" orbits; you were in fact responding to a parent describing a need for these to be precisely matched with a negation. While you're correct that there are orbital changes that are cheap, there are many that are not as well, and your statement without that qualification seemed likely to mislead.

I wasn't assuming bad faith on your part; I merely believed that your response could be confusing. You might consider that guideline as well.

The context of the conversation was that OP's comment was questioning the utility of a depot since they believed spacecraft in similar orbits would not be able to access it, only those that happened to be in precisely aligned orbits could. Interpreting "spacecraft can change their orbits" to be a claim that there are no limitations on the maneuvers a spacecraft can perform is very unreasonable.

You're focusing on the first sentence; the reply was, I think, focusing on the second.

"The delta-V for such an orbit change is exceedingly small compared to the cost of a launch."

In that context, I think it seems appropriate to point out that it's not true as a categorical statement.

the "for such an orbit change" is the qualifying statement there, as opposed to "for any orbit change". It is clearly referring to the same category of orbit change being discussed in the preceding sentence.

You don't have to be able to get all jobs. You just need to be able to get enough jobs.

I'm not getting where folks are coming from with this analysis.

Delta-V to launch to orbit is measured in km/s. If you are in a servicing orbit for a given orbit - how do you spend km/s repositioning (!)?

Plane changes can exceed the cost of putting a satellite in orbit, and even if they don’t they’re often more expensive than spacecraft can afford. The formula for changing a circular orbit without changing the altitude is 2V * sin(delta degrees / 2). This results in some pretty surprisingly high numbers; a 45 degree plane change costs 76% of your current delta v, a 90 degree change is 141%. Even a measly 5 degree change should cost 8% of the craft’s current velocity.

8% doesn’t sound like a lot, until you start doing the math on how much that is compared to orbital velocity. The Space Shuttle’s Orbital Maneuvering System was good for 300 m/s of delta v, which is only 3.8% of the minimum speed to keep orbit. If the shuttle was flying faster, those numbers start to fall.

I think at best the shuttle could probably afford to change its orbital plane by maybe 2 degrees.

> Even a measly 5 degree change

Let's remember that a 5 degree change in plane shifts the satellite over 590 km. If we pretend for a moment that satellites in LEO were evenly distributed, that 5 degree slice would contain the orbits of 53 of them. In reality, satellites tend to be placed in similar orbits.

Also your equation is for circular orbit inclination change at constant altitude, in practice you can do much more efficient plane change maneuvers by going into a higher and more elliptical orbit, changing inclination, and then returning to your original altitude and eccentricity.

I always assumed changes to altitude rather than plane for servicing orbits. Not an expert in this. I guess it depends on where high value orbits would be concentrated and space is big unfortunately.

Anyways, it'll be interesting to see where this all develops. Starship is clearly going to do in orbit refueling for their moon and other missions so there are clear situations where the delta-v for in orbit isn't too bad - match the plane on launch etc.

A larger, non-experimental depot could use electric propulsion to bring itself to satellites that need to be refueled. It's energetically expensive but likely still worth it to extend the life of expensive satellites.

> non-experimental depot could use electric propulsion to bring itself to satellites that need to be refueled. It's energetically expensive but likely still worth it to extend the life of expensive satellites.

I'd have to do the math, but I'd be curious about the confluence of launch market dynamics that would make pre-launched depots which need to change orbital planes cheaper than depots launched on demand into the right orbit from the start.

I don’t know how the math turns out, either, but because this thing exists, I would guess it’s so much cheaper to launch one huge depot than a hundred smaller ones (reasons could be higher cargo/total weight ratio and/or lower launch overhead) that it compensates for the extra maneuvering.

Of course, that’s gambling on those hundred customers to exist, on refueling to be reliable, etc.