[HN Gopher] The first on-orbit fuel depot has been deployed
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The first on-orbit fuel depot has been deployed
Author : tectonic
Score : 107 points
Date : 2021-07-07 17:39 UTC (5 hours ago)
(HTM) web link (orbitalindex.com)
(TXT) w3m dump (orbitalindex.com)
| phreeza wrote:
| From my understanding of orbital dynamics, this will only be
| useful to spacecraft already in a precisely matched orbit. So
| it's not useful to think of this as an orbital gas station that
| spacecraft can just pull up to in order to refuel.
| LeifCarrotson wrote:
| 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.
| perihelions wrote:
| 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.
| blach wrote:
| 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
| brandmeyer wrote:
| Depends on the fuel, though. Ion thrusters with a solid
| propellant are becoming popular in the smallsat business.
| JumpCrisscross wrote:
| > _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.
| azernik wrote:
| 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
| jjk166 wrote:
| 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.
| [deleted]
| clort wrote:
| 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..
| azernik wrote:
| 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.
| pogden wrote:
| This tanker is 35kg, so it is the smaller barge.
| tectonic wrote:
| 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.
| JumpCrisscross wrote:
| > _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.
| Someone wrote:
| 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.
| [deleted]
| jjk166 wrote:
| 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.
| kyralis wrote:
| That depends a lot on the change and is not always a true
| statement.
| jjk166 wrote:
| 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."
| kyralis wrote:
| 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.
| slownews45 wrote:
| 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 (!)?
| ashtonkem wrote:
| 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.
| slownews45 wrote:
| 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.
| hughes wrote:
| 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??
| bob1029 wrote:
| Probably something like this:
|
| https://www.youtube.com/watch?v=MKF0KXMha2E
| ashtonkem wrote:
| 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.
| bpye wrote:
| 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.
| jjk166 wrote:
| 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.
| ChuckMcM wrote:
| 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.
| swader999 wrote:
| Hope it has nice washrooms.
| ashtonkem wrote:
| Ah, I used this exact same strategy in KSP. It's very convenient
| to lift a huge amount of fuel into space once using a disposable
| rocket, then top up smaller crafts before heading out of orbit.
|
| I assume they won't be following my "dump extra fuel here before
| re-entering the atmosphere" strategy though.
| zeusk wrote:
| But why would you dump extra fuel in KSP? couldn't you use it
| to reduce your surface velocity before atmospheric heating
| begins? or even use it as thermal mass and do a retrograde burn
| during landing like Falcon9?
| ashtonkem wrote:
| Fuel costs money to lift, so by dumping it in the orbital
| fuel station I can delay an expensive and tedious refueling
| mission.
|
| Deceleration in KSP is easy because the aerodynamic and
| thermal models are exceptionally forgiving. Assuming you have
| literally _any_ heat shield, you have to come in at
| interplanetary transfer speeds with a periapsis below
| airplane traffic before blowing up due to overheating is a
| concern. Most players will depend on drag to do the bulk of
| their deceleration because it's cheap and easy, and even
| Falcon 9 style approaches can use parachutes and air brakes
| to do the heavy lifting. My falcon 9 equivalent rocket just
| uses a touch of throttle to prevent the landing legs from
| compressing so much that the rocket bell hits the ground. If
| I had a bit more clearance I probably wouldn't need any fuel
| other than the deceleration burn.
| jeantherapy wrote:
| who cares. starship will do this all day long.
| mkr-hn wrote:
| A whole Starship to deliver a few tens of pounds of fuel to a
| satellite seems excessive. The whole thing weighs about as much
| as a tank of gasoline. A Starship might deploy a hundred of
| these into different orbits for different refueling tasks.
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