[karkisuni]

This seems like a big deal. Assuming it could collect more than it needs to keep itself in orbit, it could refuel a tank and skip from atmospheric body to atmospheric body. Something like this could make it to Neptune and back, though it might take an incredible amount of time.

Still, atmospheric fuel scoops were still sci-fi until now, as far as I’m aware.

[binarycoffee]

This is still very much sci-fi at the moment for anything flying above a 250 km earth orbit because atmospheric density decreases dramatically fast with altitude.

The missions targeted by this technology are GOCE-like spacecrafts which by design must fly low and need an insane amount of propellant to compensate for the high atmospheric drag at such altitude.

[ckastner]

> This is still very much sci-fi at the moment for anything flying above a 250 km earth orbit because atmospheric density decreases dramatically fast with altitude.

One of my favorite takes on this concept was Poul Anderson's Tau Zero [1], which used a Bussard ramjet [2]. Apparently, in the 70s, in was thought that there was enough hydrogen surrounding our solar system to support interstellar travel.

[1] https://en.wikipedia.org/wiki/Tau_Zero

[2] https://en.wikipedia.org/wiki/Bussard_ramjet

[kwhitefoot]

Tau Zero should be better known. I read it again recently after many years, I couldn't put it down. It's a pity that more real histories don't end like this:

"I sure as hell can. Once a crisis is past, once people can manage for themselves ... what better can a king do for them than take off his crown?"

[binarycoffee]

Somewhat related is the E-sail [1] concept, a perhaps less ambitious but (probably) feasible idea to harness the momentum of solar wind particles with very long charged wires.

[1] https://en.wikipedia.org/wiki/Electric_sail

[wizardforhire]

A personal favorite feature on federation vessels.

http://memory-alpha.wikia.com/wiki/Bussard_collector

[gaius]

How can you decelerate with a ramjet? Wouldn't your own exhaust push the matter you needed out of the way?

[whatshisface]

You could push the exhaust single-file in a highly focused beam, leaving most of the solid angle around you unpushed.

[Retric]

Bussard Ramjets can be useful for interstellar travel. The net thrust is not great, but for very long and relatively slow trips it let's you power a very large ship without dragging along as much fuel assuming you can get hydrogen only fusion to work.

[PaulHoule]

People who have looked at particular instances of fusion-powered ramjets have found that they don't produce enough thrust to overcome drag:

https://en.wikipedia.org/wiki/Bussard_ramjet

More recent thinking on the concept has centered around magsails which turn the drag into a good thing. Decelerating a starship is an even tougher problem than accelerating one, and magsails are a great choice for that. (And might even be able to get a speed of 0.2% of light for departure on the solar wind)

[Retric]

You need fuel for more than just propulsion. A hybrid engine that provides trust to offset the drag while also powering a ship is very viable.

Remember, drag is a function of relative speeds. A hypothetical example with zero velocity would allow you to gather fuel without any drag.

Now for a very large and 'slow' generation ship you need a lot of energy to keep the crew alive, able to manufacture repair parts, keep the lights on etc. Now, say you want need 1 ounce of fuel per hour that does not seem bad but if your talking a 100,000+ year trip that's 54+ million pounds.

Sure, that kind of trip does not seem appealing, but remember taking 4x the mass at 1/2 the speed takes the same energy. Further you are going to want to bootstrap a civilization at the other end which means outside of grey goo taking a lot of stuff. With the added benefit of being able to go somewhere else.

PS: You also get more energy from hydrogen the further up the chain you go. A multi stage reactor that's spitting out lead provides more energy.

[PaulHoule]

lead is not the endpoint of fusion, iron is.

if you expect to take a 100,000 year trip you should expect to live off the land and mine Kuiper belt objects and rouge planets. And figure that once people have lived 10,000 years under those conditions they probably won't find anything interesting about terrestrial planets.

[pbhjpbhj]

> A hypothetical example with zero velocity would allow you to gather fuel without any drag. //

What do you mean by this, zero velocity within an atmosphere won't gather anything?

[ProblemFactory]

> The missions targeted by this technology are GOCE-like spacecrafts which by design must fly low

Once the technology matures, it could be used by more missions. Flying low has its benefits:

* Lower latency for communication satellites,

* Better resolution for Earth imaging / spy satellites,

* When the satellite fails, it quickly deorbits by itself.

Until now, flying low has just not been economical, but if this thruster has similar lifetime to medium and high orbit satellites, then many more missions could choose lower orbits.

[orbital-decay]

>When the satellite fails, it quickly deorbits by itself.

This also means that failure recovery will be quite tricky if possible at all. There are some downsides to other points too: such a satellite would work at very thin margins due to the thruster being inefficient with air as a propellant. Its ground swath width will be lower, coverage will be worse, requiring more ground stations (remote sensing is very often limited by the downlink bandwidth). Also, some kind of aerodynamic shape will be required, limiting its capabilities and power budget. (electric propulsion needs a lot of power itself)

[jccooper]

"Quickly" in this context is probably still weeks, and you could carry a little backup system to kick it into higher orbit in case of trouble. But really, low-flying com or imaging sats are probably parts of large, "cheap" constellations and meant to be of limited lifespan.

[alex_duf]

>This also means that failure recovery will be quite tricky if possible at all

Nowadays it's probably cheaper to send a new one than doing a whole Hubble like hot fix with a space shuttle

[orbital-decay]

I'm not talking about on-orbit servicing though. Failure recovery is done all the time with most satellites. The operator just needs some time to determine the nature of the problem, possibly doing some workaround. With a low flying satellite you don't have much time for that.

[walrus01]

failure recovery in the context of spacecraft usually means software-commands sent via TT&C (tracking, telemetry and control) channel to switch to another piece of hardware, part of the N+1 or 1+1 configuration on the spacecraft. It is incredibly exceptionally rare for a human to ever visit a satellite once in orbit.

They did it a few times in the 1980s with the shuttle, including recovery of a satellite to prove it could be done, and there were the hubble servicing missions. But other than that no human has ever touched a satellite once it's in orbit.

[baybal2]

This does not preclude the possibility of spacecraft doing repeated dips only on the perigee

[binarycoffee]

True, interesting idea.

[Long edit]

Thinking further about this idea, I realize this may even mitigate the catch 22 problem of very low orbits (<180km): the lower the orbit, the larger the drag and the required thrust power, meaning the solar arrays must be bigger, which in turn further increases the drag... Calculations suggests that with current solar array and thruster technology, flying lower than 150km with this concept is impossible.

But with an elliptic orbit, energy from the solar arrays can be stored on the low-drag portion of the orbit too and used during the perigee dip, thus decreasing the requirements in terms of solar arrays area.

[Cthulhu_]

I'm now imagining a craft that folds up its solar panels before dipping into the atmosphere to gather fuel / accelerate. I'm sure I've built that in KSP, :p.

[msl]

That is not particulary far-fetched either: the ISS already reorients its solar panels when not illuminated by the sun. They call it the "night glider mode" [1].

[1] https://en.wikipedia.org/wiki/Night_Glider_mode

[zaarn]

Night Glider sounds vaguely like some 80s TV show. I do hope everyone on board is required to wear non-functional sunglasses during night glide.

[sitkack]

> The implementation of drag-reducing flight modes of the space station resulted in saving about 1,000 kg of orbital-maintenance propellant per year.

[greeneggs]

Here's an example where the authors propose doing this for planetary gravity assists, e.g., instead of using Venus for a normal gravity assist, dig into its atmosphere. Everything would need to be folded up first.

"Hypersonic Interplanetary Flight: Aero Gravity Assist"

Al Bowers & Dan Banks, 2006

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/200900...

Discussed in a podcast here: https://theorbitalmechanics.com/show-notes/al-bowers

[nbadg]

This would have the added benefit of increased efficiency due to the Oberth effect; however, I'm still not sure you could use it for unassisted interplanetary flight. The last orbit, by definition, must occur before the craft passes Earth escape velocity -- the question is, in that last pass through perigee, can you get enough delta V to make it to another planet? Otherwise, you'd need supplemental propellant. It's still useful, it's just not something I would describe as "revolutionary" for interplanetary travel.

Since the TWR of electric thrusters tends to be pretty abysmal, my gut is that you probably couldn't scale up the thruster well enough to bounce between planets without that supplemental propellant.

That being said, as others have mentioned, this would be really quite interesting for stationkeeping at low orbital altitudes, particularly for small satellites.

[m_mueller]

ISS is at 150km and needs costly refueling, right? Wouldn’t that be the most interesting applicaton in terms of cost savings?

[exDM69]

ISS is at 400+ km altitude where the atmosphere is really thin. It's also very heavy for low thrust electric propulsion.

[Symmetry]

Electric thrusters have a low thrust to weight ratio but there's nothing stopping you, in theory, from just scaling up. The ISS only experiences a little drag so an electric drive trying to zero that out doesn't need a huge thrust. There's some interest in adapting VASIMR for ISS station keeping. It would work, in theory, to just put a large number of Hall effect thrusters on the back but the piping would be infeasible.

[duskwuff]

It also needs resupply missions for food, air, and crew anyway. Reboost is almost a footnote.

[SideburnsOfDoom]

> Assuming it could collect more than it needs to keep itself in orbit, it could refuel a tank

That's not this device though, it looks like the "collected" air runs straight into the thruster, like the flow through a jet engine. No tank involved.

[aetherspawn]

That's a cool thought. You could perhaps use the atmosphere of planets to accelerate at very high velocities with the energy stored between each body (which would be a lot .. ie 60 days of 24/7 solar harvesting).

The question is whether the thrust you produce is roughly linear with the energy you expel? Or does it taper asymptotic? What if the power system on the craft is titanium batteries that are designed to deliver 1 MW for say 2 minutes? Will that give you the needed acceleration in a given planets atmosphere? What if you use planetary lasers and don't need batteries at all?

[zaarn]

Solar light isn't that strong once you go beyond mars.

Earth gets 1400 W/m^2, at Saturn only 16 W/m^2 and on Neptune maybe 1.5 W if you get lucky.

60 days of continous harvesting, assuming the spacecraft doesn't use any power (which is not true in reality), is about 2 kWh at Neptune. Not that much. Saturn would be 23 kWh.

[aetherspawn]

Yuck, that’s miserable.

[zaarn]

It's the inverse square law that bites you here as the same amount of energy gets stretched out into a larger sphere as it travels outwards (at earth the energy is 1.4kW for a square meter, when going outwards, this square meter gets stretched)

Double the distance and you get 1/4th the energy.

Saturn is 9AU or 9 times as far as earth; 1/81th the energy. (1400 / 9^2 = 17, so math checks out; roughly)

We're quite lucky to be close enough for solar energy to be a viable source of energy.

[*]: https://en.wikipedia.org/wiki/Inverse-square_law

[blattimwind]

> We're quite lucky to be close enough for solar energy to be a viable source of energy.

If solar energy were not viable, this form would not exist.

[basementcat]

If solar energy were not viable, Hacker News posts might be written by technologically inclined chemoautotrophs hacking in the basement of their hydrothermal vent. https://en.wikipedia.org/wiki/Chemosynthesis

[zaarn]

Just because solar cells don't work doesn't mean photosynthesis wouldn't work.

[riazrizvi]

It may not work on any planet. A fundamental design challenge is that increased size of solar panels create more drag, and increasing the height to reduce drag means it must go faster which further increases drag. While the Solar Impulse has demonstrated an equilibrium of speed-to-size can be maintained at normal altitude and low speed, we'll have to wait to see if something can be built to sustain equilibrium at these heights.

[pX0r]

"Sorry data, air may be the eventual oil."

Trivia question: How many round-trips from Neptune would it take to cause a 1% dip in Earth's air content?

Bonus question: Since the Earth is not making any more Xenon, are we losing some of this resource to the deep space every time we nudge a satellite?

[Cthulhu_]

A lot; also keep in mind that air is not a finite resource, it's continually generated from e.g. electrolysis (h2o -> o) and other processes (carbon + oxide = c02, plenty of carbon on earth, plenty of oxygen). Plus as another commenter mentioned, we're already losing some air all the time anyway.

[ajuc]

Tangentially related question - what happens to the gas used as a reaction mass in thrusters in orbit - when it's used to speed up I guess it falls down cause velocities mostly cancel out, but when it's used to slow down the ship, and engines are fired retrograde - the reaction mass has orbital velocity, right?

Does it stay in some orbit forever, like a solid object would? Can it cause gas "Kessler syndrome", with gas rings around Earth's most common reaction mass orbits?

If we choose our orbits and burn times so that this gas piles up in particular place on particular orbit, can we then reuse that as "air" for these engines from the article?

[Armisael16]

The exhaust velocity in a xenon ion thruster is 20-50 km/s. Most of the time it’s on an earth escape trajectory (~11 km/s in low orbit).

[Symmetry]

The smaller an object is the less time it takes for drag from the super tenuous atmosphere up in orbit to slow it down so it falls back to Earth. Gas molecules are very low mass and I expect that the exhaust for any given thruster will be gone quickly, even in the higher levels of LEO.

[wyattpeak]

Xenon's very heavy, most of it would eventually come back down to Earth - probably sooner rather than later. Most of what we lose to deep space is hydrogen and helium. And almost none of that is from space missions, anyway, it's just Brownian motion.

[alex_duf]

Isn't it a matter of speed rather than mass? If the xenon is ejected faster than the escape velocity, it seems like it would get off Earth's gravity.

In fact I think it would have to be roughly twice the escape velocity since the spacecraft is already going near it in one direction. According to Wikipedia[1] the exhaust velocity of an ion thruster is between 20 to 50 km/s when the Earth escape velocity is 11km/s [2]

[1]: https://en.wikipedia.org/wiki/Ion_thruster [2]: https://en.wikipedia.org/wiki/Escape_velocity

so I would assume most of it is lost in space

[jtbayly]

Velocity alone doesn't answer the question. Direction matters.

My assumption (knowing nothing but basic Physics), is that the xenon is ejected in a direction slightly toward the earth, and mostly directly in the opposite direction of the current travel, because that's what would be necessary to counteract drag and keep a satellite on the same path.

This means that if the satellite is going almost 11km/s one direction, the xenon will have that much less speed compared to the earth. And the trajectory will be slightly toward the earth.

I would assume that makes it substantially more likely that the xenon falls back to earth.

[logfromblammo]

It depends on whether the propellant is used to increase or decrease the orbital velocity of the vessel.

Acceleration propellant would have to be ejected at the orbital velocity of the vessel plus escape velocity to escape. Deceleration propellant would just have to be ejected at escape velocity minus orbital velocity.

As deceleration near atmosphere is almost free just by dipping into it, or by using some form of sea anchor to pull on the atmosphere or magnetic field, it is more likely that propellant would be used preferentially for attitude control and acceleration.

It isn't impossible, but imparting enough velocity to propellant for it to escape Earth orbit--while accelerating a vessel in the opposite direction--seems unlikely for orbital station-keeping. You need at least 12000 m/s for escape velocity plus at least 8000 m/s to counteract the orbit you were already in, so the propellant would have to leave the vessel at more than 20000 m/s. That's a specific impulse of about 2000 s. Ion drives and VASIMR could do it, but the propellant is very likely to experience its own atmospheric drag and electromagnetic interactions, and the probability that any particular atom of propellant would actually escape with the minimum velocity-relative-to-vessel is very low. The propellant would spread out to a larger volume as quickly as it could, too. It's far more likely that one of those xenon ions would collide with a hydrogen atom in the upper atmosphere and randomly bounce it out, like a bowling ball hitting a billiard ball.

[orbital-decay]

Generally, electric propulsion is used for two purposes in Earth-orbiting spacecraft: for correcting the orbit (station-keeping) and for raising the orbit from an intermediate to the target one (in relatively recent all-electric GEO sats)

Station-keeping requires relatively short burns (sub-hour to several hours) in all directions. When raising the orbit, the spacecraft usually keeps itself in fixed position relative to the Sun to maximize the solar panel output. The propulsion unit keeps working at all times, both in prograde and retrograde, because efficient Hall thrusters are tricky to work with in impulse mode, and are heavily optimized for continuous operation.

So in most cases, xenon is ejected in arbitrary directions, retrograde being only one of them. Besides, some ion/plasma thrusters are so efficient that they eject the propellant at more than double escape velocity. I would guess most of the propellant actually leaves the gravity well; also, at higher altitudes where electric propulsion is mostly used there's no atmosphere to collide with.

[wyattpeak]

Wow, you're right, I vastly underestimated the exhaust velocity.

You'd still have to account for its interaction with the atmosphere, but my point is moot.

[russdill]

It's not Xenon you have to worry about, it's Helium. Once we run out we'll be too heavy and fall into the sun.