Comment by karkisuni
8 years ago
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.
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.
> 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
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?"
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
A personal favorite feature on federation vessels.
http://memory-alpha.wikia.com/wiki/Bussard_collector
How can you decelerate with a ramjet? Wouldn't your own exhaust push the matter you needed out of the way?
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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.
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> 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.
>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)
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This does not preclude the possibility of spacecraft doing repeated dips only on the perigee
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.
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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.
ISS is at 150km and needs costly refueling, right? Wouldn’t that be the most interesting applicaton in terms of cost savings?
ISS is at 400+ km altitude where the atmosphere is really thin. It's also very heavy for low thrust electric propulsion.
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> 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.
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?
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.
Yuck, that’s miserable.
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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.
"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?
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.
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?
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).
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.
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.
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
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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.