First firing of air-breathing electric thruster
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Still, atmospheric fuel scoops were still sci-fi until now, as far as I’m aware.
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?
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.
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.
If solar energy were not viable, this form would not exist.
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.
[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.
"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
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.
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.
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)
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.
What do you mean by this, zero velocity within an atmosphere won't gather anything?
Rest of the idea:
The Ramjet works by collecting hydrogen and Helium from a large area because you have a high relative velocity to the medium which also imposes drag. Think filter feeding whales. So you are collecting linearly more matter and thus energy per unit time with increased speed. However, drag is a function of matter collected AND relative speed so something like velocity ^3.
This suggests there is some point where you get less energy from collecting that you lose in drag. But, this also means below some speed you get more.
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.
"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?"
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.
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)
Nowadays it's probably cheaper to send a new one than doing a whole Hubble like hot fix with a space shuttle
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.
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?
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
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.
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.
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.
You'd still have to account for its interaction with the atmosphere, but my point is moot.
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?
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.
This is a prototype so I'm guessing the size/cost can be reduced in the future, and if it becomes small/cheap, I could see a fan without moving parts having a lot of applications.
It kind of works like an aerodynamic diode: it is much easier for the incoming particles to go through the intake tubes (because they are oriented along the spacecraft velocity vector) than to exit the collector. This is because after a they collide with surfaces inside the collector, their velocity vector is randomized and no longer aligned with the tubes.
[Edit] Mass is not such an issue when you contrast it with the mass of propellant that you save...
[Edit2] I forgot to precise that my description only pertained to the intake part: there is of course a plasma thruster at the back (no moving part either)
Edit: wrong... see below. Thx, walrus!
I'd be more worried about random Nitrogen Oxides (NOx) emissions, but again, too small of a volume to actually matter in any significant way. A couple of VWs are probably worse.
For lack of friction/bearings a magnetic bearing fan seems like a much better option. The strength of this technology is the great exhaust velocity, which is great for space applications.
Ah, that makes sense. Given the speed that these satellites are typically travelling at, the exhaust probably needs to go faster than that to work, right?
[1]: https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive
so, to-do:
* find way to scoop up more incoming material
* replace electric propulsion with sth more powerful
* write smug think piece about how sci-fi guides engineering
Seems like a tie-in, although Electric Thrusters themselves look bigger than the satellites right now.
That's everything I know about both subjects, so no idea if this is a useful comment or not :-)
[1] https://techcrunch.com/2018/03/01/astranis-emerges-from-stea...
[0] https://news.engin.umich.edu/2017/10/thruster-for-mars-missi...
And it is definitely hard, but this is just part of the problem.
The really tricky thing is to get the whole system working. For instance it is not longer possible to use a "high pressure" (relatively speaking of course) propellant feed. Even with the passive compression stage discussed in the article, an off-the-shelf thruster wouldn't be able to operate due to the low inlet pressure.
Also, the intake/collector design is a problem of its own. AFAIK this was the first real-life test for the passive intake concept (the theory of operation and trade-off to be considered are discussed here [1]).
That being said there is still a long way to go before this can work in space...
[1] http://erps.spacegrant.org/uploads/images/2015Presentations/...
Aircraft don't need propellant; they can simply "push off" of the air (via propellers, turbofan engines, etc); so this mechanism is useless for them. They still need fuel to run their engines, which this device doesn't provide.
This development gets rid of the need to carry propellant. They satellite will scoop it up from the atmosphere, saving weight and prolonging life.
No, this is actual Newtonian physics. Rockets function based on Newton's third law: every reaction creates an equal and opposite reaction. We sit on wheeled office chairs, I push you- we both move, to opposite directions.
Rockets aren't fueled by office chairs, though, but by gases. They push the gas molecules into a particular direction and themselves are pushed into another.
So, to move in space by pushing stuff, you need two things: a) mass to push away b) some power to do the pushing
(This ignores several categories of other forms of force generation on space craft like solar sails).
Chemical fuel rockets happen to strike two flys on one go - the mass they carry also generate the energy for the push, so that's only a matter of plumbing and hydrodynamics to get them going.
The problem with this approach, though, is that once you run out of fuel, you run out of fuel. No more chairs, no more acceleration. Refueling in space is really expensive, if you need to bring up the propellant from a deep gravity well like earths surface to the orbit (https://xkcd.com/681/).
There is no necessity the propellant (i.e the 'chairs') needs to generate the energy for pushing itself away. You can you Some Other Physics to push the propellant away from the vehicle. It works just as fine.
This system collects the fuel from sparse gas surrounding it, and expels it using an electric thruster, which probably get's it's energy from solar panels.
Potentially, like some other poster noted, you could design a spacecraft with this that once it reaches planetary orbit, it can hop from planet to planet and refuel itself indefinetly (just as long as the planets have an atmosphere).
So it's Way Cool, and this has been hypothesized in science fiction for decades, so it's also Genuine Scifi Space Tech :)
Disclosure: I was involved in that project.
[0] http://erps.spacegrant.org/uploads/images/images/iepc_articl...
There were also several precursor experimental studies funded by ESA, one of which [1] can be found in the same conf. proceeding.
[1] http://erps.spacegrant.org/uploads/images/images/iepc_articl...
http://hopefullyintersting.blogspot.com/2015/03/rockets-elec...