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.
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.
[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.