Comment by binarycoffee
8 years ago
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?
You could push the exhaust single-file in a highly focused beam, leaving most of the solid angle around you unpushed.
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.
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)
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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)
"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.
>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
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This does not preclude the possibility of spacecraft doing repeated dips only on the perigee
True, interesting idea.
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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.
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.
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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.
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.
It also needs resupply missions for food, air, and crew anyway. Reboost is almost a footnote.