[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?

[Retric]

Space is not an absolute vacuum which is why this works in the first place. If your ship sits in the interstellar medium so it's not being pushed around on net then by definition it's drag is zero. However, it's possible to collect some non zero amount of hydrogen and thus energy by putting a high vacuum pump to empty space. Efficiency left as an exercise to the reader. But, it would operate the same way as a vacuum pump inside an atmosphere, just vastly slower with random particle motion providing a continuous stream of new particles.

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.

[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.