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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 altitud

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.

baybal2 9y ago
This does not preclude the possibility of spacecraft doing repeated dips only on the perigee
- binarycoffee 9y ago
  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_ 9y ago
    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.
    - greeneggs 9y ago
      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/20090008672.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/200900... Discussed in a podcast here: https://theorbitalmechanics.com/show-notes/al-bowers https://theorbitalmechanics.com/show-notes/al-bowers
    - msl 9y ago
      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 https://en.wikipedia.org/wiki/Night_Glider_mode
      - sitkack 9y ago
        > The implementation of drag-reducing flight modes of the space station resulted in saving about 1,000 kg of orbital-maintenance propellant per year.
      - zaarn 9y ago
        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.
- nbadg 9y ago
  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.
ckastner 9y 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. 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 https://en.wikipedia.org/wiki/Tau_Zero [2] https://en.wikipedia.org/wiki/Bussard_ramjet https://en.wikipedia.org/wiki/Bussard_ramjet
- gaius 9y ago
  How can you decelerate with a ramjet? Wouldn't your own exhaust push the matter you needed out of the way?
  - whatshisface 9y ago
    You could push the exhaust single-file in a highly focused beam, leaving most of the solid angle around you unpushed.
- binarycoffee 9y ago
  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 https://en.wikipedia.org/wiki/Electric_sail
- wizardforhire 9y ago
  A personal favorite feature on federation vessels. http://memory-alpha.wikia.com/wiki/Bussard_collector http://memory-alpha.wikia.com/wiki/Bussard_collector
- Retric 9y ago
  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.
ProblemFactory 9y ago
> 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 9y ago
  >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 9y ago
    "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 9y ago
    >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
    - walrus01 9y ago
      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.
m_mueller 9y ago
ISS is at 150km and needs costly refueling, right? Wouldn’t that be the most interesting applicaton in terms of cost savings?
- exDM69 9y ago
  ISS is at 400+ km altitude where the atmosphere is really thin. It's also very heavy for low thrust electric propulsion.
  - Symmetry 9y ago
    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 9y ago
    It also needs resupply missions for food, air, and crew anyway. Reboost is almost a footnote.

baybal2 9y ago

This does not preclude the possibility of spacecraft doing repeated dips only on the perigee

binarycoffee 9y ago
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_ 9y ago
  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.
  - greeneggs 9y ago
    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/20090008672.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/200900... Discussed in a podcast here: https://theorbitalmechanics.com/show-notes/al-bowers https://theorbitalmechanics.com/show-notes/al-bowers
  - msl 9y ago
    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 https://en.wikipedia.org/wiki/Night_Glider_mode
    - sitkack 9y ago
      > The implementation of drag-reducing flight modes of the space station resulted in saving about 1,000 kg of orbital-maintenance propellant per year.
    - zaarn 9y ago
      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.
nbadg 9y ago
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.

ckastner 9y 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. 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 https://en.wikipedia.org/wiki/Tau_Zero [2] https://en.wikipedia.org/wiki/Bussard_ramjet https://en.wikipedia.org/wiki/Bussard_ramjet

gaius 9y ago
How can you decelerate with a ramjet? Wouldn't your own exhaust push the matter you needed out of the way?
- whatshisface 9y ago
  You could push the exhaust single-file in a highly focused beam, leaving most of the solid angle around you unpushed.
binarycoffee 9y ago
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 https://en.wikipedia.org/wiki/Electric_sail
wizardforhire 9y ago
A personal favorite feature on federation vessels. http://memory-alpha.wikia.com/wiki/Bussard_collector http://memory-alpha.wikia.com/wiki/Bussard_collector
Retric 9y ago
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.

ProblemFactory 9y ago

> 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 9y ago
>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 9y ago
  "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 9y ago
  >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
  - walrus01 9y ago
    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.

m_mueller 9y ago

ISS is at 150km and needs costly refueling, right? Wouldn’t that be the most interesting applicaton in terms of cost savings?

exDM69 9y ago
ISS is at 400+ km altitude where the atmosphere is really thin. It's also very heavy for low thrust electric propulsion.
- Symmetry 9y ago
  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 9y ago
  It also needs resupply missions for food, air, and crew anyway. Reboost is almost a footnote.