Ion Thruster Prototype Breaks Records in Tests

[Flyaway]

A thruster that's being developed for a future NASA mission to Mars broke several records during recent tests, suggesting that the technology is on track to take humans to the Red Planet within the next 20 years, project team members said.

The X3 thruster, which was designed by researchers at the University of Michigan in cooperation with NASA and the U.S. Air Force, is a Hall thruster — a system that propels spacecraft by accelerating a stream of electrically charged atoms, known as ions. In the recent demonstration conducted at NASA's Glenn Research Center in Ohio, the X3 broke records for the maximum power output, thrust and operating current achieved by a Hall thruster to date, according to the research team at the University of Michigan and representatives from NASA.

"We have shown that X3 can operate at over 100 kW of power," said Alec Gallimore, who is leading the project, in an interview with Space.com. "It operated at a huge range of power from 5 kW to 102 kW, with electrical current of up to 260 amperes. It generated 5.4 Newtons of thrust, which is the highest level of thrust achieved by any plasma thruster to date," added Gallimore, who is dean of engineering at the University of Michigan. The previous record was 3.3 Newtons, according to the school.

https://www.space.com/38444-mars-thruster-design-breaks-records.html

 

[Flyaway]

Throttling Up Ion Thruster Technologies

These developments in current Hall thruster technology are exciting in themselves and have implications for the near-term in missions to destinations like Mars. But I’m also interested in pursuing how we might move ion technologies in new directions by creating hybrid designs, with Kuiper Belt objects and the gravitational focus at 550 AU as potential destinations. With laser methods now in the spotlight as Breakthrough Starshot continues its analysis of a mission to Proxima Centauri, hybrid ion engine designs boosted by laser power are coming into consideration. I’ll take a look at the possibilities in tomorrow’s post.

https://www.centauri-dreams.org/?p=38697

 

[steelpillow]

Could one combine a solar panel with laser thrust? The laser would shine on the solar panel, which would absorb the photon and therefore also:
1. Absorb its momentum, generating immediate thrust.
2. Convert its energy to electricity, which can be stored in a battery to drive the ion engine for manoeuvering and deceleration.

 

[Forest Green]

Could one combine a solar panel with laser thrust? The laser would shine on the solar panel, which would absorb the photon and therefore also:
1. Absorb its momentum, generating immediate thrust.
2. Convert its energy to electricity, which can be stored in a battery to drive the ion engine for manoeuvering and deceleration.

Solar sail - Wikipedia

en.wikipedia.org

But a more precise answer is that in a solar cell an electron absorbs the energy and is excited, creating a flow of electricity. I do not believe the electron can absorb the energy and the photon create pressure thrust. The photon has to be reflected for this. In short, the energy of the photon can either converted to electrical energy or kinetic energy, not both at the same time (conservation of energy).

 

[DWG]

If you absorb the photon, then the momentum change equals the momentum of the photon (you'll also pick up energy as heat)

If you reflect the photon, then the momentum change is the momentum delivered by the incoming photon, plus that of the outgoing reflected photon.

So I suspect a reflective solar sail has advantages both for efficiency and for heat control. Above and beyond which I'm not sure you'd be able to tack without that outgoing momentum change.

 

[TomcatViP]

Solar sails (also called light sails or photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors. Based on the physics, a number of spaceflight missions to test solar propulsion and navigation have been proposed since the 1980s.

A useful analogy to solar sailing may be a sailing boat; the light exerting a force on the mirrors is akin to a sail being blown by the wind. High-energy laser beams could be used as an alternative light source to exert much greater force than would be possible using sunlight, a concept known as beam sailing. Solar sail craft offer the possibility of low-cost operations combined with long operating lifetimes. Since they have few moving parts and use no propellant, they can potentially be used numerous times for delivery of payloads.

Solar sails use a phenomenon that has a proven, measured effect on spacecraft. Solar pressure affects all spacecraft, whether in interplanetary space or in orbit around a planet or small body. A typical spacecraft going to Mars, for example, will be displaced thousands of kilometers by solar pressure, so the effects must be accounted for in trajectory planning, which has been done since the time of the earliest interplanetary spacecraft of the 1960s. Solar pressure also affects the orientation of a spacecraft, a factor that must be included in spacecraft design.[1]

The total force exerted on an 800 by 800 meter solar sail, for example, is about 5 newtons (1.1 lbf) at Earth's distance from the Sun,[2] making it a low-thrust propulsion system, similar to spacecraft propelled by electric engines, but as it uses no propellant, that force is exerted almost constantly and the collective effect over time is great enough to be considered a potential manner of propelling spacecraft.

.

Source:

Solar sail - Wikipedia

en.wikipedia.org

Radiation ≠ absorption

 

[Forest Green]

If you absorb the photon, then the momentum change equals the momentum of the photon (you'll also pick up energy as heat)

If you reflect the photon, then the momentum change is the momentum delivered by the incoming photon, plus that of the outgoing reflected photon.

So I suspect a reflective solar sail has advantages both for efficiency and for heat control. Above and beyond which I'm not sure you'd be able to tack without that outgoing momentum change.

Solar to electrical conversion is notoriously inefficient and, as I understand it, the momentum of the photon is transferred to the electron, since the electron must move to produce current.

Some answers to it here, basically you will get a small amount of electricity and a lot of heat.

https://www.reddit.com/r/Physics/comments/5rzt4s

 

[steelpillow]

Solar to electrical conversion is notoriously inefficient and, as I understand it, the momentum of the photon is transferred to the electron, since the electron must move to produce current.

The efficiency of Solar to electrical conversion has improved over the years, however it has long been the only way to generate electricity in space if you have no onboard power source. The electron does not escape the spacecraft and so the momentum must eventually be transferred there.

Yes the process generates waste heat, but at least it generates both thrust and electric power along the way. These are not easy things to do without carrying a lot of extra weight.

 

[RanulfC]

The efficiency of Solar to electrical conversion has improved over the years, however it has long been the only way to generate electricity in space if you have no onboard power source. The electron does not escape the spacecraft and so the momentum must eventually be transferred there.

Yes the process generates waste heat, but at least it generates both thrust and electric power along the way. These are not easy things to do without carrying a lot of extra weight.

As I understand it reflection would transfer more momentum because you get both the impact and then re-direction whereas absorbtion turns the momentum mostly into heat which only contributes a small percentage of the energy back into momentum. This is especially true with more 'massive' objects such as a solar cell compared to a section of solar sail. The sail is very thin and has low-mass per sq/cm where as even thin film solar cells have a much higher mass per sq/cm. The photon transfers essentially the same momentum but the "heavier" mass is moved less distance so the mass heats up to compensate for the transfered momentum. The heat radiates away, (or is aborbed by the structure) with a lot less overall 'power' than the original photon's mass/momentum. So while a laser beam could "propell" a light sail it's more likely to generate more 'heat' than propulsion with a solar cell.

Of coure using a laser and solar cells is the basis of beamed power, if not propulsion exactly Though put enough laser power on-target you CAN get some pretty sweet thrust... Albeit because you're vaporizing your 'target' material with enough energy to bleed it off as a propulsive force. That's "Ablative Laser Propulsion though (https://en.wikipedia.org/wiki/Laser_propulsion) and if I recall Jordin Kare, (https://en.wikipedia.org/wiki/Jordin_Kare) mentioned an intensive debate when the idea was studied at LLNL that came down to which was the better "propellant" for laser launch; cucumbers or watermelon and seeing a supervisor walk in, listen for a second and walk out shaking his head but not daring to ask any questions
(https://en.wikipedia.org/wiki/Beam-powered_propulsion)

Randy

 

[Forest Green]

When looked at logically, temperature is a measure of the average kinetic energy of a group of atoms or molecules, so for a solid that basically means the momentum of the photons goes into making the atoms/molecules translate and rotate (vibrate).

 

[Forest Green]

As I understand it reflection would transfer more momentum because you get both the impact and then re-direction whereas absorbtion turns the momentum mostly into heat which only contributes a small percentage of the energy back into momentum. This is especially true with more 'massive' objects such as a solar cell compared to a section of solar sail. The sail is very thin and has low-mass per sq/cm where as even thin film solar cells have a much higher mass per sq/cm. The photon transfers essentially the same momentum but the "heavier" mass is moved less distance so the mass heats up to compensate for the transfered momentum. The heat radiates away, (or is aborbed by the structure) with a lot less overall 'power' than the original photon's mass/momentum. So while a laser beam could "propell" a light sail it's more likely to generate more 'heat' than propulsion with a solar cell.

Of coure using a laser and solar cells is the basis of beamed power, if not propulsion exactly Though put enough laser power on-target you CAN get some pretty sweet thrust... Albeit because you're vaporizing your 'target' material with enough energy to bleed it off as a propulsive force. That's "Ablative Laser Propulsion though (https://en.wikipedia.org/wiki/Laser_propulsion) and if I recall Jordin Kare, (https://en.wikipedia.org/wiki/Jordin_Kare) mentioned an intensive debate when the idea was studied at LLNL that came down to which was the better "propellant" for laser launch; cucumbers or watermelon and seeing a supervisor walk in, listen for a second and walk out shaking his head but not daring to ask any questions
(https://en.wikipedia.org/wiki/Beam-powered_propulsion)

Randy

I like the idea of using a Dyson swarm to produce a Schwarzchild Kugelblitz and harvesting the Hawking radiation for propulsion. An added benefit might be that it would slow down time local to the spacecraft and generate angular momentum too.

 

[steelpillow]

So while a laser beam could "propell" a light sail it's more likely to generate more 'heat' than propulsion with a solar cell.

Perhaps "solar" panel is the wrong word, "cell" surely is. I envisage a huge, thin sheet with perhaps an organic (nanotube?) collector, tightly tuned to the laser wavelength. It would probably not be your old-style silicon cell. If the wavelengths of laser and light collector are matched then conversion efficiency is far higher than say an Earthbound solar light farm and waste heat is minimised. If the laser beam carries a second, slightly over-long wavelength, this will be reflected and not absorbed. Whether the halved level of thrust from each photon absorbed leaves one's glass half full or half empty is a matter of opinion.

To summarise, we are managing three situations with the light beam:

• low-energy* (long wave) photons: reflect. This is the principal laser output for lightsail mode.
• tuned photons: converted with relatively high efficiency. This is the laser output for energy collection mode and also provides reduced thrust.
• high-energy (short wave) photons: converted with relatively low efficiency, the rest released as waste heat. This is avoided by using a narrowband laser instead of broad-spectrum sunlight.

* Note that "low energy" does not imply a low-energy beam, merely that the photons are carefully de-tuned.

However the key point for the current topic is that the laser energy absorbed by the solar panel ultimately powers the ion drive, whereas the lightsail must carry an onboard power source. One can pose my question another way, is it worth sacrificing some efficiency in the solar sail, on order to avoid the need for an onboard power source?

 

[Forest Green]

Then you have the problem of the inefficiency of the laser and refocusing it once it gets scattered by the atmosphere. And you're still limited to about 50% efficiency, on top of the atmospheric attenuation (unless the laser is in space too) and beam divergence.

III-V Solar Cells, Modules and Concentrator Photovoltaics - Fraunhofer ISE

In the III-V solar cells, modules and concentrating photovoltaics business area, we focus on the development of highly efficient PV technologies.

www.ise.fraunhofer.de

Even assuming you use solar power to power the laser, basically what you're doing is introducing a string of losses and inefficiencies into the equation. You have the inefficiency of the laser, the inefficiency of the solar cells used to power the laser, and the inefficiency of the tuned solar cell and a sail that's limited to using one wavelength rather than the sun's full spectral output. It would be more useful to build a lens, or series of lens to focus the sun's output unto the sail at long distances.