ESA fires an air-breathing electric thruster for low orbits

[Gaarst]

I just found out about this (article is a couple days old) and it seems nobody made a thread about it here yet, so here we are:

http://www.esa.int/Our_Activities/Space_Engineering_Technology/World-first_firing_of_air-breathing_electric_thruster

From what I understand, it is basically your average ion thruster except instead of a Xe tank as propellant, it has an intake that collects the few air molecules at 200 km altitude which are then ionised and accelerated like conventional ion thrusters do xenon.

Applications are essentially for satellites orbiting in very low orbits (around 200km) for long periods, removing the need for internal fuel tanks and extending the lifetime: the article cites the GOCE satellite (Earth gravitational field mapper) which expanded 40 kg of xenon over its 4.5 years in a ~250 km circular orbit. I'm guessing most uses will be in various Earth observation/mapping missions where lower altitude means better resolution, but hopefully they will be able to improve on the concept to make it work at higher altitudes where drag becomes small enough that the thurster can be used to maneuver around (cheap LEO to GTO insertions in many passes comes to mind).

No testing mission is mentioned in the article though, so this is probably still in very early stages of development and perhaps not usable at the moment.

[DAL59]

Great for star link.  

[YNM]

Wouldn't catching the air in front of you makes for a somewhat bigger drag ?

Have they tried it ?

Is this basically an "electric jet" ?

[Gaarst]

16 minutes ago, YNM said:

Wouldn't catching the air in front of you makes for a somewhat bigger drag ?

Have they tried it ?

Is this basically an "electric jet" ?

Air molecules in space usually bounce off your spacecraft (they don't flow around it since you're in hypersonic flow) so I'm guessing as long as the intake is not larger than the spacecraft itself it shouldn't cause additional drag. I don't know how a regular air intake compares to a flat end but it should be similar.

They have tested it in a lab with conditions reproducing that at 200 km altitude. They have succeeded in igniting the engine several times using only the atmospheric gasses you'd find up there, and they mentioned that they could measure thrust in their experimental setup though no numbers were given.

You could compare to an electric scramjet since the airflow is not significantly slowed down but there is no compression of the air so it is not an actual jet engine.

Edited March 7, 2018 by Gaarst

[Scotius]

A first prototype of Bussard ramjet? Wasn't there a thread about experimental braking system for small sats being developed, that uses a bubble of plasma caught in electromagnetic field around the sat to slow it down against the atmosphere, while protecting the craft from the heat? I wonder if those two could be combined...

[ment18]

I'm interested to see what the power requirements are.  Since it will be using molecules much lighter than xenon, it will need a lot more power to produce the same amount of thrust as a xenon vehicle since it will have a higher Isp.  Also strange that a vehicle that weighed about 1000kg only had 40kg of propellant, but lost 200kg over the course of the mission?

[YNM]

36 minutes ago, Gaarst said:

Air molecules in space usually bounce off your spacecraft...

Ah.

But it's an interesting method. I'd still question it has enough force to do circularization though, unless for very long periods (which isn't great).

Edited March 7, 2018 by YNM

[sevenperforce]

This is so many kinds of fantastic.

[hms_warrior]

16 hours ago, ment18 said:

... Since it will be using molecules much lighter than xenon, it will need a lot more power to produce the same amount of thrust as a xenon vehicle since it will have a higher Isp...

The actual Isp of this Engine would be infinite, since it doesn't use any internal fuel/propellant.

Would be interessting to see how low they can get with it. The denser the atmosphere, the more reactionmass they have to work with --> more thrust per watt.

What would be the benefits of a 90km high "Orbit" be? Maybe for ultra-details google maps?^^

Edited March 8, 2018 by hms_warrior

[goncaloeaguiar]

There is still the issue of corrosion to be fixed due to atomic oxygen.

[Ultimate Steve]

7 hours ago, hms_warrior said:

The actual Isp of this Engine would be infinite

And that's now one of my favorite sentences.

[goncaloeaguiar]

9 hours ago, hms_warrior said:

The actual Isp of this Engine would be infinite, since it doesn't use any internal fuel/propellant.

Would be interessting to see how low they can get with it. The denser the atmosphere, the more reactionmass they have to work with --> more thrust per watt.

What would be the benefits of a 90km high "Orbit" be? Maybe for ultra-details google maps?^^

The Isp is still finite, because you will have a finite exhaust velocity... 

[StrandedonEarth]

1 hour ago, goncaloeaguiar said:

The Isp is still finite, because you will have a finite exhaust velocity... 

Hmm, yes, it’s the propellant ( or reaction mass, whichever you prefer) that’s infinite. 

[Shpaget]

I believe "dV" is the term you're looking for.

[Mad Rocket Scientist]

2 hours ago, Shpaget said:

I believe "dV" is the term you're looking for.

Yes, but it's because the mass ratio is infinite, not the effective exhaust velocity * g0.

[sevenperforce]

1 hour ago, Mad Rocket Scientist said:

Yes, but it's because the mass ratio is infinite, not the effective exhaust velocity * g0.

The exhaust velocity is finite; the effective exhaust velocity (and corresponding isp) is infinite.

In comparing airbreathing impulse engines to conventional rocket engines, the actual exhaust velocity is useless since you're using air as reaction mass. So effective exhaust velocity is calculated as the reciprocal of thrust-specific fuel consumption, because thrust-specific fuel consumption can be used to make direct comparison between airbreathers and rockets.

The fuel consumption of an electric ramjet is zero, so thrust-specific fuel consumption is zero (assuming net-positive thrust), and the reciprocal of zero is infinity. So, yes, specific impulse is infinite, even if the actual exhaust velocity is finite.

Edited March 9, 2018 by sevenperforce

[Mad Rocket Scientist]

Just now, sevenperforce said:

The exhaust velocity is finite; the effective exhaust velocity is infinite.

In comparing airbreathing impulse engines to conventional rocket engines, the actual exhaust velocity is useless since you're using air as reaction mass. So effective exhaust velocity is calculated as the reciprocal of thrust-specific fuel consumption, because thrust-specific fuel consumption can be used to make direct comparison between airbreathers and rockets.

The fuel consumption of an electric ramjet is zero, so thrust-specific fuel consumption is zero (assuming net-positive thrust), and the reciprocal of zero is infinity. So, yes, specific impulse is infinite, even if the actual exhaust velocity is finite.

That's interesting, I didn't know it was calculated that way for jet engines.

[sevenperforce]

7 minutes ago, Mad Rocket Scientist said:

That's interesting, I didn't know it was calculated that way for jet engines.

Yep. Thrust-specific fuel consumption is fuel flow rate divided by thrust. If you use the weight of your fuel flow, as is typical in the airline industry (e.g., "pounds per second"), then you have weight per time, divided by thrust. But weight and thrust are both units of force, so they cancel, and you're left with seconds^-1. Take the reciprocal, and you get the specific impulse, measured in seconds. If you use the mass of your fuel, then force does not cancel and taking the reciprocal gives you a speed in m/s.

Isn't dimensional analysis great?

 

[Northstar1989]

This story has been making headlines lately:

https://www.theguardian.com/science/2018/mar/08/air-fuelled-engine-development-low-flying-satellites

http://www.esa.int/Our_Activities/Space_Engineering_Technology/World-first_firing_of_air-breathing_electric_thruster

Basically, a European Space Agency researcher developed a working prototype of an electric thruster that can operate off of the residual atmosphere found at 200 kilometers altitude.

The Guardian's article correctly identifies the utility of this technology for maintaining the orbits of ultra-low altitude satellites flying at 200 km or so (the idea of such satellites isn't new- the ESA recently kept one in Ultra Low Earth Orbit at as low as 250 km for 4.5 years using nothing but a Xenon-Electric thruster, and NASA has designs on the books for similar satellites around Mars...) but nobody seems to have yet realized there is a much more useful type of Ultra Low Earth Orbit satellite we can build than scientific or comms satellites...

---

Propulsive Fluid Accumulators.

https://en.m.wikipedia.org/wiki/Propulsive_fluid_accumulator

Satellites which by some means (such as a Xenon-Electric Thruster, an electromagnetic tether that pushes off the Earth's magnetic field, or an Air-Electric Thruster) maintain a very low orbit (200 km is the sweet spot for Earth- which this thruster just happens to be designed for...) and collect residual atmospheric gases with a specialized intake- some of which are then compressed and cooled to cryogenic temperatures, to store supplies of Liquid Oxygen, Liquid Nitrogen, or even Liquid CO2 (if you're willing to pay the mass penalty to compress it to 6-7 atmospheres) locally collected in Low Earth Orbit...

The cryogenic liquids would then, in theory, be delivered to a fuel depot in a higher orbit- either by the Propulsive Fluid Accumulator itself (which would have to ascend to a higher orbit) or by a smaller tug/tanker/ferry of some sort.  Either way, the ultimate destination for these liquids is spacecraft destined for missions beyond Low Earth Orbit- anything from Mars or Moon missions, to transfers of comm satellites designed for Geostationary Orbit...

LOX can be burned with Kerosene, CH4, or H2; whereas N2 can be relatively easily reacted with O2 to form N2O4- a common oxidizer for Hydrazine-based hypergolic rocket propellants (intriguingly, Hydrazine is just N2H4, and it's also possible to manufacture hydrazine-derived hypergolic in LEO using the locally collected N2 as a feedstock along with CH4 and H2 launched from Earth- just significantly more complicated...) or just heated and used as propellant directly, as with compressed-nitrogen RCS systems (already use, require storing the N2 as a compressed gas though, which requires much stronger/heavier pressure vessels than cryogenic liquid...) or Nuclear/Microwave/Solar Thermal Rockets...

---

The point is this: the development of a propulsion system that doesn't require regular fuel launches to LEO (although even with such launches, it's still possible to leverage a few dozen kg of Xenon into *hundreds* of kg of LOX with a Propulsive Fluid Accumulator) massively improves the economics of such a technology.

As such, this Air-Electric Thruster which could allow satellites virtually unlimited loiter-times in 200 km orbit (well, as long as the solar panels last before they degrade...) could be the key breakthrough that finally makes Propulsive Fluid Accumulators a reality...

I can't be the only one to have noticed this (although, due to how few people even know what a Propulsive Fluid Accumulator is, and don't scoff at it as an impossible idea because they don't understand what's actually possible, the number of other people who realized this probably number in the dozens, at best) but I would certainly like to draw some attention to this idea.

If this Air-Electric Thruster really works at 200 km Low Earth Orbits like it is designed to, then there's no reason to think somebody couldn't design and build a Propulsive Fluid Accumulator (though likely only a tiny, test system) within the next decade as proof-of-concept.

If it worked, that would also really light a fire on developing functional orbital cryogenic fuel-transfer system to take full advantage of the nearly-unlimited supplies of cryogenic gases this would make available in Low Earth Orbit...

 

Regards,

Northstar

Edited March 9, 2018 by Northstar1989

[YNM]

You're aware of the other topic right ? This is quite a different direction than what's there so far though.

Edited March 9, 2018 by YNM

[Northstar1989]

On 3/7/2018 at 9:49 AM, ment18 said:

I'm interested to see what the power requirements are.  Since it will be using molecules much lighter than xenon, it will need a lot more power to produce the same amount of thrust as a xenon vehicle since it will have a higher Isp.  Also strange that a vehicle that weighed about 1000kg only had 40kg of propellant, but lost 200kg over the course of the mission?

The ISP is only higher, and Thrust lower, for a given amount of energy imparted to each molecule.  If you impart less energy to a larger number of lighter molecules, you can still get the same ISP and Thrust as with a heavier propellant...

The only reason this isn't done with electric thrusters that work off internal propellant is that it kind of defeats the purpose of using a lighter propellant- the mass and volume of fuel tanks required is determined by the number of molecules stored and their pressure, not their molecular mass.  So storing 1 ton of Hydrogen requires more mass in fuel tanks than storing 1 ton of Argon than storing 1 ton of Xenon.  Lighter propellants also tend to be harder to store long-term.  Which means the only reason to ever choose a lighter propellant (besides cost/availability) is if you're going to operate your electric thruster at a higher ISP- which lighter propellants are better suited for...

 

Edited March 9, 2018 by Northstar1989

[Northstar1989]

On 3/8/2018 at 2:21 AM, hms_warrior said:

The actual Isp of this Engine would be infinite, since it doesn't use any internal fuel/propellant.

Would be interessting to see how low they can get with it. The denser the atmosphere, the more reactionmass they have to work with --> more thrust per watt.

What would be the benefits of a 90km high "Orbit" be? Maybe for ultra-details google maps?^^

Actually, a system like this would use more electrical power the more reaction mass passes through it.  That's because an engine like this is essentially designed to ionize ALL the molecules entering the intake (or as close to 100% as it can get, anyways) and then accelerate all of them.  So, thrust and power consumption remain directly proportional until the point where the engine "chokes" on excess pressure...

The practical lower limit for altitude a satellite could maintain using this system would be determined by the availability of electrical power from the solar panels, anyways.  The lower your altitude the more solar panels you need to power the thruster, but also the more drag those solar panels generate...

This system has the widest margins at *higher* altitudes, where the power to drag ratio of the solar panels is better (the same area of panels generates the same power, but less drag, at higher altitudes...)

---

This is also my opportunity to sneak in the subject of Propulsive Fluid Accumulators...

https://en.m.wikipedia.org/wiki/Propulsive_fluid_accumulator

As part of an effort to figure out whether Propulsive Fluid Accumulators would be feasible, the math for air-electric thrusters was already pioneered DECADES ago, long before the first functional prototype air-electric thruster was ever built this year, in 2018 (the first that I know of, anyways).

Originally, when the idea was first looked at in the late 1960's through the 1970's, every proposal required a nuclear reactor to achieve a positive power:drag ratio for the electric thrusters systems at ANY altitude, nevertheless have any power left over to actually collect atmospheric gases (early solar panel designs existed, but their power output was pathetic).

But when the idea was revisited in the 2000's, solar technology had improved far enough that solar-powered systems were feasible- but only at HIGHER altitudes (nuclear designs were suggested to operate at 100 km, for reasons having to do with economics rather than efficiency- whereas solar designs were suggested operate at altitudes of 200 km or more).

Since that time, solar technology has improved further- and the Air-Electric Thruster developed by this ESA researcher could probably operate at lower altitudes (say, 180 km) than past solar air-electric thruster concepts (which were never actually built) were designed for due to power constraints and drag from the solar panels needed to power the whole thing...  But I still doubt it could maintain a 90, or even 100 km orbit with today's solar panel efficiencies...

http://www.esa.int/Our_Activities/Space_Engineering_Technology/World-first_firing_of_air-breathing_electric_thruster

Also, if a satellite can MAINTAIN orbit at 200 km using this Air-Electric Thruster (as advertised in the ESA post), then it's perfectly reasonable to think a satellite operating at a higher altitude (say, 300 km) could operate with a surplus of electrical power: enough to potentially run a Propulsive Fluid Accumulator system and generate enough extra Thrust to counteract the extra Drag it and some extra solar panels would generate...

Edited March 26, 2018 by Northstar1989

[Northstar1989]

1 hour ago, YNM said:

You're aware of the other topic right ? This is quite a different direction than what's there so far though.

Not seen that thread before, no.  But I'm not surprised nobody there brought up Propulsive Fluid Accumulators.  The only other person I know of who seems to be as obsessed as I am with the idea of low-cost access to space is Elon Musk, and well, he doesn't exactly haunt these forums...

Most people don't even know what a Propulsive Fluid Accumulator is (or what a Mass Driver assisted launch would look like, or how a Microwave Beamed Power system might work, or how an Airship-to-Orbit system might use electric thrusters to reach orbital velocities in the upper atmosphere, for that matter...)

Speaking of which, I bet this system could really benefit the Airship-to-Orbit concept as well...

https://en.m.wikipedia.org/wiki/JP_Aerospace

Instead of ion thrusters relying on internal propellant reserves, this ESA electric air thruster would allow the Airship to operate off the surrounding atmosphere- potentially drastically reducing the size of airship that would be required to attain neutral buoyancy with a given sized payload...

[Rakaydos]

Is the atmospere at 200 km the same proportions as lower, or is there gravity-separation?

If it's the same, you're collecting 78% nitrogen and 21% oxygen. The oxygen is useful, but the nitrogen tkes a lot of processing to become usful nitrogen compounds, and can probably be tossed. The question is whether your PFA can split different types of gasses in orbit, or whether it justcompresses them and lets another vessel handle separation.

[Northstar1989]

---

Also interesting: I bet this ESA-developed air electric thruster would be darn useful for the JP Aerospace "Airship-to-Orbit" project...

https://en.m.wikipedia.org/wiki/JP_Aerospace

Instead of the Ascender stage needing to carry considerable supplies of onboard propellant for ion thrusters, an airbreathing electric thruster would allow the use of the surrounding upper atmosphere as propellant: potentially significantly reducing the mass requirements for propulsion...

---

Some interesting background on airships:

Airships grow more efficient as they become larger.

For instance, an airship that grows 8-fold in volume (by doubling its length), assuming an isometric relationship between structural mass and surface area, cuts in half the structural mass per square meter of volume (as Surface Area grows with the 2nd power of length for a given shape, but volume with the third power of length, in yet another example of the Square-Cube Law), and frees up mass for purposes like propulsion and payload...

So, for instance, the current JP Aerospace design is a 1.8 km long airship... (by the way, interesting article discussing it below)

http://www.science20.com/robert_walker/can_giant_airships_accelerate_to_orbit_jp_aerospaces_idea-225058

Let's make up some numbers to illustrate how its performance might change with scaling...

If the 1.8 km design weighed 16 metric tons in structural mass and required 10 tons of propellant and engines to bring its 30 ton payload to orbit, what would happen to a half-sized design if you cut the propulsion/fuel system mass in half?

Well, a design with half the dimensions (900 meters long) and isometric scaling of the structural mass based on Surface Area would weigh 4 tons in structural mass and require around 1.25 tons of propellant and engines (assuming the same proportion of total vehicle mass for propulsion as with the 1800 meter design) to bring a maximum payload of 1.75 tons to orbit (1/8th the volume but 1/4th the structural mass leads to a much lower payload fraction...)

On the other hand, assuming isometric scaling of structural mass based on Surface Area, a double-sized (3.6 km long) design would have a structural mass of 64 tons and require 80 tons of propellant and engines to bring a 304 ton payload to orbit...

Therefore, and this is the interesting bit relevant to Electric Air Thrusters, smaller airships operate on much thinner margins than larger ones, and if Air-Electric Thruster saved the 900 meter design just 500 kg on propulsion system mass, it would increase the payload mass by more than 28% to 2250 kg...

 

Consequently, Air-Electric Thrusters might bring down the minimum size of an Airship-to-Orbit design significantly- perhaps allowing for much earlier development of a functional prototype carrying a tiny payload as proof-of-concept...