Full success of first-ever cubesat mission equipped with Hall-effect propulsion | Hacker News Reader

Full success of first-ever cubesat mission equipped with Hall-effect propulsion

exotrail.com

119 pointssohkamyung51 comments

Worth mentioning is that this is not the first demo of a cubesat with electronic propulsion, among others MIT has flown an electrospray based thrusters on several cubesat missions, for example Aerocube 8 [1]. There are also larger missions planned, with claims that it should be possible to achieve roughly 1km/s of delta-V with their devices in a 3u cubesat.

[1] https://spacepropulsion.mit.edu/news/aerocube-8-cd-launch-mi...

> Exotrail’s nanosatellite thruster runs on 50 watts of power and is equivalent in volume to 2 liters of soda.

Don't you confuse it with 2 liters of milk!

Perhaps an American will confirm but I believe that 2L bottles of soda are one of the few common items sold in metric quantities in the US. Smaller soda bottles are measured in fl. oz. and milk in gallons.

Correct.

A writeup for a cubesat I once tangentially worked on measured the propellant in "shots" (IIRC multiples of 44mL), which I always thought was hilarious.

Americans will do anything to not use the metric system, lmao.

It’s a real pity that Americans don’t talk about how awesome our base-2 volume measurement system is. All the benefits of metric and it’s computer friendly. If our length, area, and mass systems had been suitably adjusted at the same time, that’d’ve been the bees knees!

Only a a subset of our volume measurements are base-2. There are 16 tablespoons in a cup, 2 cups to a pint, 2 pints to a quart, and 4 quarts to a gallon, plus a few lesser used units in between (for example, a "gill" is apparently a half-cup) which is all well and good. But then there are 3 teaspoons to a tablespoon, and an assortment of units larger than a gallon: 63 gallons in a hogshead; a barrel, or half-hogshead, with 31.5 gallons; alternatively 42 gallons in an oil barrel.

Personally I think the "best" US unit is Fahrenheit, though only when used for human purposes. Going from 0-100 takes you from very cold to very hot, roughly the range of reasonable temperatures for a human to live in. A bit outside that range is also survivable with a bit more careful preparation. A place which experiences temperatures from 0-100 across a year isn't outrageous. Compare to Celsius where 0-100 ranges from only somewhat cold to far beyond what humans can reasonable exist in. Fahrenheit temperatures are just a bit more friendly to work with in my biased opinion.

This is exactly my take on the metric system. I'm all-in, except for Fahrenheit for human-related things.

Along with the benefits you mentioned, the Celsius scale, being a scale, does not have most of the benefits that the rest of the metric system has. You don't take advantage of powers of ten (we don't talk about kilodegrees Celsius), and you don't do conversion to other units. For those things, you need Kelvin anyways.

Fahrenheit makes it really awkward to think about whether it's wet, snowy or slippery ice outside. Celcius is great for that, emphasis on the phase transitions of water.

I think that falls under the same temperature intuition that people will naturally get about whatever system they're using. Just like you have to memorize that 70F or 20C are roughly room temperature, you have to memorize that 32F or 0C are the freezing point of water. I'll admit that this is a tiny point in favor of Celsius, but is mostly won out because thinking in terms of 0 to 100 is more convenient than thinking in terms of roughly -18 to 38, and a "tens" of Fahrenheit (like "it'll be in the 70s today"), is considerably more informative.

My understanding of the 3U cubesats that these are intended to be used on was that they weren't much bigger than a 2 litre bottle. Am I way off in my understanding, or do these just use most of the available volume?

It's simple, really. A 1U cubesat is defined as a 10 cm by 10 cm by 10 cm device, i.e. 1 litre in volume.

A volume of 2 litres therefore implies a 2U unit, in other words 20x10x10 cm (the actual dimension of the 20 cm part can be chosen arbitrarily).

Ah didn't realise the dimensions were quite that small! That does mean that this takes up 2/3rds of the cubesat, which sounds like it would be impractical for many purposes.

This one flew on a 6U (~35x20x10) and that's really the minimum size for this kind of engines. And even on a 6U, the power limitations make us think twice before putting an engine, it really has to be needed.

What does this even mean? DeltaV would have been much more understandable.

This comment isn't trying to convey the capability, it's talking about the physical size. Perhaps you want to know about the DeltaV, and perhaps at some point that should have been mentioned, but at this point, that's not what they're talking about.

You cannot really talk about deltaV capability for a thruster. You can talk about specific impulse, power usage, power dissipation, total impulse. But deltaV implies you know the size of the satellite, which varies for all their customers.

>superior efficiency compared to other electric propulsion technologies

That seems... unsupported.

They're products have ISPs of 800 and 1000. By contrast Dawn's gridded ion thrusters got 3,100. Comparing to other cubesat thrusters, Accion's electrospray thrusters get 1650.

Not sure what was meant in the article, but Isp is not the same as efficiency. Also, high Isp is not desirable for a power-limited spacecraft such as cubesats because the input power scales with the square of Isp.

The efficiency of an electric thruster is a measure of the losses related to ionization energy, beam divergence, beam temperature etc. In theory Isp and efficiency are therefore orthogonal, though Low-Isp devices indeed tend to have a lower efficiency due to higher ionization-to-kinetic energy ratio.

Isp is an efficiency, but it is specifically efficiency with respect to fuel usage. As this has commonly been the main measure of non-electric propulsion it is still valid to invite the comparison as part of the goal is still reducing mass by reducing fuel usage.

It does seem the efficiency they are referring to is efficiency in ionization.

And I agree it is a bit odd that they bring up the claim of "superior efficiency" and quote the power usage of the thruster (50W) without any notes about the actual thrust capability that it provides.

Input power can be plentiful with modern solar cells, and Sun just keeps on providing 1 kW / m² in optical emission.

Propellant, OTOH, is finite; once you use it up, you cannot maneuver any more. Solar cells may degrade, but keep providing power for decades, if not centuries.

So a long-lasting satellite which just needs to keep its orbit may be better off with a low-thrust engine not ejecting any reaction mass. A long-range mission with a finite travel time towards the outer reaches of the Solar system may be better off with a high-Isp jet engine.

So efficiency is always going to be some A divided by some B. Traditionall with rocket engines it means impulse divided by propellant mass expended. But given that there are both electric thrusters with much higher ISP and electric thrusters with much lower ISP (e.g. resistojets) I'm not seeing any reasonable efficiency metric under which I think the claim is plausible. Even in terms of losses electric thrusters that don't have to ionize their propellant have a pretty big advantage.

I am not sure I understand the objection (or maybe there wasn't one?) but since there seems to be some confusion in this thread on this point, the electrical efficiency of electric thrusters is very well defined, it is:

η = ½Q·(Isp·g₀)² / P

where Q is the mass flow rate (kg/s) and P is the input electrical power.

Hall thrusters have efficiencies typically ranging from 30% (small HTs) to 60% (large HTs). Gridded ion thrusters have slightly better efficiencies (add 5-10%), mainly because they operate at higher Isp (they would actually have worse efficiency than Hall thrusters when operating in the 1000-1500s range).

Why I object to the (admittedly common) characterization of Isp as an efficiency: it is always better to have a high efficiency, whereas Isp is always a trade-off between mass budget and power budget so optimal Isp is always mission-dependent.

Every time I see news about a new kind of space thruster, especially "electric" one, I'm a little hopeful that it would not require carrying propellant on board and then shooting it out the nozzle.

I've resigned myself to the fact that we are more likely going to have a The Expanse space future than a Star Trek one but I don't think it's too bad either. Because if I understand it correctly those type (not exactl that one) of thrusters propel "stuff" out of the engine not by combustion but by in this case ionizing it and pushing/pulling it along the thruster.

That way you can create higher thrust by increasing the speed you shoot it out without the need to increase propellant and it's not something that needs to be refined but you could gather it along the way.

So while it's not a reactionless thruster but it's main concern is still more on the energy side than on the reaction mass side

Electromagnetic tethers can do this, in an environment with a magnetic field.

https://en.wikipedia.org/wiki/Space_tether

How far away from earth can the operate? Do these only work in low earth orbit or higher orbits too?

This would be a cool concept for a solar powered space tug. Just goes to leo and picks up a payload and boosts to geo. Then comes back for the next one.

No consumables at all. Just a big solar array and giant wire.

GEO is well within the Earth's magnetosphere, so the question is more "how well does it work at GEO" and not whether it works at all.

AFAIK, nobody has managed to make it work well enough to use it at LEO, so nobody will give you an answer to that.

Given that no one is really trying to use them, I think there's some aspect that make them not very practical, like for example the weight required is so high that they can't counteract atmospheric drag.

Also you linked the wrong article. https://en.wikipedia.org/wiki/Electrodynamic_tether

That violates physics and is impossible, so you shouldn't be hopeful for that.

Instead you should be looking for new improvements on efficiency (exhaust velocity), cost, and thrust-to-weight-ratio. Continued marginal improvements on those things (and there's a lot to improve upon) causes massive changes in the future.

That violates physics as we know it, but there might be effects we don't know about just yet.

Marginal improvements are boring. And besides, having to carry propellant is a significant limitation to the travel distance and one of the biggest nuisances of spaceflight. If you only needed electricity or other form of energy, but not matter, it would be much, much easier.

Sorry, I can't keep track - is this the thruster that everyone said was fake, or is this a different one?

Nope, this is real thruster and is (somewhat) commonly used. Their claim is usage on smaller spacecraft. You may be thinking of the VASIMR thruster which while it is undergoing testing of initial versions is often thrown around as the cure-all for any potential space mission.

"How are we going to get humans to Mars with current thruster technology?" "Oh it'll be a lot easier when the VASIMR drive is completed"

https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magn...

https://en.wikipedia.org/wiki/EmDrive I think this was the one thought to be fake/ineffective.

As far as I can tell, this is basically an ion thruster, though I've never heard ion thrusters called Hall-effect thrusters before, so this may be a somewhat atypical sort of them. It ionizes gas to propel itself. The EM drive was basically a microwave in a sealed container that was hoped/theorized to create thrust through an unspecified/unknown mechanism, with no reaction mass coming out the back of the craft.

Compare https://en.wikipedia.org/wiki/Hall-effect_thruster to https://en.wikipedia.org/wiki/EmDrive

The Hall-effect thruster is a specific type of ion thruster. By far I am not an expect, but the Hall-effect is has something specifically to due with the movement of electrons creating a current in the thruster, which I believe causes the ionization. Others ionize the propellant by bombarding it with electrons to create the ionization.

And as another commenter pointed out, this is not the first ion thruster on a cubesat.

Don't Starlink sats use hall thrusters as well?

Starlink satellites are MUCH larger than cubesats, which are usually 1.4kg per U and usually about 3U. A Starlink satellite is about 150kg and nowhere close to the cubesat form factor.

>ExoMG™ is the first ever Hall-effect thruster operating on a sub-100kg spacecraft.

Small size definitely seems to be the main claim in this.

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Hey, here is our previous newsletters: https://us17.campaign-archive.com/home/?u=9c1db628330a8f7126...

Probably same engine as one on Dawn mission. From total dry mass of 750kg it had 30sq.m of solar panels and lots of other stuff. https://en.m.wikipedia.org/wiki/Dawn_(spacecraft)

How much ΔV do the sats get?

We'd have to know both the wet mass and the dry mass to figure that out. They mention 10 kg which I assume is the wet mass but don't give another mass number.

For electric thrusters on cubesats, you can basically ignore the mass variations for most high level calculations. Propellant mass is so little and the specific impulse so high that unless you are using all the propellant in one giant maneuver the difference will be minimal.

But it does not eject any mass, so it's all dry mass once in orbit, I assume.

Hall effect is pretty weak, though.

It does eject mass just like any other thruster. It is just a lot less mass ejected at a lot higher velocity which is why the efficiency is higher.

https://en.wikipedia.org/wiki/Hall-effect_thruster

The ionized xenon exhaust plume is certainly ejected mass. Unless I am misunderstanding what you mean.

No; I was just wrong.

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