Microthrusters For Small Satellites 75
An anonymous reader writes "A research team led by Paulo Lozano at MIT's Space Propulsion Lab and Microsystems Technology Lab have shown off a microthruster array capable of powering small satellites. The tiny, flat design could obviate the need for bulky propellant tanks. 'To explain how the thruster works, Lozano invokes the analogy of a tree: Water from the ground is pulled up a tree through a succession of smaller and smaller pores, first in the roots, then up the trunk, and finally through the leaves, where sunshine evaporates the water as gas. Lozano's microthruster works by a similar capillary action: Each layer of metal contains smaller and smaller pores, which passively suck the ionic liquid up through the chip, to the tops of the metallic tips. The group engineered a gold-coated plate over the chip, then applied a voltage, generating an electric field between the plate and the thruster's tips. In response, beams of ions escaped the tips, creating a thrust. The researchers found that an array of 500 tips produces 50 micronewtons of force — an amount of thrust that, on Earth, could only support a small shred of paper. But in zero-gravity space, this tiny force would be enough to propel a two-pound satellite.'"
Satellites in zero-gravity? (Score:4, Interesting)
Re:Satellites in zero-gravity? (Score:4, Informative)
"zero-gravity" is usually used synonymously to mean effective weightlessness, neglecting tidal effects.
So, yes, there is plenty of gravity acting upon satellites in orbits, but they are in free fall, so there isn't a significant gravitational force experienced by the components of the satellite due to their accelerating reference frame. Thanks to our somewhat sloppy terminology, this is zero-gravity.
Re: (Score:1)
'zero-gravity" No such thing.
"microgravity"
Re: (Score:2)
If we're going to be pendantic though, is that really much better a term for satellites? Sure, something as far out as the Moon is only experiencing ~0.0003g due to Earth, and that probably qualifies (as milli-gravity at least...). But a satellite in LEO is less than 30% further from Earth's center of mass than we are, and will be experiencing over 0.5g, which I think clearly does not.
Personally weightlessness gets my vote since weight is a sloppy term to begin with, a measurement of mass-scaled gravitati
Re: (Score:2)
God only knows, and He's not telling 'cause he's got a sweet zero-G spa set up and doesn't want to have to start clearing out vermin.
On a more serious note, it's at least theoretically possible there are points (geometric of course) in the universe where the sum total of all gravitational forces is exactly zero. Somewhere near the epicenter of the Big Bang would be one likely spot to look.
Re:I'm in a nitpicking mood. (Score:4, Informative)
Not really - ion propulsion trades "energy squared" for "propellant flow rate". So the large fuel tank (100 kg, say) has been replaced with a much smaller one, around 1 kg, for the same thrust. Here's the trade details:
1. Trading a 350 Isp Hydrazine thruster for a 35,000 Isp ion drive - it uses 1/100th the propellant for the same total impulse (thrust*time)
2. The thrust decreases dramatically typically - but for small satellite station-keeping that doesn't matter
3. The power requirement/N thrust gets ridiculous - from about 3.5KW/N to 350KW/N
Nanotube Array (Score:2)
Actually, I'd been wondering if an array of large-diameter nanotubes could be used to eject ions faster and more efficiently.
Re: (Score:2)
Thank You, Captain Obviate (Score:2)
But this thruster seems to be just a new variant on the ion thruster, albeit in a compact form
Re: (Score:2)
If the analogy doesn't fit, you must (shut the hell up)
2 pounds in zero-gravity? (Score:5, Funny)
How large, in zero-gravity, is a 2 pound satellite?
Re: (Score:2)
$7.23
Re: (Score:2)
Thank you. :)
Re: (Score:1)
I think it is as large as a comparable satellite in normal gravity.
Re: (Score:2)
if you had a sense of humour, you'd be familiar with the sound a 2 pound satellite makes over your head (even in a low earth orbit)
Re: (Score:1)
you know what's funny, it's how stupid you are while thinking that you have said something intelligent or insightful.
There is pound-mass and there is pound-force, but that is irrelevant even, because the joke had nothing to do with it, so you are also very dim.
Re: (Score:1)
It depends Is 2 pounds the satellites mass or its weight?
Re: (Score:2)
I'd say that it depends on if the satellite is made out of 2 pounds of feathers or 2 pounds of lead.
Re: (Score:2)
Weight vs.s. mass (Score:2, Informative)
Re: (Score:2)
So do you complain when people give their weight in kilograms instead of newtons?
Re: (Score:3)
Re:Weight vs.s. mass (Score:4, Informative)
http://en.wikipedia.org/wiki/United_States_customary_units#Units_of_mass [wikipedia.org]
The pound avoirdupois, which forms the basis of the U.S. customary system of mass, is defined as exactly 453.59237 grams by agreement between the U.S., the U.K. and other English-speaking countries in 1959. Other units of mass are defined in terms of it.
The avoirdupois pound is legally defined as a measure of mass[15], but the name pound is also applied to measures of force. For instance, in many contexts, the pound avoirdupois is used as a unit of mass, but in some contexts, the term "pound" is used to refer to "pound-force". The slug is another unit of mass derived from pound-force.
Re: (Score:2)
A satelliote with a MASS of 2 pounds is fairly small though.
Pound can refer to the pound avoirdupois, a unit of mass OR to a unit of force. So which one do you figure is more applicable in free fall?
Re: (Score:1)
A satelliote with a MASS of 2 pounds is fairly small though.
That all depends on what the satellite is made of. An aerogel satellite would be 0.907 cubic metres. If it's made out of lead... not so much.
Re: (Score:2)
From the summary (and the article):
But in zero-gravity space, this tiny force would be enough to propel a two-pound satellite.
Its doesn't say "weigh" two pounds, its says a two-pound satellite. In that context, it pretty easy to see that the implication is its a 2 lbm satellite. Its slightly ambiguous, but not wrong. I don't know why its hyphenated. Any by some magical coincidence, it would happen to weigh 2 lbf pretty much anywhere on the surface of the Earth.
Also, this isn't from the text of peer reviewed, journal quality technical paper. It's an article trying to related cutting edge res
Re: (Score:2)
The word "pound" is used for both weight and mass. One pound-force (lbf) is the weight of an object whose mass is one pound-mass (lbm) under standard gravity. In popular usage, they're both just called pounds.
The same situation exists in the SI (metric) system: one kilogram-force (kgf) is the weight of an object whose mass is one kilogram (kg) under standard gravity.
Is it confusing? Yes. Does it excuse public ignorance of science? Yes. Is it going away? Not bloody likely. Live with it.
Re: (Score:2)
I am a mechanical engineer and this is how I keep it straight in my head.
Just remember F=m a
SI is easy.
N = kg*m/s^2
English is trickier you need a constant
1 lbf = 1lbm * 32.2 ft/s^2
so the constant gc is 32.2 lbm*ft/lbf*s^2
F=m a / gc
The slug is used to get rid of the constant
1lbf = 1 slug * 1 ft/s^2
Hooray for meaningless "facts"! (Score:2)
But in zero-gravity space, this tiny force would be enough to propel a two-pound satellite.
Or a one-pound satellite. Or a ten-pound satellite. Or a hundred-kilogram satellite. Or a planet.
Re:Hooray for meaningless "facts"! (Score:4, Informative)
What is the specific impulse? (Score:3, Insightful)
There are three factors that are important for any propulsion system:
1) Thrust - check
2) Wheight - probably very low, hopefully not too important
3) Specific impulse - how much fuel do you need to get that thrust? The higher the velocity of the exhaust, the less fuel you need for a given thrust. And that is exactly what is missing from all sources. Who knows what they had to compromise with in order to scale the whole thing down?
Mod parent up (Score:3)
Thank you for being the first person to post something useful and constructive on the thread. Shame I had to scroll through the usual pedantic know-it-all minutiae about units and mass and weight to get to it.
I swear, /. is going severely downhill in the quality of discussion these days.
Re: (Score:2)
It went downhill far enough that somebody moded the "mod parent up" post up, but neglected to mod the parent up ...
Anyway ... unfortunately my questions still wants an answer.
Re: (Score:1)
Also: Power requirements (covered vaguely in TFA), efficiency, and thermal requirements, since this is an electric ion thruster. There are a lot of SEP designs, although I don't recall any others quite this small.
What I also didn't see in the article is whether they have or need some sort of MEMs equivalent of valves... will the ionic fluid boil off in vacuum without the voltage applied? At what rate? Does the ionic fluid degrade with storage, and will it clog the capillaries like an inkjet printer? How big
Re: (Score:2)
Also: Power requirements (covered vaguely in TFA), efficiency, and thermal requirements, since this is an electric ion thruster. There are a lot of SEP designs, although I don't recall any others quite this small.
What I also didn't see in the article is whether they have or need some sort of MEMs equivalent of valves... will the ionic fluid boil off in vacuum without the voltage applied? At what rate? Does the ionic fluid degrade with storage, and will it clog the capillaries like an inkjet printer? How big are the pores, and are they sensitive to cosmic radiation like silicon?
-R C
Forgot to log in. Ah well.
-R C
Re: (Score:1)
This was presented at the Small Satellite conference in Logan, UT, and I was in attendance. The Isp (specific impulse) was on the order of 2000-3000 seconds. It's basically a new way of doing a standard ion engine, only with less dry mass and less power; it's a really good thing for applications where that low a thrust is tolerable. It also has issues with how much propellant can be stored, as of right now, putting a definite limit on usable lifetime (though this wasn't specified). Another drawback is the n
Re: (Score:2)
Thanks.
Well, depending on how long the actual lifetime will be and what the propellant storage looks like, this seems like it's just what the doctor ordered for cube sats. But beyond that, they will need a way to recharge those capillaries and they'll need to mass-produce those things to get the costs down - which is kind of hard given the limited numbers of cube sats currently being launched. (Afaik, there are still fewer cube sats being launched each year than regular ones ...)
Slashdotters are not rocket scientists... (Score:2, Informative)
In the torque-free environment of space, any amount of force will overcome inertia in an appreciable way. Gravity gradients, solar pressure, lunar and solar gravitational influences, atmospheric drag.
All they're saying is that 50 microNewtons is about the right size for small satellites, regardless of how many pounds-force that satellite might be at sea level. More than likely this is intended for attitude control maneuvers rather than orbit changes, but even in the latter case, small thrusts for long perio
TAANSTAFL (Score:2)
"The tiny, flat design could obviate the need for bulky propellant tanks."
And instead, replaces them with bulky electrical power sources. There's no free lunch - whether the thrust comes from electricity or from reacting chemicals, it's got to come from somewhere.
Though, if you actually read TFA, you find there actually is a "fuel tank" built into the thruster... But, as the tank is very small, means a very limited lifetime.
Re: (Score:2)
These satellites don't have to carry the sun.
Re: (Score:2)
Solar panels are still heavy and bulky. Duh.
Re: (Score:2)
Nuclear power sources can be extremely high power, for relatively little size and weight. Hence, nuclear submarines.
NASA is upgrading RTG to more efficient Stirling radioisotope generators (SRG), and full-fledged nuclear reactors are sometimes used, with an even better power to weight ratio.
Re: (Score:2)
Maybe you should actually look up the weight of some actual spaceborne nuclear power sources, they're quite heavy for any significant power. (Curiosity's weigh over 100lb each, with an output on only 150 watts.) Not to mention that none exist down in the size range of the satellites this thruster system is designed for.
Re: (Score:2)
Okay!
SNAP-10A: Output 30 kW; weight 650 lbs.
That's completely inaccurate.
Nuclear power sources are just about as cheap as solar panels. Much cheaper than hauling up lots of fuel.
Re: (Score:2)
That's 30kw thermal - only about 500w electrical. Learn to read you moron.
OK, you have me there, it's been a while since I looked into them.
Not wrong on both counts...
I want to know one thing: (Score:2)
Specific Impulse. Discussing a new thruster without giving Isp is about as useful as announcing a "revolutionary networking technology" without giving a bits per second figure: interesting hack, but is it vaguely practical?
Re: (Score:2)
Sweet! That's completely respectable for such a tiny and lightweight design. It's good enough to make maneuvering possible in tiny satellites, and perhaps make stationkeeping more efficient in moderate sized ones assuming it's reliable and has a good service life.