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back to article NASA to flip ion engine's 'OFF' switch after brilliant 5.5 year burn

NASA's Evolutionary Xenon Thruster (NEXT), an ion-propulsion engine that has been firing continuously for five and one-half years, is due to be shut down at the end of this month. NASA's Evolutionary Xenon Thruster (NEXT) at the Glenn Research Center in Cleveland, Ohio The world's record holder for ion-propulsion longevity: …

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Mass of NEXT and power source ?

What is the mass of the NEXT engine and the required solar array (plus the xenon propellant) compared to the mass of a thruster and its propellant for the same total impulse ? (A quick calculation suggests that over 8kW of electrical power is needed which is a large array and the NEXT engine is large compared to a low output thruster.)

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Boffin

Re: Mass of NEXT and power source ?

A quick calculation suggests that over 8kW of electrical power is needed...

From the NASA press release:

"The 7-kilowatt class thruster could be used in a wide range of science missions..."

So, same order of magnitude.

Also from the press release:

"...the engine consumed about 1,918 pounds (870 kilograms) of xenon propellant, providing an amount of total impulse that would take more than 22,000 (10,000 kilograms) of conventional rocket propellant..."

So, the engine + powerplant mass could be up to 9,130 kilograms (20,086 pounds) more than the mass of a comparable rocket thruster and the entire system would still be less massive for the 5.5 year mission.

According to http://esto.nasa.gov/conferences/nstc2007/papers/Patterson_Michael_D10P3_NSTC-07-0014.pdf (Section III table II), a single thruster string is about 56.3kg. Add in the High Pressure Assembly and you get a total of 58.2kg. Call it 60, and there's still 9,070kg left over for the solar array.

According to http://www.asertti.org/events/fall/2011/presentations-workshop/Landis.pdfthe ISS solar arrays have a total mass of ~1,000kg, and according to http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html, they produce 84 kW.

Assume we need this extra solar panel mass because the probe is heading to the outer solar system, and we still have over 8,000kg of mass savings or additional payload available.

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Re: Mass of NEXT and power source ?

I couldn't see any reference to the thrust produced by the engine. Is it just my reading skills, or wasn't it mentioned?

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Go

Re: Mass of NEXT and power source ?

Did somebody just say "5 year mission"?

GJC

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Boffin

Re: Mass of NEXT and power source ?

Worth noting that when you get to your destination you've effectively got a 'free' power source to drive your science payload and communications gear, with chemical engines you still need a power supply for the instruments but the mass has to come out of the payload.

And for comparison, the Dawn spacecraft currently travelling between Vesta and Ceres runs its ion engine from a solar array that produced 10kW around Earth and about 1300W in the asteroid belt.

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Re: Mass of NEXT and power source ?

You're not the only one. I was idly wondering how many of these you would need to produce 1g of thrust. I expect some insanely large number, but just wanted to know to see what it would take to get "artificial gravity" along the axis of acceleration once in space.

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If you're going into deep space you'll be using nuclear power

If you're going into deep space you'll be using nuclear power, like Voyager 1 & 2, not a solar array.

So two new questions:

1. What would the required nuclear power generator (plus shielding) weight?

2. At what distance from the sun does it become cheaper to use nuclear than solar?

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Boffin

Re: If you're going into deep space you'll be using nuclear power

Voyager type RTGs won't do the job. At launch they only produce 470W from three units massing about 38kg each. Back of an envelope gives about 250kg for 1kW so a couple of tonnes to get the 8kW needed. A Russian TOPAZ gets closer, 300-1000kg for 5kW but at the low end there's no (or very minimal) shielding so you'd not want to put it on a long duration flight.

The cut-over between solar and nuclear power for payload operations is generally the asteroid belt, although the Juno mission currently on its way to Jupiter is solar powered due in part to there not being enough of the relevant Plutonium isotope to build the RTGs needed. The Curiosity rover had first call on it and even then there wasn't quite enough and a couple of instruments got dropped. There's a second Curiosity type rover due to go to Mars in a few years time, it looks like that will have to use solar panels as Spirit and Opportunity did.

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Boffin

Re: If you're going into deep space you'll be using nuclear power

As you get beyond Mars, RTG or Fission power becomes much more practical, Voyager 1&2, Cassini-Huygens, Galilieo. the Pioneers, New Horizons are all RTG powered.

The exceptions being Juno and Dawn, which are solar.

For 8Kw+ you're probably looking at a proper fission reactor as the mass of an RTG at that output would probably get too high and given the electrical conversion efficiency of around 3-7% you have over 100kW of heat to get rid of as a minimum and potentially up to 250kW!

As the heat can only be radiated, it makes cooling a significant challenge, it would also vastly increase weight.

NASA is (or was) working on a 40kW reactor for space, but I've not seen much news recently.

It's not just electrical power, out here with less sunlight, it is also relatively 'cooler' for the spacecraft, so they require more heating capability than something operating in the inner solar system.

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Rol
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Re: If you're going into deep space you'll be using nuclear power

If the craft used its solar panels in the vicinity of the Sun by doing an outwardly spiralling orbit, then with the addition of a gravity nudge from one of our larger neighbours it would have the craft up to speed before it had left the influence of the Sun behind.

Maybe?

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Mushroom

Re: Mass of NEXT and power source ?

It's likely for missions to the outer planets that NASA would use some form of radiothermal generator rather than solar panels. Cue the usual hysteria from the, er, "differently scientific".

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Rol
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Re: If you're going into deep space you'll be using nuclear power

The clever way to do this, is have Wall-e do baseball practise off the stern with the Higgs particles scavenged from the craft.

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Re: Mass of NEXT and power source ?

I couldn't see any reference to the thrust produced by the engine. Is it just my reading skills, or wasn't it mentioned?

Not in the article or press release. But the first paper I linked to has the maximum thrust per thruster at 236mN.

I was idly wondering how many of these you would need to produce 1g of thrust.

One g ~= 9.8 N per kg of mass. Going back to our 10,000kg example (yes I know it'd actually be more than 10,000kg but it's an easy starting point) that would be 98,000N of force needed. At 236mN per thruster, that would nominally be ~415,536 thrusters required to produce 1 g -- except, of course, that many thrusters will significantly increase the mass, significantly increasing the amount force required to reach 1 g, leading to a runaway situation which makes the conundrum of escaping Earth with a chemical rocket look like child's play.

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Boffin

Re: Mass of NEXT and power source ?

Actually, now that I think about it, there's no way these thrusters will ever reach 1g.

If we assume absolutely no payload, no fuel mass, and no power supply mass, each thruster masses 60kg and produces 235mN of thrust.

That's 0.000 401g. Adding more thrusters increases the mass and thrust linearly and proportionally, leaving the g-force constant.

So that's your absolute theoretical maximum g-force produced by any size bank of these thrusters.

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Anonymous Coward

you'd not want to put it on a long duration fligh

I'm not sure about that - the inverse square law means that you just need a long, rigid cable between the power source and the payload.

Is it thanks to the weapons program that there is too much of the wrong kind of plutonium about? Or did the US, for patriotic reasons, spend too much time trying to make americium RTGs work?

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Anonymous Coward

Re: Mass of NEXT and power source ?

I am reminded that when the isotope of plutonium used in RTGs was discovered, there was speculation that every household would have a thermal source to provide central heating, run the cooker and so on.

Growing up in the 50s and 60s it all seemed so optimistic. I was watching Tomorrow's World when Raymond Baxter explained why a handheld computer was technically impossible, but all the predictions on fusion power and the takeover in the next 20 years by fission reactors were still taken almost for granted - nobody was discouraged from using electric heaters because electricity would soon be almost free.

Clarke's Law duly intervened, and we have tiny handheld computers that could emulate a 60s mainframe with a higher throughput while making a phone call and posting to Facebook, but fission is now further in the future than it was in 1963.

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Meh

Once you get to top speed

Once you get to top speed.... How the hell do you stop?

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Alien

Re: If you're going into deep space you'll be using nuclear power

Or give Wall-E a pallet full of fire extinguishers.....

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Re: Once you get to top speed

Quite easily, flip the craft 180 degrees and your thruster becomes a brake. This is has been proposed as a means of moving humans through deep space, you accelerate the craft at around 1G until you achieve half-light speed then flip the craft around and decelerate at 1G, the advantage being that a similar gravity to the Earth is maintained throughout the vogage the avoiding muscle atrophy of the occupants.

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Re: If you're going into deep space you'll be using nuclear power

Nuclear is the answer, however not in the rtg form. The stirling engine is more efficient, and is said to be the answer (if perfected for radioisotope energy and years in deep space)

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Re: Mass of NEXT and power source ?

thrust is roughly nought-point-bugger all, but 24/7 after 5.5 years the area under the curve is pretty effing large

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Stirling (Re: If you're going into deep space you'll be using nuclear power)

Like this : http://youtu.be/3-eH1M0EcXA

(a piece of news of a small, simple reactor with stirling engine, developed at Los Alamos)

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Childcatcher

Re: Once you get to top speed

The answer to that is: SLOWLY.

I am still wondering how they get to 200,000km/h with a jet only doing 90,000km/h. Superman might be involved.

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Happy

Re: Once you get to top speed

"I am still wondering how they get to 200,000km/h with a jet only doing 90,000km/h. Superman might be involved."

Because the 200,000 km/h is relative to the speed of the start point. The 90,000 km/h is relative to the spacecraft.

Basics

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Rol
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Re: Once you get to top speed

So 200,000km/h being the limit, is based on the impact with the very thin spread of particles in space retarding acceleration?

In other words, the craft regardless of its thrust would accelerate to the speed of light in a total vacuum, it is only friction that sets a limit relative to the thrusters ability to overcome it.

My head hurts.

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Re: If you're going into deep space you'll be using nuclear power

Heat is useful; it can be used to help speed reaction mass on its way.

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Re: Mass of NEXT and power source ?

-- fission is now further in the future than it was in 1963. --

Due to human factors, rational and otherwise. Dioxin has no half life.

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Re: Once you get to top speed

To gain credence, Goddard had to show that rockets did not require something to push against to work.

quote: The prestigious New York Times dismissed Goddard's ideas and said that he didn't even possess an elementary knowledge of physics. The Times' editor incorrectly thought that rockets could not work in space. He thought the exhaust from the vehicle would have nothing to push against; he did not realize that the rocket exhaust would be acting against the inner walls of the rocket itself, :quote

http://scienceworld.wolfram.com/biography/Goddard.html

He also built ion rockets.

quote" From 1916 to 1917, Goddard built and tested experimental ion thrusters, which he thought might be used for propulsion in the near-vacuum conditions of outer space. The small glass engines he built were tested at atmospheric pressure, where they generated a stream of ionized air.[27] :quote

https://en.wikipedia.org/wiki/Robert_H._Goddard

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Perhaps this is the tech that takes us to Mars... since the 6 months journey is considered borderline too dangerous due to radiation.

Providing acceleration gets to 200K mph within a decent timeframe.

Exciting stuff though. For all the time it's took Voyager to reach the edge of the Solar System, at 200K mph it can be done within a decade.

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Nuclear propulsion

If you want to go to Mars in a month, an ion drive isn't going to help all that much as it just doesn't have enough thrust. You're going to have to bring a nuclear thermal or a nuclear electric (VASIMR, for example) rocket to get the required combination of thrust and impulse.

You could theoretically build an electric rocket using a VASIMR or similar technology powered by solar panels but that would require some pretty damned large solar panels with all the requisite expenses and mass

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Within 500 years someone will have collected one of the Voyager probes and bought it back to Earth

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Been there, done that, seen the movie

Or it comes back on its own accord. "Slightly" modified.

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WTF?

Re: Nuclear propulsion

VASIMR is not Nuclear, it's just a different type of Ion engine.

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Go

Re: Nuclear propulsion

One tiny detail ... the acceleration region develops a negative net charge because of the excess electrons not fired with their atoms, but there is a pretty simple solution :

Fire the excess electrons with another beam (an X ray beam, for example, like your old CRT tv).

That way, you can use the electrons to give some really tiny impulse to the craft. More than the impulse obtained, the main advantage will be the ability of not attracting the expelled ions with the spacecraft charge, that would otherwise provide a braking force ....

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Happy

Neat

It is really too bad the politicians keep undercutting NASA's budget. They really can get some neat things done when they are allowed to.

Also, that picture features someone who has never looked at that machine before. His face is all 'holy shit look at this thing, I wonder what it does'.

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Pint

Re: Neat

By the look of it, I believe that is the inside of the vacuum chamber so I'd wager that pic was taken at the start of the project and there is a very good possibility the gentleman in question was thinking something very like you describe only perhaps 'I wonder if it will work'.

With that, I pint to the NASA folks for another job well done.

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JLV
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>It is really too bad the politicians keep undercutting NASA's budget.

No, it is too bad that NASA's budget is only very slightly geared towards things like Kepler, Curiosity and Next.*

Most of the budget goes to manned spacecraft, i.e. the shuttle and ISS (75/25 iirc). If the Mars manned mission planning ever takes off, it will crowd out scientific research even more.

Think of what would have been achieved if the $100B of ISS had funded 40 Curiosity equivalents.

* to be fair, the politicians are still to blame, because they favor the manned aerospace lobby over real science.

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Re: >It is really too bad the politicians keep undercutting NASA's budget.

I agree, because I think our intentions should be science and discovery.

I feel there is too much money spent on the cowboy hero aspect and not enough on the science.

It is incredibly expensive to send humans into space, with all the incredible amount of shielding, food and energy we require.

If our intentions are to create American (Russian, etc.) heroes, then yes, the only way is with manned missions. Why create national heroes, and the answers might be to motivate youth in the nation concerned or it might be nationalism and prestige.

If we had unlimited money, I'd say do both and use international teams. But since it is one or the other, I say go for the science.

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Re: >It is really too bad the politicians keep undercutting NASA's budget.

Its worth doing both. As much as doing the hard science experiments is useful, and unmanned missions teach us much. Surely one day we ourselves want to reach for the stars .

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@Triggerfish

Absolutely! I grew up in the Apollo era, and as a seven-year-old nagged my parent's (successfully) to see the first moon landing in the middle of the night. Astronauts and cosmonauts were heroes that transcended national boundaries. They showed us we could reach beyond Earth. Almost every boy in my class wanted to follow Gagarin and Armstrong. I consider that a rather better aim in life than to become filthy rich in finance. If astronauts inspire a next generation of kids to reset their ambitions and aim for the stars (literally, not in the X-factor sense) it is money well spent.

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Re: Neat

Manned missions are fscking cool, and if the Space Race had carried on we'd be on Mars already.

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Happy

>Its worth doing both.

I won't downvote, since after all, everyone is entitled to their opinion.

But, currently we DON'T have the budget to do both. We wouldn't have the budget to do both if it was increased by 50%. You know that, I know that, we all know that.

That's the way it is and wishful thinking will not change that in the medium term (barring a Chinese space challenge leading to a fully-funded Apollo-type space race).

What we are doing is underfunding unmanned missions, and not advancing technology for manned missions all that much. For all the Apollo nostalgia, much of what goes into current manned missions would not be unfamiliar to Skylab and Saturn V folks. Our computers are better and we have the Shuttle to warn us about the perils of "reusable savings".

In order to put up manned missions and fulfill our dream to explore and colonize space, we need to upgrade our technologies, very significantly.

Things like NEXT, Dragon, _anything_ that brings down $/KG to orbit, space-based automated manufacturing (cracking O2 and H out of water in meteorites or the moon. Heck, even space elevators and blimps that launch rockets from near space. We also need to encourage experimentation by private enterprise - the Google guys extracting platinum out of asteroids and Virgin space hotels.

In coding equivalents, we are hand-writing in assembler right now. We need to come up with compilers to make things cheaper, otherwise there will ever only be a need for 5 computers in the world.

That's what gets me excited and that's what I believe should get a better piece of our limited budget pie.

Downvote away, folks :-)

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"If mankind ever wants to explore deep, deep space, ion propulsion and other solar-electric thrusters might better be described as a necessity – which is why work on ion propulsion has been underway at the Glenn Research Center since the 1950s."

Ironically though it's exactly this kind of work that is stopping us from leaving. So long as propulsion technology has the prospect of improving fast enough there will be no incentive to launch anything into deep space.

If we launched a probe with this ion-engine today at Proxima Centauri 4.2 lightyears it would take 14,000 years to reach the target. Obviously we won't do that though because aside from the time period being ridiculously long it's almost certain that by 2100 we'll be able to launch a much faster probe packed with far more advanced equipment that would get there first (it would only have to be about 1% faster) and so rendering the original launch pointless.

The danger is that if propulsion advances average out to about eg 2% gain in max speed per 30 years it could take us centuries (or longer) to feel it's worth sending anything into deep space which increases the amount of time we have to survive together in one place without blowing ourselves up.

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MrT
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Sad but true...

... but the bold move would be to send it anyway. If a later mission catches it up then pull over, clear the cans from the back seat and give it a lift. The time it's spent out there would make it useful to study, and if the faster future missions never get off the ground then it'll still serve the original purpose.

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Boffin

...propulsion advances average out to about eg 2% gain in max speed per 30 years it could take us centuries (or longer) to feel it's worth sending anything into deep space which increases the amount of time we have to survive together in one place without blowing ourselves up...

You can pick any figure you like to be an 'average'. But progress usually proceeds in jumps as new technologies get invented.

For instance, an average shows us gradually learning to fly from about 1800-2000. But in fact we did the base theoretical work from 1800-1850, developed the first practical machines around 1900, had a huge technology jump between 1930-1950, and have been refining things ever since...

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not really, there just is not a good electric power source, unless they make a mine perpetual generator for all extra system you need on a space craft

atom vacuums for hydrogen fuel instead of xenon gas http://en.wikipedia.org/wiki/Hydrogen_fuel

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i suspect some super condensed heat being pumped out and super cooled like a ramjet could push you along

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and if you build a perpetual generator that is started up with a small amount of solar power, all other electrical systems onboard are not classed as energy waste, the only energy that is being wasted is the small amount looping back to the motor on the generator

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And it's that same reason I'm still using my trusty Atari ST, if I started upgrading computers I'd never finish this level of Xenon 2...

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