Author Topic: High end electric/ion engines - current status?  (Read 19107 times)

Offline vda

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High end electric/ion engines - current status?
« on: 02/12/2007 06:18 am »
Hi,

So far almost all Solar System missions were using chemical engines, and frankly, they are far too inefficient for the task. Nine years to Pluto? Come on... Exploring/settling solar system using this technology will take centuries...

DS1 was a successful technology demonstrator for higher-ISP engine. What's next? Are there any plans to use something "bigger"?
Huh, probably my question should be separated into two:

1. What are theoretical performance limits of electric/ion engines (including powerplant)? For example,
 1a. If one puts 25 ton Pluto bound space probe in LEO, what reactor should be there (mass, type, etc). Propellant? Ion engine (which type)?
 1b. What are current estimates for maximum achievable T/W for "big" ion engine? Are they practical for relatively high-thrust tasks (e.g. orbit insertion at Neptune)? Imagine that you have "unlimited" amount of electricity (many megawatts). What type of ion engine will perform best? What propellant is best? (Re xenon: IIUC it's best for "small" probes like DS1 because it is using electricity most efficiently, but resulting ISP is rather low I guess?)

2. Is there any plans to actually build and fly something more advanced ("bigger") than DS1 in next 10-15 years?

Offline lambda0

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RE: High end electric/ion engines - current status?
« Reply #1 on: 02/12/2007 08:29 am »
Gridded ion engines, such as the one used on Deep Space 1, have some physical limitations that prevent to use them with high power : the fact that the plasma is not neutral between the grids introduce a limitation on the thrust density (about 1 N/mē). The gridded ion thruster of DS1 had a power of about 2 kW.
The Hall effect thruster is better from this point of view, it could be used for higher power, maybe up to a few hundreds of kW  (NASA is testing a 150 kW Hall thruster). Initially, gridded ion engines had a higher specific impulse (5000 s) than Hall thrusters, but the difference is shrinking.
Sending "big" probes to the outer planets require at least 100 kW : it was the level required for JIMO. This amount of power can be supplied by a nuclear generator; like the SAFE-400. JIMO was designed with gridded ion thrusters, using xenon. There are also some propositions to send smaller orbiters to Neptune, based on gridded thrusters, or Hall thrusters, with a power in the order of 10 kW, but the payload is less than 1 ton, and a flight time of about 15 years.

For megawatts level, both gridded ion thruster and Hall thrusters are not adapted : thrusters that accelerate a neutral plasma are superior  and allow high thrust density. This category includes the VASIMR, the MPDT (magnetoplasmadynamic thruster), the LiLFA (Lithium Lorentz force accelerator), but they require more R&D.
According to Chang-Diaz, from Ad Astra Rocket, a small 200 kW VASIMR should be ready for flight test in 2010. For the MPDT, lab tests have been done at high power : up to 30 MW, which is the level required for interplanetary manned flight.

Most of those development are still at lab level, but I think that the next ten years will be very interesting, as some of those concepts emerge as credible solutions, and also as a high power electric generators become available. However, I don't think that very high power electric propulsion required for manned flight (10 to 100 MW) will be available before 2030-2040. The applications in the next 10 years are mostly for probes, and maybe for sending cargo to the moon on slow economical trajectories.


Offline mong'

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RE: High end electric/ion engines - current status?
« Reply #2 on: 02/12/2007 11:08 am »
Quote
vda - 12/2/2007  8:18 AM

So far almost all Solar System missions were using chemical engines, and frankly, they are far too inefficient for the task. Nine years to Pluto? Come on... Exploring/settling solar system using this technology will take centuries...

problem with elctric propulsion is thrust, it is very low, a few milinewtons, it would take forever to accelerate any decent spacecraft (let' say a few tons)to earth escape velocity. and if you plan to send one to pluto it can't use solar power. that leaves only a small nuclear reactor as the power source, and those are heavy, reducing the acceleration of the spacecraft even more.

the only thing ion engine are good at is orbital manoeuvering. a couple of ion thrusters powered by a small nuclear power source would do wonders for an orbiter, especially around saturn /jupiter, it could slowly alter its orbit to get closer to every moon in those systems. the nuclear power source could also power more capable instruments and higher datarate communications.
but this probe should use good old chemical propulsion and planetary flybys to get to its target, it may take a few years but it will always be shorter(and safer since you'd have to turn-on your reactor for the whole duration of the flight) than with ion thrusters only.

there are more powerful forms of ion propulsion with higher ISP and moderate thrust, like the magnetoplama rockets but they need much bigger power source, several tens of megawatts. the mass of such reactors compared with their low to moderate thrust (10-100N) also plagues them with very low accleration

Offline Jim

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Re: High end electric/ion engines - current status?
« Reply #3 on: 02/12/2007 11:18 am »
The Dawn spacecraft is using an ion thruster

Offline meiza

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Re: High end electric/ion engines - current status?
« Reply #4 on: 02/12/2007 12:12 pm »
The biggest problem with ion engines is the power source.

If you look at JIMO, most of the mass was those huge radiators which are needed to cool the working fluid to extract energy from the reactor. That was partly because the operating temperature of the reactor was quite low.

Ion engines with ISP of 20,000 s have been lab demonstrated, but the high ISP only hurts (for now) because it takes more energy to accelerate to same speed. (The lower fuel consumption doesn't help as the fuel is a negligible mass anyway). Also the acceleration is slow.

It'd take a mission that used decades to accelerate to huge speeds, which only would have that high isp thruster as the optimal solution.

It is not sensible to analyze the propulsion systems with an assumption of "infinite power", when the choice of a propulsion system is rigidly linked to the power source. It's the same as comparing a ferrari and a lorry - either one is useless for the other's job, and you can't say which one is better without any context.

Offline vda

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Re: High end electric/ion engines - current status?
« Reply #5 on: 02/12/2007 01:47 pm »
Quote
meiza - 11/2/2007  2:12 PM
It is not sensible to analyze the propulsion systems with an assumption of "infinite power", when the choice of a propulsion system is rigidly linked to the power source. It's the same as comparing a ferrari and a lorry - either one is useless for the other's job, and you can't say which one is better without any context.

Ok, I can rephrase it. I am interested in high-power ion engines, ones which are capable of orbit insertion job. DS1 class engine is clearly far too feeble for that. Ok, so let's presume we have a ~1MW reactor (i.e. a lot of electricity). What type of engine can use that much electric current? What are expected thrust figures?

Of course this info will not be applicable to crafts with 10kW power source, I understand that.

Offline mong'

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Re: High end electric/ion engines - current status?
« Reply #6 on: 02/12/2007 02:30 pm »
I assume you mean 1 megawatt. that's low end MPD thrusters, and the thrust is pretty low, 100 newtons (22 lbs) at the most. a 1 MW reactor is going to weigh at least 30 tons.
it could perform interplanetary transfer manoeuvers albeit very slow ones (several years to reach mars)

Offline meiza

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Re: High end electric/ion engines - current status?
« Reply #7 on: 02/12/2007 02:30 pm »
It depends on the mass of the 1 MW (?) reactor and the power conversion equipment plus the required mission acceleration and delta vee and the size of the required payload. You could bolt on a lots of ion engines etc...
If you want to conduct a mission to the Oort cloud and need 100 km/s delta vee then you have no choice but to use high isp ion engines and it'll take a decade to just accelerate to the required velocity. Or something like that. Depends on power source a lot.
If you just want to go to Mars perhaps use vasimr or if to the moon then hall thrusters. Depends on the power source mass again.

For short trips and when in a hurry, use low ISP. For long trips or if you have a lot of time, use high ISP. If you have huge amounts of cheap and light power, use higher isp.

Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #8 on: 02/12/2007 09:34 pm »
Just for grins, converting 150 kW, that's 201 horsepower.  So that puts the present "big" ion engine at a power comparable to a Cessna 182.  Which is a big improvement over the last one I heard of, which was an Air Force experiment from the 1990's that was doing orbital maneuvers with less power than a Piper Cub.

In attempting to look up the power of a Saturn V first stage, I found "Its energy output was equivalent to that of 85 Hoover Dams, and its roar at liftoff equaled that of 8 million hi-fi sets (over 200 million watts). "

http://edition.cnn.com/interactive/space/9907/apollo11.saturn/sic.html

Come on, Dr. Bussard!  We need those 10 gigawatt reactors!

Offline meiza

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Re: High end electric/ion engines - current status?
« Reply #9 on: 02/12/2007 09:49 pm »
It's not the ion engine that produces the power. In a sense, it doesn't "have" any power, it only "uses" that 150 kW. It always needs an external power source. The ion engine could rather be seen not as a Cessna engine but as a Cessna propeller.

The very real questions are ~always about the power sources and their masses.

Offline Tom Ligon

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Re: High end electric/ion engines - current status?
« Reply #10 on: 02/12/2007 10:11 pm »
Exactly.  Actually, I think ion motor is the correct term, for that reason.  But the overall system, including a reactor to provide the power, would qualify as an engine by the definition I'm familiar with.

In the early jet age, people always insisted on knowing the equivalent horsepower of jet engines since that was the traditional figure of merit for piston/prop aircraft.  Expressions of jet horsepower were always impressively higher, but the designers themselves preferred to stick to thrust.  That also persisted thru chemical rockets.

Now, with solar or nuclear power sources, we're getting back to converting electrical or thermal power and a reaction mass flow to thrust.

I find occasional lapses to horsepower to be useful just to gain some perspective.  150 kW, or even 1 MW, is pretty pathetic compared to what we really want for Flash Gordon space travel.  Gigawatts start to get useful.

I once "designed" an air-breathing 4-seat jet called a Sunfire for a story.  I modeled it as having a 40 MWe fusion power source, based on a particular subsonic jet fighter's power output (55,000 HP).  By SF magic and some ion-propulsion hand-waving, this little aircraft was the same weight as the fighter, and hit the same speed at the operating altitude of the original jet.  But this hypothetical machine could continue to operate in rarified atmosphere to the fringes of space, then switch to on-board water to continue to LEO and circularize.  I constructed a thrust-lift-drag-weight model of this which ran on a Radio Shack Model III (this was back in the distant past when computers used Roman numerals).  The little jet could reach orbital velocity and 75 miles of altitude after going about 3/4 of the way around the equator.  Acceleration was pretty pathetic toward the end, though.

Offline mong'

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Re: High end electric/ion engines - current status?
« Reply #11 on: 02/12/2007 10:26 pm »
Quote
meiza - 12/2/2007  11:49 PM

It's not the ion engine that produces the power. In a sense, it doesn't "have" any power, it only "uses" that 150 kW. It always needs an external power source. The ion engine could rather be seen not as a Cessna engine but as a Cessna propeller.

The very real questions are ~always about the power sources and their masses.

the way I see it personnally is that chemical rocket engines are both an engine and a power source combined in one device, the power source being the propellant itself. in the strict sense of the term, the engine (i.e: the device that transform energy into thrust for motion) is actually the nozzle, just like the ion engine transforms electricity into thrust (using propellant)

Offline scienceguy

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RE: High end electric/ion engines - current status?
« Reply #12 on: 02/13/2007 02:40 am »
Let's just speculate that sometime in the future we have super efficient helium-3 fusion and the protons produced can be used to generate electricity directly.

Could such an apparatus be light enough and yet generate enough power for something like a VASIMR?
e^(pi*i) = -1

Offline wingod

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Re: High end electric/ion engines - current status?
« Reply #13 on: 02/13/2007 04:33 am »
Quote
vda - 12/2/2007  8:47 AM

Quote
meiza - 11/2/2007  2:12 PM
It is not sensible to analyze the propulsion systems with an assumption of "infinite power", when the choice of a propulsion system is rigidly linked to the power source. It's the same as comparing a ferrari and a lorry - either one is useless for the other's job, and you can't say which one is better without any context.

Ok, I can rephrase it. I am interested in high-power ion engines, ones which are capable of orbit insertion job. DS1 class engine is clearly far too feeble for that. Ok, so let's presume we have a ~1MW reactor (i.e. a lot of electricity). What type of engine can use that much electric current? What are expected thrust figures?

Of course this info will not be applicable to crafts with 10kW power source, I understand that.

When we did our 500 kW Hall thruster system (about 21-25 newtons) the dry mass was right at 10 metric tons.

We looked at a hybrid system for Jimo that would have used a Solar Electric Hall Thruster powered tug to get Jimo out to Mars and then let it go on to Jupiter.  I would have worked much better than jimo by itself.  In the end JIMO was its own worst enemy.


Offline lambda0

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Re: High end electric/ion engines - current status?
« Reply #14 on: 02/13/2007 09:08 am »
Quote
When we did our 500 kW Hall thruster system (about 21-25 newtons) the dry mass was right at 10 metric tons.

Hello

I didn't know that a 500 kW Hall thruster have been developed. Was it the power of a single thruster or for an array ? I thought that the most powerful Hall thruster being tested now was 150 kW (the NASA-1000M at Glenn Research Center).
Do you have some doc about this 500 kW thruster ?


Offline vda

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Re: High end electric/ion engines - current status?
« Reply #15 on: 02/13/2007 10:54 am »
Quote
meiza - 11/2/2007  4:30 PM
It depends on the mass of the 1 MW (?) reactor and the power conversion equipment plus the required mission acceleration and delta vee and the size of the required payload. You could bolt on a lots of ion engines etc...

Trying to collect some data on Internet and do basic math:
We can build fission reactors at 2.2kW per kilogram of reactor mass [src: http://en.wikipedia.org/wiki/Nuclear_photonic_rocket]
Probably the most energy-efficient use is to directly expell fission fragments. [http://en.wikipedia.org/wiki/Fission-fragment_rocket]

Reactor power: 1MW
Reactor mass: ~500kg
Exhaust, m/s: 1000000 (ISP ~100000)
Exhaust, kg per sec: 0.000002 (2 mg)
Thrust: 2N
Acceleration: 0.004 m/s^2 (4mm/s^2)
T/W ratio: 0.0004

Doesn't look that bad at all. Questions:
1. How far is this stretched from really achievable parameters? (really achievable ISP? how much of energy will be absorbed as heat instead of being expelled as exhaust? do we need radiators and how much they will weigh? etc)
2. Fission fragment rocket engine was never built IIRC, even as a small scale prototype. We only ever tried other, less 'direct' nuclear-to-exhaust convertors - I mean, reactor -> electricity -> ion engine.

[http://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster]
Let's say we use MPD. ISP is ten times lower, so exhaust mass should go up x100 in order to use the same amount of energy.

Reactor power: 1MW
Reactor mass: ~500kg
Exhaust, m/s: 100000 (ISP ~10000)
Exhaust, kg per sec: 0.0002 (200 mg)
Thrust: 20N
Acceleration: 0.04 m/s^2 (40mm/s^2)
T/W ratio: 0.004

Above numbers are incorrect because (a) conversion efficiency is not taken into account and (b) MPD thruster's own mass is assumed to be zero. I suppose I should divide those numbers at least by four in order to compare it to the first set of numbers (remembering that realistic numbers for both "engines" are even lower).

So far it doesn't look too bad - hey, 1 centimeter/second acceleration is not so feeble. In one month it gives you ~25 km/s delta-v (and will consume 500 kg of propellant, so... less than 25km/s :) , but still...).
Where am I wrong? How much lower are "realistic" numbers?

Offline lambda0

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Re: High end electric/ion engines - current status?
« Reply #16 on: 02/13/2007 11:30 am »
Quote
...
 we use MPD. ISP is ten times lower, so exhaust mass should go up x100 in order to use the same amount of energy.
Reactor power: 1MW
Reactor mass: ~500kg
Exhaust, m/s: 100000 (ISP ~10000)
Exhaust, kg per sec: 0.0002 (200 mg)
Thrust: 20 kg*m/s^2 (20N)
Acceleration: 0.04 m/s (40mm/s)
T/W ratio: 0.004

Above numbers are incorrect because (a) conversion efficiency is not taken into account and (b) MPD thruster's own mass is assumed to be zero. I suppose I should divide those numbers at least by four in order to compare it to the first set of numbers (remembering that realistic numbers for both "engines" are even lower).

So far it doesn't look too bad - hey, 1 centimeter/second acceleration is not so feeble. In one month it gives you ~25 km/s delta-v (and will consume 500 kg of propellant, so... less than 25km/s :) , but still...).
Where am I wrong? How much lower are "realistic" numbers?

This reactor mass is not realistic, I think it should be at least 10 or 20 times higher with current technology. And you cannot compare directly the mass of nuclear reactor that generates electricity to the mass of a nuclear thermal thruster (such as the fission fragments reactor) : the first one is much heavier because of all the equipment necessary to generate electricity.
However, with a high power electronuclear reactor (100-200 MW), with gaseous core and advanced MHD conversion system, it might be possible to obtain specific power of 2 or 3 kW/kg. With a 200 MW reactor of this kind, a VASIMR or MPD thruster could send a spacecraft to Mars in 2 months, even less.
But all this remains quite speculative, I don't think that this nuclear reactor exists for the moment.

Example of a high power vapor core reactor for electric propulsion :
http://www.inspi.ufl.edu/gcr.pdf
http://adsabs.harvard.edu/abs/2004AIPC..699..379K

Offline meiza

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Re: High end electric/ion engines - current status?
« Reply #17 on: 02/13/2007 12:21 pm »
Yeah. Nuclear reactors in space are not easy for generating electricity since you need big radiators. You don't need that on earth or if you use nuclear thermal rockets.
With current solar electric we're talking perhaps in the very rough ballpark of 100 W/kg.
There are a few threads already about these kinds of things

http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1139&start=1
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1218&start=1

Offline meiza

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Offline lambda0

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Re: High end electric/ion engines - current status?
« Reply #19 on: 02/13/2007 12:45 pm »
This should be interesting :
http://www.hapcos.org/DOCS/download.php?id=81&type=pdf

In fact, there may be another possibility : a few months ago, I made a few calculations for a manned Mars mission based on a MPD propulsion and a solar power plant. A solar power plant is generally considered to be more massive than the equivalent nuclear reactor, but some recent developments on thin films may change this and allow to build a very low mass power plant.
In this simulation, the specific mass of the power plant+PPU is 0.9 kg/kW, the thruster is a LiLFA which is a variant of the MPD that uses lithium as propellant : easier to store than hydrogen, that contributes also to reduce the structure mass of the spacecraft.
With a 100 MW power plant, I found that it may be possible to send a 40 t payload to Mars in about 110 days.
This calculation takes into account the efficiency, the mass of the thrusters,...
http://img504.imageshack.us/my.php?image=mpd5ug.gif
(it's in french, sorry, I didn't have time to translate).

That's very approximate calculations, not optimized, to be confirmed by more serious studies, but I suspect that there might exist "non-nuclear" solutions for short manned Mars missions : if the flight time can be reduced to no more than 100-110 days, it is possible to come back to Earth after a 1-2 months stay on Mars (which implies a 8-9 months mission), instead of waiting for the next conjonction (which implies an almost 3 years mission).
And of course, all that depends on the real performances of  those solar thin films...


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