VASIMR Ion Engine Could Cut Mars Trip To 39 Days 356
An anonymous reader writes "It would take about 39 days to reach Mars, compared to six months by conventional rocket power. 'This engine is in fact going to be tested on the International Space Station, launched about 2013,' astronaut Chris Hadfield said. The Variable Specific Impulse Magnetoplasma Rocket (VASIMR®) system encompasses three linked magnetic cells. The 'Plasma Source' cell involves the main injection of neutral gas (typically hydrogen, or other light gases) to be turned into plasma and the ionization subsystem. The 'RF Booster' cell acts as an amplifier to further energize the plasma to the desired temperature using electromagnetic waves. The 'Magnetic Nozzle' cell converts the energy of the plasma into directed motion and ultimately useful thrust."
No quite yet. (Score:2, Informative)
No stated in this article.
But I'm pretty sure the engine discussed will need to be roughly 100x more powerful to make that 39 day trip a reality.
Re:I'm dizzy. (Score:5, Informative)
Actually... (Score:5, Informative)
It already is. [wikipedia.org]
Re:"A new NASA rocket engine" (Score:5, Informative)
This started as a NASA project, at the Advanced Space Propulsion Laboratory [nasa.gov] at the Johnson Space Center.
Dr Franklin R. Chang Diaz (the other former astronaut involved, and not mantioned in this Canada-centric article) took the project to private industry in 2005 [adastrarocket.com]
Re:Can this be the primary engine of a space shutt (Score:4, Informative)
It can't manage a positive thrust to weight ratio in any mode, and in any case can only operate in a vacuum, so it would end up being launched from ground on top of a chemical rocket. In theory once in space you shouldn't need other types of engine.
Not in space (Score:2, Informative)
Other than perhaps getting the eco-hippies to shut up about lofting lots of highly enriched nuclear fuel.
From what I gathered from Googling, the only thing the "eco-hippies" have a problem with is when those nuclear reactors fall back to Earth - or when they're sunk during a nuclear submarine or ship accident.
I don't think anyone will have any problem launching a nuclear reactor into space other than the astronauts who are on board with it. And considering the long track record of such things, I don't think they will have a problem either.
Re:Primary power source? (Score:4, Informative)
Re:No quite yet. (Score:1, Informative)
There is no free lunch. VASIMR is not radically more efficient than a chemical rocket. Its advantage is that it can run off electricity. But the electricity available from solar panels is slight.
This particular VASIMR is an improvement because it can handle more power... more than solar panels could provide. It will require a nuclear powersource -- a fission plant, or a very very powerful RTG.
But if you are willing to heft a fission plant into orbit, then you could just use it as a conventional nuclear rocket (i.e. superheated steam).
Re:I'm dizzy. (Score:5, Informative)
Depends on how much he weighs. The drive provides a force of 0.5N. A typical car plus passengers masses around 1000kg. F=ma, so, 0.5 / 1000 gives him an acceleration of 0.0005 m/s/s (ignoring friction and air resistance). 800 miles is a little under 1,300 km, or 1,300,000m. Assuming a stationary start, and accelerating for the whole time, we get sqrt(2s/a), which is around 51,000 seconds, or around 14 hours. Of course, after that time he'd be going quite quickly, so he'd probably want to be slowing down for the second half of the journey which increases the total travel time to about 20 hours.
Ion drives are not (yet) fast. They provide a much lower acceleration than conventional rockets, which is why no one is talking about using them to get to orbit. They use a lot less propellant to produce this thrust than an equivalent chemical rocket though, which means that they can provide this thrust for longer. After 14 hours, the car would be going at 25m/s. Not particularly fast; a chemical rocket can get to that speed in a couple of seconds. After a week it would be going at over 300m/s, which is a lot more respectable.
Your distance from earth to mars looks sensible, and makes the average speed 16.3km/s. Assuming linear acceleration and deceleration (which is incredibly wrong when we're talking orbital mechanics, because this would be a transfer orbit so you'd actually be accelerating for most of it), that would mean that the top speed would be 32.6km/s and you'd spend half of the time accelerating to this speed and half slowing down from it. That gives a delta v of just under 0.02m/s/s, which means that either they have more than one ion engine on the craft, or they are using something that weighs a lot less than a car. At that acceleration it would take just over 3 hours to travel 800 miles, which is close to what the grandparent said. I'm not sure where you get your 1.7km/s/s from, but I I think you dropped a 'k' somewhere in your calculations.
Re:No quite yet. (Score:5, Informative)
The VASMIR can accelerate a neutral gas to much, much higher velocities than a steam driven rocket would. This means significantly more impulse per unit mass so the fuel weight would be dramatically reduced. Sure the reactor is heavy but it still packs a much higher power density than combustibles so all in all a VASMIR can pull off an excellent power to weight ratio for an extended acceleration.
Re:No quite yet. (Score:3, Informative)
There is no free lunch. VASIMR is not radically more efficient than a chemical rocket.
Yes. Yes it is. It's a very efficient Ion drive.
http://www.projectrho.com/rocket/rocket3c2.html [projectrho.com]
This particular VASIMR is an improvement because it can handle more power... more than solar panels could provide. It will require a nuclear powersource -- a fission plant, or a very very powerful RTG.
Solar panels can potentially provide the entire output of the sun in relevent wavelengths / the efficency of the solar panel... though to do that, it would need to encompass the sun.
I suspect the most likely power would be a nuclear battery (thermo-couples powered by the heat of radioactive decay), but there are many options.
But if you are willing to heft a fission plant into orbit, then you could just use it as a conventional nuclear rocket (i.e. superheated steam)
You don't get the same speed out of that reaction mass, so you don't have the same efficiency.
Re:No quite yet. (Score:2, Informative)
The Kzinti lesson.
Re:Not in space (Score:3, Informative)
Nah, when the subject of launching one into space came up decades ago, they opposed it completely, on the grounds that it might fall back to Earth.
I should also note that the reactor vessel of a ship's nuclear reactor isn't going to corrode to the point of allowing the contents out in less than many centuries.
Considering the way the word "nuclear" causes panic in the minds of many Greens, I'd guess you'd have a lot more problems than that if you tried to launch one.
Re:No quite yet. (Score:4, Informative)
Vasimr is capable of Specific Impulses of 5000+... Chemical Rockets have Specific Impulses of maybe 500 at most.
That's an order of magnitude difference.
Re:Tag as SLASHVERTISEMENT (Score:5, Informative)
Actually, I've found the latest VASIMR progress quite interesting, but that article seemed more intent on promoting Canada than feeding news. Heck, the ISS mission has been known since 2007.
A google search was also able to come up with an article with a lot more meat [nextbigfuture.com]. This explains that the project is working towards 200MW ion rockets (MUCH more powerful than the earlier .3kW), would be powered by a cheap nuke drive instead of solar panels, and they believe it's doable by 2020. Similar info is in PopSci this month.
Now if they could just get that dense plasma fusion device (see Slashdot [slashdot.org] yesterday) to power the craft instead of fission, that would be cool... yeah, I know I'm pipe dreaming again, but I can't help it.
Re:No quite yet. (Score:1, Informative)
But if you are willing to heft a fission plant into orbit, then you could just use it as a conventional nuclear rocket (i.e. superheated steam).
A conventional nuclear rocket will run out of steam (reaction mass) long before a VASIMR will. There's a tradeoff between using lots of reaction mass and using lots of energy. Chemical rockets are at the first end, then nuclear rockets, then things like VASIMR and ion drives. The first end is better for high-thrust applications (like launching from a planet), and the second end is better for long-duration applications (like traveling from Earth to Mars).
Slight Problem (Score:2, Informative)
Re:No quite yet. (Score:5, Informative)
This latter kind of generator is pretty much infinitely scalable, as you say, but aren't so efficient for big power demands - most of the RTGs in the probes you mention provide a few hundred watts, even when new.
For these thrusters, you're talking about burns of 10 MJ or more, which would require a vastly bigger RTG (or, more likely, a true reactor as the scaling would make it the more efficient choice) to get a reasonable pulse rate out of it.
Re:I'm dizzy. (Score:2, Informative)
Another main reason for the length of time involved is the orbital dynamics of the positions of the two planets. There is a astro concept called a Hohmann Transfer (http://en.wikipedia.org/wiki/Hohmann_transfer_orbit), which is a specific impulse efficient way of moving from one orbit to another. But it takes time, and requires waiting until the bodies are in the right position before we do it.
So you end up having to hang around on Mars for several months.
Going just a -little- bit faster doesn't gain anything because then you just have to wait longer for the planets to align.
Since this proposes something vastly quicker, the comment in the article about being able to do it in one planetary pass is what makes the 89 days possible. Requires tons more delta-V to do an orbital transfer this way, but the amount we'd save on human sustainment would more than make up for it.
Of course, not sure yet about hauling the nuke reactor into space...
Re:No quite yet. (Score:3, Informative)
VASIMR is not radically more efficient than a chemical rocket
Yes it is. The efficiency of a rocket is tied to the velocity of the propellant. VASIMR has a much higher velocity(~ speed of light /- 10%) than chemical(liquid propelant ~4,400 m/s) rockets. On the other hand VASIMR has very litte thrust. That means it is only useful in situation where there are no forces working against you i.e. already in orbit and no atmosphere.
Re:"A new NASA rocket engine" (Score:3, Informative)
Re:No quite yet. (Score:3, Informative)
As someone else already pointed out to the last person to make that comment: Kinetic energy and momentum are not the same. Momentum is always conserved, even in the presence of special relativistic effects[1]. Kinetic energy is not. The total energy in a closed system is constant, but in a rocket a lot of it will be lost as heat, which adds nothing to the thrust.
This is actually partially the reason why ion drives are difficult to build and lower-velocity rockets are easier. The energy required scales with the square of the velocity, but the momentum scales linearly. If you double the mass of your propellant, you double the momentum and double the imparted momentum (and therefore the acceleration). If you double the velocity then you increase the power needed by a factor of four. You quickly run out of chemical energy if you try to scale this up with chemical rockets. The advantage that ion drives have is that the energy comes from outside, so the velocity is only limited by the amount of energy you are willing (and able) to pump into it. And the amount it can take before melting, of course.
If you've got a nuclear reactor then you can pump a huge amount of energy into your electromagnets and accelerate the ions to a very high speed, but (assuming no losses) you need to increase the energy you put in by a factor of four to double the acceleration.
Faster, less-massive, particles are still generally better because you have to carry fewer of them. This has a knock-on effect, because you don't have to accelerate the ones you aren't carrying, so the imparted momentum goes a lot further.
[1] You can actually derive special relativity from the principle of conservation of momentum. I had to at school; it's quite an elegant bit of mathematics, and only takes about one page. General relativity is a lot more complex and hurts my brain
Reaction mass vs. Reactor Mass (Score:3, Informative)
The features of nuclear steam and VASIMR are pretty much a list of opposite pros and cons. E.g.:
nuclear steam doesn't waste any mass with electrical generating components so it is lighter overall than VASIMR.
Contrast this with VASIMR which can run on solar arrays and can share its electrical power source with other components.
Nuclear steam has a lower exhaust velocity so the overall power source requirement is lower.
Vasimir's higher velocity mean the specific impulse of reaction mass is 5x greater than nuclear steam, reducing carried mass and power generating needs. This has significant impact when duration of thrust is very large.
These attributes define the design envelopes.
If you need occassional thrust without a lot of mass and already have an electrical power source, VASIMR is good (e.g. orbital correction for satellites and space stations).
If you want to move a payload under continous thrust for days on end, a nuclear rocket is a good choice.
If you have a payload that has a fairly beefy electrical power source that you want to move under continous thrust for weeks on end, VASIMR is worth considering but may or may not be the best choice.
If you want to move a payload under continous thrust for a many months, go with VASIMR.
E.g. a russian ERTA generator can produce 150kW for 1.5 years while weighing 7500kg. A 150kW VASIMR drive would weigh 225kg and produce 4N. Fuel for 1.5 years is 9300kg. Total starting mass for 1.5years of 4N thrust is 17,025kg.
The SNTP nuclear rocket weighs ~13kg/N so 50kg of motor. Generously assuming the nuclear fuel would last 1.5 years, it still needs 49,000kg of reaction mass. Total starting mass for 1.5 years of 4N thrust is 49,050kg.
Assuming I've done the math right (which is not guaranteed since it involves partial fraction calculus) under that whopping 4N of thrust the VASIMR rocket will crank up to yawn-inspiring 0.004 m/s while the nuclear rocket will do a pokey 0.0012 m/s. Distance wise, the Vasimir will traverse 10.8 km vs. the nuclear rocket has only covered 3.8km.
While utterly theoretical, it does show that for ultra long burns, the reactor overhead of VASIMR is outweighed by the reaction mass increase of a nuclear rocket.
Re:Light speed probes (Score:3, Informative)
Yes, as you get get going faster, your effective mass goes up. This means to get to relativistic speeds you must apply thrust exponentially. Another problem is an engine of this type will eventually spit out all of its plasma and will run out of "fuel", so to speak.
Still, since you can run a plasma or ion engine from an external power source (the sun), you do get a massive jump in specific impulse.