One thing I don't quite get is their claim that the rocket would need "just some solar panels" instead of a fuel tank when in the paragraph before that they talk about exhausting argon-based plasma.
If there is a propellant, you'd need to store that somewhere first, right? Does each particle of the propellant exit the rocket with much higher energy? Or can Argon be stored in a much denser form than other fuels?
The difference is 'momentum gained per gram of propellant'.
Traditional engines burn fuel, and the expanding, hot fuel pushes itself out the back of the engine at high speed, pushing the rest of the spacecraft forward. The fuel containing the energy is also the propellant.
Ion engines are the opposite in almost every way. You spend a lot of electrical energy (solar, nuclear, whatever) speeding up a tiny amount of propellant (which is usually something non-reactive like argon). You shoot a little bit of fuel at insanely high speeds out of the engine, and it pushes the spacecraft forward a little bit- so you do it for a very long time.
Yes, ion engines still need propellant, but they need a whole lot less of it.
Thanks to you and the others for clarifying. For some reason it didn't occur to me they were talking about an ion engine. I've heard of them and their slow-but-steady acceleration characteristics before but never actually knew how they work (and for some reason didn't quite make the connection).
Yes, each particle of the propellant exits the rocket with much higher energy. Since you can only get so much energy from chemical reactions, to get to these higher exit velocities the energy from the solar panels is used. You use a higher ratio of energy to propellant mass than chemical fuels use.
I was at a demo for a plasma engine similar to VASIMR back in the 90's at a national lab. It was a long machine running the length of a large room. There was a window in the side where you could see the plasma. When we showed up there was a faint pinkish plasma barely visible in the window. The guy who was showing us around told us they were just getting ready for the demo and we should wait a few minutes. After a few minutes of the plasma being off, one of the engineers gave a thumbs up. Our guide pulled us close to the window and said "Watch this!". Nothing happened. "One more second!", he said. Then we smelled burning electronics. "Shit!", he said and ran towards the power supplies.
My co-founder and I looked at each other and smiled, because it was so nice to be on the other side of that situation. I knew exactly how the guys felt. Usually we were the ones trying to demo an experiment that sort-of, kind-of worked.
Side note: hats off to anyone working with high energy plasma.
If I make a mistake in my code, it generally doesn't escape magnetic confinement and instantly vaporize other portions of my system with the fury of the sun.
As discussed in the article, this is a very different kind of ion thruster. That category is quite large. In particular, I believe essentially all ion thrusters flown have been electrostatic ion thrusters, including both Deep Space 1 and the Hall thrusters discussed in the article.
They all need a propellant and power feed. But, you can use those same feeds for an array of multiple engines. Thus, 'scaling up' has a wide range of options and trade-offs. Vasimr let's you scale power output a lot from the same engine, but so would an array of smaller engines.
With that in mind the specific techniques are important, but the gaps are often overstated.
(IAN a physics major) But to elaborate per my understanding, there are 2 important variables you're playing with in rocket engines:
- Specific impulse / Isp (aka how much thrust you get for a given amount of propellant)
- Maximum Thrust
Due to the rocket equation [1], adding more propellant increases the weight of your vehicle, making it harder to move, requiring more propellant, etc etc. So being efficient with your propellant is very good.
That said, given a requirement against a gravity well (e.g. a planet), there's usually a minimum total thrust required for a given maneuver to be successful (low total thrust = maneuver takes longer = more time for gravity to pull you = more propellant required).
Thus far, we generally have two types of engines. (1) High maximum thrust, lower Isp (chemical rockets) & (2) high Isp, low maximum thrust (ion/electric engines). The two are currently very far apart [2, sort by Specific Impulse decreasing, then look at the Thrust column].
As examples (Isp Vacuum / Thrust Vacuum): NEXT ion thruster 4,100s/0.236N @ 6.9 kW, VASIMR 5,000s/5.7N @ 200 kW, Space Shuttle SRBs 268s/14MN.
The hope with VASIMR is that it provides a middle ground where high Isp is available with enough total thrust to actually be useful for something other than slow orbital adjustments. An example of "something useful" would generally be anything beyond Earth orbit that covers large distances, e.g. flying to Mars.
This is correct but incomplete: Vasimr may have a higher power density (be it power per thruster mass or per thruster volume) than ion thrusters but I am pretty sure it has lower power density than a Hall thruster, while being also much more complex.
The main (and not yet verified) claim of Vasimr compared to existing and proven tech like Hall thrusters is the ability to greatly vary the specific impulse.
That's what the article claims. That you can run in a less efficient mode that produces more total thrust when you're climbing out of a gravity well, then switch to the lower thrust higher efficiency mode for the long haul.
I thought one of the issues with Hall thrusters was that, in engineering-practical terms, getting enough total thrust out of them was impossible. Or would they scale effectively?
Yes, the claim is entirely wrong. The rocket still needs propellant. It just uses electricity to eject the propellant at much higher speeds than usual chemical reactions.
"Such an engine design offers a couple of key benefits over most existing propulsion technology. Perhaps most notably, unlike chemical rockets, Vasimr operates on electricity. As it flies through space, therefore, it does not need massive fuel tanks or a huge reservoir of liquid hydrogen and oxygen fuel. Instead, the rocket just needs some solar panels".
I guess you mean the power-to-thrust ratio, which indeed scales roughly with the specific impulse.
The thing is, pretty much every plasma propulsion device has the theoretical ability to throttle specific impulse.
Gridded ion engines and Hall thrusters can for instance directly throttle specific impulse (Isp) by varying the discharge voltage. Other plasma thrusters can do this by other means like increasing RF heating, varying the magnetic field, etc.
Unfortunately, in real life the actual usable range of Isp always ends up much narrower that initially predicted because of a variety of factors that are difficult to predict theoretically (erosion and ionization efficiency in particular).
AFAIK, due to the shear size and complexity of Vasimr, its thrust has never been directly measured and we know very little about its actual performance. I would therefore wait a little and discard sensationalist articles until real, extensive testing has taken place.
There's basically four inputs to electric propulsion: power source mass efficiency (W/kg), required delta vee (m/s), required acceleration (m/s/s), and exhaust velocity (m/s). (Well there's efficiency too.)
You can approximately minimize total mass with this info. Turns out, your spacecraft will in total be heavier, if you go to higher exhaust velocities. You save on propellant but you need bigger solar arrays. Not worth it.
All this talk about "Mars in x days" assumes a fantastic power plant. It's a little bit like you're starting a car company and showing wheels and saying "my car company enables range and acceleration three times that of Tesla" while you don't have any kind of battery or motor yet.
I don't know how hard it is, but the ISS uses big radiators to dump excess heat into space.
> The Station's outstretched radiators are made of honeycomb aluminum panels. There are 14 panels, each measuring 6 by 10 feet (1.8 by 3 meters), for a total of 1680 square feet (156 square meters) of ammonia-tubing-filled heat exchange area.
There's hard theoretical limits on the rate at which you radiate heat, so without contact with an external medium or dumping mass, you're pretty much stuck with limits based on how big your radiators are
FTA: "And at a time when the national discourse assails the value of Spanish-speaking immigrants..."
What a load. Aside from the repellent need by the author to inject politics into a science story, it's flat-out wrong: the rancor isn't over people who (like Chang-Díaz) pursue legal citizenship or legal residency, nor is it over people who speak a particular language simply on the basis of that language.
With respect, I disagree. Derogatory terms like "anchor babies" describe people who are, after all, US citizens of immigrant descent. Similarly, the rancor directed at the judge in the Trump university case had nothing to do with legal citizenship and everything to do with Hispanic ancestry. And obviously refugees are legal immigrants granted that status by the US State Department. But this doesn't prevent them from being the subjects of a great deal of both fear and fury.
These controversies, taken individually, could be viewed as legitimate questions about the ethical grounds for birthright citizenship, or the appropriate scope for recusal, or the balance between mercy and safety. But in aggregate it's clear they aren't any of those things. They're expressions of a xenophobic insistence that immigrants, particularly non-white immigrants, are "the other". That they are cheaters, that they can't be trusted, that they are /dangerous/. That's xenophobia and racism, and it has nothing to do with the rule of law.
There may be some xenophobia and racism there, but I think there's also a substantive amount of anger among a sizeable amount of the populace due to how the rule of law is selectively applied, and how open-borders advocates intentionally blur the lines between lawful immigrants and individuals who illegally enter a sovereign nation.
Case in point: in the last couple of months, how much have we heard the "we are a nation of immigrants" line, especially during a border security debate...as if just about everyone, including the passengers of the Mayflower, skirted customs?
The racists and xenophobes likely don't want immigrants, but I think the vast majority of Americans are happy to welcome folks from other countries who have demonstrated respect for our laws by entering the country legally. Hell, if you've spoken to naturalized citizens, they're often the most patriotic, pro-American people you'll encounter!
And many of those Americans, who have spent time, money and energy to become citizens, are strongly against the lackadaisical border enforcement that has led to the current mess.
I appreciate that you acknowledge that racism and xenophobia taint the arguments you're making, even if you yourself are not racist or xenophobic. It's hard to stand up for a view when it is held by people who are widely reviled, and I suspect that courage is something our country needs more of.
However, I think the examples I provided compellingly demonstrate that issues of legality do not sit at the core of the anti-immigration sentiment in the US. In particular, an overwhelming concern for the sanctity of law would likely not motivate one to demand that a sitting judge recuse themselves because of their ancestry. Nor can it have motivated anti-refugee sentiment.
More broadly, the argument about rule of law does not have good explanatory power for the actions and statements of those who make it. It does not explain opposition to open borders, or to more expansive visa programs, which of course could both be made law with comparative ease. And it seems obvious to me that anti-immigrant militias like the Minutemen represent the height of contempt for the rule of law, but somehow find a warm reception among those who advance the very argument that you do.
Does this mean that nobody legitimately cares about the legality of immigrants? Of course not. It is possible that a broad coalition of people simply happen to agree on this wide range of narrow topics that have nothing whatsoever to do with race or ancestry. But Occam's razor suggests that the simpler argument is the better one: that duplicity and political convenience are at play.
Specifically, I suspect that the broader anti-immigrant movement in the US believes it has a winner of an argument in this one despite its lack of policy prescriptions. As a result this argument-- the one you make-- often gets advanced as a fig leaf for those "other" viewpoints. Which leaves you in the awkward position of carrying water for some truly despicable people, whatever your own views are.
I can't speak for Trump's attack on the judiciary. He's barking up the wrong tree there, and did little to help his case. It will likely be left for SCOTUS to decide, as should be the case; judges shouldn't be cowed by politicians to change their rulings.
We do disagree about the point about the sanctity of law. IMO selective enforcement (engagement/detainment directives for the Border Patrol) and the implicit support of "sanctuary cities" seem to encourage the violation of immigration law. I think it is specifically the former that gave rise to groups like the Minutemen.
As for carrying water for despicable people...well, so be it. I happen to think that, as with any group, most are reasonable folks of decent character but often overshadowed by the extremists and attention-seekers in their midst -- an argument, I'm happy to point out, also made in the defense of foreign individuals who come to this country under less-than-legal circumstances.
Just to be clear: selective enforcement is the law of the land. Has been since before IIRIRA passed-- and enforcement was looser, not tighter, before then. So when you say you respect the law, that's the law and I'd expect you to either respect it or say plainly that you don't and why.
Regarding the Minutemen-- a gentle reminder that regardless of whether a person is illegally in this country or not it is a felony to "point your gun right dead at them, right between the eyes", as Chris Davis publicly advocated for during the Minutemen's heyday. And of course, there's Shawna Forde, the leader of Minutemen American Defense. She's an individual so law-abiding that she has her own entry on murderpedia. And the cofounder of the original Minuteman project was later charged with three counts of child molestation including his own daughter, and is currently serving a nearly 20 year sentence resulting from those charges. Then there's the allegations of influence peddling and fraud that have embroiled the organization in litigation against another founder, Jim Gilchrist... and the fact that Simcox cofounded another related Minuteman project with Jason Ready, a known member of the American Nazi party who received a BCD from the Marines, was believed by the FBI to have been involved in the execution of at least two suspected illegal immigrants, and finally wrapped things up with a 4-on-1 murder-suicide a few years ago. Seriously law abiding folks, there-- and just as another reminder that's the leadership. That's who these supposedly lawful folks decided to follow. Those are the people whose acts you compare the civil infractions of illegal immigrants to.
You know what? I'll take the illegal immigrants, thanks.
If there is a propellant, you'd need to store that somewhere first, right? Does each particle of the propellant exit the rocket with much higher energy? Or can Argon be stored in a much denser form than other fuels?