http://spacenews.com/vasimr-hoax/
"Zubrin wrote in SpaceNews: “To achieve his much-repeated claim that VASIMR could enable a 39-day one-way transit to Mars, Chang Diaz posits a nuclear reactor system with a power of 200,000 kilowatts and a power-to-mass ratio of 1,000 watts per kilogram. In fact, the largest space nuclear reactor ever built, the Soviet[-era] Topaz, had a power of 10 kilowatts and a power-to-mass ratio of 10 watts per kilogram. There is thus no basis whatsoever for believing in the feasibility of Chang Diaz’s fantasy power system.”
Note the word used by spacenews: Hoax.
If we want to get serious about exploiting the solar system, we'll eventually have to give in and embrace nuclear technology, whether something like VASIMR or a nuclear-thermal design like NERVA. We already routinely park 100MW+ nuclear reactors in port for our navy, why not consider civilian use for space exploration? We already know many ways to mitigate risk for the launch of nuclear material.
I think Musk, Zubrin, et al. analyze propulsion technologies from the perspective of how to enable a journey to Mars now. In that light, something like Raptor makes much more sense. You still need a chemical rocket engine to lift off from Earth or Mars, so in the near term a nuclear ion thruster just adds far too much complexity to justify its inclusion. Further, it's difficult to imagine SpaceX obtaining the political backing to put nuclear tech into space as a private company. This is also why Musk would rather power Martian propellant plants with fields of solar arrays instead of the much more mass-efficient space-rated nuclear reactors that NASA has been developing.
But imagine if we actually developed a Mars colony with millions of people. The logistics of seeding a colony on Mars with chemical thrusters already boggle the mind. Economics would basically forbid meaningful interplanetary trade unless we develop new technology with much higher specific impulse. It would be even more impactful than moving from air-freight to container ships.
The real problem for a spacecraft isn't generating 700 MW of thermal heat, which is pretty small and light, but radiating it away continuously. The ISS EATCS radiates about 70 KW and each radiator is in the thousands of sq ft and thousands of pounds. So four more digits would be tens of millions of sq ft and millions of pounds, VERY superficially. However the reactor probably doesn't have to be optimized for human temps, so maybe ten times to hundred times better? It would be quite large at any rate.
There are certain engineering optimizations you make if you have an infinite liquid heatsink like a naval reactor vs incredibly expensive cooling like a space reactor. If you're willing to boil sodium your condenser can radiate a lot more per sq ft than an ammonia based refrigerator for ISS HVAC. That's a little far fetched but the VHTR/HTGR design goal was a cool 1000 C, so the radiators can run quite a bit hotter and smaller than ISS HVAC systems.
I've changed my career path because of these problems. I was a Math Major and now EE. These are the problems we need to be focusing on. Instead of using outdated and faulty technology from the 1960s.
IMHO, humanity is an inflationary species (for lack of a better term). We want to grow, spread and thrive. We aren't using a horse and a buggy. We (mostly) aren't shipping big chunks of ice around the world. We've made massive innovations happen and we should continue. It's nice that VASIMR is trying to push the envelop. Hopefully more of us can join together and tackle these big problems together.
There's no realistic space propulsion that would enable interplanetary trade to be in any way economically feasible. It's economical to ship finished goods and even raw material around the surface of Earth because it costs less than a dollar per pound by sea, rail, or road. Air freight is more expensive at around two dollars a pound. SpaceX's best price to LEO (Falcon Heavy) is $750 a pound. Just to LEO. Even if it was ten times cheaper it would still be almost forty times more expensive than air freight.
Even with magic super efficient interplanetary transport the surface-LEO portion of the trip makes it ridiculously expensive. To get a Martian colonist to LEO would cost (at our magic $75 a pound) $11k just to get their body to LEO. Assuming the water and air they need can be recycled with 100% efficiency the food for the 40 day Mars trip would cost another $13k.
If every colonist needs a ton of material to support them on Mars (far too low of a number) you're looking at $175b per million colonists. That's with a bunch of magic hand waving and completely unrealistic pricing. What in the shit are Martian colonists going to produce in any quantity that will pay down the $175b capex?
Chemical rockets will take us from 0 to 1, and the economics will drive the next technological development to take us from 1 to n. I think it will work the same way as when telegraph wires were exploding around the US and we realized copper resources were a limiting factor. Did we give up and say "well, let's wait for the next technology"? No, we kept building the wires and other industries sprouted up to handle the demand (notably recycling).
Why do you need to attach an ion thruster to a reactor, if you can turn the reactor itself into a thruster? https://forum.kerbalspaceprogram.com/index.php?/topic/151286...
If & when we get round to allowing more exotic stuff to be used I like the look of this one.
The problem becomes yes you can use a reactor or nuclear battery to power and ion rocket, which fuel efficient, very energy inefficient, and sloooow. Or you can use a nuclear rocket which avoids the need for a radiator by tossing your waste heat out the back end. Better fuel efficiency than a chemical rocket, can go faster, difficulty gamma radiation. Between those there isn't anything in between that makes sense because a high power thermal power reactor is too heavy.
Waste heat management for 200MW system in space is firmly outside the realm of present-day technology.
It really depends on the mass budget they have for that speed for if anything new needs to be discovered.
Funny - in a lot of other technology sectors, that would count as pretty good efficiency.
Now, since you you can be boiling pure hydrogen rather than the water that comes out of a chemical rocket you can make the propellant faster. But you're limited in temperature by what your reactor can take without melting so that only buys you a power of 2 or so in efficiency.
I wrote a series about how the different types of rockets compare starting here: http://hopefullyintersting.blogspot.com/2015/03/rockets-some...
155,200mph = 69,380 m/s
At 1g acceleration (10m/s/s) that is 6,938 seconds. 6,938 seconds is like 2 hours.
A ship that can sustain 1g acceleration continuously for periods measured in hours, that would indeed be an interplanetary drive. Sustain that for days/weeks/months and it will take us to other stars.
32mi/s * (5280ft/mi) = 168,960 ft/s
168,960 ft/s ÷ 32.17405 ft/s^2 = 5,251.4371053691s
5,251s ÷ 60 s/min ÷ 60 min/h = 1.45hr
I'm assuming a lot of things would go wrong as you get closer to C...
200 MW is a helluvalotta power! Disappointing to see it's so far off.
Technically, does it even matter how fast we eject? Shouldn't relativity allow us to reach speed of light with any positive thrust velocity? If the speed of the shuttle was of any concern, that should directly invalidate relativity, since passengers would suddenly not perceive any acceleration anymore, even though nothing about the spaceship and its physical reaction has changed.
Also remember that space is not dark. Unless you are behind something you are always in daylight if you are near a star.
Is this the power consumed by the device, or a measurement of the propulsion power created? It would be interesting to know the energy efficiency; i.e. what percentage of the input power is converted to thrust.
Thoughts: We need a functioning ITER -- in space...
https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion
The project Orion study anticipated a one-way trip could reach Alpha Centauri in as short as 133 years
And Mars is a rock. I am sure there are astounding discoveries to be made, but resources would be far better spent fixing ourselves and what we've done to the planet before we cause our own extinction. Maybe we can do both, but let's not ignore the fact that we have major problems, and it is unlikely we'll ever find a better home than Earth.
