Magnetoplasma drive could make Mars transit take 39 days?(orbitalindex.com) |
Magnetoplasma drive could make Mars transit take 39 days?(orbitalindex.com) |
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.
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.
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.
http://www.projectrho.com/public_html/rocket/enginelist.php
http://www.projectrho.com/public_html/rocket/enginelist2.php
http://www.projectrho.com/public_html/rocket/enginelist3.php
RD-600: Soviet bimodal GCNTR on page 2 is the one from that discussion
I think we can safely say that we are very far from 1 Martian colonist, let alone 1e6. I think we would outperform expectations to set up a very small research station by the end of the century at great expense on the Martian surface. So there's clearly a lot of magical thinking going on here. If we allow for some amount of aspiration, there are a few ways to provide a return on long time scales (much more than 1 century, with capex far exceeding $175B just looking at the cost of staging the necessary propellant in LEO):
- Such a base will have a propellant depot in a much shallower gravity well than Earth and a far thinner atmosphere. This is a huge comparative advantage for launches into deep space.
- Necessity breeds ingenuity. A Mars colony would likely generate many advances useful to Earth that do not make sense to pursue terrestrially. Think hydroponics, insulation, radiation shielding, so on and so forth.
- There are plenty of completely desolate locations on the Martian surface - ideal locations for radio telescopes, neutrino observatories, and other hyper-sensitive experiments.
- ??? (We're trying to predict centuries ahead, after all!)
On the other hand it might actually become cheaper to ship stuff to Earth orbit (even LEO) from Mars, simply because of the delta-v cost.
Even then, 200 MW is a square 560 meters on each side.
But we really aren't 1/100th of the way. That's like saying a 90miles/gallon car is 1% of the way to getting 9000 miles/gallon. Getting from one to the other cannot happen through incremental improvements. It requires totally different, as yet unexplored, areas of science.
Yeah. I think that would be apt to say.
If you're able to do some napkin math about the sheer amount of R&D to get to the other 99%, start a company ASAP because that is the billion (maybe trillion) dollar question.
Getting there is different, yes, which was entirely my original point. I'd rather not shoot an idea down before we've even started. Let's stop with the nitpicking and actually build. Incremental is probably the only way that we will get there. Think like a builder, not a complainer.
Will getting the other 99% look like how we got 1%? Of course not and if it does...then we're actually very close to a major breakthrough because we have a lot of the auxiliary technology already fleshed out.
Every journey begins with one step and we already have made a few steps. Now onto making many more.
Not a problem for a glowing plutonium plasma.
Here is a quote on one project from sixties:
> a spaceborne electric powerplant dubbed EU-610, with an electric output of circa 3.3x106 kWt, specific power 0.7x105 kWt/kg/sec (sic), relative mass 18.7 g/kWt, length 10000 mm.
The exhaust of a nuclear thermal rocket isn't waste heat. The heat from the reactor that heats everything that is not propellant is waste heat. Getting that heat to radiators without cooking people or melting/weakening load bearing structures is the challenge with nuclear reactors in space. On Earth we use literal tons of water and air to the job.
The Carnot heat engine efficiency is not nice at a cold side of 2000K and the working fluid will be a problem. Maybe gaseous helium or argon to reduce leakage, I guess.
The only place I found EU-610 was a Russian thermal rocket. Which is kind of like saying all you need for a coal electrical plant is a lightweight pile of charcoal.
Its a different type of technology, sorta like the difference between burning in a cast iron stove to boil tea, vs using a microwave oven to boil tea.
"Folding space"/"Warping space", or shrinking the space in front of the craft while expanding the space, would seem to be the only theoretical way to achieve speeds that not only would match light speed but could surpass it and not break the laws of physics as we understand them. The drive Alcubierre proposed in the 80's, I think... maybe the 90's. I'm not sure when. A Spanish mathematician or physicist who was a fan of Star Trek.
EDIT: My understanding of physics is pretty minimal. I'm more than happy for someone to correct me and explain what I got wrong. This type of stuff is fascinating.
From the perspective of an outside observer, a constantly-accelerating spaceship will approach the speed of light, but never quite get there. This will happen regardless of the level of thrust.
Alcubierre drives are pretty sweet - the only trick is they seem to require negative mass [0]... at least they've gotten down the requirement from a negative Jupiter mass to a mere -700 kg!
[0] - https://en.wikipedia.org/wiki/Alcubierre_drive#Mass%E2%80%93...
As for the rest of your second paragraph: Look into how relativistic velocity addition works.
200MW/(220w/m2) gives us an area of 1,000,000m^2, a square 1000m on each side.
(side note: a NYC block isn't a very good length reference - they vary[1] from 250ft to 750ft depending which direction you're measuring. So the square would be 4 to 12 blocks on each side)
[1] https://streeteasy.com/blog/how-many-nyc-blocks-are-in-one-m...
It would still weigh a lot though, and heat would be a major issue. You wouldn't want to melt the solar panels.
This would require someone much more knowledgable to determine if it's feasible.
Edit: I wonder if you'd get some amount of solar sail effect in addition to the plasma rocket? Of course this would help you one way and hurt you the other way.
Bringing enough fuel to decelerate the same way you accelerated isn't usually practical, inflating your rocket by 15x its original size or more.
Any sci-fi-grade navigation computer can do that.
Of course depending on hour your Sci-Fi FTL engines work you could build up speed by stopping up high in the gravity well, let the planet accelerate you to orbital velocity, then jump sideways into an orbit once you're moving fast enough.
These functions are a hard requirement for passenger transportation anyway because you need to match orbit with the station you are docking with. Protocol is to jump to a random position a couple light seconds from the port, get authorization, then short jump to your assigned docking exit point.