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.
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.
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 Instead of using outdated and faulty technology from the 1960s.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.
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.
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?
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.
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...
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
If & when we get round to allowing more exotic stuff to be used I like the look of this one.
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.