[sandworm101]

https://www.spaceflightinsider.com/conferences/vasimr-plasma...

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.

[sobellian]

The idea isn't ludicrous, although the torch-ship speeds of 39-day transits are definitely out of reach for now. The basic idea is, let's attach a nuclear reactor to an ion engine. Further, let's make the propellant hydrogen so we can refuel anywhere in the solar system.

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.

[VLM]

In all fairness modern naval reactors like the A1B are pushing more like 700 MW thermal (unclassified, probably higher) and float in what amounts to an infinite heatsink of excellent liquid coolant. Of course lifetimes and decades of experience produce the A1B, The First Torchship is not going to do as well.

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.

[baybal2]

Space is an infinite heatsink if, and becomes an actually very good one once your radiators can reach 2000K+

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.

[giantrobot]

Space acts as a thermal insulator most of the time because radiation is the only means for a spacecraft to get rid of waste heat. Thermal radiators also only work if they can be oriented that they don't face the Sun.

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.

[baybal2]

Yes, the trick there is that only MHD coils, and few chunks of tungsten carbide will be anywhere close to the plutonium jet, and most of thermal radiation will be emitted to space.

[VLM]

You'll need some electricity and that's a mere heat generator.

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.

[LargoLasskhyfv]

Sounds like https://en.wikipedia.org/wiki/Nuclear_lightbulb

[rbanffy]

If we are in the 700 MW range, we can go with an NTR as well and solve the cooling problem with propellant.

[heimatau]

This boils down to theoretical vs applied. The OP approx. stated that we're basically 1/100 of the way there and this theoretical drive is currently impossible to implement.

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.

[sandworm101]

>>> The OP approx. stated that we're basically 1/100 of the way there

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.

[heimatau]

>> That's like saying a 90miles/gallon car is 1% of the way to getting 9000 miles/gallon.

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.

[giantrobot]

Physics, logic, and economics preclude interplanetary trade.

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?

[sobellian]

For what it's worth Musk believes that Starship will bring a >10x improvement for $/kg to LEO. Not throwing away your second stage will do magical things. At any rate a Martian colony would be mostly self-sustaining with trade being a relatively small fraction of gross domestic product (like with international trade right now). On planetary scales, moving trade from 0.01% Gross Martian Product to 0.1% would more than justify the development costs for such an engine.

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!)

[adrianN]

Interplanetary trade starts to make a lot more sense once a good chunk of your economy is happening outside the gravity wells of planets.

[hackeraccount]

Presumably the people of the future will be much richer then we are. I think there's an idea that over time wealth doesn't grow in an absolute sense - it does. Shipping good overseas makes sense now in ways it didn't 100 years ago not just because shipping costs have fallen but also because incomes have gone up.

[mr_toad]

It wouldn’t make much economic sense to ship stuff back down a gravity well if you could make it locally.

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.

[Ductapemaster]

I love that we have ideas and theoretically a combination of technologies that can take us to Mars and beyond in a short timeframe. The way I see it is that while chemical rockets are un-economic, as you point out, for a large scale colony, what humanity needs is a demonstrated need and a catalyst to get us to explore the more exotic propulsion technologies like nuclear + ion drives + hydrogen.

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).

[baybal2]

> The basic idea is, let's attach a nuclear reactor to an ion engine.

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...

[latigidigital]

Amazing site linked a bit down in that post:

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

[Pixelbrick]

Totally agreed re: the necessity of doing what can be done [i]now[/i] within existing regulatory frameworks.

If & when we get round to allowing more exotic stuff to be used I like the look of this one.

https://www.nasa.gov/pdf/718391main_Werka_2011_PhI_FFRE.pdf

[pharke]

You gotta send the wagon trains West before you build the railway.

[Gibbon1]

30 years ago as a student I tried calculating how big a space reactors cooling fins would need to be be. 1MW of power means 2 MW of heat you need to dissipate via radiation. I can't remember the numbers but I remember the result was dismal.

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.

[ColanR]

Your source is ignoring the fact that limitations on nuclear power in space are largely political. If we could put something bigger up there without concern it would be interpreted as a nuke, we would.

[Gravityloss]

Cooling is hard in space. It is hard and expensive to get good power to weight ratios.

[ColanR]

There's a lot of hard problems in space, though. We've solved a few of them - enough, at least, so that talking about large nuclear reactors in space should not be dismissed as a hoax.