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Magnetoplasma drive could make Mars transit take 39 days?

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Re: Magnetoplasma drive could make Mars transit take 39 days?

#71

Post by rbanffy »

VLM wrote: post #52
sobellian wrote: post #30

Earlier quoted context omitted.

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…

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, bu…

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

Re: Magnetoplasma drive could make Mars transit take 39 days?

#72

Post by rbanffy »

Florin_Andrei wrote: post #33
d_silin wrote: post #8

From what I heard from industry people, overheating is an unsolved problem for VASIMR. It is 60-70% efficient, meaning that 60kW of thermal power has to go somewhere. Waste heat management for 200MW system in space is firmly outside the realm of present-day technology.

> It is 60-70% efficient Funny - in a lot of other technology sectors, that would count as pretty good efficiency.

True, but radiating 30-40% of your waste heat isn't that easy.

Re: Magnetoplasma drive could make Mars transit take 39 days?

#73

Post by Gibbon1 »

sandworm101 wrote: post #15

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[-…

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.

Re: Magnetoplasma drive could make Mars transit take 39 days?

#74

Post by rbanffy »

rimunroe wrote: post #59
sandworm101 wrote: post #32

Earlier quoted context omitted.

Or go right. Time dilation would start working in your favor. You might get there, and back, within your lifetime. Of course everyone on earth would be long dead.

Micrometeors and radiation would be the major problem. Also, it's not just time dilation that would help. At relativistic speeds, space contracts quite noticeably in the direction of travel, so the actual distance you need to cover diminishes as well.

That's because you won't be locally faster than light. You may cover 4 light years in one, but only because those light years will be much smaller from your point of view. You'll never "feel" faster than light.

Re: Magnetoplasma drive could make Mars transit take 39 days?

#75

Post by rbanffy »

dylan604 wrote: post #69
jandrese wrote: post #58

Earlier quoted context omitted.

I think it would be more like you time your arrival with the conjunction of some gas giants in the destination solar system and aerobrake multiple times to get your speed low enough so you can aerobrake on your destination planet. Bringing enough fuel to decelerate the same way you accelerated isn't usually practical, inflating your rocket by 15x its original size or more.

One of my pet peeves about sci-fi is how ships jump out of FTL into a perfect orbit around a planet. The concept of temporal dampeners is laughable even after suspension of disbelief to allow for FTL.

Sci-fi-grade engines are always fun. If you want to jump straight into a stable orbit, just add the right velocity component to your exit point. Or select a synchronous orbit.

Any sci-fi-grade navigation computer can do that.

Re: Magnetoplasma drive could make Mars transit take 39 days?

#76

Post by sobellian »

giantrobot wrote: post #66
sobellian wrote: post #30

Earlier quoted context omitted.

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…

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…

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!)

Re: Magnetoplasma drive could make Mars transit take 39 days?

#77

Post by chasd00 »

codeduck wrote: post #20
a_crc wrote: post #11

Earlier quoted context omitted.

It's a lot of solar panel. A quick search gives us a figure of 220 watts of power per square meter of space based solar panel. This would mean a 200MW solar collector would need to be almost a million square meters in size. For reference that would be a square the length of 155 NYC city blocks on each side.

A million square metres is one square kilometre. It could be built but it would be heavy. A circular sail with a radius of just over 600 metres would do it.

wouldn't a million square metered be a megametre and not kilometre?

Re: Magnetoplasma drive could make Mars transit take 39 days?

#78

Post by jandrese »

rbanffy wrote: post #75
dylan604 wrote: post #69

Earlier quoted context omitted.

One of my pet peeves about sci-fi is how ships jump out of FTL into a perfect orbit around a planet. The concept of temporal dampeners is laughable even after suspension of disbelief to allow for FTL.

Sci-fi-grade engines are always fun. If you want to jump straight into a stable orbit, just add the right velocity component to your exit point. Or select a synchronous orbit. Any sci-fi-grade navigation computer can do that.

Even a Geo orbit is an orbit. I guess you could drop out at a Lagrange point.

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.

Re: Magnetoplasma drive could make Mars transit take 39 days?

#79

Post by mr_toad »

giantrobot wrote: post #66
sobellian wrote: post #30

Earlier quoted context omitted.

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…

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…

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.

Re: Magnetoplasma drive could make Mars transit take 39 days?

#80

Post by adrianN »

giantrobot wrote: post #66
sobellian wrote: post #30

Earlier quoted context omitted.

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…

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…

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