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Magnetoplasma drive could make Mars transit take 39 days?

orbitalindex.com

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

#61

Post by ColanR »

Gravityloss wrote: post #50
ColanR wrote: post #36

Earlier quoted context omitted.

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.

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

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.

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

#62

Post by jandrese »

natcombs wrote: post #10

Using nuclear bombs for propulsion was estimated to make it to Mars and back in 4 weeks. First time I heard of this, I thought it was a joke. 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

I would take the figures from Project Orion with a grain of salt. It was very much a cocktail napkin exercise.

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

#63

Post by pharke »

sobellian wrote: post #30
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[-…

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…

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

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

#64

Post by TheOtherHobbes »

ryanmarsh wrote: post #51
RandallBrown wrote: post #45

Earlier quoted context omitted.

Could you use the waste heat to boil something and then spray it out the back as extra reaction mass?

Yes, but what? You have to lift that mass to orbit. What mass boils off with greater energy than, say, mass that combusts?

You don't necessarily need to lift that mass to orbit for interstellar flight.

https://en.wikipedia.org/wiki/Bussard_ramjet

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

#65

Post by Symmetry »

RandallBrown wrote: post #45
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.

Could you use the waste heat to boil something and then spray it out the back as extra reaction mass?

That's how nuclear thermal rockets work. Boiling and spraying is going to be much, much higher thrust than what you're getting out of the electric drive but the spray will be going much slower so the amount of boilable propellant you carry with you will end up being the limiting factor in how fast you can do. And at that point you might as well forget about the electric part.

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...

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

#66

Post by giantrobot »

sobellian wrote: post #30
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[-…

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 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?

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

#67

Post by catmistake »

Everyone is missing the fact that even if this technology was available 100 years ago, and worked just like we wanted, you could not get to Mars in 39 days. It would take about twice as long, because you have to turn the ship completely around half way to the destination and slow down pretty much for the same about of time it took to get to whatever velocity.

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.

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

#68

Post by giantrobot »

baybal2 wrote: post #60
VLM wrote: post #52

Earlier quoted context omitted.

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…

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.

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.

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

#69

Post by dylan604 »

jandrese wrote: post #58
dylan604 wrote: post #37

Earlier quoted context omitted.

If your ship can only handle 1G of acceleration, would that not also mean that it could only handle 1G deceleration as well? That means starting to slow down halfway to your destination.

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.

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

#70

Post by baybal2 »

giantrobot wrote: post #68
baybal2 wrote: post #60

Earlier quoted context omitted.

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.

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 me…

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.
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