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Time to Mars, rather shocking


KASASpace

How long Mars?  

  1. 1. How long Mars?

    • 6 months
      37
    • one year
      10
    • two years
      11
    • 2 weeks
      6


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Solar sails don't scale like that. Double the radius and you quadruple both the force generated and the mass of the sail. Payload fraction goes down so performance does go up, but that can only get you so far. What you need to do to get a high performance sail is to make it lighter. There are various proposals for that, including one group who think they can build a sail from carbon nanotube mesh and get 10g acceleration at 1AU from the sun.

Alternately, theres always the laser sail. Put a great big laser in orbit and shine it on the sail of departing ships.

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I never said they scaled a specific way, I just said scaled up for greater acceleration.

And I said that doesn't happen. Except that the larger sail has a smaller payload fraction, but that only lets you approach the acceleration an unladen sail would generate, which is still puny with current technology. We need a much lighter material if we want the kind of accelerations you are talking about.

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And I said that doesn't happen. Except that the larger sail has a smaller payload fraction, but that only lets you approach the acceleration an unladen sail would generate, which is still puny with current technology. We need a much lighter material if we want the kind of accelerations you are talking about.

Here is the response.......

The least dense metal is lithium, about 5 times less dense than aluminum. Fresh, unoxidized surfaces are reflective. At a thickness of 20 nm, lithium has an areal density of 0.011 g/m2. A high-performance sail could be made of lithium alone at 20 nm (no emission layer). It would have to be fabricated in space and not used to approach the sun. In the limit, a sail craft might be constructed with a total areal density of around 0.02 g/m2, giving it a lightness number of 67 and ac of about 400 mm/s2

http://en.wikipedia.org/wiki/Solar_sail

400 mm/s^2 is 0.4 m/s^2, little over the target. But you would need to build it IN space.

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It's as some of you never played KSP and don't know how travelling among orbiting bodies works.

KSP is fairly accurate when it comes to our current level of technology, but we are talking in the future, once something akin to a nuclear lightbulb is developed.

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Ion engines might be capable of this sort of trajectory, but they fall far short of "1 foot per second" that you ask.

For now, that is.

The thrust is based on a lot of things, especially mass flow, so the more you can shove out the back of the engine the more thrust, the faster that stuff goes the better Isp you have.

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For now, that is.

The thrust is based on a lot of things, especially mass flow, so the more you can shove out the back of the engine the more thrust, the faster that stuff goes the better Isp you have.

Truth. However pumping too much electricity into the engine causes electricity to arc, reducing the amount of energy imparted to each unit of exhaust, which determines the engine Isp. You could push more mass through the drive, but it would get less energy per unit, and thus be less hot and provide less average thrust

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Yet that was designed in the 1950s, NERVA in the 60s, heck even ion thrusters had their beginnings in the early 1900s!

Unless you mean politically.....

I mean purely practically. Unless those technologies propelled a real mission, they are not within reach. When something was designed or proven on paper (or even in a lab) is fairly irrelevant when you want to propel actual spacecraft. And I am quite sure Orion will not make a comeback (if you could call that coming back) anytime soon.

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I mean purely practically. Unless those technologies propelled a real mission, they are not within reach. When something was designed or proven on paper (or even in a lab) is fairly irrelevant when you want to propel actual spacecraft. And I am quite sure Orion will not make a comeback (if you could call that coming back) anytime soon.

NERVA performed a full scale test running longer than would have been needed for a mars mission. That is far from only on paper.

More likely for a new mars mission is a fusion pule rocket using inertial/magnetic ignition of fuel pellets. This design worked on a scale model and could be used as a primitive torch ship to go straight to mars and back. NASA has put funding forth for a full scale test engine and then we are on to flight tests.

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Know what'd be a great use for NERVA? Shuttles between Mars/LMO. Get yourself a space station in orbit to serve as an extended stay hab, launch ground modules from Earth and have your station crew go between the two without any need for permanent life support function on the ground for some time.

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If you want to haul literally shiploads of Hydrogen to the depots, I think fusion engines would be better suited for that job. NERVAs would be good for LEO/LMO shuttles, and for there-and-back missions to Mars.

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Currently? Nowhere. In a few years? Well, wherever fusion engines will be built. I don't know how far Z-Pinch Fusion engines are, but the Magnetoinerial Fusion engine (Fusion-Driven Rocket) is being developed with limited NASA funding, and seems to be promising.

You can't get NERVA engines anywhere either. They'd have to be built too, I doubt the NERVAs currently in storage are still in working condition.

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Currently? Nowhere. In a few years? Well, wherever fusion engines will be built. I don't know how far Z-Pinch Fusion engines are, but the Magnetoinerial Fusion engine (Fusion-Driven Rocket) is being developed with limited NASA funding, and seems to be promising.

You can't get NERVA engines anywhere either. They'd have to be built too, I doubt the NERVAs currently in storage are still in working condition.

You've somehow completely missed the point. If a spaceflight organization wanted a NTR, they could get one. They'd just need to build it or have someone else do so. But you can't build a fusion rocket, because they haven't been invented yet.

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You've somehow completely missed the point. If a spaceflight organization wanted a NTR, they could get one. They'd just need to build it or have someone else do so. But you can't build a fusion rocket, because they haven't been invented yet.

http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII_fellows_slough.html

Already "invented" and funded for a full working rocket that is being worked on right now.

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I mean purely practically. Unless those technologies propelled a real mission, they are not within reach. When something was designed or proven on paper (or even in a lab) is fairly irrelevant when you want to propel actual spacecraft. And I am quite sure Orion will not make a comeback (if you could call that coming back) anytime soon.

Does that matter?

It was proven with scale models the concept worked, and Carl Sagan commented that it was a much better approach to using nuclear technologies.

Don't say it does, because what matters is that most people don't want to take a risk, and doing that will destroy all that we ARE.

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The NTRs that were developed in the 1960s had an Isp of about 850s. Current designs for NTRs have improved that to about 1000s.

Assuming Isp of 1000s, an NTR-powered ship that goes to Mars in 2 weeks (and needs 360 km/s of delta-v) would need a ridiculous mass ratio. If your payload is 5 tons (a small capsule), the ship would need to be at least 50 quadrillion tons, or 5 times larger than Mars's moon Phobos.

With an Isp of 4000s, among the best ion engine Isp we have (although ions, or VASIMR, are very far from having the requisite thrust), the ship would only need to be 50,000 tons, or about the mass of the Titanic.

With a hypothetical fusion engine with an Isp of 30,000s, you would still need a mass ratio of 3 or 4.

The main thing about fast travel times is that it's just not worth it. Why send a tiny probe/capsule to Mars in 2 weeks when you can send something much much bigger to Mars in 6 months? Radiation isn't really that big of a problem, and it can be mitigated by shielding.

Even with the hypothetical 30,000s Isp fusion engine, you could use a 100 ton rocket to send a 25 ton payload to Mars in 2 weeks, or you could use that same 100 ton rocket to send 5000 tons of cargo to Mars in 6 months, which is about the size of 2 fully fueled Saturn V's. You could send 100 times as many people to Mars (2500 tons) and use the rest of the mass as radiation shielding to have better radiation protection than living on Earth.

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