Build the Enterprise

[Steve]

I came across a news article about a proposal to build an ion drive version of the Enterprise for travel in the solar system.

 

http://buildtheenterprise.org

[Xavier Onassiss]

Re: Build the Enterprise

 

I ran across an article on this a few days ago, but I haven't been able to find out very much because most of the links on the website are broken and the pages they link refuse to load. What I've seen so far is a bunch of wildly optimistic claims. However, "90 days to Mars" using a nuclear powered ion drive might go beyond mere optimism and into crackpot territory; I need to do some back-of-envelope figures before I write this whole thing off.

[AlHazred]

Re: Build the Enterprise

 

90 days? Definitely crackpot, with the emphasis on the "crack"! Last I checked, ion drives can only produce about a thousandth of a G acceleration; feasible for unmanned probes, but not for us humans. 90 days seems highly unlikely.

 

Of course, a fantastic breakthrough is always possible, in which case I'll be first on line for a ticket.

[Zeropoint]

Re: Build the Enterprise

 

Crackpot? Nope, that figure is realistic. I don't have time for the math right now, but a pre-worked example for 0.01 g says it's a 30-day trip. Check Nyrath's excellent Atomic Rockets site for details.

[Xavier Onassiss]

Re: Build the Enterprise

 

Crackpot? Nope' date=' that figure is realistic. I don't have time for the math right now, but a pre-worked example for 0.01 g says it's a 30-day trip. Check Nyrath's excellent Atomic Rockets site for details.

 

Yeah, the "crackpot" part is whether that .001G is do-able to begin with; it'll take a lot of power, and to sustain it for 90 days also takes quite a bit of fuel, which increases mass, which will reduce the acceleration. (Or require a bigger engine, and a bigger power plant, which adds MORE mass, and so on....)

 

Ion drives tend to be power hogs; they need a lot of power for the amount of thrust they put out. So... nuclear power? It's heavy, and with an ion drive, heavy is the last thing you want. Solar power efficiency drops off with the inverse-square of the distance from the Sun; traveling to Mars will, on average, put you 1.6 times farther away than Earth, so at your destination the solar panels have 39% of the output they started with. You can compensate by making them bigger, but then you're back to adding more mass again.

 

Yeah, I'd really like to see the figures for the mass of this "Enterprise."

[Gnaskar]

Re: Build the Enterprise

 

I ran across an article on this a few days ago' date=' but I haven't been able to find out very much because most of the links on the website are broken and the pages they link refuse to load. What I've seen so far is a bunch of wildly optimistic claims. However, "90 days to Mars" using a nuclear powered ion drive might go beyond mere optimism and into crackpot territory; I need to do some back-of-envelope figures before I write this whole thing off.[/quote']

 

It is a crackpot figure, but a crackpot figure that came from NASA back in the Bush days. It just neglects to mention the year long climb out of the Earth's gravity well that precedes it. Sure the small acceleration adds up fast, but that really doesn't help when you have to fight your way out of a gravity well to get there.

 

Yeah' date=' I'd really like to see the figures for the mass of this "Enterprise."[/quote']

 

Wet mass of 187 million pounds. The engines use 2.5GWe which by my (rough and GURPS Vehicles based) calculations would require a 20 million pound power plant.

[Xavier Onassiss]

Re: Build the Enterprise

 

It is a crackpot figure, but a crackpot figure that came from NASA back in the Bush days. It just neglects to mention the year long climb out of the Earth's gravity well that precedes it. Sure the small acceleration adds up fast, but that really doesn't help when you have to fight your way out of a gravity well to get there.

 

 

 

Wet mass of 187 million pounds. The engines use 2.5GWe which by my (rough and GURPS Vehicles based) calculations would require a 20 million pound power plant.

 

 

Okay, that's something to work with.

 

You're right: the ion drive just goes around in ever-widening circles until it reaches Earth's escape velocity. I've seen some designs with a "boost stage" using chemical or other fast-burn high-thrust rockets for this part of the trip, then using the ion drive for the rest.

 

Also nice to see some hard numbers for the power plant, although I think I'll compare them to some real-world plants. The power-to-mass ratios in RPG vehicle design systems are sometimes a little optimistic. The above example looks like 2.5 MW per ton; Ad Astra is hoping for 1MW per ton (metric) for its VASIMR project, which hasn't been done yet. Link.

[Gnaskar]

Re: Build the Enterprise

 

The above example looks like 2.5 MW per ton; Ad Astra is hoping for 1MW per ton (metric) for its VASIMR project' date=' which hasn't been done yet. Link.

 

It's 0.25 MW per ton, actually. Or, to use the actual stats 20,000+(8*kW) lb.

 

How do you guys manage without the metric system?

[Xavier Onassiss]

Re: Build the Enterprise

 

It's 0.25 MW per ton, actually. Or, to use the actual stats 20,000+(8*kW) lb.

 

How do you guys manage without the metric system?

 

It's not really a metric/imperial thing it's just an oops-I-dropped-a-zero thing. I'm equally competent (or not, in this case) with both systems. I should have dug out a proper scientific calculator and used engineering notation instead of futzing around with the POS "10-key wonder" that was sitting on my desk; that's just asking for trouble. Thanks for fixing that.

 

Personally I prefer metric, too.