Build the USS Enterprise

[David Evil Overlord]

Not sure if this goes here, mods. Please move if I've got it wrong.

An engineer claims we could build a spaceship based on the USS Enterprise in 20 years. No warp drive, but ion engines that could reach Mars in ninety days.

http://www.universetoday.com/95099/e...urce=pulsenews

[PTeppic]

How does one slow down at constant acceleration?

[chrispenycate]

Originally Posted by PTeppic

How does one slow down at constant acceleration?

Apart from continuous loss of reaction mass, maintain constant thrust.
Technically, acceleration is the rate of change of velocity; even changing direction at the same speed is accelerating, not just going faster. Yes, that means the brake pedal and steering wheel in your car are also accelerators, but mathematicians and physicists (but not, strangely chemists; accelerating a reaction always increases reaction rates) use words in slightly different ways from the common of mortals.

[Metryq]

Ion engines? I'll have to look over that site in more detail. I didn't think ion engines could produce sufficient thrust to reach Mars in 90 days—especially with the mass of four nuclear reactors and that huge ring habitat (to say nothing of all the fuel needed to push that mass). Ion engines are very efficient, but you better not be in a hurry.

[Vertigo]

This is just a dreaming StarTrek fan who's set up a website. The 'engineer' in question is a systems and electrical engineer. A computer techie, not a spaceship designer.

The Enterprise might have looked like a good spaceship design back in the early days of star trek, but not today. This guy is proposing a rotating "gravitiy wheel" whose axis is orthogonal to the direction of thrust. I haven't figured out what acceleration you would need to achieve Mars in 90 days but I'm sure it would be enough to give the most awesome 'seasickness' imaginable as your apparent gravity continuously changes as the 'wheel' rotates.

[David Evil Overlord]

Originally Posted by Vertigo

This is just a dreaming StarTrek fan who's set up a website. The 'engineer' in question is a systems and electrical engineer. A computer techie, not a spaceship designer.

The Enterprise might have looked like a good spaceship design back in the early days of star trek, but not today. This guy is proposing a rotating "gravitiy wheel" whose axis is orthogonal to the direction of thrust. I haven't figured out what acceleration you would need to achieve Mars in 90 days but I'm sure it would be enough to give the most awesome 'seasickness' imaginable as your apparent gravity continuously changes as the 'wheel' rotates.

I think he's a little too attached to the Enterprise design, and he's trying to make it work with the tech we have today.

[PTeppic]

Originally Posted by chrispenycate

Apart from continuous loss of reaction mass, maintain constant thrust.

Precisely. You can't get to Mars with constant aceleration. Half will be "constant" positive, half "constant" negative. Which is not constant.

[pyan]

Originally Posted by PTeppic

Precisely. You can't get to Mars with constant aceleration.

Umm, not quite accurate: you'd get there all right, you'd just go straight past. Constant acceleration with a 180° flip turn at the halfway point is the way to go, I'd have thought...

Originally Posted by Vertigo

I haven't figured out what acceleration you would need to achieve Mars in 90 days but I'm sure it would be enough to give the most awesome 'seasickness' imaginable as your apparent gravity continuously changes as the 'wheel' rotates.

The thing about constant acceleration is that it's constant: - no need for huge amounts of thrust. If you're spinning the habitat at 1g, would you notice a tidal effect from, say, .05g engine thrust?

[Dave]

If it is simply a 'Disneyworld' style, themed floating hotel, then I think it could possibly be built. When I read about it, there was no mention of the trip to Mars included.

Originally Posted by Vertigo

The 'engineer' in question is a systems and electrical engineer. A computer techie, not a spaceship designer.

I doubt he is a businessman either - I don't think there will be enough Star Trek fans left around by then to make a profit out of such an expensive hotel/theme park.

[Vertigo]

Originally Posted by PTeppic

Precisely. You can't get to Mars with constant aceleration. Half will be "constant" positive, half "constant" negative. Which is not constant.

In fairness to the guy he is talking about the ship using constant acceleration which is true. At half way point you simply flip over. The ship is still generating the same accleration but is now facing the opposite way. Slightly sloppy terminology but that is all.

Originally Posted by pyan

The thing about constant acceleration is that it's constant: - no need for huge amounts of thrust. If you're spinning the habitat at 1g, would you notice a tidal effect from, say, .05g engine thrust?

Fair point Pyan I hadn't figured how low his acceleration was going to be; he's actually talking of 0.002g so you probably right.

However I still come back to the basic design. I don't know the proper terminology for it but the angled design of engines and body mean that the ship would have to be built much stronger than would be necessary if everything was in a single line. The angular stress on the connecting members between each ship part would be huge for no good reason that I can see.

[Metryq]

Originally Posted by Dave

When I read about it, there was no mention of the trip to Mars included.

It's right on the front page of the "build the Enterprise" site—unless you mean you read the site before the Dual Stage 4 Grid ion thruster engine appeared in the news.

From Wikipedia:

Dual-Stage 4-Grid (DS4G) is an electrostatic ion thruster currently under development by the European Space Agency, in collaboration with the Australian National University.

A single 20 cm diameter 4-grid ion thruster could operate at 250kW power to produce a 2.5N thrust with a specific impulse of 19,300s and an exhaust velocity of 210km/s using Xenon propellant. These high power and thrust densities allow the maximum attainable power for gridded ion engines to be extended from 40 kW of the present day, to hundreds of kW. This aspect, in combination with the very high specific impulse and a lightweight power system, makes them especially suitable for large, high delta-V future missions.

Similar to the VASIMR thruster, the limiting factor is the availability of an advanced power supply; 250kW of solar arrays is equivalent to the total array on the International Space Station, to provide 250kW with Stirling radioisotope generators would require roughly 1000kg of plutonium-238 (for which the US stockpile as of 2011 was no more than 30kg) , and so a nuclear thermal reactor would be needed.

And nuclear thermal reactors are not lightweight.

A suitable power supply is one of Dr. Robert Zubrin's arguments against the claims for the VASIMR engine:

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 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.

It's ironic that "build the Enterprise" suggests the use of ion engines. In the original STAR TREK series episode "Spock's Brain," regarded by many as the worst episode (certainly the silliest), the Enterprise—a warp-driven starship powered by antimatter reactors—is approached by an ion-driven vessel. Scotty remarks, "I've never seen anything like her. And ion propulsion at that. They could teach us a thing or two!"

I think Spock wasn't the only one:

[nightdreamer]

I get a kick out of the 100MW laser. Sure, the laser in our lab delivers about 10MW ... for 100 femtoseconds. The most powerful and efficient continuous lasers available are CO2 lasers, which reach as high as 20%. Assuming you could pack enough laser power into his spaceship design (think maybe 20,000 industrial-sized lasers) and being optimistic about the 20% efficiency, in order to deliver 100MW of power to a target would mean you are delivering 400MW to yourself. Gonna need some monster cooling system, and you don't have an external source of water. And that's on top of the additional hundreds of megawatts of reactor heat you would have to dissipate to deliver that 500MW to the laser system. I wonder how much dry ice you'd have to carry.

[Metryq]

I was thinking along the same lines, Nightdreamer. And this ship needs such a powerful laser because...?

The author's whole argument is that this "Enterprise" could be built with existing technology, then includes engines and power systems that do not exist. Saying that we have nuclear reactors, but we just need to make them smaller and lighter means the technology does not exist.

Since we've never built anything in space even remotely as big as this proposed ship, engineering that should be adequate may not be. "Galloping Gertie," the Tacoma Narrows bridge that collapsed from aeroelastic flutter, is just one example where state-of-the-art is not good enough in all cases. A bridge is not just a bridge, anymore than a rocket is just a rocket. And who would have expected mold to become such a major problem on Mir?

[pyan]

Ion engines were successfully used (sub-orbitally) in 1959 - that's over fifty years ago.

Ion thrusters

[David Evil Overlord]

If you want to get really depressed, I have a book called The Space Traveller's Handbook. It was written before the first shuttle flight, and details the next hundred years or so of spaceflight from the perspective of some future academic looking back at history.

I remember the first time I read it, decades ago, I wanted one of the asteroid-tug Mass Drivers. Especially since the electromagnetic maglev system that would power it has been in use since 1973.*

We still don't have them. I even wrote a (very bad) story once, noting that the dinosaurs would still be alive today if only they'd spent money on their mass drivers instead of proxmiring their space program.

*The Japanese bullet train uses it. Yeah, I know, it's very different to a space-based asteroid mover, but the principle is the same.