On Fri, Dec 22, 2017 at 2:52 AM, Rob O'Connor <xxxxxx@ozemail.com.au> wrote:

If we assume water, mass goes to 5% (assuming the rest of the ship has an average density equal to water[!]), so ln(1/0.95) = 0.051.

I googled "average density of a freighter" and turned up an article about building boats, which mentions that the average density of a ship is about one gram per cubic centimeter (the same as water) . . . "just before it sinks."

So I'm thinking that actual ships (and thus most likely "actual" starships) are somewhat less dense that this. :)

This helps a bit.

A round-trip to 100 diameters and back at 1G continuous acceleration requires 13.44 G-hours or a delta-v of about 482 km/sec.

So exhaust velocity = 482/0.051 ~ 9460 km/sec, from the rocket equation.

So if the average density of the ship is slightly lower, then the km/sec would be slightly higher. Maybe 10,000?

>> Direct conversion of most of the fuel's mass-energy to kinetic energy.
> Would running the water through a fusion reactor core accomplish that?

No. You would produce a plasma of hydrogen and oxygen, which could be ejected at speed - but not at the required level of performance.

Fusion implies conversion of about 0.7% of the fuel's mass to energy.
Maximum exhaust velocity is about 12% of the speed of light (100% efficient, all the fuel fused and ejected out the back of the rocket).

So . . . Yes? No?

To my original question of "Is 5% of volume enough fuel for four round trips to the typical 100d limit?"

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