Re: Spaceship with constant acceleration

On Jan 5, 6:03 pm, macfrag...@xxxxxxxxxxxxxx wrote:
It might be, IF it requires more power.  But as I noted,
using a drift leg actually requires less power, as well
as less energy and less time than continuous thrust.

Ah, I think I begin to understand what you mean: the shorter burn uses
the same power to accelerate more propellant per second to a lower
exhaust velocity, for more thrust in a shorter burn, sacrificing delta-
V but drifting most of the journey at top speed.
Okay, I think I can play around with these numbers a bit.

That's not necessary. Even if you keep the exact same
exhaust velocity, you use less power and energy because
you're lugging around less propellant mass. This lets you
use a less powerful thruster even as you get better
acceleration.

What drives the cost of the ship?  With a fantastical powerful
drive, it might plausibly be the cost of the fancy drive or the
cost of the fancy fuel, or both.

Sure, the drive is going to be the most costly element of the ship.
And yes, you'll want to get away with a relative minimum of power,
regarding the lift over time (lift = cargo transported), and other
factors. Comparing to present day, there is a Learjet and there is a
Bombardier, or, there used to be a Concorde and a DC-10. Some are just
faster and less economic but exist nonetheless.

You don't have a fixed ratio between fuel and cargo.  It
will depend on your mass ratio. You reduce power and
energy requirements greatly with a large mass ratio
(up to maybe 10:1 before diminishing returns).

Interesting, I'll check this out later.

If you assume a constant exhaust velocity, the ideal mass
ratio is somewhere around 4:1. However, if you have a
variable exhaust velocity capability (even if it's just two
modes), then you do better by initially using a low exhaust
velocity and then switching to a high exhaust velocity.

In the ideal case of a continuously variable exhaust velocity
rocket, you want to tune the current exhaust velocity to
be equal to the total accumulated delta-v so far, plus some
small constant speed (without this small constant speed,
you'd never actually get going at the start without an
infinite amount of propellant). With a mass ratio of 10:1,
this small constant would be 1/10 the total delta-v. If
you were to thrust in one direction, then the exhaust
would end up with 10% of the kinetic energy, while the
ship ends up with 90% of the kinetic energy.

Isaac Kuo
.

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