[Blog] n-Body Politics

[Humabout]

Originally Posted by kreios

Actually, this is wrong. The most efficient transfer is (depending on the ratio between the two orbits) is either a Hohmann or a bi-elliptic transfer. Both assume instant velocity changes, meaning you can never actually achieve them. However, the higher your acceleration, the closer you can get (and therefore need less dV). Assuming you're able to restart engines (bringing us back to that topic, but which means you can actually to those required two burns), a high-thrust engine is always more efficient than a low-thrust one to change orbits.

At the extreme end, we can look at the real-life example of the SMART-1 mission, which expended ~3.9km/s dV from a geostationary transfer orbit to a low lunar orbit. However, an optimal trajectory (such as produced by a high-thrust system) would only need ~1.5km/s dV for such a transfer. Of course, the ion engine was far more efficient in terms of propulsion (since its Isp is more than four times a chemical rocket's) - but if they would've had a high-thrust system with those numbers available, they would've used it.

Actually, this is correct in the context in which I was speaking.

1) Eric and I are addressing cost efficiency, not dv efficiency - which itself doesn't necessarily translate into mass efficiency or cost efficiency. And what will ultimately determine which engines get used in some hypothetical futurist setting is the combination of cost and demand.

2) Hohhman, bi-elliptic (that ration is 11:1, btw), and other high-thrust transfers are the least time-efficient transfers available. This makes them highly undesirable for most applications of space flight, both because no one wants to sit on a spaceship for a decade while they drift toward Saturn and because no company wants to pay a crew a decade of wages to drift toward Saturn once.

3) The fastest transfers available use constant thrust to attain high velocities over the course of the transfer. This necessarily increases dv, which in turn, increases the fuel costs. It, however, saves time, and thus allows more trips (meaning more revenue) and fewer paid crew hours (meaning lower expenses). This is desirable when maximizing profit, and thus, monetary efficiency.

4) When dealing with brachistochrone transfers, the higher the craft's acceleration, the higher the maximum velocity attained, and thus the more dv required to execute it. Thus, a fusion torch can pull 0.5G, it would need approximately 800 mps - meaning 13 fuel tank systems - to transfer from Earth to Mars, and you would have to refuel there. But, even a regular fusion rocket could pull 0.005G, need only 100 mps and make the trip there and back again on less than 2 fuel tank systems. They use the same type of fuel, so it's pretty plain to see that a fusion rocket is more cost efficient than a fusion torch. Or in other words, the lower-thrust drive is more cost efficient than the higher-thrust drive.

5) High-thrust rocket engines have existed since 1926. They had a high-thrust system available for the SMART-1 mission and chose to use a high-efficiency, low-thrust ion engine instead. It had a higher dv because it went faster, but it had a far lower fuel-mass percentage (3.33%) specifically because the ion engine was more efficient.