[Spaceships] Breaking orbit with a low-thrust drive

[Michael Thayne]

As I understand it, if you're in LEO with a high-thrust engine, breaking orbit is pretty simple: burn ~2 mps if delta-V, and you're at escape velocity! It's trickier with a low-thrust drive: for maximum fuel-efficiency, you want to make a series of short burns near periapsis, you coast when not near periapsis.

But what if you're impatient? How much fuel efficiency do you sacrifice by just thrusting continuously to escape from a planet's gravity? I'm happy with a playable fudge like "the delta-V cost increases by 50%" or whatever. I'm most interested in something that works for thrusts in the 0.005G to 0.05G range (fusion rockets and fusion pulse drives), but something that works for ion drives (including VASIMR from Spaceships 7) would be nice too.

[AlexanderHowl]

The delta-v for breaking orbit assumes a high thrust drive that can use the Oberth effect, so you would need 2-4 times as much delta-v for a low thrust drive.

[Anthony]

The limit case seems to be delta-V equal to your orbital velocity (which is about 2.5x as much you'd need for an oberth burn).

[cvannrederode]

Originally Posted by AlexanderHowl

The delta-v for breaking orbit assumes a high thrust drive that can use the Oberth effect, so you would need 2-4 times as much delta-v for a low thrust drive.

Not necessarily, a low thrust drive can split the escape maneuver up into multiple burns at periapsis. Just do a little each time, then travel through its increasingly elliptical orbit. It can do this any many times as needed until it reaches escape velocity, then it needs burn out the rest of its maneuver.

This also works in reverse. As long as you make an orbit with your initial capture, you can use multiple burns / aerobraking passes to circularize.

[Ulzgoroth]

Originally Posted by cvannrederode

Not necessarily, a low thrust drive can split the escape maneuver up into multiple burns at periapsis. Just do a little each time, then travel through its increasingly elliptical orbit. It can do this any many times as needed until it reaches escape velocity, then it needs burn out the rest of its maneuver.

That can be extremely slow though, as the more eccentric the orbit the smaller a fraction of the time you're near periapsis.

[Dr. Beckenstein]

Originally Posted by Michael Thayne

But what if you're impatient? How much fuel efficiency do you sacrifice by just thrusting continuously to escape from a planet's gravity? I'm happy with a playable fudge like "the delta-V cost increases by 50%" or whatever. I'm most interested in something that works for thrusts in the 0.005G to 0.05G range (fusion rockets and fusion pulse drives), but something that works for ion drives (including VASIMR from Spaceships 7) would be nice too.

The best answers to questions like this is of course the atomic rocket webpage, http://www.projectrho.com/public_html/rocket/

esp. here:

http://www.projectrho.com/public_htm...ransfer_Orbits

If you scroll drown a little, you will find a paragraph "Low Thrust" that gives a thought about this.
In short:It won't work. Even with a VASIMR you will need at least a month to break free.

Maybe if the ship has some booster rockets?

[Ulzgoroth]

Originally Posted by Dr. Beckenstein

In short:It won't work. Even with a VASIMR you will need at least a month to break free.

And how does the latter translate to the former? Low-thrust interplanetary flight is slow, that's not news.

[Fred Brackin]

Originally Posted by Ulzgoroth

That can be extremely slow though, as the more eccentric the orbit the smaller a fraction of the time you're near periapsis.

_Years_ and maybe never as you'll be limited to how close you can approach Earth without losing velocity to atmospheric drag as your orbit gets more eccentric.

I would assume that any advanced/TL9 spacefaring society will probably make chemical engine booster rockets from lunar or near asteroid mining.

[Ulzgoroth]

Originally Posted by Fred Brackin

_Years_ and maybe never as you'll be limited to how close you can approach Earth without losing velocity to atmospheric drag as your orbit gets more eccentric.

Doing repeated burns at periapsis isn't going to make your periapsis get lower. If you weren't hitting the atmosphere before you started your escape burns, you won't at any later point.

[Fred Brackin]

Originally Posted by Ulzgoroth

Doing repeated burns at periapsis isn't going to make your periapsis get lower. If you weren't hitting the atmosphere before you started your escape burns, you won't at any later point.

Some forms of increasing your orbital eccentricity would. If you started from an effectively circular orbit as your orbit became more and more highly elliptical it'd get "longer" on one axis but "narrower" on the other.

That would give you two places on your orbit to burn. If you're being very careful to later to an "egg-shaped" orbit with a narrow end far from Earth you've only got one place to burn.

The plans that I've seen for leaving orbit with ion drives or other very low thrust systems are actually spirals based on continuous thrust.

The practical trade is high thrust/low endurance v. low thrust/long endurance. Reaction mass isn't ever likely to be expensive enough that low thrust/low endurance ends up being the desrable choice.