[Spaceships] Breaking orbit with a low-thrust drive

[Ulzgoroth]

Originally Posted by Fred Brackin

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.

Well, yes, but not ones that make any sense here.

There's very little flexibility about the shape of an orbit. In particular...

Originally Posted by Fred Brackin

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.

No, this isn't possible.

An orbit (well, around a single gravitaitonal point source, but close enough) is elliptical. It can't be 'egg-shaped'. One of the two (equally) narrow ends is the closest point to the orbited body, the other is the farthest.

[Fred Brackin]

Originally Posted by Ulzgoroth

No, this isn't possible.

An orbit (well, around a single gravitaitonal point source, but close enough) is elliptical. It can't be 'egg-shaped'. One of the two (equally) narrow ends is the closest point to the orbited body, the other is the farthest.

Oh no. Egg-shaped orbits are a real thing. Spy sats use them.

[Michael Thayne]

Originally Posted by Dr. Beckenstein

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?

Spending a month to get through Earth's radiation belt isn't cool for manned missions, but can work for unmanned ones. Plus, I don't think something with thrust in the 0.005G to 0.05G range is going to take a month. I'd think a few days at most should suffice.

[Ulzgoroth]

Originally Posted by Fred Brackin

Oh no. Egg-shaped orbits are a real thing. Spy sats use them.

Elliptical orbits are a real thing. Egg shaped orbits don't work, physics-wise, and I'm seeing zero evidence of them being a thing other than an occasional inaccurate description of an elliptical orbit.

some satellites use significantly eccentric elliptical orbits to get long dwell times around their apoapsis. But such an orbit does not have a big/little end distinction - both ends are narrow, one is near the planet (and thus passed through fast) the other is far away.

[Nemoricus]

While "egg-shaped" is used colloquially for elliptical orbits, it's still true that the shape of the orbit at periapsis and apoapsis is identical. Only the velocity differs.

[Dr. Beckenstein]

Originally Posted by Michael Thayne

Spending a month to get through Earth's radiation belt isn't cool for manned missions, but can work for unmanned ones. Plus, I don't think something with thrust in the 0.005G to 0.05G range is going to take a month. I'd think a few days at most should suffice.

-polite cough -
In rocket science, one does not simply "thinK". One does the math and checks the delta V.

[Humabout]

Using constamt low acceleration, you'll end up slowly spiraling outward from the planet until you reach escape velocity. The delta-v needed should roughly be the difference between your current average orbital velocity and escape velocity. You aren't using the Oberth effect to get extra oomph from your burns, so there's nothing special going on here. The lwngth of time ahould be, in seconds, dv/acceleration,with 1G = 9.81, and velocity measured in m/s. 1 m/s is about 1609.334. Thus, in GURPS units, it takes about 0.006095 * dv/acceleration seconds to break orbit.

[Ulzgoroth]

Originally Posted by Humabout

Using constamt low acceleration, you'll end up slowly spiraling outward from the planet until you reach escape velocity. The delta-v needed should roughly be the difference between your current average orbital velocity and escape velocity. You aren't using the Oberth effect to get extra oomph from your burns, so there's nothing special going on here. The lwngth of time ahould be, in seconds, dv/acceleration,with 1G = 9.81, and velocity measured in m/s. 1 m/s is about 1609.334. Thus, in GURPS units, it takes about 0.006095 * dv/acceleration seconds to break orbit.

It's actually much worse than that. That's actually what you need with the Oberth effect - an instantaneous burn from orbital to escape velocity. Nothing will be cheaper than that.

My own calculation confirms what Anthony said back on the first page:

Originally Posted by 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).

If your thrust is so low that your orbit can be treated as always circular, the delta-V required to escape is equal to your initial orbital speed. Whereas going directly to escape in a high-thrust instant would cost (sqrt(2) - 1) times your orbital speed

EDIT: I shouldn't have said "nothing will be cheaper" - I haven't checked whether it's possible for a retrograde burn to lower your periapsis prior to doing an escape burn could be efficient. Seems unlikely, but my intuition isn't reliable.

[Anthony]

Originally Posted by Dr. Beckenstein

-polite cough -
In rocket science, one does not simply "thinK". One does the math and checks the delta V.

Of course, you first have to specify exactly what you're trying to do. If we assume a 1000s burn every orbit, which is reasonable if you're trying to loop ever further out, each burn at .005G (0.05m/s^2) will give delta-V of 50m/s.

Your final loop involves an extremely elliptical orbit (escape velocity is 11.2 km/s, so your velocity at bottom of loop starts out at 11.15 km/s). This orbit has 110x the semi-major axis of a circular orbit, and thus takes a bit over a thousand times as long (around 1500 hours), which you probably do not want to do. However, it's really only the last few loops that are horribly inefficient.

[cvannrederode]

Originally Posted by Anthony

Of course, you first have to specify exactly what you're trying to do. If we assume a 1000s burn every orbit, which is reasonable if you're trying to loop ever further out, each burn at .005G (0.05m/s^2) will give delta-V of 50m/s.

Your final loop involves an extremely elliptical orbit (escape velocity is 11.2 km/s, so your velocity at bottom of loop starts out at 11.15 km/s). This orbit has 110x the semi-major axis of a circular orbit, and thus takes a bit over a thousand times as long (around 1500 hours), which you probably do not want to do. However, it's really only the last few loops that are horribly inefficient.

You need to make sure that your apoapsis doesn't leave the Hill sphere of what your orbiting.

The Oberth effect is important, but it's not an on-or-off thing. Simply put, in lower orbits you are faster, and have more momentum (including your reaction mass), and therefore can get more of a momentum change from your reaction mass.