Why circular orbits? Question 2: controlling estimated burn to save fuel?

[JackBush]

Is there any benefit to having a circular orbit, other than it's tidy? I never get one when doing the maneuver node for Kerbin. The map camera angle is slightly (or more) off centre when you start out and I get elliptical ones every time. Sometimes they're fairly close but not really.

Question 2: what dictates the Estimated Burn time, and is there any way to control it so that it's minimal (10 to 20 seconds, say) and saves fuel?

[TronX33]

1. Circular orbits are best when you're trying to rendezvous with something, or want to get to another celestial ody.

Launch your ship, at 5000m, press d and get it to about 25 degrees or so, keeping it on the line. Then, at 10km, turn to 45 degrees. At 20km, get to 60 degrees. At 40, go to 70 degrees. At 60km, turn to 90. By then your Apoapsis should be somewhere in the 100km range. At the apoapsis, create a maneuver node so that the Pe and Ap are clsoe to the same altitude. Burn at a bet less than half of the actual burn time. So, for a 60 sec burn time, burn at 23-27. For a 30 sec burn time, burn at 12-17 secs until the node

2. You can decrease it by turning lateral earlier, but the time is just how long it takes to get there. So, you can cahnge it to less by making your orbit almost an orbit then burn , using less fuel then, but not earlier, or make it less, and burn longer using more fuel then but less fuel earlier. Overall fuel use should still be similar though, as long as your ascent is not horrible inefficient.

[Starhawk]

Circular orbits are generally easier to work with when doing a rendezvous. When burning for an interplanetary transfer, if you have a circular orbit, the Transfer Window Planner will show you just how much delta-v you'll need. If the orbit is too eccentric, the delta-v will vary depending on which point in your orbit you do the burn.

TWR dictates how long your burns are. With more thrust, your burns will take less time. This comes at the cost of mass, efficiency, and control. This does not in any way save fuel.

Happy landings!

[Empiro]

Is there any benefit to having a circular orbit, other than it's tidy? I never get one when doing the maneuver node for Kerbin. The map camera angle is slightly (or more) off centre when you start out and I get elliptical ones every time. Sometimes they're fairly close but not really.

Question 2: what dictates the Estimated Burn time, and is there any way to control it so that it's minimal (10 to 20 seconds, say) and saves fuel?

A circular orbit lets you adjust maneuver nodes backwards and forwards without changing your AP / ejection velocity, letting you adjust the timing of the maneuver independently of the magnitude of the maneuver. The camera shouldn't affect how you get a circular orbit at all -- all it matters is that your predicted AP / PE is close to each other (and to get it most efficiently, you usually place the maneuver node right at the AP and burn prograde until the PE matches the AP).

Burn time is estimated by the game by dividing the delta-V of your maneuver by the maximum acceleration your engines provide the last time you fired them. It messes up when you stage or switch to / from the craft, however. You minimize the burn time by having a high TWR, though I think there was analysis done that showed burns up to 6-8 minutes would be considered reasonably efficient. If you need more, you can burn over multiple passes at PE, though it really complicates how you make the maneuver nodes. Making sure you have short burns will usually cost you (in everything except time), because you need to carry more engine, which lowers your available delta-V and raises your rocket's cost.

Edited October 22, 2015 by Empiro

[obi_juan]

A circular orbit have the minimum of energy to reach that altitude... So saves fuel... and is more easy to make a new transference from there.

Edit: Estimated Burn is directly related with delta-v... So to save fuel try to reduce the delta v of the maneuver (the velocity that is necessary to make the next node, not to reduce the time)

Edited October 22, 2015 by obi_juan

[AlextheBodacious]

TronX33's idea of a gravity turn is a bit too steep for most rockets (at least the ones I build.) So I usually go like this:

0M: Turn 5 degrees right

10,000M: Turn 45 degrees

20,000M: Turn 35 degrees

30,000M: Turn 30 degrees. Decouple fairings.

40,000M: Turn 20 degrees. Decouple escape tower.

50 to 60,000M: Slowly turn, so your AP is a little higher than where you want to be, so if you're aiming at 75K, go for 85.

At around 65K turn in so your AP is slowly shrinking. By the time you get to 75K, you're AP will be there too, so you just need to fire sideways till you're in orbit.

[Snark]

I never get one when doing the maneuver node for Kerbin. The map camera angle is slightly (or more) off centre when you start out and I get elliptical ones every time. Sometimes they're fairly close but not really.

This reply doesn't really address the questions ("why a circular orbit" and "what's the deal with estimated burn time") -- just wanted to address the problem of getting to a circular orbit in the first place, since it sounds like you're having difficulty.

Camera angle isn't the important thing-- what you should watch are the "Pe" and "Ap" markers. Here's how to get to an almost-perfectly-circular orbit after launch:

1. Do your launch, gravity turn, etc. so that you're coasting on an upward trajectory with your Ap at the altitude that you want your circular orbit to be.

2. While you're coasting upwards towards apoapsis, set a maneuver node marker right at apoapsis. Make it as close to exactly there as possible. (It's best if you can wait until you're out of the atmosphere, or at least over 45km, to do so, because if you set the node while you're still deep in atmosphere then velocity loss due to drag will mess things up.)

3. Drag the maneuver node's "prograde" marker until you get orbit. As soon as you get a Pe marker rising out of Kerbin's surface, slow down. Slowly pull more prograde. When you get close to a circular orbit, your Ap and Pe markers will start to slide around the orbit. Slowly drag until the Ap and Pe markers are 90 degrees in front and behind you.

4. You should now be pretty close to circular, and may be able to stop there. Mouse over the Ap and Pe markers to see if they're close to equal altitude. If they're off by more than 1 km, you can fine-tune it: Is Ap ahead of you, or behind you? If Ap is ahead of you, drag radial-inward; if it's behind, drag radial-outward. Do so very slowly and carefully. This will rotate the Pe marker closer to you. Stop when it gets to where you are. You're now about as perfectly circular as it's practical to get without excessive fiddling-- the difference between Pe and Ap should be a few hundred meters at most.

In practice, you don't have to be that perfectly circular. I just like doing it that way for tidiness.

Edited October 23, 2015 by Snark

[SchweinAero]

If Ap is ahead of you, drag radial-inward; if it's behind, drag radial-outward.

Of course, you can also grab the maneuver node by the central circle and slide it around until it's exactly at apoapsis. This allows you to achieve circular orbits by pure prograde burns.

[Brainlord Mesomorph]

I had a thread about this a few months ago.

A circular orbit is better for space stations, fuel depots, satellites, anything you want to get back to and rendezvous with frequently.

A highly elliptical orbit (with a high AP and low Pe) is cheaper, no circularization burn, and provides more options and opportunities for maneuvers like leaving orbit or landing.

It all depends on the purpose of your orbit.

Edited October 23, 2015 by Brainlord Mesomorph

[Red Iron Crown]

A circular orbit have the minimum of energy to reach that altitude...

How do you figure that? If I want to hit 1000km altitude from launch a 70x1000 eccentric orbit requires less energy. If I want to capture around a body and hit 1000km, a 1000xSoI-Edge eccentric orbit requires less energy.

[Brainlord Mesomorph]

A circular orbit have the minimum of energy to reach that altitude..

I think he means from the ground, going up.

[arise257]

TronX33's idea of a gravity turn is a bit too steep for most rockets (at least the ones I build.) So I usually go like this:

0M: Turn 5 degrees right

10,000M: Turn 45 degrees

20,000M: Turn 35 degrees

30,000M: Turn 30 degrees. Decouple fairings.

40,000M: Turn 20 degrees. Decouple escape tower.

50 to 60,000M: Slowly turn, so your AP is a little higher than where you want to be, so if you're aiming at 75K, go for 85.

At around 65K turn in so your AP is slowly shrinking. By the time you get to 75K, you're AP will be there too, so you just need to fire sideways till you're in orbit.

I think even yours is too severe for most rockets. I like using my predicted apoapsis for determining each point in my ascent because it compensates for the fact that different rockets will reach specific altitudes with different TWRs, speeds, and accelerations. On a 90° heading (East):

1,000m: 80°

5,000m: 75°

10,000m: 60°

25,000m: 45°

50,000m: 30°

60 to +75,000m: 10°

Getting circular from this sub-orbital point is basically a matter of waiting until you're 30 seconds - 1 minute away from your apoapsis and burning at or slightly under 0° until you've got a low periapsis (≤ 5,000m). Kill the burn and wait again until you're 3 seconds away, then burn again until you're circular.

- - - Updated - - -

Another thing to be aware of is that when people give you their ascent profiles, take them with a grain of salt -- even mine. There are a lot of ways to affect control authority, drag, weight, lift, and thrust when designing a rocket in KSP. These factors are rooted in the preferences and habits of the designer. AlextheBodacious, for instance, is likely to design rockets with greater control authority in order to achieve that 45° turn at 10,000m. Conversely, my ascent profile suggests I rely less on control authority, and emphasize meticulous piloting and efficiency. Neither of us is technically wrong in our approach, because we're both making it to space -- it's just that we're doing it based on what our designs allow or require.

Edited October 23, 2015 by arise257