What is the ideal T-Minus point to Launch Maneuvers?

[Keramane]

I've heard some people say 2 seconds, some say 1, but I'm genuinely curious as to when the best time to start accelerating for a maneuver would be. I always seem to miss my window by a little either side.

[loiske]

I usually try to time the burn equally between the node. so with 60s burntime I start at T-30 and end at T+30. It seems to work, I never got good results starting exactly at T

[rhoark]

Depends on your TWR and the deltaV of the maneuver - whatever gets half the burn before T0.

[Noticeably FAT]

I generally do about a 60-40 split, 40 before T. Not for any super scientific reason, but because I can't seem to ever time it right.

[Kromey]

I usually do the 50/50 split, but for trickier maneuvers I'll start a bit before the 50/50 T-minus time and gradually increase my thrust, and just watch what's happening on the map view and adjust things on the fly. Seems to work most of the time for me, but I haven't done any interplanetary transfers yet.

My most difficult burn to date was from a highly... dangit, blanking on the word, my orbit's angle was way off from equatorial. Anyway, from that orbit around Mun trying to get an encounter with Minmus despite not having the patience to wait for the proper window. Took several minutes messing with maneuver nodes to finally get one. Anyway, that was nearly a 3-minute burn, and I timed it to end at roughly T-0. Worked out very well, actually.

[RoboRay]

The maneuver system assumes you're applying the full impulse instantly, right on the node. The only way to get close to that is to evenly split your burn on each side of the node. So, if your maneuver requires a four-minute burn, you should start the engine two minutes before reaching the node.

[Conarr]

Just to add to RoboRay's remark, I've noticed if you're doing a really long burn, say more than 5 minutes or so, it seems better to stop the burn, run round the orbit and burn the remainder. For me, the results get way too sloppy on a long continuous burn.

[RoboRay]

Just to add to RoboRay's remark, I've noticed if you're doing a really long burn, say more than 5 minutes or so, it seems better to stop the burn, run round the orbit and burn the remainder. For me, the results get way too sloppy on a long continuous burn.

Yes, you are correct. This is referred to as "Pe kicks" are a great way to maximize the efficiency of your transfer burns to other planets. It allows you to apply more of your thrust right at the ejection angle and parallel to your prograde axis, as well as get more benefit from the Oberth effect as your propellent has higher average kinetic energy during the burns.

If your burn is much more than five minutes, definitely try Pe kicks.

[Conarr]

Neato, I didn't know what it was called. I've seen that term, but hadn't bothered to look it up yet. I just worked out the burn thing intuitively when my target approaches got sloppy

[Eji1700]

These days i rarely care. I usually set up my point(say an intercept with jool).

Then i rotate my rocket so i'm lined up on the point and bring up the map and controls. I don't actually watch the Delta V guage, and instead watch my actual plotted course as it changes in real time on the map. Once it's at the desired point, i stop burning. I've done t minus a minute or so with such setups(sometimes days for massive adjustments)

[nats]

Just to add to RoboRay's remark, I've noticed if you're doing a really long burn, say more than 5 minutes or so, it seems better to stop the burn, run round the orbit and burn the remainder. For me, the results get way too sloppy on a long continuous burn.

Or do the long burn to get a starting trajectory and then do a small accuracy burn later on to finalise the final trajectory. They did this on the Apollo missions and I often do it on my Mun trips.

[wolfedg]

Just to add to RoboRay's remark, I've noticed if you're doing a really long burn, say more than 5 minutes or so, it seems better to stop the burn, run round the orbit and burn the remainder. For me, the results get way too sloppy on a long continuous burn.

Having never tried it myself, I always wonder how you do burns on ION engines without missing the window by hours.

[Malaugrym]

My most difficult burn to date was from a highly... dangit, blanking on the word, my orbit's angle was way off from equatorial.

Eccentric, I do believe.

On topic: I usually do the 50/50 approach previously mentioned.

Edited May 15, 2013 by Malaugrym

[itsme86]

Eccentric, I do believe.

On topic: I usually do the 50/50 approach previously mentioned.

I think Kromey was referring to inclination. Eccentricity is how elliptical your orbit is. Inclination is angular deviation from the equatorial plane.

[Stealth2668]

Yes, you are correct. This is referred to as "Pe kicks" are a great way to maximize the efficiency of your transfer burns to other planets. It allows you to apply more of your thrust right at the ejection angle and parallel to your prograde axis, as well as get more benefit from the Oberth effect as your propellent has higher average kinetic energy during the burns.

If your burn is much more than five minutes, definitely try Pe kicks.

Can someone explain this in English?

[Kromey]

I think Kromey was referring to inclination. Eccentricity is how elliptical your orbit is. Inclination is angular deviation from the equatorial plane.

Yes, thank you! That's the word I was trying to remember! I wrote "highly eccentric" like a dozen times and each time said, "No, that's not the right word." Was bugging me that I couldn't remember Orbital Mechanics Vocabulary 101!

Can someone explain this in English?

"Pe kicks" is a method by which you do a series of burns on subsequent orbits to gradually move your Pe up. For example, if you need a maneuver to add 200 m/s delta-V, you can do one burn to add 100 m/s, then wait for your orbit to bring you back around and do a second burn to add the remaining 100 m/s. It's an efficient way to accurately do maneuvers that require very large delta-V, or more accurately that require very long burn times (since, as has been mentioned, maneuver nodes assume you get the entire impulse right at T-0, which is for obvious reasons impossible).

I've seen somewhere a neat graphic depicting the orbit at each stage of a maneuver done with a dozen or so Pe kicks, but I'm not sure where to find it and Googling "Pe kicks" just wants to sell me Nikes.

[Noticeably FAT]

Someone else can explain the 'why' a little clearer than me, I'm sure, but here goes. 1) the closer to your Pe you are, the less energy it takes to raise your Ap (I believe this is the Oberth Effect). 2) the closer your burn is to your proper ejection angle, the more accurate your trajectory will be. So by splitting up your long burns, you are staying closer to your ejection angle, and you're getting more bang for your buck as each small burn gets progressively more efficient.

Kromey, if you add 'kerbal' to your search the image you were looking for shows up. Here's a link, I kinda don't want to hotlink from a guy who's likely to show up in the thread.