Executing long burns?

[duckyvengance]

I'm planning a mission to Moho, with this ship >[http://imgur.com/NOOhvvF]

The problem lies in the fact that the TWR is so low that it has a 27 minute burn. When I split the node, I end up just burning straight into the atmosphere of Kerbin. Is there some way to perform this burn, or should I modify the ship to have a higher TWR?

[Panichio]

You can split the burn up into a few parts over the course of multiple orbits. You would do a little but of the burn on one pass of the maneuver node, then warp around again, and burn a little more, then continue this process until you gain enough speed to scape Kerbin. The only con of that being that it would mess up the maneuver node and you wouldn't be able to use it's information after you pass it the first time. This method will allow you to slowly gain enough speed to escape Kerbin while not forcing you to sacrifice fuel-efficiency for TWR.

[Cashen]

You could split the burn up, that would be more efficient, but you'd have to be careful that your first burn doesn't take you out of Kerbin's sphere of influence. Alternatively you could begin the burn from a higher orbital altitude to avoid hitting the atmosphere as you burn.

[TinfoilChef]

You can split the burn up into a few parts over the course of multiple orbits.

That's pretty much the way I've been doing interplanetary departure burns lately. I work out the maneuver node, then burn from t-2:00 to t+2:00 which makes the orbit elliptical. Then I remove the maneuver and create a new one at the same spot (which is now the periapsis). From there it's just rinse and repeat until you get a Kerbin escape. This way I can take full advantage of the Olberth effect and have the periapsis of my departure orbit at 80km while the apoapsis gets higher each pass.

[Cashen]

That's pretty much the way I've been doing interplanetary departure burns lately. I work out the maneuver node, then burn from t-2:00 to t+2:00 which makes the orbit elliptical. Then I remove the maneuver and create a new one at the same spot (which is now the periapsis). From there it's just rinse and repeat until you get a Kerbin escape. This way I can take full advantage of the Olberth effect and have the periapsis of my departure orbit at 80km while the apoapsis gets higher each pass.

My question about splitting burns up is that how does it affect the phase angles? As you get into those elliptical orbits, which take longer, the planets are still moving and the phase angles may be getting out of alignment. Is that increase in delta-v basically negligible? Or do you start your burns before the phase angles are properly aligned? And how far out will you push your apoapsis before committing to the final burn (to avoid getting out of Kerbin's SOI right at the end of a burn and being forced to continue the interplanetary burn at a non-optimal location/altitude)?

[lammatt]

i think a long burn is ok...

just that you (may) need to correct your path a number of times after you leave Kerbin

as long as you manage to get a reasobly low relative speed when the two encounters (so the you dont miss the orbit insertion; i'hv missed it once when i tried to force an early arrival where my approach was way off from my Ap.)

[Mr. Scruffy]

Facing the such a problem, i´d suggest to bring up some booster stage to dock with your craft and push it out of Kerbin´s SoI.

I usually have ~0.4 TWR on my exit-stages and start burning at t-5:00 from ~90km.

[Xeldrak]

My question about splitting burns up is that how does it affect the phase angles? As you get into those elliptical orbits, which take longer, the planets are still moving and the phase angles may be getting out of alignment. Is that increase in delta-v basically negligible? Or do you start your burns before the phase angles are properly aligned? And how far out will you push your apoapsis before committing to the final burn (to avoid getting out of Kerbin's SOI right at the end of a burn and being forced to continue the interplanetary burn at a non-optimal location/altitude)?

Well, if we rule out the most extreme scenarios, like 20 burns to get on a hyperbolic trajectory, the change in planetary alignment is negligible. As is the increase in delta-v. You'll probably save dv, because you can fully use the orberth effect.

[TinfoilChef]

Well, if we rule out the most extreme scenarios, like 20 burns to get on a hyperbolic trajectory, the change in planetary alignment is negligible. As is the increase in delta-v. You'll probably save dv, because you can fully use the orberth effect.

Yes. With the launcher / boost vehicle that I dock to the ship before doing this maneuver I get an apoapsis out near the orbit of Minmus usually by the second or third burn depending on destination. Then I decouple the 'pusher' vehicle and proceed with the transfer stage. Later when the pusher reaches AP it only takes a little fuel to bring PE down low enough for Kerbin re-entry.

[Audiopulse]

Even though some frown upon it - you could use MechJeb to help you with SmartASS. If you keep the Prograde-option enabled you just need to activate the engines at the right time to use the oberth-effect as effective as possible.

[KerbMav]

Later when the pusher reaches AP it only takes a little fuel to bring PE down low enough for Kerbin re-entry.

Or docked with a station for refueling?

Standardized design is key for reusability though.

[DerekL1963]

Even though some frown upon it - you could use MechJeb to help you with SmartASS. If you keep the Prograde-option enabled you just need to activate the engines at the right time to use the oberth-effect as effective as possible.

This is what I do. And, rather than trying to hit a specific window, I boost into solar orbit and then into a transfer orbit.

[draeath]

Even though some frown upon it - you could use MechJeb to help you with SmartASS.

Ignore anyone who frowns at you for using MechJeb.

Do you look down on the Apollo astronauts for using the AGU/DSKY?

Also, ducky - your image is gone:

File not found!

If you're looking for an image, then it's probably been deleted or may not have existed at all. Bummer!