What's the easiest way to increase time to apoapsis?

[Brownhair2]

Specifically when you're on a suborbital trajectory. Burn upwards or burn prograde?

Edited June 22, 2016 by Brownhair2
It's a question

[Signo]

Prograde. 

 

Fly safe

[Reactordrone]

You'll always want some prograde component to your vector but it depends how slow you are. If you're rapidly approaching the apoapsis and you're still got 1000m/s left to burn you'll have to angle up to fight gravity and push out your apoapsis. If you're getting close to orbital speed put everything into prograde (horizontal burn).

Edited June 22, 2016 by Reactordrone

[Jarin]

Halfway between prograde and radial. (so, 45 degrees "up" from path of travel). Then adjust back down depending on how fast you want your ap to be moving. If time-to-ap isn't increasing at that angle, you're probably gonna have to scratch the flight. Note, this is not delta-V efficient, but if you're trying to push your AP from suborbital, I assume you're working with low-thrust, high-isp.

[klesh]

Indeed, prograde + a little angle "up" as needed.  Often performed in map mode. 

[Violent Jeb]

If you have a high enough TWR prograde is always easiest,cheapest, most efficient. "up" is only to compensate when you don't have the thrust. With MJ I can see "time to apoapsis" in flight mode, and usually turning 10-20 degrees "up" (or west), usually brings the time to a standstill while on ascent. I try to keep the time to apo between 30s-1m.

With increasingly low TWR, you will have to increasingly point "up".

[Superfluous J]

Note that using low TWR and pointing up FEELS less efficient, but you carried less engine mass to this point so it's likely caused your rocket to be MORE efficient.

If you don't care about efficiency, though*, bringing enough TWR to be comfortable is by far the best choice. I like it to never drop below 1.5 or so once I've go to that off the pad (which is usually somewhere in the 1.3 range)

*Not caring about efficiency isn't a totally bad thing. There is also the efficiency of play time to consider. Do you want to spend 10 minutes fighting a low-twr ship into orbit, or 5 getting into orbit and another 5 ejecting to Jool?

[ajburges]

Depends on what stage you are at sub-orbital and what you goal is.

For obvious reasons, vertical thrust of about 1g will cancel the effects of gravity. But all that does near AP is change orbital phase. Generally, you want to thrust prograde as that is the only vector that will increase your kinetic energy state.

Making assumptions about you situation, I am assuming you are talking about you gravity turn. An efficient gravity turn should not see you extend time to AP by much, but instead hold it at a value (a minute is an easy number to work with) until the AP is in the desired range. If you lack the thrust to do that, your stage is lacking in thrust to achieve the desired gravity turn or you are not flying the right gravity turn for the desired orbit. If your stage lacks thrust, you may need a bigger kick from previous stages to give your engines time to work.

Diagnosing a bad gravity turn can be hard and needs much more information. Can you provide more details about your craft and flightpath?

[Brownhair2]

3 hours ago, ajburges said:

Depends on what stage you are at sub-orbital and what you goal is.

For obvious reasons, vertical thrust of about 1g will cancel the effects of gravity. But all that does near AP is change orbital phase. Generally, you want to thrust prograde as that is the only vector that will increase your kinetic energy state.

Making assumptions about you situation, I am assuming you are talking about you gravity turn. An efficient gravity turn should not see you extend time to AP by much, but instead hold it at a value (a minute is an easy number to work with) until the AP is in the desired range. If you lack the thrust to do that, your stage is lacking in thrust to achieve the desired gravity turn or you are not flying the right gravity turn for the desired orbit. If your stage lacks thrust, you may need a bigger kick from previous stages to give your engines time to work.

Diagnosing a bad gravity turn can be hard and needs much more information. Can you provide more details about your craft and flightpath?

It's an SSTO. Flight path is climb at about a 45 degree angle, then start to flatten off at about 10km or higher to build speed, then burn at a 45 degree angle with my NERVs when the jet engines go out and hope that maybe this time I'll go to space

[Jarin]

I've found that it helps a LOT to drag along a couple rapiers for the initial kick from "high-atmosphere-flight" to "coasting into space", then run on nukes to circularize.

My general flight plan is close to yours. Initial ascent varies, but build speed between 10km and 22km on airbreathers, aiming for 1200m/s or better. Optimally you should be at about a 15 or 20 degree ascent when your jets flame out, with an AP probably around 30-40km. I tend to have enough OX in the tank to kick that AP up to 60~ish, which doesn't require much, then run on nukes to orbit. 

See example craft below. I've run it with 4 whiplashes instead of the rapiers, but it's a loooong slog to orbit on the nukes alone. 2 of each can work, but you're paying micromanagement for very little gain. With rapiers, you can just let them autoswitch. Or better, hotkey the intakes, to force the autoswitch and reduce drag at the same time.

Heavily configurable, but with that forward long tank, this takes 6 kerbals to Minmus. Can be replaced with cargo bay for LKO operations.

Edited June 22, 2016 by Jarin