Launch directly to a hohman transfer?

[Sauron]

In real life, launching directly into your ejection burn is significantly more efficient than circularizing and then ejecting. So, two questions:

1: Is this also true in KSP?

2: What kind of equations do I need to run to make this happen (for starters let's assume the target is the Mun since it's the easiest thing to get to)?

[AlamoVampire]

I think its true in KSP, I did a direct launch to docking and it took minimal fuel burn to do so, as to calculations, I have no clue, when I am doing things fast tracked, its mechjeb all the way, otherwise, its the slow and steady circularize then hohmann and so forth.

[Sirine]

1) Yes

2) Learn the http://wiki.kerbalspaceprogram.com/wiki/Maneuver_node . Is a estimated forecast of the trajectory that you will go, and estimated how many Delta-V that you need.

(The Delta-V is base on the TOTAL ENGINES that you have, even they are not active.)

[AlamoVampire]

<blush> okay, so, I guess the maneuver nodes count as the equation XD

[TheHengeProphet]

Except you cannot have a maneuver node from the ground, so it's not quite accurate in that respect.

[Sauron]

Except you cannot have a maneuver node from the ground, so it's not quite accurate in that respect.

Precisely. I can launch directly to rendezvous by eye (it's not particularly difficult--Kerbin has convenient landmarks to place oneself, and everyone figures it out eventually). I'm talking about doing exactly that but with another celestial body. I need to know the math for this (can I apply the equations used for rendezvous directly?) and how to apply said math in practice (this is a technique question).

[Motokid600]

Launch to rendezvous is one of the hardest things for me to do in this game. I try and go for it everytime. to atleast get close. And other times I dfind my target floating next to me after circularization. Launching when the target rises on the eastern horizon helps. ( though varies depending on weight ) From there on its all about the right speed angle and inclination.

[Tiron]

Precisely. I can launch directly to rendezvous by eye (it's not particularly difficult--Kerbin has convenient landmarks to place oneself, and everyone figures it out eventually). I'm talking about doing exactly that but with another celestial body. I need to know the math for this (can I apply the equations used for rendezvous directly?) and how to apply said math in practice (this is a technique question).

Well, I think the only extra thing you need to know versus doing it normally would be the Launch Phase Angle of your craft: How far ahead of the ejection burn point you have to launch to end up at the ejection point when you hit that altitude. It varies per craft and is basically just another way of measuring how quickly your craft will reach the desired altitude.

Mechjeb's LPA and rendezvous features aren't quite as useful here as they might be: Mechjeb's LPA assumes you're shooting for a circular orbit, which if you're launching directly to an ejection burn there's not really any reason to bother with.

[Sauron]

Launch to rendezvous is one of the hardest things for me to do in this game. I try and go for it everytime. to atleast get close. And other times I dfind my target floating next to me after circularization. Launching when the target rises on the eastern horizon helps. ( though varies depending on weight ) From there on its all about the right speed angle and inclination.

I've always mangaged by eyeballing. In a 100x100 orbit, launching when the target is above the desert peninsula usually works. On smaller airless bodies I just guess and adjust in-flight.

But I don't want to dock with something, I want to hit the Mun (and then everything else ) without circularizing.

@Tiron:

So if I timed my launches carefully and measured phase angles carefully (by sticking a protractor to my screen or something o_O) I should be able to eyeball it? Hrmm. Will try and see if I can keep going at KSP without using any math scarier than 6th grade algebra

Edited September 3, 2013 by Sauron

[AlamoVampire]

id say, if you want to strike the Mun, launch just as you barely see it rise on the horizon, then burn like mad for it. crude, but brutal in its effectiveness. otherwise, try ksp.olex.biz, its a wonderful site, it does all the maths for you!!

[Tiron]

I've always mangaged by eyeballing. In a 100x100 orbit, launching when the target is above the desert peninsula usually works. On smaller airless bodies I just guess and adjust in-flight.

But I don't want to dock with something, I want to hit the Mun (and then everything else ) without circularizing.

@Tiron:

So if I timed my launches carefully and measured phase angles carefully (by sticking a protractor to my screen or something o_O) I should be able to eyeball it? Hrmm. Will try and see if I can keep going at KSP without using any math scarier than 6th grade algebra

I think so, at least in theory. If you timed your launch so you ended up at the right point at the right time to make the transfer, it should work. I think.

But then I'm kinda tired too.

Edit: It occurs to me there's the possibility that you might not be able to get to the right point right off launch without circularizing and at least doing a partial orbit, if it's around the other side of the planet, say. At the very least it'd be harder.

[1of6Billion]

- Look at position of Mün.

- Launch

- Burn until your apoapsis is approx 44.000km (Within Mün's SoI)

- Coast to apoapsis

- Look at position of Mün again

- Note difference between you and Mün

- Correct for difference and launch again

- Circularise in Mün's orbit

- Profit (?)

[RoboRay]

You can get pretty close by simply launching as KSC nears the proper ejection angle due to Kerbin's rotation. Get your Ap high enough to have some time to work with and set up a maneuver node to guide you through the rest of the transfer burn.

[Stochasty]

What you want to do is time your launch so that the top of your gravity turn corresponds with your correct ejection angle; then just keep burning.

Direct ascents are not difficult, although they take a little practice, and they are more efficient since you make better use of the Oberth effect by making your burn to transfer from a lower point. They can also be safer (in case of a mishap) because your periapsis will remain within Kerbin's atmosphere, so you won't be stranded if something goes wrong.

One caveat: a lot of people, when learning how to do a direct ascent, make the mistake of just going straight up. This is wrong; in order to get the efficiency benefit, you must make a gravity turn. Therefore, you want to launch a little bit before KSC reaches the appropriate ejection angle, because you will cover some distance during the ascent, and it's the top of your gravity turn (not the location of KSC) which must line up with your ejection burn.

A second caveat: if you have the flying skill to put yourself into a 70km by 70km orbit, a direct ascent gains you little, since you will need to be mostly out of the atmosphere before you start your transfer burn even for a direct ascent. The inefficiency of circularizing comes from spending fuel to raise your periapsis, but your periapsis will end up above about 60km no matter how you do it (unless you go straight up, which will cost you more in gravity losses than you save), so there isn't all that much difference with just circularizing at 70km and making a standard transfer. (It's possible to start your transfer burn lower than 60km, but the lower you go the more additional drag losses you incur, and thus you start to lose efficiency.)

[1of6Billion]

Come to think of it... When you do most of the burning/accelerating IN THE ATMOSPHERE as I described above, you may end up using more fuel then when you're doing the ascend/cicularise/transfer routine. I will give this a try tonight. (+4hrs from now).

[Spyritdragon]

Launch to rendezvous is one of the hardest things for me to do in this game. I try and go for it everytime. to atleast get close. And other times I dfind my target floating next to me after circularization. Launching when the target rises on the eastern horizon helps. ( though varies depending on weight ) From there on its all about the right speed angle and inclination.

Launch when your target just starts to rise over the horizon. That's what Scott Manley says at least - i tend to pretty much always save fuel for circularisation.

[Specialist290]

Lots of good advice in this thread.

One final note: I might be wrong on this (and someone else will almost certainly correct me if I am), but if the Mun is your target, I think that if you start a new game file, then at the beginning of the game you're pretty much perfectly placed to make a direct launch from Kerbin to the Mun with your first rocket.